Cartesian Therapeutics (RNAC) S-1/A 9 Sep 24 IPO registration (amended) Financial data

Document and Entity Information

Document and Entity Information	6 Months Ended
	Jun. 30, 2024
Cover [Abstract]
Document Type	S-1/A
Amendment Flag	false
Entity Registrant Name	Cartesian Therapeutics, Inc.
Entity Central Index Key	0001453687
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false

Consolidated Balance Sheets (Q2

Consolidated Balance Sheets (Q2) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 87,227	$ 76,911
Accounts receivable	32,039	5,870
Unbilled receivables	3,472	2,981
Prepaid expenses and other current assets	2,044	4,967
Total current assets	124,782	90,729
Non-current assets:
Property and equipment, net	6,672	2,113
Right-of-use asset, net	13,852	10,068
In-process research and development assets	150,600	150,600
Goodwill	48,163	48,163
Long-term restricted cash	1,669	1,377
Investments	2,000	2,000
Total assets	347,738	305,050
Current liabilities:
Accounts payable	2,862	3,150
Accrued expenses and other current liabilities	10,954	15,572
Lease liability	2,523	2,166
Deferred revenue	0	2,311
Loan payable		0
Warrant liabilities	1,205	720
Contingent value right liability	8,571	15,983
Forward contract liabilities	0	28,307
Total current liabilities	26,115	68,209
Non-current liabilities:
Loan payable, net of current portion		0
Lease liability, net of current portion	12,344	8,789
Deferred revenue, net of current portion	0	3,538
Warrant liabilities, net of current portion	8,055	5,674
Contingent value right liability, net of current portion	386,829	342,617
Deferred tax liabilities, net	15,853	15,853
Total liabilities	449,196	444,680
Commitments and contingencies (Note 18)
Series A Preferred Stock, $0.0001 par value; no and 548,375 shares authorized as of June 30, 2024 and December 31, 2023, respectively; no and 435,120.513 shares issued and outstanding as of June 30, 2024 and December 31, 2023, respectively	0	296,851
Options for Series A Preferred Stock	0	3,703
Stockholders' deficit:
Preferred stock, value, issued	0	0
Common stock, $0.0001 par value; 350,000,000 shares authorized as of June 30, 2024 and December 31, 2023; 17,816,238 and 5,397,597 shares issued and outstanding as of June 30, 2024 and December 31, 2023, respectively	2	1
Additional paid-in capital	560,766	179,062
Accumulated deficit	(657,635)	(614,647)
Accumulated other comprehensive loss	(4,591)	(4,600)
Total stockholders' (deficit) equity	(101,458)	(440,184)
Total liabilities, convertible preferred stock, and stockholders' (deficit) equity	347,738	305,050
Series A Preferred Stock
Stockholders' deficit:
Preferred stock, value, issued	$ 0	$ 0

Consolidated Balance Sheets (_2

Consolidated Balance Sheets (Q2) (Parenthetical) - $ / shares	Jun. 30, 2024	Dec. 31, 2023
Temporary equity, par value (in dollars per share)	$ 0.0001	$ 0.0001
Temporary equity, shares authorized (in shares)	0	548,375
Temporary equity, shares outstanding (in shares)	0	435,120.513
Temporary equity, shares issued (in shares)	0	435,120.513
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	9,819,544.247	9,451,625
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	350,000,000	350,000,000
Common stock, shares issued (in shares)	17,816,238	5,397,597
Common stock, shares outstanding (in shares)	17,816,238	5,397,597
Series A Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, shares authorized (in shares)	180,455.753	0
Preferred stock, shares issued (in shares)	166,341.592	0
Preferred stock, shares outstanding (in shares)	166,341.592	0

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Income (Loss) (Q2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Statement of Comprehensive Income [Abstract]
Collaboration and license revenue	$ 33,271	$ 5,249	$ 39,111	$ 11,187
Grant revenue	174	0	174	0
Total revenue	33,445	5,249	39,285	11,187
Operating expenses:
Research and development	12,661	17,782	22,399	36,406
General and administrative	7,027	6,105	16,477	11,800
Total operating expenses	19,688	23,887	38,876	48,206
Operating (loss) income	13,757	(18,638)	409	(37,019)
Investment income	1,195	1,394	2,359	2,725
Foreign currency transaction, net	0	23	0	42
Interest expense	0	(752)	0	(1,560)
Change in fair value of warrant liabilities	(3,908)	6,341	(2,866)	2,262
Change in fair value of contingent value right liability	2,500	0	(36,800)	0
Change in fair value of forward contract liabilities	0	0	(6,890)	0
Other income, net	292	245	800	500
Net income (loss)	13,836	(11,387)	(42,988)	(33,050)
Other comprehensive income (loss):
Foreign currency translation adjustment	14	(27)	9	(49)
Unrealized gain on marketable securities	0	0	0	11
Total comprehensive (loss) income	13,850	(11,414)	(42,979)	(33,088)
Net income (loss)	13,836	(11,387)	(42,988)	(33,050)
Less: Undistributed earnings allocable to participating securities, basic	(4,208)	0	0	0
Less: Undistributed earnings allocable to participating securities, diluted	(4,208)	0	0	0
Net (loss) income allocable to shares of common stock - basic	9,628	(11,387)	(42,988)	(33,050)
Net (loss) income allocable to shares of common stock - diluted	$ 9,628	$ (11,387)	$ (42,988)	$ (33,050)
Net income (loss) per share allocable to common stockholders:
Basic (in dollars per share)	$ 0.58	$ (2.23)	$ (3.88)	$ (6.46)
Diluted (in dollars per share)	$ 0.54	$ (2.23)	$ (3.88)	$ (6.46)
Weighted-average common shares outstanding:
Basic (in shares)	16,723,479	5,114,747	11,068,749	5,113,213
Diluted (in shares)	17,791,143	5,114,747	11,068,749	5,113,213

Consolidated Statements of Chan

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Q2) - USD ($) $ in Thousands	Total	Series A Preferred Stock	Common stock	Additional paid-in capital	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2020	0
Beginning balance at Dec. 31, 2020	$ 0
Ending balance (in shares) at Dec. 31, 2021	0
Ending balance at Dec. 31, 2021	$ 0
Beginning balance at Dec. 31, 2020	0
Ending balance at Dec. 31, 2021	0
Beginning balance (in shares) at Dec. 31, 2020			108,071,249
Beginning balance at Dec. 31, 2020	(18,006)		$ 11	$ 391,175	$ (404,629)	$ (4,563)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)			447,492
Issuance of common stock upon exercise of options	778			778
Issuance of common stock under Employee Stock Purchase Plan (in shares)			58,794
Issuance of common stock under Employee Stock Purchase Plan	161			161
Issuance of vested restricted stock units (in shares)			201,250
Stock-based compensation expense	7,720			7,720
Currency translation adjustment	(2)					(2)
Unrealized gain on marketable securities	(1)					(1)
Net (loss) income	(25,687)				(25,687)
Ending balance (in shares) at Dec. 31, 2021			123,622,965
Ending balance at Dec. 31, 2021	$ 22,521		$ 12	457,391	(430,316)	(4,566)
Ending balance (in shares) at Dec. 31, 2022	0
Ending balance at Dec. 31, 2022	$ 0
Ending balance at Dec. 31, 2022	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)			71,190
Issuance of common stock upon exercise of options	156			156
Issuance of common stock under Employee Stock Purchase Plan (in shares)			120,877
Issuance of common stock under Employee Stock Purchase Plan	189			189
Issuance of vested restricted stock units (in shares)			131,430
Stock-based compensation expense	10,194			10,194
Currency translation adjustment	18					18
Unrealized gain on marketable securities	(10)					(10)
Net (loss) income	35,379				35,379
Ending balance (in shares) at Dec. 31, 2022			5,101,459
Ending balance at Dec. 31, 2022	93,828		$ 1	493,322	(394,937)	(4,558)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)			3,584
Issuance of common stock under Employee Stock Purchase Plan	149			149
Issuance of vested restricted stock units (in shares)			9,226
Stock-based compensation expense	2,276			2,276
Currency translation adjustment	(22)					(22)
Unrealized gain on marketable securities	11					11
Net (loss) income	(21,663)				(21,663)
Ending balance (in shares) at Mar. 31, 2023			5,114,269
Ending balance at Mar. 31, 2023	$ 74,579		$ 1	495,747	(416,600)	(4,569)
Beginning balance (in shares) at Dec. 31, 2022	0
Beginning balance at Dec. 31, 2022	$ 0
Beginning balance at Dec. 31, 2022	0
Beginning balance (in shares) at Dec. 31, 2022			5,101,459
Beginning balance at Dec. 31, 2022	93,828		$ 1	493,322	(394,937)	(4,558)
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	(49)
Unrealized gain on marketable securities	11
Net (loss) income	(33,050)
Ending balance (in shares) at Jun. 30, 2023			5,114,289
Ending balance at Jun. 30, 2023	$ 65,448		$ 1	498,030	(427,987)	(4,596)
Beginning balance (in shares) at Dec. 31, 2022	0
Beginning balance at Dec. 31, 2022	$ 0
Ending balance (in shares) at Dec. 31, 2023	435,120.513
Ending balance at Dec. 31, 2023	$ 296,851
Beginning balance at Dec. 31, 2022	0
Ending balance at Dec. 31, 2023	3,703
Beginning balance (in shares) at Dec. 31, 2022			5,101,459
Beginning balance at Dec. 31, 2022	93,828		$ 1	493,322	(394,937)	(4,558)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)			186,044
Issuance of common stock under Employee Stock Purchase Plan	231			231
Issuance of vested restricted stock units (in shares)			636,418
Stock-based compensation expense	20,964			20,964
Currency translation adjustment	(53)					(53)
Unrealized gain on marketable securities	11					11
Net (loss) income	$ (219,710)				(219,710)
Ending balance (in shares) at Dec. 31, 2023	5,397,597	0
Ending balance at Dec. 31, 2023	$ (440,184)	$ 0	$ 1	179,062	(614,647)	(4,600)
Beginning balance (in shares) at Mar. 31, 2023			5,114,269
Beginning balance at Mar. 31, 2023	74,579		$ 1	495,747	(416,600)	(4,569)
Increase (Decrease) in Stockholders' Equity
Issuance of vested restricted stock units (in shares)			20
Stock-based compensation expense	2,283			2,283
Currency translation adjustment	(27)					(27)
Unrealized gain on marketable securities	0
Net (loss) income	(11,387)				(11,387)
Ending balance (in shares) at Jun. 30, 2023			5,114,289
Ending balance at Jun. 30, 2023	$ 65,448		$ 1	498,030	(427,987)	(4,596)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series A Preferred Stock forward in connection with private placement and settlement of related forward contract (in shares)	99,140.326
Issuance of Series A Preferred Stock in connection with private placement and settlement of related forward contract	$ 75,197
Transfer of Series A Preferred Stock and options for series A Preferred Stock to permanent equity (in shares)	(534,260.839)
Transfer of Series A Preferred Stock and options for Series A Preferred Stock to permanent equity	$ (372,048)
Ending balance (in shares) at Mar. 31, 2024	0
Ending balance at Mar. 31, 2024	$ 0
Increase (Decrease) In Temporary Equity, Option, Roll Forward [Roll Forward]
Transfer of Series A Preferred Stock and options for Series A Preferred Stock to permanent equity	(3,703)
Ending balance at Mar. 31, 2024	$ 0
Beginning balance (in shares) at Dec. 31, 2023	5,397,597	0
Beginning balance at Dec. 31, 2023	$ (440,184)	$ 0	$ 1	179,062	(614,647)	(4,600)
Increase (Decrease) in Stockholders' Equity
Transfer of Series A Preferred Stock and options for series A Preferred Stock to permanent equity (in shares)		534,260.839
Transfer of Series A Preferred Stock and options for Series A Preferred Stock to permanent equity	375,751			375,751
Issuance of common stock upon exercise of options (in shares)			52,558
Issuance of common stock upon exercise of options	154			154
Issuance of common stock upon exercise of warrants and Conversion of Series A Preferred Stock to common stock (in shares)			65,681
Issuance of common stock upon exercise of warrants and Conversion of Series A Preferred Stock to common stock	2,877			2,877
Stock-based compensation expense	1,431			1,431
Currency translation adjustment	(5)					(5)
Net (loss) income	(56,824)				(56,824)
Ending balance (in shares) at Mar. 31, 2024		534,260.839	5,515,836
Ending balance at Mar. 31, 2024	$ (116,800)	$ 0	$ 1	559,275	(671,471)	(4,605)
Beginning balance (in shares) at Dec. 31, 2023	435,120.513
Beginning balance at Dec. 31, 2023	$ 296,851
Ending balance (in shares) at Jun. 30, 2024	0
Ending balance at Jun. 30, 2024	$ 0
Beginning balance at Dec. 31, 2023	3,703
Ending balance at Jun. 30, 2024	$ 0
Beginning balance (in shares) at Dec. 31, 2023	5,397,597	0
Beginning balance at Dec. 31, 2023	$ (440,184)	$ 0	$ 1	179,062	(614,647)	(4,600)
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	9
Unrealized gain on marketable securities	0
Net (loss) income	$ (42,988)
Ending balance (in shares) at Jun. 30, 2024	17,816,238	166,341.592	17,816,238
Ending balance at Jun. 30, 2024	$ (101,458)	$ 0	$ 2	560,766	(657,635)	(4,591)
Beginning balance (in shares) at Mar. 31, 2024	0
Beginning balance at Mar. 31, 2024	$ 0
Ending balance (in shares) at Jun. 30, 2024	0
Ending balance at Jun. 30, 2024	$ 0
Beginning balance at Mar. 31, 2024	0
Ending balance at Jun. 30, 2024	0
Beginning balance (in shares) at Mar. 31, 2024		534,260.839	5,515,836
Beginning balance at Mar. 31, 2024	(116,800)	$ 0	$ 1	559,275	(671,471)	(4,605)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)			36,451
Issuance of common stock upon exercise of options	120			120
Issuance of common stock upon exercise of warrants and Conversion of Series A Preferred Stock to common stock (in shares)		(367,919.247)	12,263,951
Issuance of common stock upon exercise of warrants and Conversion of Series A Preferred Stock to common stock	0		$ 1	(1)
Equity offering costs	(219)				(219)
Stock-based compensation expense	1,591					1,591
Currency translation adjustment	14					14
Unrealized gain on marketable securities	0
Net (loss) income	$ 13,836				13,836
Ending balance (in shares) at Jun. 30, 2024	17,816,238	166,341.592	17,816,238
Ending balance at Jun. 30, 2024	$ (101,458)	$ 0	$ 2	$ 560,766	$ (657,635)	$ (4,591)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (Q2) (Parenthetical)	Apr. 04, 2024
Statement of Stockholders' Equity [Abstract]
Reverse stock split conversion ratio	0.0333

Consolidated Statements of Cash

Consolidated Statements of Cash Flows (Q2) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023
Cash flows from operating activities
Net (loss) income	$ (42,988)	$ (33,050)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	379	382
Amortization of premiums and discounts on marketable securities	0	(79)
Non-cash lease expense	1,113	842
Loss on disposal of property and equipment	(2)	0
Stock-based compensation expense	3,022	6,059
Non-cash interest expense	0	533
Warrant liabilities revaluation	2,866	(2,262)
Contingent value right liability revaluation	36,800	0
Forward contract liabilities revaluation	6,890	0
Changes in operating assets and liabilities:
Accounts receivable	(26,169)	1,211
Unbilled receivable	(491)	2,107
Prepaid expenses, deposits and other assets	2,848	815
Accounts payable	(290)	(49)
Deferred revenue	(5,849)	8,504
Accrued expenses and other liabilities	(8,496)	(3,673)
Net cash used in operating activities	(30,363)	(18,660)
Cash flows from investing activities
Proceeds from maturities of marketable securities	0	28,254
Purchases of property and equipment	(2,189)	(142)
Net cash (used in) provided by investing activities	(2,189)	28,112
Cash flows from financing activities
Repayments of principal, final payment fee, and prepayment penalty on debt	0	(2,586)
Proceeds from exercise of common warrants	2,877	0
Proceeds from issuance of Series A Preferred Stock, gross in private placement	40,000	0
Proceeds from exercise of stock options	274	0
Proceeds from issuance of common stock under Employee Stock Purchase Plan	0	149
Net cash provided by (used in) financing activities	43,151	(2,437)
Effect of exchange rate changes on cash	9	(49)
Net change in cash, cash equivalents, and restricted cash	10,608	6,966
Cash, cash equivalents, and restricted cash at beginning of period	78,288	108,038
Cash, cash equivalents, and restricted cash at end of period	88,896	115,004
Supplemental cash flow information
Cash paid for interest	0	1,242
Noncash investing and financing activities
Stock-based compensation expense in accrued liabilities	0	1,500
Purchase of property and equipment not yet paid	2,879	48
Equity offering costs in accrued liabilities	$ 219	$ 0

Consolidated Balance Sheets (FY

Consolidated Balance Sheets (FY) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 76,911	$ 106,438
Marketable securities	0	28,164
Accounts receivable	5,870	6,596
Unbilled receivables	2,981	3,162
Prepaid expenses and other current assets	4,967	3,778
Total current assets	90,729	148,138
Non-current assets:
Property and equipment, net	2,113	2,794
Right-of-use asset, net	10,068	11,617
In-process research and development assets	150,600	0
Goodwill	48,163	0
Long-term restricted cash	1,377	1,311
Investments	2,000	2,000
Other assets	0	26
Total assets	305,050	165,886
Current liabilities:
Accounts payable	3,150	316
Accrued expenses and other current liabilities	15,572	14,084
Loan payable	0	8,476
Lease liability	2,166	1,608
Deferred revenue	2,311	593
Warrant liabilities	720	0
Contingent value right liability	15,983	0
Forward contract liabilities	28,307	0
Total current liabilities	68,209	25,077
Non-current liabilities:
Loan payable, net of current portion	0	17,786
Lease liability, net of current portion	8,789	10,055
Deferred revenue, net of current portion	3,538	0
Warrant liabilities, net of current portion	5,674	19,140
Contingent value right liability, net of current portion	342,617	0
Deferred tax liabilities, net	15,853	0
Total liabilities	444,680	72,058
Commitments and contingencies (Note 19)
Series A Preferred Stock, $0.0001 par value; 548,375 and no shares authorized as of December 31, 2023 and December 31, 2022, respectively; 435,120.513 and no shares issued and outstanding as of December 31, 2023 and December 31, 2022, respectively	296,851	0
Options for Series A Preferred Stock	3,703	0
Stockholders' (deficit) equity:
Preferred stock, $0.0001 par value; 9,451,625 and 10,000,000 shares authorized as of December 31, 2023 and December 31, 2022, respectively; no shares issued and outstanding as of December 31, 2023 and December 31, 2022	0	0
Common stock, $0.0001 par value; 350,000,000 shares authorized as of December 31, 2023 and December 31, 2022; 161,927,821 and 153,042,435 shares issued and outstanding as of December 31, 2023 and December 31, 2022, respectively	1
Additional paid-in capital	179,062
Accumulated deficit	(614,647)	(394,937)
Accumulated other comprehensive loss	(4,600)	(4,558)
Total stockholders' (deficit) equity	(440,184)	93,828
Total liabilities, convertible preferred stock, and stockholders' (deficit) equity	305,050	165,886
Previously Reported [Member]
Stockholders' (deficit) equity:
Common stock, $0.0001 par value; 350,000,000 shares authorized as of December 31, 2023 and December 31, 2022; 161,927,821 and 153,042,435 shares issued and outstanding as of December 31, 2023 and December 31, 2022, respectively	16	15
Additional paid-in capital	$ 179,047	$ 493,308

Consolidated Balance Sheets (_3

Consolidated Balance Sheets (FY) (Parenthetical) - $ / shares	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Nov. 13, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Temporary equity, par value (in dollars per share)	$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Temporary equity, shares authorized (in shares)	0		548,375	548,375	0
Temporary equity, shares issued (in shares)	0		435,120.513		0
Temporary equity, shares outstanding (in shares)	0	0	435,120.513		0	0	0
Preferred stock, par value (in dollars per share)	$ 0.0001		$ 0.0001		$ 0.0001
Preferred stock, shares authorized (in shares)	9,819,544.247		9,451,625		10,000,000
Preferred stock, shares issued (in shares)	0		0		0
Preferred stock, shares outstanding (in shares)	0		0		0
Common stock, par value (in dollars per share)	$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized (in shares)	350,000,000		350,000,000		350,000,000
Common stock, shares issued (in shares)	17,816,238		5,397,597
Common stock, shares outstanding (in shares)	17,816,238		5,397,597
Previously Reported [Member]
Common stock, shares issued (in shares)			161,927,821		153,042,435
Common stock, shares outstanding (in shares)			161,927,821		153,042,435

Consolidated Statements of Op_2

Consolidated Statements of Operations and Comprehensive Income (Loss) (FY) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Collaboration and license revenue	$ 26,004	$ 110,777	$ 85,077
Operating expenses:
Research and development	71,839	72,377	68,736
General and administrative	40,581	23,862	20,938
Total operating expenses	112,420	96,239	89,674
Operating (loss) income	(86,416)	14,538	(4,597)
Investment income	4,964	2,073	44
Foreign currency transaction gain (loss), net	38	(22)	0
Interest expense	(2,833)	(3,031)	(2,844)
Change in fair value of warrant liabilities	12,746	20,882	(2,339)
Change in fair value of contingent value right liability	(18,300)	0	0
Change in fair value of forward contract liabilities	(149,600)	0	0
Other income, net	691	330	15
(Loss) income before income taxes	(238,710)	34,770	(9,721)
Income tax benefit (expense)	19,000	609	(15,966)
Net income (loss)	(219,710)	35,379	(25,687)
Other comprehensive (loss) income:
Foreign currency translation adjustment	(53)	18	(2)
Unrealized gain (loss) on marketable securities	11	(10)	(1)
Total comprehensive (loss) income	$ (219,752)	$ 35,387	$ (25,690)
Net (loss) income per share:
Basic (in dollars per share)	$ (1.66)	$ 0.24	$ (0.22)
Diluted (in dollars per share)	$ (1.66)	$ 0.1	$ (0.22)
Weighted-average common shares outstanding:
Basic (in shares)	155,109,561	144,758,555	114,328,798
Diluted (in shares)	155,109,561	145,874,889	114,328,798

Consolidated Statements of Ch_3

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity (Deficit) (FY) - USD ($) $ in Thousands	Total	Previously Reported [Member]	Private Placement One	Private Placement Two	Common stock	Common stock Previously Reported [Member]	Additional paid-in capital	Additional paid-in capital Previously Reported [Member]	Accumulated deficit	Accumulated other comprehensive loss
Beginning balance (in shares) at Dec. 31, 2020	0
Beginning balance at Dec. 31, 2020	$ 0
Ending balance (in shares) at Dec. 31, 2021	0
Ending balance at Dec. 31, 2021	$ 0
Beginning balance at Dec. 31, 2020	0
Ending balance at Dec. 31, 2021	0
Beginning balance (in shares) at Dec. 31, 2020					108,071,249
Beginning balance at Dec. 31, 2020	(18,006)				$ 11		$ 391,175		$ (404,629)	$ (4,563)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)					58,794
Issuance of common stock under Employee Stock Purchase Plan	161						161
Issuance of common stock upon exercise of options (in shares)					447,492
Issuance of common stock upon exercise of options	778						778
Issuance of vested restricted stock units (in shares)					201,250
Issuance of common stock through at-the-market offering, net (in shares)					13,767,511
Issuance of common stock through at-the-market offering, net	51,934				$ 1		51,933
Issuance of common stock upon exercise of common warrants (in shares)					1,076,669
Issuance of common stock upon exercise of warrants	5,624				$ 0		5,624
Stock-based compensation expense	7,720						7,720
Currency translation adjustment	(2)									(2)
Unrealized gain (loss) on marketable securities	(1)									(1)
Net (loss) income	(25,687)								(25,687)
Ending balance (in shares) at Dec. 31, 2021					123,622,965
Ending balance at Dec. 31, 2021	$ 22,521				$ 12		457,391		(430,316)	(4,566)
Ending balance (in shares) at Dec. 31, 2022	0
Ending balance at Dec. 31, 2022	$ 0
Ending balance at Dec. 31, 2022	0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)					120,877
Issuance of common stock under Employee Stock Purchase Plan	189						189
Issuance of common stock upon exercise of options (in shares)					71,190
Issuance of common stock upon exercise of options	156						156
Issuance of vested restricted stock units (in shares)					131,430
Issuance of common stock through at-the-market offering, net (in shares)					774,544
Issuance of common stock through at-the-market offering, net	2,121				$ 0		2,121
Issuance of common stock and common warrants (in shares)					27,428,572
Issuance of common stock and common warrants	21,480				$ 3		21,477
Issuance of common stock, license agreement (in shares)					892,857
Issuance of common stock, license agreement	1,000						1,000
Reclassification of warrant liabilities	780						780
Stock-based compensation expense	10,194						10,194
Currency translation adjustment	18									18
Unrealized gain (loss) on marketable securities	(10)									(10)
Net (loss) income	35,379								35,379
Ending balance (in shares) at Dec. 31, 2022		153,042,435			5,101,459	153,042,435
Ending balance at Dec. 31, 2022	93,828				$ 1	$ 15	493,322	$ 493,308	(394,937)	(4,558)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)					3,584
Issuance of common stock under Employee Stock Purchase Plan	149						149
Issuance of vested restricted stock units (in shares)					9,226
Stock-based compensation expense	2,276						2,276
Currency translation adjustment	(22)									(22)
Unrealized gain (loss) on marketable securities	11									11
Net (loss) income	(21,663)								(21,663)
Ending balance (in shares) at Mar. 31, 2023					5,114,269
Ending balance at Mar. 31, 2023	$ 74,579				$ 1		495,747		(416,600)	(4,569)
Beginning balance (in shares) at Dec. 31, 2022	0
Beginning balance at Dec. 31, 2022	$ 0
Beginning balance at Dec. 31, 2022	0
Beginning balance (in shares) at Dec. 31, 2022		153,042,435			5,101,459	153,042,435
Beginning balance at Dec. 31, 2022	93,828				$ 1	$ 15	493,322	493,308	(394,937)	(4,558)
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	(49)
Unrealized gain (loss) on marketable securities	11
Net (loss) income	(33,050)
Ending balance (in shares) at Jun. 30, 2023					5,114,289
Ending balance at Jun. 30, 2023	$ 65,448				$ 1		498,030		(427,987)	(4,596)
Beginning balance (in shares) at Dec. 31, 2022	0
Beginning balance at Dec. 31, 2022	$ 0
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series A Preferred Stock forward in connection with private placement and settlement of related forward contract (in shares)			619.627	49,570.162
Issuance of Series A Preferred Stock forward in connection with private placement and settlement of related forward contract			$ 250	$ 34,848
Issuance of Series A Preferred Stock in connection with the Merger and settlement of related forward contract (in shares)	384,930.724
Issuance of Series A Preferred Stock in connection with the Merger and settlement of related forward contract	$ 261,753
Ending balance (in shares) at Dec. 31, 2023	435,120.513
Ending balance at Dec. 31, 2023	$ 296,851
Beginning balance at Dec. 31, 2022	0
Increase (Decrease) In Temporary Equity, Option, Roll Forward [Roll Forward]
Issuance of replacement options in Merger	3,643
Stock-based compensation expense	60
Ending balance at Dec. 31, 2023	3,703
Beginning balance (in shares) at Dec. 31, 2022		153,042,435			5,101,459	153,042,435
Beginning balance at Dec. 31, 2022	93,828				$ 1	$ 15	493,322	493,308	(394,937)	(4,558)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock under Employee Stock Purchase Plan (in shares)					186,044
Issuance of common stock under Employee Stock Purchase Plan	231						231
Issuance of vested restricted stock units (in shares)					636,418
Issuance of common stock forward in connection with the Merger	2,713						2,713
Issuance of common stock in connection with the Merger and settlement of related forward contract (in shares)					6,723,639
Issuance of common stock in connection with the Merger and settlement of related forward contract	0				$ 1		(1)
Issuance of replacement options in Merger	6,801						6,801
Issuance of common stock, license agreement (in shares)					1,339,285
Issuance of common stock, license agreement	1,500						1,500
Settlement of outstanding equity awards at Merger	(6,169)						(6,169)
Distribution of contingent value rights	(340,300)						(340,300)
Stock-based compensation expense	20,964						20,964
Currency translation adjustment	(53)									(53)
Unrealized gain (loss) on marketable securities	11									11
Net (loss) income	$ (219,710)								(219,710)
Ending balance (in shares) at Dec. 31, 2023	5,397,597	161,927,821				161,927,821
Ending balance at Dec. 31, 2023	$ (440,184)				$ 1	$ 16	179,062	179,047	(614,647)	(4,600)
Beginning balance (in shares) at Mar. 31, 2023					5,114,269
Beginning balance at Mar. 31, 2023	74,579				$ 1		495,747		(416,600)	(4,569)
Increase (Decrease) in Stockholders' Equity
Issuance of vested restricted stock units (in shares)					20
Stock-based compensation expense	2,283						2,283
Currency translation adjustment	(27)									(27)
Unrealized gain (loss) on marketable securities	0
Net (loss) income	(11,387)								(11,387)
Ending balance (in shares) at Jun. 30, 2023					5,114,289
Ending balance at Jun. 30, 2023	$ 65,448				$ 1		498,030		(427,987)	(4,596)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series A Preferred Stock forward in connection with private placement and settlement of related forward contract (in shares)	99,140.326
Issuance of Series A Preferred Stock forward in connection with private placement and settlement of related forward contract	$ 75,197
Ending balance (in shares) at Mar. 31, 2024	0
Ending balance at Mar. 31, 2024	$ 0
Ending balance at Mar. 31, 2024	$ 0
Beginning balance (in shares) at Dec. 31, 2023	5,397,597	161,927,821				161,927,821
Beginning balance at Dec. 31, 2023	$ (440,184)				$ 1	$ 16	179,062	179,047	(614,647)	(4,600)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)					52,558
Issuance of common stock upon exercise of options	154						154
Stock-based compensation expense	1,431						1,431
Currency translation adjustment	(5)									(5)
Net (loss) income	(56,824)								(56,824)
Ending balance (in shares) at Mar. 31, 2024					5,515,836
Ending balance at Mar. 31, 2024	$ (116,800)				$ 1		559,275		(671,471)	(4,605)
Beginning balance (in shares) at Dec. 31, 2023	435,120.513
Beginning balance at Dec. 31, 2023	$ 296,851
Ending balance (in shares) at Jun. 30, 2024	0
Ending balance at Jun. 30, 2024	$ 0
Beginning balance at Dec. 31, 2023	3,703
Ending balance at Jun. 30, 2024	$ 0
Beginning balance (in shares) at Dec. 31, 2023	5,397,597	161,927,821				161,927,821
Beginning balance at Dec. 31, 2023	$ (440,184)				$ 1	$ 16	179,062	$ 179,047	(614,647)	(4,600)
Increase (Decrease) in Stockholders' Equity
Currency translation adjustment	9
Unrealized gain (loss) on marketable securities	0
Net (loss) income	$ (42,988)
Ending balance (in shares) at Jun. 30, 2024	17,816,238				17,816,238
Ending balance at Jun. 30, 2024	$ (101,458)				$ 2		560,766		(657,635)	(4,591)
Beginning balance (in shares) at Mar. 31, 2024	0
Beginning balance at Mar. 31, 2024	$ 0
Ending balance (in shares) at Jun. 30, 2024	0
Ending balance at Jun. 30, 2024	$ 0
Beginning balance at Mar. 31, 2024	0
Ending balance at Jun. 30, 2024	0
Beginning balance (in shares) at Mar. 31, 2024					5,515,836
Beginning balance at Mar. 31, 2024	(116,800)				$ 1		559,275		(671,471)	(4,605)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options (in shares)					36,451
Issuance of common stock upon exercise of options	120						120
Stock-based compensation expense	1,591									1,591
Currency translation adjustment	14									14
Unrealized gain (loss) on marketable securities	0
Net (loss) income	$ 13,836								13,836
Ending balance (in shares) at Jun. 30, 2024	17,816,238				17,816,238
Ending balance at Jun. 30, 2024	$ (101,458)				$ 2		$ 560,766		$ (657,635)	$ (4,591)

Consolidated Statements of Ca_2

Consolidated Statements of Cash Flows (FY) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities
Net (loss) income	$ (219,710)	$ 35,379	$ (25,687)
Adjustments to reconcile net (loss) income to net cash used in operating activities:
Depreciation and amortization	843	1,287	1,252
Amortization of premiums and discounts on marketable securities	(79)	(375)	57
Non-cash lease expense	1,754	1,337	1,119
Impairment of Right of use asset	710	0	0
Loss (gain) on disposal of property and equipment	477	(147)	0
Stock-based compensation expense	22,524	11,194	7,720
Non-cash interest expense	455	953	1,012
Warrant liabilities revaluation	(12,746)	(20,882)	2,339
Contingent value right liability revaluation	18,300	0	0
Forward contract liabilities revaluation	149,600	0	0
Loss on extinguishment of debt	740	0	0
Provision (benefit) for deferred taxes	(19,000)	0	0
Changes in operating assets and liabilities:
Accounts receivable	726	3,318	(2,690)
Unbilled receivable	181	(3,162)	0
Prepaid expenses, deposits and other assets	(1,265)	2,471	(1,451)
Accounts payable	2,834	92	(219)
Income taxes payable	0	(601)	601
Deferred revenue	5,256	(64,707)	(45,496)
Accrued expenses and other liabilities	(2,761)	2,212	1,061
Net cash used in operating activities	(51,161)	(31,631)	(60,382)
Cash flows from investing activities
Cash assumed in acquisition of Old Cartesian	6,561	0	0
Proceeds from maturities of marketable securities	28,254	19,700	16,400
Payment made for investments	0	0	(2,000)
Purchases of marketable securities	0	(33,501)	(30,455)
Purchases of property and equipment	(206)	(1,201)	(1,085)
Net cash (used in) provided by investing activities	34,609	(15,002)	(17,140)
Cash flows from financing activities
Proceeds from issuance of Series A Preferred Stock, gross in private placement	20,250	0	0
Repayments of principal, final payment fee, and prepayment penalty on debt	(27,457)	0	0
Debt amendment fee included in debt discount	0	(110)	0
Net proceeds from issuance of common stock and common warrants	0	36,859	0
Settlement of outstanding equity awards at Merger	(6,169)	0	0
Proceeds from exercise of stock options	0	156	778
Proceeds from issuance of common stock under Employee Stock Purchase Plan	231	189	161
Net cash provided by (used in) financing activities	(13,145)	39,215	52,897
Effect of exchange rate changes on cash	(53)	20	(3)
Net change in cash, cash equivalents, and restricted cash	(29,750)	(7,398)	(24,628)
Cash, cash equivalents, and restricted cash at beginning of period	108,038	115,436	140,064
Cash, cash equivalents, and restricted cash at end of period	78,288	108,038	115,436
Supplement cash flow information
Cash paid for interest	1,853	2,248	2,002
Non-cash investing and financing activities
Issuance of common stock, license agreement in stock-based compensation expense	1,500	1,000	0
Cashless warrant exercise	0	0	5,624
Reclassification of warrant liability to equity	0	780	0
Purchase of property and equipment not yet paid	128	17	224
At-The-Market Offering
Cash flows from financing activities
Net proceeds from issuance of common stock- at-the-market offering	$ 0	$ 2,121	$ 51,958

Description of the Business (Q2

Description of the Business (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the Business	Description of the Business Cartesian Therapeutics, Inc., or the Company (formerly known as Selecta Biosciences, Inc., or Selecta), was incorporated in Delaware on December 10, 2007, and is headquartered in Gaithersburg, Maryland. The Company is a clinical-stage biotechnology company developing mRNA cell therapies for the treatment of autoimmune diseases leveraging its proprietary technology and manufacturing platform to introduce one or more mRNA molecules into cells to enhance their function. The Company believes its mRNA cell therapies have the potential to deliver deep, durable clinical benefit to a broad group of patients with autoimmune diseases because they can be administered over a short period of time, in an outpatient setting, and without pre-treatment chemotherapy. On November 13, 2023, the Company acquired, in accordance with the terms of the Agreement and Plan of Merger, or the Merger Agreement, the assets of the Delaware corporation which, immediately prior to the Merger (as defined below), was known as Cartesian Therapeutics, Inc., or Old Cartesian, as disclosed in Note 3. The transaction was structured as a stock-for-stock transaction pursuant to which all of Old Cartesian’s outstanding shares of capital stock were exchanged based on a fixed exchange ratio for consideration of 224,099 shares of the common stock, par value $0.0001 per share, of the Company, or the common stock, and 384,930.724 shares of the newly designated Series A Non-Voting Convertible Preferred Stock, par value $0.0001 per share, or the Series A Preferred Stock. The Series A Preferred Stock is intended to have economic rights similar to the common stock, but with only limited voting rights. Additionally, the Company assumed all outstanding stock options of Old Cartesian. The common stock and Series A Preferred Stock related to the Merger were issued on December 5, 2023. For additional information, see Note 3. In connection with the Merger, the Company entered into a definitive agreement, or the Securities Purchase Agreement, for a private investment in public equity transaction, or the November 2023 Private Placement, with the Investors (as defined below). The Securities Purchase Agreement provides for the issuance to the Investors of an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of approximately $60.25 million. For additional information, see Note 10. In connection with the Merger, a contractual contingent value right, or CVR, was distributed to the holders of record of the Company's common stock and 2022 Warrants (as defined below) as of the close of business on December 4, 2023, but was not distributed to holders of shares of common stock or Series A Preferred Stock issued to stockholders of Old Cartesian or the Investors in the transactions. Holders of the CVRs will be entitled to receive certain payments from proceeds received by the Company, if any, related to the disposition or monetization of the Company's legacy assets following the issuance of the CVRs. For additional information, see Note 5. On March 27, 2024, the Company’s stockholders approved the Conversion Proposal (as defined below). For additional information, see Note 10. Additionally, on March 27, 2024, the Company’s stockholders approved an amendment to the Company’s restated certificate of incorporation, as amended, or the Charter, to effect a reverse stock split of the Company’s issued and outstanding common stock, at a ratio in the range of 1-for-20 and 1-for-30, with such ratio to be determined at the discretion of the Company’s board of directors, or the Board of Directors. The Board of Directors subsequently approved a final reverse stock split ratio of 1-for-30, and the Company effected the Reverse Stock Split on April 4, 2024. As a result of the Reverse Stock Split, all figures in this Quarterly Report on Form 10-Q relating to shares of the Company’s common stock (such as share amounts, per share amounts, and conversion rates and prices), have been adjusted to reflect the Reverse Stock Split for all periods presented, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. Shares of common stock underlying outstanding stock options, restricted stock units and warrants were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with their terms. Additionally, the conversion ratio of the Company’s Series A Preferred Stock was proportionally adjusted. Stockholders entitled to fractional shares as a result of the Reverse Stock Split received a cash payment in lieu of receiving fractional shares. On July 2, 2024, the Company entered into a securities purchase agreement, or the July 2024 Purchase Agreement, for a private investment in public equity financing, or the July 2024 Private Placement, which provides for the issuance of 3,563,247 shares of common stock and 2,937,903 shares of Series B Non-Voting Convertible Preferred Stock, par value $0.0001 per share, or the Series B Preferred Stock, each at a purchase price of $20.00 per share. The July 2024 Private Placement resulted in gross proceeds of approximately $130.0 million before deducting placement agent fees and other offering expenses. For additional information, see Note 20. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in pre-clinical and clinical development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three and six months ended June 30, 2024 and 2023 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC, for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America, or U.S. GAAP, have been condensed or omitted pursuant to such rules and regulations. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2023 included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 7, 2024. The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments that are necessary for a fair statement of the Company’s financial position as of June 30, 2024, the consolidated results of operations for the three and six months ended June 30, 2024, and cash flows for the six months ended June 30, 2024. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2024 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2024. Liquidity and Management’s Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain and sustain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital, which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through public offerings and private placements of its securities, funding received from research grants, collaboration and license arrangements and a credit facility. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, the Company’s revenue has primarily been from collaboration agreements. The Company has devoted substantially all of its financial resources and efforts to developing its existing product candidates, identifying potential product candidates and conducting preclinical studies and clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any product candidates. As of June 30, 2024, the Company’s cash, cash equivalents, and restricted cash were $88.9 million, of which $1.7 million was restricted cash related to lease commitments and $0.2 million was held by its Russian subsidiary designated solely for use in its operations. The Company believes the cash, cash equivalents and restricted cash as of June 30, 2024, combined with net proceeds of $124.4 million from the July 2024 Private Placement received subsequent to June 30, 2024, will enable it to fund its current planned operations for at least the next twelve months from the date of issuance of these financial statements, though it may pursue additional cash resources through public or private equity or debt financings or by establishing collaborations with other companies. Management’s expectations with respect to its ability to fund current and long term planned operations are based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional strategic or financing opportunities sooner than would otherwise be expected. However, there is no guarantee that any collaboration milestones will be achieved or that any of these strategic or financing opportunities will be executed on favorable terms, and some could be dilutive to existing stockholders. Further, the liability associated with the CVR Agreement (as defined below) will be settled solely through cash flow received under the Company's License and Development Agreement, or as so amended, the Sobi License, with Swedish Orphan Biovitrum AB (publ.), or Sobi, and any other Gross Proceeds (as defined in the CVR Agreement) net of certain agreed deductions. Under the CVR Agreement, 100% of all milestone payments, royalties and other amounts paid to the Company or controlled entities under the Sobi License, and any other Gross Proceeds will be distributed, net of specified deductions, to holders of the CVRs. There is no obligation to the Company to fund any amount related to the CVR liability. See Note 5. If the Company is unable to obtain additional funding on a timely basis, it may be forced to significantly curtail, delay, or discontinue one or more of its planned research or development programs or be unable to expand its operations or otherwise capitalize on its commercialization of its product candidates. As of June 30, 2024, the Company had an accumulated deficit of $657.6 million. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research and development of its product candidates and its administrative organization. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at the Company. Through June 30, 2024, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.	Description of the Business Cartesian Therapeutics, Inc., or the Company, (formerly known as Selecta Biosciences, Inc., or Selecta) was incorporated in Delaware on December 10, 2007, and is headquartered in Gaithersburg, Maryland. The Company is a clinical-stage biotechnology company developing mRNA cell therapies for the treatment of autoimmune diseases leveraging its proprietary technology and manufacturing platform to introduce one or more mRNA molecules into cells to enhance their function. The Company believes its mRNA cell therapies have the potential to deliver deep, durable clinical benefit to a broad group of patients with autoimmune diseases because they can be administered over a short period of time, in an outpatient setting, and without pre-treatment chemotherapy. On November 13, 2023, the Company acquired, in accordance with the terms of the Agreement and Plan of Merger, or the Merger Agreement, the assets of the Delaware corporation which, immediately prior to the Merger (as defined below), was known as Cartesian Therapeutics, Inc., or Old Cartesian, as disclosed in Note 3. The transaction was structured as a stock-for-stock transaction pursuant to which all of Old Cartesian’s outstanding shares of capital stock were exchanged based on a fixed exchange ratio for consideration of 6,723,639 shares of the common stock, $0.0001 per share, of the Company and 384,930.724 shares of the newly designated Series A Non-Voting Convertible Preferred Stock, $0.0001 per share, or the Series A Preferred Stock. The Series A Preferred Stock is intended to have economic rights similar to the common stock, but with only limited voting rights. Additionally, the Company assumed all outstanding stock options of Old Cartesian. The common stock and Series A Preferred Stock related to the Merger were issued on December 5, 2023. For additional information, see Note 3. In connection with the Merger, the Company entered into a definitive agreement, or the Securities Purchase Agreement, for a private investment in public equity transaction, or the November 2023 Private Placement, with the Investors (as defined below). The Securities Purchase Agreement provides for the issuance to the Investors of an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of approximately $60.25 million. For additional information, see Note 11. In connection with the Merger, a contractual contingent value right, or CVR, was distributed to the holders of record of the Company's common stock and 2022 Warrants as of the close of business on December 4, 2023, but was not distributed to holders of shares of common stock or Series A Preferred Stock issued to stockholders of Old Cartesian or the Investors in the transactions. Holders of the CVRs will be entitled to receive certain payments from proceeds received by the Company, if any, related to the disposition or monetization of the Company's legacy assets following the issuance of the CVRs. For additional information, see Note 6. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in pre-clinical and clinical development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Liquidity and Management’s Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain and sustain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital, which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through public offerings and private placements of its securities, funding received from research grants, collaboration and license arrangements and a credit facility. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, the Company’s revenue has primarily been from collaboration agreements. The Company has devoted substantially all of its financial resources and efforts to developing its existing product candidates, identifying potential product candidates and conducting preclinical studies and clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any product candidates. As of December 31, 2023, the Company’s cash, cash equivalents, and restricted cash were $78.3 million, of which $1.4 million was restricted cash related to lease commitments and $0.2 million was held by its Russian subsidiary designated solely for use in its operations. The Company believes the cash, cash equivalents and restricted cash as of December 31, 2023 combined with net proceeds of $40.0 million received subsequent to December 31, 2023 from the November 2023 Private Placement will enable it to fund its current planned operations for at least the next twelve months from the date of issuance of these financial statements, though it may pursue additional cash resources through public or private equity or debt financings or by establishing collaborations with other companies. Management’s expectations with respect to its ability to fund current and long term planned operations are based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional strategic or financing opportunities sooner than would otherwise be expected. However, there is no guarantee that any collaboration milestones will be achieved or that any of these strategic or financing opportunities will be executed on favorable terms, and some could be dilutive to existing stockholders. Further, the liability associated with the CVR Agreement (as defined below) will be settled solely through cash flow received under the Company's License and Development Agreement, or as so amended, the Sobi License, with Swedish Orphan Biovitrum AB (publ.), or Sobi, and any other Gross Proceeds (as defined in the CVR Agreement) net of certain agreed deductions. Under the CVR Agreement, 100% of all milestone payments, royalties and other amounts paid to the Company or controlled entities under the Sobi License, and any other Gross Proceeds will be distributed, net of specified deductions, to holders of the CVRs. There is no obligation to the Company to fund any amount related to the CVR liability. See Note 6. The Certificate of Designation of Preferences, Rights, and Limitations of the Series A Non-Voting Convertible Preferred Stock, or the Certificate of Designation, contains a provision granting each holder of the Series A Preferred Stock the option to require the Company to redeem any or all of such holder’s then-outstanding shares of Series A Preferred Stock beginning on the date that is 18 months following the date of the closing of the Merger, November 13, 2023, at a price per share equal to the ten-day trailing average closing trading price of the common stock at such time; provided, however, that no holder will have the right to seek redemption of any shares of Series A Preferred Stock to the extent that such holder would otherwise be unable to convert such shares of Series A Preferred Stock due to the common stock beneficial ownership limitation applicable to such holder. The Company could be required to use a significant amount of its cash resources on hand to satisfy this redemption obligation, particularly if its stockholders do not ever approve a proposal to convert the Company’s Series A Preferred Stock into common stock, or generally if holders of Series A Preferred Stock exercise their redemption right with respect to a significant number of shares of Series A Preferred Stock or at a time when the trading price of the Company’s common stock is elevated. Further, in the event that the Company does not have sufficient cash on hand to satisfy its redemption obligations, the Company may need to raise additional capital to satisfy these potential obligations. Any redemption payments could materially limit the amount of cash the Company has available to fund our operations and the potential need to redeem shares of Series A Preferred Stock may limit the flexibility with which the Company seeks to operate its business. If the Company is unable to obtain additional funding on a timely basis, it may be forced to significantly curtail, delay, or discontinue one or more of its planned research or development programs or be unable to expand its operations or otherwise capitalize on its commercialization of its product candidates. As of December 31, 2023, the Company had an accumulated deficit of $614.6 million. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research and development of its product candidates and its administrative organization. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2023, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Summary of Significant Accounti

Summary of Significant Accounting Policies (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The Company disclosed its significant accounting policies in Note 2 – Summary of Significant Accounting Policies included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. There have been no material changes to the Company’s significant accounting policies during the six months ended June 30, 2024, except as noted below. Grant Revenue The Company has contracts with government-sponsored organizations for research and development related activities that provide for payments for reimbursable costs. The Company recognizes grant revenue from these contracts as it performs services under these arrangements when the funding is committed. Expenses associated with these contracts are recognized when incurred as research and development expense. Grant revenue and related expenses are presented gross in the consolidated statements of operations and comprehensive income (loss) as the Company has determined it is the primary obligor under the arrangements relative to the research and development services it performs as lead technical expert. Amounts incurred that are subject to reimbursement from the sponsor are recorded as accounts receivable on the consolidated balance sheets. Recent Accounting Pronouncements Not Yet Adopted In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures (ASU 2023-07), which requires an enhanced disclosure of significant segment expenses on an annual and interim basis. This guidance will be effective for the annual periods beginning the year ended December 31, 2024, and for interim periods beginning January 1, 2025. Early adoption is permitted. Upon adoption, the guidance should be applied retrospectively to all prior periods presented in the financial statements. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements. In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (ASU 2023-09), which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the effective tax rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. This guidance will be effective for the annual periods beginning the year ended December 31, 2025. Early adoption is permitted. Upon adoption, the guidance can be applied prospectively or retrospectively. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements.	Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta (RUS), LLC, or Selecta (RUS), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts securities corporation, and Cartesian Bio, LLC, a Delaware limited liability company, which is a variable interest entity for which the Company is the primary beneficiary. All significant intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: estimated fair value of the intangible assets acquired in connection with the Merger, estimated fair value of the CVRs, deferred income taxes, revenue recognition and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. Segment Information The Company views its operations and manages its business in one operating segment, which prior to the Merger related to the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases and subsequent to the Merger relates to the research and development of cell therapy product candidates. Cash Equivalents, Restricted Cash, Marketable Securities and Investments Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Marketable securities consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these marketable securities as available-for-sale and records them at fair value in the accompanying consolidated balance sheets. Marketable securities with less than one year until maturity are classified as short term, while marketable securities with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. The Company has also in the past invested in equity securities of a company whose securities are not publicly traded and where fair value is not readily available. This investment is recorded using cost minus impairment adjusted for changes in observable prices, depending on our ownership percentage and other factors that suggest we have significant influence. The Company monitors this investment to evaluate whether any increase or decline in its value has occurred, based on the implied value of recent company financings, public market prices of comparable companies and general market conditions. This investment is included in investments in the consolidated balance sheets. Concentrations of Credit Risk and Off-Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, short-term deposits and marketable securities, investments, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially creditworthy and, accordingly, minimal risk exists with respect to those balances. The Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of December 31, 2023, the Company maintained approximately $0.2 million in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, marketable securities, investments, warrants to purchase common stock, forward contract liabilities, and contingent value rights. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three-level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of warrant liabilities and contingent value rights are determined using Level 3 inputs. Fair value is a market-based measure considered from the perspective of a market participant rather than an entity-specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. The carrying amounts reflected in the consolidated balance sheet for investments approximate fair value and are assessed for impairment quarterly. Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use. Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available. An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. The Company recognized a $0.7 million impairment charge on a right-of-use asset during the year ended December 31, 2023. Debt Issuance Costs Debt issuance costs and fees paid to lenders are recorded as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the effective interest method and recorded as interest expense. Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non-owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net income (loss) and (ii) all components of comprehensive income (loss) other than net income (loss), referred to as other comprehensive income (loss). Other comprehensive income (loss) is comprised of unrealized gains and losses on debt securities and foreign currency translation adjustments. Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC Topic 606, Revenue from Contracts with Customers (ASC 606) Collaboration and License Revenue: The Company currently generates its revenue through collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Collaboration and license agreements with customers are generally accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, ), and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC Topic 730, , and records reimbursements from counterparties as an offset to the related research and development costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The terms of the Company’s arrangements typically include one or more of the following: (i) upfront fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of research and development expenses; and (v) profit/loss sharing arising from co-promotion arrangements. Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other promised goods and services identified in the arrangement, the Company recognizes revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other promised goods and services in the contract. For licenses that are combined with other promised goods and services, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint. Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, stock-based compensation expenses, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancellable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future research and development activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs. In June 2020, the Company and Sobi entered into a License and Development Agreement, which was amended in October 2023. Pursuant to the Sobi License, clinical trial costs incurred to complete development of the SEL-212 product candidate, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, were reimbursed by Sobi. These costs, when reimbursed, were recognized as revenue consistent with the revenue recognition methodology disclosed in Note 14. The reimbursable costs exclude any costs of additional development activities required that are related to the ImmTOR platform and that are unrelated to SEL-212. In January 2023, the Company and Astellas entered into the Astellas Agreement. Pursuant to the Astellas Agreement, Astellas will reimburse the Company for 25% of all budgeted costs incurred to complete the development of Xork for use in Pompe disease with an Astellas gene therapy investigational or authorized product. These costs, when reimbursed, will be recognized as revenue consistent with the revenue recognition methodology disclosed in Note 14. Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions. Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date. Stock-Based Compensation The Company accounts for all stock-based compensation granted to employees and non-employees using a fair value method. Stock-based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis, net of estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock-based compensation expense recognized in the consolidated financial statements is based on awards that ultimately vest. Net (Loss) Income Per Share The Company applies the two-class method to compute basic and diluted net (loss) income per share attributable to common stockholders when it has issued shares that meet the definition of participating securities. The two-class method determines net (loss) income per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires (loss) income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to share in the earnings as if all (loss) income for the period had been distributed. The Company's Series A Preferred Stock and 2022 Warrants participate in any dividends declared by the Company and are therefore considered to be participating securities. The participating securities are not required to participate in the losses of the Company, and therefore during periods of loss there is no allocation required under the two-class method. Basic net (loss) income per share attributable to common stockholders is computed by dividing the net (loss) income attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net (loss) income attributable to common stockholders is computed by adjusting net (loss) income per share attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net (loss) income per share attributable to common stockholders is computed by dividing the diluted net (loss) income attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options to purchase common stock and Series A Preferred Stock, forward contracts to issue Series A Preferred Stock, restricted stock units, warrants to purchase common stock, employee stock purchase plan stock, contingently issuable shares, and Series A Preferred Stock are considered potential dilutive common shares. Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies Leases The Company accounts for its leases in accordance with ASC Topic 842, Leases (ASC 842) In accordance with the guidance in ASC 842, the fixed and in-substance fixed contract consideration must be allocated to lease and non-lease components based on their relative fair values. Non-components of a contract (e.g., administrative tasks that do not transfer a good or service to the Company, reimbursement or payment of a lessor’s cost, etc.) do not receive an allocation of the consideration in the contract. Although allocation of consideration of lease and non-lease components is required, the Company elected the practical expedient to not separate lease components (e.g. land, building, etc.) and non-lease components (e.g., common area maintenance, consumables, etc.). The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense. Right-of-use assets and operating lease liabilities are remeasured upon certain modifications to leases using the present value of remaining lease payments and the estimated incremental borrowing rate upon lease modification. The Company enters into lease agreements with terms generally ranging from two one Acquisitions The Company evaluates acquisitions of assets and other similar transactions to assess whether or not the transaction should be accounted for as a business combination or asset acquisition by first applying a screen to determine if substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or group of similar identifiable assets. If the screen is met, the transaction is accounted for as an asset acquisition. If the screen is not met, further determination is required as to whether or not the Company has acquired inputs and processes that have the ability to create outputs, which would meet the requirements of a business. If determined to be a business combination, the Company accounts for the transaction under the acquisition method of accounting which requires the acquiring entity in a business combination to recognize the fair value of all assets acquired, liabilities assumed, and any non-controlling interest in the acquiree and establishes the acquisition date as the fair value measurement point. Accordingly, the Company recognizes assets acquired and liabilities assumed in business combinations based on the fair value estimates as of the date of acquisition. In accordance with ASC 805, Business Combinations, or ASC 805, the Company recognizes and measures goodwill as of the acquisition date, as the excess of the fair value of the consideration paid over the fair value of the identified net assets acquired. Goodwill Goodwill represents the amount of consideration paid in excess of the fair value of the identified net assets acquired as a result of the Company’s business acquisitions accounted for using the acquisition method of accounting. Goodwill is not amortized and is subject to impairment testing at a reporting unit level on an annual basis or when a triggering event occurs that may indicate the carrying value of the goodwill is impaired. An entity is permitted to first assess qualitative factors to determine if a quantitative impairment test is necessary. Such qualitative factors include macroeconomic conditions, industry and market considerations, cost factors, overall financial performance and other relevant events. Further testing is only required if the entity determines, based on the qualitative assessment, that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. The Company evaluates goodwill for impairment at least annually on October 1 and whenever facts and circumstances indicate that their carrying amounts may not be recoverable. For the year ended December 31, 2023, the Company determined that there was no impairment to goodwill. Indefinite-Lived Intangible Assets Indefinite-lived intangible assets consist of in-process research and development, or IPR&D. The fair values of IPR&D assets acquired in business combinations are capitalized. These assets are treated as indefinite-lived intangible assets until completion or abandonment of the projects, at which time the assets are amortized over the remaining useful life or written off, as appropriate. Intangible assets with indefinite lives, including IPR&D, are tested for impairment if impairment indicators arise and, at a minimum, annually. However, an entity is permitted to first assess qualitative factors to determine if a quantitative impairment test is necessary. Further testing is only required if the entity determines, based on the qualitative assessment, that it is more likely than not that an indefinite-lived intangible asset’s fair value is less than its carrying amount. Otherwise, no further impairment testing is required. The indefinite-lived intangible asset impairment test consists of a one-step analysis that compares the fair value of the intangible asset with its carrying amount. If the carrying amount of an intangible asset exceeds its fair value, an impairment loss is recognized in an amount equal to that excess. The Company considers many factors in evaluating whether the value of its intangible assets with indefinite lives may not be recoverable, including, but not limited to, expected growth rates, the cost of equity and debt capital, general economic conditions, the Company’s outlook and market performance of the Company’s industry and recent and forecasted financial performance. The Company evaluates indefinite-lived intangible assets for impairment at least annually on October 1 and whenever facts and circumstances indicate that their carrying amounts may not be recoverable. For the year ended December 31, 2023, the Company determined that there was no impairment to the IPR&D assets. Series A Preferred Stock The Company records the Series A Preferred Stock upon issuance at its fair value. The fair value includes the original issuance price, the settlement of any re

Merger (Q2)

Merger (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Merger	Merger On November 13, 2023, the Company merged with Old Cartesian in accordance with the terms of the Merger Agreement, by and among Selecta, Sakura Merger Sub I, Inc., a wholly owned subsidiary of Selecta, or First Merger Sub, Sakura Merger Sub II, LLC, a wholly owned subsidiary of Selecta, or Second Merger Sub, and Old Cartesian. Pursuant to the Merger Agreement, First Merger Sub merged with and into Old Cartesian, pursuant to which Old Cartesian was the surviving corporation and became a wholly owned subsidiary of Selecta, or the First Merger. Immediately following the First Merger, Old Cartesian merged with and into Second Merger Sub, pursuant to which Second Merger Sub was the surviving entity, or the Second Merger and, together with the First Merger, the Merger. In connection with the Second Merger, Old Cartesian changed its name to Cartesian Bio, LLC. The Merger was intended to qualify as a tax-free reorganization for U.S. federal income tax purposes. As a result of the Merger, Selecta changed its corporate name to Cartesian Therapeutics, Inc. and its common stock began trading on the Nasdaq Global Market under the new trading symbol “RNAC” beginning on November 14, 2023. The Merger Agreement was unanimously approved by the board of directors of Selecta and the board of directors of Old Cartesian. The Merger was consummated substantially concurrently with the entry into the Merger Agreement and was not subject to approval of the Company's stockholders. Under the terms of the Merger Agreement, following the consummation of the Merger on November 13, 2023, or the Closing Date, in exchange for 100% of the outstanding shares of capital stock of Old Cartesian immediately prior to the effective time of the First Merger, the Company agreed to issue to the stockholders of Old Cartesian (i) 224,099 shares of the Company’s common stock and (ii) 384,930.724 shares of Series A Preferred Stock. The issuance of the shares of common stock and Series A Preferred Stock occurred on December 5, 2023 which was after the December 4, 2023 record date for the distribution of the CVRs (see Note 5); as such, the Old Cartesian stockholders did not have rights as holders of common stock or holders of Series A Preferred Stock until such issuance on December 5, 2023. In addition, all outstanding stock options to purchase Old Cartesian common stock were assumed by the Company and converted into stock options to purchase (i) shares of the Company’s common stock or (ii) shares of the Company’s Series A Preferred Stock on terms substantially identical to those in effect prior to Merger Agreement, except for adjustments to the underlying number of shares and the exercise price based on the Merger Agreement exchange ratio. Pursuant to the Merger Agreement, the Company agreed to hold a stockholders’ meeting, or the Special Meeting, to submit the following proposals to a vote of its stockholders: (i) the approval of the conversion of shares of Series A Preferred Stock into shares of common stock, or the Conversion Proposal, and (ii) either or both of (A) the approval of an amendment to the Company’s Charter to increase the number of shares of common stock authorized under the Charter and (B) the approval of an amendment to the Charter to effect a reverse stock split of all outstanding shares of common stock, in either case (A) or (B) by a number of authorized shares or at a stock split ratio, as the case may be, sufficient to allow the conversion of all shares of Series A Preferred Stock issued in the Merger. The Special Meeting was held on March 27, 2024 in which the Company’s stockholders approved the Conversion Proposal, among other matters (see Note 10). The Company concluded the acquisition resulted in the Company obtaining a controlling financial interest in a variable interest entity, or VIE, in accordance with ASC 810, Consolidation , or ASC 810. The Company determined that Old Cartesian was considered to be a VIE as it did not have sufficient equity to finance its activities without additional subordinated financial support. Prior to the Closing Date, the primary source of funding for Old Cartesian had been preferred stock financings. The Company acquired all of the outstanding shares of Old Cartesian and, therefore, is the sole equity holder and primary beneficiary. The Company has the obligation to absorb the losses and right to receive the benefits of Old Cartesian, and the power to direct the activities that most significantly affect the economic performance of Old Cartesian which the Company considers to be its development activities. Therefore, the Company is the primary beneficiary. Further, the Company concluded the VIE qualified as a business and accounted for the transaction as the acquisition of a business in accordance with ASC 805, Business Combinations , or ASC 805. As the primary beneficiary, the Company was the acquirer in the transaction. The Company exchanged the right to receive shares of common stock and Series A Preferred Stock for all of the outstanding equity of Old Cartesian. The Company determined the rights to receive shares exchanged in the Merger represent a forward contract. The fair value of the forward contracts was determined based on the fair value of shares of common stock and Series A Preferred Stock underlying the forward contracts as of the acquisition date. The total purchase price consists of the fair value of the forward contracts in addition to a portion of the fair value of options exchanged in the transaction related to prior service. Under the acquisition method, the total purchase price of the acquisition was allocated to the net tangible and identifiable intangible assets acquired and liabilities assumed based on their estimated fair values as of the date of the acquisition. The total fair value of the consideration of $168.5 million as of the Closing Date is summarized as follows (in thousands): Forward contract to issue common stock ​ ​ $ 2,713 Forward contract to issue Series A Preferred Stock ​ ​ 155,308 Stock options allocated to consideration paid ​ ​ 10,444 Total consideration ​ ​ $168,465 The Company recorded the assets acquired and liabilities assumed as of the Closing Date based on the information available at that date. The following table presents the allocation of the purchase price to the estimated fair values of the assets acquired and liabilities assumed as of the Closing Date (in thousands): ​ ​ As of November 13, 2023 Assets acquired: ​ ​ Cash and cash equivalents ​ ​ $ 6,561 Prepaid expenses and other current assets ​ ​ 309 Property and equipment, net ​ ​ 215 Right-of-use asset, net ​ ​ 915 In-process research and development assets ​ ​ 150,600 Goodwill ​ ​ 48,163 ​ ​ $206,763 Liabilities assumed ​ ​ Accrued expenses and other current liabilities ​ ​ $ 2,530 Lease liability ​ ​ 292 Lease liability, net of current portion ​ ​ 623 Deferred tax liability ​ ​ 34,853 ​ ​ $ 38,298 Net assets acquired ​ ​ $168,465 The fair value of the in-process research and development, or IPR&D, assets were capitalized as of the Closing Date and will be accounted for as indefinite-lived intangible assets until completion or disposition of the assets or abandonment of the associated research and development efforts. Upon successful completion of the development efforts, the carrying value of each respective IPR&D asset will be amortized over its estimated useful life. Until that time, the IPR&D assets will be subject to impairment testing and will not be amortized. The goodwill recorded related to the Merger is the excess of the fair value of the consideration transferred by the acquirer over the fair value of tangible assets, identifiable intangible assets and assumed liabilities as of the Closing Date and is not deductible for tax purposes. The goodwill balance is primarily attributable to the value of the assembled workforce and deferred tax liabilities associated with the transaction. The following summarizes the Company’s intangible assets acquired in the Merger (in thousands): ​ ​ Acquisition Date Fair Value Descartes-08 for MG ​ ​ $ 93,900 Descartes-08 for SLE ​ ​ 56,700 Total in-process research and development assets ​ ​ $150,600 The fair value of the intangible assets was estimated using the income approach in which the after-tax cash flows were discounted to present value. The cash flows are based on estimates used to price the transaction, and the discount rates applied were benchmarked with reference to the implied rate of return from the transaction model as well as the weighted average cost of capital. The forward contract related to the common stock was recorded as additional paid-in capital as the instrument is indexed to the Company’s common stock. The forward contract related to the Series A Preferred Stock was recorded as a liability as the underlying Series A Preferred Stock has a redemption feature that may require the Company to settle the instrument by transferring an asset. The forward contract was measured at fair value through the date of settlement through the issuance of the shares of Series A Preferred Stock on December 5, 2023.	Merger On November 13, 2023, the Company merged with Old Cartesian in accordance with the terms of the Merger Agreement, by and among Selecta, Sakura Merger Sub I, Inc., a wholly owned subsidiary of Selecta, or First Merger Sub, Sakura Merger Sub II, LLC, a wholly owned subsidiary of Selecta, or Second Merger Sub, and Old Cartesian. Pursuant to the Merger Agreement, First Merger Sub merged with and into Old Cartesian, pursuant to which Old Cartesian was the surviving corporation and became a wholly owned subsidiary of Selecta, or the First Merger. Immediately following the First Merger, Old Cartesian merged with and into Second Merger Sub, pursuant to which Second Merger Sub was the surviving entity, or the Second Merger and, together with the First Merger, the Merger. In connection with the Second Merger, Old Cartesian changed its name to Cartesian Bio, LLC. The Merger was intended to qualify as a tax-free reorganization for U.S. federal income tax purposes. As a result of the Merger, Selecta changed its corporate name to Cartesian Therapeutics, Inc. and its common stock began trading on the Nasdaq Global Market under the new trading symbol “RNAC” beginning on November 14, 2023. The Merger Agreement was unanimously approved by the board of directors, or the Board of Directors, of Selecta and the board of directors of Old Cartesian. The Merger was consummated substantially concurrently with the entry into the Merger Agreement and was not subject to approval of the Company's stockholders. Under the terms of the Merger Agreement, following the consummation of the Merger on November 13, 2023, or the Closing Date, in exchange for 100% of the outstanding shares of capital stock of Old Cartesian immediately prior to the effective time of the First Merger, the Company agreed to issue to the stockholders of Old Cartesian (i) 6,723,639 shares of the Company’s common stock and (ii) 384,930.724 shares of Series A Preferred Stock. The issuance of the shares of common stock and Series A Preferred Stock occurred on December 5, 2023 which was after the December 4, 2023 record date for the distribution of the CVRs (see Note 6); as such, the Old Cartesian stockholders did not have rights as holders of common stock or holders of Series A Preferred Stock until such issuance on December 5, 2023. In addition, all outstanding stock options to purchase Old Cartesian common stock were assumed by the Company and converted into stock options to purchase (i) shares of the Company’s common stock or (ii) shares of the Company’s Series A Preferred Stock on terms substantially identical to those in effect prior to Merger Agreement, except for adjustments to the underlying number of shares and the exercise price based on the Merger Agreement exchange ratio. Pursuant to the Merger Agreement, the Company agreed to hold a stockholders’ meeting to submit the following proposals to a vote of its stockholders: (i) the approval of the conversion of shares of Series A Preferred Stock into shares of common stock, or the Conversion Proposal, and (ii) either or both of (A) the approval of an amendment to the Company’s restated certificate of incorporation, as amended, or the Charter, to increase the number of shares of common stock authorized under the Charter and (B) the approval of an amendment to the Charter to effect a reverse stock split of all outstanding shares of common stock, in either case (A) or (B) by a number of authorized shares or at a stock split ratio, as the case may be, sufficient to allow the conversion of all shares of Series A Preferred Stock issued in the Merger. The Company concluded the acquisition resulted in the Company obtaining a controlling financial interest in a VIE in accordance with ASC 810, Consolidation The Company exchanged the right to receive shares of common stock and Series A Preferred Stock for all of the outstanding equity of Old Cartesian. The Company determined the rights to receive shares exchanged in the Merger represent a forward contract. The fair value of the forward contracts was determined based on the fair value of shares of common stock and Series A Preferred Stock underlying the forward contracts as of the acquisition date. The total purchase price consists of the fair value of the forward contracts in addition to a portion of the fair value of options exchanged in the transaction related to prior service. Under the acquisition method, the total purchase price of the acquisition was allocated to the net tangible and identifiable intangible assets acquired and liabilities assumed based on their estimated fair values as of the date of the acquisition. The total fair value of the consideration of $168.5 million as of the Closing Date is summarized as follows (in thousands): Forward contract to issue common stock ​ ​ $ 2,713 Forward contract to issue Series A Preferred Stock ​ ​ 155,308 Stock options allocated to consideration paid ​ ​ 10,444 Total consideration ​ ​ $168,465 The Company recorded the assets acquired and liabilities assumed as of the Closing Date based on the information available at that date. The following table presents the allocation of the purchase price to the estimated fair values of the assets acquired and liabilities assumed as of the Closing Date (in thousands): ​ ​ As of November 13, 2023 Assets acquired: ​ ​ Cash and cash equivalents ​ ​ $ 6,561 Prepaid expenses and other current assets ​ ​ 309 Property and equipment, net ​ ​ 215 Right-of-use asset, net ​ ​ 915 In-process research and development assets ​ ​ 150,600 Goodwill ​ ​ 48,163 ​ ​ $206,763 Liabilities assumed ​ ​ Accrued expenses and other current liabilities ​ ​ $ 2,530 Lease liability ​ ​ $ 292 Lease liability, net of current portion ​ ​ $ 623 Deferred tax liability ​ ​ $ 34,853 ​ ​ $ 38,298 Net assets acquired ​ ​ $168,465 The fair value of IPR&D assets were capitalized as of the Closing Date and will be accounted for as indefinite-lived intangible assets until completion or disposition of the assets or abandonment of the associated research and development efforts. Upon successful completion of the development efforts, the carrying value of the respective IPR&D asset will be amortized over its estimated useful life. Until that time, the IPR&D assets will be subject to impairment testing and will not be amortized. The goodwill recorded related to the Merger is the excess of the fair value of the consideration transferred by the acquirer over the fair value of tangible assets, identifiable intangible assets and assumed liabilities as of the Closing Date and is not deductible for tax purposes. The goodwill balance is primarily attributable to the value of the assembled workforce and deferred tax liabilities associated with the transaction. The following summarizes the Company’s intangible assets acquired in the Merger and their carrying value as of December 31, 2023 (in thousands): ​ ​ Acquisition Date Fair Value ​ ​ Impairment ​ ​ Carrying Value at December 31, 2023 Descartes-08 for MG ​ ​ $ 93,900 ​ ​ $— ​ ​ $ 93,900 Descartes-08 for SLE ​ ​ 56,700 ​ ​ — ​ ​ 56,700 Total in-process research and development assets ​ ​ $150,600 ​ ​ $— ​ ​ $150,600 The fair value of the intangible assets was estimated using the income approach in which the after-tax cash flows were discounted to present value. The cash flows are based on estimates used to price the transaction, and the discount rates applied were benchmarked with reference to the implied rate of return from the transaction model as well as the weighted average cost of capital. For the period from November 13, 2023 to December 31, 2023, Old Cartesian’s revenue and net loss within the consolidated statements of operations and comprehensive (loss) income were $0.0 million and $1.6 million, respectively. The following unaudited pro forma financial information reflects the consolidated results of operations of the Company as if the Merger had taken place on January 1, 2022. The unaudited pro forma financial information is not necessarily indicative of the results of operations as they would have been had the transactions been effected on the assumed date (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 Revenue ​ ​ $ 26,004 ​ ​ $112,226 Net (loss) income ​ ​ $(232,259) ​ ​ $ 29,607 The Company’s transaction costs of $4.9 million were expensed as incurred and included in general and administrative expense in the consolidated statements of operations and comprehensive (loss) income. The forward contract related to the common stock was recorded as additional paid-in capital as the instrument is indexed to the Company’s common stock. The forward contract related to the Series A Preferred Stock was recorded as a liability as the underlying Series A Preferred Stock has a redemption feature that may require the Company to settle the instrument by transferring an asset. The forward contract was measured at fair value through the date of settlement through the issuance of the shares of Series A Preferred Stock on December 5, 2023.

Net Income (Loss) Per Share All

Net Income (Loss) Per Share Allocable to Common Stockholders (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Earnings Per Share [Abstract]
Net Income (Loss) Per Share Allocable to Common Stockholders	Net Income (Loss) Per Share Allocable to Common Stockholders The following table sets forth the computation of basic and diluted net income (loss) per share allocable to common stockholders for the three and six months ended June 30, 2024 and 2023 (in thousands, except share and per-share data): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Numerator: ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ ​ $ 13,836 ​ ​ $ (11,387) ​ ​ $ (42,988) ​ ​ $ (33,050) Less: Undistributed earnings allocable to participating securities ​ ​ (4,208) ​ ​ — ​ ​ — ​ ​ — Net income (loss) allocable to shares of common stock - basic and diluted ​ ​ $ 9,628 ​ ​ $ (11,387) ​ ​ $ (42,988) ​ ​ $ (33,050) Denominator: ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ ​ 16,723,479 ​ ​ 5,114,747 ​ ​ 11,068,749 ​ ​ 5,113,213 Dilutive effect of employee equity incentive plans ​ ​ 1,067,664 ​ ​ — ​ ​ — ​ ​ — Weighted-average common shares outstanding - diluted ​ ​ 17,791,143 ​ ​ 5,114,747 ​ ​ 11,068,749 ​ ​ 5,113,213 Net income (loss) per share allocable to common stockholders: ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ ​ $ 0.58 ​ ​ $ (2.23) ​ ​ $ (3.88) ​ ​ $ (6.46) Diluted ​ ​ $ 0.54 ​ ​ $ (2.23) ​ ​ $ (3.88) ​ ​ $ (6.46) The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net income (loss) per share allocable to common stockholders for all periods presented, as the effect would have been anti-dilutive: ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Common stock options, restricted stock units and ESPP shares ​ ​ 740,211 ​ ​ 753,590 ​ ​ 2,344,017 ​ ​ 753,590 Warrants to purchase common stock ​ ​ 975,132 ​ ​ 1,040,943 ​ ​ 975,132 ​ ​ 1,040,943 Series A Preferred Stock ​ ​ 5,544,719 ​ ​ — ​ ​ 5,544,719 ​ ​ — Total ​ ​ 7,260,062 ​ ​ 1,794,533 ​ ​ 8,863,868 ​ ​ 1,794,533	Net (Loss) Income Per Share The Company reported a net loss for the years ended December 31, 2023 and 2021, and net income for the year ended December 31, 2022. The Company used the treasury stock method to determine the number of dilutive shares. The following table sets forth the computation of basic and diluted net (loss) income per share (in thousands, except share and per-share data): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Numerator: ​ ​ ​ ​ ​ ​ Net (loss) income ​ ​ $(219,710) ​ ​ $ 35,379 ​ ​ $(25,687) Less: CVR distribution to participating securities ​ ​ (37,550) ​ ​ — ​ ​ — Net (loss) income allocable to shares of common stock - basic ​ ​ (257,260) ​ ​ 35,379 ​ ​ (25,687) Less: Change in fair value of warrants ​ ​ — ​ ​ (20,882) ​ ​ — Net (loss) income allocable to shares of common stock - diluted ​ ​ $(257,260) ​ ​ $ 14,497 ​ ​ $(25,687) ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Denominator: ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ ​ 155,109,561 ​ ​ 144,758,555 ​ ​ 114,328,798 Dilutive effect of employee equity incentive plans and outstanding warrants ​ ​ — ​ ​ 1,116,334 ​ ​ — Weighted-average common shares used in per share calculations - diluted ​ ​ 155,109,561 ​ ​ 145,874,889 ​ ​ 114,328,798 Net (loss) income per share: ​ ​ ​ ​ ​ ​ Basic ​ ​ $ (1.66) ​ ​ $ 0.24 ​ ​ $ (0.22) Diluted ​ ​ $ (1.66) ​ ​ $ 0.10 ​ ​ $ (0.22) The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net (loss) income per share for all periods presented, as the effect would have been anti-dilutive: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Warrants to purchase common stock ​ ​ 31,224,703 ​ ​ 213,339 ​ ​ 10,735,980 Series A Preferred Stock ​ ​ 435,120,513 ​ ​ — ​ ​ — Forward contract to issue Series A Preferred Stock ​ ​ 99,140,326 ​ ​ — ​ ​ — Common stock options, RSUs and ESPP shares ​ ​ 23,306,661 ​ ​ 17,800,034 ​ ​ 11,492,002 Series A Preferred Stock options ​ ​ 14,112,299 ​ ​ — ​ ​ — Total ​ ​ 602,904,502 ​ ​ 18,013,373 ​ ​ 22,227,982

Fair Value Measurements (Q2)

Fair Value Measurements (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2024 and December 31, 2023 (in thousands): ​ ​ June 30, 2024 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 39,181 ​ ​ $39,181 ​ ​ $— ​ ​ $ — Total assets ​ ​ $ 39,181 ​ ​ $39,181 ​ ​ $— ​ ​ $ — Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 9,260 ​ ​ $ — ​ ​ $— ​ ​ $ 9,260 Contingent value right liability ​ ​ $395,400 ​ ​ $ — ​ ​ $— ​ ​ $395,400 Total liabilities ​ ​ $404,660 ​ ​ $ — ​ ​ $— ​ ​ $404,660 ​ ​ December 31, 2023 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Total assets ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 6,394 ​ ​ $ — ​ ​ $ — ​ ​ $ 6,394 Contingent value right liability ​ ​ $358,600 ​ ​ $ — ​ ​ $ — ​ ​ $358,600 Forward contract liabilities ​ ​ $ 28,307 ​ ​ $ — ​ ​ $28,307 ​ ​ $ — Total liabilities ​ ​ $393,301 ​ ​ $ — ​ ​ $28,307 ​ ​ $364,994 There were no transfers within the fair value hierarchy during the six months ended June 30, 2024 or year ended December 31, 2023. Cash, Cash Equivalents, and Restricted Cash As of June 30, 2024 and December 31, 2023, money market funds were classified as cash and cash equivalents on the accompanying consolidated balance sheets as they mature within 90 days from the date of purchase. As of June 30, 2024, the Company had restricted cash balances relating to secured letters of credit in connection with its real estate leases. Short-term restricted cash is included within prepaid expenses and other current assets in the consolidated balance sheets. The Company’s consolidated statements of cash flows include the following as of June 30, 2024 and 2023 (in thousands): ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Cash and cash equivalents ​ ​ $87,227 ​ ​ $112,027 Short-term restricted cash ​ ​ — ​ ​ 1,600 Long-term restricted cash ​ ​ 1,669 ​ ​ 1,377 Total cash, cash equivalents, and restricted cash ​ ​ $88,896 ​ ​ $115,004 Warrants to Purchase Common Stock In December 2019, the Company issued warrants to purchase common stock in connection with a private placement, or the 2019 Warrants. Pursuant to the terms of the 2019 Warrants, the Company could be required to settle the 2019 Warrants in cash in the event of certain acquisitions of the Company and, as a result, the 2019 Warrants are required to be measured at fair value and reported as a liability on the balance sheet. On December 20, 2022, the Company amended the terms of the outstanding 2019 Warrants held by certain members of the Board of Directors, or the Amended 2019 Warrants, to remove the cash settlement provision. As a result, the Amended 2019 Warrants were remeasured at fair value on December 20, 2022 and reclassified from a liability to equity on the balance sheet. See Note 12 to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023 for further discussion on the equity-classified Amended 2019 Warrants. In April 2022, the Company issued warrants in connection with an underwritten offering, or the 2022 Warrants. Pursuant to the terms of the 2022 Warrants, the Company could be required to settle the 2022 Warrants in cash in the event of an acquisition of the Company under certain circumstances and, as a result, the 2022 Warrants are required to be measured at fair value and reported as a liability on the balance sheet. The Company recorded the fair value of the 2019 Warrants and the 2022 Warrants upon issuance using the Black-Scholes valuation model and is required to revalue the 2019 Warrants and the 2022 Warrants at each reporting date, with any changes in fair value recorded in the statement of operations and comprehensive income (loss). The valuations of the 2019 Warrants and the 2022 Warrants are classified as Level 3 of the fair value hierarchy due to the need to use assumptions in the valuations that are both significant to the fair value measurement and unobservable, including the stock price volatility and the expected life of the 2019 Warrants and the 2022 Warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement. The estimated fair values of the 2019 Warrants and the 2022 Warrants were determined using the following inputs to the Black-Scholes simulation valuation: Estimated fair value of the underlying stock . The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period. Risk-free interest rate . The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption. Dividend rate . The dividend rate is based on the historical rate, which the Company anticipates will remain at zero. Expected life . The expected life of the 2019 Warrants and the 2022 Warrants is assumed to be equivalent to their remaining contractual terms which expire on December 23, 2024 and April 11, 2027, respectively. Volatility . The Company estimates stock price volatility based on the Company’s historical volatility for a period of time commensurate with the expected remaining life of the warrants. A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2019 Warrants liability is as follows: ​ ​ June 30, 2024 ​ ​ December 31, 2023 Risk-free interest rate ​ ​ 5.33% ​ ​ 4.79% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 0.49 ​ ​ 0.98 Expected volatility ​ ​ 110.52% ​ ​ 83.67% A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2022 Warrants liability is as follows: ​ ​ June 30, 2024 ​ ​ December 31, 2023 Risk-free interest rate ​ ​ 4.52% ​ ​ 4.01% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 2.78 ​ ​ 3.28 Expected volatility ​ ​ 87.23% ​ ​ 84.09% The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 11 to these unaudited consolidated financial statements) for the six months ended June 30, 2024 (in thousands): ​ ​ Warrant liabilities Fair value as of December 31, 2023 ​ ​ $6,394 Change in fair value ​ ​ 2,866 Fair value as of June 30, 2024 ​ ​ $9,260 Contingent Value Right On December 6, 2023, as contemplated by the Merger Agreement, the Company entered into a contingent value rights agreement, or the CVR Agreement, pursuant to which each holder of common stock or a 2022 Warrant as of December 4, 2023 was distributed a CVR, issued by the Company for each share of common stock held directly or underlying a 2022 Warrant held by such holder as of December 4, 2023. Holders of warrants other than the 2022 Warrants will be entitled to receive, upon exercise of such warrants and in accordance with the terms of the warrants, one CVR per each share of common stock underlying such warrants. Each CVR entitles its holder to distributions of the following, pro-rated on a per-CVR basis, during the period ending on the date on which the Royalty Term (as defined in the Sobi License) ends, or the Termination Date: • 100% of all milestone payments, royalties and other amounts paid to the Company or its controlled affiliates, or the Company Entities, under the Sobi License or, following certain terminations of the Sobi License, any agreement a Company Entity enters into that provides for the development and commercialization of SEL-212; and • 100% of all cash consideration and the actual liquidation value of any and all non-cash consideration of any kind that is paid to or is actually received by any Company Entity prior to the Termination Date pursuant to an agreement relating to a sale, license, transfer or other disposition of any transferable asset of the Company existing as of immediately prior to the Merger, other than those exclusively licensed under the Sobi License or which the Company Entities are required to continue to own in order to comply with the Sobi License. The distributions in respect of the CVRs will be made on a semi-annual basis, and will be subject to a number of deductions, subject to certain exceptions or limitations, including for (i) certain taxes payable on the proceeds subject to the CVR distribution, (ii) certain out of pocket costs incurred by the Company Entities, including audit and accounting fees incurred in connection with reporting obligations relating to the CVRs and other expenses incurred in the performance of their obligations and other actions under the CVR Agreement, (iii) a fixed semi-annual amount of $0.75 million for general and administrative overhead, (iv) payments made and remaining obligations on lease liabilities of Selecta immediately prior to the Merger and (v) amounts paid and remaining obligations with regard to the Xork product candidate. Each of the deductions described in (iv) and (v) will be made only if certain milestone payments under the Sobi License are made and are also subject to certain adjustments as contemplated in the CVR Agreement. Upon the achievement of a development milestone in June 2024, Sobi became obligated to make a $30.0 million payment to the Company and made such payment in July 2024. The proceeds from this payment, net of deductions specified in the CVR Agreement, is expected to be included in the next scheduled distribution to the holders of the CVR in March 2025. The CVRs represent financial instruments that are accounted for under the fair value option election in ASC 825 , Financial Instruments , or ASC 825. Under the fair value option election, the CVRs are initially measured at the aggregate estimated fair value of the CVRs and will be subsequently remeasured at estimated fair value on a recurring basis at each reporting period date. The liability was recorded at the date of approval, November 13, 2023, as a dividend. The estimated fair value of the CVR liability was determined using a discounted cash flow methodology as of December 31, 2023 and a Monte Carlo simulation model as of June 30, 2024 to estimate future cash flows associated with the legacy assets, including the expected milestone and royalty payments under the Sobi License, net of deductions. Changes in fair value of the CVR liability are presented in the consolidated statements of operations and comprehensive income (loss). The liability value is based on significant inputs not observable in the market such as estimated cash flows, estimated probabilities of success, expected volatility of future revenues (Monte Carlo simulation model) and risk-adjustment discount rates (discounted cash flow methodology), which represent a Level 3 measurement within the fair value hierarchy. The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows: ​ ​ June 30, 2024 Estimated cash flow dates ​ ​ 2024-2038 Estimated probability of success ​ ​ 95.0% - 100.0% Expected volatility of future revenues ​ ​ 22.0% ​ ​ December 31, 2023 Estimated cash flow dates ​ ​ 2024 - 2038 Estimated probability of success ​ ​ 95.0% Risk-adjusted discount rate ​ ​ 13.7% The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the six months ended June 30, 2024 (in thousands): ​ ​ CVR liability Fair value as of December 31, 2023 ​ ​ $358,600 Change in fair value ​ ​ 36,800 Fair value as of June 30, 2024 ​ ​ $395,400 Forward Contract Liabilities Merger Consideration In connection with the Merger, the Company entered into a contract for the issuance of 384,930.724 shares of Series A Preferred Stock as part of the consideration transferred. The fair value of the forward contract at the Closing Date was $155.3 million. The non-cash settlement of this liability occurred on December 5, 2023 with the issuance of the Series A Preferred Stock for $261.8 million. November 2023 Private Placement The Company entered into a contract for the issuance of 149,330.115 shares of Series A Preferred Stock as part of the November 2023 Private Placement which was settled in multiple tranches. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Timothy A. Springer, a member of the Company’s Board of Directors, and TAS Partners LLC, an affiliate of Dr. Springer, represented a forward contract. See Note 10. The initial fair value of the forward contract liability on November 13, 2023 was insignificant as the fair value of the underlying Series A Preferred Stock was equal to the purchase price of the Series A Preferred Stock as agreed upon in the November 2023 Private Placement. Subsequent measurement of the fair value of the forward contract liability was based on the market price of the Company’s common stock, which represented the redemption and conversion value of the Series A Preferred Stock, less the purchase price, on an as-converted basis. The non-cash settlement of a portion of the liability occurred on December 13, 2023 with the issuance of the first tranche of the Series A Preferred Stock for $14.8 million. The non-cash settlement of the remaining second and third tranches occurred on January 12, 2024 and February 11, 2024, respectively, for a total of $35.2 million. The following table presents changes in the forward contract liabilities for the periods presented (in thousands): ​ ​ Forward contract liabilities Fair value as of December 31, 2023 ​ ​ $ 28,307 Settlements ​ ​ (35,197) Change in fair value ​ ​ 6,890 Fair value as of June 30, 2024 ​ ​ $ —	Fair Value Measurements The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands): ​ ​ December 31, 2023 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Total assets ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 6,394 ​ ​ $ — ​ ​ $ — ​ ​ $ 6,394 Contingent value right liability ​ ​ 358,600 ​ ​ — ​ ​ — ​ ​ 358,600 Forward contract liabilities ​ ​ 28,307 ​ ​ — ​ ​ 28,307 ​ ​ — Total liabilities ​ ​ $393,301 ​ ​ $ — ​ ​ $28,307 ​ ​ $364,994 ​ ​ December 31, 2022 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $53,552 ​ ​ $53,552 ​ ​ $ — ​ ​ $— Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ U.S. government agency securities and treasuries ​ ​ 13,557 ​ ​ — ​ ​ 13,557 ​ ​ — Corporate bonds ​ ​ 1,951 ​ ​ — ​ ​ 1,951 ​ ​ — Commercial paper ​ ​ 12,656 ​ ​ — ​ ​ 12,656 ​ ​ — Total assets ​ ​ $81,716 ​ ​ $53,552 ​ ​ $28,164 ​ ​ $— ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $19,140 ​ ​ $— ​ ​ $— ​ ​ $19,140 Total liabilities ​ ​ $19,140 ​ ​ $— ​ ​ $— ​ ​ $19,140 There were no transfers within the fair value hierarchy during the years ended December 31, 2023 or 2022. Cash, Cash Equivalents, and Restricted Cash As of December 31, 2023 and 2022, money market funds were classified as cash and cash equivalents on the accompanying consolidated balance sheets as they mature within 90 days from the date of purchase. As of December 31, 2023, the Company had restricted cash balances relating to a secured letter of credit in connection with its lease for the Company’s prior headquarters (see Note 9 included elsewhere in this Annual Report). Short-term restricted cash is included within prepaid expenses and other current assets in the consolidated balance sheets. The Company’s consolidated statement of cash flows includes the following as of December 31, 2023, 2022 and 2021 (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Cash and cash equivalents ​ ​ $76,911 ​ ​ $106,438 ​ ​ $114,057 Short-term restricted cash ​ ​ — ​ ​ 289 ​ ​ — Long-term restricted cash ​ ​ 1,377 ​ ​ 1,311 ​ ​ 1,379 Total cash, cash equivalents, and restricted cash ​ ​ $78,288 ​ ​ $108,038 ​ ​ $115,436 Marketable Securities No marketable securities were held as of December 31, 2023. Marketable securities held as of December 31, 2022 and classified as Level 2 within the valuation hierarchy consist of U.S. government agency securities and treasuries, corporate bonds and commercial paper. Marketable securities represent holdings of available-for-sale marketable debt securities in accordance with the Company’s investment policy. The Company estimates the fair value of these marketable securities by taking into consideration valuations that include market pricing based on real-time trade data for the same or similar securities, and other observable inputs. The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to the earliest call date for premiums or to maturity for discounts. Warrants to Purchase Common Stock In December 2019, the Company issued warrants to purchase common stock in connection with a private placement, or the 2019 Warrants. Pursuant to the terms of the 2019 Warrants, the Company could be required to settle the 2019 Warrants in cash in the event of certain acquisitions of the Company and, as a result, the common warrants are required to be measured at fair value and reported as a liability on the balance sheet. On December 20, 2022, the Company amended the terms of the outstanding 2019 Warrants held by certain members of its Board of Directors, or the Amended 2019 Warrants, to remove the cash settlement provision. As a result, the Amended 2019 Warrants were remeasured at fair value on December 20, 2022 and reclassified from a liability to equity on the balance sheet. Refer to Note 12 for further discussion on the equity-classified Amended 2019 Warrants. In April 2022, the Company issued warrants in connection with an underwritten offering, or the 2022 Warrants. Pursuant to the terms of the 2022 Warrants, the Company could be required to settle the 2022 Warrants in cash in the event of an acquisition of the Company under certain circumstances and, as a result, the 2022 Warrants are required to be measured at fair value and reported as a liability on the balance sheet. The Company recorded the fair value of the 2019 Warrants and the 2022 Warrants upon issuance using the Black-Scholes valuation model and is required to revalue the 2019 Warrants and the 2022 Warrants at each reporting date, with any changes in fair value recorded in the statement of operations and comprehensive income (loss). The valuations of the 2019 Warrants and the 2022 Warrants are classified as Level 3 of the fair value hierarchy due to the need to use assumptions in the valuations that are both significant to the fair value measurement and unobservable, including the stock price volatility and the expected life of the 2019 Warrants and the 2022 Warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement. The changes in the fair values of the warrants are reflected in the statement of operations and comprehensive income (loss) for the years ended December 31, 2023, 2022 and 2021. The estimated fair values of the 2019 Warrants and the 2022 Warrants were determined using the following inputs to the Black-Scholes simulation valuation: Estimated fair value of the underlying stock . The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period. Risk-free interest rate . The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption. Dividend rate . The dividend rate is based on the historical rate, which the Company anticipates will remain at zero. Expected life . The expected life of the 2019 Warrants and the 2022 Warrants is assumed to be equivalent to their remaining contractual terms which expire on December 23, 2024 and April 11, 2027, respectively. Volatility . The Company estimates stock price volatility based on the Company’s historical volatility for a period of time commensurate with the expected remaining life of the warrants. A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2019 Warrants liability is as follows: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Risk-free interest rate ​ ​ 4.79% ​ ​ 4.74% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 0.98 ​ ​ 1.98 Expected volatility ​ ​ 83.67% ​ ​ 79.92% A summary of the Black-Scholes valuation model assumptions used to record the fair value of the 2022 Warrants liability is as follows: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Risk-free interest rate ​ ​ 4.01% ​ ​ 4.22% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 3.28 ​ ​ 4.28 Expected volatility ​ ​ 84.09% ​ ​ 98.05% The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 12), for the year ended December 31, 2023 (in thousands): ​ ​ Warrant liabilities Fair value as of December 31, 2022 ​ ​ $ 19,140 Change in fair value ​ ​ (12,746) Fair value as of December 31, 2023 ​ ​ $ 6,394 Contingent Value Right On December 6, 2023, as contemplated by the Merger Agreement, the Company entered into the CVR Agreement, pursuant to which each holder of common stock as of December 4, 2023 or a 2022 Warrant was distributed a CVR, issued by the Company for each share of common stock held directly or underlying a 2022 Warrant held by such holder as of December 4, 2023. Holders of warrants other than the 2022 Warrants will be entitled to receive, upon exercise of such warrants and in accordance with the terms of the warrants, one CVR per each share of common stock underlying such warrants. Each CVR entitles its holder to distributions of the following, pro-rated on a per-CVR basis, during the period ending on the date on which the Royalty Term (as defined in the Sobi License) ends, or the Termination Date: • 100% of all milestone payments, royalties and other amounts paid to the Company or its controlled affiliates, or the Company Entities, under the Sobi License or, following certain terminations of the Sobi License, any agreement a Company Entity enters into that provides for the development and commercialization of SEL-212; and • 100% of all cash consideration and the actual liquidation value of any and all non-cash consideration of any kind that is paid to or is actually received by any Company Entity prior to the Termination Date pursuant to an agreement relating to a sale, license, transfer or other disposition of any transferable asset of the Company existing as of immediately prior to the Merger, other than those exclusively licensed under the Sobi License or which the Company Entities are required to continue to own in order to comply with the Sobi License. The distributions in respect of the CVRs will be made on a semi-annual basis, and will be subject to a number of deductions, subject to certain exceptions or limitations, including for (i) certain taxes payable on the proceeds subject to the CVR distribution, (ii) certain out of pocket costs incurred by the Company Entities, including audit and accounting fees incurred in connection with reporting obligations relating to the CVRs and other expenses incurred in the performance of their obligations and other actions under the CVR Agreement, (iii) a fixed semi-annual amount of $0.75 million for general and administrative overhead, (iv) payments made and remaining obligations on lease liabilities of Selecta immediately prior to the Merger and (v) amounts paid and remaining obligations with regard to the Xork product candidate. Each of the deductions described in (iv) and (v) will be made only if certain milestone payments under the Sobi License are made and are also subject to certain adjustments as contemplated in the CVR Agreement. The CVRs represent financial instruments that are accounted for under the fair value option election in ASC 825, Financial Instruments The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows: ​ ​ December 31, 2023 ​ ​ At Issuance November 13, 2023 Estimated cash flow dates ​ ​ 2024 - 2038 ​ ​ 2024 - 2038 Estimated probability of success ​ ​ 95.0% ​ ​ 95.0% Risk-adjusted discount rate ​ ​ 13.7% ​ ​ 14.4% The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the year ended December 31, 2023 (in thousands): ​ ​ CVR liability Fair value as of December 31, 2022 ​ ​ $ — Issuances ​ ​ 340,300 Change in fair value ​ ​ 18,300 Fair value as of December 31, 2023 ​ ​ $358,600 Forward Contract Liabilities Merger Consideration In connection with the Merger, the Company entered into a contract for the issuance of 384,930.724 shares of Series A Preferred Stock as part of the consideration transferred. The fair value of the forward contract at the Closing Date (defined below) was $155.3 million. The non-cash settlement of this liability occurred on December 5, 2023 with the issuance of the Series A Preferred Stock for $261.8 million. November 2023 Private Placement The Company entered into a contract for the issuance of 149,330.115 shares of Series A Preferred Stock as part of the November 2023 Private Placement which was settled in multiple tranches. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Timothy A. Springer, a member of the Company’s Board of Directors, and TAS Partners LLC, an affiliate of Dr. Springer, represented a forward contract. See Note 11. The fair value of the forward contract liability on November 13, 2023 was insignificant as the fair value of the underlying Series A Preferred Stock was equal to the purchase price of the Series A Preferred Stock as agreed upon in the November 2023 Private Placement. The non-cash settlement of a portion of the liability occurred on December 13, 2023 with the issuance of the first tranche of the Series A Preferred Stock for $14.8 million. The following table presents changes in the forward contract liabilities for the periods presented (in thousands): ​ ​ Forward contract liabilities Fair value as of December 31, 2022 ​ ​ $ — Issuances ​ ​ 155,308 Settlements ​ ​ (276,601) Change in fair value ​ ​ 149,600 Fair value as of December 31, 2023 ​ ​ $ 28,307

Property and Equipment (Q2)

Property and Equipment (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consists of the following (in thousands): ​ ​ June 30, 2024 ​ ​ December 31, 2023 Laboratory equipment ​ ​ $ 6,944 ​ ​ $ 6,280 Computer equipment and software ​ ​ 621 ​ ​ 702 Leasehold improvements ​ ​ 61 ​ ​ 61 Furniture and fixtures ​ ​ 462 ​ ​ 452 Office equipment ​ ​ 196 ​ ​ 196 Construction in process ​ ​ 4,417 ​ ​ 150 Total property and equipment ​ ​ 12,701 ​ ​ 7,841 Less: Accumulated depreciation ​ ​ (6,029) ​ ​ (5,728) Property and equipment, net ​ ​ $ 6,672 ​ ​ $ 2,113 Depreciation expense was $0.2 million for each of the three months ended June 30, 2024 and 2023, and $0.4 million for each of the six months ended June 30, 2024 and 2023.	Property and Equipment Property and equipment consists of the following (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Laboratory equipment ​ ​ $ 6,280 ​ ​ $ 6,001 Computer equipment and software ​ ​ 702 ​ ​ 697 Leasehold improvements ​ ​ 61 ​ ​ 57 Furniture and fixtures ​ ​ 452 ​ ​ 453 Office equipment ​ ​ 196 ​ ​ 192 Construction in process ​ ​ 150 ​ ​ 599 Total property and equipment ​ ​ 7,841 ​ ​ 7,999 Less accumulated depreciation ​ ​ (5,728) ​ ​ (5,205) Property and equipment, net ​ ​ $ 2,113 ​ ​ $ 2,794 Depreciation expense was $0.7 million, $0.7 million and $0.6 million for the years ended December 31, 2023, 2022 and 2021, respectively.

Accrued Expenses (Q2)

Accrued Expenses (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): ​ ​ June 30, 2024 ​ ​ December 31, 2023 Payroll and employee related expenses ​ ​ $ 1,402 ​ ​ $ 4,390 Accrued patent fees ​ ​ 689 ​ ​ 472 Accrued external research and development costs ​ ​ 3,467 ​ ​ 4,896 Accrued professional and consulting services ​ ​ 1,871 ​ ​ 4,331 Property and equipment ​ ​ 2,877 ​ ​ 128 Other ​ ​ 309 ​ ​ 516 Accrued expenses ​ ​ $10,615 ​ ​ $14,733	Accrued Expenses Accrued expenses consist of the following (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Payroll and employee related expenses ​ ​ $ 4,390 ​ ​ $ 4,242 Accrued patent fees ​ ​ 472 ​ ​ 696 Accrued external research and development costs ​ ​ 4,896 ​ ​ 7,274 Accrued professional and consulting services ​ ​ 4,331 ​ ​ 985 Accrued interest ​ ​ — ​ ​ 222 Other ​ ​ 644 ​ ​ 665 Accrued expenses ​ ​ $14,733 ​ ​ $14,084

Leases (Q2)

Leases (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Leases [Abstract]
Leases	Leases 65 Grove Street Lease In July 2019, the Company entered into a lease with BRE-BMR Grove LLC for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts, or the Watertown Lease. On September 1, 2022, the Company entered into an amendment to the Watertown Lease, or the Lease Agreement Amendment, to expand the Company’s laboratory and office space located at 65 Grove Street, Watertown, Massachusetts by approximately 7,216 square feet. In connection with the Lease Agreement Amendment, the Company secured a letter of credit for the Watertown Lease from Silicon Valley Bank, a division of First-Citizens Bank & Trust Company (successor by purchase to the Federal Deposit Insurance Corporation as Receiver for Silicon Valley Bridge Bank, N.A. (as successor to Silicon Valley Bank)), or SVB, for $1.6 million as of December 31, 2022. In May 2023, the Company received notice from BRE-BMR Grove LLC that the requirements to reduce the amount of the letter of credit for the Watertown Lease had been met. In connection therewith, in June 2023, the Company secured a letter of credit from JPMorgan Chase Bank, N.A. for $1.4 million, which is recognized as long-term restricted cash as of June 30, 2024 and December 31, 2023, and renews automatically each year. The $1.6 million letter of credit with SVB was released from restriction and returned to the Company on July 17, 2023, and therefore was reclassified into cash and cash equivalents in the consolidated balance sheets. On October 6, 2022, the Company entered into a sublease agreement to sublease 7,216 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts. The sublease commenced on October 24, 2022, and the term expired on March 31, 2024. On October 31, 2023, in connection with entering into Amendment No. 1 to the Sobi License as described in Note 13, the Company entered into a sublease agreement with Sobi to sublease approximately 5,600 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts for which Sobi paid $1.0 million upfront rental payment. The sublease commenced on November 6, 2023, when the Company, Sobi, and BRE-BMR Grove LLC, executed a Consent to Sublease. The term of the sublease expires on November 5, 2024 with no option to extend the sublease term. As of June 30, 2024 and December 31, 2023, deferred rent of $0.3 million and $0.8 million is included within accrued expenses and other current liabilities in the consolidated balance sheets. Sublease income is included within other income, net in the consolidated statements of operations and comprehensive income (loss). During the year ended December 31, 2023, the Company determined that the right-of-use asset related to the operating lease for approximately 7,216 square feet at 65 Grove Street was partially impaired as of November 30, 2023. As a result, the Company recognized a $0.7 million right-of-use asset impairment charge in the fourth quarter of 2023. 704 Quince Orchard Road Leases In connection with the Merger, the Company acquired two operating leases for office and laboratory space in Gaithersburg, Maryland. The leases expire in January 2027 and do not contain any renewal rights. The discount rate of 11.5% was determined based on the Company’s incremental borrowing rate adjusted for the lease term. 7495 New Horizon Way Leases On February 28, 2024, the Company entered into a lease agreement with 7495 RP, LLC, or the Landlord, pursuant to which it agreed to lease from the Landlord the manufacturing space located at 7495 New Horizon Way, Frederick, Maryland, or the Frederick Lease Agreement. The space consists of 19,199 leasable square feet of integrated manufacturing and office space. The lease commenced on May 1, 2024 which was the date the Landlord delivered full possession of the premises to the Company. The Frederick Lease Agreement will terminate approximately 7.2 years following the commencement date. The Company will have one option to extend the term of the Frederick Lease Agreement for a period of five years at a cost of 100% of the then-fair market value, not to exceed 103% of the then-current base rent. Base rent, which was due beginning on July 1, 2024, is $0.9 million annually and is subject to an annual upward adjustment of 3% of the then-current rental rate. In addition, the Company is obligated to pay its share of operating costs and taxes related to the property. The Company paid the first month’s rent of $0.1 million upon execution of the Frederick Lease Agreement. The Company assessed the classification of the lease at the commencement date and concluded it should be accounted for as an operating lease. The Company recorded a lease liability and right-of-use asset of $3.6 million and $3.7 million, respectively, on the commencement date. The Frederick Lease Agreement includes a tenant improvement allowance of up to $0.7 million which was recognized as a reduction in the right-of-use asset and lease liability at the commencement date as the Company was reasonably certain to incur reimbursable costs related to alterations equal to or exceeding the amount. Additionally, the prepaid rent was included as an adjustment to the right-of-use asset. The discount rate of 14% was determined based on the Company’s incremental borrowing rate adjusted for the lease term, including any reasonably certain renewal periods. Effective May 7, 2024, the Company and the Landlord entered into the first amendment to the Frederick Lease Agreement, or the Amended Frederick Lease Agreement, providing for the expansion of the premises leased pursuant to the Frederick Lease Agreement by approximately 7,842 square feet. In connection with the expansion of the leased premises, the Company is obligated to pay $0.3 million in additional annual base rent for the first year of the term, which is subject to an annual upward adjustment of 3% of the then-current rental rate, as well as its share of operating costs and taxes. The lease commenced on May 7, 2024 which was the date the Landlord delivered full possession of the premises to the Company and will be coterminous with the Frederick Lease Agreement. The rent commencement date is expected to be September 1, 2024. The Company assessed the classification of the lease at the commencement date and concluded it should be accounted for as an operating lease. The Company recorded a lease liability and right-of-use asset each of $1.2 million on the commencement date. The Amended Frederick Lease Agreement includes a tenant improvement allowance of up to $0.1 million which was recognized as a reduction in the right-of-use asset and lease liability at the commencement date as the Company was reasonably certain to incur reimbursable costs related to alterations equal to or exceeding the amount. The discount rate of 14% was determined based on the Company’s incremental borrowing rate adjusted for the lease term. The Company secured a letter of credit from SVB for $0.3 million for the Frederick Lease Agreement and the Amended Frederick Lease Agreement, which is recognized as long-term restricted cash as of June 30, 2024, and renews automatically each year. For the three and six months ended June 30, 2024 and 2023, the components of lease costs were as follows (in thousands): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 ​ Operating lease cost ​ ​ $ 956 ​ ​ $ 696 ​ ​ $1,731 ​ ​ $1,392 ​ Variable lease cost ​ ​ 352 ​ ​ 270 ​ ​ 749 ​ ​ 412 ​ Short-term lease cost ​ ​ 1 ​ ​ 2 ​ ​ 4 ​ ​ 5 ​ Less: Sublease income ​ ​ (250 ) ​ ​ (251 ) ​ ​ (760 ) ​ ​ (506 ) ​ Total lease cost ​ ​ $1,059 ​ ​ $ 717 ​ ​ $1,724 ​ ​ $1,303 ​ The maturity of the Company’s operating lease liabilities as of June 30, 2024 were as follows (in thousands): ​ ​ June 30, 2024 2024 (remainder) ​ ​ $ 1,239 2025 ​ ​ 4,350 2026 ​ ​ 4,471 2027 ​ ​ 4,276 2028 ​ ​ 2,243 Thereafter ​ ​ 3,409 Total future minimum lease payments ​ ​ 19,988 Less: Imputed interest ​ ​ 5,121 Total operating lease liabilities ​ ​ $14,867 The supplemental disclosure for the statement of cash flows related to operating leases was as follows (in thousands): ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ $1,604 ​ ​ $1,319 Other than the initial recording of the right-of-use assets and lease liabilities for the Frederick Lease Agreement and Amended Frederick Lease Agreement, which were non-cash, the changes in the Company’s right-of-use assets and lease liabilities for the six months ended June 30, 2024 and 2023 are reflected in the non-cash lease expense and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows. The following summarizes additional information related to operating leases: ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Weighted-average remaining lease term ​ ​ 4.9 years ​ ​ 4.9 years Weighted-average discount rate ​ ​ 11.5% ​ ​ 9.7%	Leases 65 Grove Street Lease In July 2019, the Company entered into a lease with BRE-BMR Grove LLC for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts, or the Watertown Lease. As part of the Watertown Lease, the Company incurred $0.8 million in non-reimbursable construction costs. The lease began in March 2020, when the Company took control of the office space, and the lease term is 8 years. The discount rate of 8.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term, including any reasonably certain renewal periods. In connection with the Watertown Lease, the Company secured a letter of credit from Silicon Valley Bank, a division of First-Citizens Bank & Trust Company (successor by purchase to the Federal Deposit Insurance Corporation as Receiver for Silicon Valley Bridge Bank, N.A. (as successor to Silicon Valley Bank)), or SVB, for $1.6 million, of which $0.3 million is recognized as short-term restricted cash and $1.3 million is recognized as long-term restricted cash, as of December 31, 2022. On September 1, 2022, the Company entered into an amendment, or the Lease Agreement Amendment, to its lease agreement with BRE-BMR Grove LLC, originally entered into on July 23, 2019, or the Lease Agreement, to expand the Company’s laboratory and office space located at 65 Grove Street, Watertown, Massachusetts by 7,216 square feet. The lease term began on September 1, 2022, consistent with when the Company took control of the office space and the expected lease term is 5.7 years. The discount rate of 11.3% was determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods. Rent payments began in November 2022, and the base rent for the first year is $0.1 million per month. The Company recorded the right-of-use asset and operating lease liabilities of $3.2 million during the year ended December 31, 2022 as control of the premises was transferred to the Company. On October 6, 2022, the Company entered into a sublease agreement to sublease 7,216 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts. The sublease commenced on October 24, 2022, when the Company, the sublessee and BRE-BMR Grove LLC, executed a Consent to Sublease. The term of the sublease expires on March 31, 2024 with no option to extend the sublease term. Sublease income is included within other income, net in the consolidated statements of operations and comprehensive income (loss). As a result of the sublease agreement and Consent to Sublease, rent payments to BRE-BMR Grove LLC for the lease of the office space increased. The change of consideration in the contract was accounted for as a lease modification and the right-of-use asset and lease liability were remeasured at the modification date of October 24, 2022. The discount rate of 11.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods as of October 24, 2022, resulting in a decrease of less than $0.1 million to both the right-of-use asset and lease liabilities. In May 2023, the Company received notice from BRE-BMR Grove LLC that the requirements to reduce the amount of the letter of credit for the Watertown Lease had been met. In connection therewith, in June 2023, the Company secured a letter of credit from JPMorgan Chase Bank, N.A. for $1.4 million, which is recognized as long-term restricted cash as of December 31, 2023, and renews automatically each year. The $1.6 million letter of credit with SVB was released from restriction and returned to the Company on July 17, 2023, and therefore was reclassified into cash and cash equivalents in the consolidated balance sheets. On October 31, 2023, in connection with entering into Amendment No. 1 to the License and Development Agreement with Sobi as described in Note 14, the Company entered into a sublease agreement with Sobi to sublease approximately 5,600 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts for which Sobi paid $1.0 million upfront rental payment. The sublease commenced on November 6, 2023, when the Company, Sobi, and BRE-BMR Grove LLC, executed a Consent to Sublease. The term of the sublease expires on November 5, 2024 with no option to extend the sublease term. As of December 31, 2023, deferred rent of $0.8 million is included within accrued expenses and other current liabilities in the consolidated balance sheets. During the year ended December 31, 2023, the Company determined that the right-of-use asset related to the operating lease for approximately 7,216 square feet at 65 Grove Street was partially impaired as of November 30, 2023. As a result, the Company recognized a $0.7 million right-of-use asset impairment charge with $0.6 million and $0.1 million recognized in research and development and general and administrative operating expense categories, respectively, on its consolidated statements of operations and comprehensive income (loss) during the year ended December 31, 2023. 704 Quince Orchard Road Leases In connection with the Merger, the Company acquired two operating leases for office and laboratory space in Gaithersburg, Maryland. The leases expire in January 2027 and do not contain any renewal rights. The discount rate of 11.5% was determined based on the Company’s incremental borrowing rate adjusted for the lease term. Moscow, Russia Lease The Company has a month-to-month facility agreement for Selecta (RUS)'s Moscow, Russia office. Rent expense is recognized as incurred. Rent expense for the years ended December 31, 2023, 2022 and 2021 was $3.8 million, $3.2 million, and $2.9 million, respectively. For the years ended December 31, 2023, 2022 and 2021, the components of lease costs were as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Operating lease cost ​ ​ $ 2,828 ​ ​ $2,276 ​ ​ $2,023 Variable lease cost ​ ​ 965 ​ ​ 910 ​ ​ 834 Short-term lease cost ​ ​ 8 ​ ​ 11 ​ ​ 10 Less sublease income ​ ​ (1,172) ​ ​ (176) ​ ​ — Total lease cost ​ ​ $ 2,629 ​ ​ $3,021 ​ ​ $2,867 The maturity of the Company’s operating lease liabilities as of December 31, 2023 were as follows (in thousands): ​ ​ December 31, 2023 2024 ​ ​ $ 3,077 2025 ​ ​ 3,164 2026 ​ ​ 3,248 2027 ​ ​ 3,017 2028 ​ ​ 946 Thereafter ​ ​ — Total future minimum lease payments ​ ​ 13,452 Less: Imputed interest ​ ​ 2,497 Total operating lease liabilities ​ ​ $10,955 The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ $2,696 ​ ​ $2,048 Other than the initial recording and modification of the right-of-use asset and lease liability for the Watertown Lease during the year ended December 31, 2022 and the impairment on the right-of-use asset for the Watertown Lease and the assumption of the right-of-use assets and lease liabilities in connection with the Merger during the year ended December 31, 2023, which were non-cash, the changes in the Company’s right-of-use asset and lease liability for the years ended December 31, 2023 and 2022 are reflected in the non-cash lease expense and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows. The following summarizes additional information related to operating leases: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Weighted-average remaining lease term ​ ​ 4.3 years ​ ​ 5.4 years Weighted-average discount rate ​ ​ 9.9 % ​ ​ 9.7 %

Debt (Q2)

Debt (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Debt Disclosure [Abstract]
Debt	Debt 2020 Term Loan On August 31, 2020, the Company entered into a Loan and Security Agreement with Oxford Finance LLC, or Oxford, and Silicon Valley Bank, or the Loan and Security Agreement, and such facility, the 2020 Term Loan. On March 10, 2023, Silicon Valley Bank was closed by the California Department of Financial Protection and Innovation, and the Federal Deposit Insurance Corporation, or the FDIC, was appointed as receiver. On March 13, 2023, the FDIC announced that all of Silicon Valley Bank’s deposits and substantially all of its assets had been transferred to a newly created, full-service, FDIC-operated bridge bank, Silicon Valley Bridge Bank, N.A., or SVBB. SVBB assumed all loans that were previously held by Silicon Valley Bank. On March 27, 2023, First-Citizens Bank & Trust Company assumed all of SVBB’s customer deposits and certain other liabilities and acquired substantially all of SVBB’s loans and certain other assets from the FDIC, including the 2020 Term Loan. On September 11, 2023, the Company entered into a payoff letter with Oxford and SVB, pursuant to which the Company paid all outstanding amounts under the 2020 Term Loan, together with accrued interest and a prepayment penalty, resulting in the full extinguishment of the 2020 Term Loan. The total payoff amount was $22.3 million, consisting of the remaining principal amount due of $19.8 million, the final payment fee of $2.3 million, the prepayment penalty of $0.2 million, and less than $0.1 million of accrued interest. During the third quarter of 2023, the Company recorded a loss of $0.7 million on the extinguishment of the 2020 Term Loan, consisting of the prepayment penalty of $0.2 million and the write-off of $0.5 million of unamortized debt issuance costs and venture debt termination fee. As of June 30, 2024 and December 31, 2023, the Company had no outstanding borrowings.	Debt 2020 Term Loan On August 31, 2020, the Company entered into a Loan and Security Agreement with Oxford Finance LLC, or Oxford, and Silicon Valley Bank, or the Loan and Security Agreement, and such facility, the 2020 Term Loan. On March 10, 2023, Silicon Valley Bank was closed by the California Department of Financial Protection and Innovation, and the Federal Deposit Insurance Corporation, or the FDIC, was appointed as receiver. On March 13, 2023, the FDIC announced that all of Silicon Valley Bank’s deposits and substantially all of its assets had been transferred to a newly created, full-service, FDIC-operated bridge bank, Silicon Valley Bridge Bank, N.A., or SVBB. SVBB assumed all loans that were previously held by Silicon Valley Bank. On March 27, 2023, First-Citizens Bank & Trust Company assumed all of SVBB’s customer deposits and certain other liabilities and acquired substantially all of SVBB’s loans and certain other assets from the FDIC, including the 2020 Term Loan. On September 11, 2023, the Company entered into a payoff letter with Oxford and SVB, pursuant to which the Company paid all outstanding amounts under the 2020 Term Loan, together with accrued interest and a prepayment penalty, resulting in the full extinguishment of the 2020 Term Loan. The total payoff amount was $22.3 million, consisting of the remaining principal amount due of $19.8 million, the final payment fee of $2.3 million, the prepayment penalty of $0.2 million, and less than $0.1 million of accrued interest. During the year ended December 31, 2023, the Company recorded a loss of $0.7 million on the extinguishment of the 2020 Term Loan, consisting of the prepayment penalty of $0.2 million and the write-off of $0.5 million of unamortized debt issuance costs and venture debt termination fee, which was included within interest expense in the consolidated statements of operations and comprehensive income (loss). As of December 31, 2023, the Company had no outstanding borrowings, and as of December 31, 2022, the outstanding principal balance under the 2020 Term Loan was $25.0 million. During the years ended December 31, 2023, 2022 and 2021, the Company recognized $2.1 million, $3.0 million and $2.8 million respectively of interest expense related to the 2020 Term Loan.

Series A Preferred Stock (Q2)

Series A Preferred Stock (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Temporary Equity Disclosure [Abstract]
Series A Preferred Stock	Series A Preferred Stock The Certificate of Designation of Preferences, Rights, and Limitations of the Series A Non-Voting Convertible Preferred Stock, or the Certificate of Designation, was filed on November 13, 2023, which provided for the designation of shares of the Series A Preferred Stock and authorized the issuance of 548,375 shares of Series A Preferred Stock. Additionally on November 13, 2023, the Company entered into the Securities Purchase Agreement with (i) Dr. Timothy A. Springer, a member of the Company’s Board of Directors; (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, a co-founder and the former chief executive officer of Old Cartesian, who joined the Company’s Board of Directors effective immediately after the effective time of the Merger, or the Investors. Pursuant to the Securities Purchase Agreement, the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million in the November 2023 Private Placement. In the November 2023 Private Placement, Dr. Timothy A. Springer agreed to settle his purchases in three tranches of shares of Series A Preferred Stock, the first for a purchase price of $10.0 million and each thereafter for a purchase price of approximately $20.0 million, with the three tranches settling 30, 60, and 90 days, respectively, following the Closing Date. TAS Partners LLC agreed to settle its purchase for approximately $10.0 million within 30 days following the Closing Date. The first, second and third tranches were settled on December 13, 2023, January 12, 2024 and February 11, 2024, respectively, under which (i) 24,785.081 shares of Series A Preferred Stock were issued to each of TAS Partners LLC and Dr. Timothy A. Springer in the first tranche, (ii) 49,570.163 shares of Series A Preferred Stock were issued to Dr. Timothy A. Springer in the second tranche, and (iii) 49,570.163 shares of Series A Preferred Stock were issued to Dr. Timothy A. Springer in the third tranche. On November 15, 2023, the Company issued 619.627 shares of Series A Preferred Stock to Seven One Eight Three Four Irrevocable Trust for $0.25 million. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Springer and TAS Partners LLC represented a forward contract and was accounted for as a liability with changes in fair value recorded in earnings. A portion of the liability was settled with the initial issuance of 49,570.162 shares of Series A Preferred Stock on December 13, 2023. The remaining portion of the forward contract liability was settled upon the issuance of 49,570.163 shares of Series A Preferred Stock each on January 12, 2024 and February 11, 2024, respectively (see Note 5). On December 5, 2023, the Company issued 384,930.724 shares of Series A Preferred Stock as part of its consideration transferred in connection with the Merger which settled the related forward contract liability (see Note 5). On March 26, 2024, the Company, with the consent of the requisite holders of Series A Preferred Stock, amended the Certificate of Designation such that the automatic conversion of the Series A Preferred Stock into common stock, or the Automatic Conversion, would occur eight business days following stockholder approval of the Conversion Proposal. Upon such date, each share of Series A Preferred Stock automatically converted into 33.333 shares of common stock, subject to certain limitations, including that a holder of Series A Preferred Stock is prohibited from converting shares of Series A Preferred Stock into shares of common stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than a specified percentage (to be established by the holder between 0% and 19.9%) of the total number of shares of common stock issued and outstanding immediately after giving effect to such conversion; provided, however, that such beneficial ownership limitation does not apply to Dr. Springer, TAS Partners LLC, or any of their respective affiliates. Each share of Series A Preferred Stock outstanding that was not automatically converted into common stock as a result of the beneficial ownership limitation is convertible at any time at the option of the holder, only to the extent the beneficial ownership limitation does not apply to the shares of Series A Preferred Stock to be converted. On March 27, 2024, the Company’s stockholders approved the Conversion Proposal, among other matters, at the Special Meeting. As a result of the approval of the Conversion Proposal, all conditions that could have required cash redemption of the Series A Preferred Stock were satisfied. Since the Series A Preferred Stock is no longer redeemable, the associated balances of the Series A Preferred Stock were reclassified from mezzanine equity to permanent equity during the first quarter of 2024. On April 8, 2024, pursuant to the terms of the Certificate of Designation, as amended, 367,919.247 shares of Series A Preferred Stock automatically converted into 12,263,951 shares of common stock, including the non-cash reclassification of an amount equal to the increase in par value of common stock from additional paid-in capital; 166,341.592 shares of Series A Preferred Stock did not automatically convert due to beneficial ownership limitations. As of June 30, 2024, the Company had 166,341.592 shares of Series A Preferred Stock issued and outstanding, which are convertible into 5,544,719 shares of common stock.	Series A Preferred Stock The Certificate of Designation was filed on November 13, 2023, which provided for the designation of shares of the Series A Preferred Stock and authorized the issuance of 548,375 shares of Series A Preferred Stock. Additionally on November 13, 2023, the Company entered into the Securities Purchase Agreement with (i) Dr. Timothy A. Springer, a member of the Company’s Board of Directors; (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, a co-founder and the former chief executive officer of Old Cartesian, who joined the Company’s Board of Directors effective immediately after the effective time of the Merger, or the Investors. Pursuant to the Securities Purchase Agreement, the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million in the November 2023 Private Placement. In the November 2023 Private Placement Dr. Timothy A. Springer agreed to settle his purchases in three tranches of shares of Series A Preferred Stock, the first for a purchase price of $10.0 million and each thereafter for a purchase price of approximately $20.0 million, with the three tranches settling 30, 60, and 90 days, respectively, following the Closing Date. TAS Partners LLC agreed to settle its purchase for approximately $10.0 million within 30 days following the Closing Date. The first, second and third tranches were settled on December 13, 2023, January 12, 2024 and February 11, 2024, respectively, under which (i) 24,785.081 shares of Series A Preferred Stock were issued to each of TAS Partners LLC and Dr. Timothy A. Springer in the first tranche, (ii) 49,570.163 shares of Series A Preferred Stock were issued to Dr. Timothy A. Springer in the second tranche, and (iii) 49,570.163 shares of Series A Preferred Stock were issued to Dr. Timothy A. Springer in the third tranche. On November 15, 2023, the Company issued 619.627 shares of Series A Preferred Stock to Seven One Eight Three Four Irrevocable Trust for $0.25 million. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Springer and TAS Partners LLC represented a forward contract and was accounted for as a liability with changes in fair value recorded in earnings. A portion of the liability was settled with the initial issuance of 49,570.162 shares of Series A Preferred Stock on December 13, 2023 (see Note 6). On December 5, 2023, the Company issued 384,930.724 shares of Series A Preferred Stock as part of its consideration transferred in connection with the Merger which settled the related forward contract liability (see Note 6). As of December 31, 2023, the Company had 435,120.513 shares of Series A Preferred Stock issued and outstanding. In accordance with the guidance in ASC 480, Distinguishing Liabilities from Equity The Series A Preferred Stock had the following rights and preferences as of December 31, 2023: Conversion Prior to the stockholder approval of the Conversion Proposal, the Series A Preferred Shares are not convertible. Following the stockholder approval of the Conversion Proposal, each share of Series A Preferred Stock will automatically convert into 1,000 shares of common stock, subject to certain limitations, including that a holder of Series A Preferred Stock is prohibited from converting shares of Series A Preferred Stock into shares of common stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than a specified percentage (to be established by the holder between 0% and 19.9%) of the total number of shares of common stock issued and outstanding immediately after giving effect to such conversion; provided, however, that such beneficial ownership limitation does not apply to Dr. Springer, TAS Partners LLC, or any of their respective affiliates. Each share of Series A Preferred Stock outstanding that is not otherwise automatically converted into common stock as a result of the beneficial ownership limitation shall be convertible at any time at the option of the holder following stockholder approval of the Conversion Proposal, only to the extent the beneficial ownership limitation does not apply to the shares of Series A Preferred Stock to be converted. Redemption Each share of Series A Preferred Stock will be redeemable at the option of the holder at any time following the date that is 18 months after the initial issuance date of the Series A Preferred Stock, other than any shares of Series A Preferred Stock that would not be convertible into shares of common stock as a result of the beneficial ownership limitation referred to above. The amount payable upon redemption will be equal to the average closing sale price of the common stock listed over the ten consecutive trading days ending on, and including, the day immediately prior to the redemption date multiplied by the number of shares of common stock the Series A Preferred Stock would be convertible into. Dividends Holders of Series A Preferred Stock are entitled to receive dividends on shares of Series A Preferred Stock on an as-converted basis equal to the dividends paid on shares of the common stock; provided, however, that holders of Series A Preferred Stock (or any shares of common stock into which the Series A Preferred Stock are convertible) are not entitled to any CVRs or any amounts paid under the CVR Agreement. Voting Except as otherwise required by law, the Series A Preferred Stock does not have voting rights. However, as long as any shares of Series A Preferred Stock are outstanding, the Company will not, without the affirmative vote of the holders of a majority of the then-outstanding shares of the Series A Preferred Stock, (a) alter or change adversely the powers, preferences or rights given to the Series A Preferred Stock, (b) alter or amend the Certificate of Designation, (c) amend the Charter or other organizational documents in any manner that adversely affects any rights of the holders of Series A Preferred Stock, (d) issue further shares of Series A Preferred Stock (other than in connection with the exercise of the stock options to purchase Series A Preferred Stock) or increase or decrease (other than by conversion) the number of authorized shares of Series A Preferred Stock, (e) prior to the stockholder approval of the Conversion Proposal or at any time while at least 30% of the originally issued Series A Preferred Stock remains issued and outstanding, consummate either (A) a Fundamental Transaction (as defined in the Certificate of Designation) or (B) any merger or consolidation of the Company or other business combination in which the stockholders of the Company immediately before such transaction do not hold at least a majority of the capital stock of the Company immediately after such transaction, (f) amend or fail to comply with, in any manner that would be reasonably likely to prevent, impede or materially delay the conversion (or the stockholder approval thereof), or terminate, any of the stockholder support agreements entered into in connection with the Merger, or the Support Agreements, or agree to any transfer, sale or disposition of such shares subject to the Support Agreements (except for such transfers, sales or dispositions with respect to which the approval of the Company is not required pursuant to the applicable Support Agreement) or (g) enter into any agreement with respect to any of the foregoing. Liquidation The holders of Series A Preferred Stock shall rank on parity with the common stockholders as to distributions of assets upon liquidation, dissolution or winding up of the Company, whether voluntarily or involuntarily. Upon any liquidation, dissolution or winding-up of the Company, whether voluntary or involuntary each holder of Series A Preferred Stock shall be entitled to receive out the assets of the Company equal to of the same amount that a holder of common stock would receive if the Series A Preferred Stock were fully converted, which shall be paid pari passu with holders of common stock, plus an amount equal to any dividends declared but unpaid. If the assets available for distribution are not sufficient to pay the holders of the Series A Preferred Stock pursuant to the preceding sentence, the assets will be distributed ratably to the holders of the Series A Preferred Stock and common stock. Reserved Shares As of December 31, 2023, the Company has authorized shares of Series A Preferred Stock for future issuance as follows: ​ ​ As of December 31, 2023 Shares reserved for issuance in November 2023 Private Placement ​ ​ 99,140.326 Outstanding Series A Preferred Stock options ​ ​ 14,112.299 Total ​ ​ 113,252.625

Equity (Q2)

Equity (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Equity [Abstract]
Equity	Equity Equity Financings “At-the-Market” Offerings On October 25, 2021, the Company entered into a sales agreement, or the 2021 Sales Agreement, with Leerink Partners LLC (then known as SVB Leerink LLC), or Leerink Partners, pursuant to which the Company may sell shares of the Company’s common stock, from time to time, through an “at the market” equity offering program under which Leerink Partners will act as sales agent. The shares of common stock sold pursuant to the 2021 Sales Agreement, if any, would be issued and sold pursuant to a registration statement to be filed by the Company with the SEC, for aggregate remaining gross sales proceeds of up to $51.0 million. During the six months ended June 30, 2024 and the year ended December 31, 2023, the Company sold no shares of its common stock pursuant to the 2021 Sales Agreement. Warrants The following is a summary of warrant activity for the six months ended June 30, 2024: ​ ​ Number of Warrants ​ ​ Weighted- average exercise price ​ ​ Equity classified ​ ​ Liability classified ​ ​ Total ​ Outstanding at December 31, 2023 ​ ​ 74,420 ​ ​ 966,393 ​ ​ 1,040,813 ​ ​ $45.98 Exercises ​ ​ (65,681) ​ ​ — ​ ​ (65,681) ​ ​ 43.80 Outstanding at June 30, 2024 ​ ​ 8,739 ​ ​ 966,393 ​ ​ 975,132 ​ ​ $46.12 See Note 14 for further discussion on the exercise of the 65,681 warrants during the six months ended June 30, 2024. See Note 12 to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023 for further discussion of the terms related to the Company’s warrants. Common Stock On April 4, 2024, the Company implemented the Reverse Stock Split. The Reverse Stock Split became effective at 4:30 p.m. Eastern Time on April 4, 2024. On April 5, 2024, the Company’s common stock began trading on The Nasdaq Global Market on a split-adjusted basis under the symbol “RNAC” with a new CUSIP number, 816212302. As a result of the Reverse Stock Split, every 30 shares of common stock outstanding were combined, automatically and without any action on the part of the Company or its stockholders, into one share of common stock. Stockholders entitled to fractional shares as a result of the Reverse Stock Split received a cash payment in lieu of receiving fractional shares. The Reverse Stock Split did not change the number of authorized shares or par value of the Company’s common or preferred stock. Reserved Shares The Company has authorized shares of common stock for future issuance as of June 30, 2024 as follows: Exercise of warrants ​ ​ 975,132 Shares available for future stock incentive awards ​ ​ 3,930,990 Unvested restricted stock units ​ ​ 454,456 Outstanding common stock options ​ ​ 1,889,561 Series A Preferred Stock ​ ​ 5,544,719 Total ​ ​ 12,794,858	Equity Equity Financings Merger On December 5, 2023, the Company issued 6,723,639 shares of common stock as part of its consideration transferred in connection with the Merger which settled the related equity-classified forward contract (see Note 3). Underwritten Offering On April 6, 2022, the Company entered into an underwriting agreement with SVB Securities LLC (now known as Leerink Partners LLC), as representative of the several underwriters named therein, relating to an underwritten offering of 27,428,572 shares of the Company’s common stock and 2022 Warrants to purchase up to 20,571,429 shares of common stock. The offering of such shares and the 2022 Warrants is referred to as the 2022 Offering. Each share and accompanying 2022 Warrant to purchase 0.75 shares of common stock was sold at a combined offering price of $1.41. The exercise price for the 2022 Warrants is $1.55 per share. The Company received net proceeds from the 2022 Offering of approximately $36.9 million. The 2022 Warrants are subject to adjustment in the event of certain stock dividends and distributions, stock splits, stock combinations, reclassifications or similar events affecting the Company’s common stock and also upon any distributions for no consideration of assets to the Company’s stockholders. Each 2022 Warrant is exercisable at any time and from time to time after issuance. In the event of certain corporate transactions, the holders of the 2022 Warrants will be entitled to receive the kind and amount of securities, cash or other property that the holders would have received had they exercised the Warrants immediately prior to such transaction. Therefore, the Company is required to account for the 2022 Warrants as liabilities and record the 2022 Warrants at fair value. The 2022 Warrants do not entitle the holders thereof to any voting rights or any of the other rights or privileges to which holders of Common Stock are entitled. “At-the-Market” Offerings 2020 Sales Agreement On August 6, 2020, the Company entered into a sales agreement, or the 2020 Sales Agreement with Jefferies LLC, as sales agent, pursuant to which the Company was permitted, from time to time, to issue and sell common stock with an aggregate value of up to $50.0 million in an “at the market offering.” On October 8, 2021, the Company delivered notice to Jefferies LLC that the Company was terminating the 2020 Sales Agreement, with effect as of October 19, 2021. 2021 Sales Agreement On October 25, 2021, the Company entered into a Sales Agreement, or the 2021 Sales Agreement, with Leerink Partners LLC (then known as SVB Leerink LLC), or Leerink Partners, pursuant to which the Company may sell shares of the Company’s common stock, from time to time, through an “at the market” equity offering program under which Leerink Partners will act as sales agent. The shares of common stock sold pursuant to the 2021 Sales Agreement, if any, would be issued and sold pursuant to a registration statement to be filed by the Company with the SEC, for aggregate remaining gross sales proceeds of up to $51.0 million. During the year ended December 31, 2023, the Company sold no shares of its common stock pursuant to the 2021 Sales Agreement. During the year ended December 31, 2022, the Company sold 774,544 shares of its common stock pursuant to the 2021 Sales Agreement for aggregate net proceeds of $2.1 million, after deducting commissions and other transaction costs. June 2020 Sobi Stock Purchase On June 11, 2020, the Company entered into a stock purchase agreement with Sobi, pursuant to which the Company sold an aggregate of 5,416,390 shares of its common stock at a purchase price equal to $4.6156 per share, which represented 120% of the 10-day volume-weighted average price of the Company’s common stock prior to signing, for aggregate gross proceeds of $25.0 million, or the Sobi Private Placement. The closing of the Sobi Private Placement occurred on July 31, 2020. In accordance with ASC 815, this forward sale treatment qualified as equity classification as the shares are not within the scope of ASC 480. The gross proceeds of $25.0 million were determined to include a premium to the fair value of the Company’s shares as of July 28, 2020 of approximately $14.5 million. As a result, such amount was included in the transaction price for revenue recognition of the Sobi License. See Note 14 for details. Also on June 11, 2020, the Company entered into a registration rights agreement, as amended by that certain letter agreement, dated as of November 4, 2020, or the Sobi Registration Rights Agreement, with Sobi, pursuant to which the Company agreed to prepare and file a registration statement with respect to the resale of the shares of common stock acquired in the Sobi Private Placement. The Company will be required to file this resale registration statement within 30 days following receipt by the Company of a written request from Sobi to file such resale registration statement, and to have the registration statement declared effective within ten business days after the SEC informs the Company that no review of such resale registration statement will be made or that the SEC has no further comments on such resale registration statement. December 2019 Financing On December 18, 2019, the Company entered into a securities purchase agreement, or the 2019 Purchase Agreement, with a group of institutional investors and certain members of the Board of Directors. Pursuant to the 2019 Purchase Agreement, the Company sold an aggregate of 37,634,883 shares of its common stock at a purchase price of $1.46 per share, warrants to purchase an aggregate of 22,988,501 shares of common stock at a purchase price of $0.125 per share underlying each common warrant, and pre-funded warrants to purchase an aggregate of 8,342,128 shares of common stock at a purchase price of $1.46 per share, all with five year terms, or the 2019 PIPE. The closing of the 2019 PIPE occurred on December 23, 2019. The exercise price of the pre-funded warrants is $0.0001 per share and the exercise price for the common warrants is $1.46 per share. In the event of a certain sale of the Company, the terms of the common warrants require us to make a payment to such common warrant holders based on a Black-Scholes valuation (using variables as specified in the warrants). This provision does not apply to the pre-funded warrants. Therefore, the Company is required to account for the common warrants as liabilities and record them at fair value, while the pre-funded warrants met the criteria to be classified as permanent equity. The Company recorded the fair value of the 2019 Warrants of $40.7 million upon issuance using the Black-Scholes valuation model. Issuance costs were allocated between the equity component with an offset to additional paid-in capital and the liability component recorded as expense on a relative fair value basis. Total net proceeds from the equity offering was $65.6 million, after deducting transaction costs and commissions of $4.4 million. As discussed in Note 6, the Company remeasured the Amended 2019 Warrants at the fair value of $0.8 million on December 20, 2022 and reclassified this amount to additional paid-in capital. The remaining 2019 Warrants liability and the 2022 Warrants liability were revalued as of December 31, 2023 at $6.4 million. During the years ended December 31, 2023, 2022 and 2021, the Company recorded a decrease of $12.7 million and $20.9 million and an increase of $2.3 million, respectively, in the fair value of the warrants in the consolidated statements of operations and comprehensive income (loss). June 2017 Financing In June 2017, the Company entered into a securities purchase agreement, or the Institutional Purchase Agreement, with certain institutional investors and a securities purchase agreement with Timothy A. Springer, Ph.D., a member of the Board of Directors, or the Springer Purchase Agreement, for a private placement of the Company’s securities, or the 2017 PIPE. Pursuant to the Institutional Purchase Agreement, the Company sold an aggregate of 2,750,000 shares of its common stock at a purchase price equal to $16.00 per share. Pursuant to the Springer Purchase Agreement, the Company sold to Dr. Springer an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Stock Market on June 23, 2017, and warrants to purchase up to 79,130 shares of common stock, or the Warrant Shares, exercisable at $17.71 per Warrant Share, and with a term of five years. The equity-classified warrants expired during the year ended December 31, 2022. Warrants The following is a summary of warrant activity for the years ended December 31, 2023 and 2022: ​ ​ Number of Warrants ​ ​ ​ ​ Equity classified ​ ​ Liability classified ​ ​ Total ​ ​ Weighted average exercise price Outstanding at December 31, 2021 ​ ​ 292,469 ​ ​ 10,443,511 ​ ​ 10,735,980 ​ ​ $ 1.62 Issuance ​ ​ — ​ ​ 20,571,429 ​ ​ 20,571,429 ​ ​ 1.55 Canceled ​ ​ (79,130) ​ ​ — ​ ​ (79,130) ​ ​ $17.71 Reclassification of warrant liability to equity on modification ​ ​ 2,022,987 ​ ​ (2,022,987) ​ ​ — ​ ​ $ 1.46 Outstanding at December 31, 2022 ​ ​ 2,236,326 ​ ​ 28,991,953 ​ ​ 31,228,279 ​ ​ $ 1.53 Canceled ​ ​ (3,576) ​ ​ — ​ ​ (3,576) ​ ​ 16.77 Outstanding at December 31, 2023 ​ ​ 2,232,750 ​ ​ 28,991,953 ​ ​ 31,224,703 ​ ​ $ 1.53 Common Stock As of December 31, 2023, the Company had 350,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 161,927,821 shares issued and outstanding. The voting, dividend and liquidation rights of the common stockholders are subject to and qualified by the rights, powers and preferences of the preferred stock. The common stock has the following characteristics: Voting Common stockholders are entitled to one vote for each share of common stock held with respect to all matters voted on by the stockholders of the Company. Dividends Common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. Through December 31, 2023, no cash dividends have been declared or paid on common stock. Liquidation Upon liquidation of the Company, common stockholders are entitled to receive all assets of the Company available for distribution to such stockholders. Reserved Shares The Company has authorized shares of common stock for future issuance as follows: ​ ​ As of December 31, 2023 Exercise of warrants ​ ​ 31,224,703 Shares available for future stock incentive awards ​ ​ 35,836,268 Outstanding common stock options ​ ​ 23,306,661 Total ​ ​ 90,367,632 As described in Note 11, prior to the stockholder approval of the Conversion Proposal, the Series A Preferred Shares are not convertible. Following the stockholder approval of the Conversion Proposal, each share of Series A Preferred Stock will automatically convert into 1,000 shares of common stock.

Stock Incentive Plans (Q2)

Stock Incentive Plans (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Stock Incentive Plans	Stock Incentive Plans The Company maintained the 2008 Stock Incentive Plan, or the 2008 Plan, for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board of Directors. In connection with the Merger, all outstanding awards issued under the 2008 Plan were cancelled, and the Board of Directors formally terminated the 2008 Plan. In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 40,341 shares of common stock for future issuance under the 2016 Plan and the Company ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remained subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expired, lapsed or terminated became available under the 2016 Plan as shares available for future grants. Additionally, pursuant to the terms of the 2016 Plan, the Board of Directors is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board of Directors or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board of Directors established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In January 2024, the number of shares of common stock that may be issued under the 2016 Plan was increased by 215,903. In June 2024, the Company’s stockholders approved an amendment and restatement of the 2016 Plan to reserve an additional 3,466,544 shares of its common stock for issuance. As of June 30, 2024, 3,496,828 shares remain available for future issuance under the 2016 Plan. In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan, was adopted by the Board of Directors without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 39,166 shares of its common stock for issuance. In March 2019, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 66,667 shares of the Company’s common stock for issuance thereunder. In December 2023, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 60,833 shares of the Company’s common stock for issuance thereunder. In June 2024, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 360,000 shares of the Company’s common stock for issuance thereunder. As of June 30, 2024, there are 364,660 shares available for future grant under the 2018 Inducement Incentive Award Plan. In accordance with the Merger Agreement, the Company assumed Old Cartesian’s 2016 Stock Incentive Plan, or the Old Cartesian Plan. The Old Cartesian Plan permits the granting of options or restricted stock to employees, officers, directors, consultants and advisors to the Company. The unvested common stock options and Series A Preferred Stock options assumed by the Company in connection with the Merger generally vest over a four-year period. Additionally, the stock options granted have a contractual term of ten years and only full shares can be exercised as per the individual award agreements. As of June 30, 2024, there are 23,707 shares available for future grant under the Old Cartesian Plan. In connection with the Merger, the outstanding stock options to purchase Old Cartesian common stock were converted into stock options to purchase 776,865 shares of common stock and 14,112.299 shares of Series A Preferred Stock of the Company. These replacement awards were revalued at their acquisition-date fair value and then attributed to pre and post-combination service. This resulted in $2.6 million attributed to post-combination service to be recognized as stock-based compensation expense over the remaining terms of the replacement awards, of which $0.3 million and $0.7 million was recognized as research and development expense in the consolidated statements of operations and comprehensive income (loss) during the three and six months ended June 30, 2024, respectively. Following the Automatic Conversion, the options exercisable for shares of Series A Preferred Stock became exercisable for shares of common stock. Stock-Based Compensation Expense Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive income (loss), including $1.5 million recognized as stock-based compensation expense upon the achievement of a technical milestone by Ginkgo Bioworks Holdings, Inc., or Ginkgo, during the three and six months ended June 30, 2023 as described in Note 15, was as follows (in thousands): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Research and development ​ ​ $ 776 ​ ​ $2,677 ​ ​ $1,488 ​ ​ $3,869 General and administrative ​ ​ 815 ​ ​ 1,106 ​ ​ 1,534 ​ ​ 2,190 Total stock-based compensation expense ​ ​ $1,591 ​ ​ $3,783 ​ ​ $3,022 ​ ​ $6,059 Stock Options The estimated grant date fair values of stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model based on the following weighted-average assumptions: ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Risk-free interest rate ​ ​ 4.53% ​ ​ 3.38% ​ ​ 4.04% ​ ​ 3.95% Dividend yield ​ ​ — ​ ​ — ​ ​ — ​ ​ — Expected term (in years) ​ ​ 6.18 ​ ​ 6.00 ​ ​ 6.20 ​ ​ 5.94 Expected volatility ​ ​ 93.57% ​ ​ 94.40% ​ ​ 95.09% ​ ​ 94.64% Weighted-average fair value of common stock ​ ​ $22.08 ​ ​ $39.30 ​ ​ $20.14 ​ ​ $34.50 The expected term of the Company's stock options granted has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Under the simplified method, the expected term is presumed to be the midpoint between the vesting date and the end of the contractual term. The Company utilizes this method due to lack of historical exercise data and the plain nature of its stock-based awards. The weighted-average grant date fair value of stock options granted was $17.40 and $30.51 during the three months ended June 30, 2024 and 2023, respectively, and $16.05 and $26.89 during the six months ended June 30, 2024 and 2023, respectively. As of June 30, 2024, total unrecognized compensation expense related to unvested common stock options was $9.9 million, which is expected to be recognized over a weighted average period of 3.0 years. The following table summarizes the stock option activity under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: ​ ​ Number of common stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Outstanding at December 31, 2023 ​ ​ 776,865 ​ ​ $ 2.97 ​ ​ 6.50 ​ ​ $13,760 Granted ​ ​ 761,556 ​ ​ $20.14 ​ ​ ​ ​ Converted from options for Series A Preferred Stock ​ ​ 470,403 ​ ​ $ 2.40 ​ ​ ​ ​ Exercised ​ ​ (89,009) ​ ​ $ 3.08 ​ ​ ​ ​ Forfeited ​ ​ (30,254) ​ ​ $14.84 ​ ​ ​ ​ Outstanding at June 30, 2024 ​ ​ 1,889,561 ​ ​ $ 9.56 ​ ​ 7.24 ​ ​ $33,081 Vested at June 30, 2024 ​ ​ 944,490 ​ ​ $ 2.74 ​ ​ 5.43 ​ ​ $22,925 Vested and expected to vest at June 30, 2024 ​ ​ 1,743,749 ​ ​ $ 8.66 ​ ​ 7.12 ​ ​ $32,070 The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Outstanding at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 Converted to options for common stock ​ ​ (14,112.299) ​ ​ $79.94 ​ ​ ​ ​ Outstanding at June 30, 2024 ​ ​ — ​ ​ $ — ​ ​ ​ As a result of the approval of the Conversion Proposal on March 27, 2024, all conditions that could have required cash redemption of the Series A Preferred Stock underlying the stock options were satisfied. Since the Series A Preferred Stock is no longer redeemable, the associated balances of the stock options to purchase Series A Preferred Stock were reclassified from mezzanine equity to additional paid-in capital during the first quarter of 2024. Following the Automatic Conversion, all options to purchase Series A Preferred Stock were converted into options to purchase common stock with adjustments to the underlying number of shares of common stock determined by multiplying the number of shares of Series A Preferred Stock by 33.333 and rounding down to the nearest whole number of shares and the per-share exercise price by dividing the per-share exercise price of Series A Preferred Stock by 33.333 and rounding the resulting exercise price up to the nearest whole cent. Restricted Stock Units During the six months ended June 30, 2024, the Company granted 471,104 restricted stock unit awards with a weighted-average fair value of $19.80 per share based on the closing price of the Company’s common stock on the date of grant under the 2016 Plan and the Old Cartesian Plan, which generally vest over a four-year term. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% for restricted stock unit awards based on historical experience. Unrecognized compensation expense related to the restricted stock units was $6.1 million as of June 30, 2024, which is expected to be recognized over a weighted-average period of 3.4 years. The following table summarizes the Company’s restricted stock units under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and the Old Cartesian Plan: ​ ​ Number of shares ​ ​ Weighted-average grant date fair value ($) Unvested at December 31, 2023 ​ ​ — ​ ​ $ — Granted ​ ​ 471,104 ​ ​ 19.80 Forfeited ​ ​ (16,648) ​ ​ 19.80 Unvested at June 30, 2024 ​ ​ 454,456 ​ ​ $19.80 Employee Stock Purchase Plan In June 2016, the Company approved the 2016 Employee Stock Purchase Plan, or the ESPP, which authorized 5,769 shares of common stock for future issuance under the ESPP to participating employees. As of June 30, 2024, 45,795 shares remain available for future issuance under the ESPP. In connection with the Merger, the Board of Directors suspended the offerings under the ESPP. The Company recognized no stock-based compensation expense under the ESPP for the three and six months ended June 30, 2024 and less than $0.1 million stock-based compensation expense under the ESPP for each of the three and six months ended June 30, 2023.	Stock Incentive Plans The Company maintained the 2008 Stock Incentive Plan, or the 2008 Plan, for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board. In connection with the Merger, all outstanding awards issued under the 2008 Plan were cancelled, and the Board formally terminated the 2008 Plan. In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 1,210,256 shares of common stock for future issuance under the 2016 Plan and the Company ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remained subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expired, lapsed or terminated became available under the 2016 Plan as shares available for future grants. Additionally, pursuant to the terms of the 2016 Plan, the Board is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In January 2023 and 2022, the number of shares of common stock that may be issued under the 2016 Plan was increased by 6,121,697 and 4,944,919 shares, respectively. As of December 31, 2023, 22,504,503 shares remain available for future issuance under the 2016 Plan. In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan was adopted by the Board without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 1,175,000 shares of its common stock for issuance. In March 2019, the Board approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. In December 2023, the Board approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 1,825,000 shares of the Company’s common stock for issuance thereunder. As of December 31, 2023, there are 4,500,858 shares available for future grant under the 2018 Inducement Incentive Award Plan. In accordance with the Merger Agreement, the Company assumed Old Cartesian’s 2016 Stock Incentive Plan, or the Old Cartesian Plan. The Old Cartesian Plan permits the granting of options or restricted stock to employees, officers, directors, consultants and advisors to the Company. The unvested common stock options and Series A Preferred Stock options assumed by the Company in connection with the Merger generally vest over a four-year period. Additionally, the stock options granted have a contractual term of ten years and only full shares can be exercised as per the individual award agreements. As of December 31, 2023, there are 3,848,809 shares available for future grant under the Old Cartesian Plan. In connection with the Merger, the outstanding stock options to purchase Old Cartesian common stock were converted into stock options to purchase 23,306,661 shares of common stock and 14,112.299 shares of Series A Preferred Stock of the Company. These replacement awards were revalued at their acquisition-date fair value and then attributed to pre and post-combination service. This resulted in $2.6 million attributed to post-combination service to be recognized as stock-based compensation expense over the remaining terms of the replacement awards, of which $0.2 million was recognized as research and development expense in the consolidated statements of operations and comprehensive (loss) income during the year ended December 31, 2023. Settlement of Equity Compensation Awards Upon consummation of the First Merger, the equity compensation awards of the Company outstanding as of the date of the Merger were settled as follows: (i) each unvested option to acquire shares of common stock and each unvested restricted stock unit award with respect to shares of common stock was accelerated and vested in full at the effective time of the First Merger; (ii) each option to acquire shares of common stock was canceled and in exchange therefore, former holders became entitled to receive an amount in cash equal to the product of (A) the total number of shares of common stock subject to the unexercised portion the stock option (determined after giving effect to the accelerated vesting) multiplied by (B) the excess, if any, of $2.06, or the Cash-out Amount, over the applicable exercise price per share of common stock under such stock option; and (iii) each restricted stock unit award with respect to shares of common stock was cancelled and the former holder of such canceled restricted stock unit became entitled, in exchange therefor, to receive an amount in cash equal to the product of (A) the total number of shares of common stock deliverable under such restricted stock unit (determined after giving effect to the accelerated vesting) multiplied by (B) the Cash-out Amount. Stock options with an exercise price in excess of the Cash-out Amount received no cash payment. The modification to accelerate the vesting of all awards upon the Merger resulted in full recognition of unrecognized compensation of $13.1 million, of which $5.9 million and $7.2 million was classified as research and development expense and general and administrative expense, respectively, in the consolidated statements of operations and comprehensive (loss) income. In addition, with the exception of any options with an exercise price greater than $2.06 per share, all awards were settled in cash for an amount equal to $2.06 less any exercise price associated with the awards. The total cash payment made to the holders of stock options and restricted stock units was $9.4 million. The fair value of the awards prior to the settlement was recorded to additional paid-in capital in an amount of $6.2 million and the amount in excess of fair value was recognized as additional stock-based compensation expense in an amount of $3.2 million, of which $1.5 million and $1.7 million was classified as research and development expense and general and administrative expense, respectively, in the consolidated statements of operations and comprehensive (loss) income. Stock-Based Compensation Expense Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive income (loss), including $1.5 million recognized as stock-based compensation expense upon the achievement of a technical milestone by Ginkgo Bioworks Holdings, Inc., or Ginkgo, during the year ended December 31, 2023 and $1.0 million recognized as stock-based compensation expense upon the issuance of common stock to Ginkgo during the year ended December 31, 2022 as described in Note 16, was as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Research and development ​ ​ $12,985 ​ ​ $ 5,061 ​ ​ $3,204 General and administrative ​ ​ 12,793 ​ ​ 6,133 ​ ​ 4,516 Total stock-based compensation expense ​ ​ $25,778 ​ ​ $11,194 ​ ​ $7,720 Stock Options The fair value of the stock options assumed in connection with the Merger was calculated using a Black-Scholes option pricing model based on the following weighted-average assumptions: ​ ​ Common Stock ​ ​ Series A Preferred Stock Risk-free interest rate ​ ​ 4.83 % ​ ​ 4.92 % Dividend yield ​ ​ — ​ ​ — Expected term ​ ​ 3.59 ​ ​ 3.29 Expected volatility ​ ​ 83.77 % ​ ​ 83.87 % Weighted-average fair value of common stock or Series A Preferred Stock, as applicable ​ ​ $ 0.40 ​ ​ $403.47 The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Risk-free interest rate ​ ​ 3.95 % ​ ​ 2.24 % ​ ​ 0.79 % ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Dividend yield ​ ​ — ​ ​ — ​ ​ — Expected term ​ ​ 5.94 ​ ​ 6.02 ​ ​ 6.03 Expected volatility ​ ​ 94.64 % ​ ​ 92.21 % ​ ​ 95.04 % Weighted-average fair value of common stock ​ ​ $ 1.15 ​ ​ $ 2.63 ​ ​ $ 3.58 The expected term of the Company's stock options granted to employees has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Under the simplified method, the expected term is presumed to be the midpoint between the vesting date and the end of the contractual term. The Company utilizes this method due to lack of historical exercise data and the plain nature of its stock-based awards. Expected volatilities are based on the Company’s historical volatility. The weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2023, 2022 and 2021 was $0.90, $1.99, and $2.73 respectively. As of December 31, 2023, total unrecognized compensation expense related to unvested common stock options and Series A Preferred Stock options was $1.4 million and $1.0 million, respectively, which is expected to be recognized over a weighted average period of 2.4 years and 2.5 years, respectively. The following table summarizes the stock option activity under the 2008 Plan, the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: ​ ​ Number of Common Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Employees ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ 15,578,412 ​ ​ $3.44 ​ ​ 7.57 ​ ​ $ 4 Granted ​ ​ 5,477,200 ​ ​ $1.15 ​ ​ ​ ​ Assumed in connection with Merger ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ ​ ​ Exercised ​ ​ — ​ ​ $ — ​ ​ ​ ​ Forfeited ​ ​ (2,215,020) ​ ​ $2.68 ​ ​ ​ ​ Cancelled/settled in connection with the Merger ​ ​ (18,840,592) ​ ​ $2.86 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ 6.50 ​ ​ $13,760 Vested at December 31, 2023 ​ ​ 18,067,999 ​ ​ $0.10 ​ ​ 6.13 ​ ​ $10,725 Vested and expected to vest at December 31, 2023 ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ 6.50 ​ ​ $13,760 ​ ​ ​ ​ ​ ​ ​ ​ Non-employee consultants ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ 266,239 ​ ​ $8.05 ​ ​ 5.08 ​ ​ $ — Forfeited ​ ​ — ​ ​ $ — ​ ​ ​ ​ Cancelled/settled in connection with the Merger ​ ​ (266,239) ​ ​ $8.05 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ — ​ ​ $ — ​ ​ — ​ ​ $ — The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Employees ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ — ​ ​ $ — ​ ​ — ​ ​ $ — Assumed in connection with Merger ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Vested at December 31, 2023 ​ ​ 10,860.441 ​ ​ $71.67 ​ ​ 5.15 ​ ​ $6,709 Vested and expected to vest at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 Restricted Stock Units During the year ended December 31, 2023, the Company granted 1,054,600 restricted stock awards with a weighted average fair value of $1.13 per share based on the closing price of the Company’s common stock on the date of grant to employees under the 2016 Plan, which vested over a four-year term . Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% for restricted stock awards to employees based on historical experience. There was no unrecognized compensation expense and no outstanding restricted stock units as of December 31, 2023. The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan: ​ ​ Number of shares ​ ​ Weighted average grant date fair value ($) Unvested at December 31, 2022 ​ ​ 1,705,558 ​ ​ $2.62 Granted ​ ​ 1,054,600 ​ ​ 1.13 Vested ​ ​ (636,418) ​ ​ 2.40 Forfeited ​ ​ (446,108) ​ ​ 1.91 Cancelled/settled in connection with the Merger ​ ​ (1,677,632) ​ ​ 1.96 Unvested at December 31, 2023 ​ ​ — ​ ​ $ — Employee Stock Purchase Plan In June 2016, the Company approved the 2016 Employee Stock Purchase Plan, or the ESPP, which authorized 173,076 shares of common stock for future issuance under the ESPP to participating employees. In January 2023 and 2022, the number of shares of common stock authorized for issuance under the ESPP was increased by 1,530,424 shares and 1,236,229 shares, respectively. During the year ended December 31, 2023, the Company issued 186,044 shares of common stock under the ESPP. As of December 31, 2023, 4,982,098 shares remain available for future issuance under the ESPP. In connection with the Merger, the Board suspended the current ESPP offering period. For each of the years ended December 31, 2023 and 2022, the Company recognized $0.1 million of stock-based compensation expense under the ESPP.

Revenue Arrangements (Q2)

Revenue Arrangements (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
Revenue Arrangements	Revenue Arrangements Collaboration and license revenue Astellas Gene Therapies In January 2023, the Company entered into a License and Development Agreement, or the Astellas Agreement, with Audentes Therapeutics, Inc., doing business as Astellas Gene Therapies, or Astellas. Under the Astellas Agreement, the Company granted Astellas an exclusive license to the Company’s IdeXork technology arising from Xork (defined below), to develop and commercialize Xork for use in Pompe disease in combination with an Astellas gene therapy investigational or authorized product. Xork, Genovis’ IgG Protease, is licensed by the Genovis Agreement, as described in Note 15 to these consolidated financial statements. Astellas paid a $10.0 million upfront payment to the Company upon signing of the Astellas Agreement, and the Company was entitled to receive up to $340.0 million in future additional payments over the course of the partnership that were contingent on the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales where Xork is used as a pre-treatment for an Astellas investigational or authorized product. The Company was also eligible for tiered royalty payments ranging from low to high single digits. Any proceeds received from milestone payments or royalties relating to Xork would have been required to be distributed to holders of CVRs, net of certain deductions. A more detailed description of the Astellas Agreement and the Company's evaluation of this agreement under ASC 606 can be found in Note 14 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. In March 2024, the Company was notified by Astellas of its intention to terminate the Astellas Agreement, which occurred effective June 6, 2024. As of June 30, 2024, there were no unsatisfied performance obligations related to the Astellas Agreement. As of December 31, 2023, the Company recorded $2.3 million and $3.5 million as a short-term and a long-term contract liability, respectively, representing deferred revenue associated with the Astellas Agreement. As of June 30, 2024 and December 31, 2023, the Company recorded a receivable of $0.1 million and $0.3 million, respectively, representing billings for the Xork Development Services (as defined in the Astellas Agreement) that are subject to reimbursement by Astellas. Revenue of $0.5 million and $6.3 million related to the Astellas Agreement was recognized during the three and six months ended June 30, 2024, inclusive of $3.2 million of revenue recognized from performance obligations related to prior periods as a result of the change in transaction price during the six months ended June 30, 2024. Revenue of $0.8 million and $1.4 million related to the Astellas Agreement was recognized during the three and six months ended June 30, 2023, respectively. Takeda Pharmaceuticals USA, Inc. License and Development Agreement In October 2021, the Company entered into a License Agreement, or the Takeda Agreement, with Takeda Pharmaceuticals USA, Inc., or Takeda. Under the Takeda Agreement, the Company granted Takeda an exclusive license to the Company’s ImmTOR technology initially for two specified disease indications within the field of lysosomal storage disorders. Takeda paid a $3.0 million upfront payment to the Company upon signing of the Takeda Agreement, and the Company was entitled to receive up to $1.124 billion in future additional payments over the course of the partnership that were contingent on the achievement of development or commercial milestones or Takeda’s election to continue its activities at specified development stages. The Company was also eligible for tiered royalties on future commercial sales of any licensed products. A more detailed description of the Takeda Agreement and the Company's evaluation of this agreement under ASC 606 can be found in Note 14 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. On March 9, 2023, the Company was notified by Takeda of the achievement of the milestone event related to the completion of a non-clinical milestone for one of the specified disease indications within the field of lysosomal storage disorders under the Takeda Agreement. Accordingly, the Company received a milestone payment of $0.5 million during the three months ended June 30, 2023. The Takeda Agreement was terminated effective July 25, 2023, following Takeda’s decision to discontinue discovery and pre-clinical activities in adeno-associated virus, or AAV, gene therapy. As of June 30, 2024 and December 31, 2023, there were no unsatisfied performance obligations related to the Takeda Agreement. No revenue related to the Takeda Agreement was recognized during the three and six months ended June 30, 2024. Revenue of $0.1 million and $0.6 million related to the Takeda Agreement was recognized during the three and six months ended June 30, 2023, respectively. Swedish Orphan Biovitrum AB (publ.) License and Development Agreement In June 2020, the Company and Sobi entered into the Sobi License, which was subsequently amended. Pursuant to the Sobi License, the Company agreed to grant Sobi an exclusive, worldwide (except as to Greater China) license to develop, manufacture and commercialize the SEL-212 drug candidate, which is currently in development for the treatment of chronic refractory gout. The SEL-212 drug candidate is a pharmaceutical composition containing a combination of SEL-037, or the Compound, and ImmTOR. Pursuant to the Sobi License, in consideration of the license, Sobi agreed to pay the Company a one-time, upfront payment of $75.0 million. Sobi has also agreed to make milestone payments totaling up to $630.0 million to the Company upon the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier. A more detailed description of the Sobi License and the Company's evaluation of this agreement under ASC 606 can be found in Note 14 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. On October 31, 2023, the Company and Sobi entered into Amendment No. 1 to the Sobi License, pursuant to which the Company granted Sobi an exclusive license to manufacture ImmTOR solely in connection with Sobi’s development of SEL-212 under the License and Development Agreement and transferred certain contracts and manufacturing equipment to Sobi. Additionally, in connection with entry into the amendment, Sobi agreed to make employment offers to certain of the Company’s employees engaged in ImmTOR manufacturing activities on or prior to a specified date, and the Company agreed not to terminate the employment of such employees prior to such specified date. The Company maintains no responsibilities to Sobi to manufacture, or supply Sobi with, ImmTOR under the Sobi License. On June 28, 2024, Sobi initiated a rolling biologics license application to the FDA for SEL-212 for the potential treatment of chronic refractory gout which resulted in the achievement of a development milestone and a $30.0 million payment obligation from Sobi to the Company. As a result, the development milestone was no longer constrained and $30.0 million was recognized as revenue during each of the three and six months ended June 30, 2024 as there were no remaining performance obligations under the Sobi License. The proceeds from the achievement of the development milestone were received from Sobi in July 2024 and are expected to be included, net of deductions as specified in the CVR Agreement, in the next scheduled distribution to holders of the CVRs in March 2025. As of June 30, 2024 and December 31, 2023, the Company recorded total outstanding receivables of $31.7 million and $4.6 million, respectively, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi and, as of June 30, 2024, the unpaid milestone. Additionally, as of June 30, 2024 and December 31, 2023, the Company recorded a total unbilled receivable of $3.5 million and $3.0 million, respectively, representing revenue earned but not yet billed for the Phase 3 DISSOLVE program. Revenue of $32.8 million, inclusive of the $30.0 million development milestone, related to the Sobi License was recognized during each of the three and six months ended June 30, 2024. Revenue of $4.3 million and $8.7 million related to the Sobi License was recognized during the three and six months ended June 30, 2023, respectively. Sarepta Therapeutics, Inc. Research License and Option Agreement In June 2020, the Company and Sarepta Therapeutics, Inc., or Sarepta, entered into a Research License and Option Agreement, or the Sarepta Agreement. Pursuant to the Sarepta Agreement, the Company agreed to grant Sarepta a license under the Company’s intellectual property rights covering the Company’s antigen-specific biodegradable nanoparticle encapsulating ImmTOR to research and evaluate ImmTOR in combination with Sarepta’s adeno-associated virus gene therapy technology, or gene editing technology, using viral or non-viral delivery, to treat Duchenne Muscular Dystrophy and certain Limb-Girdle Muscular Dystrophy subtypes, or the Indications. Sarepta initially had an option term of 24 months during which it could opt-in to obtain an exclusive license to further develop and commercialize the product to treat at least one indication, with a potential to extend the option term for an additional fee. The Company agreed to supply ImmTOR to Sarepta for clinical supply on a cost-plus basis under the Sarepta Agreement. A more detailed description of the Sarepta Agreement and the Company's evaluation of this agreement under ASC 606 can be found in Note 14 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. On March 13, 2023, the Company was notified by Sarepta that Sarepta would not be exercising its exclusive option under the Sarepta Agreement. Therefore, the remaining deferred revenue balance as of December 31, 2022 of $0.5 million was recognized as revenue during the six months ended June 30, 2023. No revenue related to the Sarepta Agreement was recognized during the three and six months ended June 30, 2024 or the three months ended June 30, 2023. Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represent the transaction price of contracts for which work has not been performed, or has been partially performed. As of June 30, 2024, there were no unsatisfied performance obligations from contracts with customers. Contract Balances from Contracts with Customers The following table presents changes in the Company’s contract liabilities during the six months ended June 30, 2024 (in thousands): ​ ​ Balance at beginning of period ​ ​ Additions ​ ​ Deductions ​ ​ Balance at end of period Six Months Ended June 30, 2024 ​ ​ ​ ​ ​ ​ ​ ​ Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ ​ $5,849 ​ ​ $— ​ ​ $(5,849) ​ ​ $— Total contract liabilities ​ ​ $5,849 ​ ​ $— ​ ​ $(5,849) ​ ​ $— Grant revenue National Institute of Neurological Disorders and Stroke of the National Institutes of Health In June 2024, the Company received funding approval from the National Institute of Neurological Disorders and Stroke of the National Institutes of Health, or NINDS, for an award of $1.5 million granted for the budget period, which runs from June 2024 through May 2025. Subject to the availability of funds and satisfactory progress of the project, an additional $1.5 million is recommended to be awarded for the budget period June 2025 through May 2026. The funding was provided by NINDS to further the Company’s use of RNA-based CAR-T cells to combat autoantibody-associated autoimmune disorders. Grant funding is to be used solely for manufacturing of RNA-based CAR-T cells and analysis of samples to inform mechanism of action. The award period runs through May 31, 2026. The Company will recognize grant revenue when expenses reimbursable under the grant have been incurred during the budget period. As of June 30, 2024, the Company recorded a receivable of $0.2 million that is subject to reimbursement by NINDS. The Company recognized grant revenue of $0.2 million during each of the three and six months ended June 30, 2024.	Revenue Arrangements Astellas Gene Therapies In January 2023, the Company entered into the Astellas Agreement, with Astellas. Under the Astellas Agreement, the Company granted Astellas an exclusive license to the Company’s IdeXork technology arising from Xork, to develop and commercialize Xork for use in Pompe disease in combination with an Astellas gene therapy investigational or authorized product. Xork, Genovis’ IgG Protease, is licensed pursuant to an Exclusive License Agreement with Genovis, or the Genovis Agreement, as described in Note 16 to these consolidated financial statements. Astellas paid a $10.0 million upfront payment to the Company upon signing of the Astellas Agreement, and the Company is entitled to receive up to $340.0 million in future additional payments over the course of the partnership that are contingent on the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales where Xork is used as a pre-treatment for an Astellas investigational or authorized product. The Company is also eligible for tiered royalty payments ranging from low to high single digits. Any proceeds received from milestone payments or royalties relating to Xork would be required to be distributed to holders of CVRs, net of certain deductions. Pursuant to the Astellas Agreement, the Company will have the exclusive right and responsibility to complete research and development of Xork products and to conduct all preclinical studies and clinical trials for Xork for use in Pompe disease with an Astellas gene therapy investigational or authorized product, or the Xork Development Services. Astellas will reimburse the Company for 25% of all budgeted costs incurred to complete the development of Xork for use in Pompe disease with an Astellas gene therapy investigational or authorized product. The Company will have control and responsibility over regulatory filings, including any investigational drug applications, biologics license applications, and marketing authorization applications relating to the licensed product. Astellas will have the exclusive right and responsibility to research, develop, and commercialize Astellas products used in combination with Xork and will have control and responsibility over all regulatory filings, including any investigational drug applications, biologics license applications, and marketing authorization applications, relating to Astellas products and Astellas products used in combination with Xork. The Company determined the Astellas Agreement represents a service arrangement under the scope of ASC 606. The Company determined that the sublicense of Xork to Astellas, the licensed know-how, and the Xork Development Services represent a single promise and performance obligation to be transferred to Astellas over time due to the nature of the promises in the contract. As such, the Company will recognize the transaction price as revenue utilizing the input method to measure the progress of satisfying the single performance obligation to Astellas. In determining the transaction price, the Company concluded the upfront payment of $10.0 million and development cost reimbursements of $5.5 million will be included in the initial transaction price. All other development milestones will be fully constrained and will only be included in the transaction price when the applicable milestone is deemed probable of achievement. Each of these variable consideration items were evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt and timing of such development milestones is outside the control of the Company and probability of success criteria is estimated. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur. In accordance with ASC 606, the Company will only recognize revenue associated with sales-based milestones and royalties when the subsequent sales thresholds are reached and underlying sales occur, respectively. The Company determined that a significant financing component does not exist in its arrangement with Astellas. The Company also determined the options to negotiate additional fields, enter into a clinical supply agreement, and enter into a commercial supply agreement do not represent material rights under the Astellas Agreement. Astellas has the right to terminate the Astellas Agreement in its entirety or on a field-by-field basis, upon 90 days’ written notice to the Company. As of December 31, 2023, the Company recorded $2.3 million as a short-term contract liability and $3.5 million as a long-term contract liability, representing deferred revenue associated with the Astellas Agreement. As of December 31, 2023, the Company recorded a receivable of $0.3 million, representing billings for the Xork Development Services that are subject to reimbursement by Astellas. Revenue of $5.5 million related to the Astellas Agreement was recognized during the year ended December 31, 2023. Takeda Pharmaceuticals USA, Inc. License and Development Agreement On October 1, 2021, the Company entered into a License Agreement, or the Takeda Agreement, with Takeda. Under the Takeda Agreement, the Company granted Takeda an exclusive license to the Company’s ImmTOR technology initially for two specified disease indications within the field of lysosomal storage disorders. Takeda paid a $3.0 million upfront payment to the Company upon signing of the Takeda Agreement, and the Company was entitled to receive up to $1.124 billion in future additional payments over the course of the partnership that were contingent on the achievement of development or commercial milestones or Takeda’s election to continue its activities at specified development stages. The Company was also eligible for tiered royalties on future commercial sales of any licensed products. Pursuant to the Takeda Agreement, the Company determined the Takeda Agreement represented a service arrangement under the scope of ASC 606, and given the reversion of the rights under the Takeda Agreement represented a penalty in substance for a termination by Takeda, the contract term would remain the stated term of the Takeda Agreement. The Company determined that the research license, the licensed know-how, and the manufactured supply and delivery of materials represented a single promise and performance obligation to be transferred to Takeda over time due to the nature of the promises in the contract. The delivery of the manufactured supply was the predominant promise within the arrangement, as it was essential to the utility of the licensed intellectual property. The material supplied by the Company to Takeda was unique to the Company and cannot be obtained by other vendors. As such, consideration in the initial transaction price was allocated to the single performance obligation and the recognition period would not extend beyond the initial contractual period. The Company recognized the revenue associated with the upfront payment and combined single performance obligation utilizing the output method over the term that manufactured supply was delivered to Takeda. In determining the transaction price, the Company concluded the payment associated with all the performance milestones was fully constrained and only included in the transaction price when the respective milestone was deemed probable of achievement. Each of these variable consideration items were evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt and timing of such study milestones is outside the control of the Company and probability of success criteria is estimated. The Company re-evaluated the transaction price in each reporting period, as uncertain events were resolved, or as other changes in circumstances occurred. Takeda had the right to exercise covenant release rights on a field-by-field basis. If Takeda exercised its covenant release rights, we could have received exercise payments per indication and would have been entitled to significant development and commercial milestone payments and tiered royalties on commercial sales. The Company determined that a significant financing component did not exist in its arrangement with Takeda. The Company also determined the options to negotiate additional fields, pursue other products, enter into a supply agreement explore additional fields, and pursue additional development under the initial fields did not represent material rights under the agreement. Takeda had the right to terminate the Takeda Agreement in its entirety or on a field-by-field basis, upon 90 days’ written notice to the Company. On March 9, 2023, the Company was notified by Takeda of the achievement of the milestone event related to the completion of a non-clinical milestone for one of the specified disease indications within the field of lysosomal storage disorders under the Takeda Agreement. Accordingly, the Company received a milestone payment of $0.5 million during the year ended December 31, 2023. The Takeda Agreement was terminated effective July 25, 2023, following Takeda’s decision to discontinue discovery and pre-clinical activities in adeno-associated virus, or AAV, gene therapy. As of December 31, 2023, the Company recorded no contract liability. As of December 31, 2022, the Company recorded $0.1 million as a short-term contract liability and no long-term contract liability representing deferred revenue associated with this agreement. Revenue of $0.6 million and $1.8 million related to the Takeda Agreement was recognized during the years ended December 31, 2023 and 2022, respectively. Swedish Orphan Biovitrum License and Development Agreement On June 11, 2020, the Company and Sobi entered into a License and Development Agreement. Pursuant to the Sobi License, the Company agreed to grant Sobi an exclusive, worldwide (except as to Greater China) license to develop, manufacture and commercialize the SEL-212 drug candidate, which is currently in development for the treatment of chronic refractory gout. The SEL-212 drug candidate is a pharmaceutical composition containing a combination of SEL-037, or the Compound, and ImmTOR. Pursuant to the Sobi License, in consideration of the license, Sobi agreed to pay the Company a one-time, upfront payment of $75.0 million. Sobi has also agreed to make milestone payments totaling up to $630.0 million to the Company upon the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier. Any proceeds received from milestone payments or royalties relating to the Sobi License would be required to be distributed to holders of CVRs, net of certain deductions. Pursuant to the Sobi License, the Company agreed to supply (at cost) quantities of the Compound and ImmTOR as necessary for completion of the two Phase 3 clinical trials of SEL-212 (DISSOLVE I and DISSOLVE II) and a six-month placebo extension. The Company was required to supply quantities of the Compound until all rights to the Compound and any materials needed to manufacture the Compound were transferred to Sobi, which transfer occurred upon the execution of Amendment No. 1 to the License and Development Agreement on October 31, 2023. Sobi has agreed to reimburse the Company for all budgeted costs incurred to complete development of SEL-212, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, except for any costs of additional development activities required that are related to ImmTOR and that are unrelated to SEL-212. Sobi will have control and responsibility over all regulatory filings, including any investigational drug applications (IND), biologics license applications (BLA), and marketing authorization applications (MAA) relating to the licensed product. The transactions contemplated by the Sobi License were consummated on July 28, 2020. Sobi may terminate the Sobi License for any reason upon 180 days’ written notice to the Company, whereby all rights granted under the Sobi License would revert back to the Company. In addition, if Sobi were to terminate the Sobi License, the Company has the option to obtain a license to all patents and know-how necessary to exploit SEL-212 in existence as of the termination date from Sobi in return for making an equitable royalty payment to Sobi. Additionally, on June 11, 2020, the Company entered into the Sobi Purchase Agreement in connection with the Sobi License. The closing of the Sobi Private Placement occurred on July 31, 2020, following the closing of the transactions contemplated under the Sobi License. See Note 12 for details. The Company determined that the Sobi License represents a service arrangement under the scope of ASC 606. In addition, given the Sobi License and Sobi Purchase Agreement were executed contemporaneously and negotiated as a package with a single commercial objective, the Company will account for the two agreements as a single contract. The term of the Sobi License commenced upon the effective date of July 28, 2020 and will continue on a product-by-product basis until the royalty terms for each country have expired. The royalty term for a given product begins upon the first commercial sale of the product in a country and ends at the later of ten years from the first commercial sale, expiration of the last valid patent claim covering the product and expiration of all regulatory exclusivity periods for the product in a country. Given the reversion of the rights under the Sobi License represents a penalty in substance for a termination by Sobi, the contract term would remain the stated term of the Sobi License. The Company determined that the Sobi License contains three distinct performance obligations due to the nature of the promises in the contract, which includes conducting the Phase 3 DISSOLVE trials, Sobi’s option to set-up a second source supplier, and a combined obligation comprised of the delivery of the license to SEL-212, transfer of the know-how and the manufacturing and delivery of SEL-212 supply for development, or the Combined License Obligation. As the set-up of a second source supplier is optional for Sobi and the Company will be reimbursed at cost for its efforts in the subsequent set-up and technology transfer, the option for this future service was determined to be at a significant and incremental discount to its standalone selling price and treated as a material right in the arrangement, namely a distinct performance obligation. In determining the transaction price, the Company concluded the upfront payment of $75.0 million and the $5.0 million development milestone associated with the dosing of the first patient in the Phase 3 DISSOLVE trials were included in the transaction price. All other development milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of the evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company and probability of success criteria is estimated. The Company re-evaluates the transaction price in each reporting period, as uncertain events are resolved. In accordance with ASC 606, the Company will only recognize revenue associated with sales-based milestones and royalties when the subsequent sales thresholds are reached and underlying sales occur, respectively. In connection with the Sobi Purchase Agreement, the Company determined that the gross proceeds of $25.0 million from the Sobi Private Placement included a premium to the fair value of the Company’s shares as of July 28, 2020 equal to approximately $14.5 million. The premium amount is included in the transaction price for revenue recognition. The Company estimates and includes in the transaction price the total reimbursements to be received from Sobi for both the manufacturing and delivery of the Compound and ImmTOR as well as conducting the Phase 3 DISSOLVE trials. The Company determined that a significant financing component does not exist in its arrangement with Sobi. The Company allocated the transaction price based on the relative standalone selling prices of the three distinct performance obligations. The Company estimated the standalone selling price of conducting the Phase 3 DISSOLVE trials by forecasting its anticipated costs and applying a margin reflective of the industry. The Company must determine the standalone selling price of the second source supplier option by determining the discount given to Sobi multiplied by the likelihood that Sobi will exercise the option in the future. Similar to the Phase 3 program estimate, the Company estimated the discount of the option by forecasting the set-up costs and applying a margin that is reflective of the industry. As the Company will be providing the set-up and technology transfer services and the future supply at cost, the discount of the option is equal to the margin amount. The Company considered discussions with Sobi as well as probability of regulatory success of SEL-212 in determining the likelihood of exercise. The Company estimated the standalone selling price of the Combined License Obligation by utilizing a discounted cash flow model. The Company determined that the delivery of the supply to Sobi best represents the pattern of delivery of the Combined License Obligation as the supply is essential to the utility of the license and know-how. The Company will recognize the revenue allocated to the Combined License Obligation by utilizing the output method. The Company estimated the total supply of the Compound and ImmTOR to be required during the clinical trial period and will recognize revenue as this supply is shipped for use in the clinical trials. The Company will recognize the revenue allocated to the conducting of the Phase 3 DISSOLVE trials obligation by utilizing the input method. The Company estimated the total budgeted costs to be incurred over the Phase 3 DISSOLVE trials and will recognize revenue as these costs are incurred. The Company’s costs best represent the pattern of transfer as these will capture all performance of the trials completed to date and can be readily measured. The Company will recognize the revenue allocated to the second source supplier option when the future services and goods are transferred. On June 29, 2022, the Company completed enrollment of the DISSOLVE II trial. The completion of enrollment of the DISSOLVE II trial resulted in the achievement of a development milestone and a $10.0 million payment obligation from Sobi to the Company. This amount was added to the overall transaction price and payment was received during the year ended December 31, 2022. On October 31, 2023, the Company and Sobi entered into Amendment No. 1 to the License and Development Agreement, pursuant to which the Company granted Sobi an exclusive license to manufacture ImmTOR solely in connection with Sobi’s development of SEL-212 under the License and Development Agreement and transferred certain contracts and manufacturing equipment to Sobi. Additionally, in connection with entry into the amendment, Sobi agreed to make employment offers to certain of the Company’s employees engaged in ImmTOR manufacturing activities on or prior to a specified date, and the Company agreed not to terminate the employment of such employees prior to such specified date. The Company maintains no responsibilities to Sobi to manufacture, or supply Sobi with, ImmTOR under the Sobi License. As of December 31, 2023 and 2022, the Company recorded a total outstanding receivable of $4.6 million and $5.0 million, respectively, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi. Additionally, as of December 31, 2023 and 2022, the Company recorded a total unbilled receivable of $3.0 million and $3.2 million, respectively, representing revenue earned but not yet billed for the Phase 3 DISSOLVE program. Revenue of $19.4 million and $82.6 million related to the Sobi License was recognized during the years ended December 31, 2023 and 2022, respectively, inclusive of $1.1 million of revenue recognized from performance obligations related to prior periods as a result of the change in transaction price during the year ended December 31, 2023. Sarepta Therapeutics, Inc. Research License and Option Agreement In June 2020, the Company and Sarepta Therapeutics, Inc., or Sarepta, entered into a Research License and Option Agreement, or the Sarepta Agreement. Pursuant to the Sarepta Agreement, the Company agreed to grant Sarepta a license under the Company’s intellectual property rights covering the Company’s antigen-specific biodegradable nanoparticle encapsulating ImmTOR to research and evaluate ImmTOR in combination with Sarepta’s adeno-associated virus gene therapy technology, or gene editing technology, using viral or non-viral delivery, to treat Duchenne Muscular Dystrophy and certain Limb-Girdle Muscular Dystrophy subtypes, or the Indications. Sarepta initially had an option term of 24 months during which it could opt-in to obtain an exclusive license to further develop and commercialize the Product to treat at least one Indication, with a potential to extend the option term for an additional fee. The Company will supply ImmTOR to Sarepta for clinical supply on a cost-plus basis. Sarepta paid a $2.0 million upfront payment to the Company upon signing of the Sarepta Agreement, and the Company is eligible to receive additional preclinical payments during the option term. If Sarepta opts-in to an exclusive license agreement, the Company could receive option exercise payments per Indication upon execution of the exclusive license, and the Company would be entitled to significant development and commercial milestone payments and tiered royalties ranging from the mid-to-high single digits based on net sales. Pursuant to the Sarepta Agreement, the Company determined the Sarepta Agreement represents a service arrangement under the scope of ASC 606, with a 24-month contract duration. Given the reversion of the rights under the Sarepta Agreement represents a penalty in substance for a termination by Sarepta, the contract term would remain the stated term of the Sarepta Agreement. The Company determined that the Sarepta Agreement and supply obligation including the delivery of the research license, the licensed know-how, the manufactured supply and delivery of materials represent a single promise and performance obligation to be transferred to Sarepta over time due to the nature of the promises in the contract. The delivery of the manufactured supply is the predominant promise within the arrangement, as it is essential to the utility of the licensed intellectual property. As such, consideration in the initial transaction price will be allocated to the single performance obligation based on the contractual price. In determining the transaction price, the Company concluded the payment associated with all the performance milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such study milestones is outside the control of the Company and probability of success criteria is estimated. The Company also determined the option to enter into a future commercial license agreement and extend the term of the option does not represent a material right since it was not priced at an incremental discount. Sarepta may terminate the Sarepta Agreement for any reason upon 30 days’ written notice to the Company. The Sarepta Agreement contains other customary terms and conditions, including representations and warranties, covenants, termination, and indemnification obligations in favor of each party. On April 13, 2021, the Company was notified by Sarepta of the achievement of the milestone event related to the completion of a non-clinical study for Duchenne muscular dystrophy and certain limb-girdle muscular dystrophies under the Sarepta Agreement. Accordingly, the Company received a milestone payment of $3.0 million during the three months ended June 30, 2021. On June 10, 2022, the Company was notified by Sarepta that Sarepta would be extending their options under the Sarepta Agreement. In exchange for a nine-month extension to Sarepta’s options to both Duchenne muscular dystrophy and certain limb-girdle muscular dystrophies, the Company received a milestone payment of $2.0 million during the year ended December 31, 2022. On June 15, 2022, the Company was notified by Sarepta of the achievement of a milestone event related to certain preclinical study milestones under the Sarepta Agreement. Accordingly, the Company received a milestone payment of $4.0 million during the year ended December 31, 2022. On March 13, 2023, the Company was notified by Sarepta that Sarepta would not be exercising its exclusive option under the Sarepta Agreement. The Sarepta Agreement terminated upon the expiration of the option in March 2023. As of December 31, 2023, the Company recorded no contract liability. As of December 31, 2022, the Company recorded $0.5 million as a short-term contract liability representing deferred revenue associated with this agreement. Revenue of $0.5 million and $10.2 million related to the Sarepta Agreement was recognized during the years ended December 31, 2023 and 2022, respectively. Asklepios Biopharmaceutical, Inc. License Agreement for Pompe Disease In December 2019, the Company and Asklepios Biopharmaceutical, Inc., or AskBio, entered into a license agreement, or the AskBio License Agreement. Pursuant to the AskBio License Agreement, AskBio exercised its option to exclusively license the Company’s intellectual property rights covering the Company’s ImmTOR platform to research, develop, and commercialize certain AAV gene therapy products utilizing ImmTOR, and targeting the GAA gene, or derivatives thereof, to treat Pompe Disease. On November 18, 2022, both parties agreed to mutually terminate the AskBio License Agreement. Therefore, the remaining contract liability of $7.0 million was recognized as revenue during the period ended December 31, 2022. Spark Therapeutics, Inc. In December 2016, the Company entered into a license and option agreement, or the Spark License Agreement, with Spark Therapeutics, Inc., or Spark, pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the ImmTOR platform. The Spark License Agreement provided Spark with certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property, allowing Spark to develop and commercialize gene therapies in combination with ImmTOR for Factor VIII, an essential blood clotting protein relevant to the treatment of hemophilia A, the initial target. On January 18, 2022, both parties agreed to mutually terminate the Spark License Agreement. Therefore, the remaining contract liability of $9.2 million was recognized as revenue during the year ended December 31, 2022. Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represent the transaction price of contracts for which work has not been performed (or has been partially performed). As of December 31, 2023, the aggregate amount of the transaction price allocated to remaining performance obligations was $5.8 million. Contract Balances from Contracts with Customers ( Astellas, Takeda, Sobi, Sarepta, AskBio, and Spark ) The following table presents changes in the Company’s contract liabilities during the year ended December 31, 2023 (in thousands): ​ ​ Balance at beginning of period ​ ​ Additions ​ ​ Deductions ​ ​ Balance at end of period Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ ​ $593 ​ ​ $10,500 ​ ​ $(5,244) ​ ​ $5,849 Total contract liabilities ​ ​ $593 ​ ​ $10,500 ​ ​ $(5,244) ​ ​ $5,849

Related-Party Transactions (Q2)

Related-Party Transactions (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Related Party Transactions [Abstract]
Related-Party Transactions	Related-Party Transactions November 2023 Securities Purchase Agreement On November 13, 2023, the Company entered into the Securities Purchase Agreement with (i) Dr. Timothy A. Springer, (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, in which the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million (see Note 10). The November 2023 Private Placement includes a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the six months ended June 30, 2024. Name ​ ​ Shares of Series A Preferred Stock purchased ​ ​ Total aggregate purchase price Timothy A. Springer, Ph.D. ​ ​ 99,140.326 ​ ​ $40,000,000 Exercise of Amended 2019 Warrants On March 26, 2024, TAS Partners LLC, an affiliate of Dr. Springer, exercised 65,681 Amended 2019 Warrants, paid the per-share exercise price of $43.80 in cash for an aggregate exercise price of $2.9 million, and received 65,681 shares of common stock and 1,970,443 CVRs. During the three and six months ended June 30, 2023, there were no related party transactions.	Related-Party Transactions November 2023 Securities Purchase Agreement On November 13, 2023, the Company entered into the Securities Purchase Agreement with (i) Dr. Timothy A. Springer, (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, in which the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million (see Note 11). The November 2023 Private Placement includes a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the year ended December 31, 2023. Name ​ ​ Shares of Series A Preferred Stock purchased ​ ​ Total aggregate purchase price Timothy A. Springer, Ph.D. ​ ​ 24,785.081 ​ ​ $10,000,000 TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) ​ ​ 24,785.081 ​ ​ $10,000,000 Seven One Eight Three Four Irrevocable Trust (affiliate of Murat Kalayoglu, MD, Ph.D.) ​ ​ 619.627 ​ ​ $ 250,000 April 2022 Offering During the year ended December 31, 2022, the Company completed the 2022 Offering as described in Note 12. The following table sets forth the number of shares of Common Stock and 2022 Warrants purchased in the 2022 Offering by directors and executive officers, as of the time of the Offering, and related parties thereto: Name ​ ​ Shares of Common Stock purchased ​ ​ 2022 Warrants purchased ​ ​ Total aggregate purchase price TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) ​ ​ 6,681,600 ​ ​ 5,011,200 ​ ​ $9,421,056 Warrant liability reclassification During the year ended December 31, 2022, the Company amended the terms of certain of the outstanding 2019 Warrants held by members of the Company's Board of Directors and remeasured the Amended 2019 Warrants as described in Note 6. Consulting Services The Company entered into consulting agreements with its founders to serve on its Scientific Advisory Board, effective January 1, 2020 to December 31, 2021, under which they were paid quarterly for their services. The Company incurred expenses for consulting services provided by its founders totaling $0.1 million during the year ended December 31, 2021. No expenses were incurred for the years ended December 31, 2023 and 2022.

Collaboration and License Agree

Collaboration and License Agreements (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Collaboration and License Agreements	Collaboration and License Agreements Biogen MA, Inc. On September 8, 2023, the Company entered into a non-exclusive, sublicensable, worldwide, perpetual patent license agreement, or the Biogen Agreement, with Biogen MA, Inc., or Biogen, to research, develop, make, use, offer, sell and import products or processes containing or using an engineering T-cell modified with an mRNA comprising, or encoding a protein comprising, certain sequences licensed under the Biogen Agreement for the prevention, treatment, palliation and management of autoimmune diseases and disorders, excluding cancers, neoplastic disorders, and paraneoplastic disorders. The Company is not obligated to pay Biogen any expenses, fees, or royalties. The Company may terminate the Biogen Agreement for any reason or no reason, and Biogen may terminate the agreement after a notice-and-cure period of 30 days if the Company fails to pay a fee owed to Biogen or for any other material breach of the agreement. The Biogen Agreement will otherwise expire when all claims of all issued patents within the patents and patent applications licensed to the Company under the Biogen Agreement have expired or been finally rendered revoked, invalid or unenforceable by a decision of a court or government agency. National Cancer Institute of the National Institutes of Health Effective September 16, 2019, the Company entered into a nonexclusive, worldwide license agreement, or the NCI Agreement, with the U.S. Department of Health and Human Services, represented by the National Cancer Institute of the National Institutes of Health, or NCI. Under the NCI Agreement, the Company was granted a license under certain NCI patents and patent applications designated in the agreement, to make, use, sell, offer and import products and processes within the scope of the patents and applications licensed under the NCI Agreement when developing and manufacturing anti-BCMA CAR-T cell products for the treatment of myasthenia gravis, pemphigus vulgaris, and immune thrombocytopenic purpura according to methods designated in the NCI Agreement. In connection with the Company’s entry into the NCI Agreement, Old Cartesian paid to NCI a one-time $0.1 million license royalty payment. Under the NCI Agreement, the Company is further required to pay NCI a low five-digit annual royalty. The Company must also pay earned royalties on net sales in a low single-digit percentage and pay up to $0.8 million in benchmark royalties upon the Company's achievement of designated benchmarks that are based on the commercial development plan agreed between the parties. Under the NCI Agreement, the Company must use reasonable commercial efforts to bring licensed products and licensed processes to the point of Practical Application (as defined in the NCI Agreement). Upon the Company’s first commercial sale, the Company must use reasonable commercial efforts to make licensed products and licensed processes reasonably accessible to the United States public. After the Company's first commercial sale, the Company must make reasonable quantities of licensed products or materials produced via licensed processes available to patient assistance programs and develop educational materials detailing the licensed products. Unless the Company obtains a waiver from NCI, the Company must have licensed products and licensed processes manufactured substantially in the United States. Prior to the first commercial sale, upon NCI’s request, the Company is obligated to provide NCI with commercially reasonable quantities of licensed products made through licensed processes to be used for in vitro research. Additionally, the Company must use reasonable commercial efforts to initiate a Phase 3 clinical trial of a licensed product by the fourth quarter of 2024, submit a BLA with respect to a licensed product by the fourth quarter of 2026, and make a first commercial sale of a licensed product by the fourth quarter of 2028. The NCI Agreement terminates upon the expiration of the last to expire of the patent rights licensed thereunder, if not sooner terminated. NCI has the right to terminate this agreement, after giving written notice and providing a cure period in accordance with its terms, if the Company is in default of a material obligation. The Company has the unilateral right to terminate the agreement in any country or territory by giving NCI 60 days’ written notice. The Company agreed to indemnify NCI against any liability arising out of the Company’s, sublicensees’ or third parties’ use of the licensed patent rights and licensed products or licensed processes developed in connection with the licensed patent rights. Ginkgo Bioworks Holdings, Inc. Collaboration and License Agreements On October 25, 2021, the Company entered into a Collaboration and License Agreement, or the First Ginkgo Agreement, with Ginkgo. Under the First Ginkgo Agreement, Ginkgo will design next generation IgA proteases with potentially transformative therapeutic potential. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments for fixed fair values in the form of the Company's common stock, clinical and commercial milestone payments of up to $85.0 million in cash. The First Ginkgo Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808, Collaborative Arrangements , or ASC 808, as the risk and rewards are not shared by both parties. The Company will expense costs related to the First Ginkgo Agreement as incurred until regulatory approval is received in accordance with ASC 730, Research and Development , or ASC 730. The Company is accounting for the contingently issuable shares to be issued in exchange for the license obtained from Ginkgo as a liability classified stock-based compensation arrangement with a non-employee which will be recognized when achievement of the milestones is probable. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Ginkgo tiered royalties ranging from low-single digit to high-single digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. On January 3, 2022, the Company entered into a Collaboration and License Agreement, or the Second Ginkgo Agreement, with Ginkgo. Under this agreement, the Company will engage with Ginkgo to develop AAV capsids designed to enhance transduction efficiency and transgene expression. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments in the form of shares of the Company’s common stock, clinical and commercial milestone payments of up to $207 million in cash. The Second Ginkgo Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the Second Ginkgo Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company is accounting for the contingently issuable shares of common stock to be issued in exchange for the license obtained from Ginkgo as a liability-classified, stock-based compensation arrangement with a non-employee which will be recognized when achievement of the milestones is probable. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Ginkgo tiered royalties ranging from low-single digit to high-single digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. On June 13, 2022, the Company was notified of the achievement of the midpoint of the technical development plan under the First Ginkgo Agreement by Ginkgo. This milestone resulted in the payment of $0.5 million and issuance of 29,761 shares of the Company’s common stock then-valued at $1.0 million to Ginkgo during the year ended December 31, 2022. On July 19, 2023, the Company and Ginkgo mutually agreed that the completion of the technical development plan’s midpoint task under the Second Ginkgo Agreement had been achieved as of June 30, 2023. This milestone resulted in the payment of $1.0 million and issuance of 44,642 shares of the Company’s common stock then-valued at $1.5 million to Ginkgo during the year ended December 31, 2023. Genovis AB (publ.) License Agreement In October 2021, the Company entered into an Exclusive License Agreement, or the Genovis Agreement, with Genovis AB (publ.), or Genovis. Under the Genovis Agreement, the Company paid to Genovis an upfront payment in exchange for an exclusive license to the Xork enzyme technology across all therapeutic uses in humans, excluding research, preclinical, diagnostic and other potential non-therapeutic applications of the enzyme. Genovis is eligible to earn from the Company development and sales-based milestones and sublicensing fees. The Genovis Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the Genovis Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Genovis tiered royalties of low double digit percentages of worldwide annual net sales of collaboration products which will be expensed as the commercial sales occur. In February 2023, the Company made a $4.0 million payment to Genovis as a result of the sublicense of Xork to Astellas. See Note 13 to these unaudited consolidated financial statements for further discussion on the Astellas Agreement. In March 2024, the Company notified Genovis of its intention to terminate the Genovis Agreement effective September 13, 2024. Cyrus Biotechnology, Inc. Collaboration and License Agreement In September 2021, the Company and Cyrus Biotechnology, Inc., or Cyrus, entered into a collaboration and license agreement, or the Cyrus Agreement. Pursuant to the Cyrus Agreement, Cyrus agreed to grant the Company an exclusive, worldwide license to certain intellectual property to form a protein engineering collaboration combining the Company’s ImmTOR platform with Cyrus’ ability to redesign protein therapeutics. The lead program was a proprietary interleukin-2, or IL-2, protein agonist designed to selectively promote expansion of regulatory T cells for treatment of patients with autoimmune diseases and other deleterious immune conditions. In return for the licensed intellectual property, the Company made an upfront payment and was obligated to pay certain discovery, development, and sales-based milestones which could have potentially totaled up to approximately $1.5 billion across multiple programs. The Cyrus Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company expensed costs related to the Cyrus Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company assessed the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, would have amortized these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company was also obligated to pay Cyrus tiered royalties ranging from mid-single digit to low-double digit percentages of annual net sales of collaboration products which would have been expensed as commercial sales occur. On June 13, 2022, the Company and Cyrus mutually agreed that the preclinical key in-vitro success milestone had been achieved. In October 2023, the Company notified Cyrus of its termination of the Cyrus Agreement effective December 29, 2023. Stock Purchase Agreement Additionally, on September 7, 2021, the Company entered into a stock purchase agreement, or the Series B Preferred Stock Purchase Agreement, in connection with the Cyrus Agreement. Pursuant to the Series B Preferred Stock Purchase Agreement, the Company purchased 2,326,934 shares of Cyrus’ Series B Preferred Stock, par value $0.0001 per share, at a purchase price of $0.8595 per share, for $2.0 million. In accordance with ASC 810, the Company has a variable interest in Cyrus resulting from its equity investment. The Company will share in Cyrus’ expected losses or receive a portion of its expected returns and absorb the variability associated with changes in the entity’s net assets. However, the Company is not the primary beneficiary as it does not have the power to direct the activities most significant to Cyrus, and therefore it is not required to consolidate Cyrus. The Company has recognized the $2.0 million investment of Cyrus’ Series B Preferred Stock at cost on the purchase date. As of June 30, 2024, no impairment indicators were present and there were no observable price changes. Therefore, the carrying value of the investment in Cyrus is $2.0 million on the accompanying consolidated balance sheets. The Company’s maximum exposure to loss related to this variable interest entity is limited to the carrying value of the investment. The Company has not provided financing to Cyrus other than the amount contractually required by the Series B Preferred Stock Purchase Agreement. Asklepios Biopharmaceutical, Inc. Feasibility Study and License Agreement In August 2019, the Company entered into a feasibility study and license agreement, or the AskBio Collaboration Agreement, with Asklepios Biopharmaceutical, Inc., or AskBio. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain AAV gene therapy products utilizing the Company’s ImmTOR platform to enable re-dosing of such AAV gene therapy products to treat serious rare and orphan genetic diseases for which there is a significant unmet medical need. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AskBio’s AAV gene therapy, or SEL-302, (previously disclosed as MMA-101, in combination with ImmTOR) for the treatment of methylmalonic acidemia, or MMA, to mitigate the formation of neutralizing anti-AAV capsid antibodies. On April 29, 2021, the Company was notified by AskBio that it intended to opt-out of development of the MMA indication. The Company and AskBio shared responsibility for the research, development and commercialization of products developed under the SEL-399 program collaboration. The parties also shared research, development, and commercialization costs equally for all collaboration products, but with a right of either party to opt out of certain products, and thereby not be required to share costs for such products. Each party would have received a percentage of net profits under the collaboration equal to the percentage of shared costs borne by such party in the development of such product. Pursuant to the AskBio Collaboration Agreement, AskBio was responsible for manufacturing the AAV capsids and AAV vectors and the Company was responsible for manufacturing ImmTOR. The Company and AskBio mutually agreed to the termination of the AskBio Collaboration Agreement, effective December 13, 2023. No collaboration expense under the AskBio Collaboration Agreement was recognized during the three and six months ended June 30, 2024. For the three and six months ended June 30, 2023, the Company recognized less than $0.1 million and $0.1 million, respectively, of collaboration expense under the AskBio Collaboration Agreement in which actual costs incurred by both parties approximate a 50% cost share. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement, or the 3SBio License, with Shenyang Sunshine Pharmaceutical Co., Ltd., or 3SBio. The Company has paid to 3SBio an aggregate of $7.0 million in upfront and milestone-based payments under the 3SBio License as of June 30, 2024. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $15.0 million for products containing the Company’s ImmTOR platform.	Collaboration and License Agreements Biogen MA, Inc. On September 8, 2023, the Company entered into a non-exclusive, sublicensable, worldwide, perpetual patent license agreement, or the Biogen Agreement with Biogen MA, Inc., or Biogen to research, develop, make, use, offer, sell and import products or processes containing or using an engineering T-cell modified with an mRNA comprising, or encoding a protein comprising, certain sequences licensed under the Biogen Agreement for the prevention, treatment, palliation and management of autoimmune diseases and disorders, excluding cancers, neoplastic disorders, and paraneoplastic disorders. The Company is not obligated to pay Biogen any expenses, fees, or royalties. The Company may terminate the Biogen Agreement for any reason or no reason, and Biogen may terminate the agreement after a notice-and-cure period of 30 days if the Company fails to pay a fee owed to Biogen or for any other material breach of the agreement. The Biogen Agreement will otherwise expire when all claims of all issued patents within the patents and patent applications licensed to the Company under the Biogen Agreement have expired or been finally rendered revoked, invalid or unenforceable by a decision of a court or government agency. National Cancer Institute of the National Institutes of Health Effective September 16, 2019, the Company entered into a nonexclusive, worldwide license agreement, or the NCI Agreement with the U.S. Department of Health and Human Services, represented by the National Cancer Institute of the National Institutes of Health, or NCI. Under the NCI Agreement, the Company was granted a license under certain NCI patents and patent applications designated in the agreement, to make, use, sell, offer and import products and processes within the scope of the patents and applications licensed under the NCI Agreement when developing and manufacturing anti-BCMA CAR-T cell products for the treatment of myasthenia gravis, pemphigus vulgaris, and immune thrombocytopenic purpura according to methods designated in the NCI Agreement. In connection with the Company's entry into the NCI Agreement, Old Cartesian paid to NCI a one-time $0.1 million license royalty payment. Under the NCI Agreement, the Company is further required to pay NCI a low five-digit annual royalty. The Company must also pay earned royalties on net sales in a low single-digit percentage and pay up to $0.8 million in benchmark royalties upon the Company's achievement of designated benchmarks that are based on the commercial development plan agreed between the parties. Under the NCI Agreement, the Company must use reasonable commercial efforts to bring licensed products and licensed processes to the point of Practical Application (as defined in the NCI Agreement). Upon the Company's first commercial sale, the Company must use reasonable commercial efforts to make licensed products and licensed processes reasonably accessible to the United States public. After the Company's first commercial sale, the Company must make reasonable quantities of licensed products or materials produced via licensed processes available to patient assistance programs and develop educational materials detailing the licensed products. Unless the Company obtains a waiver from NCI, the Company must have licensed products and licensed processes manufactured substantially in the United States. Prior to the first commercial sale, upon NCI’s request, the Company is obligated to provide NCI with commercially reasonable quantities of licensed products made through licensed processes to be used for in vitro research. Additionally, the Company must use reasonable commercial efforts to initiate a Phase 3 clinical trial of a licensed product by the fourth quarter of 2024, submit a BLA with respect to a licensed product by the fourth quarter of 2026, and make a first commercial sale of a licensed product by the fourth quarter of 2028. The NCI Agreement terminates upon the expiration of the last to expire of the patent rights licensed thereunder, if not sooner terminated. NCI has the right to terminate this agreement, after giving written notice and providing a cure period in accordance with its terms, if the Company is in default of a material obligation. The Company has the unilateral right to terminate the agreement in any country or territory by giving NCI 60 days’ written notice. The Company agreed to indemnify NCI against any liability arising out of the Company's, sublicensees’ or third parties’ use of the licensed patent rights and licensed products or licensed processes developed in connection with the licensed patent rights. Ginkgo Bioworks Holdings, Inc. Collaboration and License Agreements On October 25, 2021, the Company entered into a Collaboration and License Agreement, or the First Ginkgo Agreement, with Ginkgo. Under the First Ginkgo Agreement, Ginkgo will design next generation IgA proteases with potentially transformative therapeutic potential. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments for fixed fair values in the form of the Company's common stock, clinical and commercial milestone payments of up to $85.0 million in cash. The First Ginkgo Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the First Ginkgo Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company is accounting for the contingently issuable shares to be issued in exchange for the license obtained from Ginkgo as a liability classified stock-based compensation arrangement with a non-employee which will be recognized when achievement of the milestones is probable. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Ginkgo tiered royalties ranging from low-single digit to high-single digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. On January 3, 2022, the Company entered into a Collaboration and License Agreement, or the Second Ginkgo Agreement, with Ginkgo. Under this agreement, the Company will engage with Ginkgo to develop AAV capsids designed to enhance transduction efficiency and transgene expression. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments in the form of shares of the Company’s common stock, clinical and commercial milestone payments of up to $207 million in cash. The Second Ginkgo Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the Second Ginkgo Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company is accounting for the contingently issuable shares of common stock to be issued in exchange for the license obtained from Ginkgo as a liability-classified, stock-based compensation arrangement with a non-employee which will be recognized when achievement of the milestones is probable. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Ginkgo tiered royalties ranging from low-single digit to high-single digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. On June 13, 2022, the Company was notified of the achievement of the midpoint of the technical development plan under the First Ginkgo Agreement by Ginkgo. This milestone resulted in the payment of $0.5 million and issuance of 892,857 shares of the Company’s common stock then-valued at $1.0 million to Ginkgo during the year ended December 31, 2022. On July 19, 2023, the Company and Ginkgo mutually agreed that the completion of the technical development plan’s midpoint task under the Second Ginkgo Agreement had been achieved as of June 30, 2023. This milestone resulted in the payment of $1.0 million and issuance of 1,339,285 shares of the Company’s common stock then-valued at $1.5 million to Ginkgo during the year ended December 31, 2023. Genovis AB (publ.) License Agreement On October 21, 2021, the Company entered into the Genovis Agreement with Genovis. Under the Genovis Agreement, the Company paid to Genovis an upfront payment in exchange for an exclusive license to Genovis’ IgG Protease, Xork, enzyme technology across all therapeutic uses in humans, excluding research, preclinical, diagnostic and other potential non-therapeutic applications of the enzyme. Genovis is eligible to earn from the Company development and sales-based milestones and sublicensing fees. The Genovis Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the Genovis Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Genovis tiered royalties of low double digit percentages of worldwide annual net sales of collaboration products which will be expensed as the commercial sales occur. In February 2023, the Company made a $4.0 million payment to Genovis as a result of the sublicense of Xork to Astellas. See Note 14 to these consolidated financial statements for further discussion on the Astellas Agreement. Cyrus Biotechnology, Inc. Collaboration and License Agreement On September 7, 2021, the Company and Cyrus entered into the Cyrus Agreement. Pursuant to the Cyrus Agreement, Cyrus agreed to grant the Company an exclusive, worldwide license to certain intellectual property to form a protein engineering collaboration combining the Company’s ImmTOR platform with Cyrus’ ability to redesign protein therapeutics. The lead program was a proprietary interleukin-2, or IL-2, protein agonist designed to selectively promote expansion of regulatory T cells for treatment of patients with autoimmune diseases and other deleterious immune conditions. In return for the licensed intellectual property, the Company made an upfront payment and was obligated to pay certain discovery, development, and sales-based milestones which could have potentially totaled up to approximately $1.5 billion across multiple programs. The Cyrus Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company expensed costs related to the Cyrus Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company assessed the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, would have amortized these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company was also obligated to pay Cyrus tiered royalties ranging from mid-single digit to low-double digit percentages of annual net sales of collaboration products which would have been expensed as commercial sales occur. On June 13, 2022, the Company and Cyrus mutually agreed that the preclinical key in-vitro success milestone had been achieved. In October 2023, the Company notified Cyrus of its termination of the Cyrus Agreement, effective December 29, 2023. Stock Purchase Agreement Additionally, on September 7, 2021, the Company entered into a stock purchase agreement, or the Series B Preferred Stock Purchase Agreement, in connection with the Cyrus Agreement. Pursuant to the Series B Preferred Stock Purchase Agreement, the Company purchased 2,326,934 shares of Cyrus’ Series B Preferred Stock, par value $0.0001 per share, at a purchase price of $0.8595 per share for $2.0 million. In accordance with ASC 810, the Company has a variable interest in Cyrus resulting from its equity investment. The Company will share in Cyrus’ expected losses or receive a portion of its expected returns and absorb the variability associated with changes in the entity’s net assets. However, the Company is not the primary beneficiary as it does not have the power to direct the activities most significant to Cyrus, and therefore it is not required to consolidate Cyrus. The Company has recognized the $2.0 million investment of Cyrus’ Series B Preferred Stock at cost on the purchase date. As of December 31, 2023, no impairment indicators are present and therefore the carrying value of the investment in Cyrus is $2.0 million on the accompanying consolidated balance sheet. The Company’s maximum exposure to loss related to this VIE is limited to the carrying value of the investment. The Company has not provided financing to Cyrus other than the amount contractually required by the Series B Preferred Stock Purchase Agreement. Asklepios Biopharmaceutical, Inc. Feasibility Study and License Agreement In August 2019, the Company entered into a feasibility study and license agreement with AskBio, or the AskBio Collaboration Agreement. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain AAV gene therapy products utilizing the Company’s ImmTOR platform to enable re-dosing of such AAV gene therapy products to treat serious rare and orphan genetic diseases for which there is a significant unmet medical need. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AskBio’s AAV gene therapy, or SEL-302, (previously disclosed as MMA-101, in combination with ImmTOR) for the treatment of methylmalonic acidemia, or MMA, to mitigate the formation of neutralizing anti-AAV capsid antibodies. On April 29, 2021, the Company was notified by AskBio that it intended to opt-out of development of the MMA indication. The Company and AskBio shared responsibility for the research, development and commercialization of products developed under the SEL-399 program collaboration. The parties also shared research, development, and commercialization costs equally for all collaboration products, but with a right of either party to opt out of certain products, and thereby not be required to share costs for such products. Each party would have received a percentage of net profits under the collaboration equal to the percentage of shared costs borne by such party in the development of such product. Pursuant to the AskBio Collaboration Agreement, AskBio was responsible for manufacturing the AAV capsids and AAV vectors and the Company was responsible for manufacturing ImmTOR. The Company and AskBio mutually agreed to the termination of the AskBio Collaboration Agreement, effective December 13, 2023. For the years ended December 31, 2023 and 2022, the Company recognized $0.1 million and $0.9 million, respectively, of collaboration expense under the AskBio Collaboration Agreement in which actual costs incurred by both parties approximate a 50% cost share. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement, or the 3SBio License, with Shenyang Sunshine Pharmaceutical Co., Ltd., or 3SBio. The Company has paid to 3SBio an aggregate of $7.0 million in upfront and milestone-based payments under the 3SBio License as of December 31, 2023. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $15.0 million for products containing the Company’s ImmTOR platform.

Income Taxes (Q2)

Income Taxes (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes As of June 30, 2024, the Company has not recorded any U.S. federal or state income tax benefits for either the net losses the Company has incurred or its earned research and orphan drug credits, due to the uncertainty of realizing a benefit from those items in the future.	Income Taxes The Company provides for income taxes under ASC 740. Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. On November 13, 2023, the Company acquired, in accordance with the terms of the Merger Agreement, the assets of Old Cartesian. In accordance with ASC 805-740-25-3, recognition of deferred tax assets and liabilities is required for substantially all temporary differences and acquired tax carryforwards and credits. The Company has computed estimated temporary differences and acquired tax carryforwards and credits as of the transaction date. The Company will not have tax basis in IPR&D booked as part of the purchase accounting. For accounting purposes, the IPR&D will not be amortized and only subject to impairment review and testing. Though the tax effects may be delayed indefinitely, ASC 740-10-55-63 states that “deferred tax liabilities may not be eliminated or reduced because a reporting entity may be able to delay the settlement of those liabilities by delaying the events that would cause taxable temporary differences to reverse.” As such, the Company can potentially only utilize indefinite-lived assets as it relates to this indefinite lived intangible deferred tax liability reversal. As such, the Company has booked a deferred tax liability for the portion of the liability that cannot be reduced based on scheduling. Additionally, a portion of this target deferred tax liability is offset with the Company's pre-Merger deferred tax assets on a combined basis, and as such the portion of deferred tax liability reduced by the Company's pre-Merger deferred tax assets has been charged to income rather than to goodwill. For the year ended December 31, 2023, the Company recognized a current tax benefit, of $19.0 million. For the year ended December 31, 2022, the Company recognized a current tax benefit for penalty abatements received of $0.6 million. For the year ended December 31, 2021, the Company had recorded a tax expense of $16.0 million, inclusive of penalties and interest of $1.3 million assessed as of December 31, 2021. The following table reconciles the federal statutory income tax rate to the Company’s effective income tax rate: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Statutory U.S. federal rate ​ ​ 21.0 % ​ ​ 21.0 % ​ ​ 21.0 % State income taxes - net of federal benefit ​ ​ 2.3 % ​ ​ 1.6 % ​ ​ (166.0)% Permanent items ​ ​ (1.6)% ​ ​ (18.6)% ​ ​ 8.3 % Research tax credits ​ ​ 0.6 % ​ ​ (3.2)% ​ ​ 55.0 % Deferred revenue ​ ​ — % ​ ​ — % ​ ​ 156.5 % Other ​ ​ — % ​ ​ — % ​ ​ (3.7)% Change in fair value of forward contract liabilities ​ ​ (13.2)% ​ ​ — % ​ ​ — % Valuation allowance, net ​ ​ 2.8 % ​ ​ (4.4)% ​ ​v (230.1)% Stock-based compensation ​ ​ (3.9)% ​ ​ 1.8 % ​ ​ (5.2)% Effective income tax rate ​ ​ 8.0% ​ ​ (1.8)% ​ ​ (164.2)% The tax effects of temporary differences that give rise to the Company’s net deferred tax assets are as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 Deferred Tax Assets ​ ​ ​ ​ Net operating loss carryforwards ​ ​ $ 29,841 ​ ​ $ 17,015 Research and development credits ​ ​ 5,649 ​ ​ 2,806 Stock-based compensation expense ​ ​ 8 ​ ​ 5,892 Other expenses ​ ​ 705 ​ ​ 1,697 Deferred revenue ​ ​ 84,626 ​ ​ 83,417 Operating lease liabilities ​ ​ 2,718 ​ ​ 3,186 R&E Capitalization ​ ​ 19,778 ​ ​ 9,588 Patent and license costs ​ ​ 9,140 ​ ​ 7,472 Gross deferred tax assets ​ ​ 152,465 ​ ​ 131,073 Deferred Tax Liabilities ​ ​ ​ ​ Intangible assets ​ ​ $ (41,144) ​ ​ $ — Depreciation ​ ​ (128) ​ ​ (81) Operating lease right-of-use assets ​ ​ (2,751) ​ ​ (3,174) Gross deferred tax liabilities ​ ​ (44,023) ​ ​ (3,255) Net deferred tax assets before valuation allowance ​ ​ 108,442 ​ ​ 127,818 Valuation allowance ​ ​ (124,295) ​ ​ (127,818) Net deferred tax assets/(liabilities) ​ ​ $ (15,853) ​ ​ $ — The Company has provided a full valuation allowance against its net deferred tax assets, outside of the indefinite tax liability booked as part of the Merger. The Company believes that it is more likely than not that the net deferred tax assets will not be realized. Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets and concluded that it is more likely than not that the Company will not realize the benefit of its net deferred tax assets. The valuation allowance decreased by $3.5 million for the year ended December 31, 2023, primarily as a result of tax benefit booked as part of the Merger. The valuation allowance decreased by $1.5 million for the year ended December 31, 2022, primarily as a result of pre-tax income and credits. As of December 31, 2023, the Company is in the process of winding down operations in Russia and does not expect any tax liability relating to such operations. At December 31, 2023, the Company has federal net operating loss carryforward of $108.7 million, which can be carried forward indefinitely, subject to an 80% limitation and state net operating loss carryforward of $110.3 million, which will expire at various times through 2043. The Company has $4.9 million and $0.9 million, respectively, of federal and state research and development tax credit carryforwards, which will expire at various times through 2043. Utilization of the NOL carryforwards and research and orphan drug credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended, or the Code, and similar state law due to ownership changes that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. If the Company experiences a change of control, as defined by Section 382 of the Code and similar state law, utilization of the NOL carryforwards or research and orphan drug credit carryforwards may be subject to an annual limitation under Section 382 of the Code, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the NOL carryforwards or research and orphan drug credit carryforwards before utilization. The Company performed an analysis of ownership changes through December 31, 2023. Based on this analysis, the Company does not believe that any of its tax attributes through December 31, 2023 will expire unutilized due to Section 382 limitations. To the extent the Company enters into future equity transactions, there could be a limitation on the Company’s tax attributes. The Company applies ASC 740, Income Taxes During 2023, the Company completed a detailed study of its research and development and orphan drug credits through December 31, 2022. As a result, the Company adjusted its deferred tax asset balances and the impacts are included in the research tax credits and state income taxes - net of federal benefit lines in the effective rate reconciliation above. The Tax Cuts and Jobs Act requires taxpayers to capitalize and amortize, rather than deduct, research and experimental, or R&E, expenditures under section 174 for tax years beginning after December 31, 2021. These rules became effective for the Company during the year ended December 31, 2022. As a result, the Company has capitalized R&E costs of $44.7 million and $29.3 million for the years ended December 31, 2023 and December 31, 2022, respectively. The Company will amortize these costs for tax purposes over five years if the R&E was performed in the U.S. and over 15 years if the R&E was performed outside the U.S. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statement of operations. As of December 31, 2023, the Company had no accrued interest related to uncertain tax positions. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities is open for tax years since inception as the Company claimed research tax credits on its 2020 tax return which remains open for examination for the 2020 year as well as for any year in which a credit has been claimed for. The Company files income tax returns in the United States and Massachusetts. There are currently no federal, state or foreign audits in progress.

Defined Contribution Plan (Q2)

Defined Contribution Plan (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution Plan The Company maintains a defined contribution plan, or the 401(k) Plan, under Section 401(k) of the Internal Revenue Code. The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. As of January 2022, all matching contributions vest ratably over two years and participant contributions vest immediately. Contributions by the Company totaled less than $0.1 million and $0.1 million during the three months ended June 30, 2024 and 2023, respectively, and $0.1 million and $0.2 million during the six months ended June 30, 2024 and 2023, respectively.	Defined Contribution Plan The Company maintains a defined contribution plan, or the 401(k) Plan, under Section 401(k) of the Internal Revenue Code. The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. As of January 2022, all matching contributions vest ratably over two years and participant contributions vest immediately. Contributions by the Company totaled $0.3 million, $0.3 million, and $0.2 million during each of the years ended December 31, 2023, 2022 and 2021, respectively.

Commitments and Contingencies (

Commitments and Contingencies (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of June 30, 2024, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non-performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect on the Company’s business, financial position, results of operations or cash flows.	Commitments and Contingencies As of December 31, 2023, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non-performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect the Company’s business, financial position, results of operations or cash flows.

Restructuring (Q2)

Restructuring (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Restructuring	Restructuring In April 2023, in light of current market conditions, the Board of Directors took steps to extend the Company’s cash runway by pausing further development of SEL-302 for the treatment of MMA, and conducting a targeted headcount reduction. On August 17, 2023, the Company announced additional steps to extend cash runway and maximize value for stockholders by continuing to prioritize development of SEL-212 and support of its collaboration with Astellas for Xork, and pausing further development of all of the Company’s other clinical and preclinical product candidates that it was no longer actively advancing. As a result of these measures, the Company implemented a restructuring plan resulting in an approximate 89% reduction of the Company's existing headcount by July 1, 2024. The Company recognized restructuring expenses consisting of one-time cash severance payments and other employee-related costs of $6.4 million for the year ended December 31, 2023 with $5.6 million and $0.8 million recorded to research and development and general and administrative operating expense categories, respectively, on its consolidated statements of operations and comprehensive income (loss) based on each employee's role. Cash payments for employee related restructuring charges of $2.5 million were paid as of December 31, 2023. For the six months ended June 30, 2024, the Company recorded restructuring expenses consisting of one-time cash severance payments and other employee-related costs of $0.8 million, with $0.2 million and $0.6 million recorded to research and development and general and administrative operating expense categories, respectively, on its consolidated statements of operations and comprehensive income (loss) based on each employee’s role. Cash payments for employee related restructuring charges of $4.5 million were paid during the six months ended June 30, 2024. The Company expects that the payments for the restructuring plan will be substantially complete by June 30, 2024. The following table summarizes the change in the Company's accrued restructuring balance (in thousands): ​ ​ Beginning Balance December 31, 2023 ​ ​ Charges ​ ​ Payments ​ ​ Ending Balance June 30, 2024 Severance liability ​ ​ $3,896 ​ ​ $805 ​ ​ $(4,527) ​ ​ $174	Restructuring In April 2023, in light of current market conditions, the Board of Directors, took steps to extend the Company's cash runway by pausing further development of SEL-302 for the treatment of MMA, and conducting a targeted headcount reduction. On August 17, 2023, the Company announced additional steps to extend cash runway and maximize value for stockholders by continuing to prioritize development of SEL-212 and support of its collaboration with Astellas for Xork, and pausing further development of all of the Company’s other clinical and preclinical product candidates that it was no longer actively advancing. As a result of these measures, the Company implemented a restructuring plan resulting in an approximate 79% reduction of the Company's existing headcount by December 31, 2023. The Company recognized restructuring expenses consisting of one-time cash severance payments and other employee-related costs of $6.4 million during the year ended December 31, 2023. Cash payments for employee related restructuring charges of $2.5 million were paid as of December 31, 2023. The Company recorded $5.6 million and $0.8 million based on each employee's role to research and development and general and administrative operating expense categories, respectively, on its consolidated statements of operations and comprehensive income (loss) for the year ended December 31, 2023. The following table summarizes the change in the Company's accrued restructuring balance (in thousands): ​ ​ Beginning Balance December 31, 2022 ​ ​ Charges ​ ​ Payments ​ ​ Ending Balance December 31, 2023 Severance liability ​ ​ $— ​ ​ $6,431 ​ ​ $2,535 ​ ​ $3,896

Subsequent Events (Q2)

Subsequent Events (Q2)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On July 2, 2024, the Company entered into the July 2024 Purchase Agreement for the July 2024 Private Placement, with certain institutional and accredited investors, or the Purchasers. Pursuant to the July 2024 Purchase Agreement, the Purchasers agreed to purchase an aggregate of 3,563,247 shares of common stock and 2,937,903 shares of Series B Preferred Stock, inclusive of 2,359,500 shares of Series B Preferred Stock purchased by directors and executive officers, and related parties thereto, each at a price per share of $20.00. Each share of Series B Preferred Stock is convertible into one share of the Company’s common stock subject to stockholder approval of a proposal to issue such shares of common stock upon conversion of such shares of Series B Preferred Stock in accordance with the Listing Rules of the Nasdaq Stock Market LLC, or the Series B Conversion Proposal. The July 2024 Private Placement resulted in gross proceeds of approximately $130.0 million before deducting placement agent fees and other offering expenses. Pursuant to the July 2024 Purchase Agreement, the Company has agreed to submit to its stockholders the approval of the Series B Conversion Proposal, at a special meeting of stockholders, which is to be held no later than October 31, 2024. Holders of shares of Series B Preferred Stock are entitled to receive dividends on shares of Series B Preferred Stock equal to, on an as-if-converted-to-common stock basis, and in the same form as dividends actually paid on shares of the common stock. Except as otherwise required by law, the Series B Preferred Stock does not have voting rights. However, as long as any shares of Series B Preferred Stock are outstanding, the Company will not, without the affirmative vote of the holders of a majority of the then outstanding shares of the Series B Preferred Stock, (a) alter or change adversely the powers, preferences or rights given to the Series B Preferred Stock, (b) alter or amend the Certificate of Designation of Preferences, Rights and Limitations of the Series B Non-Voting Convertible Preferred Stock, or the Series B Certificate of Designation, or (c) amend its restated certificate of incorporation, as amended, or other charter documents in any manner that adversely affects any rights of the holders of Series B Preferred Stock. The Series B Preferred Stock does not have a preference upon any liquidation, dissolution or winding-up of the Company. Following stockholder approval of the Series B Conversion Proposal, each share of Series B Preferred Stock will automatically convert into one share of common stock, subject to certain limitations, including that a holder of Series B Preferred Stock is prohibited from converting shares of Series B Preferred Stock into shares of common stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than a specified percentage (to be established by the holder between 0% and 19.9%) of the total number of shares of common stock issued and outstanding immediately after giving effect to such conversion; provided, however, that such beneficial ownership limitation does not apply to TAS Partners LLC or any of its affiliates. Based on the 21,382,485 shares of common stock outstanding on August 7, 2024, there would be 29,865,107 shares of common stock outstanding if all shares of Series A Preferred Stock and all shares of Series B Preferred Stock converted into common stock on such date.	Subsequent Events On February 28, 2024, the Company entered into a lease agreement with 7495 RP, LLC, or the Landlord, pursuant to which it agreed to lease from the Landlord the manufacturing space located at 7495 New Horizon Way, Frederick, Maryland, or the Frederick Lease Agreement. The space consists of 19,199 leasable square feet of integrated manufacturing and office space. The initial term of the Frederick Lease Agreement is expected to commence no later than April 1, 2024, once the Landlord has obtained legal possession of the premises free of the existing tenant and delivered full possession of the premises to the Company, or the Commencement Date. The Frederick Lease Agreement will terminate seven

Description of the Business (FY

Description of the Business (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the Business	Description of the Business Cartesian Therapeutics, Inc., or the Company (formerly known as Selecta Biosciences, Inc., or Selecta), was incorporated in Delaware on December 10, 2007, and is headquartered in Gaithersburg, Maryland. The Company is a clinical-stage biotechnology company developing mRNA cell therapies for the treatment of autoimmune diseases leveraging its proprietary technology and manufacturing platform to introduce one or more mRNA molecules into cells to enhance their function. The Company believes its mRNA cell therapies have the potential to deliver deep, durable clinical benefit to a broad group of patients with autoimmune diseases because they can be administered over a short period of time, in an outpatient setting, and without pre-treatment chemotherapy. On November 13, 2023, the Company acquired, in accordance with the terms of the Agreement and Plan of Merger, or the Merger Agreement, the assets of the Delaware corporation which, immediately prior to the Merger (as defined below), was known as Cartesian Therapeutics, Inc., or Old Cartesian, as disclosed in Note 3. The transaction was structured as a stock-for-stock transaction pursuant to which all of Old Cartesian’s outstanding shares of capital stock were exchanged based on a fixed exchange ratio for consideration of 224,099 shares of the common stock, par value $0.0001 per share, of the Company, or the common stock, and 384,930.724 shares of the newly designated Series A Non-Voting Convertible Preferred Stock, par value $0.0001 per share, or the Series A Preferred Stock. The Series A Preferred Stock is intended to have economic rights similar to the common stock, but with only limited voting rights. Additionally, the Company assumed all outstanding stock options of Old Cartesian. The common stock and Series A Preferred Stock related to the Merger were issued on December 5, 2023. For additional information, see Note 3. In connection with the Merger, the Company entered into a definitive agreement, or the Securities Purchase Agreement, for a private investment in public equity transaction, or the November 2023 Private Placement, with the Investors (as defined below). The Securities Purchase Agreement provides for the issuance to the Investors of an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of approximately $60.25 million. For additional information, see Note 10. In connection with the Merger, a contractual contingent value right, or CVR, was distributed to the holders of record of the Company's common stock and 2022 Warrants (as defined below) as of the close of business on December 4, 2023, but was not distributed to holders of shares of common stock or Series A Preferred Stock issued to stockholders of Old Cartesian or the Investors in the transactions. Holders of the CVRs will be entitled to receive certain payments from proceeds received by the Company, if any, related to the disposition or monetization of the Company's legacy assets following the issuance of the CVRs. For additional information, see Note 5. On March 27, 2024, the Company’s stockholders approved the Conversion Proposal (as defined below). For additional information, see Note 10. Additionally, on March 27, 2024, the Company’s stockholders approved an amendment to the Company’s restated certificate of incorporation, as amended, or the Charter, to effect a reverse stock split of the Company’s issued and outstanding common stock, at a ratio in the range of 1-for-20 and 1-for-30, with such ratio to be determined at the discretion of the Company’s board of directors, or the Board of Directors. The Board of Directors subsequently approved a final reverse stock split ratio of 1-for-30, and the Company effected the Reverse Stock Split on April 4, 2024. As a result of the Reverse Stock Split, all figures in this Quarterly Report on Form 10-Q relating to shares of the Company’s common stock (such as share amounts, per share amounts, and conversion rates and prices), have been adjusted to reflect the Reverse Stock Split for all periods presented, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. Shares of common stock underlying outstanding stock options, restricted stock units and warrants were proportionately reduced and the respective exercise prices, if applicable, were proportionately increased in accordance with their terms. Additionally, the conversion ratio of the Company’s Series A Preferred Stock was proportionally adjusted. Stockholders entitled to fractional shares as a result of the Reverse Stock Split received a cash payment in lieu of receiving fractional shares. On July 2, 2024, the Company entered into a securities purchase agreement, or the July 2024 Purchase Agreement, for a private investment in public equity financing, or the July 2024 Private Placement, which provides for the issuance of 3,563,247 shares of common stock and 2,937,903 shares of Series B Non-Voting Convertible Preferred Stock, par value $0.0001 per share, or the Series B Preferred Stock, each at a purchase price of $20.00 per share. The July 2024 Private Placement resulted in gross proceeds of approximately $130.0 million before deducting placement agent fees and other offering expenses. For additional information, see Note 20. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in pre-clinical and clinical development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Unaudited Interim Financial Information The accompanying unaudited consolidated financial statements for the three and six months ended June 30, 2024 and 2023 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission, or the SEC, for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America, or U.S. GAAP, have been condensed or omitted pursuant to such rules and regulations. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2023 included in the Company’s Annual Report on Form 10-K that was filed with the SEC on March 7, 2024. The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements. In the opinion of management, the accompanying unaudited interim consolidated financial statements contain all adjustments that are necessary for a fair statement of the Company’s financial position as of June 30, 2024, the consolidated results of operations for the three and six months ended June 30, 2024, and cash flows for the six months ended June 30, 2024. Such adjustments are of a normal and recurring nature. The results of operations for the three and six months ended June 30, 2024 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2024. Liquidity and Management’s Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain and sustain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital, which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through public offerings and private placements of its securities, funding received from research grants, collaboration and license arrangements and a credit facility. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, the Company’s revenue has primarily been from collaboration agreements. The Company has devoted substantially all of its financial resources and efforts to developing its existing product candidates, identifying potential product candidates and conducting preclinical studies and clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any product candidates. As of June 30, 2024, the Company’s cash, cash equivalents, and restricted cash were $88.9 million, of which $1.7 million was restricted cash related to lease commitments and $0.2 million was held by its Russian subsidiary designated solely for use in its operations. The Company believes the cash, cash equivalents and restricted cash as of June 30, 2024, combined with net proceeds of $124.4 million from the July 2024 Private Placement received subsequent to June 30, 2024, will enable it to fund its current planned operations for at least the next twelve months from the date of issuance of these financial statements, though it may pursue additional cash resources through public or private equity or debt financings or by establishing collaborations with other companies. Management’s expectations with respect to its ability to fund current and long term planned operations are based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional strategic or financing opportunities sooner than would otherwise be expected. However, there is no guarantee that any collaboration milestones will be achieved or that any of these strategic or financing opportunities will be executed on favorable terms, and some could be dilutive to existing stockholders. Further, the liability associated with the CVR Agreement (as defined below) will be settled solely through cash flow received under the Company's License and Development Agreement, or as so amended, the Sobi License, with Swedish Orphan Biovitrum AB (publ.), or Sobi, and any other Gross Proceeds (as defined in the CVR Agreement) net of certain agreed deductions. Under the CVR Agreement, 100% of all milestone payments, royalties and other amounts paid to the Company or controlled entities under the Sobi License, and any other Gross Proceeds will be distributed, net of specified deductions, to holders of the CVRs. There is no obligation to the Company to fund any amount related to the CVR liability. See Note 5. If the Company is unable to obtain additional funding on a timely basis, it may be forced to significantly curtail, delay, or discontinue one or more of its planned research or development programs or be unable to expand its operations or otherwise capitalize on its commercialization of its product candidates. As of June 30, 2024, the Company had an accumulated deficit of $657.6 million. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research and development of its product candidates and its administrative organization. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at the Company. Through June 30, 2024, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.	Description of the Business Cartesian Therapeutics, Inc., or the Company, (formerly known as Selecta Biosciences, Inc., or Selecta) was incorporated in Delaware on December 10, 2007, and is headquartered in Gaithersburg, Maryland. The Company is a clinical-stage biotechnology company developing mRNA cell therapies for the treatment of autoimmune diseases leveraging its proprietary technology and manufacturing platform to introduce one or more mRNA molecules into cells to enhance their function. The Company believes its mRNA cell therapies have the potential to deliver deep, durable clinical benefit to a broad group of patients with autoimmune diseases because they can be administered over a short period of time, in an outpatient setting, and without pre-treatment chemotherapy. On November 13, 2023, the Company acquired, in accordance with the terms of the Agreement and Plan of Merger, or the Merger Agreement, the assets of the Delaware corporation which, immediately prior to the Merger (as defined below), was known as Cartesian Therapeutics, Inc., or Old Cartesian, as disclosed in Note 3. The transaction was structured as a stock-for-stock transaction pursuant to which all of Old Cartesian’s outstanding shares of capital stock were exchanged based on a fixed exchange ratio for consideration of 6,723,639 shares of the common stock, $0.0001 per share, of the Company and 384,930.724 shares of the newly designated Series A Non-Voting Convertible Preferred Stock, $0.0001 per share, or the Series A Preferred Stock. The Series A Preferred Stock is intended to have economic rights similar to the common stock, but with only limited voting rights. Additionally, the Company assumed all outstanding stock options of Old Cartesian. The common stock and Series A Preferred Stock related to the Merger were issued on December 5, 2023. For additional information, see Note 3. In connection with the Merger, the Company entered into a definitive agreement, or the Securities Purchase Agreement, for a private investment in public equity transaction, or the November 2023 Private Placement, with the Investors (as defined below). The Securities Purchase Agreement provides for the issuance to the Investors of an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of approximately $60.25 million. For additional information, see Note 11. In connection with the Merger, a contractual contingent value right, or CVR, was distributed to the holders of record of the Company's common stock and 2022 Warrants as of the close of business on December 4, 2023, but was not distributed to holders of shares of common stock or Series A Preferred Stock issued to stockholders of Old Cartesian or the Investors in the transactions. Holders of the CVRs will be entitled to receive certain payments from proceeds received by the Company, if any, related to the disposition or monetization of the Company's legacy assets following the issuance of the CVRs. For additional information, see Note 6. The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, new technological innovations, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. The Company’s product candidates are in pre-clinical and clinical development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, or maintained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees and consultants. Liquidity and Management’s Plan The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain and sustain profitable operations. The Company is subject to a number of risks similar to other early-stage life science companies, including, but not limited to, successful development of its product candidates, raising additional capital with favorable terms, protection of proprietary technology and market acceptance of any approved future products. The successful development of product candidates requires substantial working capital, which may not be available to the Company on favorable terms or at all. To date, the Company has financed its operations primarily through public offerings and private placements of its securities, funding received from research grants, collaboration and license arrangements and a credit facility. The Company currently has no source of product revenue, and it does not expect to generate product revenue for the foreseeable future. To date, the Company’s revenue has primarily been from collaboration agreements. The Company has devoted substantially all of its financial resources and efforts to developing its existing product candidates, identifying potential product candidates and conducting preclinical studies and clinical trials. The Company is in the early stages of development of its product candidates, and it has not completed development of any product candidates. As of December 31, 2023, the Company’s cash, cash equivalents, and restricted cash were $78.3 million, of which $1.4 million was restricted cash related to lease commitments and $0.2 million was held by its Russian subsidiary designated solely for use in its operations. The Company believes the cash, cash equivalents and restricted cash as of December 31, 2023 combined with net proceeds of $40.0 million received subsequent to December 31, 2023 from the November 2023 Private Placement will enable it to fund its current planned operations for at least the next twelve months from the date of issuance of these financial statements, though it may pursue additional cash resources through public or private equity or debt financings or by establishing collaborations with other companies. Management’s expectations with respect to its ability to fund current and long term planned operations are based on estimates that are subject to risks and uncertainties. If actual results are different from management’s estimates, the Company may need to seek additional strategic or financing opportunities sooner than would otherwise be expected. However, there is no guarantee that any collaboration milestones will be achieved or that any of these strategic or financing opportunities will be executed on favorable terms, and some could be dilutive to existing stockholders. Further, the liability associated with the CVR Agreement (as defined below) will be settled solely through cash flow received under the Company's License and Development Agreement, or as so amended, the Sobi License, with Swedish Orphan Biovitrum AB (publ.), or Sobi, and any other Gross Proceeds (as defined in the CVR Agreement) net of certain agreed deductions. Under the CVR Agreement, 100% of all milestone payments, royalties and other amounts paid to the Company or controlled entities under the Sobi License, and any other Gross Proceeds will be distributed, net of specified deductions, to holders of the CVRs. There is no obligation to the Company to fund any amount related to the CVR liability. See Note 6. The Certificate of Designation of Preferences, Rights, and Limitations of the Series A Non-Voting Convertible Preferred Stock, or the Certificate of Designation, contains a provision granting each holder of the Series A Preferred Stock the option to require the Company to redeem any or all of such holder’s then-outstanding shares of Series A Preferred Stock beginning on the date that is 18 months following the date of the closing of the Merger, November 13, 2023, at a price per share equal to the ten-day trailing average closing trading price of the common stock at such time; provided, however, that no holder will have the right to seek redemption of any shares of Series A Preferred Stock to the extent that such holder would otherwise be unable to convert such shares of Series A Preferred Stock due to the common stock beneficial ownership limitation applicable to such holder. The Company could be required to use a significant amount of its cash resources on hand to satisfy this redemption obligation, particularly if its stockholders do not ever approve a proposal to convert the Company’s Series A Preferred Stock into common stock, or generally if holders of Series A Preferred Stock exercise their redemption right with respect to a significant number of shares of Series A Preferred Stock or at a time when the trading price of the Company’s common stock is elevated. Further, in the event that the Company does not have sufficient cash on hand to satisfy its redemption obligations, the Company may need to raise additional capital to satisfy these potential obligations. Any redemption payments could materially limit the amount of cash the Company has available to fund our operations and the potential need to redeem shares of Series A Preferred Stock may limit the flexibility with which the Company seeks to operate its business. If the Company is unable to obtain additional funding on a timely basis, it may be forced to significantly curtail, delay, or discontinue one or more of its planned research or development programs or be unable to expand its operations or otherwise capitalize on its commercialization of its product candidates. As of December 31, 2023, the Company had an accumulated deficit of $614.6 million. The Company anticipates operating losses to continue for the foreseeable future due to, among other things, costs related to research and development of its product candidates and its administrative organization. Guarantees and Indemnifications As permitted under Delaware law, the Company indemnifies its officers, directors, consultants and employees for certain events or occurrences that happen by reason of the relationship with, or position held at, the Company. Through December 31, 2023, the Company had not experienced any losses related to these indemnification obligations, and no claims were outstanding. The Company does not expect significant claims related to these indemnification obligations and, consequently, concluded that the fair value of these obligations is negligible, and no related reserves were established.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The Company disclosed its significant accounting policies in Note 2 – Summary of Significant Accounting Policies included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. There have been no material changes to the Company’s significant accounting policies during the six months ended June 30, 2024, except as noted below. Grant Revenue The Company has contracts with government-sponsored organizations for research and development related activities that provide for payments for reimbursable costs. The Company recognizes grant revenue from these contracts as it performs services under these arrangements when the funding is committed. Expenses associated with these contracts are recognized when incurred as research and development expense. Grant revenue and related expenses are presented gross in the consolidated statements of operations and comprehensive income (loss) as the Company has determined it is the primary obligor under the arrangements relative to the research and development services it performs as lead technical expert. Amounts incurred that are subject to reimbursement from the sponsor are recorded as accounts receivable on the consolidated balance sheets. Recent Accounting Pronouncements Not Yet Adopted In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures (ASU 2023-07), which requires an enhanced disclosure of significant segment expenses on an annual and interim basis. This guidance will be effective for the annual periods beginning the year ended December 31, 2024, and for interim periods beginning January 1, 2025. Early adoption is permitted. Upon adoption, the guidance should be applied retrospectively to all prior periods presented in the financial statements. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements. In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (ASU 2023-09), which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the effective tax rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. This guidance will be effective for the annual periods beginning the year ended December 31, 2025. Early adoption is permitted. Upon adoption, the guidance can be applied prospectively or retrospectively. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements.	Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta (RUS), LLC, or Selecta (RUS), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts securities corporation, and Cartesian Bio, LLC, a Delaware limited liability company, which is a variable interest entity for which the Company is the primary beneficiary. All significant intercompany accounts and transactions have been eliminated. Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: estimated fair value of the intangible assets acquired in connection with the Merger, estimated fair value of the CVRs, deferred income taxes, revenue recognition and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates. Segment Information The Company views its operations and manages its business in one operating segment, which prior to the Merger related to the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases and subsequent to the Merger relates to the research and development of cell therapy product candidates. Cash Equivalents, Restricted Cash, Marketable Securities and Investments Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Marketable securities consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these marketable securities as available-for-sale and records them at fair value in the accompanying consolidated balance sheets. Marketable securities with less than one year until maturity are classified as short term, while marketable securities with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. The Company has also in the past invested in equity securities of a company whose securities are not publicly traded and where fair value is not readily available. This investment is recorded using cost minus impairment adjusted for changes in observable prices, depending on our ownership percentage and other factors that suggest we have significant influence. The Company monitors this investment to evaluate whether any increase or decline in its value has occurred, based on the implied value of recent company financings, public market prices of comparable companies and general market conditions. This investment is included in investments in the consolidated balance sheets. Concentrations of Credit Risk and Off-Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, short-term deposits and marketable securities, investments, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially creditworthy and, accordingly, minimal risk exists with respect to those balances. The Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of December 31, 2023, the Company maintained approximately $0.2 million in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, marketable securities, investments, warrants to purchase common stock, forward contract liabilities, and contingent value rights. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three-level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of warrant liabilities and contingent value rights are determined using Level 3 inputs. Fair value is a market-based measure considered from the perspective of a market participant rather than an entity-specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. The carrying amounts reflected in the consolidated balance sheet for investments approximate fair value and are assessed for impairment quarterly. Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use. Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available. An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. The Company recognized a $0.7 million impairment charge on a right-of-use asset during the year ended December 31, 2023. Debt Issuance Costs Debt issuance costs and fees paid to lenders are recorded as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the effective interest method and recorded as interest expense. Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non-owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net income (loss) and (ii) all components of comprehensive income (loss) other than net income (loss), referred to as other comprehensive income (loss). Other comprehensive income (loss) is comprised of unrealized gains and losses on debt securities and foreign currency translation adjustments. Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC Topic 606, Revenue from Contracts with Customers (ASC 606) Collaboration and License Revenue: The Company currently generates its revenue through collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Collaboration and license agreements with customers are generally accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, ), and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC Topic 730, , and records reimbursements from counterparties as an offset to the related research and development costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The terms of the Company’s arrangements typically include one or more of the following: (i) upfront fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of research and development expenses; and (v) profit/loss sharing arising from co-promotion arrangements. Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other promised goods and services identified in the arrangement, the Company recognizes revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other promised goods and services in the contract. For licenses that are combined with other promised goods and services, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint. Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, stock-based compensation expenses, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made. Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancellable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future research and development activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs. In June 2020, the Company and Sobi entered into a License and Development Agreement, which was amended in October 2023. Pursuant to the Sobi License, clinical trial costs incurred to complete development of the SEL-212 product candidate, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, were reimbursed by Sobi. These costs, when reimbursed, were recognized as revenue consistent with the revenue recognition methodology disclosed in Note 14. The reimbursable costs exclude any costs of additional development activities required that are related to the ImmTOR platform and that are unrelated to SEL-212. In January 2023, the Company and Astellas entered into the Astellas Agreement. Pursuant to the Astellas Agreement, Astellas will reimburse the Company for 25% of all budgeted costs incurred to complete the development of Xork for use in Pompe disease with an Astellas gene therapy investigational or authorized product. These costs, when reimbursed, will be recognized as revenue consistent with the revenue recognition methodology disclosed in Note 14. Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions. Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date. Stock-Based Compensation The Company accounts for all stock-based compensation granted to employees and non-employees using a fair value method. Stock-based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis, net of estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock-based compensation expense recognized in the consolidated financial statements is based on awards that ultimately vest. Net (Loss) Income Per Share The Company applies the two-class method to compute basic and diluted net (loss) income per share attributable to common stockholders when it has issued shares that meet the definition of participating securities. The two-class method determines net (loss) income per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires (loss) income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to share in the earnings as if all (loss) income for the period had been distributed. The Company's Series A Preferred Stock and 2022 Warrants participate in any dividends declared by the Company and are therefore considered to be participating securities. The participating securities are not required to participate in the losses of the Company, and therefore during periods of loss there is no allocation required under the two-class method. Basic net (loss) income per share attributable to common stockholders is computed by dividing the net (loss) income attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net (loss) income attributable to common stockholders is computed by adjusting net (loss) income per share attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net (loss) income per share attributable to common stockholders is computed by dividing the diluted net (loss) income attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options to purchase common stock and Series A Preferred Stock, forward contracts to issue Series A Preferred Stock, restricted stock units, warrants to purchase common stock, employee stock purchase plan stock, contingently issuable shares, and Series A Preferred Stock are considered potential dilutive common shares. Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies Leases The Company accounts for its leases in accordance with ASC Topic 842, Leases (ASC 842) In accordance with the guidance in ASC 842, the fixed and in-substance fixed contract consideration must be allocated to lease and non-lease components based on their relative fair values. Non-components of a contract (e.g., administrative tasks that do not transfer a good or service to the Company, reimbursement or payment of a lessor’s cost, etc.) do not receive an allocation of the consideration in the contract. Although allocation of consideration of lease and non-lease components is required, the Company elected the practical expedient to not separate lease components (e.g. land, building, etc.) and non-lease components (e.g., common area maintenance, consumables, etc.). The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense. Right-of-use assets and operating lease liabilities are remeasured upon certain modifications to leases using the present value of remaining lease payments and the estimated incremental borrowing rate upon lease modification. The Company enters into lease agreements with terms generally ranging from two one Acquisitions The Company evaluates acquisitions of assets and other similar transactions to assess whether or not the transaction should be accounted for as a business combination or asset acquisition by first applying a screen to determine if substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or group of similar identifiable assets. If the screen is met, the transaction is accounted for as an asset acquisition. If the screen is not met, further determination is required as to whether or not the Company has acquired inputs and processes that have the ability to create outputs, which would meet the requirements of a business. If determined to be a business combination, the Company accounts for the transaction under the acquisition method of accounting which requires the acquiring entity in a business combination to recognize the fair value of all assets acquired, liabilities assumed, and any non-controlling interest in the acquiree and establishes the acquisition date as the fair value measurement point. Accordingly, the Company recognizes assets acquired and liabilities assumed in business combinations based on the fair value estimates as of the date of acquisition. In accordance with ASC 805, Business Combinations, or ASC 805, the Company recognizes and measures goodwill as of the acquisition date, as the excess of the fair value of the consideration paid over the fair value of the identified net assets acquired. Goodwill Goodwill represents the amount of consideration paid in excess of the fair value of the identified net assets acquired as a result of the Company’s business acquisitions accounted for using the acquisition method of accounting. Goodwill is not amortized and is subject to impairment testing at a reporting unit level on an annual basis or when a triggering event occurs that may indicate the carrying value of the goodwill is impaired. An entity is permitted to first assess qualitative factors to determine if a quantitative impairment test is necessary. Such qualitative factors include macroeconomic conditions, industry and market considerations, cost factors, overall financial performance and other relevant events. Further testing is only required if the entity determines, based on the qualitative assessment, that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. The Company evaluates goodwill for impairment at least annually on October 1 and whenever facts and circumstances indicate that their carrying amounts may not be recoverable. For the year ended December 31, 2023, the Company determined that there was no impairment to goodwill. Indefinite-Lived Intangible Assets Indefinite-lived intangible assets consist of in-process research and development, or IPR&D. The fair values of IPR&D assets acquired in business combinations are capitalized. These assets are treated as indefinite-lived intangible assets until completion or abandonment of the projects, at which time the assets are amortized over the remaining useful life or written off, as appropriate. Intangible assets with indefinite lives, including IPR&D, are tested for impairment if impairment indicators arise and, at a minimum, annually. However, an entity is permitted to first assess qualitative factors to determine if a quantitative impairment test is necessary. Further testing is only required if the entity determines, based on the qualitative assessment, that it is more likely than not that an indefinite-lived intangible asset’s fair value is less than its carrying amount. Otherwise, no further impairment testing is required. The indefinite-lived intangible asset impairment test consists of a one-step analysis that compares the fair value of the intangible asset with its carrying amount. If the carrying amount of an intangible asset exceeds its fair value, an impairment loss is recognized in an amount equal to that excess. The Company considers many factors in evaluating whether the value of its intangible assets with indefinite lives may not be recoverable, including, but not limited to, expected growth rates, the cost of equity and debt capital, general economic conditions, the Company’s outlook and market performance of the Company’s industry and recent and forecasted financial performance. The Company evaluates indefinite-lived intangible assets for impairment at least annually on October 1 and whenever facts and circumstances indicate that their carrying amounts may not be recoverable. For the year ended December 31, 2023, the Company determined that there was no impairment to the IPR&D assets. Series A Preferred Stock The Company records the Series A Preferred Stock upon issuance at its fair value. The fair value includes the original issuance price, the settlement of any re

Merger (FY)

Merger (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Merger	Merger On November 13, 2023, the Company merged with Old Cartesian in accordance with the terms of the Merger Agreement, by and among Selecta, Sakura Merger Sub I, Inc., a wholly owned subsidiary of Selecta, or First Merger Sub, Sakura Merger Sub II, LLC, a wholly owned subsidiary of Selecta, or Second Merger Sub, and Old Cartesian. Pursuant to the Merger Agreement, First Merger Sub merged with and into Old Cartesian, pursuant to which Old Cartesian was the surviving corporation and became a wholly owned subsidiary of Selecta, or the First Merger. Immediately following the First Merger, Old Cartesian merged with and into Second Merger Sub, pursuant to which Second Merger Sub was the surviving entity, or the Second Merger and, together with the First Merger, the Merger. In connection with the Second Merger, Old Cartesian changed its name to Cartesian Bio, LLC. The Merger was intended to qualify as a tax-free reorganization for U.S. federal income tax purposes. As a result of the Merger, Selecta changed its corporate name to Cartesian Therapeutics, Inc. and its common stock began trading on the Nasdaq Global Market under the new trading symbol “RNAC” beginning on November 14, 2023. The Merger Agreement was unanimously approved by the board of directors of Selecta and the board of directors of Old Cartesian. The Merger was consummated substantially concurrently with the entry into the Merger Agreement and was not subject to approval of the Company's stockholders. Under the terms of the Merger Agreement, following the consummation of the Merger on November 13, 2023, or the Closing Date, in exchange for 100% of the outstanding shares of capital stock of Old Cartesian immediately prior to the effective time of the First Merger, the Company agreed to issue to the stockholders of Old Cartesian (i) 224,099 shares of the Company’s common stock and (ii) 384,930.724 shares of Series A Preferred Stock. The issuance of the shares of common stock and Series A Preferred Stock occurred on December 5, 2023 which was after the December 4, 2023 record date for the distribution of the CVRs (see Note 5); as such, the Old Cartesian stockholders did not have rights as holders of common stock or holders of Series A Preferred Stock until such issuance on December 5, 2023. In addition, all outstanding stock options to purchase Old Cartesian common stock were assumed by the Company and converted into stock options to purchase (i) shares of the Company’s common stock or (ii) shares of the Company’s Series A Preferred Stock on terms substantially identical to those in effect prior to Merger Agreement, except for adjustments to the underlying number of shares and the exercise price based on the Merger Agreement exchange ratio. Pursuant to the Merger Agreement, the Company agreed to hold a stockholders’ meeting, or the Special Meeting, to submit the following proposals to a vote of its stockholders: (i) the approval of the conversion of shares of Series A Preferred Stock into shares of common stock, or the Conversion Proposal, and (ii) either or both of (A) the approval of an amendment to the Company’s Charter to increase the number of shares of common stock authorized under the Charter and (B) the approval of an amendment to the Charter to effect a reverse stock split of all outstanding shares of common stock, in either case (A) or (B) by a number of authorized shares or at a stock split ratio, as the case may be, sufficient to allow the conversion of all shares of Series A Preferred Stock issued in the Merger. The Special Meeting was held on March 27, 2024 in which the Company’s stockholders approved the Conversion Proposal, among other matters (see Note 10). The Company concluded the acquisition resulted in the Company obtaining a controlling financial interest in a variable interest entity, or VIE, in accordance with ASC 810, Consolidation , or ASC 810. The Company determined that Old Cartesian was considered to be a VIE as it did not have sufficient equity to finance its activities without additional subordinated financial support. Prior to the Closing Date, the primary source of funding for Old Cartesian had been preferred stock financings. The Company acquired all of the outstanding shares of Old Cartesian and, therefore, is the sole equity holder and primary beneficiary. The Company has the obligation to absorb the losses and right to receive the benefits of Old Cartesian, and the power to direct the activities that most significantly affect the economic performance of Old Cartesian which the Company considers to be its development activities. Therefore, the Company is the primary beneficiary. Further, the Company concluded the VIE qualified as a business and accounted for the transaction as the acquisition of a business in accordance with ASC 805, Business Combinations , or ASC 805. As the primary beneficiary, the Company was the acquirer in the transaction. The Company exchanged the right to receive shares of common stock and Series A Preferred Stock for all of the outstanding equity of Old Cartesian. The Company determined the rights to receive shares exchanged in the Merger represent a forward contract. The fair value of the forward contracts was determined based on the fair value of shares of common stock and Series A Preferred Stock underlying the forward contracts as of the acquisition date. The total purchase price consists of the fair value of the forward contracts in addition to a portion of the fair value of options exchanged in the transaction related to prior service. Under the acquisition method, the total purchase price of the acquisition was allocated to the net tangible and identifiable intangible assets acquired and liabilities assumed based on their estimated fair values as of the date of the acquisition. The total fair value of the consideration of $168.5 million as of the Closing Date is summarized as follows (in thousands): Forward contract to issue common stock ​ ​ $ 2,713 Forward contract to issue Series A Preferred Stock ​ ​ 155,308 Stock options allocated to consideration paid ​ ​ 10,444 Total consideration ​ ​ $168,465 The Company recorded the assets acquired and liabilities assumed as of the Closing Date based on the information available at that date. The following table presents the allocation of the purchase price to the estimated fair values of the assets acquired and liabilities assumed as of the Closing Date (in thousands): ​ ​ As of November 13, 2023 Assets acquired: ​ ​ Cash and cash equivalents ​ ​ $ 6,561 Prepaid expenses and other current assets ​ ​ 309 Property and equipment, net ​ ​ 215 Right-of-use asset, net ​ ​ 915 In-process research and development assets ​ ​ 150,600 Goodwill ​ ​ 48,163 ​ ​ $206,763 Liabilities assumed ​ ​ Accrued expenses and other current liabilities ​ ​ $ 2,530 Lease liability ​ ​ 292 Lease liability, net of current portion ​ ​ 623 Deferred tax liability ​ ​ 34,853 ​ ​ $ 38,298 Net assets acquired ​ ​ $168,465 The fair value of the in-process research and development, or IPR&D, assets were capitalized as of the Closing Date and will be accounted for as indefinite-lived intangible assets until completion or disposition of the assets or abandonment of the associated research and development efforts. Upon successful completion of the development efforts, the carrying value of each respective IPR&D asset will be amortized over its estimated useful life. Until that time, the IPR&D assets will be subject to impairment testing and will not be amortized. The goodwill recorded related to the Merger is the excess of the fair value of the consideration transferred by the acquirer over the fair value of tangible assets, identifiable intangible assets and assumed liabilities as of the Closing Date and is not deductible for tax purposes. The goodwill balance is primarily attributable to the value of the assembled workforce and deferred tax liabilities associated with the transaction. The following summarizes the Company’s intangible assets acquired in the Merger (in thousands): ​ ​ Acquisition Date Fair Value Descartes-08 for MG ​ ​ $ 93,900 Descartes-08 for SLE ​ ​ 56,700 Total in-process research and development assets ​ ​ $150,600 The fair value of the intangible assets was estimated using the income approach in which the after-tax cash flows were discounted to present value. The cash flows are based on estimates used to price the transaction, and the discount rates applied were benchmarked with reference to the implied rate of return from the transaction model as well as the weighted average cost of capital. The forward contract related to the common stock was recorded as additional paid-in capital as the instrument is indexed to the Company’s common stock. The forward contract related to the Series A Preferred Stock was recorded as a liability as the underlying Series A Preferred Stock has a redemption feature that may require the Company to settle the instrument by transferring an asset. The forward contract was measured at fair value through the date of settlement through the issuance of the shares of Series A Preferred Stock on December 5, 2023.	Merger On November 13, 2023, the Company merged with Old Cartesian in accordance with the terms of the Merger Agreement, by and among Selecta, Sakura Merger Sub I, Inc., a wholly owned subsidiary of Selecta, or First Merger Sub, Sakura Merger Sub II, LLC, a wholly owned subsidiary of Selecta, or Second Merger Sub, and Old Cartesian. Pursuant to the Merger Agreement, First Merger Sub merged with and into Old Cartesian, pursuant to which Old Cartesian was the surviving corporation and became a wholly owned subsidiary of Selecta, or the First Merger. Immediately following the First Merger, Old Cartesian merged with and into Second Merger Sub, pursuant to which Second Merger Sub was the surviving entity, or the Second Merger and, together with the First Merger, the Merger. In connection with the Second Merger, Old Cartesian changed its name to Cartesian Bio, LLC. The Merger was intended to qualify as a tax-free reorganization for U.S. federal income tax purposes. As a result of the Merger, Selecta changed its corporate name to Cartesian Therapeutics, Inc. and its common stock began trading on the Nasdaq Global Market under the new trading symbol “RNAC” beginning on November 14, 2023. The Merger Agreement was unanimously approved by the board of directors, or the Board of Directors, of Selecta and the board of directors of Old Cartesian. The Merger was consummated substantially concurrently with the entry into the Merger Agreement and was not subject to approval of the Company's stockholders. Under the terms of the Merger Agreement, following the consummation of the Merger on November 13, 2023, or the Closing Date, in exchange for 100% of the outstanding shares of capital stock of Old Cartesian immediately prior to the effective time of the First Merger, the Company agreed to issue to the stockholders of Old Cartesian (i) 6,723,639 shares of the Company’s common stock and (ii) 384,930.724 shares of Series A Preferred Stock. The issuance of the shares of common stock and Series A Preferred Stock occurred on December 5, 2023 which was after the December 4, 2023 record date for the distribution of the CVRs (see Note 6); as such, the Old Cartesian stockholders did not have rights as holders of common stock or holders of Series A Preferred Stock until such issuance on December 5, 2023. In addition, all outstanding stock options to purchase Old Cartesian common stock were assumed by the Company and converted into stock options to purchase (i) shares of the Company’s common stock or (ii) shares of the Company’s Series A Preferred Stock on terms substantially identical to those in effect prior to Merger Agreement, except for adjustments to the underlying number of shares and the exercise price based on the Merger Agreement exchange ratio. Pursuant to the Merger Agreement, the Company agreed to hold a stockholders’ meeting to submit the following proposals to a vote of its stockholders: (i) the approval of the conversion of shares of Series A Preferred Stock into shares of common stock, or the Conversion Proposal, and (ii) either or both of (A) the approval of an amendment to the Company’s restated certificate of incorporation, as amended, or the Charter, to increase the number of shares of common stock authorized under the Charter and (B) the approval of an amendment to the Charter to effect a reverse stock split of all outstanding shares of common stock, in either case (A) or (B) by a number of authorized shares or at a stock split ratio, as the case may be, sufficient to allow the conversion of all shares of Series A Preferred Stock issued in the Merger. The Company concluded the acquisition resulted in the Company obtaining a controlling financial interest in a VIE in accordance with ASC 810, Consolidation The Company exchanged the right to receive shares of common stock and Series A Preferred Stock for all of the outstanding equity of Old Cartesian. The Company determined the rights to receive shares exchanged in the Merger represent a forward contract. The fair value of the forward contracts was determined based on the fair value of shares of common stock and Series A Preferred Stock underlying the forward contracts as of the acquisition date. The total purchase price consists of the fair value of the forward contracts in addition to a portion of the fair value of options exchanged in the transaction related to prior service. Under the acquisition method, the total purchase price of the acquisition was allocated to the net tangible and identifiable intangible assets acquired and liabilities assumed based on their estimated fair values as of the date of the acquisition. The total fair value of the consideration of $168.5 million as of the Closing Date is summarized as follows (in thousands): Forward contract to issue common stock ​ ​ $ 2,713 Forward contract to issue Series A Preferred Stock ​ ​ 155,308 Stock options allocated to consideration paid ​ ​ 10,444 Total consideration ​ ​ $168,465 The Company recorded the assets acquired and liabilities assumed as of the Closing Date based on the information available at that date. The following table presents the allocation of the purchase price to the estimated fair values of the assets acquired and liabilities assumed as of the Closing Date (in thousands): ​ ​ As of November 13, 2023 Assets acquired: ​ ​ Cash and cash equivalents ​ ​ $ 6,561 Prepaid expenses and other current assets ​ ​ 309 Property and equipment, net ​ ​ 215 Right-of-use asset, net ​ ​ 915 In-process research and development assets ​ ​ 150,600 Goodwill ​ ​ 48,163 ​ ​ $206,763 Liabilities assumed ​ ​ Accrued expenses and other current liabilities ​ ​ $ 2,530 Lease liability ​ ​ $ 292 Lease liability, net of current portion ​ ​ $ 623 Deferred tax liability ​ ​ $ 34,853 ​ ​ $ 38,298 Net assets acquired ​ ​ $168,465 The fair value of IPR&D assets were capitalized as of the Closing Date and will be accounted for as indefinite-lived intangible assets until completion or disposition of the assets or abandonment of the associated research and development efforts. Upon successful completion of the development efforts, the carrying value of the respective IPR&D asset will be amortized over its estimated useful life. Until that time, the IPR&D assets will be subject to impairment testing and will not be amortized. The goodwill recorded related to the Merger is the excess of the fair value of the consideration transferred by the acquirer over the fair value of tangible assets, identifiable intangible assets and assumed liabilities as of the Closing Date and is not deductible for tax purposes. The goodwill balance is primarily attributable to the value of the assembled workforce and deferred tax liabilities associated with the transaction. The following summarizes the Company’s intangible assets acquired in the Merger and their carrying value as of December 31, 2023 (in thousands): ​ ​ Acquisition Date Fair Value ​ ​ Impairment ​ ​ Carrying Value at December 31, 2023 Descartes-08 for MG ​ ​ $ 93,900 ​ ​ $— ​ ​ $ 93,900 Descartes-08 for SLE ​ ​ 56,700 ​ ​ — ​ ​ 56,700 Total in-process research and development assets ​ ​ $150,600 ​ ​ $— ​ ​ $150,600 The fair value of the intangible assets was estimated using the income approach in which the after-tax cash flows were discounted to present value. The cash flows are based on estimates used to price the transaction, and the discount rates applied were benchmarked with reference to the implied rate of return from the transaction model as well as the weighted average cost of capital. For the period from November 13, 2023 to December 31, 2023, Old Cartesian’s revenue and net loss within the consolidated statements of operations and comprehensive (loss) income were $0.0 million and $1.6 million, respectively. The following unaudited pro forma financial information reflects the consolidated results of operations of the Company as if the Merger had taken place on January 1, 2022. The unaudited pro forma financial information is not necessarily indicative of the results of operations as they would have been had the transactions been effected on the assumed date (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 Revenue ​ ​ $ 26,004 ​ ​ $112,226 Net (loss) income ​ ​ $(232,259) ​ ​ $ 29,607 The Company’s transaction costs of $4.9 million were expensed as incurred and included in general and administrative expense in the consolidated statements of operations and comprehensive (loss) income. The forward contract related to the common stock was recorded as additional paid-in capital as the instrument is indexed to the Company’s common stock. The forward contract related to the Series A Preferred Stock was recorded as a liability as the underlying Series A Preferred Stock has a redemption feature that may require the Company to settle the instrument by transferring an asset. The forward contract was measured at fair value through the date of settlement through the issuance of the shares of Series A Preferred Stock on December 5, 2023.

Marketable Securities and Inves

Marketable Securities and Investments (FY)	12 Months Ended
	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Marketable Securities and Investments	Marketable Securities and Investments No marketable securities were held as of December 31, 2023. The following table summarizes the marketable securities held as of December 31, 2022 (in thousands): ​ ​ Amortized cost ​ ​ Unrealized gains ​ ​ Unrealized losses ​ ​ Fair value December 31, 2022 ​ ​ ​ ​ ​ ​ ​ ​ U.S. government agency securities and treasuries ​ ​ $13,566 ​ ​ $— ​ ​ $ (9) ​ ​ $13,557 Corporate bonds ​ ​ $ 1,953 ​ ​ $— ​ ​ $ (2) ​ ​ $ 1,951 Commercial paper ​ ​ 12,656 ​ ​ — ​ ​ — ​ ​ 12,656 Total ​ ​ $28,175 ​ ​ $— ​ ​ $(11) ​ ​ $28,164 Investments As of December 31, 2023 and 2022, the Company has a $2.0 million investment in Cyrus Biotechnology, Inc., or Cyrus, pursuant to the Company's Collaboration and License Agreement with Cyrus, or the Cyrus Agreement. The Company’s maximum exposure to loss related to this VIE is limited to the carrying value of the investment. See Note 16 for details.

Net (Loss) Income Per Share (FY

Net (Loss) Income Per Share (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Earnings Per Share [Abstract]
Net (Loss) Income Per Share	Net Income (Loss) Per Share Allocable to Common Stockholders The following table sets forth the computation of basic and diluted net income (loss) per share allocable to common stockholders for the three and six months ended June 30, 2024 and 2023 (in thousands, except share and per-share data): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Numerator: ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ ​ $ 13,836 ​ ​ $ (11,387) ​ ​ $ (42,988) ​ ​ $ (33,050) Less: Undistributed earnings allocable to participating securities ​ ​ (4,208) ​ ​ — ​ ​ — ​ ​ — Net income (loss) allocable to shares of common stock - basic and diluted ​ ​ $ 9,628 ​ ​ $ (11,387) ​ ​ $ (42,988) ​ ​ $ (33,050) Denominator: ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ ​ 16,723,479 ​ ​ 5,114,747 ​ ​ 11,068,749 ​ ​ 5,113,213 Dilutive effect of employee equity incentive plans ​ ​ 1,067,664 ​ ​ — ​ ​ — ​ ​ — Weighted-average common shares outstanding - diluted ​ ​ 17,791,143 ​ ​ 5,114,747 ​ ​ 11,068,749 ​ ​ 5,113,213 Net income (loss) per share allocable to common stockholders: ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ ​ $ 0.58 ​ ​ $ (2.23) ​ ​ $ (3.88) ​ ​ $ (6.46) Diluted ​ ​ $ 0.54 ​ ​ $ (2.23) ​ ​ $ (3.88) ​ ​ $ (6.46) The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net income (loss) per share allocable to common stockholders for all periods presented, as the effect would have been anti-dilutive: ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Common stock options, restricted stock units and ESPP shares ​ ​ 740,211 ​ ​ 753,590 ​ ​ 2,344,017 ​ ​ 753,590 Warrants to purchase common stock ​ ​ 975,132 ​ ​ 1,040,943 ​ ​ 975,132 ​ ​ 1,040,943 Series A Preferred Stock ​ ​ 5,544,719 ​ ​ — ​ ​ 5,544,719 ​ ​ — Total ​ ​ 7,260,062 ​ ​ 1,794,533 ​ ​ 8,863,868 ​ ​ 1,794,533	Net (Loss) Income Per Share The Company reported a net loss for the years ended December 31, 2023 and 2021, and net income for the year ended December 31, 2022. The Company used the treasury stock method to determine the number of dilutive shares. The following table sets forth the computation of basic and diluted net (loss) income per share (in thousands, except share and per-share data): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Numerator: ​ ​ ​ ​ ​ ​ Net (loss) income ​ ​ $(219,710) ​ ​ $ 35,379 ​ ​ $(25,687) Less: CVR distribution to participating securities ​ ​ (37,550) ​ ​ — ​ ​ — Net (loss) income allocable to shares of common stock - basic ​ ​ (257,260) ​ ​ 35,379 ​ ​ (25,687) Less: Change in fair value of warrants ​ ​ — ​ ​ (20,882) ​ ​ — Net (loss) income allocable to shares of common stock - diluted ​ ​ $(257,260) ​ ​ $ 14,497 ​ ​ $(25,687) ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Denominator: ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ ​ 155,109,561 ​ ​ 144,758,555 ​ ​ 114,328,798 Dilutive effect of employee equity incentive plans and outstanding warrants ​ ​ — ​ ​ 1,116,334 ​ ​ — Weighted-average common shares used in per share calculations - diluted ​ ​ 155,109,561 ​ ​ 145,874,889 ​ ​ 114,328,798 Net (loss) income per share: ​ ​ ​ ​ ​ ​ Basic ​ ​ $ (1.66) ​ ​ $ 0.24 ​ ​ $ (0.22) Diluted ​ ​ $ (1.66) ​ ​ $ 0.10 ​ ​ $ (0.22) The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net (loss) income per share for all periods presented, as the effect would have been anti-dilutive: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Warrants to purchase common stock ​ ​ 31,224,703 ​ ​ 213,339 ​ ​ 10,735,980 Series A Preferred Stock ​ ​ 435,120,513 ​ ​ — ​ ​ — Forward contract to issue Series A Preferred Stock ​ ​ 99,140,326 ​ ​ — ​ ​ — Common stock options, RSUs and ESPP shares ​ ​ 23,306,661 ​ ​ 17,800,034 ​ ​ 11,492,002 Series A Preferred Stock options ​ ​ 14,112,299 ​ ​ — ​ ​ — Total ​ ​ 602,904,502 ​ ​ 18,013,373 ​ ​ 22,227,982

Fair Value Measurements (FY)

Fair Value Measurements (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2024 and December 31, 2023 (in thousands): ​ ​ June 30, 2024 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 39,181 ​ ​ $39,181 ​ ​ $— ​ ​ $ — Total assets ​ ​ $ 39,181 ​ ​ $39,181 ​ ​ $— ​ ​ $ — Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 9,260 ​ ​ $ — ​ ​ $— ​ ​ $ 9,260 Contingent value right liability ​ ​ $395,400 ​ ​ $ — ​ ​ $— ​ ​ $395,400 Total liabilities ​ ​ $404,660 ​ ​ $ — ​ ​ $— ​ ​ $404,660 ​ ​ December 31, 2023 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Total assets ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 6,394 ​ ​ $ — ​ ​ $ — ​ ​ $ 6,394 Contingent value right liability ​ ​ $358,600 ​ ​ $ — ​ ​ $ — ​ ​ $358,600 Forward contract liabilities ​ ​ $ 28,307 ​ ​ $ — ​ ​ $28,307 ​ ​ $ — Total liabilities ​ ​ $393,301 ​ ​ $ — ​ ​ $28,307 ​ ​ $364,994 There were no transfers within the fair value hierarchy during the six months ended June 30, 2024 or year ended December 31, 2023. Cash, Cash Equivalents, and Restricted Cash As of June 30, 2024 and December 31, 2023, money market funds were classified as cash and cash equivalents on the accompanying consolidated balance sheets as they mature within 90 days from the date of purchase. As of June 30, 2024, the Company had restricted cash balances relating to secured letters of credit in connection with its real estate leases. Short-term restricted cash is included within prepaid expenses and other current assets in the consolidated balance sheets. The Company’s consolidated statements of cash flows include the following as of June 30, 2024 and 2023 (in thousands): ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Cash and cash equivalents ​ ​ $87,227 ​ ​ $112,027 Short-term restricted cash ​ ​ — ​ ​ 1,600 Long-term restricted cash ​ ​ 1,669 ​ ​ 1,377 Total cash, cash equivalents, and restricted cash ​ ​ $88,896 ​ ​ $115,004 Warrants to Purchase Common Stock In December 2019, the Company issued warrants to purchase common stock in connection with a private placement, or the 2019 Warrants. Pursuant to the terms of the 2019 Warrants, the Company could be required to settle the 2019 Warrants in cash in the event of certain acquisitions of the Company and, as a result, the 2019 Warrants are required to be measured at fair value and reported as a liability on the balance sheet. On December 20, 2022, the Company amended the terms of the outstanding 2019 Warrants held by certain members of the Board of Directors, or the Amended 2019 Warrants, to remove the cash settlement provision. As a result, the Amended 2019 Warrants were remeasured at fair value on December 20, 2022 and reclassified from a liability to equity on the balance sheet. See Note 12 to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023 for further discussion on the equity-classified Amended 2019 Warrants. In April 2022, the Company issued warrants in connection with an underwritten offering, or the 2022 Warrants. Pursuant to the terms of the 2022 Warrants, the Company could be required to settle the 2022 Warrants in cash in the event of an acquisition of the Company under certain circumstances and, as a result, the 2022 Warrants are required to be measured at fair value and reported as a liability on the balance sheet. The Company recorded the fair value of the 2019 Warrants and the 2022 Warrants upon issuance using the Black-Scholes valuation model and is required to revalue the 2019 Warrants and the 2022 Warrants at each reporting date, with any changes in fair value recorded in the statement of operations and comprehensive income (loss). The valuations of the 2019 Warrants and the 2022 Warrants are classified as Level 3 of the fair value hierarchy due to the need to use assumptions in the valuations that are both significant to the fair value measurement and unobservable, including the stock price volatility and the expected life of the 2019 Warrants and the 2022 Warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement. The estimated fair values of the 2019 Warrants and the 2022 Warrants were determined using the following inputs to the Black-Scholes simulation valuation: Estimated fair value of the underlying stock . The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period. Risk-free interest rate . The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption. Dividend rate . The dividend rate is based on the historical rate, which the Company anticipates will remain at zero. Expected life . The expected life of the 2019 Warrants and the 2022 Warrants is assumed to be equivalent to their remaining contractual terms which expire on December 23, 2024 and April 11, 2027, respectively. Volatility . The Company estimates stock price volatility based on the Company’s historical volatility for a period of time commensurate with the expected remaining life of the warrants. A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2019 Warrants liability is as follows: ​ ​ June 30, 2024 ​ ​ December 31, 2023 Risk-free interest rate ​ ​ 5.33% ​ ​ 4.79% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 0.49 ​ ​ 0.98 Expected volatility ​ ​ 110.52% ​ ​ 83.67% A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2022 Warrants liability is as follows: ​ ​ June 30, 2024 ​ ​ December 31, 2023 Risk-free interest rate ​ ​ 4.52% ​ ​ 4.01% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 2.78 ​ ​ 3.28 Expected volatility ​ ​ 87.23% ​ ​ 84.09% The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 11 to these unaudited consolidated financial statements) for the six months ended June 30, 2024 (in thousands): ​ ​ Warrant liabilities Fair value as of December 31, 2023 ​ ​ $6,394 Change in fair value ​ ​ 2,866 Fair value as of June 30, 2024 ​ ​ $9,260 Contingent Value Right On December 6, 2023, as contemplated by the Merger Agreement, the Company entered into a contingent value rights agreement, or the CVR Agreement, pursuant to which each holder of common stock or a 2022 Warrant as of December 4, 2023 was distributed a CVR, issued by the Company for each share of common stock held directly or underlying a 2022 Warrant held by such holder as of December 4, 2023. Holders of warrants other than the 2022 Warrants will be entitled to receive, upon exercise of such warrants and in accordance with the terms of the warrants, one CVR per each share of common stock underlying such warrants. Each CVR entitles its holder to distributions of the following, pro-rated on a per-CVR basis, during the period ending on the date on which the Royalty Term (as defined in the Sobi License) ends, or the Termination Date: • 100% of all milestone payments, royalties and other amounts paid to the Company or its controlled affiliates, or the Company Entities, under the Sobi License or, following certain terminations of the Sobi License, any agreement a Company Entity enters into that provides for the development and commercialization of SEL-212; and • 100% of all cash consideration and the actual liquidation value of any and all non-cash consideration of any kind that is paid to or is actually received by any Company Entity prior to the Termination Date pursuant to an agreement relating to a sale, license, transfer or other disposition of any transferable asset of the Company existing as of immediately prior to the Merger, other than those exclusively licensed under the Sobi License or which the Company Entities are required to continue to own in order to comply with the Sobi License. The distributions in respect of the CVRs will be made on a semi-annual basis, and will be subject to a number of deductions, subject to certain exceptions or limitations, including for (i) certain taxes payable on the proceeds subject to the CVR distribution, (ii) certain out of pocket costs incurred by the Company Entities, including audit and accounting fees incurred in connection with reporting obligations relating to the CVRs and other expenses incurred in the performance of their obligations and other actions under the CVR Agreement, (iii) a fixed semi-annual amount of $0.75 million for general and administrative overhead, (iv) payments made and remaining obligations on lease liabilities of Selecta immediately prior to the Merger and (v) amounts paid and remaining obligations with regard to the Xork product candidate. Each of the deductions described in (iv) and (v) will be made only if certain milestone payments under the Sobi License are made and are also subject to certain adjustments as contemplated in the CVR Agreement. Upon the achievement of a development milestone in June 2024, Sobi became obligated to make a $30.0 million payment to the Company and made such payment in July 2024. The proceeds from this payment, net of deductions specified in the CVR Agreement, is expected to be included in the next scheduled distribution to the holders of the CVR in March 2025. The CVRs represent financial instruments that are accounted for under the fair value option election in ASC 825 , Financial Instruments , or ASC 825. Under the fair value option election, the CVRs are initially measured at the aggregate estimated fair value of the CVRs and will be subsequently remeasured at estimated fair value on a recurring basis at each reporting period date. The liability was recorded at the date of approval, November 13, 2023, as a dividend. The estimated fair value of the CVR liability was determined using a discounted cash flow methodology as of December 31, 2023 and a Monte Carlo simulation model as of June 30, 2024 to estimate future cash flows associated with the legacy assets, including the expected milestone and royalty payments under the Sobi License, net of deductions. Changes in fair value of the CVR liability are presented in the consolidated statements of operations and comprehensive income (loss). The liability value is based on significant inputs not observable in the market such as estimated cash flows, estimated probabilities of success, expected volatility of future revenues (Monte Carlo simulation model) and risk-adjustment discount rates (discounted cash flow methodology), which represent a Level 3 measurement within the fair value hierarchy. The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows: ​ ​ June 30, 2024 Estimated cash flow dates ​ ​ 2024-2038 Estimated probability of success ​ ​ 95.0% - 100.0% Expected volatility of future revenues ​ ​ 22.0% ​ ​ December 31, 2023 Estimated cash flow dates ​ ​ 2024 - 2038 Estimated probability of success ​ ​ 95.0% Risk-adjusted discount rate ​ ​ 13.7% The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the six months ended June 30, 2024 (in thousands): ​ ​ CVR liability Fair value as of December 31, 2023 ​ ​ $358,600 Change in fair value ​ ​ 36,800 Fair value as of June 30, 2024 ​ ​ $395,400 Forward Contract Liabilities Merger Consideration In connection with the Merger, the Company entered into a contract for the issuance of 384,930.724 shares of Series A Preferred Stock as part of the consideration transferred. The fair value of the forward contract at the Closing Date was $155.3 million. The non-cash settlement of this liability occurred on December 5, 2023 with the issuance of the Series A Preferred Stock for $261.8 million. November 2023 Private Placement The Company entered into a contract for the issuance of 149,330.115 shares of Series A Preferred Stock as part of the November 2023 Private Placement which was settled in multiple tranches. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Timothy A. Springer, a member of the Company’s Board of Directors, and TAS Partners LLC, an affiliate of Dr. Springer, represented a forward contract. See Note 10. The initial fair value of the forward contract liability on November 13, 2023 was insignificant as the fair value of the underlying Series A Preferred Stock was equal to the purchase price of the Series A Preferred Stock as agreed upon in the November 2023 Private Placement. Subsequent measurement of the fair value of the forward contract liability was based on the market price of the Company’s common stock, which represented the redemption and conversion value of the Series A Preferred Stock, less the purchase price, on an as-converted basis. The non-cash settlement of a portion of the liability occurred on December 13, 2023 with the issuance of the first tranche of the Series A Preferred Stock for $14.8 million. The non-cash settlement of the remaining second and third tranches occurred on January 12, 2024 and February 11, 2024, respectively, for a total of $35.2 million. The following table presents changes in the forward contract liabilities for the periods presented (in thousands): ​ ​ Forward contract liabilities Fair value as of December 31, 2023 ​ ​ $ 28,307 Settlements ​ ​ (35,197) Change in fair value ​ ​ 6,890 Fair value as of June 30, 2024 ​ ​ $ —	Fair Value Measurements The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands): ​ ​ December 31, 2023 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Total assets ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 6,394 ​ ​ $ — ​ ​ $ — ​ ​ $ 6,394 Contingent value right liability ​ ​ 358,600 ​ ​ — ​ ​ — ​ ​ 358,600 Forward contract liabilities ​ ​ 28,307 ​ ​ — ​ ​ 28,307 ​ ​ — Total liabilities ​ ​ $393,301 ​ ​ $ — ​ ​ $28,307 ​ ​ $364,994 ​ ​ December 31, 2022 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $53,552 ​ ​ $53,552 ​ ​ $ — ​ ​ $— Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ U.S. government agency securities and treasuries ​ ​ 13,557 ​ ​ — ​ ​ 13,557 ​ ​ — Corporate bonds ​ ​ 1,951 ​ ​ — ​ ​ 1,951 ​ ​ — Commercial paper ​ ​ 12,656 ​ ​ — ​ ​ 12,656 ​ ​ — Total assets ​ ​ $81,716 ​ ​ $53,552 ​ ​ $28,164 ​ ​ $— ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $19,140 ​ ​ $— ​ ​ $— ​ ​ $19,140 Total liabilities ​ ​ $19,140 ​ ​ $— ​ ​ $— ​ ​ $19,140 There were no transfers within the fair value hierarchy during the years ended December 31, 2023 or 2022. Cash, Cash Equivalents, and Restricted Cash As of December 31, 2023 and 2022, money market funds were classified as cash and cash equivalents on the accompanying consolidated balance sheets as they mature within 90 days from the date of purchase. As of December 31, 2023, the Company had restricted cash balances relating to a secured letter of credit in connection with its lease for the Company’s prior headquarters (see Note 9 included elsewhere in this Annual Report). Short-term restricted cash is included within prepaid expenses and other current assets in the consolidated balance sheets. The Company’s consolidated statement of cash flows includes the following as of December 31, 2023, 2022 and 2021 (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Cash and cash equivalents ​ ​ $76,911 ​ ​ $106,438 ​ ​ $114,057 Short-term restricted cash ​ ​ — ​ ​ 289 ​ ​ — Long-term restricted cash ​ ​ 1,377 ​ ​ 1,311 ​ ​ 1,379 Total cash, cash equivalents, and restricted cash ​ ​ $78,288 ​ ​ $108,038 ​ ​ $115,436 Marketable Securities No marketable securities were held as of December 31, 2023. Marketable securities held as of December 31, 2022 and classified as Level 2 within the valuation hierarchy consist of U.S. government agency securities and treasuries, corporate bonds and commercial paper. Marketable securities represent holdings of available-for-sale marketable debt securities in accordance with the Company’s investment policy. The Company estimates the fair value of these marketable securities by taking into consideration valuations that include market pricing based on real-time trade data for the same or similar securities, and other observable inputs. The amortized cost of available-for-sale debt securities is adjusted for amortization of premiums and accretion of discounts to the earliest call date for premiums or to maturity for discounts. Warrants to Purchase Common Stock In December 2019, the Company issued warrants to purchase common stock in connection with a private placement, or the 2019 Warrants. Pursuant to the terms of the 2019 Warrants, the Company could be required to settle the 2019 Warrants in cash in the event of certain acquisitions of the Company and, as a result, the common warrants are required to be measured at fair value and reported as a liability on the balance sheet. On December 20, 2022, the Company amended the terms of the outstanding 2019 Warrants held by certain members of its Board of Directors, or the Amended 2019 Warrants, to remove the cash settlement provision. As a result, the Amended 2019 Warrants were remeasured at fair value on December 20, 2022 and reclassified from a liability to equity on the balance sheet. Refer to Note 12 for further discussion on the equity-classified Amended 2019 Warrants. In April 2022, the Company issued warrants in connection with an underwritten offering, or the 2022 Warrants. Pursuant to the terms of the 2022 Warrants, the Company could be required to settle the 2022 Warrants in cash in the event of an acquisition of the Company under certain circumstances and, as a result, the 2022 Warrants are required to be measured at fair value and reported as a liability on the balance sheet. The Company recorded the fair value of the 2019 Warrants and the 2022 Warrants upon issuance using the Black-Scholes valuation model and is required to revalue the 2019 Warrants and the 2022 Warrants at each reporting date, with any changes in fair value recorded in the statement of operations and comprehensive income (loss). The valuations of the 2019 Warrants and the 2022 Warrants are classified as Level 3 of the fair value hierarchy due to the need to use assumptions in the valuations that are both significant to the fair value measurement and unobservable, including the stock price volatility and the expected life of the 2019 Warrants and the 2022 Warrants. Generally, increases (decreases) in the fair value of the underlying stock and estimated term would result in a directionally similar impact to the fair value measurement. The changes in the fair values of the warrants are reflected in the statement of operations and comprehensive income (loss) for the years ended December 31, 2023, 2022 and 2021. The estimated fair values of the 2019 Warrants and the 2022 Warrants were determined using the following inputs to the Black-Scholes simulation valuation: Estimated fair value of the underlying stock . The Company estimates the fair value of the common stock based on the closing stock price at the end of each reporting period. Risk-free interest rate . The risk-free interest rate is based on the U.S. Treasury at the valuation date commensurate with the expected remaining life assumption. Dividend rate . The dividend rate is based on the historical rate, which the Company anticipates will remain at zero. Expected life . The expected life of the 2019 Warrants and the 2022 Warrants is assumed to be equivalent to their remaining contractual terms which expire on December 23, 2024 and April 11, 2027, respectively. Volatility . The Company estimates stock price volatility based on the Company’s historical volatility for a period of time commensurate with the expected remaining life of the warrants. A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2019 Warrants liability is as follows: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Risk-free interest rate ​ ​ 4.79% ​ ​ 4.74% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 0.98 ​ ​ 1.98 Expected volatility ​ ​ 83.67% ​ ​ 79.92% A summary of the Black-Scholes valuation model assumptions used to record the fair value of the 2022 Warrants liability is as follows: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Risk-free interest rate ​ ​ 4.01% ​ ​ 4.22% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 3.28 ​ ​ 4.28 Expected volatility ​ ​ 84.09% ​ ​ 98.05% The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 12), for the year ended December 31, 2023 (in thousands): ​ ​ Warrant liabilities Fair value as of December 31, 2022 ​ ​ $ 19,140 Change in fair value ​ ​ (12,746) Fair value as of December 31, 2023 ​ ​ $ 6,394 Contingent Value Right On December 6, 2023, as contemplated by the Merger Agreement, the Company entered into the CVR Agreement, pursuant to which each holder of common stock as of December 4, 2023 or a 2022 Warrant was distributed a CVR, issued by the Company for each share of common stock held directly or underlying a 2022 Warrant held by such holder as of December 4, 2023. Holders of warrants other than the 2022 Warrants will be entitled to receive, upon exercise of such warrants and in accordance with the terms of the warrants, one CVR per each share of common stock underlying such warrants. Each CVR entitles its holder to distributions of the following, pro-rated on a per-CVR basis, during the period ending on the date on which the Royalty Term (as defined in the Sobi License) ends, or the Termination Date: • 100% of all milestone payments, royalties and other amounts paid to the Company or its controlled affiliates, or the Company Entities, under the Sobi License or, following certain terminations of the Sobi License, any agreement a Company Entity enters into that provides for the development and commercialization of SEL-212; and • 100% of all cash consideration and the actual liquidation value of any and all non-cash consideration of any kind that is paid to or is actually received by any Company Entity prior to the Termination Date pursuant to an agreement relating to a sale, license, transfer or other disposition of any transferable asset of the Company existing as of immediately prior to the Merger, other than those exclusively licensed under the Sobi License or which the Company Entities are required to continue to own in order to comply with the Sobi License. The distributions in respect of the CVRs will be made on a semi-annual basis, and will be subject to a number of deductions, subject to certain exceptions or limitations, including for (i) certain taxes payable on the proceeds subject to the CVR distribution, (ii) certain out of pocket costs incurred by the Company Entities, including audit and accounting fees incurred in connection with reporting obligations relating to the CVRs and other expenses incurred in the performance of their obligations and other actions under the CVR Agreement, (iii) a fixed semi-annual amount of $0.75 million for general and administrative overhead, (iv) payments made and remaining obligations on lease liabilities of Selecta immediately prior to the Merger and (v) amounts paid and remaining obligations with regard to the Xork product candidate. Each of the deductions described in (iv) and (v) will be made only if certain milestone payments under the Sobi License are made and are also subject to certain adjustments as contemplated in the CVR Agreement. The CVRs represent financial instruments that are accounted for under the fair value option election in ASC 825, Financial Instruments The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows: ​ ​ December 31, 2023 ​ ​ At Issuance November 13, 2023 Estimated cash flow dates ​ ​ 2024 - 2038 ​ ​ 2024 - 2038 Estimated probability of success ​ ​ 95.0% ​ ​ 95.0% Risk-adjusted discount rate ​ ​ 13.7% ​ ​ 14.4% The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the year ended December 31, 2023 (in thousands): ​ ​ CVR liability Fair value as of December 31, 2022 ​ ​ $ — Issuances ​ ​ 340,300 Change in fair value ​ ​ 18,300 Fair value as of December 31, 2023 ​ ​ $358,600 Forward Contract Liabilities Merger Consideration In connection with the Merger, the Company entered into a contract for the issuance of 384,930.724 shares of Series A Preferred Stock as part of the consideration transferred. The fair value of the forward contract at the Closing Date (defined below) was $155.3 million. The non-cash settlement of this liability occurred on December 5, 2023 with the issuance of the Series A Preferred Stock for $261.8 million. November 2023 Private Placement The Company entered into a contract for the issuance of 149,330.115 shares of Series A Preferred Stock as part of the November 2023 Private Placement which was settled in multiple tranches. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Timothy A. Springer, a member of the Company’s Board of Directors, and TAS Partners LLC, an affiliate of Dr. Springer, represented a forward contract. See Note 11. The fair value of the forward contract liability on November 13, 2023 was insignificant as the fair value of the underlying Series A Preferred Stock was equal to the purchase price of the Series A Preferred Stock as agreed upon in the November 2023 Private Placement. The non-cash settlement of a portion of the liability occurred on December 13, 2023 with the issuance of the first tranche of the Series A Preferred Stock for $14.8 million. The following table presents changes in the forward contract liabilities for the periods presented (in thousands): ​ ​ Forward contract liabilities Fair value as of December 31, 2022 ​ ​ $ — Issuances ​ ​ 155,308 Settlements ​ ​ (276,601) Change in fair value ​ ​ 149,600 Fair value as of December 31, 2023 ​ ​ $ 28,307

Property and Equipment (FY)

Property and Equipment (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consists of the following (in thousands): ​ ​ June 30, 2024 ​ ​ December 31, 2023 Laboratory equipment ​ ​ $ 6,944 ​ ​ $ 6,280 Computer equipment and software ​ ​ 621 ​ ​ 702 Leasehold improvements ​ ​ 61 ​ ​ 61 Furniture and fixtures ​ ​ 462 ​ ​ 452 Office equipment ​ ​ 196 ​ ​ 196 Construction in process ​ ​ 4,417 ​ ​ 150 Total property and equipment ​ ​ 12,701 ​ ​ 7,841 Less: Accumulated depreciation ​ ​ (6,029) ​ ​ (5,728) Property and equipment, net ​ ​ $ 6,672 ​ ​ $ 2,113 Depreciation expense was $0.2 million for each of the three months ended June 30, 2024 and 2023, and $0.4 million for each of the six months ended June 30, 2024 and 2023.	Property and Equipment Property and equipment consists of the following (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Laboratory equipment ​ ​ $ 6,280 ​ ​ $ 6,001 Computer equipment and software ​ ​ 702 ​ ​ 697 Leasehold improvements ​ ​ 61 ​ ​ 57 Furniture and fixtures ​ ​ 452 ​ ​ 453 Office equipment ​ ​ 196 ​ ​ 192 Construction in process ​ ​ 150 ​ ​ 599 Total property and equipment ​ ​ 7,841 ​ ​ 7,999 Less accumulated depreciation ​ ​ (5,728) ​ ​ (5,205) Property and equipment, net ​ ​ $ 2,113 ​ ​ $ 2,794 Depreciation expense was $0.7 million, $0.7 million and $0.6 million for the years ended December 31, 2023, 2022 and 2021, respectively.

Accrued Expenses (FY)

Accrued Expenses (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): ​ ​ June 30, 2024 ​ ​ December 31, 2023 Payroll and employee related expenses ​ ​ $ 1,402 ​ ​ $ 4,390 Accrued patent fees ​ ​ 689 ​ ​ 472 Accrued external research and development costs ​ ​ 3,467 ​ ​ 4,896 Accrued professional and consulting services ​ ​ 1,871 ​ ​ 4,331 Property and equipment ​ ​ 2,877 ​ ​ 128 Other ​ ​ 309 ​ ​ 516 Accrued expenses ​ ​ $10,615 ​ ​ $14,733	Accrued Expenses Accrued expenses consist of the following (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Payroll and employee related expenses ​ ​ $ 4,390 ​ ​ $ 4,242 Accrued patent fees ​ ​ 472 ​ ​ 696 Accrued external research and development costs ​ ​ 4,896 ​ ​ 7,274 Accrued professional and consulting services ​ ​ 4,331 ​ ​ 985 Accrued interest ​ ​ — ​ ​ 222 Other ​ ​ 644 ​ ​ 665 Accrued expenses ​ ​ $14,733 ​ ​ $14,084

Leases (FY)

Leases (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Leases [Abstract]
Leases	Leases 65 Grove Street Lease In July 2019, the Company entered into a lease with BRE-BMR Grove LLC for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts, or the Watertown Lease. On September 1, 2022, the Company entered into an amendment to the Watertown Lease, or the Lease Agreement Amendment, to expand the Company’s laboratory and office space located at 65 Grove Street, Watertown, Massachusetts by approximately 7,216 square feet. In connection with the Lease Agreement Amendment, the Company secured a letter of credit for the Watertown Lease from Silicon Valley Bank, a division of First-Citizens Bank & Trust Company (successor by purchase to the Federal Deposit Insurance Corporation as Receiver for Silicon Valley Bridge Bank, N.A. (as successor to Silicon Valley Bank)), or SVB, for $1.6 million as of December 31, 2022. In May 2023, the Company received notice from BRE-BMR Grove LLC that the requirements to reduce the amount of the letter of credit for the Watertown Lease had been met. In connection therewith, in June 2023, the Company secured a letter of credit from JPMorgan Chase Bank, N.A. for $1.4 million, which is recognized as long-term restricted cash as of June 30, 2024 and December 31, 2023, and renews automatically each year. The $1.6 million letter of credit with SVB was released from restriction and returned to the Company on July 17, 2023, and therefore was reclassified into cash and cash equivalents in the consolidated balance sheets. On October 6, 2022, the Company entered into a sublease agreement to sublease 7,216 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts. The sublease commenced on October 24, 2022, and the term expired on March 31, 2024. On October 31, 2023, in connection with entering into Amendment No. 1 to the Sobi License as described in Note 13, the Company entered into a sublease agreement with Sobi to sublease approximately 5,600 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts for which Sobi paid $1.0 million upfront rental payment. The sublease commenced on November 6, 2023, when the Company, Sobi, and BRE-BMR Grove LLC, executed a Consent to Sublease. The term of the sublease expires on November 5, 2024 with no option to extend the sublease term. As of June 30, 2024 and December 31, 2023, deferred rent of $0.3 million and $0.8 million is included within accrued expenses and other current liabilities in the consolidated balance sheets. Sublease income is included within other income, net in the consolidated statements of operations and comprehensive income (loss). During the year ended December 31, 2023, the Company determined that the right-of-use asset related to the operating lease for approximately 7,216 square feet at 65 Grove Street was partially impaired as of November 30, 2023. As a result, the Company recognized a $0.7 million right-of-use asset impairment charge in the fourth quarter of 2023. 704 Quince Orchard Road Leases In connection with the Merger, the Company acquired two operating leases for office and laboratory space in Gaithersburg, Maryland. The leases expire in January 2027 and do not contain any renewal rights. The discount rate of 11.5% was determined based on the Company’s incremental borrowing rate adjusted for the lease term. 7495 New Horizon Way Leases On February 28, 2024, the Company entered into a lease agreement with 7495 RP, LLC, or the Landlord, pursuant to which it agreed to lease from the Landlord the manufacturing space located at 7495 New Horizon Way, Frederick, Maryland, or the Frederick Lease Agreement. The space consists of 19,199 leasable square feet of integrated manufacturing and office space. The lease commenced on May 1, 2024 which was the date the Landlord delivered full possession of the premises to the Company. The Frederick Lease Agreement will terminate approximately 7.2 years following the commencement date. The Company will have one option to extend the term of the Frederick Lease Agreement for a period of five years at a cost of 100% of the then-fair market value, not to exceed 103% of the then-current base rent. Base rent, which was due beginning on July 1, 2024, is $0.9 million annually and is subject to an annual upward adjustment of 3% of the then-current rental rate. In addition, the Company is obligated to pay its share of operating costs and taxes related to the property. The Company paid the first month’s rent of $0.1 million upon execution of the Frederick Lease Agreement. The Company assessed the classification of the lease at the commencement date and concluded it should be accounted for as an operating lease. The Company recorded a lease liability and right-of-use asset of $3.6 million and $3.7 million, respectively, on the commencement date. The Frederick Lease Agreement includes a tenant improvement allowance of up to $0.7 million which was recognized as a reduction in the right-of-use asset and lease liability at the commencement date as the Company was reasonably certain to incur reimbursable costs related to alterations equal to or exceeding the amount. Additionally, the prepaid rent was included as an adjustment to the right-of-use asset. The discount rate of 14% was determined based on the Company’s incremental borrowing rate adjusted for the lease term, including any reasonably certain renewal periods. Effective May 7, 2024, the Company and the Landlord entered into the first amendment to the Frederick Lease Agreement, or the Amended Frederick Lease Agreement, providing for the expansion of the premises leased pursuant to the Frederick Lease Agreement by approximately 7,842 square feet. In connection with the expansion of the leased premises, the Company is obligated to pay $0.3 million in additional annual base rent for the first year of the term, which is subject to an annual upward adjustment of 3% of the then-current rental rate, as well as its share of operating costs and taxes. The lease commenced on May 7, 2024 which was the date the Landlord delivered full possession of the premises to the Company and will be coterminous with the Frederick Lease Agreement. The rent commencement date is expected to be September 1, 2024. The Company assessed the classification of the lease at the commencement date and concluded it should be accounted for as an operating lease. The Company recorded a lease liability and right-of-use asset each of $1.2 million on the commencement date. The Amended Frederick Lease Agreement includes a tenant improvement allowance of up to $0.1 million which was recognized as a reduction in the right-of-use asset and lease liability at the commencement date as the Company was reasonably certain to incur reimbursable costs related to alterations equal to or exceeding the amount. The discount rate of 14% was determined based on the Company’s incremental borrowing rate adjusted for the lease term. The Company secured a letter of credit from SVB for $0.3 million for the Frederick Lease Agreement and the Amended Frederick Lease Agreement, which is recognized as long-term restricted cash as of June 30, 2024, and renews automatically each year. For the three and six months ended June 30, 2024 and 2023, the components of lease costs were as follows (in thousands): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 ​ Operating lease cost ​ ​ $ 956 ​ ​ $ 696 ​ ​ $1,731 ​ ​ $1,392 ​ Variable lease cost ​ ​ 352 ​ ​ 270 ​ ​ 749 ​ ​ 412 ​ Short-term lease cost ​ ​ 1 ​ ​ 2 ​ ​ 4 ​ ​ 5 ​ Less: Sublease income ​ ​ (250 ) ​ ​ (251 ) ​ ​ (760 ) ​ ​ (506 ) ​ Total lease cost ​ ​ $1,059 ​ ​ $ 717 ​ ​ $1,724 ​ ​ $1,303 ​ The maturity of the Company’s operating lease liabilities as of June 30, 2024 were as follows (in thousands): ​ ​ June 30, 2024 2024 (remainder) ​ ​ $ 1,239 2025 ​ ​ 4,350 2026 ​ ​ 4,471 2027 ​ ​ 4,276 2028 ​ ​ 2,243 Thereafter ​ ​ 3,409 Total future minimum lease payments ​ ​ 19,988 Less: Imputed interest ​ ​ 5,121 Total operating lease liabilities ​ ​ $14,867 The supplemental disclosure for the statement of cash flows related to operating leases was as follows (in thousands): ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ $1,604 ​ ​ $1,319 Other than the initial recording of the right-of-use assets and lease liabilities for the Frederick Lease Agreement and Amended Frederick Lease Agreement, which were non-cash, the changes in the Company’s right-of-use assets and lease liabilities for the six months ended June 30, 2024 and 2023 are reflected in the non-cash lease expense and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows. The following summarizes additional information related to operating leases: ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Weighted-average remaining lease term ​ ​ 4.9 years ​ ​ 4.9 years Weighted-average discount rate ​ ​ 11.5% ​ ​ 9.7%	Leases 65 Grove Street Lease In July 2019, the Company entered into a lease with BRE-BMR Grove LLC for 25,078 square feet of laboratory and office space located at 65 Grove Street, Watertown, Massachusetts, or the Watertown Lease. As part of the Watertown Lease, the Company incurred $0.8 million in non-reimbursable construction costs. The lease began in March 2020, when the Company took control of the office space, and the lease term is 8 years. The discount rate of 8.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term, including any reasonably certain renewal periods. In connection with the Watertown Lease, the Company secured a letter of credit from Silicon Valley Bank, a division of First-Citizens Bank & Trust Company (successor by purchase to the Federal Deposit Insurance Corporation as Receiver for Silicon Valley Bridge Bank, N.A. (as successor to Silicon Valley Bank)), or SVB, for $1.6 million, of which $0.3 million is recognized as short-term restricted cash and $1.3 million is recognized as long-term restricted cash, as of December 31, 2022. On September 1, 2022, the Company entered into an amendment, or the Lease Agreement Amendment, to its lease agreement with BRE-BMR Grove LLC, originally entered into on July 23, 2019, or the Lease Agreement, to expand the Company’s laboratory and office space located at 65 Grove Street, Watertown, Massachusetts by 7,216 square feet. The lease term began on September 1, 2022, consistent with when the Company took control of the office space and the expected lease term is 5.7 years. The discount rate of 11.3% was determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods. Rent payments began in November 2022, and the base rent for the first year is $0.1 million per month. The Company recorded the right-of-use asset and operating lease liabilities of $3.2 million during the year ended December 31, 2022 as control of the premises was transferred to the Company. On October 6, 2022, the Company entered into a sublease agreement to sublease 7,216 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts. The sublease commenced on October 24, 2022, when the Company, the sublessee and BRE-BMR Grove LLC, executed a Consent to Sublease. The term of the sublease expires on March 31, 2024 with no option to extend the sublease term. Sublease income is included within other income, net in the consolidated statements of operations and comprehensive income (loss). As a result of the sublease agreement and Consent to Sublease, rent payments to BRE-BMR Grove LLC for the lease of the office space increased. The change of consideration in the contract was accounted for as a lease modification and the right-of-use asset and lease liability were remeasured at the modification date of October 24, 2022. The discount rate of 11.9% was determined based on the Company’s incremental borrowing rate adjusted for the lease term including any reasonably certain renewal periods as of October 24, 2022, resulting in a decrease of less than $0.1 million to both the right-of-use asset and lease liabilities. In May 2023, the Company received notice from BRE-BMR Grove LLC that the requirements to reduce the amount of the letter of credit for the Watertown Lease had been met. In connection therewith, in June 2023, the Company secured a letter of credit from JPMorgan Chase Bank, N.A. for $1.4 million, which is recognized as long-term restricted cash as of December 31, 2023, and renews automatically each year. The $1.6 million letter of credit with SVB was released from restriction and returned to the Company on July 17, 2023, and therefore was reclassified into cash and cash equivalents in the consolidated balance sheets. On October 31, 2023, in connection with entering into Amendment No. 1 to the License and Development Agreement with Sobi as described in Note 14, the Company entered into a sublease agreement with Sobi to sublease approximately 5,600 square feet of space currently rented by the Company at 65 Grove Street, Watertown, Massachusetts for which Sobi paid $1.0 million upfront rental payment. The sublease commenced on November 6, 2023, when the Company, Sobi, and BRE-BMR Grove LLC, executed a Consent to Sublease. The term of the sublease expires on November 5, 2024 with no option to extend the sublease term. As of December 31, 2023, deferred rent of $0.8 million is included within accrued expenses and other current liabilities in the consolidated balance sheets. During the year ended December 31, 2023, the Company determined that the right-of-use asset related to the operating lease for approximately 7,216 square feet at 65 Grove Street was partially impaired as of November 30, 2023. As a result, the Company recognized a $0.7 million right-of-use asset impairment charge with $0.6 million and $0.1 million recognized in research and development and general and administrative operating expense categories, respectively, on its consolidated statements of operations and comprehensive income (loss) during the year ended December 31, 2023. 704 Quince Orchard Road Leases In connection with the Merger, the Company acquired two operating leases for office and laboratory space in Gaithersburg, Maryland. The leases expire in January 2027 and do not contain any renewal rights. The discount rate of 11.5% was determined based on the Company’s incremental borrowing rate adjusted for the lease term. Moscow, Russia Lease The Company has a month-to-month facility agreement for Selecta (RUS)'s Moscow, Russia office. Rent expense is recognized as incurred. Rent expense for the years ended December 31, 2023, 2022 and 2021 was $3.8 million, $3.2 million, and $2.9 million, respectively. For the years ended December 31, 2023, 2022 and 2021, the components of lease costs were as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Operating lease cost ​ ​ $ 2,828 ​ ​ $2,276 ​ ​ $2,023 Variable lease cost ​ ​ 965 ​ ​ 910 ​ ​ 834 Short-term lease cost ​ ​ 8 ​ ​ 11 ​ ​ 10 Less sublease income ​ ​ (1,172) ​ ​ (176) ​ ​ — Total lease cost ​ ​ $ 2,629 ​ ​ $3,021 ​ ​ $2,867 The maturity of the Company’s operating lease liabilities as of December 31, 2023 were as follows (in thousands): ​ ​ December 31, 2023 2024 ​ ​ $ 3,077 2025 ​ ​ 3,164 2026 ​ ​ 3,248 2027 ​ ​ 3,017 2028 ​ ​ 946 Thereafter ​ ​ — Total future minimum lease payments ​ ​ 13,452 Less: Imputed interest ​ ​ 2,497 Total operating lease liabilities ​ ​ $10,955 The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ $2,696 ​ ​ $2,048 Other than the initial recording and modification of the right-of-use asset and lease liability for the Watertown Lease during the year ended December 31, 2022 and the impairment on the right-of-use asset for the Watertown Lease and the assumption of the right-of-use assets and lease liabilities in connection with the Merger during the year ended December 31, 2023, which were non-cash, the changes in the Company’s right-of-use asset and lease liability for the years ended December 31, 2023 and 2022 are reflected in the non-cash lease expense and accrued expenses and other liabilities, respectively, in the consolidated statements of cash flows. The following summarizes additional information related to operating leases: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Weighted-average remaining lease term ​ ​ 4.3 years ​ ​ 5.4 years Weighted-average discount rate ​ ​ 9.9 % ​ ​ 9.7 %

Debt (FY)

Debt (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Debt Disclosure [Abstract]
Debt	Debt 2020 Term Loan On August 31, 2020, the Company entered into a Loan and Security Agreement with Oxford Finance LLC, or Oxford, and Silicon Valley Bank, or the Loan and Security Agreement, and such facility, the 2020 Term Loan. On March 10, 2023, Silicon Valley Bank was closed by the California Department of Financial Protection and Innovation, and the Federal Deposit Insurance Corporation, or the FDIC, was appointed as receiver. On March 13, 2023, the FDIC announced that all of Silicon Valley Bank’s deposits and substantially all of its assets had been transferred to a newly created, full-service, FDIC-operated bridge bank, Silicon Valley Bridge Bank, N.A., or SVBB. SVBB assumed all loans that were previously held by Silicon Valley Bank. On March 27, 2023, First-Citizens Bank & Trust Company assumed all of SVBB’s customer deposits and certain other liabilities and acquired substantially all of SVBB’s loans and certain other assets from the FDIC, including the 2020 Term Loan. On September 11, 2023, the Company entered into a payoff letter with Oxford and SVB, pursuant to which the Company paid all outstanding amounts under the 2020 Term Loan, together with accrued interest and a prepayment penalty, resulting in the full extinguishment of the 2020 Term Loan. The total payoff amount was $22.3 million, consisting of the remaining principal amount due of $19.8 million, the final payment fee of $2.3 million, the prepayment penalty of $0.2 million, and less than $0.1 million of accrued interest. During the third quarter of 2023, the Company recorded a loss of $0.7 million on the extinguishment of the 2020 Term Loan, consisting of the prepayment penalty of $0.2 million and the write-off of $0.5 million of unamortized debt issuance costs and venture debt termination fee. As of June 30, 2024 and December 31, 2023, the Company had no outstanding borrowings.	Debt 2020 Term Loan On August 31, 2020, the Company entered into a Loan and Security Agreement with Oxford Finance LLC, or Oxford, and Silicon Valley Bank, or the Loan and Security Agreement, and such facility, the 2020 Term Loan. On March 10, 2023, Silicon Valley Bank was closed by the California Department of Financial Protection and Innovation, and the Federal Deposit Insurance Corporation, or the FDIC, was appointed as receiver. On March 13, 2023, the FDIC announced that all of Silicon Valley Bank’s deposits and substantially all of its assets had been transferred to a newly created, full-service, FDIC-operated bridge bank, Silicon Valley Bridge Bank, N.A., or SVBB. SVBB assumed all loans that were previously held by Silicon Valley Bank. On March 27, 2023, First-Citizens Bank & Trust Company assumed all of SVBB’s customer deposits and certain other liabilities and acquired substantially all of SVBB’s loans and certain other assets from the FDIC, including the 2020 Term Loan. On September 11, 2023, the Company entered into a payoff letter with Oxford and SVB, pursuant to which the Company paid all outstanding amounts under the 2020 Term Loan, together with accrued interest and a prepayment penalty, resulting in the full extinguishment of the 2020 Term Loan. The total payoff amount was $22.3 million, consisting of the remaining principal amount due of $19.8 million, the final payment fee of $2.3 million, the prepayment penalty of $0.2 million, and less than $0.1 million of accrued interest. During the year ended December 31, 2023, the Company recorded a loss of $0.7 million on the extinguishment of the 2020 Term Loan, consisting of the prepayment penalty of $0.2 million and the write-off of $0.5 million of unamortized debt issuance costs and venture debt termination fee, which was included within interest expense in the consolidated statements of operations and comprehensive income (loss). As of December 31, 2023, the Company had no outstanding borrowings, and as of December 31, 2022, the outstanding principal balance under the 2020 Term Loan was $25.0 million. During the years ended December 31, 2023, 2022 and 2021, the Company recognized $2.1 million, $3.0 million and $2.8 million respectively of interest expense related to the 2020 Term Loan.

Series A Preferred Stock (FY)

Series A Preferred Stock (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Temporary Equity Disclosure [Abstract]
Series A Preferred Stock	Series A Preferred Stock The Certificate of Designation of Preferences, Rights, and Limitations of the Series A Non-Voting Convertible Preferred Stock, or the Certificate of Designation, was filed on November 13, 2023, which provided for the designation of shares of the Series A Preferred Stock and authorized the issuance of 548,375 shares of Series A Preferred Stock. Additionally on November 13, 2023, the Company entered into the Securities Purchase Agreement with (i) Dr. Timothy A. Springer, a member of the Company’s Board of Directors; (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, a co-founder and the former chief executive officer of Old Cartesian, who joined the Company’s Board of Directors effective immediately after the effective time of the Merger, or the Investors. Pursuant to the Securities Purchase Agreement, the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million in the November 2023 Private Placement. In the November 2023 Private Placement, Dr. Timothy A. Springer agreed to settle his purchases in three tranches of shares of Series A Preferred Stock, the first for a purchase price of $10.0 million and each thereafter for a purchase price of approximately $20.0 million, with the three tranches settling 30, 60, and 90 days, respectively, following the Closing Date. TAS Partners LLC agreed to settle its purchase for approximately $10.0 million within 30 days following the Closing Date. The first, second and third tranches were settled on December 13, 2023, January 12, 2024 and February 11, 2024, respectively, under which (i) 24,785.081 shares of Series A Preferred Stock were issued to each of TAS Partners LLC and Dr. Timothy A. Springer in the first tranche, (ii) 49,570.163 shares of Series A Preferred Stock were issued to Dr. Timothy A. Springer in the second tranche, and (iii) 49,570.163 shares of Series A Preferred Stock were issued to Dr. Timothy A. Springer in the third tranche. On November 15, 2023, the Company issued 619.627 shares of Series A Preferred Stock to Seven One Eight Three Four Irrevocable Trust for $0.25 million. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Springer and TAS Partners LLC represented a forward contract and was accounted for as a liability with changes in fair value recorded in earnings. A portion of the liability was settled with the initial issuance of 49,570.162 shares of Series A Preferred Stock on December 13, 2023. The remaining portion of the forward contract liability was settled upon the issuance of 49,570.163 shares of Series A Preferred Stock each on January 12, 2024 and February 11, 2024, respectively (see Note 5). On December 5, 2023, the Company issued 384,930.724 shares of Series A Preferred Stock as part of its consideration transferred in connection with the Merger which settled the related forward contract liability (see Note 5). On March 26, 2024, the Company, with the consent of the requisite holders of Series A Preferred Stock, amended the Certificate of Designation such that the automatic conversion of the Series A Preferred Stock into common stock, or the Automatic Conversion, would occur eight business days following stockholder approval of the Conversion Proposal. Upon such date, each share of Series A Preferred Stock automatically converted into 33.333 shares of common stock, subject to certain limitations, including that a holder of Series A Preferred Stock is prohibited from converting shares of Series A Preferred Stock into shares of common stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than a specified percentage (to be established by the holder between 0% and 19.9%) of the total number of shares of common stock issued and outstanding immediately after giving effect to such conversion; provided, however, that such beneficial ownership limitation does not apply to Dr. Springer, TAS Partners LLC, or any of their respective affiliates. Each share of Series A Preferred Stock outstanding that was not automatically converted into common stock as a result of the beneficial ownership limitation is convertible at any time at the option of the holder, only to the extent the beneficial ownership limitation does not apply to the shares of Series A Preferred Stock to be converted. On March 27, 2024, the Company’s stockholders approved the Conversion Proposal, among other matters, at the Special Meeting. As a result of the approval of the Conversion Proposal, all conditions that could have required cash redemption of the Series A Preferred Stock were satisfied. Since the Series A Preferred Stock is no longer redeemable, the associated balances of the Series A Preferred Stock were reclassified from mezzanine equity to permanent equity during the first quarter of 2024. On April 8, 2024, pursuant to the terms of the Certificate of Designation, as amended, 367,919.247 shares of Series A Preferred Stock automatically converted into 12,263,951 shares of common stock, including the non-cash reclassification of an amount equal to the increase in par value of common stock from additional paid-in capital; 166,341.592 shares of Series A Preferred Stock did not automatically convert due to beneficial ownership limitations. As of June 30, 2024, the Company had 166,341.592 shares of Series A Preferred Stock issued and outstanding, which are convertible into 5,544,719 shares of common stock.	Series A Preferred Stock The Certificate of Designation was filed on November 13, 2023, which provided for the designation of shares of the Series A Preferred Stock and authorized the issuance of 548,375 shares of Series A Preferred Stock. Additionally on November 13, 2023, the Company entered into the Securities Purchase Agreement with (i) Dr. Timothy A. Springer, a member of the Company’s Board of Directors; (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, a co-founder and the former chief executive officer of Old Cartesian, who joined the Company’s Board of Directors effective immediately after the effective time of the Merger, or the Investors. Pursuant to the Securities Purchase Agreement, the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million in the November 2023 Private Placement. In the November 2023 Private Placement Dr. Timothy A. Springer agreed to settle his purchases in three tranches of shares of Series A Preferred Stock, the first for a purchase price of $10.0 million and each thereafter for a purchase price of approximately $20.0 million, with the three tranches settling 30, 60, and 90 days, respectively, following the Closing Date. TAS Partners LLC agreed to settle its purchase for approximately $10.0 million within 30 days following the Closing Date. The first, second and third tranches were settled on December 13, 2023, January 12, 2024 and February 11, 2024, respectively, under which (i) 24,785.081 shares of Series A Preferred Stock were issued to each of TAS Partners LLC and Dr. Timothy A. Springer in the first tranche, (ii) 49,570.163 shares of Series A Preferred Stock were issued to Dr. Timothy A. Springer in the second tranche, and (iii) 49,570.163 shares of Series A Preferred Stock were issued to Dr. Timothy A. Springer in the third tranche. On November 15, 2023, the Company issued 619.627 shares of Series A Preferred Stock to Seven One Eight Three Four Irrevocable Trust for $0.25 million. The Company determined the obligation to issue 148,710.488 shares of Series A Preferred Stock to Dr. Springer and TAS Partners LLC represented a forward contract and was accounted for as a liability with changes in fair value recorded in earnings. A portion of the liability was settled with the initial issuance of 49,570.162 shares of Series A Preferred Stock on December 13, 2023 (see Note 6). On December 5, 2023, the Company issued 384,930.724 shares of Series A Preferred Stock as part of its consideration transferred in connection with the Merger which settled the related forward contract liability (see Note 6). As of December 31, 2023, the Company had 435,120.513 shares of Series A Preferred Stock issued and outstanding. In accordance with the guidance in ASC 480, Distinguishing Liabilities from Equity The Series A Preferred Stock had the following rights and preferences as of December 31, 2023: Conversion Prior to the stockholder approval of the Conversion Proposal, the Series A Preferred Shares are not convertible. Following the stockholder approval of the Conversion Proposal, each share of Series A Preferred Stock will automatically convert into 1,000 shares of common stock, subject to certain limitations, including that a holder of Series A Preferred Stock is prohibited from converting shares of Series A Preferred Stock into shares of common stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than a specified percentage (to be established by the holder between 0% and 19.9%) of the total number of shares of common stock issued and outstanding immediately after giving effect to such conversion; provided, however, that such beneficial ownership limitation does not apply to Dr. Springer, TAS Partners LLC, or any of their respective affiliates. Each share of Series A Preferred Stock outstanding that is not otherwise automatically converted into common stock as a result of the beneficial ownership limitation shall be convertible at any time at the option of the holder following stockholder approval of the Conversion Proposal, only to the extent the beneficial ownership limitation does not apply to the shares of Series A Preferred Stock to be converted. Redemption Each share of Series A Preferred Stock will be redeemable at the option of the holder at any time following the date that is 18 months after the initial issuance date of the Series A Preferred Stock, other than any shares of Series A Preferred Stock that would not be convertible into shares of common stock as a result of the beneficial ownership limitation referred to above. The amount payable upon redemption will be equal to the average closing sale price of the common stock listed over the ten consecutive trading days ending on, and including, the day immediately prior to the redemption date multiplied by the number of shares of common stock the Series A Preferred Stock would be convertible into. Dividends Holders of Series A Preferred Stock are entitled to receive dividends on shares of Series A Preferred Stock on an as-converted basis equal to the dividends paid on shares of the common stock; provided, however, that holders of Series A Preferred Stock (or any shares of common stock into which the Series A Preferred Stock are convertible) are not entitled to any CVRs or any amounts paid under the CVR Agreement. Voting Except as otherwise required by law, the Series A Preferred Stock does not have voting rights. However, as long as any shares of Series A Preferred Stock are outstanding, the Company will not, without the affirmative vote of the holders of a majority of the then-outstanding shares of the Series A Preferred Stock, (a) alter or change adversely the powers, preferences or rights given to the Series A Preferred Stock, (b) alter or amend the Certificate of Designation, (c) amend the Charter or other organizational documents in any manner that adversely affects any rights of the holders of Series A Preferred Stock, (d) issue further shares of Series A Preferred Stock (other than in connection with the exercise of the stock options to purchase Series A Preferred Stock) or increase or decrease (other than by conversion) the number of authorized shares of Series A Preferred Stock, (e) prior to the stockholder approval of the Conversion Proposal or at any time while at least 30% of the originally issued Series A Preferred Stock remains issued and outstanding, consummate either (A) a Fundamental Transaction (as defined in the Certificate of Designation) or (B) any merger or consolidation of the Company or other business combination in which the stockholders of the Company immediately before such transaction do not hold at least a majority of the capital stock of the Company immediately after such transaction, (f) amend or fail to comply with, in any manner that would be reasonably likely to prevent, impede or materially delay the conversion (or the stockholder approval thereof), or terminate, any of the stockholder support agreements entered into in connection with the Merger, or the Support Agreements, or agree to any transfer, sale or disposition of such shares subject to the Support Agreements (except for such transfers, sales or dispositions with respect to which the approval of the Company is not required pursuant to the applicable Support Agreement) or (g) enter into any agreement with respect to any of the foregoing. Liquidation The holders of Series A Preferred Stock shall rank on parity with the common stockholders as to distributions of assets upon liquidation, dissolution or winding up of the Company, whether voluntarily or involuntarily. Upon any liquidation, dissolution or winding-up of the Company, whether voluntary or involuntary each holder of Series A Preferred Stock shall be entitled to receive out the assets of the Company equal to of the same amount that a holder of common stock would receive if the Series A Preferred Stock were fully converted, which shall be paid pari passu with holders of common stock, plus an amount equal to any dividends declared but unpaid. If the assets available for distribution are not sufficient to pay the holders of the Series A Preferred Stock pursuant to the preceding sentence, the assets will be distributed ratably to the holders of the Series A Preferred Stock and common stock. Reserved Shares As of December 31, 2023, the Company has authorized shares of Series A Preferred Stock for future issuance as follows: ​ ​ As of December 31, 2023 Shares reserved for issuance in November 2023 Private Placement ​ ​ 99,140.326 Outstanding Series A Preferred Stock options ​ ​ 14,112.299 Total ​ ​ 113,252.625

Equity (FY)

Equity (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Equity [Abstract]
Equity	Equity Equity Financings “At-the-Market” Offerings On October 25, 2021, the Company entered into a sales agreement, or the 2021 Sales Agreement, with Leerink Partners LLC (then known as SVB Leerink LLC), or Leerink Partners, pursuant to which the Company may sell shares of the Company’s common stock, from time to time, through an “at the market” equity offering program under which Leerink Partners will act as sales agent. The shares of common stock sold pursuant to the 2021 Sales Agreement, if any, would be issued and sold pursuant to a registration statement to be filed by the Company with the SEC, for aggregate remaining gross sales proceeds of up to $51.0 million. During the six months ended June 30, 2024 and the year ended December 31, 2023, the Company sold no shares of its common stock pursuant to the 2021 Sales Agreement. Warrants The following is a summary of warrant activity for the six months ended June 30, 2024: ​ ​ Number of Warrants ​ ​ Weighted- average exercise price ​ ​ Equity classified ​ ​ Liability classified ​ ​ Total ​ Outstanding at December 31, 2023 ​ ​ 74,420 ​ ​ 966,393 ​ ​ 1,040,813 ​ ​ $45.98 Exercises ​ ​ (65,681) ​ ​ — ​ ​ (65,681) ​ ​ 43.80 Outstanding at June 30, 2024 ​ ​ 8,739 ​ ​ 966,393 ​ ​ 975,132 ​ ​ $46.12 See Note 14 for further discussion on the exercise of the 65,681 warrants during the six months ended June 30, 2024. See Note 12 to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023 for further discussion of the terms related to the Company’s warrants. Common Stock On April 4, 2024, the Company implemented the Reverse Stock Split. The Reverse Stock Split became effective at 4:30 p.m. Eastern Time on April 4, 2024. On April 5, 2024, the Company’s common stock began trading on The Nasdaq Global Market on a split-adjusted basis under the symbol “RNAC” with a new CUSIP number, 816212302. As a result of the Reverse Stock Split, every 30 shares of common stock outstanding were combined, automatically and without any action on the part of the Company or its stockholders, into one share of common stock. Stockholders entitled to fractional shares as a result of the Reverse Stock Split received a cash payment in lieu of receiving fractional shares. The Reverse Stock Split did not change the number of authorized shares or par value of the Company’s common or preferred stock. Reserved Shares The Company has authorized shares of common stock for future issuance as of June 30, 2024 as follows: Exercise of warrants ​ ​ 975,132 Shares available for future stock incentive awards ​ ​ 3,930,990 Unvested restricted stock units ​ ​ 454,456 Outstanding common stock options ​ ​ 1,889,561 Series A Preferred Stock ​ ​ 5,544,719 Total ​ ​ 12,794,858	Equity Equity Financings Merger On December 5, 2023, the Company issued 6,723,639 shares of common stock as part of its consideration transferred in connection with the Merger which settled the related equity-classified forward contract (see Note 3). Underwritten Offering On April 6, 2022, the Company entered into an underwriting agreement with SVB Securities LLC (now known as Leerink Partners LLC), as representative of the several underwriters named therein, relating to an underwritten offering of 27,428,572 shares of the Company’s common stock and 2022 Warrants to purchase up to 20,571,429 shares of common stock. The offering of such shares and the 2022 Warrants is referred to as the 2022 Offering. Each share and accompanying 2022 Warrant to purchase 0.75 shares of common stock was sold at a combined offering price of $1.41. The exercise price for the 2022 Warrants is $1.55 per share. The Company received net proceeds from the 2022 Offering of approximately $36.9 million. The 2022 Warrants are subject to adjustment in the event of certain stock dividends and distributions, stock splits, stock combinations, reclassifications or similar events affecting the Company’s common stock and also upon any distributions for no consideration of assets to the Company’s stockholders. Each 2022 Warrant is exercisable at any time and from time to time after issuance. In the event of certain corporate transactions, the holders of the 2022 Warrants will be entitled to receive the kind and amount of securities, cash or other property that the holders would have received had they exercised the Warrants immediately prior to such transaction. Therefore, the Company is required to account for the 2022 Warrants as liabilities and record the 2022 Warrants at fair value. The 2022 Warrants do not entitle the holders thereof to any voting rights or any of the other rights or privileges to which holders of Common Stock are entitled. “At-the-Market” Offerings 2020 Sales Agreement On August 6, 2020, the Company entered into a sales agreement, or the 2020 Sales Agreement with Jefferies LLC, as sales agent, pursuant to which the Company was permitted, from time to time, to issue and sell common stock with an aggregate value of up to $50.0 million in an “at the market offering.” On October 8, 2021, the Company delivered notice to Jefferies LLC that the Company was terminating the 2020 Sales Agreement, with effect as of October 19, 2021. 2021 Sales Agreement On October 25, 2021, the Company entered into a Sales Agreement, or the 2021 Sales Agreement, with Leerink Partners LLC (then known as SVB Leerink LLC), or Leerink Partners, pursuant to which the Company may sell shares of the Company’s common stock, from time to time, through an “at the market” equity offering program under which Leerink Partners will act as sales agent. The shares of common stock sold pursuant to the 2021 Sales Agreement, if any, would be issued and sold pursuant to a registration statement to be filed by the Company with the SEC, for aggregate remaining gross sales proceeds of up to $51.0 million. During the year ended December 31, 2023, the Company sold no shares of its common stock pursuant to the 2021 Sales Agreement. During the year ended December 31, 2022, the Company sold 774,544 shares of its common stock pursuant to the 2021 Sales Agreement for aggregate net proceeds of $2.1 million, after deducting commissions and other transaction costs. June 2020 Sobi Stock Purchase On June 11, 2020, the Company entered into a stock purchase agreement with Sobi, pursuant to which the Company sold an aggregate of 5,416,390 shares of its common stock at a purchase price equal to $4.6156 per share, which represented 120% of the 10-day volume-weighted average price of the Company’s common stock prior to signing, for aggregate gross proceeds of $25.0 million, or the Sobi Private Placement. The closing of the Sobi Private Placement occurred on July 31, 2020. In accordance with ASC 815, this forward sale treatment qualified as equity classification as the shares are not within the scope of ASC 480. The gross proceeds of $25.0 million were determined to include a premium to the fair value of the Company’s shares as of July 28, 2020 of approximately $14.5 million. As a result, such amount was included in the transaction price for revenue recognition of the Sobi License. See Note 14 for details. Also on June 11, 2020, the Company entered into a registration rights agreement, as amended by that certain letter agreement, dated as of November 4, 2020, or the Sobi Registration Rights Agreement, with Sobi, pursuant to which the Company agreed to prepare and file a registration statement with respect to the resale of the shares of common stock acquired in the Sobi Private Placement. The Company will be required to file this resale registration statement within 30 days following receipt by the Company of a written request from Sobi to file such resale registration statement, and to have the registration statement declared effective within ten business days after the SEC informs the Company that no review of such resale registration statement will be made or that the SEC has no further comments on such resale registration statement. December 2019 Financing On December 18, 2019, the Company entered into a securities purchase agreement, or the 2019 Purchase Agreement, with a group of institutional investors and certain members of the Board of Directors. Pursuant to the 2019 Purchase Agreement, the Company sold an aggregate of 37,634,883 shares of its common stock at a purchase price of $1.46 per share, warrants to purchase an aggregate of 22,988,501 shares of common stock at a purchase price of $0.125 per share underlying each common warrant, and pre-funded warrants to purchase an aggregate of 8,342,128 shares of common stock at a purchase price of $1.46 per share, all with five year terms, or the 2019 PIPE. The closing of the 2019 PIPE occurred on December 23, 2019. The exercise price of the pre-funded warrants is $0.0001 per share and the exercise price for the common warrants is $1.46 per share. In the event of a certain sale of the Company, the terms of the common warrants require us to make a payment to such common warrant holders based on a Black-Scholes valuation (using variables as specified in the warrants). This provision does not apply to the pre-funded warrants. Therefore, the Company is required to account for the common warrants as liabilities and record them at fair value, while the pre-funded warrants met the criteria to be classified as permanent equity. The Company recorded the fair value of the 2019 Warrants of $40.7 million upon issuance using the Black-Scholes valuation model. Issuance costs were allocated between the equity component with an offset to additional paid-in capital and the liability component recorded as expense on a relative fair value basis. Total net proceeds from the equity offering was $65.6 million, after deducting transaction costs and commissions of $4.4 million. As discussed in Note 6, the Company remeasured the Amended 2019 Warrants at the fair value of $0.8 million on December 20, 2022 and reclassified this amount to additional paid-in capital. The remaining 2019 Warrants liability and the 2022 Warrants liability were revalued as of December 31, 2023 at $6.4 million. During the years ended December 31, 2023, 2022 and 2021, the Company recorded a decrease of $12.7 million and $20.9 million and an increase of $2.3 million, respectively, in the fair value of the warrants in the consolidated statements of operations and comprehensive income (loss). June 2017 Financing In June 2017, the Company entered into a securities purchase agreement, or the Institutional Purchase Agreement, with certain institutional investors and a securities purchase agreement with Timothy A. Springer, Ph.D., a member of the Board of Directors, or the Springer Purchase Agreement, for a private placement of the Company’s securities, or the 2017 PIPE. Pursuant to the Institutional Purchase Agreement, the Company sold an aggregate of 2,750,000 shares of its common stock at a purchase price equal to $16.00 per share. Pursuant to the Springer Purchase Agreement, the Company sold to Dr. Springer an aggregate of 338,791 shares of common stock at a purchase price equal to $17.71 per share, which was equal to the most recent consolidated closing bid price on the Nasdaq Stock Market on June 23, 2017, and warrants to purchase up to 79,130 shares of common stock, or the Warrant Shares, exercisable at $17.71 per Warrant Share, and with a term of five years. The equity-classified warrants expired during the year ended December 31, 2022. Warrants The following is a summary of warrant activity for the years ended December 31, 2023 and 2022: ​ ​ Number of Warrants ​ ​ ​ ​ Equity classified ​ ​ Liability classified ​ ​ Total ​ ​ Weighted average exercise price Outstanding at December 31, 2021 ​ ​ 292,469 ​ ​ 10,443,511 ​ ​ 10,735,980 ​ ​ $ 1.62 Issuance ​ ​ — ​ ​ 20,571,429 ​ ​ 20,571,429 ​ ​ 1.55 Canceled ​ ​ (79,130) ​ ​ — ​ ​ (79,130) ​ ​ $17.71 Reclassification of warrant liability to equity on modification ​ ​ 2,022,987 ​ ​ (2,022,987) ​ ​ — ​ ​ $ 1.46 Outstanding at December 31, 2022 ​ ​ 2,236,326 ​ ​ 28,991,953 ​ ​ 31,228,279 ​ ​ $ 1.53 Canceled ​ ​ (3,576) ​ ​ — ​ ​ (3,576) ​ ​ 16.77 Outstanding at December 31, 2023 ​ ​ 2,232,750 ​ ​ 28,991,953 ​ ​ 31,224,703 ​ ​ $ 1.53 Common Stock As of December 31, 2023, the Company had 350,000,000 shares of common stock authorized for issuance, $0.0001 par value per share, with 161,927,821 shares issued and outstanding. The voting, dividend and liquidation rights of the common stockholders are subject to and qualified by the rights, powers and preferences of the preferred stock. The common stock has the following characteristics: Voting Common stockholders are entitled to one vote for each share of common stock held with respect to all matters voted on by the stockholders of the Company. Dividends Common stockholders are entitled to receive dividends, if and when declared by the Board of Directors. Through December 31, 2023, no cash dividends have been declared or paid on common stock. Liquidation Upon liquidation of the Company, common stockholders are entitled to receive all assets of the Company available for distribution to such stockholders. Reserved Shares The Company has authorized shares of common stock for future issuance as follows: ​ ​ As of December 31, 2023 Exercise of warrants ​ ​ 31,224,703 Shares available for future stock incentive awards ​ ​ 35,836,268 Outstanding common stock options ​ ​ 23,306,661 Total ​ ​ 90,367,632 As described in Note 11, prior to the stockholder approval of the Conversion Proposal, the Series A Preferred Shares are not convertible. Following the stockholder approval of the Conversion Proposal, each share of Series A Preferred Stock will automatically convert into 1,000 shares of common stock.

Stock Incentive Plans (FY)

Stock Incentive Plans (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Stock Incentive Plans	Stock Incentive Plans The Company maintained the 2008 Stock Incentive Plan, or the 2008 Plan, for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board of Directors. In connection with the Merger, all outstanding awards issued under the 2008 Plan were cancelled, and the Board of Directors formally terminated the 2008 Plan. In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 40,341 shares of common stock for future issuance under the 2016 Plan and the Company ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remained subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expired, lapsed or terminated became available under the 2016 Plan as shares available for future grants. Additionally, pursuant to the terms of the 2016 Plan, the Board of Directors is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board of Directors or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board of Directors established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In January 2024, the number of shares of common stock that may be issued under the 2016 Plan was increased by 215,903. In June 2024, the Company’s stockholders approved an amendment and restatement of the 2016 Plan to reserve an additional 3,466,544 shares of its common stock for issuance. As of June 30, 2024, 3,496,828 shares remain available for future issuance under the 2016 Plan. In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan, was adopted by the Board of Directors without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 39,166 shares of its common stock for issuance. In March 2019, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 66,667 shares of the Company’s common stock for issuance thereunder. In December 2023, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 60,833 shares of the Company’s common stock for issuance thereunder. In June 2024, the Board of Directors approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 360,000 shares of the Company’s common stock for issuance thereunder. As of June 30, 2024, there are 364,660 shares available for future grant under the 2018 Inducement Incentive Award Plan. In accordance with the Merger Agreement, the Company assumed Old Cartesian’s 2016 Stock Incentive Plan, or the Old Cartesian Plan. The Old Cartesian Plan permits the granting of options or restricted stock to employees, officers, directors, consultants and advisors to the Company. The unvested common stock options and Series A Preferred Stock options assumed by the Company in connection with the Merger generally vest over a four-year period. Additionally, the stock options granted have a contractual term of ten years and only full shares can be exercised as per the individual award agreements. As of June 30, 2024, there are 23,707 shares available for future grant under the Old Cartesian Plan. In connection with the Merger, the outstanding stock options to purchase Old Cartesian common stock were converted into stock options to purchase 776,865 shares of common stock and 14,112.299 shares of Series A Preferred Stock of the Company. These replacement awards were revalued at their acquisition-date fair value and then attributed to pre and post-combination service. This resulted in $2.6 million attributed to post-combination service to be recognized as stock-based compensation expense over the remaining terms of the replacement awards, of which $0.3 million and $0.7 million was recognized as research and development expense in the consolidated statements of operations and comprehensive income (loss) during the three and six months ended June 30, 2024, respectively. Following the Automatic Conversion, the options exercisable for shares of Series A Preferred Stock became exercisable for shares of common stock. Stock-Based Compensation Expense Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive income (loss), including $1.5 million recognized as stock-based compensation expense upon the achievement of a technical milestone by Ginkgo Bioworks Holdings, Inc., or Ginkgo, during the three and six months ended June 30, 2023 as described in Note 15, was as follows (in thousands): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Research and development ​ ​ $ 776 ​ ​ $2,677 ​ ​ $1,488 ​ ​ $3,869 General and administrative ​ ​ 815 ​ ​ 1,106 ​ ​ 1,534 ​ ​ 2,190 Total stock-based compensation expense ​ ​ $1,591 ​ ​ $3,783 ​ ​ $3,022 ​ ​ $6,059 Stock Options The estimated grant date fair values of stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model based on the following weighted-average assumptions: ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Risk-free interest rate ​ ​ 4.53% ​ ​ 3.38% ​ ​ 4.04% ​ ​ 3.95% Dividend yield ​ ​ — ​ ​ — ​ ​ — ​ ​ — Expected term (in years) ​ ​ 6.18 ​ ​ 6.00 ​ ​ 6.20 ​ ​ 5.94 Expected volatility ​ ​ 93.57% ​ ​ 94.40% ​ ​ 95.09% ​ ​ 94.64% Weighted-average fair value of common stock ​ ​ $22.08 ​ ​ $39.30 ​ ​ $20.14 ​ ​ $34.50 The expected term of the Company's stock options granted has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Under the simplified method, the expected term is presumed to be the midpoint between the vesting date and the end of the contractual term. The Company utilizes this method due to lack of historical exercise data and the plain nature of its stock-based awards. The weighted-average grant date fair value of stock options granted was $17.40 and $30.51 during the three months ended June 30, 2024 and 2023, respectively, and $16.05 and $26.89 during the six months ended June 30, 2024 and 2023, respectively. As of June 30, 2024, total unrecognized compensation expense related to unvested common stock options was $9.9 million, which is expected to be recognized over a weighted average period of 3.0 years. The following table summarizes the stock option activity under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: ​ ​ Number of common stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Outstanding at December 31, 2023 ​ ​ 776,865 ​ ​ $ 2.97 ​ ​ 6.50 ​ ​ $13,760 Granted ​ ​ 761,556 ​ ​ $20.14 ​ ​ ​ ​ Converted from options for Series A Preferred Stock ​ ​ 470,403 ​ ​ $ 2.40 ​ ​ ​ ​ Exercised ​ ​ (89,009) ​ ​ $ 3.08 ​ ​ ​ ​ Forfeited ​ ​ (30,254) ​ ​ $14.84 ​ ​ ​ ​ Outstanding at June 30, 2024 ​ ​ 1,889,561 ​ ​ $ 9.56 ​ ​ 7.24 ​ ​ $33,081 Vested at June 30, 2024 ​ ​ 944,490 ​ ​ $ 2.74 ​ ​ 5.43 ​ ​ $22,925 Vested and expected to vest at June 30, 2024 ​ ​ 1,743,749 ​ ​ $ 8.66 ​ ​ 7.12 ​ ​ $32,070 The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Outstanding at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 Converted to options for common stock ​ ​ (14,112.299) ​ ​ $79.94 ​ ​ ​ ​ Outstanding at June 30, 2024 ​ ​ — ​ ​ $ — ​ ​ ​ As a result of the approval of the Conversion Proposal on March 27, 2024, all conditions that could have required cash redemption of the Series A Preferred Stock underlying the stock options were satisfied. Since the Series A Preferred Stock is no longer redeemable, the associated balances of the stock options to purchase Series A Preferred Stock were reclassified from mezzanine equity to additional paid-in capital during the first quarter of 2024. Following the Automatic Conversion, all options to purchase Series A Preferred Stock were converted into options to purchase common stock with adjustments to the underlying number of shares of common stock determined by multiplying the number of shares of Series A Preferred Stock by 33.333 and rounding down to the nearest whole number of shares and the per-share exercise price by dividing the per-share exercise price of Series A Preferred Stock by 33.333 and rounding the resulting exercise price up to the nearest whole cent. Restricted Stock Units During the six months ended June 30, 2024, the Company granted 471,104 restricted stock unit awards with a weighted-average fair value of $19.80 per share based on the closing price of the Company’s common stock on the date of grant under the 2016 Plan and the Old Cartesian Plan, which generally vest over a four-year term. Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% for restricted stock unit awards based on historical experience. Unrecognized compensation expense related to the restricted stock units was $6.1 million as of June 30, 2024, which is expected to be recognized over a weighted-average period of 3.4 years. The following table summarizes the Company’s restricted stock units under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and the Old Cartesian Plan: ​ ​ Number of shares ​ ​ Weighted-average grant date fair value ($) Unvested at December 31, 2023 ​ ​ — ​ ​ $ — Granted ​ ​ 471,104 ​ ​ 19.80 Forfeited ​ ​ (16,648) ​ ​ 19.80 Unvested at June 30, 2024 ​ ​ 454,456 ​ ​ $19.80 Employee Stock Purchase Plan In June 2016, the Company approved the 2016 Employee Stock Purchase Plan, or the ESPP, which authorized 5,769 shares of common stock for future issuance under the ESPP to participating employees. As of June 30, 2024, 45,795 shares remain available for future issuance under the ESPP. In connection with the Merger, the Board of Directors suspended the offerings under the ESPP. The Company recognized no stock-based compensation expense under the ESPP for the three and six months ended June 30, 2024 and less than $0.1 million stock-based compensation expense under the ESPP for each of the three and six months ended June 30, 2023.	Stock Incentive Plans The Company maintained the 2008 Stock Incentive Plan, or the 2008 Plan, for employees, consultants, advisors, and directors. The 2008 Plan provided for the granting of incentive and non-qualified stock option and restricted stock awards as determined by the Board. In connection with the Merger, all outstanding awards issued under the 2008 Plan were cancelled, and the Board formally terminated the 2008 Plan. In June 2016, the Company’s stockholders approved the 2016 Incentive Award Plan, or the 2016 Plan, which authorized 1,210,256 shares of common stock for future issuance under the 2016 Plan and the Company ceased granting awards under the 2008 Plan. Upon the effective date of the 2016 Plan, awards issued under the 2008 Plan remained subject to the terms of the 2008 Plan. Awards granted under the 2008 Plan that expired, lapsed or terminated became available under the 2016 Plan as shares available for future grants. Additionally, pursuant to the terms of the 2016 Plan, the Board is authorized to grant awards with respect to common stock, and may delegate to a committee of one or more members of the Board or executive officers of the Company the authority to grant options and restricted stock units. On December 9, 2020, the Board established a Stock Option Committee authorized to grant awards to certain employees and consultants subject to conditions and limitations within the 2016 Plan. In January 2023 and 2022, the number of shares of common stock that may be issued under the 2016 Plan was increased by 6,121,697 and 4,944,919 shares, respectively. As of December 31, 2023, 22,504,503 shares remain available for future issuance under the 2016 Plan. In September 2018, the Company’s 2018 Employment Inducement Incentive Award Plan, or the 2018 Inducement Incentive Award Plan was adopted by the Board without stockholder approval pursuant to Rule 5635(c)(4) of the Nasdaq Stock Market LLC listing rules, which authorized 1,175,000 shares of its common stock for issuance. In March 2019, the Board approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 2,000,000 shares of the Company’s common stock for issuance thereunder. In December 2023, the Board approved an amendment and restatement of the 2018 Inducement Incentive Award Plan to reserve an additional 1,825,000 shares of the Company’s common stock for issuance thereunder. As of December 31, 2023, there are 4,500,858 shares available for future grant under the 2018 Inducement Incentive Award Plan. In accordance with the Merger Agreement, the Company assumed Old Cartesian’s 2016 Stock Incentive Plan, or the Old Cartesian Plan. The Old Cartesian Plan permits the granting of options or restricted stock to employees, officers, directors, consultants and advisors to the Company. The unvested common stock options and Series A Preferred Stock options assumed by the Company in connection with the Merger generally vest over a four-year period. Additionally, the stock options granted have a contractual term of ten years and only full shares can be exercised as per the individual award agreements. As of December 31, 2023, there are 3,848,809 shares available for future grant under the Old Cartesian Plan. In connection with the Merger, the outstanding stock options to purchase Old Cartesian common stock were converted into stock options to purchase 23,306,661 shares of common stock and 14,112.299 shares of Series A Preferred Stock of the Company. These replacement awards were revalued at their acquisition-date fair value and then attributed to pre and post-combination service. This resulted in $2.6 million attributed to post-combination service to be recognized as stock-based compensation expense over the remaining terms of the replacement awards, of which $0.2 million was recognized as research and development expense in the consolidated statements of operations and comprehensive (loss) income during the year ended December 31, 2023. Settlement of Equity Compensation Awards Upon consummation of the First Merger, the equity compensation awards of the Company outstanding as of the date of the Merger were settled as follows: (i) each unvested option to acquire shares of common stock and each unvested restricted stock unit award with respect to shares of common stock was accelerated and vested in full at the effective time of the First Merger; (ii) each option to acquire shares of common stock was canceled and in exchange therefore, former holders became entitled to receive an amount in cash equal to the product of (A) the total number of shares of common stock subject to the unexercised portion the stock option (determined after giving effect to the accelerated vesting) multiplied by (B) the excess, if any, of $2.06, or the Cash-out Amount, over the applicable exercise price per share of common stock under such stock option; and (iii) each restricted stock unit award with respect to shares of common stock was cancelled and the former holder of such canceled restricted stock unit became entitled, in exchange therefor, to receive an amount in cash equal to the product of (A) the total number of shares of common stock deliverable under such restricted stock unit (determined after giving effect to the accelerated vesting) multiplied by (B) the Cash-out Amount. Stock options with an exercise price in excess of the Cash-out Amount received no cash payment. The modification to accelerate the vesting of all awards upon the Merger resulted in full recognition of unrecognized compensation of $13.1 million, of which $5.9 million and $7.2 million was classified as research and development expense and general and administrative expense, respectively, in the consolidated statements of operations and comprehensive (loss) income. In addition, with the exception of any options with an exercise price greater than $2.06 per share, all awards were settled in cash for an amount equal to $2.06 less any exercise price associated with the awards. The total cash payment made to the holders of stock options and restricted stock units was $9.4 million. The fair value of the awards prior to the settlement was recorded to additional paid-in capital in an amount of $6.2 million and the amount in excess of fair value was recognized as additional stock-based compensation expense in an amount of $3.2 million, of which $1.5 million and $1.7 million was classified as research and development expense and general and administrative expense, respectively, in the consolidated statements of operations and comprehensive (loss) income. Stock-Based Compensation Expense Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive income (loss), including $1.5 million recognized as stock-based compensation expense upon the achievement of a technical milestone by Ginkgo Bioworks Holdings, Inc., or Ginkgo, during the year ended December 31, 2023 and $1.0 million recognized as stock-based compensation expense upon the issuance of common stock to Ginkgo during the year ended December 31, 2022 as described in Note 16, was as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Research and development ​ ​ $12,985 ​ ​ $ 5,061 ​ ​ $3,204 General and administrative ​ ​ 12,793 ​ ​ 6,133 ​ ​ 4,516 Total stock-based compensation expense ​ ​ $25,778 ​ ​ $11,194 ​ ​ $7,720 Stock Options The fair value of the stock options assumed in connection with the Merger was calculated using a Black-Scholes option pricing model based on the following weighted-average assumptions: ​ ​ Common Stock ​ ​ Series A Preferred Stock Risk-free interest rate ​ ​ 4.83 % ​ ​ 4.92 % Dividend yield ​ ​ — ​ ​ — Expected term ​ ​ 3.59 ​ ​ 3.29 Expected volatility ​ ​ 83.77 % ​ ​ 83.87 % Weighted-average fair value of common stock or Series A Preferred Stock, as applicable ​ ​ $ 0.40 ​ ​ $403.47 The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Risk-free interest rate ​ ​ 3.95 % ​ ​ 2.24 % ​ ​ 0.79 % ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Dividend yield ​ ​ — ​ ​ — ​ ​ — Expected term ​ ​ 5.94 ​ ​ 6.02 ​ ​ 6.03 Expected volatility ​ ​ 94.64 % ​ ​ 92.21 % ​ ​ 95.04 % Weighted-average fair value of common stock ​ ​ $ 1.15 ​ ​ $ 2.63 ​ ​ $ 3.58 The expected term of the Company's stock options granted to employees has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. Under the simplified method, the expected term is presumed to be the midpoint between the vesting date and the end of the contractual term. The Company utilizes this method due to lack of historical exercise data and the plain nature of its stock-based awards. Expected volatilities are based on the Company’s historical volatility. The weighted average grant date fair value of stock options granted to employees during the years ended December 31, 2023, 2022 and 2021 was $0.90, $1.99, and $2.73 respectively. As of December 31, 2023, total unrecognized compensation expense related to unvested common stock options and Series A Preferred Stock options was $1.4 million and $1.0 million, respectively, which is expected to be recognized over a weighted average period of 2.4 years and 2.5 years, respectively. The following table summarizes the stock option activity under the 2008 Plan, the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: ​ ​ Number of Common Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Employees ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ 15,578,412 ​ ​ $3.44 ​ ​ 7.57 ​ ​ $ 4 Granted ​ ​ 5,477,200 ​ ​ $1.15 ​ ​ ​ ​ Assumed in connection with Merger ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ ​ ​ Exercised ​ ​ — ​ ​ $ — ​ ​ ​ ​ Forfeited ​ ​ (2,215,020) ​ ​ $2.68 ​ ​ ​ ​ Cancelled/settled in connection with the Merger ​ ​ (18,840,592) ​ ​ $2.86 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ 6.50 ​ ​ $13,760 Vested at December 31, 2023 ​ ​ 18,067,999 ​ ​ $0.10 ​ ​ 6.13 ​ ​ $10,725 Vested and expected to vest at December 31, 2023 ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ 6.50 ​ ​ $13,760 ​ ​ ​ ​ ​ ​ ​ ​ Non-employee consultants ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ 266,239 ​ ​ $8.05 ​ ​ 5.08 ​ ​ $ — Forfeited ​ ​ — ​ ​ $ — ​ ​ ​ ​ Cancelled/settled in connection with the Merger ​ ​ (266,239) ​ ​ $8.05 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ — ​ ​ $ — ​ ​ — ​ ​ $ — The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Employees ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ — ​ ​ $ — ​ ​ — ​ ​ $ — Assumed in connection with Merger ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Vested at December 31, 2023 ​ ​ 10,860.441 ​ ​ $71.67 ​ ​ 5.15 ​ ​ $6,709 Vested and expected to vest at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 Restricted Stock Units During the year ended December 31, 2023, the Company granted 1,054,600 restricted stock awards with a weighted average fair value of $1.13 per share based on the closing price of the Company’s common stock on the date of grant to employees under the 2016 Plan, which vested over a four-year term . Forfeitures are estimated at the time of grant and are adjusted, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The Company has estimated a forfeiture rate of 10% for restricted stock awards to employees based on historical experience. There was no unrecognized compensation expense and no outstanding restricted stock units as of December 31, 2023. The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan: ​ ​ Number of shares ​ ​ Weighted average grant date fair value ($) Unvested at December 31, 2022 ​ ​ 1,705,558 ​ ​ $2.62 Granted ​ ​ 1,054,600 ​ ​ 1.13 Vested ​ ​ (636,418) ​ ​ 2.40 Forfeited ​ ​ (446,108) ​ ​ 1.91 Cancelled/settled in connection with the Merger ​ ​ (1,677,632) ​ ​ 1.96 Unvested at December 31, 2023 ​ ​ — ​ ​ $ — Employee Stock Purchase Plan In June 2016, the Company approved the 2016 Employee Stock Purchase Plan, or the ESPP, which authorized 173,076 shares of common stock for future issuance under the ESPP to participating employees. In January 2023 and 2022, the number of shares of common stock authorized for issuance under the ESPP was increased by 1,530,424 shares and 1,236,229 shares, respectively. During the year ended December 31, 2023, the Company issued 186,044 shares of common stock under the ESPP. As of December 31, 2023, 4,982,098 shares remain available for future issuance under the ESPP. In connection with the Merger, the Board suspended the current ESPP offering period. For each of the years ended December 31, 2023 and 2022, the Company recognized $0.1 million of stock-based compensation expense under the ESPP.

Revenue Arrangements (FY)

Revenue Arrangements (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
Revenue Arrangements	Revenue Arrangements Collaboration and license revenue Astellas Gene Therapies In January 2023, the Company entered into a License and Development Agreement, or the Astellas Agreement, with Audentes Therapeutics, Inc., doing business as Astellas Gene Therapies, or Astellas. Under the Astellas Agreement, the Company granted Astellas an exclusive license to the Company’s IdeXork technology arising from Xork (defined below), to develop and commercialize Xork for use in Pompe disease in combination with an Astellas gene therapy investigational or authorized product. Xork, Genovis’ IgG Protease, is licensed by the Genovis Agreement, as described in Note 15 to these consolidated financial statements. Astellas paid a $10.0 million upfront payment to the Company upon signing of the Astellas Agreement, and the Company was entitled to receive up to $340.0 million in future additional payments over the course of the partnership that were contingent on the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales where Xork is used as a pre-treatment for an Astellas investigational or authorized product. The Company was also eligible for tiered royalty payments ranging from low to high single digits. Any proceeds received from milestone payments or royalties relating to Xork would have been required to be distributed to holders of CVRs, net of certain deductions. A more detailed description of the Astellas Agreement and the Company's evaluation of this agreement under ASC 606 can be found in Note 14 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. In March 2024, the Company was notified by Astellas of its intention to terminate the Astellas Agreement, which occurred effective June 6, 2024. As of June 30, 2024, there were no unsatisfied performance obligations related to the Astellas Agreement. As of December 31, 2023, the Company recorded $2.3 million and $3.5 million as a short-term and a long-term contract liability, respectively, representing deferred revenue associated with the Astellas Agreement. As of June 30, 2024 and December 31, 2023, the Company recorded a receivable of $0.1 million and $0.3 million, respectively, representing billings for the Xork Development Services (as defined in the Astellas Agreement) that are subject to reimbursement by Astellas. Revenue of $0.5 million and $6.3 million related to the Astellas Agreement was recognized during the three and six months ended June 30, 2024, inclusive of $3.2 million of revenue recognized from performance obligations related to prior periods as a result of the change in transaction price during the six months ended June 30, 2024. Revenue of $0.8 million and $1.4 million related to the Astellas Agreement was recognized during the three and six months ended June 30, 2023, respectively. Takeda Pharmaceuticals USA, Inc. License and Development Agreement In October 2021, the Company entered into a License Agreement, or the Takeda Agreement, with Takeda Pharmaceuticals USA, Inc., or Takeda. Under the Takeda Agreement, the Company granted Takeda an exclusive license to the Company’s ImmTOR technology initially for two specified disease indications within the field of lysosomal storage disorders. Takeda paid a $3.0 million upfront payment to the Company upon signing of the Takeda Agreement, and the Company was entitled to receive up to $1.124 billion in future additional payments over the course of the partnership that were contingent on the achievement of development or commercial milestones or Takeda’s election to continue its activities at specified development stages. The Company was also eligible for tiered royalties on future commercial sales of any licensed products. A more detailed description of the Takeda Agreement and the Company's evaluation of this agreement under ASC 606 can be found in Note 14 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. On March 9, 2023, the Company was notified by Takeda of the achievement of the milestone event related to the completion of a non-clinical milestone for one of the specified disease indications within the field of lysosomal storage disorders under the Takeda Agreement. Accordingly, the Company received a milestone payment of $0.5 million during the three months ended June 30, 2023. The Takeda Agreement was terminated effective July 25, 2023, following Takeda’s decision to discontinue discovery and pre-clinical activities in adeno-associated virus, or AAV, gene therapy. As of June 30, 2024 and December 31, 2023, there were no unsatisfied performance obligations related to the Takeda Agreement. No revenue related to the Takeda Agreement was recognized during the three and six months ended June 30, 2024. Revenue of $0.1 million and $0.6 million related to the Takeda Agreement was recognized during the three and six months ended June 30, 2023, respectively. Swedish Orphan Biovitrum AB (publ.) License and Development Agreement In June 2020, the Company and Sobi entered into the Sobi License, which was subsequently amended. Pursuant to the Sobi License, the Company agreed to grant Sobi an exclusive, worldwide (except as to Greater China) license to develop, manufacture and commercialize the SEL-212 drug candidate, which is currently in development for the treatment of chronic refractory gout. The SEL-212 drug candidate is a pharmaceutical composition containing a combination of SEL-037, or the Compound, and ImmTOR. Pursuant to the Sobi License, in consideration of the license, Sobi agreed to pay the Company a one-time, upfront payment of $75.0 million. Sobi has also agreed to make milestone payments totaling up to $630.0 million to the Company upon the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier. A more detailed description of the Sobi License and the Company's evaluation of this agreement under ASC 606 can be found in Note 14 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. On October 31, 2023, the Company and Sobi entered into Amendment No. 1 to the Sobi License, pursuant to which the Company granted Sobi an exclusive license to manufacture ImmTOR solely in connection with Sobi’s development of SEL-212 under the License and Development Agreement and transferred certain contracts and manufacturing equipment to Sobi. Additionally, in connection with entry into the amendment, Sobi agreed to make employment offers to certain of the Company’s employees engaged in ImmTOR manufacturing activities on or prior to a specified date, and the Company agreed not to terminate the employment of such employees prior to such specified date. The Company maintains no responsibilities to Sobi to manufacture, or supply Sobi with, ImmTOR under the Sobi License. On June 28, 2024, Sobi initiated a rolling biologics license application to the FDA for SEL-212 for the potential treatment of chronic refractory gout which resulted in the achievement of a development milestone and a $30.0 million payment obligation from Sobi to the Company. As a result, the development milestone was no longer constrained and $30.0 million was recognized as revenue during each of the three and six months ended June 30, 2024 as there were no remaining performance obligations under the Sobi License. The proceeds from the achievement of the development milestone were received from Sobi in July 2024 and are expected to be included, net of deductions as specified in the CVR Agreement, in the next scheduled distribution to holders of the CVRs in March 2025. As of June 30, 2024 and December 31, 2023, the Company recorded total outstanding receivables of $31.7 million and $4.6 million, respectively, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi and, as of June 30, 2024, the unpaid milestone. Additionally, as of June 30, 2024 and December 31, 2023, the Company recorded a total unbilled receivable of $3.5 million and $3.0 million, respectively, representing revenue earned but not yet billed for the Phase 3 DISSOLVE program. Revenue of $32.8 million, inclusive of the $30.0 million development milestone, related to the Sobi License was recognized during each of the three and six months ended June 30, 2024. Revenue of $4.3 million and $8.7 million related to the Sobi License was recognized during the three and six months ended June 30, 2023, respectively. Sarepta Therapeutics, Inc. Research License and Option Agreement In June 2020, the Company and Sarepta Therapeutics, Inc., or Sarepta, entered into a Research License and Option Agreement, or the Sarepta Agreement. Pursuant to the Sarepta Agreement, the Company agreed to grant Sarepta a license under the Company’s intellectual property rights covering the Company’s antigen-specific biodegradable nanoparticle encapsulating ImmTOR to research and evaluate ImmTOR in combination with Sarepta’s adeno-associated virus gene therapy technology, or gene editing technology, using viral or non-viral delivery, to treat Duchenne Muscular Dystrophy and certain Limb-Girdle Muscular Dystrophy subtypes, or the Indications. Sarepta initially had an option term of 24 months during which it could opt-in to obtain an exclusive license to further develop and commercialize the product to treat at least one indication, with a potential to extend the option term for an additional fee. The Company agreed to supply ImmTOR to Sarepta for clinical supply on a cost-plus basis under the Sarepta Agreement. A more detailed description of the Sarepta Agreement and the Company's evaluation of this agreement under ASC 606 can be found in Note 14 to the consolidated financial statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023. On March 13, 2023, the Company was notified by Sarepta that Sarepta would not be exercising its exclusive option under the Sarepta Agreement. Therefore, the remaining deferred revenue balance as of December 31, 2022 of $0.5 million was recognized as revenue during the six months ended June 30, 2023. No revenue related to the Sarepta Agreement was recognized during the three and six months ended June 30, 2024 or the three months ended June 30, 2023. Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represent the transaction price of contracts for which work has not been performed, or has been partially performed. As of June 30, 2024, there were no unsatisfied performance obligations from contracts with customers. Contract Balances from Contracts with Customers The following table presents changes in the Company’s contract liabilities during the six months ended June 30, 2024 (in thousands): ​ ​ Balance at beginning of period ​ ​ Additions ​ ​ Deductions ​ ​ Balance at end of period Six Months Ended June 30, 2024 ​ ​ ​ ​ ​ ​ ​ ​ Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ ​ $5,849 ​ ​ $— ​ ​ $(5,849) ​ ​ $— Total contract liabilities ​ ​ $5,849 ​ ​ $— ​ ​ $(5,849) ​ ​ $— Grant revenue National Institute of Neurological Disorders and Stroke of the National Institutes of Health In June 2024, the Company received funding approval from the National Institute of Neurological Disorders and Stroke of the National Institutes of Health, or NINDS, for an award of $1.5 million granted for the budget period, which runs from June 2024 through May 2025. Subject to the availability of funds and satisfactory progress of the project, an additional $1.5 million is recommended to be awarded for the budget period June 2025 through May 2026. The funding was provided by NINDS to further the Company’s use of RNA-based CAR-T cells to combat autoantibody-associated autoimmune disorders. Grant funding is to be used solely for manufacturing of RNA-based CAR-T cells and analysis of samples to inform mechanism of action. The award period runs through May 31, 2026. The Company will recognize grant revenue when expenses reimbursable under the grant have been incurred during the budget period. As of June 30, 2024, the Company recorded a receivable of $0.2 million that is subject to reimbursement by NINDS. The Company recognized grant revenue of $0.2 million during each of the three and six months ended June 30, 2024.	Revenue Arrangements Astellas Gene Therapies In January 2023, the Company entered into the Astellas Agreement, with Astellas. Under the Astellas Agreement, the Company granted Astellas an exclusive license to the Company’s IdeXork technology arising from Xork, to develop and commercialize Xork for use in Pompe disease in combination with an Astellas gene therapy investigational or authorized product. Xork, Genovis’ IgG Protease, is licensed pursuant to an Exclusive License Agreement with Genovis, or the Genovis Agreement, as described in Note 16 to these consolidated financial statements. Astellas paid a $10.0 million upfront payment to the Company upon signing of the Astellas Agreement, and the Company is entitled to receive up to $340.0 million in future additional payments over the course of the partnership that are contingent on the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales where Xork is used as a pre-treatment for an Astellas investigational or authorized product. The Company is also eligible for tiered royalty payments ranging from low to high single digits. Any proceeds received from milestone payments or royalties relating to Xork would be required to be distributed to holders of CVRs, net of certain deductions. Pursuant to the Astellas Agreement, the Company will have the exclusive right and responsibility to complete research and development of Xork products and to conduct all preclinical studies and clinical trials for Xork for use in Pompe disease with an Astellas gene therapy investigational or authorized product, or the Xork Development Services. Astellas will reimburse the Company for 25% of all budgeted costs incurred to complete the development of Xork for use in Pompe disease with an Astellas gene therapy investigational or authorized product. The Company will have control and responsibility over regulatory filings, including any investigational drug applications, biologics license applications, and marketing authorization applications relating to the licensed product. Astellas will have the exclusive right and responsibility to research, develop, and commercialize Astellas products used in combination with Xork and will have control and responsibility over all regulatory filings, including any investigational drug applications, biologics license applications, and marketing authorization applications, relating to Astellas products and Astellas products used in combination with Xork. The Company determined the Astellas Agreement represents a service arrangement under the scope of ASC 606. The Company determined that the sublicense of Xork to Astellas, the licensed know-how, and the Xork Development Services represent a single promise and performance obligation to be transferred to Astellas over time due to the nature of the promises in the contract. As such, the Company will recognize the transaction price as revenue utilizing the input method to measure the progress of satisfying the single performance obligation to Astellas. In determining the transaction price, the Company concluded the upfront payment of $10.0 million and development cost reimbursements of $5.5 million will be included in the initial transaction price. All other development milestones will be fully constrained and will only be included in the transaction price when the applicable milestone is deemed probable of achievement. Each of these variable consideration items were evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt and timing of such development milestones is outside the control of the Company and probability of success criteria is estimated. The Company will re-evaluate the transaction price in each reporting period, as uncertain events are resolved, or as other changes in circumstances occur. In accordance with ASC 606, the Company will only recognize revenue associated with sales-based milestones and royalties when the subsequent sales thresholds are reached and underlying sales occur, respectively. The Company determined that a significant financing component does not exist in its arrangement with Astellas. The Company also determined the options to negotiate additional fields, enter into a clinical supply agreement, and enter into a commercial supply agreement do not represent material rights under the Astellas Agreement. Astellas has the right to terminate the Astellas Agreement in its entirety or on a field-by-field basis, upon 90 days’ written notice to the Company. As of December 31, 2023, the Company recorded $2.3 million as a short-term contract liability and $3.5 million as a long-term contract liability, representing deferred revenue associated with the Astellas Agreement. As of December 31, 2023, the Company recorded a receivable of $0.3 million, representing billings for the Xork Development Services that are subject to reimbursement by Astellas. Revenue of $5.5 million related to the Astellas Agreement was recognized during the year ended December 31, 2023. Takeda Pharmaceuticals USA, Inc. License and Development Agreement On October 1, 2021, the Company entered into a License Agreement, or the Takeda Agreement, with Takeda. Under the Takeda Agreement, the Company granted Takeda an exclusive license to the Company’s ImmTOR technology initially for two specified disease indications within the field of lysosomal storage disorders. Takeda paid a $3.0 million upfront payment to the Company upon signing of the Takeda Agreement, and the Company was entitled to receive up to $1.124 billion in future additional payments over the course of the partnership that were contingent on the achievement of development or commercial milestones or Takeda’s election to continue its activities at specified development stages. The Company was also eligible for tiered royalties on future commercial sales of any licensed products. Pursuant to the Takeda Agreement, the Company determined the Takeda Agreement represented a service arrangement under the scope of ASC 606, and given the reversion of the rights under the Takeda Agreement represented a penalty in substance for a termination by Takeda, the contract term would remain the stated term of the Takeda Agreement. The Company determined that the research license, the licensed know-how, and the manufactured supply and delivery of materials represented a single promise and performance obligation to be transferred to Takeda over time due to the nature of the promises in the contract. The delivery of the manufactured supply was the predominant promise within the arrangement, as it was essential to the utility of the licensed intellectual property. The material supplied by the Company to Takeda was unique to the Company and cannot be obtained by other vendors. As such, consideration in the initial transaction price was allocated to the single performance obligation and the recognition period would not extend beyond the initial contractual period. The Company recognized the revenue associated with the upfront payment and combined single performance obligation utilizing the output method over the term that manufactured supply was delivered to Takeda. In determining the transaction price, the Company concluded the payment associated with all the performance milestones was fully constrained and only included in the transaction price when the respective milestone was deemed probable of achievement. Each of these variable consideration items were evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt and timing of such study milestones is outside the control of the Company and probability of success criteria is estimated. The Company re-evaluated the transaction price in each reporting period, as uncertain events were resolved, or as other changes in circumstances occurred. Takeda had the right to exercise covenant release rights on a field-by-field basis. If Takeda exercised its covenant release rights, we could have received exercise payments per indication and would have been entitled to significant development and commercial milestone payments and tiered royalties on commercial sales. The Company determined that a significant financing component did not exist in its arrangement with Takeda. The Company also determined the options to negotiate additional fields, pursue other products, enter into a supply agreement explore additional fields, and pursue additional development under the initial fields did not represent material rights under the agreement. Takeda had the right to terminate the Takeda Agreement in its entirety or on a field-by-field basis, upon 90 days’ written notice to the Company. On March 9, 2023, the Company was notified by Takeda of the achievement of the milestone event related to the completion of a non-clinical milestone for one of the specified disease indications within the field of lysosomal storage disorders under the Takeda Agreement. Accordingly, the Company received a milestone payment of $0.5 million during the year ended December 31, 2023. The Takeda Agreement was terminated effective July 25, 2023, following Takeda’s decision to discontinue discovery and pre-clinical activities in adeno-associated virus, or AAV, gene therapy. As of December 31, 2023, the Company recorded no contract liability. As of December 31, 2022, the Company recorded $0.1 million as a short-term contract liability and no long-term contract liability representing deferred revenue associated with this agreement. Revenue of $0.6 million and $1.8 million related to the Takeda Agreement was recognized during the years ended December 31, 2023 and 2022, respectively. Swedish Orphan Biovitrum License and Development Agreement On June 11, 2020, the Company and Sobi entered into a License and Development Agreement. Pursuant to the Sobi License, the Company agreed to grant Sobi an exclusive, worldwide (except as to Greater China) license to develop, manufacture and commercialize the SEL-212 drug candidate, which is currently in development for the treatment of chronic refractory gout. The SEL-212 drug candidate is a pharmaceutical composition containing a combination of SEL-037, or the Compound, and ImmTOR. Pursuant to the Sobi License, in consideration of the license, Sobi agreed to pay the Company a one-time, upfront payment of $75.0 million. Sobi has also agreed to make milestone payments totaling up to $630.0 million to the Company upon the achievement of various development and regulatory milestones and, if commercialized, sales thresholds for annual net sales of SEL-212, and tiered royalty payments ranging from the low double digits on the lowest sales tier to the high teens on the highest sales tier. Any proceeds received from milestone payments or royalties relating to the Sobi License would be required to be distributed to holders of CVRs, net of certain deductions. Pursuant to the Sobi License, the Company agreed to supply (at cost) quantities of the Compound and ImmTOR as necessary for completion of the two Phase 3 clinical trials of SEL-212 (DISSOLVE I and DISSOLVE II) and a six-month placebo extension. The Company was required to supply quantities of the Compound until all rights to the Compound and any materials needed to manufacture the Compound were transferred to Sobi, which transfer occurred upon the execution of Amendment No. 1 to the License and Development Agreement on October 31, 2023. Sobi has agreed to reimburse the Company for all budgeted costs incurred to complete development of SEL-212, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, except for any costs of additional development activities required that are related to ImmTOR and that are unrelated to SEL-212. Sobi will have control and responsibility over all regulatory filings, including any investigational drug applications (IND), biologics license applications (BLA), and marketing authorization applications (MAA) relating to the licensed product. The transactions contemplated by the Sobi License were consummated on July 28, 2020. Sobi may terminate the Sobi License for any reason upon 180 days’ written notice to the Company, whereby all rights granted under the Sobi License would revert back to the Company. In addition, if Sobi were to terminate the Sobi License, the Company has the option to obtain a license to all patents and know-how necessary to exploit SEL-212 in existence as of the termination date from Sobi in return for making an equitable royalty payment to Sobi. Additionally, on June 11, 2020, the Company entered into the Sobi Purchase Agreement in connection with the Sobi License. The closing of the Sobi Private Placement occurred on July 31, 2020, following the closing of the transactions contemplated under the Sobi License. See Note 12 for details. The Company determined that the Sobi License represents a service arrangement under the scope of ASC 606. In addition, given the Sobi License and Sobi Purchase Agreement were executed contemporaneously and negotiated as a package with a single commercial objective, the Company will account for the two agreements as a single contract. The term of the Sobi License commenced upon the effective date of July 28, 2020 and will continue on a product-by-product basis until the royalty terms for each country have expired. The royalty term for a given product begins upon the first commercial sale of the product in a country and ends at the later of ten years from the first commercial sale, expiration of the last valid patent claim covering the product and expiration of all regulatory exclusivity periods for the product in a country. Given the reversion of the rights under the Sobi License represents a penalty in substance for a termination by Sobi, the contract term would remain the stated term of the Sobi License. The Company determined that the Sobi License contains three distinct performance obligations due to the nature of the promises in the contract, which includes conducting the Phase 3 DISSOLVE trials, Sobi’s option to set-up a second source supplier, and a combined obligation comprised of the delivery of the license to SEL-212, transfer of the know-how and the manufacturing and delivery of SEL-212 supply for development, or the Combined License Obligation. As the set-up of a second source supplier is optional for Sobi and the Company will be reimbursed at cost for its efforts in the subsequent set-up and technology transfer, the option for this future service was determined to be at a significant and incremental discount to its standalone selling price and treated as a material right in the arrangement, namely a distinct performance obligation. In determining the transaction price, the Company concluded the upfront payment of $75.0 million and the $5.0 million development milestone associated with the dosing of the first patient in the Phase 3 DISSOLVE trials were included in the transaction price. All other development milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of the evaluation of the constraint, the Company considered numerous factors, including that receipt of such milestones is outside the control of the Company and probability of success criteria is estimated. The Company re-evaluates the transaction price in each reporting period, as uncertain events are resolved. In accordance with ASC 606, the Company will only recognize revenue associated with sales-based milestones and royalties when the subsequent sales thresholds are reached and underlying sales occur, respectively. In connection with the Sobi Purchase Agreement, the Company determined that the gross proceeds of $25.0 million from the Sobi Private Placement included a premium to the fair value of the Company’s shares as of July 28, 2020 equal to approximately $14.5 million. The premium amount is included in the transaction price for revenue recognition. The Company estimates and includes in the transaction price the total reimbursements to be received from Sobi for both the manufacturing and delivery of the Compound and ImmTOR as well as conducting the Phase 3 DISSOLVE trials. The Company determined that a significant financing component does not exist in its arrangement with Sobi. The Company allocated the transaction price based on the relative standalone selling prices of the three distinct performance obligations. The Company estimated the standalone selling price of conducting the Phase 3 DISSOLVE trials by forecasting its anticipated costs and applying a margin reflective of the industry. The Company must determine the standalone selling price of the second source supplier option by determining the discount given to Sobi multiplied by the likelihood that Sobi will exercise the option in the future. Similar to the Phase 3 program estimate, the Company estimated the discount of the option by forecasting the set-up costs and applying a margin that is reflective of the industry. As the Company will be providing the set-up and technology transfer services and the future supply at cost, the discount of the option is equal to the margin amount. The Company considered discussions with Sobi as well as probability of regulatory success of SEL-212 in determining the likelihood of exercise. The Company estimated the standalone selling price of the Combined License Obligation by utilizing a discounted cash flow model. The Company determined that the delivery of the supply to Sobi best represents the pattern of delivery of the Combined License Obligation as the supply is essential to the utility of the license and know-how. The Company will recognize the revenue allocated to the Combined License Obligation by utilizing the output method. The Company estimated the total supply of the Compound and ImmTOR to be required during the clinical trial period and will recognize revenue as this supply is shipped for use in the clinical trials. The Company will recognize the revenue allocated to the conducting of the Phase 3 DISSOLVE trials obligation by utilizing the input method. The Company estimated the total budgeted costs to be incurred over the Phase 3 DISSOLVE trials and will recognize revenue as these costs are incurred. The Company’s costs best represent the pattern of transfer as these will capture all performance of the trials completed to date and can be readily measured. The Company will recognize the revenue allocated to the second source supplier option when the future services and goods are transferred. On June 29, 2022, the Company completed enrollment of the DISSOLVE II trial. The completion of enrollment of the DISSOLVE II trial resulted in the achievement of a development milestone and a $10.0 million payment obligation from Sobi to the Company. This amount was added to the overall transaction price and payment was received during the year ended December 31, 2022. On October 31, 2023, the Company and Sobi entered into Amendment No. 1 to the License and Development Agreement, pursuant to which the Company granted Sobi an exclusive license to manufacture ImmTOR solely in connection with Sobi’s development of SEL-212 under the License and Development Agreement and transferred certain contracts and manufacturing equipment to Sobi. Additionally, in connection with entry into the amendment, Sobi agreed to make employment offers to certain of the Company’s employees engaged in ImmTOR manufacturing activities on or prior to a specified date, and the Company agreed not to terminate the employment of such employees prior to such specified date. The Company maintains no responsibilities to Sobi to manufacture, or supply Sobi with, ImmTOR under the Sobi License. As of December 31, 2023 and 2022, the Company recorded a total outstanding receivable of $4.6 million and $5.0 million, respectively, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi. Additionally, as of December 31, 2023 and 2022, the Company recorded a total unbilled receivable of $3.0 million and $3.2 million, respectively, representing revenue earned but not yet billed for the Phase 3 DISSOLVE program. Revenue of $19.4 million and $82.6 million related to the Sobi License was recognized during the years ended December 31, 2023 and 2022, respectively, inclusive of $1.1 million of revenue recognized from performance obligations related to prior periods as a result of the change in transaction price during the year ended December 31, 2023. Sarepta Therapeutics, Inc. Research License and Option Agreement In June 2020, the Company and Sarepta Therapeutics, Inc., or Sarepta, entered into a Research License and Option Agreement, or the Sarepta Agreement. Pursuant to the Sarepta Agreement, the Company agreed to grant Sarepta a license under the Company’s intellectual property rights covering the Company’s antigen-specific biodegradable nanoparticle encapsulating ImmTOR to research and evaluate ImmTOR in combination with Sarepta’s adeno-associated virus gene therapy technology, or gene editing technology, using viral or non-viral delivery, to treat Duchenne Muscular Dystrophy and certain Limb-Girdle Muscular Dystrophy subtypes, or the Indications. Sarepta initially had an option term of 24 months during which it could opt-in to obtain an exclusive license to further develop and commercialize the Product to treat at least one Indication, with a potential to extend the option term for an additional fee. The Company will supply ImmTOR to Sarepta for clinical supply on a cost-plus basis. Sarepta paid a $2.0 million upfront payment to the Company upon signing of the Sarepta Agreement, and the Company is eligible to receive additional preclinical payments during the option term. If Sarepta opts-in to an exclusive license agreement, the Company could receive option exercise payments per Indication upon execution of the exclusive license, and the Company would be entitled to significant development and commercial milestone payments and tiered royalties ranging from the mid-to-high single digits based on net sales. Pursuant to the Sarepta Agreement, the Company determined the Sarepta Agreement represents a service arrangement under the scope of ASC 606, with a 24-month contract duration. Given the reversion of the rights under the Sarepta Agreement represents a penalty in substance for a termination by Sarepta, the contract term would remain the stated term of the Sarepta Agreement. The Company determined that the Sarepta Agreement and supply obligation including the delivery of the research license, the licensed know-how, the manufactured supply and delivery of materials represent a single promise and performance obligation to be transferred to Sarepta over time due to the nature of the promises in the contract. The delivery of the manufactured supply is the predominant promise within the arrangement, as it is essential to the utility of the licensed intellectual property. As such, consideration in the initial transaction price will be allocated to the single performance obligation based on the contractual price. In determining the transaction price, the Company concluded the payment associated with all the performance milestones will be fully constrained and only be included in the transaction price when the respective milestone is deemed probable of achievement. Each of these variable consideration items was evaluated under the most likely amount method to determine whether such amounts were probable of occurrence, or whether such amounts should be constrained until they become probable. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of such study milestones is outside the control of the Company and probability of success criteria is estimated. The Company also determined the option to enter into a future commercial license agreement and extend the term of the option does not represent a material right since it was not priced at an incremental discount. Sarepta may terminate the Sarepta Agreement for any reason upon 30 days’ written notice to the Company. The Sarepta Agreement contains other customary terms and conditions, including representations and warranties, covenants, termination, and indemnification obligations in favor of each party. On April 13, 2021, the Company was notified by Sarepta of the achievement of the milestone event related to the completion of a non-clinical study for Duchenne muscular dystrophy and certain limb-girdle muscular dystrophies under the Sarepta Agreement. Accordingly, the Company received a milestone payment of $3.0 million during the three months ended June 30, 2021. On June 10, 2022, the Company was notified by Sarepta that Sarepta would be extending their options under the Sarepta Agreement. In exchange for a nine-month extension to Sarepta’s options to both Duchenne muscular dystrophy and certain limb-girdle muscular dystrophies, the Company received a milestone payment of $2.0 million during the year ended December 31, 2022. On June 15, 2022, the Company was notified by Sarepta of the achievement of a milestone event related to certain preclinical study milestones under the Sarepta Agreement. Accordingly, the Company received a milestone payment of $4.0 million during the year ended December 31, 2022. On March 13, 2023, the Company was notified by Sarepta that Sarepta would not be exercising its exclusive option under the Sarepta Agreement. The Sarepta Agreement terminated upon the expiration of the option in March 2023. As of December 31, 2023, the Company recorded no contract liability. As of December 31, 2022, the Company recorded $0.5 million as a short-term contract liability representing deferred revenue associated with this agreement. Revenue of $0.5 million and $10.2 million related to the Sarepta Agreement was recognized during the years ended December 31, 2023 and 2022, respectively. Asklepios Biopharmaceutical, Inc. License Agreement for Pompe Disease In December 2019, the Company and Asklepios Biopharmaceutical, Inc., or AskBio, entered into a license agreement, or the AskBio License Agreement. Pursuant to the AskBio License Agreement, AskBio exercised its option to exclusively license the Company’s intellectual property rights covering the Company’s ImmTOR platform to research, develop, and commercialize certain AAV gene therapy products utilizing ImmTOR, and targeting the GAA gene, or derivatives thereof, to treat Pompe Disease. On November 18, 2022, both parties agreed to mutually terminate the AskBio License Agreement. Therefore, the remaining contract liability of $7.0 million was recognized as revenue during the period ended December 31, 2022. Spark Therapeutics, Inc. In December 2016, the Company entered into a license and option agreement, or the Spark License Agreement, with Spark Therapeutics, Inc., or Spark, pursuant to which the Company and Spark agreed to collaborate on the development of gene therapies for certain targets utilizing the ImmTOR platform. The Spark License Agreement provided Spark with certain exclusive, worldwide, royalty bearing licenses to the Company’s intellectual property, allowing Spark to develop and commercialize gene therapies in combination with ImmTOR for Factor VIII, an essential blood clotting protein relevant to the treatment of hemophilia A, the initial target. On January 18, 2022, both parties agreed to mutually terminate the Spark License Agreement. Therefore, the remaining contract liability of $9.2 million was recognized as revenue during the year ended December 31, 2022. Transaction Price Allocated to Future Performance Obligations Remaining performance obligations represent the transaction price of contracts for which work has not been performed (or has been partially performed). As of December 31, 2023, the aggregate amount of the transaction price allocated to remaining performance obligations was $5.8 million. Contract Balances from Contracts with Customers ( Astellas, Takeda, Sobi, Sarepta, AskBio, and Spark ) The following table presents changes in the Company’s contract liabilities during the year ended December 31, 2023 (in thousands): ​ ​ Balance at beginning of period ​ ​ Additions ​ ​ Deductions ​ ​ Balance at end of period Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ ​ $593 ​ ​ $10,500 ​ ​ $(5,244) ​ ​ $5,849 Total contract liabilities ​ ​ $593 ​ ​ $10,500 ​ ​ $(5,244) ​ ​ $5,849

Related-Party Transactions (FY)

Related-Party Transactions (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Related Party Transactions [Abstract]
Related-Party Transactions	Related-Party Transactions November 2023 Securities Purchase Agreement On November 13, 2023, the Company entered into the Securities Purchase Agreement with (i) Dr. Timothy A. Springer, (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, in which the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million (see Note 10). The November 2023 Private Placement includes a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the six months ended June 30, 2024. Name ​ ​ Shares of Series A Preferred Stock purchased ​ ​ Total aggregate purchase price Timothy A. Springer, Ph.D. ​ ​ 99,140.326 ​ ​ $40,000,000 Exercise of Amended 2019 Warrants On March 26, 2024, TAS Partners LLC, an affiliate of Dr. Springer, exercised 65,681 Amended 2019 Warrants, paid the per-share exercise price of $43.80 in cash for an aggregate exercise price of $2.9 million, and received 65,681 shares of common stock and 1,970,443 CVRs. During the three and six months ended June 30, 2023, there were no related party transactions.	Related-Party Transactions November 2023 Securities Purchase Agreement On November 13, 2023, the Company entered into the Securities Purchase Agreement with (i) Dr. Timothy A. Springer, (ii) TAS Partners LLC, an affiliate of Dr. Springer, and (iii) Seven One Eight Three Four Irrevocable Trust, a trust associated with Dr. Murat Kalayoglu, in which the Company agreed to issue and sell an aggregate of 149,330.115 shares of Series A Preferred Stock for an aggregate purchase price of $60.25 million (see Note 11). The November 2023 Private Placement includes a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the year ended December 31, 2023. Name ​ ​ Shares of Series A Preferred Stock purchased ​ ​ Total aggregate purchase price Timothy A. Springer, Ph.D. ​ ​ 24,785.081 ​ ​ $10,000,000 TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) ​ ​ 24,785.081 ​ ​ $10,000,000 Seven One Eight Three Four Irrevocable Trust (affiliate of Murat Kalayoglu, MD, Ph.D.) ​ ​ 619.627 ​ ​ $ 250,000 April 2022 Offering During the year ended December 31, 2022, the Company completed the 2022 Offering as described in Note 12. The following table sets forth the number of shares of Common Stock and 2022 Warrants purchased in the 2022 Offering by directors and executive officers, as of the time of the Offering, and related parties thereto: Name ​ ​ Shares of Common Stock purchased ​ ​ 2022 Warrants purchased ​ ​ Total aggregate purchase price TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) ​ ​ 6,681,600 ​ ​ 5,011,200 ​ ​ $9,421,056 Warrant liability reclassification During the year ended December 31, 2022, the Company amended the terms of certain of the outstanding 2019 Warrants held by members of the Company's Board of Directors and remeasured the Amended 2019 Warrants as described in Note 6. Consulting Services The Company entered into consulting agreements with its founders to serve on its Scientific Advisory Board, effective January 1, 2020 to December 31, 2021, under which they were paid quarterly for their services. The Company incurred expenses for consulting services provided by its founders totaling $0.1 million during the year ended December 31, 2021. No expenses were incurred for the years ended December 31, 2023 and 2022.

Collaboration and License Agr_2

Collaboration and License Agreements (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Collaboration and License Agreements	Collaboration and License Agreements Biogen MA, Inc. On September 8, 2023, the Company entered into a non-exclusive, sublicensable, worldwide, perpetual patent license agreement, or the Biogen Agreement, with Biogen MA, Inc., or Biogen, to research, develop, make, use, offer, sell and import products or processes containing or using an engineering T-cell modified with an mRNA comprising, or encoding a protein comprising, certain sequences licensed under the Biogen Agreement for the prevention, treatment, palliation and management of autoimmune diseases and disorders, excluding cancers, neoplastic disorders, and paraneoplastic disorders. The Company is not obligated to pay Biogen any expenses, fees, or royalties. The Company may terminate the Biogen Agreement for any reason or no reason, and Biogen may terminate the agreement after a notice-and-cure period of 30 days if the Company fails to pay a fee owed to Biogen or for any other material breach of the agreement. The Biogen Agreement will otherwise expire when all claims of all issued patents within the patents and patent applications licensed to the Company under the Biogen Agreement have expired or been finally rendered revoked, invalid or unenforceable by a decision of a court or government agency. National Cancer Institute of the National Institutes of Health Effective September 16, 2019, the Company entered into a nonexclusive, worldwide license agreement, or the NCI Agreement, with the U.S. Department of Health and Human Services, represented by the National Cancer Institute of the National Institutes of Health, or NCI. Under the NCI Agreement, the Company was granted a license under certain NCI patents and patent applications designated in the agreement, to make, use, sell, offer and import products and processes within the scope of the patents and applications licensed under the NCI Agreement when developing and manufacturing anti-BCMA CAR-T cell products for the treatment of myasthenia gravis, pemphigus vulgaris, and immune thrombocytopenic purpura according to methods designated in the NCI Agreement. In connection with the Company’s entry into the NCI Agreement, Old Cartesian paid to NCI a one-time $0.1 million license royalty payment. Under the NCI Agreement, the Company is further required to pay NCI a low five-digit annual royalty. The Company must also pay earned royalties on net sales in a low single-digit percentage and pay up to $0.8 million in benchmark royalties upon the Company's achievement of designated benchmarks that are based on the commercial development plan agreed between the parties. Under the NCI Agreement, the Company must use reasonable commercial efforts to bring licensed products and licensed processes to the point of Practical Application (as defined in the NCI Agreement). Upon the Company’s first commercial sale, the Company must use reasonable commercial efforts to make licensed products and licensed processes reasonably accessible to the United States public. After the Company's first commercial sale, the Company must make reasonable quantities of licensed products or materials produced via licensed processes available to patient assistance programs and develop educational materials detailing the licensed products. Unless the Company obtains a waiver from NCI, the Company must have licensed products and licensed processes manufactured substantially in the United States. Prior to the first commercial sale, upon NCI’s request, the Company is obligated to provide NCI with commercially reasonable quantities of licensed products made through licensed processes to be used for in vitro research. Additionally, the Company must use reasonable commercial efforts to initiate a Phase 3 clinical trial of a licensed product by the fourth quarter of 2024, submit a BLA with respect to a licensed product by the fourth quarter of 2026, and make a first commercial sale of a licensed product by the fourth quarter of 2028. The NCI Agreement terminates upon the expiration of the last to expire of the patent rights licensed thereunder, if not sooner terminated. NCI has the right to terminate this agreement, after giving written notice and providing a cure period in accordance with its terms, if the Company is in default of a material obligation. The Company has the unilateral right to terminate the agreement in any country or territory by giving NCI 60 days’ written notice. The Company agreed to indemnify NCI against any liability arising out of the Company’s, sublicensees’ or third parties’ use of the licensed patent rights and licensed products or licensed processes developed in connection with the licensed patent rights. Ginkgo Bioworks Holdings, Inc. Collaboration and License Agreements On October 25, 2021, the Company entered into a Collaboration and License Agreement, or the First Ginkgo Agreement, with Ginkgo. Under the First Ginkgo Agreement, Ginkgo will design next generation IgA proteases with potentially transformative therapeutic potential. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments for fixed fair values in the form of the Company's common stock, clinical and commercial milestone payments of up to $85.0 million in cash. The First Ginkgo Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808, Collaborative Arrangements , or ASC 808, as the risk and rewards are not shared by both parties. The Company will expense costs related to the First Ginkgo Agreement as incurred until regulatory approval is received in accordance with ASC 730, Research and Development , or ASC 730. The Company is accounting for the contingently issuable shares to be issued in exchange for the license obtained from Ginkgo as a liability classified stock-based compensation arrangement with a non-employee which will be recognized when achievement of the milestones is probable. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Ginkgo tiered royalties ranging from low-single digit to high-single digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. On January 3, 2022, the Company entered into a Collaboration and License Agreement, or the Second Ginkgo Agreement, with Ginkgo. Under this agreement, the Company will engage with Ginkgo to develop AAV capsids designed to enhance transduction efficiency and transgene expression. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments in the form of shares of the Company’s common stock, clinical and commercial milestone payments of up to $207 million in cash. The Second Ginkgo Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the Second Ginkgo Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company is accounting for the contingently issuable shares of common stock to be issued in exchange for the license obtained from Ginkgo as a liability-classified, stock-based compensation arrangement with a non-employee which will be recognized when achievement of the milestones is probable. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Ginkgo tiered royalties ranging from low-single digit to high-single digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. On June 13, 2022, the Company was notified of the achievement of the midpoint of the technical development plan under the First Ginkgo Agreement by Ginkgo. This milestone resulted in the payment of $0.5 million and issuance of 29,761 shares of the Company’s common stock then-valued at $1.0 million to Ginkgo during the year ended December 31, 2022. On July 19, 2023, the Company and Ginkgo mutually agreed that the completion of the technical development plan’s midpoint task under the Second Ginkgo Agreement had been achieved as of June 30, 2023. This milestone resulted in the payment of $1.0 million and issuance of 44,642 shares of the Company’s common stock then-valued at $1.5 million to Ginkgo during the year ended December 31, 2023. Genovis AB (publ.) License Agreement In October 2021, the Company entered into an Exclusive License Agreement, or the Genovis Agreement, with Genovis AB (publ.), or Genovis. Under the Genovis Agreement, the Company paid to Genovis an upfront payment in exchange for an exclusive license to the Xork enzyme technology across all therapeutic uses in humans, excluding research, preclinical, diagnostic and other potential non-therapeutic applications of the enzyme. Genovis is eligible to earn from the Company development and sales-based milestones and sublicensing fees. The Genovis Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the Genovis Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Genovis tiered royalties of low double digit percentages of worldwide annual net sales of collaboration products which will be expensed as the commercial sales occur. In February 2023, the Company made a $4.0 million payment to Genovis as a result of the sublicense of Xork to Astellas. See Note 13 to these unaudited consolidated financial statements for further discussion on the Astellas Agreement. In March 2024, the Company notified Genovis of its intention to terminate the Genovis Agreement effective September 13, 2024. Cyrus Biotechnology, Inc. Collaboration and License Agreement In September 2021, the Company and Cyrus Biotechnology, Inc., or Cyrus, entered into a collaboration and license agreement, or the Cyrus Agreement. Pursuant to the Cyrus Agreement, Cyrus agreed to grant the Company an exclusive, worldwide license to certain intellectual property to form a protein engineering collaboration combining the Company’s ImmTOR platform with Cyrus’ ability to redesign protein therapeutics. The lead program was a proprietary interleukin-2, or IL-2, protein agonist designed to selectively promote expansion of regulatory T cells for treatment of patients with autoimmune diseases and other deleterious immune conditions. In return for the licensed intellectual property, the Company made an upfront payment and was obligated to pay certain discovery, development, and sales-based milestones which could have potentially totaled up to approximately $1.5 billion across multiple programs. The Cyrus Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company expensed costs related to the Cyrus Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company assessed the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, would have amortized these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company was also obligated to pay Cyrus tiered royalties ranging from mid-single digit to low-double digit percentages of annual net sales of collaboration products which would have been expensed as commercial sales occur. On June 13, 2022, the Company and Cyrus mutually agreed that the preclinical key in-vitro success milestone had been achieved. In October 2023, the Company notified Cyrus of its termination of the Cyrus Agreement effective December 29, 2023. Stock Purchase Agreement Additionally, on September 7, 2021, the Company entered into a stock purchase agreement, or the Series B Preferred Stock Purchase Agreement, in connection with the Cyrus Agreement. Pursuant to the Series B Preferred Stock Purchase Agreement, the Company purchased 2,326,934 shares of Cyrus’ Series B Preferred Stock, par value $0.0001 per share, at a purchase price of $0.8595 per share, for $2.0 million. In accordance with ASC 810, the Company has a variable interest in Cyrus resulting from its equity investment. The Company will share in Cyrus’ expected losses or receive a portion of its expected returns and absorb the variability associated with changes in the entity’s net assets. However, the Company is not the primary beneficiary as it does not have the power to direct the activities most significant to Cyrus, and therefore it is not required to consolidate Cyrus. The Company has recognized the $2.0 million investment of Cyrus’ Series B Preferred Stock at cost on the purchase date. As of June 30, 2024, no impairment indicators were present and there were no observable price changes. Therefore, the carrying value of the investment in Cyrus is $2.0 million on the accompanying consolidated balance sheets. The Company’s maximum exposure to loss related to this variable interest entity is limited to the carrying value of the investment. The Company has not provided financing to Cyrus other than the amount contractually required by the Series B Preferred Stock Purchase Agreement. Asklepios Biopharmaceutical, Inc. Feasibility Study and License Agreement In August 2019, the Company entered into a feasibility study and license agreement, or the AskBio Collaboration Agreement, with Asklepios Biopharmaceutical, Inc., or AskBio. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain AAV gene therapy products utilizing the Company’s ImmTOR platform to enable re-dosing of such AAV gene therapy products to treat serious rare and orphan genetic diseases for which there is a significant unmet medical need. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AskBio’s AAV gene therapy, or SEL-302, (previously disclosed as MMA-101, in combination with ImmTOR) for the treatment of methylmalonic acidemia, or MMA, to mitigate the formation of neutralizing anti-AAV capsid antibodies. On April 29, 2021, the Company was notified by AskBio that it intended to opt-out of development of the MMA indication. The Company and AskBio shared responsibility for the research, development and commercialization of products developed under the SEL-399 program collaboration. The parties also shared research, development, and commercialization costs equally for all collaboration products, but with a right of either party to opt out of certain products, and thereby not be required to share costs for such products. Each party would have received a percentage of net profits under the collaboration equal to the percentage of shared costs borne by such party in the development of such product. Pursuant to the AskBio Collaboration Agreement, AskBio was responsible for manufacturing the AAV capsids and AAV vectors and the Company was responsible for manufacturing ImmTOR. The Company and AskBio mutually agreed to the termination of the AskBio Collaboration Agreement, effective December 13, 2023. No collaboration expense under the AskBio Collaboration Agreement was recognized during the three and six months ended June 30, 2024. For the three and six months ended June 30, 2023, the Company recognized less than $0.1 million and $0.1 million, respectively, of collaboration expense under the AskBio Collaboration Agreement in which actual costs incurred by both parties approximate a 50% cost share. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement, or the 3SBio License, with Shenyang Sunshine Pharmaceutical Co., Ltd., or 3SBio. The Company has paid to 3SBio an aggregate of $7.0 million in upfront and milestone-based payments under the 3SBio License as of June 30, 2024. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $15.0 million for products containing the Company’s ImmTOR platform.	Collaboration and License Agreements Biogen MA, Inc. On September 8, 2023, the Company entered into a non-exclusive, sublicensable, worldwide, perpetual patent license agreement, or the Biogen Agreement with Biogen MA, Inc., or Biogen to research, develop, make, use, offer, sell and import products or processes containing or using an engineering T-cell modified with an mRNA comprising, or encoding a protein comprising, certain sequences licensed under the Biogen Agreement for the prevention, treatment, palliation and management of autoimmune diseases and disorders, excluding cancers, neoplastic disorders, and paraneoplastic disorders. The Company is not obligated to pay Biogen any expenses, fees, or royalties. The Company may terminate the Biogen Agreement for any reason or no reason, and Biogen may terminate the agreement after a notice-and-cure period of 30 days if the Company fails to pay a fee owed to Biogen or for any other material breach of the agreement. The Biogen Agreement will otherwise expire when all claims of all issued patents within the patents and patent applications licensed to the Company under the Biogen Agreement have expired or been finally rendered revoked, invalid or unenforceable by a decision of a court or government agency. National Cancer Institute of the National Institutes of Health Effective September 16, 2019, the Company entered into a nonexclusive, worldwide license agreement, or the NCI Agreement with the U.S. Department of Health and Human Services, represented by the National Cancer Institute of the National Institutes of Health, or NCI. Under the NCI Agreement, the Company was granted a license under certain NCI patents and patent applications designated in the agreement, to make, use, sell, offer and import products and processes within the scope of the patents and applications licensed under the NCI Agreement when developing and manufacturing anti-BCMA CAR-T cell products for the treatment of myasthenia gravis, pemphigus vulgaris, and immune thrombocytopenic purpura according to methods designated in the NCI Agreement. In connection with the Company's entry into the NCI Agreement, Old Cartesian paid to NCI a one-time $0.1 million license royalty payment. Under the NCI Agreement, the Company is further required to pay NCI a low five-digit annual royalty. The Company must also pay earned royalties on net sales in a low single-digit percentage and pay up to $0.8 million in benchmark royalties upon the Company's achievement of designated benchmarks that are based on the commercial development plan agreed between the parties. Under the NCI Agreement, the Company must use reasonable commercial efforts to bring licensed products and licensed processes to the point of Practical Application (as defined in the NCI Agreement). Upon the Company's first commercial sale, the Company must use reasonable commercial efforts to make licensed products and licensed processes reasonably accessible to the United States public. After the Company's first commercial sale, the Company must make reasonable quantities of licensed products or materials produced via licensed processes available to patient assistance programs and develop educational materials detailing the licensed products. Unless the Company obtains a waiver from NCI, the Company must have licensed products and licensed processes manufactured substantially in the United States. Prior to the first commercial sale, upon NCI’s request, the Company is obligated to provide NCI with commercially reasonable quantities of licensed products made through licensed processes to be used for in vitro research. Additionally, the Company must use reasonable commercial efforts to initiate a Phase 3 clinical trial of a licensed product by the fourth quarter of 2024, submit a BLA with respect to a licensed product by the fourth quarter of 2026, and make a first commercial sale of a licensed product by the fourth quarter of 2028. The NCI Agreement terminates upon the expiration of the last to expire of the patent rights licensed thereunder, if not sooner terminated. NCI has the right to terminate this agreement, after giving written notice and providing a cure period in accordance with its terms, if the Company is in default of a material obligation. The Company has the unilateral right to terminate the agreement in any country or territory by giving NCI 60 days’ written notice. The Company agreed to indemnify NCI against any liability arising out of the Company's, sublicensees’ or third parties’ use of the licensed patent rights and licensed products or licensed processes developed in connection with the licensed patent rights. Ginkgo Bioworks Holdings, Inc. Collaboration and License Agreements On October 25, 2021, the Company entered into a Collaboration and License Agreement, or the First Ginkgo Agreement, with Ginkgo. Under the First Ginkgo Agreement, Ginkgo will design next generation IgA proteases with potentially transformative therapeutic potential. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments for fixed fair values in the form of the Company's common stock, clinical and commercial milestone payments of up to $85.0 million in cash. The First Ginkgo Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the First Ginkgo Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company is accounting for the contingently issuable shares to be issued in exchange for the license obtained from Ginkgo as a liability classified stock-based compensation arrangement with a non-employee which will be recognized when achievement of the milestones is probable. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Ginkgo tiered royalties ranging from low-single digit to high-single digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. On January 3, 2022, the Company entered into a Collaboration and License Agreement, or the Second Ginkgo Agreement, with Ginkgo. Under this agreement, the Company will engage with Ginkgo to develop AAV capsids designed to enhance transduction efficiency and transgene expression. In return, Ginkgo is eligible to earn both upfront research and development fees and milestone payments, including certain milestone payments in the form of shares of the Company’s common stock, clinical and commercial milestone payments of up to $207 million in cash. The Second Ginkgo Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the Second Ginkgo Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company is accounting for the contingently issuable shares of common stock to be issued in exchange for the license obtained from Ginkgo as a liability-classified, stock-based compensation arrangement with a non-employee which will be recognized when achievement of the milestones is probable. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Ginkgo tiered royalties ranging from low-single digit to high-single digit percentages of annual net sales of collaboration products which will be expensed as the commercial sales occur. On June 13, 2022, the Company was notified of the achievement of the midpoint of the technical development plan under the First Ginkgo Agreement by Ginkgo. This milestone resulted in the payment of $0.5 million and issuance of 892,857 shares of the Company’s common stock then-valued at $1.0 million to Ginkgo during the year ended December 31, 2022. On July 19, 2023, the Company and Ginkgo mutually agreed that the completion of the technical development plan’s midpoint task under the Second Ginkgo Agreement had been achieved as of June 30, 2023. This milestone resulted in the payment of $1.0 million and issuance of 1,339,285 shares of the Company’s common stock then-valued at $1.5 million to Ginkgo during the year ended December 31, 2023. Genovis AB (publ.) License Agreement On October 21, 2021, the Company entered into the Genovis Agreement with Genovis. Under the Genovis Agreement, the Company paid to Genovis an upfront payment in exchange for an exclusive license to Genovis’ IgG Protease, Xork, enzyme technology across all therapeutic uses in humans, excluding research, preclinical, diagnostic and other potential non-therapeutic applications of the enzyme. Genovis is eligible to earn from the Company development and sales-based milestones and sublicensing fees. The Genovis Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company will expense costs related to the Genovis Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company will assess the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, will amortize these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company is also obligated to pay Genovis tiered royalties of low double digit percentages of worldwide annual net sales of collaboration products which will be expensed as the commercial sales occur. In February 2023, the Company made a $4.0 million payment to Genovis as a result of the sublicense of Xork to Astellas. See Note 14 to these consolidated financial statements for further discussion on the Astellas Agreement. Cyrus Biotechnology, Inc. Collaboration and License Agreement On September 7, 2021, the Company and Cyrus entered into the Cyrus Agreement. Pursuant to the Cyrus Agreement, Cyrus agreed to grant the Company an exclusive, worldwide license to certain intellectual property to form a protein engineering collaboration combining the Company’s ImmTOR platform with Cyrus’ ability to redesign protein therapeutics. The lead program was a proprietary interleukin-2, or IL-2, protein agonist designed to selectively promote expansion of regulatory T cells for treatment of patients with autoimmune diseases and other deleterious immune conditions. In return for the licensed intellectual property, the Company made an upfront payment and was obligated to pay certain discovery, development, and sales-based milestones which could have potentially totaled up to approximately $1.5 billion across multiple programs. The Cyrus Agreement was assessed for collaboration components and was determined not to be within the scope of ASC 808 as the risk and rewards are not shared by both parties. The Company expensed costs related to the Cyrus Agreement as incurred until regulatory approval is received in accordance with ASC 730. The Company assessed the capitalization of costs incurred after the receipt of regulatory approval and, if applicable, would have amortized these payments based on the expected useful life of each asset, typically based on the expected commercial exclusivity period. The Company was also obligated to pay Cyrus tiered royalties ranging from mid-single digit to low-double digit percentages of annual net sales of collaboration products which would have been expensed as commercial sales occur. On June 13, 2022, the Company and Cyrus mutually agreed that the preclinical key in-vitro success milestone had been achieved. In October 2023, the Company notified Cyrus of its termination of the Cyrus Agreement, effective December 29, 2023. Stock Purchase Agreement Additionally, on September 7, 2021, the Company entered into a stock purchase agreement, or the Series B Preferred Stock Purchase Agreement, in connection with the Cyrus Agreement. Pursuant to the Series B Preferred Stock Purchase Agreement, the Company purchased 2,326,934 shares of Cyrus’ Series B Preferred Stock, par value $0.0001 per share, at a purchase price of $0.8595 per share for $2.0 million. In accordance with ASC 810, the Company has a variable interest in Cyrus resulting from its equity investment. The Company will share in Cyrus’ expected losses or receive a portion of its expected returns and absorb the variability associated with changes in the entity’s net assets. However, the Company is not the primary beneficiary as it does not have the power to direct the activities most significant to Cyrus, and therefore it is not required to consolidate Cyrus. The Company has recognized the $2.0 million investment of Cyrus’ Series B Preferred Stock at cost on the purchase date. As of December 31, 2023, no impairment indicators are present and therefore the carrying value of the investment in Cyrus is $2.0 million on the accompanying consolidated balance sheet. The Company’s maximum exposure to loss related to this VIE is limited to the carrying value of the investment. The Company has not provided financing to Cyrus other than the amount contractually required by the Series B Preferred Stock Purchase Agreement. Asklepios Biopharmaceutical, Inc. Feasibility Study and License Agreement In August 2019, the Company entered into a feasibility study and license agreement with AskBio, or the AskBio Collaboration Agreement. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to license intellectual property rights to each other as part of a collaboration to research, develop, and commercialize certain AAV gene therapy products utilizing the Company’s ImmTOR platform to enable re-dosing of such AAV gene therapy products to treat serious rare and orphan genetic diseases for which there is a significant unmet medical need. Pursuant to the AskBio Collaboration Agreement, the Company and AskBio agreed to conduct proof of concept studies to potentially validate the use of ImmTOR in conjunction with AskBio’s AAV gene therapy, or SEL-302, (previously disclosed as MMA-101, in combination with ImmTOR) for the treatment of methylmalonic acidemia, or MMA, to mitigate the formation of neutralizing anti-AAV capsid antibodies. On April 29, 2021, the Company was notified by AskBio that it intended to opt-out of development of the MMA indication. The Company and AskBio shared responsibility for the research, development and commercialization of products developed under the SEL-399 program collaboration. The parties also shared research, development, and commercialization costs equally for all collaboration products, but with a right of either party to opt out of certain products, and thereby not be required to share costs for such products. Each party would have received a percentage of net profits under the collaboration equal to the percentage of shared costs borne by such party in the development of such product. Pursuant to the AskBio Collaboration Agreement, AskBio was responsible for manufacturing the AAV capsids and AAV vectors and the Company was responsible for manufacturing ImmTOR. The Company and AskBio mutually agreed to the termination of the AskBio Collaboration Agreement, effective December 13, 2023. For the years ended December 31, 2023 and 2022, the Company recognized $0.1 million and $0.9 million, respectively, of collaboration expense under the AskBio Collaboration Agreement in which actual costs incurred by both parties approximate a 50% cost share. Shenyang Sunshine Pharmaceutical Co., Ltd In May 2014, the Company entered into a license agreement, or the 3SBio License, with Shenyang Sunshine Pharmaceutical Co., Ltd., or 3SBio. The Company has paid to 3SBio an aggregate of $7.0 million in upfront and milestone-based payments under the 3SBio License as of December 31, 2023. The Company is required to make future payments to 3SBio contingent upon the occurrence of events related to the achievement of clinical and regulatory approval milestones of up to an aggregate of $15.0 million for products containing the Company’s ImmTOR platform.

Income Taxes (FY)

Income Taxes (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes As of June 30, 2024, the Company has not recorded any U.S. federal or state income tax benefits for either the net losses the Company has incurred or its earned research and orphan drug credits, due to the uncertainty of realizing a benefit from those items in the future.	Income Taxes The Company provides for income taxes under ASC 740. Under ASC 740, the Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax bases of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. On November 13, 2023, the Company acquired, in accordance with the terms of the Merger Agreement, the assets of Old Cartesian. In accordance with ASC 805-740-25-3, recognition of deferred tax assets and liabilities is required for substantially all temporary differences and acquired tax carryforwards and credits. The Company has computed estimated temporary differences and acquired tax carryforwards and credits as of the transaction date. The Company will not have tax basis in IPR&D booked as part of the purchase accounting. For accounting purposes, the IPR&D will not be amortized and only subject to impairment review and testing. Though the tax effects may be delayed indefinitely, ASC 740-10-55-63 states that “deferred tax liabilities may not be eliminated or reduced because a reporting entity may be able to delay the settlement of those liabilities by delaying the events that would cause taxable temporary differences to reverse.” As such, the Company can potentially only utilize indefinite-lived assets as it relates to this indefinite lived intangible deferred tax liability reversal. As such, the Company has booked a deferred tax liability for the portion of the liability that cannot be reduced based on scheduling. Additionally, a portion of this target deferred tax liability is offset with the Company's pre-Merger deferred tax assets on a combined basis, and as such the portion of deferred tax liability reduced by the Company's pre-Merger deferred tax assets has been charged to income rather than to goodwill. For the year ended December 31, 2023, the Company recognized a current tax benefit, of $19.0 million. For the year ended December 31, 2022, the Company recognized a current tax benefit for penalty abatements received of $0.6 million. For the year ended December 31, 2021, the Company had recorded a tax expense of $16.0 million, inclusive of penalties and interest of $1.3 million assessed as of December 31, 2021. The following table reconciles the federal statutory income tax rate to the Company’s effective income tax rate: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Statutory U.S. federal rate ​ ​ 21.0 % ​ ​ 21.0 % ​ ​ 21.0 % State income taxes - net of federal benefit ​ ​ 2.3 % ​ ​ 1.6 % ​ ​ (166.0)% Permanent items ​ ​ (1.6)% ​ ​ (18.6)% ​ ​ 8.3 % Research tax credits ​ ​ 0.6 % ​ ​ (3.2)% ​ ​ 55.0 % Deferred revenue ​ ​ — % ​ ​ — % ​ ​ 156.5 % Other ​ ​ — % ​ ​ — % ​ ​ (3.7)% Change in fair value of forward contract liabilities ​ ​ (13.2)% ​ ​ — % ​ ​ — % Valuation allowance, net ​ ​ 2.8 % ​ ​ (4.4)% ​ ​v (230.1)% Stock-based compensation ​ ​ (3.9)% ​ ​ 1.8 % ​ ​ (5.2)% Effective income tax rate ​ ​ 8.0% ​ ​ (1.8)% ​ ​ (164.2)% The tax effects of temporary differences that give rise to the Company’s net deferred tax assets are as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 Deferred Tax Assets ​ ​ ​ ​ Net operating loss carryforwards ​ ​ $ 29,841 ​ ​ $ 17,015 Research and development credits ​ ​ 5,649 ​ ​ 2,806 Stock-based compensation expense ​ ​ 8 ​ ​ 5,892 Other expenses ​ ​ 705 ​ ​ 1,697 Deferred revenue ​ ​ 84,626 ​ ​ 83,417 Operating lease liabilities ​ ​ 2,718 ​ ​ 3,186 R&E Capitalization ​ ​ 19,778 ​ ​ 9,588 Patent and license costs ​ ​ 9,140 ​ ​ 7,472 Gross deferred tax assets ​ ​ 152,465 ​ ​ 131,073 Deferred Tax Liabilities ​ ​ ​ ​ Intangible assets ​ ​ $ (41,144) ​ ​ $ — Depreciation ​ ​ (128) ​ ​ (81) Operating lease right-of-use assets ​ ​ (2,751) ​ ​ (3,174) Gross deferred tax liabilities ​ ​ (44,023) ​ ​ (3,255) Net deferred tax assets before valuation allowance ​ ​ 108,442 ​ ​ 127,818 Valuation allowance ​ ​ (124,295) ​ ​ (127,818) Net deferred tax assets/(liabilities) ​ ​ $ (15,853) ​ ​ $ — The Company has provided a full valuation allowance against its net deferred tax assets, outside of the indefinite tax liability booked as part of the Merger. The Company believes that it is more likely than not that the net deferred tax assets will not be realized. Realization of future tax benefits is dependent on many factors, including the Company’s ability to generate taxable income. The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets and concluded that it is more likely than not that the Company will not realize the benefit of its net deferred tax assets. The valuation allowance decreased by $3.5 million for the year ended December 31, 2023, primarily as a result of tax benefit booked as part of the Merger. The valuation allowance decreased by $1.5 million for the year ended December 31, 2022, primarily as a result of pre-tax income and credits. As of December 31, 2023, the Company is in the process of winding down operations in Russia and does not expect any tax liability relating to such operations. At December 31, 2023, the Company has federal net operating loss carryforward of $108.7 million, which can be carried forward indefinitely, subject to an 80% limitation and state net operating loss carryforward of $110.3 million, which will expire at various times through 2043. The Company has $4.9 million and $0.9 million, respectively, of federal and state research and development tax credit carryforwards, which will expire at various times through 2043. Utilization of the NOL carryforwards and research and orphan drug credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, as amended, or the Code, and similar state law due to ownership changes that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. If the Company experiences a change of control, as defined by Section 382 of the Code and similar state law, utilization of the NOL carryforwards or research and orphan drug credit carryforwards may be subject to an annual limitation under Section 382 of the Code, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the NOL carryforwards or research and orphan drug credit carryforwards before utilization. The Company performed an analysis of ownership changes through December 31, 2023. Based on this analysis, the Company does not believe that any of its tax attributes through December 31, 2023 will expire unutilized due to Section 382 limitations. To the extent the Company enters into future equity transactions, there could be a limitation on the Company’s tax attributes. The Company applies ASC 740, Income Taxes During 2023, the Company completed a detailed study of its research and development and orphan drug credits through December 31, 2022. As a result, the Company adjusted its deferred tax asset balances and the impacts are included in the research tax credits and state income taxes - net of federal benefit lines in the effective rate reconciliation above. The Tax Cuts and Jobs Act requires taxpayers to capitalize and amortize, rather than deduct, research and experimental, or R&E, expenditures under section 174 for tax years beginning after December 31, 2021. These rules became effective for the Company during the year ended December 31, 2022. As a result, the Company has capitalized R&E costs of $44.7 million and $29.3 million for the years ended December 31, 2023 and December 31, 2022, respectively. The Company will amortize these costs for tax purposes over five years if the R&E was performed in the U.S. and over 15 years if the R&E was performed outside the U.S. Interest and penalty charges, if any, related to unrecognized tax benefits would be classified as income tax expense in the accompanying statement of operations. As of December 31, 2023, the Company had no accrued interest related to uncertain tax positions. The statute of limitations for assessment by the Internal Revenue Service and Massachusetts tax authorities is open for tax years since inception as the Company claimed research tax credits on its 2020 tax return which remains open for examination for the 2020 year as well as for any year in which a credit has been claimed for. The Company files income tax returns in the United States and Massachusetts. There are currently no federal, state or foreign audits in progress.

Defined Contribution Plan (FY)

Defined Contribution Plan (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Retirement Benefits [Abstract]
Defined Contribution Plan	Defined Contribution Plan The Company maintains a defined contribution plan, or the 401(k) Plan, under Section 401(k) of the Internal Revenue Code. The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. As of January 2022, all matching contributions vest ratably over two years and participant contributions vest immediately. Contributions by the Company totaled less than $0.1 million and $0.1 million during the three months ended June 30, 2024 and 2023, respectively, and $0.1 million and $0.2 million during the six months ended June 30, 2024 and 2023, respectively.	Defined Contribution Plan The Company maintains a defined contribution plan, or the 401(k) Plan, under Section 401(k) of the Internal Revenue Code. The 401(k) Plan covers all employees who meet defined minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pretax basis. The 401(k) Plan provides for matching contributions on a portion of participant contributions pursuant to the 401(k) Plan’s matching formula. As of January 2022, all matching contributions vest ratably over two years and participant contributions vest immediately. Contributions by the Company totaled $0.3 million, $0.3 million, and $0.2 million during each of the years ended December 31, 2023, 2022 and 2021, respectively.

Commitments and Contingencies_2

Commitments and Contingencies (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of June 30, 2024, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non-performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect on the Company’s business, financial position, results of operations or cash flows.	Commitments and Contingencies As of December 31, 2023, the Company was not a party to any litigation that could have a material adverse effect on the Company’s business, financial position, results of operations or cash flows. Other As permitted under Delaware law, the Company indemnifies its directors for certain events or occurrences while the director is, or was, serving at the Company’s request in such capacity. The term of the indemnification is for the director’s lifetime. The maximum potential amount of future payments the Company could be required to make is unlimited; however, the Company has directors’ insurance coverage that limits its exposure and enables it to recover a portion of any future amounts paid. The Company also has indemnification arrangements under certain of its facility leases that require it to indemnify the landlord against certain costs, expenses, fines, suits, claims, demands, liabilities, and actions directly resulting from certain breaches, violations, or non-performance of any covenant or condition of the Company’s lease. The term of the indemnification is for the term of the related lease agreement. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is unlimited. To date, the Company had not experienced any material losses related to any of its indemnification obligations, and no material claims with respect thereto were outstanding. The Company is a party in various other contractual disputes and potential claims arising in the ordinary course of business. The Company does not believe that the resolution of these matters will have a material adverse effect the Company’s business, financial position, results of operations or cash flows.

Restructuring (FY)

Restructuring (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Restructuring	Restructuring In April 2023, in light of current market conditions, the Board of Directors took steps to extend the Company’s cash runway by pausing further development of SEL-302 for the treatment of MMA, and conducting a targeted headcount reduction. On August 17, 2023, the Company announced additional steps to extend cash runway and maximize value for stockholders by continuing to prioritize development of SEL-212 and support of its collaboration with Astellas for Xork, and pausing further development of all of the Company’s other clinical and preclinical product candidates that it was no longer actively advancing. As a result of these measures, the Company implemented a restructuring plan resulting in an approximate 89% reduction of the Company's existing headcount by July 1, 2024. The Company recognized restructuring expenses consisting of one-time cash severance payments and other employee-related costs of $6.4 million for the year ended December 31, 2023 with $5.6 million and $0.8 million recorded to research and development and general and administrative operating expense categories, respectively, on its consolidated statements of operations and comprehensive income (loss) based on each employee's role. Cash payments for employee related restructuring charges of $2.5 million were paid as of December 31, 2023. For the six months ended June 30, 2024, the Company recorded restructuring expenses consisting of one-time cash severance payments and other employee-related costs of $0.8 million, with $0.2 million and $0.6 million recorded to research and development and general and administrative operating expense categories, respectively, on its consolidated statements of operations and comprehensive income (loss) based on each employee’s role. Cash payments for employee related restructuring charges of $4.5 million were paid during the six months ended June 30, 2024. The Company expects that the payments for the restructuring plan will be substantially complete by June 30, 2024. The following table summarizes the change in the Company's accrued restructuring balance (in thousands): ​ ​ Beginning Balance December 31, 2023 ​ ​ Charges ​ ​ Payments ​ ​ Ending Balance June 30, 2024 Severance liability ​ ​ $3,896 ​ ​ $805 ​ ​ $(4,527) ​ ​ $174	Restructuring In April 2023, in light of current market conditions, the Board of Directors, took steps to extend the Company's cash runway by pausing further development of SEL-302 for the treatment of MMA, and conducting a targeted headcount reduction. On August 17, 2023, the Company announced additional steps to extend cash runway and maximize value for stockholders by continuing to prioritize development of SEL-212 and support of its collaboration with Astellas for Xork, and pausing further development of all of the Company’s other clinical and preclinical product candidates that it was no longer actively advancing. As a result of these measures, the Company implemented a restructuring plan resulting in an approximate 79% reduction of the Company's existing headcount by December 31, 2023. The Company recognized restructuring expenses consisting of one-time cash severance payments and other employee-related costs of $6.4 million during the year ended December 31, 2023. Cash payments for employee related restructuring charges of $2.5 million were paid as of December 31, 2023. The Company recorded $5.6 million and $0.8 million based on each employee's role to research and development and general and administrative operating expense categories, respectively, on its consolidated statements of operations and comprehensive income (loss) for the year ended December 31, 2023. The following table summarizes the change in the Company's accrued restructuring balance (in thousands): ​ ​ Beginning Balance December 31, 2022 ​ ​ Charges ​ ​ Payments ​ ​ Ending Balance December 31, 2023 Severance liability ​ ​ $— ​ ​ $6,431 ​ ​ $2,535 ​ ​ $3,896

Subsequent Events (FY)

Subsequent Events (FY)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On July 2, 2024, the Company entered into the July 2024 Purchase Agreement for the July 2024 Private Placement, with certain institutional and accredited investors, or the Purchasers. Pursuant to the July 2024 Purchase Agreement, the Purchasers agreed to purchase an aggregate of 3,563,247 shares of common stock and 2,937,903 shares of Series B Preferred Stock, inclusive of 2,359,500 shares of Series B Preferred Stock purchased by directors and executive officers, and related parties thereto, each at a price per share of $20.00. Each share of Series B Preferred Stock is convertible into one share of the Company’s common stock subject to stockholder approval of a proposal to issue such shares of common stock upon conversion of such shares of Series B Preferred Stock in accordance with the Listing Rules of the Nasdaq Stock Market LLC, or the Series B Conversion Proposal. The July 2024 Private Placement resulted in gross proceeds of approximately $130.0 million before deducting placement agent fees and other offering expenses. Pursuant to the July 2024 Purchase Agreement, the Company has agreed to submit to its stockholders the approval of the Series B Conversion Proposal, at a special meeting of stockholders, which is to be held no later than October 31, 2024. Holders of shares of Series B Preferred Stock are entitled to receive dividends on shares of Series B Preferred Stock equal to, on an as-if-converted-to-common stock basis, and in the same form as dividends actually paid on shares of the common stock. Except as otherwise required by law, the Series B Preferred Stock does not have voting rights. However, as long as any shares of Series B Preferred Stock are outstanding, the Company will not, without the affirmative vote of the holders of a majority of the then outstanding shares of the Series B Preferred Stock, (a) alter or change adversely the powers, preferences or rights given to the Series B Preferred Stock, (b) alter or amend the Certificate of Designation of Preferences, Rights and Limitations of the Series B Non-Voting Convertible Preferred Stock, or the Series B Certificate of Designation, or (c) amend its restated certificate of incorporation, as amended, or other charter documents in any manner that adversely affects any rights of the holders of Series B Preferred Stock. The Series B Preferred Stock does not have a preference upon any liquidation, dissolution or winding-up of the Company. Following stockholder approval of the Series B Conversion Proposal, each share of Series B Preferred Stock will automatically convert into one share of common stock, subject to certain limitations, including that a holder of Series B Preferred Stock is prohibited from converting shares of Series B Preferred Stock into shares of common stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than a specified percentage (to be established by the holder between 0% and 19.9%) of the total number of shares of common stock issued and outstanding immediately after giving effect to such conversion; provided, however, that such beneficial ownership limitation does not apply to TAS Partners LLC or any of its affiliates. Based on the 21,382,485 shares of common stock outstanding on August 7, 2024, there would be 29,865,107 shares of common stock outstanding if all shares of Series A Preferred Stock and all shares of Series B Preferred Stock converted into common stock on such date.	Subsequent Events On February 28, 2024, the Company entered into a lease agreement with 7495 RP, LLC, or the Landlord, pursuant to which it agreed to lease from the Landlord the manufacturing space located at 7495 New Horizon Way, Frederick, Maryland, or the Frederick Lease Agreement. The space consists of 19,199 leasable square feet of integrated manufacturing and office space. The initial term of the Frederick Lease Agreement is expected to commence no later than April 1, 2024, once the Landlord has obtained legal possession of the premises free of the existing tenant and delivered full possession of the premises to the Company, or the Commencement Date. The Frederick Lease Agreement will terminate seven

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Q2) (Policies)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Grand Revenue	Grant Revenue The Company has contracts with government-sponsored organizations for research and development related activities that provide for payments for reimbursable costs. The Company recognizes grant revenue from these contracts as it performs services under these arrangements when the funding is committed. Expenses associated with these contracts are recognized when incurred as research and development expense. Grant revenue and related expenses are presented gross in the consolidated statements of operations and comprehensive income (loss) as the Company has determined it is the primary obligor under the arrangements relative to the research and development services it performs as lead technical expert. Amounts incurred that are subject to reimbursement from the sponsor are recorded as accounts receivable on the consolidated balance sheets.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Not Yet Adopted In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures (ASU 2023-07), which requires an enhanced disclosure of significant segment expenses on an annual and interim basis. This guidance will be effective for the annual periods beginning the year ended December 31, 2024, and for interim periods beginning January 1, 2025. Early adoption is permitted. Upon adoption, the guidance should be applied retrospectively to all prior periods presented in the financial statements. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements. In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (ASU 2023-09), which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the effective tax rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. This guidance will be effective for the annual periods beginning the year ended December 31, 2025. Early adoption is permitted. Upon adoption, the guidance can be applied prospectively or retrospectively. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements.	Recent Accounting Pronouncements Recently Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments Codification Improvements to Topic 326, Financial Instruments-Credit Losses ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2022, with early adoption permitted. The Company adopted the new standard effective January 1, 2023, using a modified retrospective transition method, and there was no impact on its consolidated financial statements or results of operations upon adoption. In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350), Not Yet Adopted In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (FY) (Policies)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Accounting Policies [Abstract]
Principles of Consolidation		Principles of Consolidation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, Selecta (RUS), LLC, or Selecta (RUS), a Russian limited liability corporation, and Selecta Biosciences Security Corporation, a Massachusetts securities corporation, and Cartesian Bio, LLC, a Delaware limited liability company, which is a variable interest entity for which the Company is the primary beneficiary. All significant intercompany accounts and transactions have been eliminated.
Use of Estimates		Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities as of the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. The Company’s management considers many factors in selecting appropriate financial accounting policies and controls, and bases its estimates on historical experience and other market-specific or other relevant assumptions that it believes to be reasonable under the circumstances. In preparing these consolidated financial statements, management used significant estimates in the following areas, among others: estimated fair value of the intangible assets acquired in connection with the Merger, estimated fair value of the CVRs, deferred income taxes, revenue recognition and estimating accrued research and development expenses. The Company assesses the above estimates on an ongoing basis; however, actual results could materially differ from those estimates.
Segment Information		Segment Information The Company views its operations and manages its business in one operating segment, which prior to the Merger related to the research and development of nanoparticle immunomodulatory drugs for the treatment and prevention of human diseases and subsequent to the Merger relates to the research and development of cell therapy product candidates.
Cash Equivalents, Restricted Cash, Marketable Securities and Investments		Cash Equivalents, Restricted Cash, Marketable Securities and Investments Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Marketable securities consist of securities with remaining maturities greater than 90 days when purchased. The Company classifies these marketable securities as available-for-sale and records them at fair value in the accompanying consolidated balance sheets. Marketable securities with less than one year until maturity are classified as short term, while marketable securities with maturities greater than one year are classified as long term. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment. Although available to be sold to meet operating needs or otherwise, securities are generally held through maturity. The cost of securities sold is determined based on the specific identification method for purposes of recording realized gains and losses. The Company has also in the past invested in equity securities of a company whose securities are not publicly traded and where fair value is not readily available. This investment is recorded using cost minus impairment adjusted for changes in observable prices, depending on our ownership percentage and other factors that suggest we have significant influence. The Company monitors this investment to evaluate whether any increase or decline in its value has occurred, based on the implied value of recent company financings, public market prices of comparable companies and general market conditions. This investment is included in investments in the consolidated balance sheets.
Concentrations of Credit Risk and Off-Balance Sheet Risk		Concentrations of Credit Risk and Off-Balance Sheet Risk Financial instruments that potentially subject the Company to concentration of credit risk consist primarily of cash, cash equivalents, short-term deposits and marketable securities, investments, and accounts receivable. Cash and cash equivalents are deposited with federally insured financial institutions in the United States and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially creditworthy and, accordingly, minimal risk exists with respect to those balances. The Company also maintains cash in Russian bank accounts in denominations of both Russian rubles and U.S. dollars. As of December 31, 2023, the Company maintained approximately $0.2 million in Russian bank accounts in denominations of both Russian rubles and U.S. dollars.
Fair Value of Financial Instruments		Fair Value of Financial Instruments The Company’s financial instruments consist mainly of cash equivalents, restricted cash, accounts payable, loans payable, marketable securities, investments, warrants to purchase common stock, forward contract liabilities, and contingent value rights. The carrying amounts of cash equivalents, restricted cash, accounts receivable, and accounts payable approximate their estimated fair value due to their short-term maturities. Accounting standards define fair value as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. A three-level hierarchy is used to prioritize the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements), and the lowest priority to unobservable inputs (Level 3 measurements). The three levels of the fair value hierarchy are described below: Level 1 —Level 1 inputs are quoted prices in active markets for identical assets or liabilities that the reporting entity has the ability to access at the measurement date. Level 2 —Level 2 inputs are inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. If the asset or liability has a specified (contractual) term, a Level 2 input must be observable for substantially the full term of the asset or liability. Level 3 —Level 3 inputs are unobservable inputs for the asset or liability in which there is little, if any, market activity for the asset or liability at the measurement date. To the extent that a valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The fair value of warrant liabilities and contingent value rights are determined using Level 3 inputs. Fair value is a market-based measure considered from the perspective of a market participant rather than an entity-specific measure. Therefore, even when market assumptions are not readily available, the Company’s own assumptions are set to reflect those that market participants would use in pricing the asset or liability at the measurement date. The Company uses prices and inputs that are current as of the measurement date, including during periods of market dislocation. In periods of market dislocation, the observability of prices and inputs may change for many instruments. This condition could cause an instrument to be reclassified within levels in the fair value hierarchy. The carrying amounts reflected in the consolidated balance sheet for investments approximate fair value and are assessed for impairment quarterly.
Property and Equipment		Property and Equipment Property and equipment are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the respective assets, generally seven years for furniture and fixtures, five years for laboratory equipment, software and office equipment and three years for computer equipment. Leasehold improvements are amortized over their useful life or the life of the lease, whichever is shorter. Major additions and betterments are capitalized. Maintenance and repairs, which do not improve or extend the life of the respective assets, are charged to operations as incurred. Costs incurred for construction in progress are recorded as assets and are not amortized until the construction is substantially complete and the assets are ready for their intended use.
Impairment of Long-Lived Assets		Impairment of Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. In order to determine if assets have been impaired, assets are tested at the lowest level for which identifiable independent cash flows are available. An impairment loss is recognized when the sum of projected undiscounted cash flows is less than the carrying value of the asset group. The measurement of the impairment loss to be recognized is based on the difference between the fair value and the carrying value of the asset group. The Company recognized a $0.7 million impairment charge on a right-of-use asset during the year ended December 31, 2023.
Debt Issuance Costs		Debt Issuance Costs Debt issuance costs and fees paid to lenders are recorded as a direct deduction from the face amount of the related debt. Debt issuance costs are amortized over the term of the related debt using the effective interest method and recorded as interest expense.
Accumulated Other Comprehensive Income (Loss)		Accumulated Other Comprehensive Income (Loss) Comprehensive income (loss) is defined as the change in the equity of a business entity during a period from transactions and other events and circumstances from non-owner sources. It includes all changes in equity during a period except those resulting from investments by owners and distributions to owners. Comprehensive income (loss) consists of: (i) all components of net income (loss) and (ii) all components of comprehensive income (loss) other than net income (loss), referred to as other comprehensive income (loss). Other comprehensive income (loss) is comprised of unrealized gains and losses on debt securities and foreign currency translation adjustments.
Revenue Recognition		Revenue Recognition Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. Pursuant to ASC Topic 606, Revenue from Contracts with Customers (ASC 606) Collaboration and License Revenue: The Company currently generates its revenue through collaboration and license agreements with strategic collaborators for the development and commercialization of product candidates. Collaboration and license agreements with customers are generally accounted for in accordance with ASC 606. The Company analyzes collaboration arrangements by first assessing whether they are within the scope of ASC Topic 808, ), and evaluates whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards that are dependent on the commercial success of such activities. Collaboration agreements with customers that are not within the scope of ASC 808 are accounted for in accordance with ASC 606. To the extent the collaboration agreement is within the scope of ASC 808, the Company also assesses whether any aspects of the agreement are within the scope of other accounting literature (specifically ASC 606). If the Company concludes that some or all aspects of the agreement are distinct and represent a transaction with a customer, the Company accounts for those aspects of the arrangement within the scope of ASC 606. The Company recognizes the shared costs incurred that are not within the scope of other accounting literature as a component of the related expense in the period incurred by analogy to ASC Topic 730, , and records reimbursements from counterparties as an offset to the related research and development costs. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements in accordance with ASC 606, the Company performs the five steps above. As part of the accounting for the arrangement, the Company must develop assumptions that require judgment to determine the stand-alone selling price for each performance obligation identified in the contract. The Company uses key assumptions to determine the stand-alone selling price, which may include market conditions, reimbursement rates for personnel costs, development timelines and probabilities of regulatory success. The terms of the Company’s arrangements typically include one or more of the following: (i) upfront fees; (ii) milestone payments related to the achievement of development, regulatory, or commercial goals; (iii) royalties on net sales of licensed products; (iv) reimbursements or cost-sharing of research and development expenses; and (v) profit/loss sharing arising from co-promotion arrangements. Licenses of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other promised goods and services identified in the arrangement, the Company recognizes revenues from non-refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. If not distinct, the license is combined with other promised goods and services in the contract. For licenses that are combined with other promised goods and services, the Company assesses the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Optional licenses are evaluated to determine if they are issued at a discount, and therefore, represent material rights and accounted for as separate performance obligations. Milestone Payments: At the inception of each arrangement that includes developmental and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. The Company also evaluates the milestone to determine whether they are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated, otherwise, such amounts are constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint, and if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. Manufacturing Supply Services: Arrangements that include a promise for future supply of drug substance or drug product for either clinical development or commercial supply at the customer’s discretion are evaluated to determine if they are distinct and optional. For optional services that are distinct, the Company assesses if they are priced at a discount, and therefore, provide a material right to the licensee to be accounted for as separate performance obligations. Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint.
Research and Development Costs		Research and Development Costs Costs incurred in the research and development of the Company’s products are expensed as incurred. Research and development expenses include costs incurred in performing research and development activities, including salaries and benefits, stock-based compensation expenses, facilities cost, overhead costs, contract services, supplies and other outside costs. Nonrefundable advance payments for goods and services that will be used in future research and development activities are expensed when the activity has been performed or when the goods have been received rather than when the payment is made.
Clinical Trial Costs		Clinical Trial Costs Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include patient costs, clinical research organization costs and costs for data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected on the consolidated balance sheets as a prepaid asset or accrued clinical trial cost. These third party agreements are generally cancellable, and related costs are recorded as research and development expenses as incurred. Non-refundable advance clinical payments for goods or services that will be used or rendered for future research and development activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. The Company also records accruals for estimated ongoing clinical research and development costs. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates may be made in determining the accrued balances at the end of any reporting period. Actual results could differ from the estimates made by the Company. The historical clinical accrual estimates made by the Company have not been materially different from the actual costs. In June 2020, the Company and Sobi entered into a License and Development Agreement, which was amended in October 2023. Pursuant to the Sobi License, clinical trial costs incurred to complete development of the SEL-212 product candidate, including but not limited to costs incurred while conducting and completing the Phase 3 DISSOLVE trials, were reimbursed by Sobi. These costs, when reimbursed, were recognized as revenue consistent with the revenue recognition methodology disclosed in Note 14. The reimbursable costs exclude any costs of additional development activities required that are related to the ImmTOR platform and that are unrelated to SEL-212. In January 2023, the Company and Astellas entered into the Astellas Agreement. Pursuant to the Astellas Agreement, Astellas will reimburse the Company for 25% of all budgeted costs incurred to complete the development of Xork for use in Pompe disease with an Astellas gene therapy investigational or authorized product. These costs, when reimbursed, will be recognized as revenue consistent with the revenue recognition methodology disclosed in Note 14.
Income Taxes		Income Taxes The Company provides deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the Company’s financial statement carrying amounts and the tax basis of assets and liabilities using enacted tax rates expected to be in effect in the years in which the differences are expected to reverse. A valuation allowance is provided to reduce the deferred tax assets to the amount that will more-likely-than-not be realized. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more-likely-than-not that a position will be sustained, none of the benefit attributable to the position is recognized. The tax benefit to be recognized for any tax position that meets the more-likely-than-not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred interest and penalties related to uncertain tax positions.
Warrants		Warrants The Company determines the accounting classification of warrants that are issued, as either liability or equity, by first assessing whether the warrants meet liability classification in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock If warrants do not meet liability classification under ASC 480-10, the Company assesses the requirements under ASC 815-40, which states that contracts that require or may require the issuer to settle the contract for cash are liabilities recorded at fair value, irrespective of the likelihood of the transaction occurring that triggers the net cash settlement feature. If the warrants do not require liability classification under ASC 815-40, in order to conclude equity classification, the Company assesses whether the warrants are indexed to its common stock and whether the warrants are classified as equity under ASC 815-40 or other applicable GAAP. After all relevant assessments are made, the Company concludes whether the warrants are classified as liability or equity. Liability classified warrants are required to be accounted for at fair value both on the date of issuance and on subsequent accounting period ending dates, with all changes in fair value after the issuance date recorded in the statements of operations as a gain or loss. Equity classified warrants are accounted for at fair value on the issuance date with no changes in fair value recognized after the issuance date.
Stock-Based Compensation		Stock-Based Compensation The Company accounts for all stock-based compensation granted to employees and non-employees using a fair value method. Stock-based compensation is measured at the grant date fair value and is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis, net of estimated forfeitures. To the extent that actual forfeitures differ from the Company’s estimates, the differences are recorded as a cumulative adjustment in the period the estimates were adjusted. Stock-based compensation expense recognized in the consolidated financial statements is based on awards that ultimately vest.
Net (Loss) Income Per Share		Net (Loss) Income Per Share The Company applies the two-class method to compute basic and diluted net (loss) income per share attributable to common stockholders when it has issued shares that meet the definition of participating securities. The two-class method determines net (loss) income per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires (loss) income available to common stockholders for the period to be allocated between common and participating securities based upon their respective rights to share in the earnings as if all (loss) income for the period had been distributed. The Company's Series A Preferred Stock and 2022 Warrants participate in any dividends declared by the Company and are therefore considered to be participating securities. The participating securities are not required to participate in the losses of the Company, and therefore during periods of loss there is no allocation required under the two-class method. Basic net (loss) income per share attributable to common stockholders is computed by dividing the net (loss) income attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net (loss) income attributable to common stockholders is computed by adjusting net (loss) income per share attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities. Diluted net (loss) income per share attributable to common stockholders is computed by dividing the diluted net (loss) income attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period, including potential dilutive common shares. For purpose of this calculation, outstanding options to purchase common stock and Series A Preferred Stock, forward contracts to issue Series A Preferred Stock, restricted stock units, warrants to purchase common stock, employee stock purchase plan stock, contingently issuable shares, and Series A Preferred Stock are considered potential dilutive common shares.
Contingent Liabilities		Contingent Liabilities The Company accounts for its contingent liabilities in accordance with ASC No. 450, Contingencies
Leases		Leases The Company accounts for its leases in accordance with ASC Topic 842, Leases (ASC 842) In accordance with the guidance in ASC 842, the fixed and in-substance fixed contract consideration must be allocated to lease and non-lease components based on their relative fair values. Non-components of a contract (e.g., administrative tasks that do not transfer a good or service to the Company, reimbursement or payment of a lessor’s cost, etc.) do not receive an allocation of the consideration in the contract. Although allocation of consideration of lease and non-lease components is required, the Company elected the practical expedient to not separate lease components (e.g. land, building, etc.) and non-lease components (e.g., common area maintenance, consumables, etc.). The lease component results in an operating right-of-use asset being recorded on the balance sheet and amortized on a straight-line basis as lease expense. Right-of-use assets and operating lease liabilities are remeasured upon certain modifications to leases using the present value of remaining lease payments and the estimated incremental borrowing rate upon lease modification. The Company enters into lease agreements with terms generally ranging from two one
Acquisitions		Acquisitions The Company evaluates acquisitions of assets and other similar transactions to assess whether or not the transaction should be accounted for as a business combination or asset acquisition by first applying a screen to determine if substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or group of similar identifiable assets. If the screen is met, the transaction is accounted for as an asset acquisition. If the screen is not met, further determination is required as to whether or not the Company has acquired inputs and processes that have the ability to create outputs, which would meet the requirements of a business. If determined to be a business combination, the Company accounts for the transaction under the acquisition method of accounting which requires the acquiring entity in a business combination to recognize the fair value of all assets acquired, liabilities assumed, and any non-controlling interest in the acquiree and establishes the acquisition date as the fair value measurement point. Accordingly, the Company recognizes assets acquired and liabilities assumed in business combinations based on the fair value estimates as of the date of acquisition. In accordance with ASC 805, Business Combinations, or ASC 805, the Company recognizes and measures goodwill as of the acquisition date, as the excess of the fair value of the consideration paid over the fair value of the identified net assets acquired.
Goodwill		Goodwill Goodwill represents the amount of consideration paid in excess of the fair value of the identified net assets acquired as a result of the Company’s business acquisitions accounted for using the acquisition method of accounting. Goodwill is not amortized and is subject to impairment testing at a reporting unit level on an annual basis or when a triggering event occurs that may indicate the carrying value of the goodwill is impaired. An entity is permitted to first assess qualitative factors to determine if a quantitative impairment test is necessary. Such qualitative factors include macroeconomic conditions, industry and market considerations, cost factors, overall financial performance and other relevant events. Further testing is only required if the entity determines, based on the qualitative assessment, that it is more likely than not that the fair value of the reporting unit is less than its carrying amount. The Company evaluates goodwill for impairment at least annually on October 1 and whenever facts and circumstances indicate that their carrying amounts may not be recoverable. For the year ended December 31, 2023, the Company determined that there was no impairment to goodwill.
Indefinite-Lived Intangible Assets		Indefinite-Lived Intangible Assets Indefinite-lived intangible assets consist of in-process research and development, or IPR&D. The fair values of IPR&D assets acquired in business combinations are capitalized. These assets are treated as indefinite-lived intangible assets until completion or abandonment of the projects, at which time the assets are amortized over the remaining useful life or written off, as appropriate. Intangible assets with indefinite lives, including IPR&D, are tested for impairment if impairment indicators arise and, at a minimum, annually. However, an entity is permitted to first assess qualitative factors to determine if a quantitative impairment test is necessary. Further testing is only required if the entity determines, based on the qualitative assessment, that it is more likely than not that an indefinite-lived intangible asset’s fair value is less than its carrying amount. Otherwise, no further impairment testing is required. The indefinite-lived intangible asset impairment test consists of a one-step analysis that compares the fair value of the intangible asset with its carrying amount. If the carrying amount of an intangible asset exceeds its fair value, an impairment loss is recognized in an amount equal to that excess. The Company considers many factors in evaluating whether the value of its intangible assets with indefinite lives may not be recoverable, including, but not limited to, expected growth rates, the cost of equity and debt capital, general economic conditions, the Company’s outlook and market performance of the Company’s industry and recent and forecasted financial performance. The Company evaluates indefinite-lived intangible assets for impairment at least annually on October 1 and whenever facts and circumstances indicate that their carrying amounts may not be recoverable. For the year ended December 31, 2023, the Company determined that there was no impairment to the IPR&D assets.
Series A Preferred Stock and Options		Series A Preferred Stock The Company records the Series A Preferred Stock upon issuance at its fair value. The fair value includes the original issuance price, the settlement of any related forward contract, and is less issuance costs. The Company classifies its Series A Preferred Stock outside of stockholders’ equity as the redemption of such shares is outside the Company’s control. The Company does not adjust the carrying value of the Series A Preferred Stock to redemption value until it is probable of becoming redeemable, which the Company did not conclude was probable as of December 31, 2023. Series A Preferred Stock Options The Company classifies a portion of the fair value of the vested stock options for Series A Preferred Stock equal to the estimated redemption value on the measurement date outside of stockholders’ equity, as the redemption of the shares underlying the options are outside the Company’s control. Any fair value in excess of the estimated redemption value is recognized as additional paid-in capital. The estimated redemption value is based on the intrinsic value of the option. The Company does not adjust the carrying value of the stock options for Series A Preferred Stock until the underlying Series A Preferred Stock is probable of becoming redeemable. The Company concluded the redemption was not probable of occurring as of December 31, 2023. The Company records the stock options for Series A Preferred Stock based on the intrinsic value of the vested options.
Variable Interest Entities		Variable Interest Entities The Company evaluates its variable interests in variable interest entities, or VIEs, and consolidates VIEs when the Company is the primary beneficiary. The Company determines whether it is the primary beneficiary of a VIE based on its assessment of whether the Company possesses both (i) the power to direct the activities that most significantly affect the VIE’s economic performance and (ii) the obligation to absorb losses that could be significant to the VIE or the right to receive benefits that could be significant to the VIE. The Company reevaluates the accounting for its VIEs upon the occurrence of events that could change the primary beneficiary conclusion.
Contingent Value Right Liability		Contingent Value Right Liability The CVRs distributed by the Company pursuant to the terms of the CVR Agreement represent financial instruments that are accounted for under the fair value option election in ASC 825, Financial Instruments
Forward Contract Liabilities		Forward Contract Liabilities The Company accounts for contracts related to the future issuance of Series A Preferred Stock as a liability because the underlying shares of Series A Preferred Stock include a redemption feature that may require the Company to settle the instrument by transferring an asset. The forward contract liability is carried at fair value through the date the underlying Series A Preferred Stock are issued. The fair value of the forward contract liability was initially measured based on the fair value of the Series A Preferred Stock issued in the November 2023 Private Placement (see Note 11), less the purchase price, if any. Subsequent measurement of the fair value of the forward contract liability is based on the market price of the Company’s common stock, which represents the redemption and conversion value of the Series A Preferred Stock, less the purchase price, if any, on an as-converted basis. The remeasurement of the forward contract liability is based on Level 2 inputs within the fair value hierarchy as it’s based on observable market data. Changes in fair value of the liability are presented within change in fair value of forward contract liabilities in the consolidated statements of operations and comprehensive income (loss).
Recent Accounting Pronouncements	Recent Accounting Pronouncements Not Yet Adopted In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures (ASU 2023-07), which requires an enhanced disclosure of significant segment expenses on an annual and interim basis. This guidance will be effective for the annual periods beginning the year ended December 31, 2024, and for interim periods beginning January 1, 2025. Early adoption is permitted. Upon adoption, the guidance should be applied retrospectively to all prior periods presented in the financial statements. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements. In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures (ASU 2023-09), which improves the transparency of income tax disclosures by requiring consistent categories and greater disaggregation of information in the effective tax rate reconciliation and income taxes paid disaggregated by jurisdiction. It also includes certain other amendments to improve the effectiveness of income tax disclosures. This guidance will be effective for the annual periods beginning the year ended December 31, 2025. Early adoption is permitted. Upon adoption, the guidance can be applied prospectively or retrospectively. The Company does not expect the adoption of this guidance to have a material impact on its consolidated financial statements.	Recent Accounting Pronouncements Recently Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments Codification Improvements to Topic 326, Financial Instruments-Credit Losses ASU 2016-13 also requires enhanced disclosures to help financial statement users better understand significant estimates and judgments used in estimating credit losses. This ASU is effective for smaller reporting companies for fiscal years beginning after December 15, 2022, with early adoption permitted. The Company adopted the new standard effective January 1, 2023, using a modified retrospective transition method, and there was no impact on its consolidated financial statements or results of operations upon adoption. In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350), Not Yet Adopted In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures In December 2023, the FASB issued ASU 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures

Merger (Q2) (Tables)

Merger (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Schedule of Fair Value of Consideration	The total fair value of the consideration of $168.5 million as of the Closing Date is summarized as follows (in thousands): Forward contract to issue common stock ​ ​ $ 2,713 Forward contract to issue Series A Preferred Stock ​ ​ 155,308 Stock options allocated to consideration paid ​ ​ 10,444 Total consideration ​ ​ $168,465	The total fair value of the consideration of $168.5 million as of the Closing Date is summarized as follows (in thousands): Forward contract to issue common stock ​ ​ $ 2,713 Forward contract to issue Series A Preferred Stock ​ ​ 155,308 Stock options allocated to consideration paid ​ ​ 10,444 Total consideration ​ ​ $168,465
Schedule of Assets Acquired and Liabilities Assumed	The following table presents the allocation of the purchase price to the estimated fair values of the assets acquired and liabilities assumed as of the Closing Date (in thousands): ​ ​ As of November 13, 2023 Assets acquired: ​ ​ Cash and cash equivalents ​ ​ $ 6,561 Prepaid expenses and other current assets ​ ​ 309 Property and equipment, net ​ ​ 215 Right-of-use asset, net ​ ​ 915 In-process research and development assets ​ ​ 150,600 Goodwill ​ ​ 48,163 ​ ​ $206,763 Liabilities assumed ​ ​ Accrued expenses and other current liabilities ​ ​ $ 2,530 Lease liability ​ ​ 292 Lease liability, net of current portion ​ ​ 623 Deferred tax liability ​ ​ 34,853 ​ ​ $ 38,298 Net assets acquired ​ ​ $168,465	The following table presents the allocation of the purchase price to the estimated fair values of the assets acquired and liabilities assumed as of the Closing Date (in thousands): ​ ​ As of November 13, 2023 Assets acquired: ​ ​ Cash and cash equivalents ​ ​ $ 6,561 Prepaid expenses and other current assets ​ ​ 309 Property and equipment, net ​ ​ 215 Right-of-use asset, net ​ ​ 915 In-process research and development assets ​ ​ 150,600 Goodwill ​ ​ 48,163 ​ ​ $206,763 Liabilities assumed ​ ​ Accrued expenses and other current liabilities ​ ​ $ 2,530 Lease liability ​ ​ $ 292 Lease liability, net of current portion ​ ​ $ 623 Deferred tax liability ​ ​ $ 34,853 ​ ​ $ 38,298 Net assets acquired ​ ​ $168,465
Schedule of Intangible Assets Acquired	The following summarizes the Company’s intangible assets acquired in the Merger (in thousands): ​ ​ Acquisition Date Fair Value Descartes-08 for MG ​ ​ $ 93,900 Descartes-08 for SLE ​ ​ 56,700 Total in-process research and development assets ​ ​ $150,600	The following summarizes the Company’s intangible assets acquired in the Merger and their carrying value as of December 31, 2023 (in thousands): ​ ​ Acquisition Date Fair Value ​ ​ Impairment ​ ​ Carrying Value at December 31, 2023 Descartes-08 for MG ​ ​ $ 93,900 ​ ​ $— ​ ​ $ 93,900 Descartes-08 for SLE ​ ​ 56,700 ​ ​ — ​ ​ 56,700 Total in-process research and development assets ​ ​ $150,600 ​ ​ $— ​ ​ $150,600

Net Income (Loss) Per Share A_2

Net Income (Loss) Per Share Allocable to Common Stockholders (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted Net (Loss) Income Per Share	The following table sets forth the computation of basic and diluted net income (loss) per share allocable to common stockholders for the three and six months ended June 30, 2024 and 2023 (in thousands, except share and per-share data): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Numerator: ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ ​ $ 13,836 ​ ​ $ (11,387) ​ ​ $ (42,988) ​ ​ $ (33,050) Less: Undistributed earnings allocable to participating securities ​ ​ (4,208) ​ ​ — ​ ​ — ​ ​ — Net income (loss) allocable to shares of common stock - basic and diluted ​ ​ $ 9,628 ​ ​ $ (11,387) ​ ​ $ (42,988) ​ ​ $ (33,050) Denominator: ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ ​ 16,723,479 ​ ​ 5,114,747 ​ ​ 11,068,749 ​ ​ 5,113,213 Dilutive effect of employee equity incentive plans ​ ​ 1,067,664 ​ ​ — ​ ​ — ​ ​ — Weighted-average common shares outstanding - diluted ​ ​ 17,791,143 ​ ​ 5,114,747 ​ ​ 11,068,749 ​ ​ 5,113,213 Net income (loss) per share allocable to common stockholders: ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ ​ $ 0.58 ​ ​ $ (2.23) ​ ​ $ (3.88) ​ ​ $ (6.46) Diluted ​ ​ $ 0.54 ​ ​ $ (2.23) ​ ​ $ (3.88) ​ ​ $ (6.46)	The following table sets forth the computation of basic and diluted net (loss) income per share (in thousands, except share and per-share data): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Numerator: ​ ​ ​ ​ ​ ​ Net (loss) income ​ ​ $(219,710) ​ ​ $ 35,379 ​ ​ $(25,687) Less: CVR distribution to participating securities ​ ​ (37,550) ​ ​ — ​ ​ — Net (loss) income allocable to shares of common stock - basic ​ ​ (257,260) ​ ​ 35,379 ​ ​ (25,687) Less: Change in fair value of warrants ​ ​ — ​ ​ (20,882) ​ ​ — Net (loss) income allocable to shares of common stock - diluted ​ ​ $(257,260) ​ ​ $ 14,497 ​ ​ $(25,687) ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Denominator: ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ ​ 155,109,561 ​ ​ 144,758,555 ​ ​ 114,328,798 Dilutive effect of employee equity incentive plans and outstanding warrants ​ ​ — ​ ​ 1,116,334 ​ ​ — Weighted-average common shares used in per share calculations - diluted ​ ​ 155,109,561 ​ ​ 145,874,889 ​ ​ 114,328,798 Net (loss) income per share: ​ ​ ​ ​ ​ ​ Basic ​ ​ $ (1.66) ​ ​ $ 0.24 ​ ​ $ (0.22) Diluted ​ ​ $ (1.66) ​ ​ $ 0.10 ​ ​ $ (0.22)
Schedule of Potential Common Shares Issuable Upon Conversion of Warrants	The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net income (loss) per share allocable to common stockholders for all periods presented, as the effect would have been anti-dilutive: ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Common stock options, restricted stock units and ESPP shares ​ ​ 740,211 ​ ​ 753,590 ​ ​ 2,344,017 ​ ​ 753,590 Warrants to purchase common stock ​ ​ 975,132 ​ ​ 1,040,943 ​ ​ 975,132 ​ ​ 1,040,943 Series A Preferred Stock ​ ​ 5,544,719 ​ ​ — ​ ​ 5,544,719 ​ ​ — Total ​ ​ 7,260,062 ​ ​ 1,794,533 ​ ​ 8,863,868 ​ ​ 1,794,533	The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net (loss) income per share for all periods presented, as the effect would have been anti-dilutive: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Warrants to purchase common stock ​ ​ 31,224,703 ​ ​ 213,339 ​ ​ 10,735,980 Series A Preferred Stock ​ ​ 435,120,513 ​ ​ — ​ ​ — Forward contract to issue Series A Preferred Stock ​ ​ 99,140,326 ​ ​ — ​ ​ — Common stock options, RSUs and ESPP shares ​ ​ 23,306,661 ​ ​ 17,800,034 ​ ​ 11,492,002 Series A Preferred Stock options ​ ​ 14,112,299 ​ ​ — ​ ​ — Total ​ ​ 602,904,502 ​ ​ 18,013,373 ​ ​ 22,227,982

Fair Value Measurements (Q2) (T

Fair Value Measurements (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on a Recurring Basis	The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2024 and December 31, 2023 (in thousands): ​ ​ June 30, 2024 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 39,181 ​ ​ $39,181 ​ ​ $— ​ ​ $ — Total assets ​ ​ $ 39,181 ​ ​ $39,181 ​ ​ $— ​ ​ $ — Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 9,260 ​ ​ $ — ​ ​ $— ​ ​ $ 9,260 Contingent value right liability ​ ​ $395,400 ​ ​ $ — ​ ​ $— ​ ​ $395,400 Total liabilities ​ ​ $404,660 ​ ​ $ — ​ ​ $— ​ ​ $404,660 ​ ​ December 31, 2023 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Total assets ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 6,394 ​ ​ $ — ​ ​ $ — ​ ​ $ 6,394 Contingent value right liability ​ ​ $358,600 ​ ​ $ — ​ ​ $ — ​ ​ $358,600 Forward contract liabilities ​ ​ $ 28,307 ​ ​ $ — ​ ​ $28,307 ​ ​ $ — Total liabilities ​ ​ $393,301 ​ ​ $ — ​ ​ $28,307 ​ ​ $364,994	The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands): ​ ​ December 31, 2023 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Total assets ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 6,394 ​ ​ $ — ​ ​ $ — ​ ​ $ 6,394 Contingent value right liability ​ ​ 358,600 ​ ​ — ​ ​ — ​ ​ 358,600 Forward contract liabilities ​ ​ 28,307 ​ ​ — ​ ​ 28,307 ​ ​ — Total liabilities ​ ​ $393,301 ​ ​ $ — ​ ​ $28,307 ​ ​ $364,994 ​ ​ December 31, 2022 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $53,552 ​ ​ $53,552 ​ ​ $ — ​ ​ $— Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ U.S. government agency securities and treasuries ​ ​ 13,557 ​ ​ — ​ ​ 13,557 ​ ​ — Corporate bonds ​ ​ 1,951 ​ ​ — ​ ​ 1,951 ​ ​ — Commercial paper ​ ​ 12,656 ​ ​ — ​ ​ 12,656 ​ ​ — Total assets ​ ​ $81,716 ​ ​ $53,552 ​ ​ $28,164 ​ ​ $— ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $19,140 ​ ​ $— ​ ​ $— ​ ​ $19,140 Total liabilities ​ ​ $19,140 ​ ​ $— ​ ​ $— ​ ​ $19,140
Schedule of Cash and Cash Equivalents	The Company’s consolidated statements of cash flows include the following as of June 30, 2024 and 2023 (in thousands): ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Cash and cash equivalents ​ ​ $87,227 ​ ​ $112,027 Short-term restricted cash ​ ​ — ​ ​ 1,600 Long-term restricted cash ​ ​ 1,669 ​ ​ 1,377 Total cash, cash equivalents, and restricted cash ​ ​ $88,896 ​ ​ $115,004	The Company’s consolidated statement of cash flows includes the following as of December 31, 2023, 2022 and 2021 (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Cash and cash equivalents ​ ​ $76,911 ​ ​ $106,438 ​ ​ $114,057 Short-term restricted cash ​ ​ — ​ ​ 289 ​ ​ — Long-term restricted cash ​ ​ 1,377 ​ ​ 1,311 ​ ​ 1,379 Total cash, cash equivalents, and restricted cash ​ ​ $78,288 ​ ​ $108,038 ​ ​ $115,436
Schedule of Restricted Cash and Cash Equivalents	The Company’s consolidated statements of cash flows include the following as of June 30, 2024 and 2023 (in thousands): ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Cash and cash equivalents ​ ​ $87,227 ​ ​ $112,027 Short-term restricted cash ​ ​ — ​ ​ 1,600 Long-term restricted cash ​ ​ 1,669 ​ ​ 1,377 Total cash, cash equivalents, and restricted cash ​ ​ $88,896 ​ ​ $115,004	The Company’s consolidated statement of cash flows includes the following as of December 31, 2023, 2022 and 2021 (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Cash and cash equivalents ​ ​ $76,911 ​ ​ $106,438 ​ ​ $114,057 Short-term restricted cash ​ ​ — ​ ​ 289 ​ ​ — Long-term restricted cash ​ ​ 1,377 ​ ​ 1,311 ​ ​ 1,379 Total cash, cash equivalents, and restricted cash ​ ​ $78,288 ​ ​ $108,038 ​ ​ $115,436
Schedule of Fair Value Measurement Inputs and Valuation Techniques	A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2019 Warrants liability is as follows: ​ ​ June 30, 2024 ​ ​ December 31, 2023 Risk-free interest rate ​ ​ 5.33% ​ ​ 4.79% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 0.49 ​ ​ 0.98 Expected volatility ​ ​ 110.52% ​ ​ 83.67% A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2022 Warrants liability is as follows: ​ ​ June 30, 2024 ​ ​ December 31, 2023 Risk-free interest rate ​ ​ 4.52% ​ ​ 4.01% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 2.78 ​ ​ 3.28 Expected volatility ​ ​ 87.23% ​ ​ 84.09% The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows: ​ ​ June 30, 2024 Estimated cash flow dates ​ ​ 2024-2038 Estimated probability of success ​ ​ 95.0% - 100.0% Expected volatility of future revenues ​ ​ 22.0% ​ ​ December 31, 2023 Estimated cash flow dates ​ ​ 2024 - 2038 Estimated probability of success ​ ​ 95.0% Risk-adjusted discount rate ​ ​ 13.7%	A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2019 Warrants liability is as follows: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Risk-free interest rate ​ ​ 4.79% ​ ​ 4.74% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 0.98 ​ ​ 1.98 Expected volatility ​ ​ 83.67% ​ ​ 79.92% A summary of the Black-Scholes valuation model assumptions used to record the fair value of the 2022 Warrants liability is as follows: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Risk-free interest rate ​ ​ 4.01% ​ ​ 4.22% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 3.28 ​ ​ 4.28 Expected volatility ​ ​ 84.09% ​ ​ 98.05% The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows: ​ ​ December 31, 2023 ​ ​ At Issuance November 13, 2023 Estimated cash flow dates ​ ​ 2024 - 2038 ​ ​ 2024 - 2038 Estimated probability of success ​ ​ 95.0% ​ ​ 95.0% Risk-adjusted discount rate ​ ​ 13.7% ​ ​ 14.4%
Schedule of Changes in the Warrant Liabilities	The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 11 to these unaudited consolidated financial statements) for the six months ended June 30, 2024 (in thousands): ​ ​ Warrant liabilities Fair value as of December 31, 2023 ​ ​ $6,394 Change in fair value ​ ​ 2,866 Fair value as of June 30, 2024 ​ ​ $9,260 The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the six months ended June 30, 2024 (in thousands): ​ ​ CVR liability Fair value as of December 31, 2023 ​ ​ $358,600 Change in fair value ​ ​ 36,800 Fair value as of June 30, 2024 ​ ​ $395,400 The following table presents changes in the forward contract liabilities for the periods presented (in thousands): ​ ​ Forward contract liabilities Fair value as of December 31, 2023 ​ ​ $ 28,307 Settlements ​ ​ (35,197) Change in fair value ​ ​ 6,890 Fair value as of June 30, 2024 ​ ​ $ —	The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 12), for the year ended December 31, 2023 (in thousands): ​ ​ Warrant liabilities Fair value as of December 31, 2022 ​ ​ $ 19,140 Change in fair value ​ ​ (12,746) Fair value as of December 31, 2023 ​ ​ $ 6,394 The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the year ended December 31, 2023 (in thousands): ​ ​ CVR liability Fair value as of December 31, 2022 ​ ​ $ — Issuances ​ ​ 340,300 Change in fair value ​ ​ 18,300 Fair value as of December 31, 2023 ​ ​ $358,600 The following table presents changes in the forward contract liabilities for the periods presented (in thousands): ​ ​ Forward contract liabilities Fair value as of December 31, 2022 ​ ​ $ — Issuances ​ ​ 155,308 Settlements ​ ​ (276,601) Change in fair value ​ ​ 149,600 Fair value as of December 31, 2023 ​ ​ $ 28,307

Property and Equipment (Q2) (Ta

Property and Equipment (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): ​ ​ June 30, 2024 ​ ​ December 31, 2023 Laboratory equipment ​ ​ $ 6,944 ​ ​ $ 6,280 Computer equipment and software ​ ​ 621 ​ ​ 702 Leasehold improvements ​ ​ 61 ​ ​ 61 Furniture and fixtures ​ ​ 462 ​ ​ 452 Office equipment ​ ​ 196 ​ ​ 196 Construction in process ​ ​ 4,417 ​ ​ 150 Total property and equipment ​ ​ 12,701 ​ ​ 7,841 Less: Accumulated depreciation ​ ​ (6,029) ​ ​ (5,728) Property and equipment, net ​ ​ $ 6,672 ​ ​ $ 2,113	Property and equipment consists of the following (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Laboratory equipment ​ ​ $ 6,280 ​ ​ $ 6,001 Computer equipment and software ​ ​ 702 ​ ​ 697 Leasehold improvements ​ ​ 61 ​ ​ 57 Furniture and fixtures ​ ​ 452 ​ ​ 453 Office equipment ​ ​ 196 ​ ​ 192 Construction in process ​ ​ 150 ​ ​ 599 Total property and equipment ​ ​ 7,841 ​ ​ 7,999 Less accumulated depreciation ​ ​ (5,728) ​ ​ (5,205) Property and equipment, net ​ ​ $ 2,113 ​ ​ $ 2,794

Accrued Expenses (Q2) (Tables)

Accrued Expenses (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): ​ ​ June 30, 2024 ​ ​ December 31, 2023 Payroll and employee related expenses ​ ​ $ 1,402 ​ ​ $ 4,390 Accrued patent fees ​ ​ 689 ​ ​ 472 Accrued external research and development costs ​ ​ 3,467 ​ ​ 4,896 Accrued professional and consulting services ​ ​ 1,871 ​ ​ 4,331 Property and equipment ​ ​ 2,877 ​ ​ 128 Other ​ ​ 309 ​ ​ 516 Accrued expenses ​ ​ $10,615 ​ ​ $14,733	Accrued expenses consist of the following (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Payroll and employee related expenses ​ ​ $ 4,390 ​ ​ $ 4,242 Accrued patent fees ​ ​ 472 ​ ​ 696 Accrued external research and development costs ​ ​ 4,896 ​ ​ 7,274 Accrued professional and consulting services ​ ​ 4,331 ​ ​ 985 Accrued interest ​ ​ — ​ ​ 222 Other ​ ​ 644 ​ ​ 665 Accrued expenses ​ ​ $14,733 ​ ​ $14,084

Leases (Q2) (Tables)

Leases (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Leases [Abstract]
Schedule of Lease, Cost	For the three and six months ended June 30, 2024 and 2023, the components of lease costs were as follows (in thousands): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 ​ Operating lease cost ​ ​ $ 956 ​ ​ $ 696 ​ ​ $1,731 ​ ​ $1,392 ​ Variable lease cost ​ ​ 352 ​ ​ 270 ​ ​ 749 ​ ​ 412 ​ Short-term lease cost ​ ​ 1 ​ ​ 2 ​ ​ 4 ​ ​ 5 ​ Less: Sublease income ​ ​ (250 ) ​ ​ (251 ) ​ ​ (760 ) ​ ​ (506 ) ​ Total lease cost ​ ​ $1,059 ​ ​ $ 717 ​ ​ $1,724 ​ ​ $1,303 ​ The following summarizes additional information related to operating leases: ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Weighted-average remaining lease term ​ ​ 4.9 years ​ ​ 4.9 years Weighted-average discount rate ​ ​ 11.5% ​ ​ 9.7%	For the years ended December 31, 2023, 2022 and 2021, the components of lease costs were as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Operating lease cost ​ ​ $ 2,828 ​ ​ $2,276 ​ ​ $2,023 Variable lease cost ​ ​ 965 ​ ​ 910 ​ ​ 834 Short-term lease cost ​ ​ 8 ​ ​ 11 ​ ​ 10 Less sublease income ​ ​ (1,172) ​ ​ (176) ​ ​ — Total lease cost ​ ​ $ 2,629 ​ ​ $3,021 ​ ​ $2,867 The following summarizes additional information related to operating leases: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Weighted-average remaining lease term ​ ​ 4.3 years ​ ​ 5.4 years Weighted-average discount rate ​ ​ 9.9 % ​ ​ 9.7 %
Schedule of Lessee, Operating Lease, Liability, Maturity	The maturity of the Company’s operating lease liabilities as of June 30, 2024 were as follows (in thousands): ​ ​ June 30, 2024 2024 (remainder) ​ ​ $ 1,239 2025 ​ ​ 4,350 2026 ​ ​ 4,471 2027 ​ ​ 4,276 2028 ​ ​ 2,243 Thereafter ​ ​ 3,409 Total future minimum lease payments ​ ​ 19,988 Less: Imputed interest ​ ​ 5,121 Total operating lease liabilities ​ ​ $14,867	The maturity of the Company’s operating lease liabilities as of December 31, 2023 were as follows (in thousands): ​ ​ December 31, 2023 2024 ​ ​ $ 3,077 2025 ​ ​ 3,164 2026 ​ ​ 3,248 2027 ​ ​ 3,017 2028 ​ ​ 946 Thereafter ​ ​ — Total future minimum lease payments ​ ​ 13,452 Less: Imputed interest ​ ​ 2,497 Total operating lease liabilities ​ ​ $10,955
Schedule of Operating Lease, Lease Income	The supplemental disclosure for the statement of cash flows related to operating leases was as follows (in thousands): ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ $1,604 ​ ​ $1,319	The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ $2,696 ​ ​ $2,048

Equity (Q2) (Tables)

Equity (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Equity [Abstract]
Schedule of Stockholders' Equity Note, Warrants or Rights	The following is a summary of warrant activity for the six months ended June 30, 2024: ​ ​ Number of Warrants ​ ​ Weighted- average exercise price ​ ​ Equity classified ​ ​ Liability classified ​ ​ Total ​ Outstanding at December 31, 2023 ​ ​ 74,420 ​ ​ 966,393 ​ ​ 1,040,813 ​ ​ $45.98 Exercises ​ ​ (65,681) ​ ​ — ​ ​ (65,681) ​ ​ 43.80 Outstanding at June 30, 2024 ​ ​ 8,739 ​ ​ 966,393 ​ ​ 975,132 ​ ​ $46.12	The following is a summary of warrant activity for the years ended December 31, 2023 and 2022: ​ ​ Number of Warrants ​ ​ ​ ​ Equity classified ​ ​ Liability classified ​ ​ Total ​ ​ Weighted average exercise price Outstanding at December 31, 2021 ​ ​ 292,469 ​ ​ 10,443,511 ​ ​ 10,735,980 ​ ​ $ 1.62 Issuance ​ ​ — ​ ​ 20,571,429 ​ ​ 20,571,429 ​ ​ 1.55 Canceled ​ ​ (79,130) ​ ​ — ​ ​ (79,130) ​ ​ $17.71 Reclassification of warrant liability to equity on modification ​ ​ 2,022,987 ​ ​ (2,022,987) ​ ​ — ​ ​ $ 1.46 Outstanding at December 31, 2022 ​ ​ 2,236,326 ​ ​ 28,991,953 ​ ​ 31,228,279 ​ ​ $ 1.53 Canceled ​ ​ (3,576) ​ ​ — ​ ​ (3,576) ​ ​ 16.77 Outstanding at December 31, 2023 ​ ​ 2,232,750 ​ ​ 28,991,953 ​ ​ 31,224,703 ​ ​ $ 1.53
Schedule of Authorized Shares of Common Stock for Future Issuance	The Company has authorized shares of common stock for future issuance as of June 30, 2024 as follows: Exercise of warrants ​ ​ 975,132 Shares available for future stock incentive awards ​ ​ 3,930,990 Unvested restricted stock units ​ ​ 454,456 Outstanding common stock options ​ ​ 1,889,561 Series A Preferred Stock ​ ​ 5,544,719 Total ​ ​ 12,794,858	The Company has authorized shares of common stock for future issuance as follows: ​ ​ As of December 31, 2023 Exercise of warrants ​ ​ 31,224,703 Shares available for future stock incentive awards ​ ​ 35,836,268 Outstanding common stock options ​ ​ 23,306,661 Total ​ ​ 90,367,632

Stock Incentive Plans (Q2) (Tab

Stock Incentive Plans (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Schedule of Stock-Based Compensation	Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive income (loss), including $1.5 million recognized as stock-based compensation expense upon the achievement of a technical milestone by Ginkgo Bioworks Holdings, Inc., or Ginkgo, during the three and six months ended June 30, 2023 as described in Note 15, was as follows (in thousands): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Research and development ​ ​ $ 776 ​ ​ $2,677 ​ ​ $1,488 ​ ​ $3,869 General and administrative ​ ​ 815 ​ ​ 1,106 ​ ​ 1,534 ​ ​ 2,190 Total stock-based compensation expense ​ ​ $1,591 ​ ​ $3,783 ​ ​ $3,022 ​ ​ $6,059	Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive income (loss), including $1.5 million recognized as stock-based compensation expense upon the achievement of a technical milestone by Ginkgo Bioworks Holdings, Inc., or Ginkgo, during the year ended December 31, 2023 and $1.0 million recognized as stock-based compensation expense upon the issuance of common stock to Ginkgo during the year ended December 31, 2022 as described in Note 16, was as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Research and development ​ ​ $12,985 ​ ​ $ 5,061 ​ ​ $3,204 General and administrative ​ ​ 12,793 ​ ​ 6,133 ​ ​ 4,516 Total stock-based compensation expense ​ ​ $25,778 ​ ​ $11,194 ​ ​ $7,720
Schedule of Weighted Average Assumptions Used	The estimated grant date fair values of stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model based on the following weighted-average assumptions: ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Risk-free interest rate ​ ​ 4.53% ​ ​ 3.38% ​ ​ 4.04% ​ ​ 3.95% Dividend yield ​ ​ — ​ ​ — ​ ​ — ​ ​ — Expected term (in years) ​ ​ 6.18 ​ ​ 6.00 ​ ​ 6.20 ​ ​ 5.94 Expected volatility ​ ​ 93.57% ​ ​ 94.40% ​ ​ 95.09% ​ ​ 94.64% Weighted-average fair value of common stock ​ ​ $22.08 ​ ​ $39.30 ​ ​ $20.14 ​ ​ $34.50	The fair value of the stock options assumed in connection with the Merger was calculated using a Black-Scholes option pricing model based on the following weighted-average assumptions: ​ ​ Common Stock ​ ​ Series A Preferred Stock Risk-free interest rate ​ ​ 4.83 % ​ ​ 4.92 % Dividend yield ​ ​ — ​ ​ — Expected term ​ ​ 3.59 ​ ​ 3.29 Expected volatility ​ ​ 83.77 % ​ ​ 83.87 % Weighted-average fair value of common stock or Series A Preferred Stock, as applicable ​ ​ $ 0.40 ​ ​ $403.47 The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Risk-free interest rate ​ ​ 3.95 % ​ ​ 2.24 % ​ ​ 0.79 % ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Dividend yield ​ ​ — ​ ​ — ​ ​ — Expected term ​ ​ 5.94 ​ ​ 6.02 ​ ​ 6.03 Expected volatility ​ ​ 94.64 % ​ ​ 92.21 % ​ ​ 95.04 % Weighted-average fair value of common stock ​ ​ $ 1.15 ​ ​ $ 2.63 ​ ​ $ 3.58
Schedule of Options Activity	The following table summarizes the stock option activity under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: ​ ​ Number of common stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Outstanding at December 31, 2023 ​ ​ 776,865 ​ ​ $ 2.97 ​ ​ 6.50 ​ ​ $13,760 Granted ​ ​ 761,556 ​ ​ $20.14 ​ ​ ​ ​ Converted from options for Series A Preferred Stock ​ ​ 470,403 ​ ​ $ 2.40 ​ ​ ​ ​ Exercised ​ ​ (89,009) ​ ​ $ 3.08 ​ ​ ​ ​ Forfeited ​ ​ (30,254) ​ ​ $14.84 ​ ​ ​ ​ Outstanding at June 30, 2024 ​ ​ 1,889,561 ​ ​ $ 9.56 ​ ​ 7.24 ​ ​ $33,081 Vested at June 30, 2024 ​ ​ 944,490 ​ ​ $ 2.74 ​ ​ 5.43 ​ ​ $22,925 Vested and expected to vest at June 30, 2024 ​ ​ 1,743,749 ​ ​ $ 8.66 ​ ​ 7.12 ​ ​ $32,070 The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Outstanding at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 Converted to options for common stock ​ ​ (14,112.299) ​ ​ $79.94 ​ ​ ​ ​ Outstanding at June 30, 2024 ​ ​ — ​ ​ $ — ​ ​ ​	The following table summarizes the stock option activity under the 2008 Plan, the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: ​ ​ Number of Common Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Employees ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ 15,578,412 ​ ​ $3.44 ​ ​ 7.57 ​ ​ $ 4 Granted ​ ​ 5,477,200 ​ ​ $1.15 ​ ​ ​ ​ Assumed in connection with Merger ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ ​ ​ Exercised ​ ​ — ​ ​ $ — ​ ​ ​ ​ Forfeited ​ ​ (2,215,020) ​ ​ $2.68 ​ ​ ​ ​ Cancelled/settled in connection with the Merger ​ ​ (18,840,592) ​ ​ $2.86 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ 6.50 ​ ​ $13,760 Vested at December 31, 2023 ​ ​ 18,067,999 ​ ​ $0.10 ​ ​ 6.13 ​ ​ $10,725 Vested and expected to vest at December 31, 2023 ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ 6.50 ​ ​ $13,760 ​ ​ ​ ​ ​ ​ ​ ​ Non-employee consultants ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ 266,239 ​ ​ $8.05 ​ ​ 5.08 ​ ​ $ — Forfeited ​ ​ — ​ ​ $ — ​ ​ ​ ​ Cancelled/settled in connection with the Merger ​ ​ (266,239) ​ ​ $8.05 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ — ​ ​ $ — ​ ​ — ​ ​ $ — The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Employees ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ — ​ ​ $ — ​ ​ — ​ ​ $ — Assumed in connection with Merger ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Vested at December 31, 2023 ​ ​ 10,860.441 ​ ​ $71.67 ​ ​ 5.15 ​ ​ $6,709 Vested and expected to vest at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601
Schedule of Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	The following table summarizes the Company’s restricted stock units under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and the Old Cartesian Plan: ​ ​ Number of shares ​ ​ Weighted-average grant date fair value ($) Unvested at December 31, 2023 ​ ​ — ​ ​ $ — Granted ​ ​ 471,104 ​ ​ 19.80 Forfeited ​ ​ (16,648) ​ ​ 19.80 Unvested at June 30, 2024 ​ ​ 454,456 ​ ​ $19.80	The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan: ​ ​ Number of shares ​ ​ Weighted average grant date fair value ($) Unvested at December 31, 2022 ​ ​ 1,705,558 ​ ​ $2.62 Granted ​ ​ 1,054,600 ​ ​ 1.13 Vested ​ ​ (636,418) ​ ​ 2.40 Forfeited ​ ​ (446,108) ​ ​ 1.91 Cancelled/settled in connection with the Merger ​ ​ (1,677,632) ​ ​ 1.96 Unvested at December 31, 2023 ​ ​ — ​ ​ $ —

Revenue Arrangements (Q2) (Tabl

Revenue Arrangements (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
Schedule of Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities during the six months ended June 30, 2024 (in thousands): ​ ​ Balance at beginning of period ​ ​ Additions ​ ​ Deductions ​ ​ Balance at end of period Six Months Ended June 30, 2024 ​ ​ ​ ​ ​ ​ ​ ​ Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ ​ $5,849 ​ ​ $— ​ ​ $(5,849) ​ ​ $— Total contract liabilities ​ ​ $5,849 ​ ​ $— ​ ​ $(5,849) ​ ​ $—	The following table presents changes in the Company’s contract liabilities during the year ended December 31, 2023 (in thousands): ​ ​ Balance at beginning of period ​ ​ Additions ​ ​ Deductions ​ ​ Balance at end of period Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ ​ $593 ​ ​ $10,500 ​ ​ $(5,244) ​ ​ $5,849 Total contract liabilities ​ ​ $593 ​ ​ $10,500 ​ ​ $(5,244) ​ ​ $5,849

Related-Party Transactions (Q_2

Related-Party Transactions (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The November 2023 Private Placement includes a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the six months ended June 30, 2024. Name ​ ​ Shares of Series A Preferred Stock purchased ​ ​ Total aggregate purchase price Timothy A. Springer, Ph.D. ​ ​ 99,140.326 ​ ​ $40,000,000	The November 2023 Private Placement includes a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the year ended December 31, 2023. Name ​ ​ Shares of Series A Preferred Stock purchased ​ ​ Total aggregate purchase price Timothy A. Springer, Ph.D. ​ ​ 24,785.081 ​ ​ $10,000,000 TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) ​ ​ 24,785.081 ​ ​ $10,000,000 Seven One Eight Three Four Irrevocable Trust (affiliate of Murat Kalayoglu, MD, Ph.D.) ​ ​ 619.627 ​ ​ $ 250,000 The following table sets forth the number of shares of Common Stock and 2022 Warrants purchased in the 2022 Offering by directors and executive officers, as of the time of the Offering, and related parties thereto: Name ​ ​ Shares of Common Stock purchased ​ ​ 2022 Warrants purchased ​ ​ Total aggregate purchase price TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) ​ ​ 6,681,600 ​ ​ 5,011,200 ​ ​ $9,421,056

Restructuring (Q2) (Tables)

Restructuring (Q2) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Schedule of Change in Accrued Restructuring Balance	The following table summarizes the change in the Company's accrued restructuring balance (in thousands): ​ ​ Beginning Balance December 31, 2023 ​ ​ Charges ​ ​ Payments ​ ​ Ending Balance June 30, 2024 Severance liability ​ ​ $3,896 ​ ​ $805 ​ ​ $(4,527) ​ ​ $174	The following table summarizes the change in the Company's accrued restructuring balance (in thousands): ​ ​ Beginning Balance December 31, 2022 ​ ​ Charges ​ ​ Payments ​ ​ Ending Balance December 31, 2023 Severance liability ​ ​ $— ​ ​ $6,431 ​ ​ $2,535 ​ ​ $3,896

Merger (FY) (Tables)

Merger (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Schedule of Fair Value of Consideration	The total fair value of the consideration of $168.5 million as of the Closing Date is summarized as follows (in thousands): Forward contract to issue common stock ​ ​ $ 2,713 Forward contract to issue Series A Preferred Stock ​ ​ 155,308 Stock options allocated to consideration paid ​ ​ 10,444 Total consideration ​ ​ $168,465	The total fair value of the consideration of $168.5 million as of the Closing Date is summarized as follows (in thousands): Forward contract to issue common stock ​ ​ $ 2,713 Forward contract to issue Series A Preferred Stock ​ ​ 155,308 Stock options allocated to consideration paid ​ ​ 10,444 Total consideration ​ ​ $168,465
Schedule of Assets Acquired and Liabilities Assumed	The following table presents the allocation of the purchase price to the estimated fair values of the assets acquired and liabilities assumed as of the Closing Date (in thousands): ​ ​ As of November 13, 2023 Assets acquired: ​ ​ Cash and cash equivalents ​ ​ $ 6,561 Prepaid expenses and other current assets ​ ​ 309 Property and equipment, net ​ ​ 215 Right-of-use asset, net ​ ​ 915 In-process research and development assets ​ ​ 150,600 Goodwill ​ ​ 48,163 ​ ​ $206,763 Liabilities assumed ​ ​ Accrued expenses and other current liabilities ​ ​ $ 2,530 Lease liability ​ ​ 292 Lease liability, net of current portion ​ ​ 623 Deferred tax liability ​ ​ 34,853 ​ ​ $ 38,298 Net assets acquired ​ ​ $168,465	The following table presents the allocation of the purchase price to the estimated fair values of the assets acquired and liabilities assumed as of the Closing Date (in thousands): ​ ​ As of November 13, 2023 Assets acquired: ​ ​ Cash and cash equivalents ​ ​ $ 6,561 Prepaid expenses and other current assets ​ ​ 309 Property and equipment, net ​ ​ 215 Right-of-use asset, net ​ ​ 915 In-process research and development assets ​ ​ 150,600 Goodwill ​ ​ 48,163 ​ ​ $206,763 Liabilities assumed ​ ​ Accrued expenses and other current liabilities ​ ​ $ 2,530 Lease liability ​ ​ $ 292 Lease liability, net of current portion ​ ​ $ 623 Deferred tax liability ​ ​ $ 34,853 ​ ​ $ 38,298 Net assets acquired ​ ​ $168,465
Schedule of Intangible Assets Acquired	The following summarizes the Company’s intangible assets acquired in the Merger (in thousands): ​ ​ Acquisition Date Fair Value Descartes-08 for MG ​ ​ $ 93,900 Descartes-08 for SLE ​ ​ 56,700 Total in-process research and development assets ​ ​ $150,600	The following summarizes the Company’s intangible assets acquired in the Merger and their carrying value as of December 31, 2023 (in thousands): ​ ​ Acquisition Date Fair Value ​ ​ Impairment ​ ​ Carrying Value at December 31, 2023 Descartes-08 for MG ​ ​ $ 93,900 ​ ​ $— ​ ​ $ 93,900 Descartes-08 for SLE ​ ​ 56,700 ​ ​ — ​ ​ 56,700 Total in-process research and development assets ​ ​ $150,600 ​ ​ $— ​ ​ $150,600
Schedule of Pro forma Financial Information		The following unaudited pro forma financial information reflects the consolidated results of operations of the Company as if the Merger had taken place on January 1, 2022. The unaudited pro forma financial information is not necessarily indicative of the results of operations as they would have been had the transactions been effected on the assumed date (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 Revenue ​ ​ $ 26,004 ​ ​ $112,226 Net (loss) income ​ ​ $(232,259) ​ ​ $ 29,607

Marketable Securities and Inv_2

Marketable Securities and Investments (FY) (Tables)	12 Months Ended
	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Schedule of Available-for-Sale Marketable Securities	The following table summarizes the marketable securities held as of December 31, 2022 (in thousands): ​ ​ Amortized cost ​ ​ Unrealized gains ​ ​ Unrealized losses ​ ​ Fair value December 31, 2022 ​ ​ ​ ​ ​ ​ ​ ​ U.S. government agency securities and treasuries ​ ​ $13,566 ​ ​ $— ​ ​ $ (9) ​ ​ $13,557 Corporate bonds ​ ​ $ 1,953 ​ ​ $— ​ ​ $ (2) ​ ​ $ 1,951 Commercial paper ​ ​ 12,656 ​ ​ — ​ ​ — ​ ​ 12,656 Total ​ ​ $28,175 ​ ​ $— ​ ​ $(11) ​ ​ $28,164

Net (Loss) Income Per Share (_2

Net (Loss) Income Per Share (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted Net (Loss) Income Per Share	The following table sets forth the computation of basic and diluted net income (loss) per share allocable to common stockholders for the three and six months ended June 30, 2024 and 2023 (in thousands, except share and per-share data): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Numerator: ​ ​ ​ ​ ​ ​ ​ ​ Net income (loss) ​ ​ $ 13,836 ​ ​ $ (11,387) ​ ​ $ (42,988) ​ ​ $ (33,050) Less: Undistributed earnings allocable to participating securities ​ ​ (4,208) ​ ​ — ​ ​ — ​ ​ — Net income (loss) allocable to shares of common stock - basic and diluted ​ ​ $ 9,628 ​ ​ $ (11,387) ​ ​ $ (42,988) ​ ​ $ (33,050) Denominator: ​ ​ ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ ​ 16,723,479 ​ ​ 5,114,747 ​ ​ 11,068,749 ​ ​ 5,113,213 Dilutive effect of employee equity incentive plans ​ ​ 1,067,664 ​ ​ — ​ ​ — ​ ​ — Weighted-average common shares outstanding - diluted ​ ​ 17,791,143 ​ ​ 5,114,747 ​ ​ 11,068,749 ​ ​ 5,113,213 Net income (loss) per share allocable to common stockholders: ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ ​ $ 0.58 ​ ​ $ (2.23) ​ ​ $ (3.88) ​ ​ $ (6.46) Diluted ​ ​ $ 0.54 ​ ​ $ (2.23) ​ ​ $ (3.88) ​ ​ $ (6.46)	The following table sets forth the computation of basic and diluted net (loss) income per share (in thousands, except share and per-share data): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Numerator: ​ ​ ​ ​ ​ ​ Net (loss) income ​ ​ $(219,710) ​ ​ $ 35,379 ​ ​ $(25,687) Less: CVR distribution to participating securities ​ ​ (37,550) ​ ​ — ​ ​ — Net (loss) income allocable to shares of common stock - basic ​ ​ (257,260) ​ ​ 35,379 ​ ​ (25,687) Less: Change in fair value of warrants ​ ​ — ​ ​ (20,882) ​ ​ — Net (loss) income allocable to shares of common stock - diluted ​ ​ $(257,260) ​ ​ $ 14,497 ​ ​ $(25,687) ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Denominator: ​ ​ ​ ​ ​ ​ Weighted-average common shares outstanding - basic ​ ​ 155,109,561 ​ ​ 144,758,555 ​ ​ 114,328,798 Dilutive effect of employee equity incentive plans and outstanding warrants ​ ​ — ​ ​ 1,116,334 ​ ​ — Weighted-average common shares used in per share calculations - diluted ​ ​ 155,109,561 ​ ​ 145,874,889 ​ ​ 114,328,798 Net (loss) income per share: ​ ​ ​ ​ ​ ​ Basic ​ ​ $ (1.66) ​ ​ $ 0.24 ​ ​ $ (0.22) Diluted ​ ​ $ (1.66) ​ ​ $ 0.10 ​ ​ $ (0.22)
Schedule of Potential Common Shares Issuable Upon Conversion of Warrants	The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net income (loss) per share allocable to common stockholders for all periods presented, as the effect would have been anti-dilutive: ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Common stock options, restricted stock units and ESPP shares ​ ​ 740,211 ​ ​ 753,590 ​ ​ 2,344,017 ​ ​ 753,590 Warrants to purchase common stock ​ ​ 975,132 ​ ​ 1,040,943 ​ ​ 975,132 ​ ​ 1,040,943 Series A Preferred Stock ​ ​ 5,544,719 ​ ​ — ​ ​ 5,544,719 ​ ​ — Total ​ ​ 7,260,062 ​ ​ 1,794,533 ​ ​ 8,863,868 ​ ​ 1,794,533	The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net (loss) income per share for all periods presented, as the effect would have been anti-dilutive: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Warrants to purchase common stock ​ ​ 31,224,703 ​ ​ 213,339 ​ ​ 10,735,980 Series A Preferred Stock ​ ​ 435,120,513 ​ ​ — ​ ​ — Forward contract to issue Series A Preferred Stock ​ ​ 99,140,326 ​ ​ — ​ ​ — Common stock options, RSUs and ESPP shares ​ ​ 23,306,661 ​ ​ 17,800,034 ​ ​ 11,492,002 Series A Preferred Stock options ​ ​ 14,112,299 ​ ​ — ​ ​ — Total ​ ​ 602,904,502 ​ ​ 18,013,373 ​ ​ 22,227,982

Fair Value Measurements (FY) (T

Fair Value Measurements (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on a Recurring Basis	The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2024 and December 31, 2023 (in thousands): ​ ​ June 30, 2024 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 39,181 ​ ​ $39,181 ​ ​ $— ​ ​ $ — Total assets ​ ​ $ 39,181 ​ ​ $39,181 ​ ​ $— ​ ​ $ — Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 9,260 ​ ​ $ — ​ ​ $— ​ ​ $ 9,260 Contingent value right liability ​ ​ $395,400 ​ ​ $ — ​ ​ $— ​ ​ $395,400 Total liabilities ​ ​ $404,660 ​ ​ $ — ​ ​ $— ​ ​ $404,660 ​ ​ December 31, 2023 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Total assets ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 6,394 ​ ​ $ — ​ ​ $ — ​ ​ $ 6,394 Contingent value right liability ​ ​ $358,600 ​ ​ $ — ​ ​ $ — ​ ​ $358,600 Forward contract liabilities ​ ​ $ 28,307 ​ ​ $ — ​ ​ $28,307 ​ ​ $ — Total liabilities ​ ​ $393,301 ​ ​ $ — ​ ​ $28,307 ​ ​ $364,994	The following tables present the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2023 and 2022 (in thousands): ​ ​ December 31, 2023 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — Total assets ​ ​ $ 41,161 ​ ​ $41,161 ​ ​ $ — ​ ​ $ — ​ ​ ​ ​ ​ ​ ​ ​ Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $ 6,394 ​ ​ $ — ​ ​ $ — ​ ​ $ 6,394 Contingent value right liability ​ ​ 358,600 ​ ​ — ​ ​ — ​ ​ 358,600 Forward contract liabilities ​ ​ 28,307 ​ ​ — ​ ​ 28,307 ​ ​ — Total liabilities ​ ​ $393,301 ​ ​ $ — ​ ​ $28,307 ​ ​ $364,994 ​ ​ December 31, 2022 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Assets: ​ ​ ​ ​ ​ ​ ​ ​ Money market funds (included in cash equivalents) ​ ​ $53,552 ​ ​ $53,552 ​ ​ $ — ​ ​ $— Marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ U.S. government agency securities and treasuries ​ ​ 13,557 ​ ​ — ​ ​ 13,557 ​ ​ — Corporate bonds ​ ​ 1,951 ​ ​ — ​ ​ 1,951 ​ ​ — Commercial paper ​ ​ 12,656 ​ ​ — ​ ​ 12,656 ​ ​ — Total assets ​ ​ $81,716 ​ ​ $53,552 ​ ​ $28,164 ​ ​ $— ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2022 ​ ​ Total ​ ​ Level 1 ​ ​ Level 2 ​ ​ Level 3 Liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Warrant liabilities ​ ​ $19,140 ​ ​ $— ​ ​ $— ​ ​ $19,140 Total liabilities ​ ​ $19,140 ​ ​ $— ​ ​ $— ​ ​ $19,140
Schedule of Cash and Cash Equivalents	The Company’s consolidated statements of cash flows include the following as of June 30, 2024 and 2023 (in thousands): ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Cash and cash equivalents ​ ​ $87,227 ​ ​ $112,027 Short-term restricted cash ​ ​ — ​ ​ 1,600 Long-term restricted cash ​ ​ 1,669 ​ ​ 1,377 Total cash, cash equivalents, and restricted cash ​ ​ $88,896 ​ ​ $115,004	The Company’s consolidated statement of cash flows includes the following as of December 31, 2023, 2022 and 2021 (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Cash and cash equivalents ​ ​ $76,911 ​ ​ $106,438 ​ ​ $114,057 Short-term restricted cash ​ ​ — ​ ​ 289 ​ ​ — Long-term restricted cash ​ ​ 1,377 ​ ​ 1,311 ​ ​ 1,379 Total cash, cash equivalents, and restricted cash ​ ​ $78,288 ​ ​ $108,038 ​ ​ $115,436
Schedule of Restricted Cash and Cash Equivalents	The Company’s consolidated statements of cash flows include the following as of June 30, 2024 and 2023 (in thousands): ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Cash and cash equivalents ​ ​ $87,227 ​ ​ $112,027 Short-term restricted cash ​ ​ — ​ ​ 1,600 Long-term restricted cash ​ ​ 1,669 ​ ​ 1,377 Total cash, cash equivalents, and restricted cash ​ ​ $88,896 ​ ​ $115,004	The Company’s consolidated statement of cash flows includes the following as of December 31, 2023, 2022 and 2021 (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Cash and cash equivalents ​ ​ $76,911 ​ ​ $106,438 ​ ​ $114,057 Short-term restricted cash ​ ​ — ​ ​ 289 ​ ​ — Long-term restricted cash ​ ​ 1,377 ​ ​ 1,311 ​ ​ 1,379 Total cash, cash equivalents, and restricted cash ​ ​ $78,288 ​ ​ $108,038 ​ ​ $115,436
Schedule of Significant Inputs	A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2019 Warrants liability is as follows: ​ ​ June 30, 2024 ​ ​ December 31, 2023 Risk-free interest rate ​ ​ 5.33% ​ ​ 4.79% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 0.49 ​ ​ 0.98 Expected volatility ​ ​ 110.52% ​ ​ 83.67% A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2022 Warrants liability is as follows: ​ ​ June 30, 2024 ​ ​ December 31, 2023 Risk-free interest rate ​ ​ 4.52% ​ ​ 4.01% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 2.78 ​ ​ 3.28 Expected volatility ​ ​ 87.23% ​ ​ 84.09% The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows: ​ ​ June 30, 2024 Estimated cash flow dates ​ ​ 2024-2038 Estimated probability of success ​ ​ 95.0% - 100.0% Expected volatility of future revenues ​ ​ 22.0% ​ ​ December 31, 2023 Estimated cash flow dates ​ ​ 2024 - 2038 Estimated probability of success ​ ​ 95.0% Risk-adjusted discount rate ​ ​ 13.7%	A summary of the Black-Scholes pricing model assumptions used to record the fair value of the 2019 Warrants liability is as follows: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Risk-free interest rate ​ ​ 4.79% ​ ​ 4.74% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 0.98 ​ ​ 1.98 Expected volatility ​ ​ 83.67% ​ ​ 79.92% A summary of the Black-Scholes valuation model assumptions used to record the fair value of the 2022 Warrants liability is as follows: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Risk-free interest rate ​ ​ 4.01% ​ ​ 4.22% Dividend yield ​ ​ — ​ ​ — Expected life (in years) ​ ​ 3.28 ​ ​ 4.28 Expected volatility ​ ​ 84.09% ​ ​ 98.05% The significant inputs used to estimate the fair value of the CVR liability, which represented a financial instrument being accounted for under the fair value option, were as follows: ​ ​ December 31, 2023 ​ ​ At Issuance November 13, 2023 Estimated cash flow dates ​ ​ 2024 - 2038 ​ ​ 2024 - 2038 Estimated probability of success ​ ​ 95.0% ​ ​ 95.0% Risk-adjusted discount rate ​ ​ 13.7% ​ ​ 14.4%
Schedule of Changes in the Fair Value of Liabilities	The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 11 to these unaudited consolidated financial statements) for the six months ended June 30, 2024 (in thousands): ​ ​ Warrant liabilities Fair value as of December 31, 2023 ​ ​ $6,394 Change in fair value ​ ​ 2,866 Fair value as of June 30, 2024 ​ ​ $9,260 The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the six months ended June 30, 2024 (in thousands): ​ ​ CVR liability Fair value as of December 31, 2023 ​ ​ $358,600 Change in fair value ​ ​ 36,800 Fair value as of June 30, 2024 ​ ​ $395,400 The following table presents changes in the forward contract liabilities for the periods presented (in thousands): ​ ​ Forward contract liabilities Fair value as of December 31, 2023 ​ ​ $ 28,307 Settlements ​ ​ (35,197) Change in fair value ​ ​ 6,890 Fair value as of June 30, 2024 ​ ​ $ —	The following table reflects a roll-forward of fair value for the Company’s Level 3 warrant liabilities (see Note 12), for the year ended December 31, 2023 (in thousands): ​ ​ Warrant liabilities Fair value as of December 31, 2022 ​ ​ $ 19,140 Change in fair value ​ ​ (12,746) Fair value as of December 31, 2023 ​ ​ $ 6,394 The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the year ended December 31, 2023 (in thousands): ​ ​ CVR liability Fair value as of December 31, 2022 ​ ​ $ — Issuances ​ ​ 340,300 Change in fair value ​ ​ 18,300 Fair value as of December 31, 2023 ​ ​ $358,600 The following table presents changes in the forward contract liabilities for the periods presented (in thousands): ​ ​ Forward contract liabilities Fair value as of December 31, 2022 ​ ​ $ — Issuances ​ ​ 155,308 Settlements ​ ​ (276,601) Change in fair value ​ ​ 149,600 Fair value as of December 31, 2023 ​ ​ $ 28,307

Property and Equipment (FY) (Ta

Property and Equipment (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): ​ ​ June 30, 2024 ​ ​ December 31, 2023 Laboratory equipment ​ ​ $ 6,944 ​ ​ $ 6,280 Computer equipment and software ​ ​ 621 ​ ​ 702 Leasehold improvements ​ ​ 61 ​ ​ 61 Furniture and fixtures ​ ​ 462 ​ ​ 452 Office equipment ​ ​ 196 ​ ​ 196 Construction in process ​ ​ 4,417 ​ ​ 150 Total property and equipment ​ ​ 12,701 ​ ​ 7,841 Less: Accumulated depreciation ​ ​ (6,029) ​ ​ (5,728) Property and equipment, net ​ ​ $ 6,672 ​ ​ $ 2,113	Property and equipment consists of the following (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Laboratory equipment ​ ​ $ 6,280 ​ ​ $ 6,001 Computer equipment and software ​ ​ 702 ​ ​ 697 Leasehold improvements ​ ​ 61 ​ ​ 57 Furniture and fixtures ​ ​ 452 ​ ​ 453 Office equipment ​ ​ 196 ​ ​ 192 Construction in process ​ ​ 150 ​ ​ 599 Total property and equipment ​ ​ 7,841 ​ ​ 7,999 Less accumulated depreciation ​ ​ (5,728) ​ ​ (5,205) Property and equipment, net ​ ​ $ 2,113 ​ ​ $ 2,794

Accrued Expenses (FY) (Tables)

Accrued Expenses (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): ​ ​ June 30, 2024 ​ ​ December 31, 2023 Payroll and employee related expenses ​ ​ $ 1,402 ​ ​ $ 4,390 Accrued patent fees ​ ​ 689 ​ ​ 472 Accrued external research and development costs ​ ​ 3,467 ​ ​ 4,896 Accrued professional and consulting services ​ ​ 1,871 ​ ​ 4,331 Property and equipment ​ ​ 2,877 ​ ​ 128 Other ​ ​ 309 ​ ​ 516 Accrued expenses ​ ​ $10,615 ​ ​ $14,733	Accrued expenses consist of the following (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Payroll and employee related expenses ​ ​ $ 4,390 ​ ​ $ 4,242 Accrued patent fees ​ ​ 472 ​ ​ 696 Accrued external research and development costs ​ ​ 4,896 ​ ​ 7,274 Accrued professional and consulting services ​ ​ 4,331 ​ ​ 985 Accrued interest ​ ​ — ​ ​ 222 Other ​ ​ 644 ​ ​ 665 Accrued expenses ​ ​ $14,733 ​ ​ $14,084

Leases (FY) (Tables)

Leases (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Leases [Abstract]
Schedule of Lease, Cost	For the three and six months ended June 30, 2024 and 2023, the components of lease costs were as follows (in thousands): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 ​ Operating lease cost ​ ​ $ 956 ​ ​ $ 696 ​ ​ $1,731 ​ ​ $1,392 ​ Variable lease cost ​ ​ 352 ​ ​ 270 ​ ​ 749 ​ ​ 412 ​ Short-term lease cost ​ ​ 1 ​ ​ 2 ​ ​ 4 ​ ​ 5 ​ Less: Sublease income ​ ​ (250 ) ​ ​ (251 ) ​ ​ (760 ) ​ ​ (506 ) ​ Total lease cost ​ ​ $1,059 ​ ​ $ 717 ​ ​ $1,724 ​ ​ $1,303 ​ The following summarizes additional information related to operating leases: ​ ​ June 30, ​ ​ 2024 ​ ​ 2023 Weighted-average remaining lease term ​ ​ 4.9 years ​ ​ 4.9 years Weighted-average discount rate ​ ​ 11.5% ​ ​ 9.7%	For the years ended December 31, 2023, 2022 and 2021, the components of lease costs were as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Operating lease cost ​ ​ $ 2,828 ​ ​ $2,276 ​ ​ $2,023 Variable lease cost ​ ​ 965 ​ ​ 910 ​ ​ 834 Short-term lease cost ​ ​ 8 ​ ​ 11 ​ ​ 10 Less sublease income ​ ​ (1,172) ​ ​ (176) ​ ​ — Total lease cost ​ ​ $ 2,629 ​ ​ $3,021 ​ ​ $2,867 The following summarizes additional information related to operating leases: ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Weighted-average remaining lease term ​ ​ 4.3 years ​ ​ 5.4 years Weighted-average discount rate ​ ​ 9.9 % ​ ​ 9.7 %
Schedule of Lessee, Operating Lease, Liability, Maturity	The maturity of the Company’s operating lease liabilities as of June 30, 2024 were as follows (in thousands): ​ ​ June 30, 2024 2024 (remainder) ​ ​ $ 1,239 2025 ​ ​ 4,350 2026 ​ ​ 4,471 2027 ​ ​ 4,276 2028 ​ ​ 2,243 Thereafter ​ ​ 3,409 Total future minimum lease payments ​ ​ 19,988 Less: Imputed interest ​ ​ 5,121 Total operating lease liabilities ​ ​ $14,867	The maturity of the Company’s operating lease liabilities as of December 31, 2023 were as follows (in thousands): ​ ​ December 31, 2023 2024 ​ ​ $ 3,077 2025 ​ ​ 3,164 2026 ​ ​ 3,248 2027 ​ ​ 3,017 2028 ​ ​ 946 Thereafter ​ ​ — Total future minimum lease payments ​ ​ 13,452 Less: Imputed interest ​ ​ 2,497 Total operating lease liabilities ​ ​ $10,955
Schedule of Operating Lease, Lease Income	The supplemental disclosure for the statement of cash flows related to operating leases was as follows (in thousands): ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ $1,604 ​ ​ $1,319	The supplemental disclosure for the statement of cash flows related to operating leases were as follows (in thousands): ​ ​ December 31, ​ ​ 2023 ​ ​ 2022 Cash paid for amounts included in the measurement of lease liabilities: ​ ​ $2,696 ​ ​ $2,048

Series A Preferred Stock (FY) (

Series A Preferred Stock (FY) (Tables)	12 Months Ended
	Dec. 31, 2023
Temporary Equity Disclosure [Abstract]
Schedule of Authorized Shares of Preferred Stock for Future Issuance	As of December 31, 2023, the Company has authorized shares of Series A Preferred Stock for future issuance as follows: ​ ​ As of December 31, 2023 Shares reserved for issuance in November 2023 Private Placement ​ ​ 99,140.326 Outstanding Series A Preferred Stock options ​ ​ 14,112.299 Total ​ ​ 113,252.625

Equity (FY) (Tables)

Equity (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Equity [Abstract]
Schedule of Stockholders' Equity Note, Warrants or Rights	The following is a summary of warrant activity for the six months ended June 30, 2024: ​ ​ Number of Warrants ​ ​ Weighted- average exercise price ​ ​ Equity classified ​ ​ Liability classified ​ ​ Total ​ Outstanding at December 31, 2023 ​ ​ 74,420 ​ ​ 966,393 ​ ​ 1,040,813 ​ ​ $45.98 Exercises ​ ​ (65,681) ​ ​ — ​ ​ (65,681) ​ ​ 43.80 Outstanding at June 30, 2024 ​ ​ 8,739 ​ ​ 966,393 ​ ​ 975,132 ​ ​ $46.12	The following is a summary of warrant activity for the years ended December 31, 2023 and 2022: ​ ​ Number of Warrants ​ ​ ​ ​ Equity classified ​ ​ Liability classified ​ ​ Total ​ ​ Weighted average exercise price Outstanding at December 31, 2021 ​ ​ 292,469 ​ ​ 10,443,511 ​ ​ 10,735,980 ​ ​ $ 1.62 Issuance ​ ​ — ​ ​ 20,571,429 ​ ​ 20,571,429 ​ ​ 1.55 Canceled ​ ​ (79,130) ​ ​ — ​ ​ (79,130) ​ ​ $17.71 Reclassification of warrant liability to equity on modification ​ ​ 2,022,987 ​ ​ (2,022,987) ​ ​ — ​ ​ $ 1.46 Outstanding at December 31, 2022 ​ ​ 2,236,326 ​ ​ 28,991,953 ​ ​ 31,228,279 ​ ​ $ 1.53 Canceled ​ ​ (3,576) ​ ​ — ​ ​ (3,576) ​ ​ 16.77 Outstanding at December 31, 2023 ​ ​ 2,232,750 ​ ​ 28,991,953 ​ ​ 31,224,703 ​ ​ $ 1.53
Schedule of Authorized Shares of Common Stock for Future Issuance	The Company has authorized shares of common stock for future issuance as of June 30, 2024 as follows: Exercise of warrants ​ ​ 975,132 Shares available for future stock incentive awards ​ ​ 3,930,990 Unvested restricted stock units ​ ​ 454,456 Outstanding common stock options ​ ​ 1,889,561 Series A Preferred Stock ​ ​ 5,544,719 Total ​ ​ 12,794,858	The Company has authorized shares of common stock for future issuance as follows: ​ ​ As of December 31, 2023 Exercise of warrants ​ ​ 31,224,703 Shares available for future stock incentive awards ​ ​ 35,836,268 Outstanding common stock options ​ ​ 23,306,661 Total ​ ​ 90,367,632

Stock Incentive Plans (FY) (Tab

Stock Incentive Plans (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Schedule of Stock-Based Compensation	Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive income (loss), including $1.5 million recognized as stock-based compensation expense upon the achievement of a technical milestone by Ginkgo Bioworks Holdings, Inc., or Ginkgo, during the three and six months ended June 30, 2023 as described in Note 15, was as follows (in thousands): ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Research and development ​ ​ $ 776 ​ ​ $2,677 ​ ​ $1,488 ​ ​ $3,869 General and administrative ​ ​ 815 ​ ​ 1,106 ​ ​ 1,534 ​ ​ 2,190 Total stock-based compensation expense ​ ​ $1,591 ​ ​ $3,783 ​ ​ $3,022 ​ ​ $6,059	Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive income (loss), including $1.5 million recognized as stock-based compensation expense upon the achievement of a technical milestone by Ginkgo Bioworks Holdings, Inc., or Ginkgo, during the year ended December 31, 2023 and $1.0 million recognized as stock-based compensation expense upon the issuance of common stock to Ginkgo during the year ended December 31, 2022 as described in Note 16, was as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Research and development ​ ​ $12,985 ​ ​ $ 5,061 ​ ​ $3,204 General and administrative ​ ​ 12,793 ​ ​ 6,133 ​ ​ 4,516 Total stock-based compensation expense ​ ​ $25,778 ​ ​ $11,194 ​ ​ $7,720
Schedule of Weighted Average Assumptions Used	The estimated grant date fair values of stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model based on the following weighted-average assumptions: ​ ​ Three Months Ended June 30, ​ ​ Six Months Ended June 30, ​ ​ 2024 ​ ​ 2023 ​ ​ 2024 ​ ​ 2023 Risk-free interest rate ​ ​ 4.53% ​ ​ 3.38% ​ ​ 4.04% ​ ​ 3.95% Dividend yield ​ ​ — ​ ​ — ​ ​ — ​ ​ — Expected term (in years) ​ ​ 6.18 ​ ​ 6.00 ​ ​ 6.20 ​ ​ 5.94 Expected volatility ​ ​ 93.57% ​ ​ 94.40% ​ ​ 95.09% ​ ​ 94.64% Weighted-average fair value of common stock ​ ​ $22.08 ​ ​ $39.30 ​ ​ $20.14 ​ ​ $34.50	The fair value of the stock options assumed in connection with the Merger was calculated using a Black-Scholes option pricing model based on the following weighted-average assumptions: ​ ​ Common Stock ​ ​ Series A Preferred Stock Risk-free interest rate ​ ​ 4.83 % ​ ​ 4.92 % Dividend yield ​ ​ — ​ ​ — Expected term ​ ​ 3.59 ​ ​ 3.29 Expected volatility ​ ​ 83.77 % ​ ​ 83.87 % Weighted-average fair value of common stock or Series A Preferred Stock, as applicable ​ ​ $ 0.40 ​ ​ $403.47 The estimated grant date fair values of employee stock option awards granted under the 2016 Plan and the 2018 Inducement Incentive Award Plan were calculated using the Black-Scholes option pricing model, based on the following weighted-average assumptions: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Risk-free interest rate ​ ​ 3.95 % ​ ​ 2.24 % ​ ​ 0.79 % ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Dividend yield ​ ​ — ​ ​ — ​ ​ — Expected term ​ ​ 5.94 ​ ​ 6.02 ​ ​ 6.03 Expected volatility ​ ​ 94.64 % ​ ​ 92.21 % ​ ​ 95.04 % Weighted-average fair value of common stock ​ ​ $ 1.15 ​ ​ $ 2.63 ​ ​ $ 3.58
Schedule of Options Activity	The following table summarizes the stock option activity under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: ​ ​ Number of common stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Outstanding at December 31, 2023 ​ ​ 776,865 ​ ​ $ 2.97 ​ ​ 6.50 ​ ​ $13,760 Granted ​ ​ 761,556 ​ ​ $20.14 ​ ​ ​ ​ Converted from options for Series A Preferred Stock ​ ​ 470,403 ​ ​ $ 2.40 ​ ​ ​ ​ Exercised ​ ​ (89,009) ​ ​ $ 3.08 ​ ​ ​ ​ Forfeited ​ ​ (30,254) ​ ​ $14.84 ​ ​ ​ ​ Outstanding at June 30, 2024 ​ ​ 1,889,561 ​ ​ $ 9.56 ​ ​ 7.24 ​ ​ $33,081 Vested at June 30, 2024 ​ ​ 944,490 ​ ​ $ 2.74 ​ ​ 5.43 ​ ​ $22,925 Vested and expected to vest at June 30, 2024 ​ ​ 1,743,749 ​ ​ $ 8.66 ​ ​ 7.12 ​ ​ $32,070 The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Outstanding at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 Converted to options for common stock ​ ​ (14,112.299) ​ ​ $79.94 ​ ​ ​ ​ Outstanding at June 30, 2024 ​ ​ — ​ ​ $ — ​ ​ ​	The following table summarizes the stock option activity under the 2008 Plan, the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: ​ ​ Number of Common Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Employees ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ 15,578,412 ​ ​ $3.44 ​ ​ 7.57 ​ ​ $ 4 Granted ​ ​ 5,477,200 ​ ​ $1.15 ​ ​ ​ ​ Assumed in connection with Merger ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ ​ ​ Exercised ​ ​ — ​ ​ $ — ​ ​ ​ ​ Forfeited ​ ​ (2,215,020) ​ ​ $2.68 ​ ​ ​ ​ Cancelled/settled in connection with the Merger ​ ​ (18,840,592) ​ ​ $2.86 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ 6.50 ​ ​ $13,760 Vested at December 31, 2023 ​ ​ 18,067,999 ​ ​ $0.10 ​ ​ 6.13 ​ ​ $10,725 Vested and expected to vest at December 31, 2023 ​ ​ 23,306,661 ​ ​ $0.10 ​ ​ 6.50 ​ ​ $13,760 ​ ​ ​ ​ ​ ​ ​ ​ Non-employee consultants ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ 266,239 ​ ​ $8.05 ​ ​ 5.08 ​ ​ $ — Forfeited ​ ​ — ​ ​ $ — ​ ​ ​ ​ Cancelled/settled in connection with the Merger ​ ​ (266,239) ​ ​ $8.05 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ — ​ ​ $ — ​ ​ — ​ ​ $ — The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Employees ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at December 31, 2022 ​ ​ — ​ ​ $ — ​ ​ — ​ ​ $ — Assumed in connection with Merger ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ ​ ​ Outstanding at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601 ​ ​ Number of Series A Preferred Stock options ​ ​ Weighted-average exercise price ($) ​ ​ Weighted-average remaining contractual term (in years) ​ ​ Aggregate intrinsic value (in thousands) Vested at December 31, 2023 ​ ​ 10,860.441 ​ ​ $71.67 ​ ​ 5.15 ​ ​ $6,709 Vested and expected to vest at December 31, 2023 ​ ​ 14,112.299 ​ ​ $79.94 ​ ​ 5.91 ​ ​ $8,601
Schedule of Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	The following table summarizes the Company’s restricted stock units under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and the Old Cartesian Plan: ​ ​ Number of shares ​ ​ Weighted-average grant date fair value ($) Unvested at December 31, 2023 ​ ​ — ​ ​ $ — Granted ​ ​ 471,104 ​ ​ 19.80 Forfeited ​ ​ (16,648) ​ ​ 19.80 Unvested at June 30, 2024 ​ ​ 454,456 ​ ​ $19.80	The following table summarizes the Company’s restricted stock units under the 2016 Plan and 2018 Inducement Incentive Award Plan: ​ ​ Number of shares ​ ​ Weighted average grant date fair value ($) Unvested at December 31, 2022 ​ ​ 1,705,558 ​ ​ $2.62 Granted ​ ​ 1,054,600 ​ ​ 1.13 Vested ​ ​ (636,418) ​ ​ 2.40 Forfeited ​ ​ (446,108) ​ ​ 1.91 Cancelled/settled in connection with the Merger ​ ​ (1,677,632) ​ ​ 1.96 Unvested at December 31, 2023 ​ ​ — ​ ​ $ —

Revenue Arrangements (FY) (Tabl

Revenue Arrangements (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Revenue from Contract with Customer [Abstract]
Schedule of Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities during the six months ended June 30, 2024 (in thousands): ​ ​ Balance at beginning of period ​ ​ Additions ​ ​ Deductions ​ ​ Balance at end of period Six Months Ended June 30, 2024 ​ ​ ​ ​ ​ ​ ​ ​ Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ ​ $5,849 ​ ​ $— ​ ​ $(5,849) ​ ​ $— Total contract liabilities ​ ​ $5,849 ​ ​ $— ​ ​ $(5,849) ​ ​ $—	The following table presents changes in the Company’s contract liabilities during the year ended December 31, 2023 (in thousands): ​ ​ Balance at beginning of period ​ ​ Additions ​ ​ Deductions ​ ​ Balance at end of period Contract liabilities: ​ ​ ​ ​ ​ ​ ​ ​ Deferred revenue ​ ​ $593 ​ ​ $10,500 ​ ​ $(5,244) ​ ​ $5,849 Total contract liabilities ​ ​ $593 ​ ​ $10,500 ​ ​ $(5,244) ​ ​ $5,849

Related-Party Transactions - (F

Related-Party Transactions - (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The November 2023 Private Placement includes a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the six months ended June 30, 2024. Name ​ ​ Shares of Series A Preferred Stock purchased ​ ​ Total aggregate purchase price Timothy A. Springer, Ph.D. ​ ​ 99,140.326 ​ ​ $40,000,000	The November 2023 Private Placement includes a delayed settlement mechanism, and as a result, the below issuances and sales to related parties of the Company were made during the year ended December 31, 2023. Name ​ ​ Shares of Series A Preferred Stock purchased ​ ​ Total aggregate purchase price Timothy A. Springer, Ph.D. ​ ​ 24,785.081 ​ ​ $10,000,000 TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) ​ ​ 24,785.081 ​ ​ $10,000,000 Seven One Eight Three Four Irrevocable Trust (affiliate of Murat Kalayoglu, MD, Ph.D.) ​ ​ 619.627 ​ ​ $ 250,000 The following table sets forth the number of shares of Common Stock and 2022 Warrants purchased in the 2022 Offering by directors and executive officers, as of the time of the Offering, and related parties thereto: Name ​ ​ Shares of Common Stock purchased ​ ​ 2022 Warrants purchased ​ ​ Total aggregate purchase price TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) ​ ​ 6,681,600 ​ ​ 5,011,200 ​ ​ $9,421,056

Income Taxes (FY) (Tables)

Income Taxes (FY) (Tables)	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	The following table reconciles the federal statutory income tax rate to the Company’s effective income tax rate: ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 ​ ​ 2021 Statutory U.S. federal rate ​ ​ 21.0 % ​ ​ 21.0 % ​ ​ 21.0 % State income taxes - net of federal benefit ​ ​ 2.3 % ​ ​ 1.6 % ​ ​ (166.0)% Permanent items ​ ​ (1.6)% ​ ​ (18.6)% ​ ​ 8.3 % Research tax credits ​ ​ 0.6 % ​ ​ (3.2)% ​ ​ 55.0 % Deferred revenue ​ ​ — % ​ ​ — % ​ ​ 156.5 % Other ​ ​ — % ​ ​ — % ​ ​ (3.7)% Change in fair value of forward contract liabilities ​ ​ (13.2)% ​ ​ — % ​ ​ — % Valuation allowance, net ​ ​ 2.8 % ​ ​ (4.4)% ​ ​v (230.1)% Stock-based compensation ​ ​ (3.9)% ​ ​ 1.8 % ​ ​ (5.2)% Effective income tax rate ​ ​ 8.0% ​ ​ (1.8)% ​ ​ (164.2)%
Schedule of Deferred Tax Assets and Liabilities	The tax effects of temporary differences that give rise to the Company’s net deferred tax assets are as follows (in thousands): ​ ​ Year Ended December 31, ​ ​ 2023 ​ ​ 2022 Deferred Tax Assets ​ ​ ​ ​ Net operating loss carryforwards ​ ​ $ 29,841 ​ ​ $ 17,015 Research and development credits ​ ​ 5,649 ​ ​ 2,806 Stock-based compensation expense ​ ​ 8 ​ ​ 5,892 Other expenses ​ ​ 705 ​ ​ 1,697 Deferred revenue ​ ​ 84,626 ​ ​ 83,417 Operating lease liabilities ​ ​ 2,718 ​ ​ 3,186 R&E Capitalization ​ ​ 19,778 ​ ​ 9,588 Patent and license costs ​ ​ 9,140 ​ ​ 7,472 Gross deferred tax assets ​ ​ 152,465 ​ ​ 131,073 Deferred Tax Liabilities ​ ​ ​ ​ Intangible assets ​ ​ $ (41,144) ​ ​ $ — Depreciation ​ ​ (128) ​ ​ (81) Operating lease right-of-use assets ​ ​ (2,751) ​ ​ (3,174) Gross deferred tax liabilities ​ ​ (44,023) ​ ​ (3,255) Net deferred tax assets before valuation allowance ​ ​ 108,442 ​ ​ 127,818 Valuation allowance ​ ​ (124,295) ​ ​ (127,818) Net deferred tax assets/(liabilities) ​ ​ $ (15,853) ​ ​ $ —

Restructuring (FY) (Tables)

Restructuring (FY) (Tables)	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Schedule of Change in Accrued Restructuring Balance	The following table summarizes the change in the Company's accrued restructuring balance (in thousands): ​ ​ Beginning Balance December 31, 2023 ​ ​ Charges ​ ​ Payments ​ ​ Ending Balance June 30, 2024 Severance liability ​ ​ $3,896 ​ ​ $805 ​ ​ $(4,527) ​ ​ $174	The following table summarizes the change in the Company's accrued restructuring balance (in thousands): ​ ​ Beginning Balance December 31, 2022 ​ ​ Charges ​ ​ Payments ​ ​ Ending Balance December 31, 2023 Severance liability ​ ​ $— ​ ​ $6,431 ​ ​ $2,535 ​ ​ $3,896

Description of the Business (_2

Description of the Business (Q2) (Details) $ / shares in Units, $ in Thousands							1 Months Ended	6 Months Ended
	Jul. 02, 2024 USD ($) $ / shares shares	Apr. 04, 2024	Dec. 13, 2023 USD ($)	Dec. 06, 2023	Dec. 05, 2023 shares	Nov. 13, 2023 USD ($) $ / shares shares	Aug. 07, 2024 USD ($)	Jun. 30, 2024 USD ($) $ / shares	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($) $ / shares
Cash and Cash Equivalents [Line Items]
Common stock, par value (in dollars per share) | $ / shares						$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Temporary equity, par value (in dollars per share) | $ / shares						$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Reverse stock split conversion ratio		0.0333
Cash, cash equivalents, restricted cash and marketable securities								$ 88,900	$ 78,300
Restricted cash and cash equivalents								1,700	1,400
Accumulated deficit								(657,635)	(614,647)	$ (394,937)
Russian Subsidiary
Cash and Cash Equivalents [Line Items]
Cash maintained in Russian bank accounts								$ 200	$ 200
Securities Purchase Agreement
Cash and Cash Equivalents [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares						149,330.115
Sale of stock, consideration received on transaction			$ 14,800			$ 60,250
July 2024 Purchase Agreement | Subsequent Event
Cash and Cash Equivalents [Line Items]
Sale of stock, gross consideration received on transaction	$ 130,000
Sale of stock, consideration received on transaction							$ 124,400
July 2024 Purchase Agreement | Common stock | Subsequent Event
Cash and Cash Equivalents [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares	3,563,247
July 2024 Purchase Agreement | Series B Preferred Stock | Subsequent Event
Cash and Cash Equivalents [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001
Sale of stock, number of shares issued in transaction (in shares) | shares	2,937,903
Sale of stock, price per share (in dollars per share) | $ / shares	$ 20
Old Cartesian
Cash and Cash Equivalents [Line Items]
Contingent value right, percentage of milestone payments, royalties and other amounts to be distributed				100%				100%
Old Cartesian | Common stock
Cash and Cash Equivalents [Line Items]
Issuance of shares (in shares) | shares					6,723,639	224,099
Old Cartesian | Series A Preferred Stock
Cash and Cash Equivalents [Line Items]
Issuance of shares (in shares) | shares					384,930.724	384,930.724

Merger - Narrative (Q2) (Detail

Merger - Narrative (Q2) (Details) - Old Cartesian - shares	Dec. 05, 2023	Nov. 13, 2023	Dec. 31, 2023
Business Acquisition [Line Items]
Percentage acquired		100%	100%
Common stock
Business Acquisition [Line Items]
Issuance of shares (in shares)	6,723,639	224,099
Series A Preferred Stock
Business Acquisition [Line Items]
Issuance of shares (in shares)	384,930.724	384,930.724

Merger - Schedule of Fair Value

Merger - Schedule of Fair Value of Consideration (Q2) (Details) - Old Cartesian $ in Thousands	Nov. 13, 2023 USD ($)
Business Acquisition [Line Items]
Total consideration	$ 168,465
Common stock
Business Acquisition [Line Items]
Forward contract to issue stock and options	2,713
Series A Preferred Stock
Business Acquisition [Line Items]
Forward contract to issue stock and options	155,308
Options
Business Acquisition [Line Items]
Forward contract to issue stock and options	$ 10,444

Merger - Schedule of Assets Acq

Merger - Schedule of Assets Acquired and Liabilities Assumed (Q2) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Nov. 13, 2023	Dec. 31, 2022
Assets acquired:
Goodwill	$ 48,163	$ 48,163		$ 0
Old Cartesian
Assets acquired:
Cash and cash equivalents			$ 6,561
Prepaid expenses and other current assets			309
Property and equipment, net			215
Right-of-use asset, net			915
In-process research and development assets			150,600
Goodwill			48,163
Total assets acquired			206,763
Liabilities assumed
Accrued expenses and other current liabilities			2,530
Lease liability			292
Lease liability, net of current portion			623
Deferred tax liability			34,853
Total liabilities assumed			38,298
Net assets acquired			$ 168,465

Merger - Schedule of Intangible

Merger - Schedule of Intangible Assets Acquired (Q2) (Details) - Old Cartesian $ in Thousands	Nov. 13, 2023 USD ($)
Business Acquisition [Line Items]
Acquisition Date Fair Value	$ 150,600
Descartes-08 for MG
Business Acquisition [Line Items]
Acquisition Date Fair Value	93,900
Descartes-08 for SLE
Business Acquisition [Line Items]
Acquisition Date Fair Value	$ 56,700

Net Income (Loss) Per Share A_3

Net Income (Loss) Per Share Allocable to Common Stockholders - Computation of Basic and Diluted Net Loss Per Share (Q2) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Numerator:
Net (loss) income	$ 13,836	$ (56,824)	$ (11,387)	$ (21,663)	$ (42,988)	$ (33,050)	$ (219,710)	$ 35,379	$ (25,687)
Less: Undistributed earnings allocable to participating securities, basic	(4,208)		0		0	0
Less: Undistributed earnings allocable to participating securities, diluted	(4,208)		0		0	0
Net loss allocable to shares of common stock - basic	9,628		(11,387)		(42,988)	(33,050)	(257,260)	35,379	(25,687)
Net loss allocable to shares of common stock - diluted	$ 9,628		$ (11,387)		$ (42,988)	$ (33,050)	$ (257,260)	$ 14,497	$ (25,687)
Denominator:
Weighted-average common shares outstanding - basic (in shares)	16,723,479		5,114,747		11,068,749	5,113,213	155,109,561	144,758,555	114,328,798
Dilutive effect of employee equity incentive plans (in shares)	1,067,664		0		0	0
Weighted-average common shares outstanding - diluted (in shares)	17,791,143		5,114,747		11,068,749	5,113,213	155,109,561	145,874,889	114,328,798
Net income (loss) per share allocable to common stockholders:
Basic (in dollars per share)	$ 0.58		$ (2.23)		$ (3.88)	$ (6.46)	$ (1.66)	$ 0.24	$ (0.22)
Diluted (in dollars per share)	$ 0.54		$ (2.23)		$ (3.88)	$ (6.46)	$ (1.66)	$ 0.1	$ (0.22)

Net Income (Loss) Per Share A_4

Net Income (Loss) Per Share Allocable to Common Stockholders - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Q2) (Details) - shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Potential common shares
Total (in shares)	7,260,062	1,794,533	8,863,868	1,794,533	602,904,502	18,013,373	22,227,982
Common stock options, restricted stock units and ESPP shares
Potential common shares
Total (in shares)	740,211	753,590	2,344,017	753,590	23,306,661	17,800,034	11,492,002
Warrants to purchase common stock
Potential common shares
Total (in shares)	975,132	1,040,943	975,132	1,040,943	31,224,703	213,339	10,735,980
Series A Preferred Stock
Potential common shares
Total (in shares)	5,544,719	0	5,544,719	0	435,120,513	0	0

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Q2) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Liabilities:
Forward contract liabilities	$ 0	$ 28,307	$ 0
Recurring
Assets:
Total assets	39,181	41,161	81,716
Liabilities:
Warrant liabilities	9,260	6,394	19,140
Contingent value right liability	395,400	358,600
Forward contract liabilities		28,307
Total liabilities	404,660	393,301	19,140
Recurring | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	39,181	41,161	53,552
Recurring | Level 1
Assets:
Total assets	39,181	41,161	53,552
Liabilities:
Warrant liabilities	0	0	0
Contingent value right liability	0	0
Forward contract liabilities		0
Total liabilities	0	0	0
Recurring | Level 1 | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	39,181	41,161	53,552
Recurring | Level 2
Assets:
Total assets	0	0	28,164
Liabilities:
Warrant liabilities	0	0	0
Contingent value right liability	0	0
Forward contract liabilities		28,307
Total liabilities	0	28,307	0
Recurring | Level 2 | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	0	0	0
Recurring | Level 3
Assets:
Total assets	0	0	0
Liabilities:
Warrant liabilities	9,260	6,394	19,140
Contingent value right liability	395,400	358,600
Forward contract liabilities		0
Total liabilities	404,660	364,994	19,140
Recurring | Level 3 | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	$ 0	$ 0	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Q2) (Details)							1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 28, 2024 USD ($)	Dec. 13, 2023 USD ($) shares	Dec. 06, 2023 USD ($) shares	Dec. 05, 2023 USD ($) shares	Nov. 15, 2023 shares	Nov. 13, 2023 USD ($) shares	Feb. 11, 2024 USD ($) shares	Jun. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers, net									$ 0	$ 0	$ 0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, liability, transfers, net									0	$ 0	$ 0
Revenue recognized									(5,849,000)
Obligation to issue stock (in shares) | shares		49,570.162				148,710.488	49,570.163
November 2023 Private Placement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares					619.627	149,330.115
Issuance of stock for settlement of liability		$ 14,800,000					$ 35,200,000
SOBI Purchase Agreement | Collaborative Arrangement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Revenue recognized	$ 30,000,000							$ 30,000,000	$ 30,000,000
Old Cartesian
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right, percentage of milestone payments, royalties and other amounts to be distributed			100%						100%
Fixed semi annual amount			$ 750,000
Old Cartesian | Series A Preferred Stock
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Issuance of shares (in shares) | shares				384,930.724		384,930.724
Forward contract to issue stock and options						$ 155,308,000
Settlement of forward contract liability				$ 261,800,000
Old Cartesian | 2022 Warrants | Contingent Value Right
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Class of warrant or right, number of securities called by each warrant or right (in shares) | shares			1
Dividend yield | Valuation technique, option pricing model
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrants and rights outstanding, measurement input								0	0	0
Dividend yield | Valuation technique, option pricing model | 2022 Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrants and rights outstanding, measurement input								0	0	0	0

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Cash and Cash Equivalents (Q2) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Cash and cash equivalents	$ 87,227	$ 76,911	$ 112,027	$ 106,438	$ 114,057
Short-term restricted cash	0	0	1,600	289	0
Long-term restricted cash	1,669	1,377	1,377	1,311	1,379
Total cash, cash equivalents, and restricted cash	$ 88,896	$ 78,288	$ 115,004	$ 108,038	$ 115,436	$ 140,064

Fair Value Measurements - Assum

Fair Value Measurements - Assumptions Used to Record the Fair Value of the Warrants (Q2) (Details) - Valuation technique, option pricing model	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Risk-free interest rate | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0533	0.0479	0.0474
Risk-free interest rate | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0452	0.0401	0.0422
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0
Dividend yield | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0	0
Dividend yield | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0	0
Expected life (in years) | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	5 months 26 days	11 months 23 days	1 year 11 months 23 days
Expected life (in years) | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	2 years 9 months 10 days	3 years 3 months 10 days	4 years 3 months 10 days
Expected volatility | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	1.1052	0.8367	0.7992
Expected volatility | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.8723	0.8409	0.9805

Fair Value Measurements - Chang

Fair Value Measurements - Change in the Warrant Liabilities (Q2) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Warrant
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value	$ 6,394	$ 19,140
Change in fair value	2,866	(12,746)
Fair value	9,260	6,394
CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value	358,600	0
Change in fair value	36,800	18,300
Fair value	$ 395,400	$ 358,600

Fair Value Measurements - Sch_3

Fair Value Measurements - Schedule of Significant Inputs Used to Estimate the Fair Value of the CVRs (Q2) (Details) - Level 3	Jun. 30, 2024	Dec. 31, 2023	Nov. 13, 2023
Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input		0.95	0.95
Estimated probability of success | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	0.95
Estimated probability of success | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	1
Expected volatility of future revenues
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	0.22
Risk-adjusted discount rate
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input		0.137	0.144

Fair Value Measurements - Cha_2

Fair Value Measurements - Change in the Forward Contract Liabilities (Q2) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value	$ 28,307	$ 0
Settlements	(35,197)	(276,601)
Change in fair value	$ 6,890	$ 149,600
Change in fair value, location	Change in fair value of forward contract liabilities	Change in fair value of forward contract liabilities
Fair value as of June 30, 2024	$ 0	$ 28,307

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Q2) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 12,701	$ 7,841	$ 7,999
Less: Accumulated depreciation	(6,029)	(5,728)	(5,205)
Property and equipment, net	6,672	2,113	2,794
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	6,944	6,280	6,001
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	621	702	697
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	61	61	57
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	462	452	453
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	196	196	192
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 4,417	$ 150	$ 599

Property and Equipment - Narrat

Property and Equipment - Narrative (Q2) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 0.2	$ 0.2	$ 0.4	$ 0.4	$ 0.7	$ 0.7	$ 0.6

Accrued Expenses (Q2) (Details)

Accrued Expenses (Q2) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 1,402	$ 4,390	$ 4,242
Accrued patent fees	689	472	696
Accrued external research and development costs	3,467	4,896	7,274
Accrued professional and consulting services	1,871	4,331	985
Accrued, Property And equipment	2,877	128
Other	309	516
Accrued expenses	$ 10,615	$ 14,733	$ 14,084

Leases - Narrative (Q2) (Detail

Leases - Narrative (Q2) (Details) $ in Thousands						3 Months Ended	12 Months Ended
	May 07, 2024 USD ($) ft²	May 01, 2024 USD ($) extension_option	Feb. 28, 2024 USD ($) ft²	Nov. 13, 2023 operating_lease	Oct. 31, 2023 USD ($) ft²	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) operating_lease	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2024 USD ($)	Jul. 17, 2023 USD ($)	Jun. 30, 2023	Oct. 24, 2022 USD ($)	Oct. 06, 2022 ft²	Sep. 01, 2022 ft²	Jul. 31, 2019 ft²
Operating Leased Assets [Line Items]
Right-of-use asset impairment expense							$ 710	$ 0	$ 0
Operating lease, weighted average discount rate, percent						9.90%	9.90%	9.70%		11.50%		9.70%
Lease liabilities						$ 10,955	$ 10,955			$ 14,867
Right-of-use asset, net						$ 10,068	$ 10,068	$ 11,617		13,852
Minimum
Operating Leased Assets [Line Items]
Lease term						2 years	2 years
Lease renewal term (in years)						1 year	1 year
Maximum
Operating Leased Assets [Line Items]
Lease term						8 years	8 years
Lease renewal term (in years)						5 years	5 years
65 Grove Street, Watertown, MA
Operating Leased Assets [Line Items]
Area of office space leased | ft²					5,600									7,216	7,216	25,078
Letter of credit outstanding											$ 1,600
Upfront rental payment					$ 1,000
Deferred rent						$ 800	$ 800			300
Right-of-use asset impairment expense						700	700
Operating lease, weighted average discount rate, percent													11.90%		11.30%	8.90%
Lease term															5 years 8 months 12 days	8 years
Lease liabilities								3,200					$ 100
Right-of-use asset, net								3,200					$ 100
65 Grove Street, Watertown, MA | Letter of Credit
Operating Leased Assets [Line Items]
Restricted cash								1,600
Long-term restricted cash						$ 1,400	$ 1,400	$ 1,300		1,400
704 Quince Orchard Road Leases
Operating Leased Assets [Line Items]
Number of acquired operating leases | operating_lease				2			2
Operating lease, weighted average discount rate, percent				11.50%		11.50%	11.50%
7495 new Horizon Way Lease
Operating Leased Assets [Line Items]
Area of office space leased | ft²	7,842		19,199
Letter of credit outstanding										$ 300
Operating lease, weighted average discount rate, percent	14%	14%
Lease term		7 years 2 months 12 days
Number of options to extend lease | extension_option		1
Lease renewal term (in years)		5 years
Lease agreement, monthly base rent		$ 900
Base rent upward annual adjustment percentage after first year	3%
Lease agreement, first month's rent paid upon execution of lease agreement			$ 100
Lease liabilities	$ 1,200	3,600
Right-of-use asset, net	1,200	3,700
Tenant improvement recognized as reduction in right-of-use asset and lease liability	100	$ 700
Additional annual base rent for first year	$ 300
7495 new Horizon Way Lease | Minimum
Operating Leased Assets [Line Items]
Operating lease, cost (as a percent)		100%
7495 new Horizon Way Lease | Maximum
Operating Leased Assets [Line Items]
Operating lease, cost (as a percent)		103%

Leases - Components of Lease Co

Leases - Components of Lease Costs (Q2) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease cost	$ 956	$ 696	$ 1,731	$ 1,392	$ 2,828	$ 2,276	$ 2,023
Variable lease cost	352	270	749	412	965	910	834
Short-term lease cost	1	2	4	5	8	11	10
Less: Sublease income	250	251	760	506	1,172	176	0
Total lease cost	$ 1,059	$ 717	$ 1,724	$ 1,303	$ 2,629	$ 3,021	$ 2,867

Leases - Operating Lease, Liabi

Leases - Operating Lease, Liability, Maturity (Q2) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Leases [Abstract]
2024 (remainder)	$ 1,239
2025	4,350	$ 3,077
2026	4,471	3,164
2027	4,276	3,248
2028	2,243	3,017
Thereafter	3,409
Total future minimum lease payments	19,988	13,452
Less: Imputed interest	5,121	2,497
Total operating lease liabilities	$ 14,867	$ 10,955

Leases - Other Information (Q2)

Leases - Other Information (Q2) (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities:	$ 1,604	$ 1,319	$ 2,696	$ 2,048

Leases - Lease Term and Discoun

Leases - Lease Term and Discount Rate (Q2) (Details)	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Leases [Abstract]
Weighted-average remaining lease term	4 years 10 months 24 days	4 years 3 months 18 days	4 years 10 months 24 days	5 years 4 months 24 days
Weighted-average discount rate	11.50%	9.90%	9.70%	9.70%

Debt (Q2) (Details)

Debt (Q2) (Details) - USD ($)		3 Months Ended	6 Months Ended		12 Months Ended
	Sep. 11, 2023	Sep. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Line of Credit Facility [Line Items]
Total payoff amount of term loan			$ 0	$ 2,586,000	$ 27,457,000	$ 0	$ 0
Loss on extinguishment of debt					740,000	0	$ 0
2020 Term Loans
Line of Credit Facility [Line Items]
Total payoff amount of term loan	$ 22,300,000
Principal amount extinguished	19,800,000
Final payment fee	2,300,000
Prepayment fee	200,000	$ 200,000			200,000
Accrued interest	$ 100,000
Loss on extinguishment of debt		700,000			700,000
Unamortized debt issuance cost		$ 500,000			500,000
Outstanding borrowings			$ 0		$ 0	$ 25,000,000

Series A Preferred Stock (Q2) (

Series A Preferred Stock (Q2) (Details) $ in Thousands								1 Months Ended	3 Months Ended
	Apr. 08, 2024 shares	Feb. 11, 2024 shares	Jan. 12, 2024 shares	Dec. 13, 2023 USD ($) shares	Dec. 05, 2023 shares	Nov. 15, 2023 USD ($) shares	Nov. 13, 2023 USD ($) shares	Nov. 30, 2023 USD ($)	Jun. 30, 2024 shares	Mar. 31, 2024 shares	Mar. 26, 2024 business_day shares	Dec. 31, 2023 shares	Jun. 30, 2023 shares	Mar. 31, 2023 shares	Dec. 31, 2022 shares	Dec. 31, 2021 shares	Dec. 31, 2020 shares
Preferred stock
Temporary equity, shares authorized (in shares)							548,375		0			548,375			0
Obligation to issue stock (in shares)		49,570.163		49,570.162			148,710.488
Number of business days | business_day											8
Convertible preferred stock, shares that did not automatically convert (in shares)	166,341.592
Convertible preferred stock, common stock issuable upon conversion (in shares)									5,544,719
Common stock, shares outstanding (in shares)									17,816,238			5,397,597
Preferred stock, shares issued (in shares)									0			0			0
Preferred stock, shares outstanding (in shares)									0			0			0
Series A Preferred Stock
Preferred stock
Issuance of common stock upon exercise of warrants and Conversion of Series A Preferred Stock to common stock (in shares)	(367,919.247)								(367,919.247)
Common stock, shares outstanding (in shares)									166,341.592	534,260.839		0
Common stock
Preferred stock
Issuance of common stock upon exercise of warrants and Conversion of Series A Preferred Stock to common stock (in shares)	12,263,951								12,263,951	65,681
Common stock, shares outstanding (in shares)									17,816,238	5,515,836			5,114,289	5,114,269	5,101,459	123,622,965	108,071,249
Series A Preferred Stock
Preferred stock
Convertible preferred stock (in shares)											33.333
Preferred stock, shares issued (in shares)									166,341.592			0
Preferred stock, shares outstanding (in shares)									166,341.592			0
Series A Preferred Stock | Minimum
Preferred stock
Beneficial ownership limitation percentage for conversion of common stock issued and outstanding											0%
Series A Preferred Stock | Maximum
Preferred stock
Beneficial ownership limitation percentage for conversion of common stock issued and outstanding											19.90%
Old Cartesian | Series A Preferred Stock
Preferred stock
Issuance of shares (in shares)					384,930.724		384,930.724
Securities Purchase Agreement
Preferred stock
Sale of stock, number of shares issued in transaction (in shares)							149,330.115
Sale of stock, consideration received on transaction | $				$ 14,800			$ 60,250
November 2023 Private Placement, Tranche One
Preferred stock
Sale of stock, number of shares issued in transaction (in shares)				24,785.081
Sale of stock, consideration received on transaction | $								$ 10,000
Sale of stock, settlement term								30 days
November 2023 Private Placement, Tranche Three
Preferred stock
Sale of stock, number of shares issued in transaction (in shares)		49,570.163
Sale of stock, consideration received on transaction | $								$ 20,000
Sale of stock, settlement term								90 days
November 2023 Private Placement, Tranche Two
Preferred stock
Sale of stock, number of shares issued in transaction (in shares)			49,570.163
Sale of stock, consideration received on transaction | $								$ 20,000
Sale of stock, settlement term								60 days
November 2023 Private Placement
Preferred stock
Sale of stock, number of shares issued in transaction (in shares)						619.627	149,330.115
Sale of stock, consideration received on transaction | $						$ 250

Equity - Narrative (Q2) (Detail

Equity - Narrative (Q2) (Details) $ in Millions				6 Months Ended	12 Months Ended
	Apr. 04, 2024	Oct. 25, 2021 USD ($)	Aug. 06, 2020 USD ($)	Jun. 30, 2024 shares	Dec. 31, 2023 shares	Dec. 31, 2022 shares
Subsidiary, Sale of Stock [Line Items]
Exercise of warrants (in shares)				65,681
Reverse stock split conversion ratio	0.0333
Warrants to purchase common stock
Subsidiary, Sale of Stock [Line Items]
Exercise of warrants (in shares)				65,681
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Sale of stock, consideration received on transaction | $		$ 51	$ 50
Sale of stock, number of shares issued in transaction (in shares)				0	0	774,544

Equity - Warranty Activity (Q2)

Equity - Warranty Activity (Q2) (Details)	6 Months Ended
	Jun. 30, 2024 $ / shares shares
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	1,040,813
Exercises (in shares)	(65,681)
Class of warrant or right, outstanding, ending balance (in shares)	975,132
Class Of Warrant Or Right Weighted Average Exercise Price [Roll Forward]
Exercise price, beginning balance (in dollars per share) | $ / shares	$ 45.98
Exercises (in dollars per share) | $ / shares	43.8
Exercise price, ending balance (in dollars per share) | $ / shares	$ 46.12
Warrants to purchase common stock
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	966,393
Class of warrant or right, outstanding, ending balance (in shares)	966,393
Warrants to purchase common stock
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	74,420
Exercises (in shares)	(65,681)
Class of warrant or right, outstanding, ending balance (in shares)	8,739

Equity - Schedule of Authorized

Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Q2) (Details) - shares	Jun. 30, 2024	Dec. 31, 2023
Class of Stock [Line Items]
Total (in shares)	12,794,858	90,367,632
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	3,930,990	35,836,268
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	454,456
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	1,889,561	23,306,661
Series A Preferred Stock
Class of Stock [Line Items]
Total (in shares)	5,544,719
Exercise of warrants
Class of Stock [Line Items]
Total (in shares)	975,132	31,224,703

Stock Incentive Plans - Narrati

Stock Incentive Plans - Narrative (Q2) (Details) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024 USD ($) shares	Jan. 31, 2024 shares	Jan. 31, 2023 shares	Jan. 31, 2022 shares	Jun. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2023 USD ($) $ / shares	Jun. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2023 USD ($) $ / shares	Dec. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Mar. 27, 2024	Nov. 13, 2023 USD ($) shares	Mar. 31, 2019 shares	Sep. 30, 2018 shares	Jun. 30, 2016 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total (in shares)	12,794,858				12,794,858		12,794,858		90,367,632
Stock based compensation expense | $					$ 1,591	$ 3,783	$ 3,022	$ 6,059	$ 25,778	$ 11,194	$ 7,720
Weighted average grant date fair value of stock options (in dollars per share) | $ / shares					$ 17.4	$ 30.51	$ 16.05	$ 26.89
Rounding down per-share exercise price (in shares)									1,000			33.333
Outstanding Series A Preferred Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total (in shares)	1,889,561				1,889,561		1,889,561		23,306,661
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term							4 years
Weighted average period							3 years 4 months 24 days
Grants in period (in shares)							471,104		1,054,600
Grants in period (in dollars per share) | $ / shares							$ 19.8		$ 1.13
Estimated forfeitures rate							10%		10%
Unrecognized compensation expense | $	$ 6,100				$ 6,100		$ 6,100		$ 0
Common stock | Outstanding Series A Preferred Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested employee stock options | $	$ 9,900				$ 9,900		$ 9,900		$ 1,400
Weighted average period							3 years		2 years 4 months 24 days
Series A Preferred Stock | Outstanding Series A Preferred Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested employee stock options | $									$ 1,000
Weighted average period									2 years 6 months
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)																40,341
Number of shares authorized, increase (in shares)	3,466,544	215,903	6,121,697	4,944,919
Total (in shares)	3,496,828				3,496,828		3,496,828		22,504,503
2016 Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term									4 years
Grants in period (in shares)									1,054,600
Grants in period (in dollars per share) | $ / shares									$ 1.13
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)	364,660				364,660		364,660		4,500,858
Total (in shares)	360,000				360,000		360,000		60,833					66,667	39,166
Old Cartesian Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)	23,707				23,707		23,707		3,848,809
Award vesting term							4 years		4 years
Award expiration term							10 years		10 years
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)	0				0		0		14,112.299
Unrecognized compensation expense related to unvested employee stock options | $													$ 2,600
Stock based compensation expense | $					$ 300		$ 700		$ 200
Old Cartesian Plan | Old Cartestian | Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)													776,865
Old Cartesian Plan | Old Cartestian | Series A Preferred Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)													14,112.299

Stock Incentive Plans - Schedul

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Q2) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,591	$ 3,783	$ 3,022	$ 6,059	$ 25,778	$ 11,194	$ 7,720
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	776	2,677	1,488	3,869
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 815	$ 1,106	1,534	$ 2,190	12,793	6,133	$ 4,516
Ginkgo Bioworks Holdings, Inc
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense			$ 1,500		$ 1,500	$ 1,000

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Assumptions (Q2) (Details) - Outstanding common stock options - $ / shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	4.53%	3.38%	4.04%	3.95%	3.95%	2.24%	0.79%
Dividend yield	0%	0%	0%	0%	0%	0%	0%
Expected term (in years)	6 years 2 months 4 days	6 years	6 years 2 months 12 days	5 years 11 months 8 days	5 years 11 months 8 days	6 years 7 days	6 years 10 days
Expected volatility	93.57%	94.40%	95.09%	94.64%	94.64%	92.21%	95.04%
Weighted-average fair value of common stock (in dollars per share)	$ 22.08	$ 39.3	$ 20.14	$ 34.5	$ 1.15	$ 2.63	$ 3.58

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Options Activity (Q2) (Details) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024 USD ($) $ / shares shares	Dec. 31, 2023 USD ($) $ / shares shares
2016 Plan, 2018 Inducement Incentive Award Plan and Old Cartesian Plan
Number of Options
Beginning balance (in shares) | shares	776,865
Granted (in shares) | shares	761,556
Converted from options for series A Preferred Stock (in shares) | shares	470,403
Exercised (in shares) | shares	(89,009)
Forfeited (in shares) | shares	(30,254)
Ending balance (in shares) | shares	1,889,561	776,865
Vested (in shares) | shares	944,490
Vested and expected to vest (in shares) | shares	1,743,749
Weighted-average exercise price
Beginning balance (in dollars per share) | $ / shares	$ 2.97
Granted (in dollars per share) | $ / shares	20.14
Converted from options for series A Preferred Stock (in dollars per share) | $ / shares	2.4
Exercised (in dollars per share) | $ / shares	3.08
Forfeited (in dollars per share) | $ / shares	14.84
Ending balance (in dollars per share) | $ / shares	9.56	$ 2.97
Vested (in dollars per share) | $ / shares	2.74
Vested and expected to vest (in dollars per share) | $ / shares	$ 8.66
Weighted-average remaining contractual term
Outstanding, term	7 years 2 months 26 days	6 years 6 months
Vested and expected to vest, term	7 years 1 month 13 days
Aggregate intrinsic value
Outstanding | $	$ 33,081	$ 13,760
Vested | $	22,925
Vested and expected to vest | $	$ 32,070
Old Cartesian Plan
Number of Options
Beginning balance (in shares) | shares	14,112.299
Converted to options for common stock (in shares) | shares	(14,112.299)
Ending balance (in shares) | shares	0	14,112.299
Weighted-average exercise price
Beginning balance (in dollars per share) | $ / shares	$ 79.94
Converted to options for common stock (in dollars per share) | $ / shares	79.94
Ending balance (in dollars per share) | $ / shares	$ 0	$ 79.94
Weighted-average remaining contractual term
Outstanding, term	5 years 10 months 28 days
Vested, term		5 years 5 months 4 days
Aggregate intrinsic value
Outstanding | $		$ 8,601

Stock Incentive Plans - Restric

Stock Incentive Plans - Restricted Stock Units (Q2) (Details) - Restricted Stock Units (RSUs) - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested at beginning of period (in shares)	0	1,705,558
Grants in period (in shares)	471,104	1,054,600
Forfeited in period (in shares)	(16,648)	(446,108)
Unvested at end of period (in shares)	454,456	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Unvested at beginning of period (in dollars per share)	$ 0	$ 2.62
Grants in period (in dollars per share)	19.8	1.13
Forfeited in period (in dollars per share)	19.8	1.91
Unvested at end of period (in dollars per share)	$ 19.8	$ 0

Stock Incentive Plans - Employe

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Q2) (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	12,794,858		12,794,858		90,367,632
Stock based compensation expense	$ 1,591,000	$ 3,783,000	$ 3,022,000	$ 6,059,000	$ 25,778,000	$ 11,194,000	$ 7,720,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	45,795		45,795		4,982,098			5,769
Stock based compensation expense	$ 0	$ 100,000	$ 0	$ 100,000	$ 100,000	$ 100,000

Revenue Arrangements - Narrativ

Revenue Arrangements - Narrative (Q2) (Details) - USD ($)				1 Months Ended			3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 28, 2024	Oct. 01, 2021	Jun. 11, 2020	Jan. 31, 2023	Oct. 31, 2021	Jun. 30, 2020	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2021	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Disaggregation of Revenue [Line Items]
Deferred revenue related to agreement							$ 0			$ 0		$ 5,849,000	$ 593,000
Deferred revenue							0			0		2,311,000	593,000
Deferred revenue, net of current portion							0			0		3,538,000	0
Accounts receivable							32,039,000			32,039,000		5,870,000	6,596,000
Revenue recognized												5,244,000
Revenue recognized										(5,849,000)
Collaboration and license revenue							33,271,000	$ 5,249,000		39,111,000	$ 11,187,000	26,004,000	110,777,000	$ 85,077,000
Remaining performance obligation							0			0		5,800,000
Grant revenue							174,000	0		174,000	0
National Institute of Neurological Disorders and Stroke of the National Institutes of Health Funding
Disaggregation of Revenue [Line Items]
Government assistance, award amount							1,500,000			1,500,000
Government assistance, potential additional award amount							1,500,000			1,500,000
Government assistance, asset, current							200,000			200,000
Grant revenue							200,000			200,000
SOBI Purchase Agreement
Disaggregation of Revenue [Line Items]
Accounts receivable							31,700,000			31,700,000		4,600,000
Collaboration and license revenue							32,800,000	4,300,000			8,700,000
SOBI Purchase Agreement | Billed Revenues
Disaggregation of Revenue [Line Items]
Accounts receivable							3,500,000			3,500,000		3,000,000
Takeda Agreement
Disaggregation of Revenue [Line Items]
Deferred revenue							0			0		0	100,000
Deferred revenue, net of current portion													0
Upfront cash payment		$ 3,000,000			$ 3,000,000
Future additional payments, expected		$ 1,124,000,000			$ 1,124,000,000
Milestone payments								500,000
Customer, liability, revenue recognized, including opening balance							0	100,000		0	600,000	600,000	1,800,000
Collaborative Arrangement
Disaggregation of Revenue [Line Items]
Upfront cash payment			$ 75,000,000
Sales milestone payments			$ 630,000,000
Collaborative Arrangement | SOBI Purchase Agreement
Disaggregation of Revenue [Line Items]
Upfront cash payment						$ 75,000,000
Sales milestone payments						630,000,000
Revenue recognized	$ 30,000,000						30,000,000			30,000,000
Astellas Gene Therapies
Disaggregation of Revenue [Line Items]
Upfront payment				$ 10,000,000
Development and commercial milestones plus royalties				$ 340,000,000
Deferred revenue related to agreement							0			0
Deferred revenue												2,300,000
Deferred revenue, net of current portion												3,500,000
Accounts receivable							100,000			100,000		300,000
Revenue recognized							500,000	800,000		6,300,000	1,400,000
Revenue, remaining performance obligation recognized										3,200,000
Revenue recognized												5,500,000
Sarepta Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
Deferred revenue												0	500,000
Upfront cash payment						$ 2,000,000
Sales milestone payments									$ 3,000,000
Revenue recognized							$ 0	$ 0		$ 0	$ 500,000
Collaboration and license revenue												$ 500,000	$ 10,200,000
Option term						24 months

Revenue Arrangements - Schedule

Revenue Arrangements - Schedule of Changes in Contract Liabilities (Q2) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Contract liabilities:
Deferred revenue, beginning of period	$ 5,849	$ 593
Deferred revenue, additions	0	10,500
Deferred revenue, deductions	(5,849)	5,244
Deferred revenue, end of period	0	5,849
Contract liabilities, beginning balance	5,849	593
Additions	0	10,500
Deductions	(5,849)
Contract liabilities, ending balance	$ 0	$ 5,849

Related-Party Transactions - Na

Related-Party Transactions - Narrative (Q2) (Details) - USD ($) $ / shares in Units, $ in Thousands				6 Months Ended
	Mar. 26, 2024	Dec. 13, 2023	Nov. 13, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Related Party Transaction [Line Items]
Weighted average exercise price (in dollars per share)				$ 46.12		$ 45.98	$ 1.53	$ 1.62
Proceeds from exercise of common warrants				$ 2,877	$ 0
Securities Purchase Agreement
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)			149,330.115
Sale of stock, consideration received on transaction		$ 14,800	$ 60,250
Related Party
Related Party Transaction [Line Items]
Issuance of common stock upon exercise of warrants and Conversion of Series A Preferred Stock to common stock (in shares)	65,681
Issuance of CVRs upon exercise of warrants (in shares)	1,970,443
Related Party | 2019 Warrants
Related Party Transaction [Line Items]
Warrants exercised (in shares)	65,681
Weighted average exercise price (in dollars per share)	$ 43.8
Proceeds from exercise of common warrants	$ 2,900
Related Party | Securities Purchase Agreement
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)			149,330.115
Sale of stock, consideration received on transaction			$ 60,250

Related-Party Transactions - Nu

Related-Party Transactions - Number of Shares of Common Stock Purchased (Q2) (Details) - November 2023 Private Placement - USD ($)			12 Months Ended
	Nov. 15, 2023	Nov. 13, 2023	Dec. 31, 2023
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	619.627	149,330.115
Sale of stock, consideration received on transaction	$ 250,000
Dr. Timothy A. Springer | Related Party
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		99,140.326	24,785.081
Sale of stock, consideration received on transaction		$ 40,000,000	$ 10,000,000

Collaboration and License Agr_3

Collaboration and License Agreements - Biogen M.A., Inc. (Q2) (Details)	Sep. 08, 2023	Sep. 16, 2019
Biogen M.A., Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Written notice for agreement termination	30 days	30 days

Collaboration and License Agr_4

Collaboration and License Agreements - National Cancer Institute of the National Institutes of Health (Q2) (Details) - NCI Agreement $ in Millions	Sep. 16, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
License and option agreement, royalty payment	$ 0.1
License and option agreement, benchmark royalties, maximum	$ 0.8
Written notice for agreement termination	60 days

Collaboration and License Agr_5

Collaboration and License Agreements - Ginkgo Agreement (Q2) (Details) - Ginkgo Agreement - USD ($) $ in Millions					12 Months Ended
	Jul. 19, 2023	Jun. 13, 2022	Jan. 03, 2022	Oct. 25, 2021	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Clinical and commercial milestone payment, expected			$ 207	$ 85
Milestone payment	$ 1	$ 0.5			$ 1
Collaborative arrangement, number of shares issued for milestone achievement (in shares)	1,339,285	29,761			44,642
Collaborative arrangement, shares issued for milestone achievement, amount					$ 1.5	$ 1

Collaboration and License Agr_6

Collaboration and License Agreements - Genovis (Q2) (Details) $ in Millions	1 Months Ended
	Feb. 28, 2023 USD ($)
Genovis AB
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 4

Collaboration and License Agr_7

Collaboration and License Agreements - Cyrus Biotechnology, Inc. (Q2) (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended
	Sep. 07, 2021	Sep. 30, 2021	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Equity securities without readily determinable fair value, amount			$ 2,000	$ 2,000	$ 2,000
Cyrus Biotechnology, Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 1,500,000	$ 1,500,000
Equity securities, stock purchase agreement, shares purchased (in shares)	2,326,934
Equity securities without readily determinable fair value, amount	$ 2,000		$ 2,000	$ 2,000	$ 2,000
Cyrus Biotechnology, Inc. | Series B Preferred Stock Purchase Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Equity securities, stock purchase agreement, par value per share (in dollars per share)	$ 0.0001
Equity securities, stock purchase agreement, purchase price per share (in dollars per share)	$ 0.8595

Collaboration and License Agr_8

Collaboration and License Agreements - AskBio (Q2) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration expense	$ 19,688	$ 23,887	$ 38,876	$ 48,206	$ 112,420	$ 96,239	$ 89,674
AskBio License
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration expense	$ 0	$ 100	$ 0	$ 100	$ 100	$ 900
Cost share percentage	50%	50%	50%	50%	50%	50%

Collaboration and License Agr_9

Collaboration and License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Q2) (Details) - 3SBio License - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate amount of upfront and milestone-based payments	$ 7	$ 7
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing ImmTOR platform	$ 15	$ 15

Defined Contribution Plan (Q2)

Defined Contribution Plan (Q2) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Retirement Benefits [Abstract]
Vesting period			2 years		2 years
Employer contribution made, net	$ 0.1	$ 0.1	$ 0.1	$ 0.2	$ 0.3	$ 0.3	$ 0.2

Restructuring - Narrative (Q2)

Restructuring - Narrative (Q2) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended	15 Months Ended
	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2024
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated, period percent		79%	89%
Restructuring charges	$ 805	$ 6,431
Cash payments for employee related restructuring charges	4,527	2,535
Research and development
Restructuring Cost and Reserve [Line Items]
Restructuring charges	200	5,600
General and administrative
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 600	$ 800

Restructuring - Schedule of Cha

Restructuring - Schedule of Change in Accrued Restructuring Balance (Q2) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Restructuring Reserve [Roll Forward]
Beginning Balance	$ 3,896	$ 0
Charges	805	6,431
Payments	(4,527)	(2,535)
Ending Balance	$ 174	$ 3,896

Subsequent Events (Q2) (Details

Subsequent Events (Q2) (Details) $ / shares in Units, $ in Millions	Jul. 02, 2024 USD ($) $ / shares shares	Aug. 07, 2024 shares	Jun. 30, 2024 shares	Dec. 31, 2023 shares
Subsequent Event [Line Items]
Common stock, shares outstanding (in shares)			17,816,238	5,397,597
Subsequent Event
Subsequent Event [Line Items]
Common stock, shares outstanding (in shares)		21,382,485
Common stock, shares, outstanding, proforma as if preferred stock converted (in shares)		29,865,107
Subsequent Event | July 2024 Purchase Agreement
Subsequent Event [Line Items]
Sale of stock, gross consideration received on transaction | $	$ 130
Subsequent Event | Common stock | July 2024 Purchase Agreement
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	3,563,247
Subsequent Event | Series B Preferred Stock
Subsequent Event [Line Items]
Convertible preferred stock (in shares)	1
Subsequent Event | Series B Preferred Stock | July 2024 Purchase Agreement
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	2,937,903
Sale of stock, price per share (in dollars per share) | $ / shares	$ 20
Subsequent Event | Series B Preferred Stock | Minimum
Subsequent Event [Line Items]
Convertible preferred stock, conversion restriction, beneficial ownership	0
Subsequent Event | Series B Preferred Stock | Maximum
Subsequent Event [Line Items]
Convertible preferred stock, conversion restriction, beneficial ownership	0.199
Subsequent Event | Series B Preferred Stock | Directors and Executive Officers | July 2024 Purchase Agreement
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	2,359,500

Description of the Business (_3

Description of the Business (FY) (Details) - USD ($) $ / shares in Units, $ in Thousands						2 Months Ended	6 Months Ended
	Dec. 13, 2023	Dec. 06, 2023	Dec. 05, 2023	Nov. 15, 2023	Nov. 13, 2023	Mar. 07, 2024	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash and Cash Equivalents [Line Items]
Common stock, par value (in dollars per share)					$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Temporary equity, par value (in dollars per share)					$ 0.0001		$ 0.0001	$ 0.0001	$ 0.0001
Cash, cash equivalents, restricted cash and marketable securities							$ 88,900	$ 78,300
Restricted cash and cash equivalents							1,700	1,400
Accumulated deficit							657,635	614,647	$ 394,937
Russian Subsidiary
Cash and Cash Equivalents [Line Items]
Cash maintained in Russian bank accounts							$ 200	$ 200
Securities Purchase Agreement
Cash and Cash Equivalents [Line Items]
Sale of stock, number of shares issued in transaction (in shares)					149,330.115
Sale common stock	$ 14,800				$ 60,250
November 2023 Private Placement
Cash and Cash Equivalents [Line Items]
Sale of stock, number of shares issued in transaction (in shares)				619.627	149,330.115
Sale common stock				$ 250
November 2023 Private Placement | Subsequent Event
Cash and Cash Equivalents [Line Items]
Sale common stock						$ 40,000
Old Cartesian
Cash and Cash Equivalents [Line Items]
Contingent value right, percentage of milestone payments, royalties and other amounts to be distributed		100%					100%
Preferred stock, convertible, redemption terms, period from merger					18 months
Old Cartesian | Common stock
Cash and Cash Equivalents [Line Items]
Issuance of shares (in shares)			6,723,639		224,099
Old Cartesian | Common stock | Previously Reported [Member]
Cash and Cash Equivalents [Line Items]
Issuance of shares (in shares)					6,723,639
Old Cartesian | Series A Preferred Stock
Cash and Cash Equivalents [Line Items]
Issuance of shares (in shares)			384,930.724		384,930.724

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Additional Information (FY) (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2023	Dec. 31, 2023 USD ($) segment
Accounting Policies [Abstract]
Number of operating segments | segment		1
Lessee, Lease, Description [Line Items]
Impairment to goodwill		$ 0
Impairment of intangible assets		$ 0
Astellas Gene Therapies
Lessee, Lease, Description [Line Items]
Reimbursement percentage, budgeted costs incurred to complete the development	25%
Minimum
Lessee, Lease, Description [Line Items]
Lease term		2 years
Lease renewal term		1 year
Maximum
Lessee, Lease, Description [Line Items]
Lease term		8 years
Lease renewal term		5 years

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (FY) (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Russian Subsidiary
Concentration Risk [Line Items]
Cash maintained in Russian bank accounts	$ 0.2	$ 0.2

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Property and Equipment (FY) (Details)	Dec. 31, 2023
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Laboratory Equipment, Software and Office Equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Computer Equipment
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years

Merger - Narrative (FY) (Detail

Merger - Narrative (FY) (Details) - Old Cartesian - USD ($) $ in Millions			2 Months Ended
	Dec. 05, 2023	Nov. 13, 2023	Dec. 31, 2023
Business Acquisition [Line Items]
Percentage acquired		100%	100%
Revenues			$ 0
Net loss			$ 1.6
Transaction costs		$ 4.9
Common stock
Business Acquisition [Line Items]
Issuance of shares (in shares)	6,723,639	224,099
Common stock | Previously Reported [Member]
Business Acquisition [Line Items]
Issuance of shares (in shares)		6,723,639
Series A Preferred Stock
Business Acquisition [Line Items]
Issuance of shares (in shares)	384,930.724	384,930.724

Merger - Schedule of Fair Val_2

Merger - Schedule of Fair Value of Consideration (FY) (Details) - Old Cartesian $ in Thousands	Nov. 13, 2023 USD ($)
Business Acquisition [Line Items]
Total consideration	$ 168,465
Common stock
Business Acquisition [Line Items]
Forward contract to issue stock and options	2,713
Series A Preferred Stock
Business Acquisition [Line Items]
Forward contract to issue stock and options	155,308
Options
Business Acquisition [Line Items]
Forward contract to issue stock and options	$ 10,444

Merger - Schedule of Assets A_2

Merger - Schedule of Assets Acquired and Liabilities Assumed (FY) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Nov. 13, 2023	Dec. 31, 2022
Assets acquired:
Goodwill	$ 48,163	$ 48,163		$ 0
Old Cartesian
Assets acquired:
Cash and cash equivalents			$ 6,561
Prepaid expenses and other current assets			309
Property and equipment, net			215
Right-of-use asset, net			915
In-process research and development assets			150,600
Goodwill			48,163
Total assets acquired			206,763
Liabilities assumed
Accrued expenses and other current liabilities			2,530
Lease liability			292
Lease liability, net of current portion			623
Deferred tax liability			34,853
Total liabilities assumed			38,298
Net assets acquired			$ 168,465

Merger - Schedule of Intangib_2

Merger - Schedule of Intangible Assets Acquired (FY) (Details) - USD ($) $ in Thousands	2 Months Ended	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2023	Jun. 30, 2024	Nov. 13, 2023	Dec. 31, 2022
Business Acquisition [Line Items]
Impairment		$ 0
Carrying Value	$ 150,600	150,600	$ 150,600		$ 0
Old Cartesian
Business Acquisition [Line Items]
Acquisition Date Fair Value				$ 150,600
Impairment	0
Carrying Value	150,600	150,600
Old Cartesian | Descartes-08 for MG
Business Acquisition [Line Items]
Acquisition Date Fair Value				93,900
Impairment	0
Carrying Value	93,900	93,900
Old Cartesian | Descartes-08 for SLE
Business Acquisition [Line Items]
Acquisition Date Fair Value				$ 56,700
Impairment	0
Carrying Value	$ 56,700	$ 56,700

Merger - Schedule of Pro forma

Merger - Schedule of Pro forma Financial Information (FY) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Business Combination and Asset Acquisition [Abstract]
Revenue	$ 26,004	$ 112,226
Net (loss) income	$ (232,259)	$ 29,607

Marketable Securities and Inv_3

Marketable Securities and Investments - Summary of Available-for-Sale Marketable Securities (FY) (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized cost		$ 28,175,000
Unrealized gains		0
Unrealized losses		(11,000)
Fair value	$ 0	28,164,000
U.S. government agency securities and treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized cost		13,566,000
Unrealized gains		0
Unrealized losses		(9,000)
Fair value		13,557,000
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost		1,953,000
Unrealized gains		0
Unrealized losses		(2,000)
Fair value		1,951,000
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized cost		12,656,000
Unrealized gains		0
Unrealized losses		0
Fair value		$ 12,656,000

Marketable Securities and Inv_4

Marketable Securities and Investments - Narrative (FY) (Details) - USD ($)	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022	Sep. 07, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Marketable securities		$ 0	$ 28,164,000
Investments	$ 2,000,000	2,000,000	2,000,000
Cyrus Biotechnology, Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Investments	$ 2,000,000	$ 2,000,000	$ 2,000,000	$ 2,000,000

Net (Loss) Income Per Share - C

Net (Loss) Income Per Share - Computation of Basic and Diluted Net (Loss) Income Per Share (FY) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Numerator:
Net (loss) income	$ 13,836	$ (56,824)	$ (11,387)	$ (21,663)	$ (42,988)	$ (33,050)	$ (219,710)	$ 35,379	$ (25,687)
Less: CVR distribution to participating securities							(37,550)	0	0
Net (loss) income allocable to shares of common stock - basic	9,628		(11,387)		(42,988)	(33,050)	(257,260)	35,379	(25,687)
Less: Change in fair value of warrants							0	(20,882)	0
Net (loss) income allocable to shares of common stock - diluted	$ 9,628		$ (11,387)		$ (42,988)	$ (33,050)	$ (257,260)	$ 14,497	$ (25,687)
Denominator:
Weighted-average common shares outstanding - basic (in shares)	16,723,479		5,114,747		11,068,749	5,113,213	155,109,561	144,758,555	114,328,798
Dilutive effect of employee equity incentive plans and outstanding warrants (in shares)							0	1,116,334	0
Weighted-average common shares used in per share calculations - diluted (in shares)	17,791,143		5,114,747		11,068,749	5,113,213	155,109,561	145,874,889	114,328,798
Net (loss) income per share:
Basic (in dollars per share)	$ 0.58		$ (2.23)		$ (3.88)	$ (6.46)	$ (1.66)	$ 0.24	$ (0.22)
Diluted (in dollars per share)	$ 0.54		$ (2.23)		$ (3.88)	$ (6.46)	$ (1.66)	$ 0.1	$ (0.22)

Net (Loss) Income Per Share - S

Net (Loss) Income Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (FY) (Details) - shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Potential common shares
Total (in shares)	7,260,062	1,794,533	8,863,868	1,794,533	602,904,502	18,013,373	22,227,982
Warrants to purchase common stock
Potential common shares
Total (in shares)	975,132	1,040,943	975,132	1,040,943	31,224,703	213,339	10,735,980
Series A Preferred Stock
Potential common shares
Total (in shares)	5,544,719	0	5,544,719	0	435,120,513	0	0
Forward contract to issue Series A Preferred Stock
Potential common shares
Total (in shares)					99,140,326	0	0
Common stock options, RSUs and ESPP shares
Potential common shares
Total (in shares)	740,211	753,590	2,344,017	753,590	23,306,661	17,800,034	11,492,002
Series A Preferred Stock options
Potential common shares
Total (in shares)					14,112,299	0	0

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Assets Measured at Fair Value on a Recurring Basis (FY) (Details) - USD ($)	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Assets:
Marketable securities:		$ 0	$ 28,164,000
Liabilities:
Forward contract liabilities	$ 0	28,307,000	0
U.S. government agency securities and treasuries
Assets:
Marketable securities:			13,557,000
Corporate bonds
Assets:
Marketable securities:			1,951,000
Commercial paper
Assets:
Marketable securities:			12,656,000
Recurring
Assets:
Total assets	39,181,000	41,161,000	81,716,000
Liabilities:
Warrant liabilities	9,260,000	6,394,000	19,140,000
Contingent value right liability	395,400,000	358,600,000
Forward contract liabilities		28,307,000
Total liabilities	404,660,000	393,301,000	19,140,000
Recurring | Level 1
Assets:
Total assets	39,181,000	41,161,000	53,552,000
Liabilities:
Warrant liabilities	0	0	0
Contingent value right liability	0	0
Forward contract liabilities		0
Total liabilities	0	0	0
Recurring | Level 2
Assets:
Total assets	0	0	28,164,000
Liabilities:
Warrant liabilities	0	0	0
Contingent value right liability	0	0
Forward contract liabilities		28,307,000
Total liabilities	0	28,307,000	0
Recurring | Level 3
Assets:
Total assets	0	0	0
Liabilities:
Warrant liabilities	9,260,000	6,394,000	19,140,000
Contingent value right liability	395,400,000	358,600,000
Forward contract liabilities		0
Total liabilities	404,660,000	364,994,000	19,140,000
Recurring | U.S. government agency securities and treasuries
Assets:
Marketable securities:			13,557,000
Recurring | U.S. government agency securities and treasuries | Level 1
Assets:
Marketable securities:			0
Recurring | U.S. government agency securities and treasuries | Level 2
Assets:
Marketable securities:			13,557,000
Recurring | U.S. government agency securities and treasuries | Level 3
Assets:
Marketable securities:			0
Recurring | Corporate bonds
Assets:
Marketable securities:			1,951,000
Recurring | Corporate bonds | Level 1
Assets:
Marketable securities:			0
Recurring | Corporate bonds | Level 2
Assets:
Marketable securities:			1,951,000
Recurring | Corporate bonds | Level 3
Assets:
Marketable securities:			0
Recurring | Commercial paper
Assets:
Marketable securities:			12,656,000
Recurring | Commercial paper | Level 1
Assets:
Marketable securities:			0
Recurring | Commercial paper | Level 2
Assets:
Marketable securities:			12,656,000
Recurring | Commercial paper | Level 3
Assets:
Marketable securities:			0
Recurring | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	39,181,000	41,161,000	53,552,000
Recurring | Money market funds (included in cash equivalents) | Level 1
Assets:
Money market funds (included in cash equivalents)	39,181,000	41,161,000	53,552,000
Recurring | Money market funds (included in cash equivalents) | Level 2
Assets:
Money market funds (included in cash equivalents)	0	0	0
Recurring | Money market funds (included in cash equivalents) | Level 3
Assets:
Money market funds (included in cash equivalents)	$ 0	$ 0	$ 0

Fair Value Measurements - Nar_2

Fair Value Measurements - Narrative (FY) (Details)					6 Months Ended	12 Months Ended
	Dec. 13, 2023 USD ($)	Dec. 06, 2023 USD ($) shares	Dec. 05, 2023 USD ($) shares	Nov. 13, 2023 USD ($) shares	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value Disclosures [Abstract]
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers, net					$ 0	$ 0	$ 0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, liability, transfers, net					$ 0	0	0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Marketable securities:						$ 0	$ 28,164,000
Securities Purchase Agreement
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Sale of stock, number of shares issued in transaction (in shares) | shares				149,330.115
Sale common stock	$ 14,800,000			$ 60,250,000
Old Cartesian
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right, percentage of milestone payments, royalties and other amounts to be distributed		100%			100%
Fixed semi annual amount		$ 750,000
Old Cartesian | Series A Preferred Stock
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Issuance of shares (in shares) | shares			384,930.724	384,930.724
Forward contract to issue stock				$ 155,308,000
Settlement of forward contract liability			$ 261,800,000
Old Cartesian | 2022 Warrants | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Class of warrant or right, number of securities called by each warrant or right (in shares) | shares		1
Dividend yield | Valuation Technique, Option Pricing Model
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrants and rights outstanding, measurement input					0	0
Dividend yield | Valuation Technique, Option Pricing Model | 2022 Warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrants and rights outstanding, measurement input					0	0	0

Fair Value Measurements - Sch_4

Fair Value Measurements - Schedule of Cash and Cash Equivalents (FY) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Cash and cash equivalents	$ 87,227	$ 76,911	$ 112,027	$ 106,438	$ 114,057
Short-term restricted cash	0	0	1,600	289	0
Long-term restricted cash	1,669	1,377	1,377	1,311	1,379
Total cash, cash equivalents, and restricted cash	$ 88,896	$ 78,288	$ 115,004	$ 108,038	$ 115,436	$ 140,064

Fair Value Measurements - Ass_2

Fair Value Measurements - Assumptions Used to Record the Fair Value of the Warrants (FY) (Details) - Valuation Technique, Option Pricing Model	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Risk-free interest rate | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0533	0.0479	0.0474
Risk-free interest rate | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0452	0.0401	0.0422
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0
Dividend yield | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0	0
Dividend yield | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0	0
Expected life (in years) | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	5 months 26 days	11 months 23 days	1 year 11 months 23 days
Expected life (in years) | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	2 years 9 months 10 days	3 years 3 months 10 days	4 years 3 months 10 days
Expected volatility | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	1.1052	0.8367	0.7992
Expected volatility | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.8723	0.8409	0.9805

Fair Value Measurements - Cha_3

Fair Value Measurements - Change in the Fair Value of Liabilities (FY) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Warrant
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value	$ 6,394	$ 19,140
Change in fair value	2,866	(12,746)
Fair value	9,260	6,394
CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value	358,600	0
Issuances		340,300
Change in fair value	36,800	18,300
Fair value	$ 395,400	$ 358,600

Fair Value Measurements - Sch_5

Fair Value Measurements - Schedule of Significant Inputs Used to Estimate the Fair Value of the CVRs (FY) (Details) - Level 3	Dec. 31, 2023	Nov. 13, 2023
Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	0.95	0.95
Risk-adjusted discount rate
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	0.137	0.144

Fair Value Measurements - Cha_4

Fair Value Measurements - Change in the Forward Contract Liabilities (FY) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value	$ 28,307	$ 0
Issuances		155,308
Settlements	(35,197)	(276,601)
Change in fair value	6,890	149,600
Fair value as of December 31, 2023	$ 0	$ 28,307
Fair Value, Net Derivative Asset (Liability), Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Change in fair value of forward contract liabilities	Change in fair value of forward contract liabilities

Property and Equipment - Sche_2

Property and Equipment - Schedule of Property and Equipment (FY) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 12,701	$ 7,841	$ 7,999
Less accumulated depreciation	(6,029)	(5,728)	(5,205)
Property and equipment, net	6,672	2,113	2,794
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	6,944	6,280	6,001
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	621	702	697
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	61	61	57
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	462	452	453
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	196	196	192
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 4,417	$ 150	$ 599

Property and Equipment - Narr_2

Property and Equipment - Narrative (FY) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 0.2	$ 0.2	$ 0.4	$ 0.4	$ 0.7	$ 0.7	$ 0.6

Accrued Expenses (FY) (Details)

Accrued Expenses (FY) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 1,402	$ 4,390	$ 4,242
Accrued patent fees	689	472	696
Accrued external research and development costs	3,467	4,896	7,274
Accrued professional and consulting services	1,871	4,331	985
Accrued interest		0	222
Other		644	665
Accrued expenses	$ 10,615	$ 14,733	$ 14,084

Leases - Narrative (FY) (Detail

Leases - Narrative (FY) (Details) $ in Thousands				1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended
	Nov. 13, 2023 operating_lease	Oct. 31, 2023 USD ($) ft²	Sep. 01, 2022 USD ($) ft²	Jul. 31, 2019 USD ($) ft²	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($) operating_lease	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jul. 17, 2023 USD ($)	Oct. 24, 2022 USD ($)	Oct. 06, 2022 ft²
Operating Leased Assets [Line Items]
Lessee-paid construction costs					$ 1,059		$ 717	$ 1,724	$ 1,303	$ 2,629	$ 3,021	$ 2,867
Weighted-average discount rate					11.50%	9.90%	9.70%	11.50%	9.70%	9.90%	9.70%
Right-of-use asset, net					$ 13,852	$ 10,068		$ 13,852		$ 10,068	$ 11,617
Lease liabilities					14,867	10,955		14,867		10,955
Right-of-use asset impairment expense										710	0	0
65 Grove Street, Watertown,MA
Operating Leased Assets [Line Items]
Area of office space leased | ft²		5,600	7,216	25,078											7,216
Lessee-paid construction costs				$ 800
Lease term			5 years 8 months 12 days	8 years
Weighted-average discount rate			11.30%	8.90%										11.90%
Base rent			$ 100
Letters of credit outstanding, amount													$ 1,600
Right-of-use asset, net											3,200			$ 100
Lease liabilities											3,200			$ 100
Upfront rental payment		$ 1,000
Deferred rent					300	800		300		800
Right-of-use asset impairment expense						700				700
65 Grove Street, Watertown,MA | Research and development
Operating Leased Assets [Line Items]
Right-of-use asset impairment expense										600
65 Grove Street, Watertown,MA | General and administrative
Operating Leased Assets [Line Items]
Right-of-use asset impairment expense										100
65 Grove Street, Watertown,MA | Letter of Credit
Operating Leased Assets [Line Items]
Restricted cash											1,600
Restricted cash, current											300
Restricted cash, noncurrent					$ 1,400	$ 1,400		$ 1,400		$ 1,400	1,300
704 Quince Orchard Road Leases
Operating Leased Assets [Line Items]
Weighted-average discount rate	11.50%					11.50%				11.50%
Number of acquired operating leases | operating_lease	2									2
Moscow Russia Lease
Operating Leased Assets [Line Items]
Lessee-paid construction costs										$ 3,800	$ 3,200	$ 2,900

Leases - Components of Lease _2

Leases - Components of Lease Costs (FY) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease cost	$ 956	$ 696	$ 1,731	$ 1,392	$ 2,828	$ 2,276	$ 2,023
Variable lease cost	352	270	749	412	965	910	834
Short-term lease cost	1	2	4	5	8	11	10
Less sublease income	(250)	(251)	(760)	(506)	(1,172)	(176)	0
Total lease cost	$ 1,059	$ 717	$ 1,724	$ 1,303	$ 2,629	$ 3,021	$ 2,867

Leases - Operating Lease, Lia_2

Leases - Operating Lease, Liability, Maturity (FY) (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Leases [Abstract]
2024	$ 4,350	$ 3,077
2025	4,471	3,164
2026	4,276	3,248
2027	2,243	3,017
2028		946
Thereafter		0
Total future minimum lease payments	19,988	13,452
Less: Imputed interest	5,121	2,497
Total operating lease liabilities	$ 14,867	$ 10,955

Leases - Other Information (FY)

Leases - Other Information (FY) (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities:	$ 1,604	$ 1,319	$ 2,696	$ 2,048

Leases - Lease Term and Disco_2

Leases - Lease Term and Discount Rate (FY) (Details)	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Leases [Abstract]
Weighted-average remaining lease term	4 years 10 months 24 days	4 years 3 months 18 days	4 years 10 months 24 days	5 years 4 months 24 days
Weighted-average discount rate	11.50%	9.90%	9.70%	9.70%

Debt (FY) (Details)

Debt (FY) (Details) - USD ($)		3 Months Ended	6 Months Ended		12 Months Ended
	Sep. 11, 2023	Sep. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Line of Credit Facility [Line Items]
Total payoff amount of term loan			$ 0	$ 2,586,000	$ 27,457,000	$ 0	$ 0
Loss on extinguishment of debt					740,000	0	0
Interest expense related to term loans					2,100,000	3,000,000	$ 2,800,000
2020 Term Loans
Line of Credit Facility [Line Items]
Total payoff amount of term loan	$ 22,300,000
Extinguishment of debt, amount	19,800,000
Debt instrument, fee amount	2,300,000
Debt instrument, prepayment fee	200,000	$ 200,000			200,000
Debt instrument, accrued interest	$ 100,000
Loss on extinguishment of debt		700,000			700,000
Deferred debt issuance cost, writeoff		$ 500,000			500,000
Term loan facility			$ 0		$ 0	$ 25,000,000

Series A Preferred Stock - Narr

Series A Preferred Stock - Narrative (FY) (Details) $ in Thousands

1 Months Ended

2 Months Ended

12 Months Ended

Feb. 11, 2024 shares

Jan. 12, 2024 shares

Dec. 13, 2023 USD ($) shares

Dec. 05, 2023 shares

Nov. 15, 2023 USD ($) shares

Nov. 13, 2023 USD ($) shares

Nov. 30, 2023 USD ($)

Mar. 07, 2024 USD ($)

Dec. 31, 2023 day shares

Jun. 30, 2024 shares

Mar. 31, 2024 shares

Mar. 27, 2024

Dec. 31, 2022 shares

Dec. 31, 2021 shares

Dec. 31, 2020 shares

Preferred stock

Authorized shares of Preferred Stock (in shares)

548,375

548,375

0

0

Derivative liability, shares (in shares)

49,570.163

49,570.162

148,710.488

Temporary equity, shares issued (in shares)

435,120.513

0

0

Temporary equity, shares outstanding (in shares)

435,120.513

0

0

0

0

0

Preferred stock, convertible, conversion ratio

1,000

33.333

Preferred stock, redemption term

18 months

Preferred stock, convertible, threshold consecutive trading days | day

10

Preferred stock, convertible, transaction restrictions, percentage of originally issued stock issued and outstanding

30%

Securities Purchase Agreement

Preferred stock

Sale of stock, number of shares issued in transaction (in shares)

149,330.115

Sale common stock | $

$ 14,800

$ 60,250

November 2023 Private Placement

Preferred stock

Sale of stock, number of shares issued in transaction (in shares)

619.627

149,330.115

Sale common stock | $

$ 250

November 2023 Private Placement, Tranche One

Preferred stock

Sale of stock, number of shares issued in transaction (in shares)

24,785.081

Sale common stock | $

$ 10,000

Sale of stock, settlement term

30 days

November 2023 Private Placement, Tranche Two

Preferred stock

Sale of stock, number of shares issued in transaction (in shares)

49,570.163

Sale common stock | $

$ 20,000

Sale of stock, settlement term

60 days

November 2023 Private Placement, Tranche Three

Preferred stock

Sale of stock, number of shares issued in transaction (in shares)

49,570.163

Sale common stock | $

$ 20,000

Sale of stock, settlement term

90 days

Subsequent Event | November 2023 Private Placement

Preferred stock

Sale common stock | $

$ 40,000

Subsequent Event | November 2023 Private Placement, Tranche Two

Preferred stock

Sale of stock, number of shares issued in transaction (in shares)

49,570.163

Subsequent Event | November 2023 Private Placement, Tranche Three

Preferred stock

Sale of stock, number of shares issued in transaction (in shares)

49,570.163

Minimum

Preferred stock

Beneficially holders owned percentage

0%

Maximum

Preferred stock

Beneficially holders owned percentage

19.90%

Old Cartesian | Series A Preferred Stock

Preferred stock

Issuance of shares (in shares)

384,930.724

384,930.724

Series A Preferred Stock - Sche

Series A Preferred Stock - Schedule of Authorized Shares of Preferred Stock for Future Issuance (FY) (Details)	Dec. 31, 2023 shares
Class of Stock [Line Items]
Shares reserved for issuance (in shares)	113,252.625
Outstanding Series A Preferred Stock options
Class of Stock [Line Items]
Shares reserved for issuance (in shares)	14,112.299
November 2023 Private Placement
Class of Stock [Line Items]
Shares reserved for issuance (in shares)	99,140.326

Equity - Narrative (FY) (Detail

Equity - Narrative (FY) (Details)										1 Months Ended	6 Months Ended	12 Months Ended
	Dec. 05, 2023 shares	Nov. 13, 2023 $ / shares shares	Dec. 20, 2022 USD ($)	Apr. 06, 2022 USD ($) $ / shares shares	Oct. 25, 2021 USD ($)	Aug. 06, 2020 USD ($)	Jul. 28, 2020 USD ($)	Jun. 11, 2020 USD ($) $ / shares shares	Dec. 18, 2019 USD ($) $ / shares shares	Jun. 30, 2017 $ / shares shares	Jun. 30, 2024 $ / shares shares	Dec. 31, 2023 USD ($) vote $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares	Mar. 27, 2024
Subsidiary, Sale of Stock [Line Items]
Exercise price (in dollars per share) | $ / shares											$ 46.12	$ 45.98	$ 1.53	$ 1.62
Class of warrant or right, outstanding (in shares)											975,132	1,040,813	31,228,279	10,735,980
Common stock, shares authorized (in shares)											350,000,000	350,000,000	350,000,000
Common stock, par value (in dollars per share) | $ / shares		$ 0.0001									$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares issued (in shares)											17,816,238	5,397,597
Common stock, shares outstanding (in shares)											17,816,238	5,397,597
Number of votes per share that common stockholders are entitled to | vote												1
Dividends declared or paid on common stock | $												$ 0
Preferred stock, convertible, conversion ratio												1,000			33.333
Previously Reported [Member]
Subsidiary, Sale of Stock [Line Items]
Exercise price (in dollars per share) | $ / shares												$ 1.53
Class of warrant or right, outstanding (in shares)												31,224,703
Common stock, shares issued (in shares)												161,927,821	153,042,435
Common stock, shares outstanding (in shares)												161,927,821	153,042,435
2019 Warrants
Subsidiary, Sale of Stock [Line Items]
Warrant issued | $			$ 800,000
Warrants to purchase common stock
Subsidiary, Sale of Stock [Line Items]
Class of warrant or right, outstanding (in shares)											966,393	966,393	28,991,953	10,443,511
Warrants to purchase common stock | Previously Reported [Member]
Subsidiary, Sale of Stock [Line Items]
Class of warrant or right, outstanding (in shares)												28,991,953
Underwriting Agreement
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)				27,428,572
Sale common stock | $				$ 36,900,000
Underwriting Agreement - 2022 Warrants
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)				20,571,429
Class of warrant or right, number of securities called by warrants or rights (in shares)				0.75
Share price (in dollars per share) | $ / shares				$ 1.41
Exercise price (in dollars per share) | $ / shares				$ 1.55
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)											0	0	774,544
Sale common stock | $					$ 51,000,000	$ 50,000,000
Net proceeds from issuance of common stock | $													$ 2,100,000
SOBI Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)								5,416,390
Share price (in dollars per share) | $ / shares								$ 4.6156
Sale common stock | $								$ 25,000,000
Sale of stock, percentage of ten-day volume weighted average price of common stock								120%
Redemption premium | $							$ 14,500,000
December 2019 Financing
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)									37,634,883
Share price (in dollars per share) | $ / shares									$ 1.46
Sale common stock | $									$ 65,600,000
Class of warrant or right, outstanding (in shares)									22,988,501
Warrant issued | $									$ 40,700,000
Issuance costs | $									$ 4,400,000
Warrant liabilities, net of current portion | $												$ 6,400,000
Warrant, increase (decrease) in equity, amount | $												$ (12,700,000)	$ (20,900,000)	$ 2,300,000
December 2019 Financing | Common Warrant
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares									$ 0.125
Exercise price (in dollars per share) | $ / shares									$ 1.46
Warrants and rights outstanding, term									5 years
December 2019 Financing | Pre-Funded Warrant
Subsidiary, Sale of Stock [Line Items]
Exercise price (in dollars per share) | $ / shares									$ 0.0001
Class of warrant or right, number of warrants exercised (in shares)									8,342,128
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)										2,750,000
Share price (in dollars per share) | $ / shares										$ 16
Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares										$ 17.71
Issuance of common stock, net of issuance costs (in shares)										338,791
Warrants to purchase common stock
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)										79,130
Exercise price (in dollars per share) | $ / shares										$ 17.71
Warrants and rights outstanding, term										5 years
Old Cartesian | Common stock
Subsidiary, Sale of Stock [Line Items]
Issuance of shares (in shares)	6,723,639	224,099
Old Cartesian | Common stock | Previously Reported [Member]
Subsidiary, Sale of Stock [Line Items]
Issuance of shares (in shares)		6,723,639

Equity - Warranty Activity (FY)

Equity - Warranty Activity (FY) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	31,228,279	10,735,980
Issuances (in shares)		20,571,429
Canceled (in shares)	(3,576)	(79,130)
Reclassification of warrant liability to equity on modification (in shares)		0
Class of warrant or right, outstanding, ending balance (in shares)	1,040,813	31,228,279
Class Of Warrant Or Right Weighted Average Exercise Price [Roll Forward]
Exercise price, beginning balance (in dollars per share)	$ 1.53	$ 1.62
Issuance (in dollars per share)		1.55
Canceled (in dollars per share)	16.77	17.71
Reclassification of warrant liability to equity on modification (in dollars per share)		1.46
Exercise price, ending balance (in dollars per share)	$ 45.98	$ 1.53
Previously Reported [Member]
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, ending balance (in shares)	31,224,703
Class Of Warrant Or Right Weighted Average Exercise Price [Roll Forward]
Exercise price, ending balance (in dollars per share)	$ 1.53
Warrant
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	28,991,953	10,443,511
Issuances (in shares)		20,571,429
Canceled (in shares)	0	0
Reclassification of warrant liability to equity on modification (in shares)		(2,022,987)
Class of warrant or right, outstanding, ending balance (in shares)	966,393	28,991,953
Warrant | Previously Reported [Member]
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, ending balance (in shares)	28,991,953
Warrant
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, beginning balance (in shares)	2,236,326	292,469
Issuances (in shares)		0
Canceled (in shares)	(3,576)	(79,130)
Reclassification of warrant liability to equity on modification (in shares)		2,022,987
Class of warrant or right, outstanding, ending balance (in shares)	74,420	2,236,326
Warrant | Previously Reported [Member]
Class Of Warrant Or Right [Roll Forward]
Class of warrant or right, outstanding, ending balance (in shares)	2,232,750

Equity - Schedule of Authoriz_2

Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (FY) (Details) - shares	Jun. 30, 2024	Dec. 31, 2023
Class of Stock [Line Items]
Total (in shares)	12,794,858	90,367,632
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	3,930,990	35,836,268
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	1,889,561	23,306,661
Warrant
Class of Stock [Line Items]
Total (in shares)	975,132	31,224,703

Stock Incentive Plans - Narra_2

Stock Incentive Plans - Narrative (FY) (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended				3 Months Ended		6 Months Ended		12 Months Ended
	Nov. 13, 2023	Jun. 30, 2024	Jan. 31, 2024	Jan. 31, 2023	Jan. 31, 2022	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		12,794,858				12,794,858		12,794,858		90,367,632
Stock based compensation expense						$ 1,591	$ 3,783	$ 3,022	$ 6,059	$ 25,778	$ 11,194	$ 7,720
Share based compensation arrangement by share based payment award, settlement of awards, amount (in dollars per share)	$ 2.06
Share-based payment arrangement, accelerated cost	$ 13,100
Total cash payment made to the holders of stock options and restricted stock units	9,400
Adjustments to additional paid in capital, settlement of outstanding equity awards at merger										6,169
Fair value was recognized as additional stock-based compensation expense in an amount										3,200
Weighted average grant date fair value of stock options (in dollars per share)						$ 17.4	$ 30.51	$ 16.05	$ 26.89
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense										12,985	5,061	3,204
Share-based payment arrangement, accelerated cost	5,900
Fair value was recognized as additional stock-based compensation expense in an amount										1,500
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock based compensation expense						$ 815	$ 1,106	$ 1,534	$ 2,190	12,793	$ 6,133	$ 4,516
Share-based payment arrangement, accelerated cost	7,200
Fair value was recognized as additional stock-based compensation expense in an amount										$ 1,700
Shares available for future stock incentive awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		3,930,990				3,930,990		3,930,990		35,836,268
Weighted average grant date fair value of stock options (in dollars per share)										$ 0.9	$ 1.99	$ 2.73
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term								4 years
Weighted average period for recognition								3 years 4 months 24 days
Grants in period (in shares)								471,104		1,054,600
Grants in period (in dollars per share)								$ 19.8		$ 1.13
Estimated forfeitures rate								10%		10%
Unrecognized compensation expense		$ 6,100				$ 6,100		$ 6,100		$ 0
Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)		1,889,561				1,889,561		1,889,561		23,306,661
Common stock | Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested employee stock options		$ 9,900				$ 9,900		$ 9,900		$ 1,400
Weighted average period for recognition								3 years		2 years 4 months 24 days
Series A Preferred Stock | Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested employee stock options										$ 1,000
Weighted average period for recognition										2 years 6 months
2016 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)															40,341
Number of shares authorized, increase (in shares)		3,466,544	215,903	6,121,697	4,944,919
Common stock authorized and reserved for future issuance (in shares)		3,496,828				3,496,828		3,496,828		22,504,503
2016 Plan | Previously Reported [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)															1,210,256
2016 Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting term										4 years
Grants in period (in shares)										1,054,600
Grants in period (in dollars per share)										$ 1.13
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)		364,660				364,660		364,660		4,500,858
Common stock authorized and reserved for future issuance (in shares)		360,000				360,000		360,000		60,833			66,667	39,166
Employment Inducement Incentive Award Plan | Previously Reported [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)										1,825,000			2,000,000	1,175,000
Old Cartesian Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)		23,707				23,707		23,707		3,848,809
Award vesting term								4 years		4 years
Award expiration term								10 years		10 years
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)		0				0		0		14,112.299
Unrecognized compensation expense related to unvested employee stock options	$ 2,600
Stock based compensation expense						$ 300		$ 700		$ 200
Old Cartesian Plan | Common stock | Old Cartestian
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)	776,865
Old Cartesian Plan | Common stock | Old Cartestian | Previously Reported [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)	23,306,661
Old Cartesian Plan | Series A Preferred Stock | Old Cartestian
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)	14,112.299

Stock Incentive Plans - Sched_3

Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (FY) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,591	$ 3,783	$ 3,022	$ 6,059	$ 25,778	$ 11,194	$ 7,720
Ginkgo Bioworks Holdings, Inc
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense			1,500		1,500	1,000
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense					12,985	5,061	3,204
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 815	$ 1,106	$ 1,534	$ 2,190	$ 12,793	$ 6,133	$ 4,516

Stock Incentive Plans - Sched_4

Stock Incentive Plans - Schedule of Assumptions (FY) (Details) - Options - $ / shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	4.53%	3.38%	4.04%	3.95%	3.95%	2.24%	0.79%
Dividend yield	0%	0%	0%	0%	0%	0%	0%
Expected term	6 years 2 months 4 days	6 years	6 years 2 months 12 days	5 years 11 months 8 days	5 years 11 months 8 days	6 years 7 days	6 years 10 days
Expected volatility	93.57%	94.40%	95.09%	94.64%	94.64%	92.21%	95.04%
Weighted-average fair value of common stock (in dollars per share)	$ 22.08	$ 39.3	$ 20.14	$ 34.5	$ 1.15	$ 2.63	$ 3.58
Common stock | Old Cartestian
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate					4.83%
Dividend yield					0%
Expected term					3 years 7 months 2 days
Expected volatility					83.77%
Weighted-average fair value of common stock (in dollars per share)					$ 0.4
Series A Preferred Stock | Old Cartestian
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate					4.92%
Dividend yield					0%
Expected term					3 years 3 months 14 days
Expected volatility					83.87%
Weighted-average fair value of common stock (in dollars per share)					$ 403.47

Stock Incentive Plans - Summa_2

Stock Incentive Plans - Summary of Options Activity (FY) (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan | Employees
Number of Options
Beginning balance (in shares)	23,306,661	15,578,412
Granted (in shares)		5,477,200
Assumed in connection with Merger (in shares)		23,306,661
Exercised (in shares)		0
Forfeited (in shares)		(2,215,020)
Cancelled/settled in connection with the Merger (in shares)		(18,840,592)
Ending balance (in shares)		23,306,661	15,578,412
Vested (in shares)		18,067,999
Vested and expected to vest (in shares)		23,306,661
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 0.1	$ 3.44
Granted (in dollars per share)		1.15
Assumed in connection with Merger (in dollars per share)		0.1
Forfeited (in dollars per share)		2.68
Cancelled/settled in connection with the Merger (in dollars per share)		2.86
Ending balance (in dollars per share)		0.1	$ 3.44
Vested (in dollars per share)		0.1
Vested and expected to vest (in dollars per share)		$ 0.1
Weighted-average remaining contractual term
Outstanding, term		6 years 6 months	7 years 6 months 25 days
Vested, term		6 years 1 month 17 days
Vested and expected to vest, term		6 years 6 months
Aggregate intrinsic value
Outstanding		$ 13,760	$ 4
Vested		10,725
Vested and expected to vest		$ 13,760
2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan | Non-employee consultants
Number of Options
Beginning balance (in shares)	0	266,239
Forfeited (in shares)		0
Cancelled/settled in connection with the Merger (in shares)		(266,239)
Ending balance (in shares)		0	266,239
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 0	$ 8.05
Forfeited (in dollars per share)		0
Cancelled/settled in connection with the Merger (in dollars per share)		8.05
Ending balance (in dollars per share)		$ 0	$ 8.05
Weighted-average remaining contractual term
Outstanding, term			5 years 29 days
Aggregate intrinsic value
Outstanding		$ 0
Old Cartesian Plan
Number of Options
Beginning balance (in shares)	14,112.299
Ending balance (in shares)	0	14,112.299
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 79.94
Ending balance (in dollars per share)	$ 0	$ 79.94
Weighted-average remaining contractual term
Outstanding, term	5 years 10 months 28 days
Vested, term		5 years 5 months 4 days
Aggregate intrinsic value
Outstanding		$ 8,601
Old Cartesian Plan | Employees
Number of Options
Beginning balance (in shares)	14,112.299	0
Assumed in connection with Merger (in shares)		14,112.299
Ending balance (in shares)		14,112.299	0
Vested (in shares)		10,860.441
Vested and expected to vest (in shares)		14,112.299
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 79.94	$ 0
Assumed in connection with Merger (in dollars per share)		79.94
Ending balance (in dollars per share)		79.94	$ 0
Vested (in dollars per share)		71.67
Vested and expected to vest (in dollars per share)		$ 79.94
Weighted-average remaining contractual term
Outstanding, term		5 years 10 months 28 days
Vested, term		5 years 1 month 24 days
Vested and expected to vest, term		5 years 10 months 28 days
Aggregate intrinsic value
Outstanding		$ 8,601	$ 0
Vested		6,709
Vested and expected to vest		$ 8,601

Stock Incentive Plans - Restr_2

Stock Incentive Plans - Restricted Stock Units (FY) (Details) - Restricted Stock Units (RSUs) - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested at beginning of period (in shares)	0	1,705,558
Grants in period (in shares)	471,104	1,054,600
Vested in period (in shares)		(636,418)
Forfeited in period (in shares)	(16,648)	(446,108)
Cancelled/settled in connection with the Merger (in shares)		(1,677,632)
Unvested at end of period (in shares)	454,456	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Unvested at beginning of period (in dollars per share)	$ 0	$ 2.62
Grants in period (in dollars per share)	19.8	1.13
Vested in period (in dollars per share)		2.4
Forfeited in period (in dollars per share)	19.8	1.91
Cancelled/settled in connection with the Merger (in dollars per share)		1.96
Unvested at end of period (in dollars per share)	$ 19.8	$ 0

Stock Incentive Plans - Emplo_2

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (FY) (Details) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 31, 2023	Jan. 31, 2022	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			12,794,858		12,794,858		90,367,632
Stock-based compensation expense			$ 1,591,000	$ 3,783,000	$ 3,022,000	$ 6,059,000	$ 25,778,000	$ 11,194,000	$ 7,720,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)			45,795		45,795		4,982,098			5,769
Number of shares authorized, increase (in shares)	1,530,424	1,236,229
Shares of common stock issued to employees under the ESPP (in shares)							186,044
Stock-based compensation expense			$ 0	$ 100,000	$ 0	$ 100,000	$ 100,000	$ 100,000
ESPP | Previously Reported [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)										173,076

Revenue Arrangements - Narrat_2

Revenue Arrangements - Narrative (FY) (Details)						1 Months Ended			3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 29, 2022 USD ($)	Jun. 10, 2022	Oct. 01, 2021 USD ($)	Jul. 28, 2020 USD ($)	Jun. 11, 2020 USD ($) obligation	Jan. 31, 2023 USD ($)	Oct. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Disaggregation of Revenue [Line Items]
Short-term contract liability									$ 0			$ 0		$ 2,311,000	$ 593,000
Long-term contract liability									0			0		3,538,000	0
Revenue recognized												(5,849,000)
Accounts receivable									32,039,000			32,039,000		5,870,000	6,596,000
Collaboration and license revenue									33,271,000	$ 5,249,000		39,111,000	$ 11,187,000	26,004,000	110,777,000	$ 85,077,000
Remaining performance obligation									0			0		5,800,000
SOBI Purchase Agreement
Disaggregation of Revenue [Line Items]
Revenue recognized														1,100,000
Sale common stock					$ 25,000,000
Redemption premium				$ 14,500,000
Collaborative arrangement, payment obligation	$ 10,000,000
Accounts receivable														4,600,000	5,000,000
Collaboration and license revenue														19,400,000	82,600,000
SOBI Purchase Agreement | Billed Revenues
Disaggregation of Revenue [Line Items]
Accounts receivable														3,000,000	3,200,000
Astellas Gene Therapies
Disaggregation of Revenue [Line Items]
Collaboration and license agreements upfront payment						$ 10,000,000
Development and commercial milestones plus royalties						$ 340,000,000
Reimbursement percentage, budgeted costs incurred to complete the development						25%
License and option agreement, development cost reimbursements						$ 5,500,000
License and option agreement, written notice period before cancellation						90 days
Short-term contract liability														2,300,000
Long-term contract liability														3,500,000
Contract with customer, asset, reclassified to receivable														300,000
Revenue recognized														5,500,000
Accounts receivable									100,000			100,000		300,000
Sarepta Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
License and option agreement, written notice period before cancellation								30 days
Short-term contract liability														0	500,000
Revenue recognized									0	0		0	500,000
Upfront cash payment								$ 2,000,000
Sales milestone payments											$ 3,000,000
Collaboration and license revenue														500,000	10,200,000
Option term								24 months
Extension term		9 months
Milestone receivable for certain dystrophies															2,000,000
Milestone receivable															4,000,000
Spark License Agreement
Disaggregation of Revenue [Line Items]
Revenue recognized															9,200,000
Takeda Agreement
Disaggregation of Revenue [Line Items]
License and option agreement, written notice period before cancellation			90 days
Short-term contract liability									0			0		0	100,000
Long-term contract liability															0
Contract with customer, asset, reclassified to receivable														500,000
Customer, liability, revenue recognized, including opening balance									$ 0	$ 100,000		$ 0	$ 600,000	$ 600,000	1,800,000
Upfront cash payment			$ 3,000,000				$ 3,000,000
Future additional payments, expected			$ 1,124,000,000				$ 1,124,000,000
Collaborative Arrangement
Disaggregation of Revenue [Line Items]
License and option agreement, written notice period before cancellation					180 days
Upfront cash payment					$ 75,000,000
Sales milestone payments					$ 630,000,000
Period after first commercial sale when the Company is eligible to receive royalties					10 years
Number of obligations | obligation					3
Development milestone					$ 5,000,000
AskBio License Agreement
Disaggregation of Revenue [Line Items]
Revenue recognized															$ 7,000,000

Revenue Arrangements - Schedu_2

Revenue Arrangements - Schedule of Changes in Contract Liabilities (FY) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Contract liabilities:
Deferred revenue, beginning of period	$ 5,849	$ 593
Deferred revenue, additions	0	10,500
Deferred revenue, deductions	5,849	(5,244)
Deferred revenue, end of period	0	5,849
Contract liabilities, beginning balance	5,849	593
Additions	0	10,500
Deductions		(5,244)
Contract liabilities, ending balance	$ 0	$ 5,849

Related-Party Transactions - _2

Related-Party Transactions - Narrative (FY) (Details) - USD ($) $ in Thousands			12 Months Ended
	Dec. 13, 2023	Nov. 13, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Securities Purchase Agreement
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		149,330.115
Total aggregate purchase price	$ 14,800	$ 60,250
Related Party
Related Party Transaction [Line Items]
Consulting services expenses			$ 0	$ 0	$ 100
Related Party | Securities Purchase Agreement
Related Party Transaction [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		149,330.115
Total aggregate purchase price		$ 60,250

Related-party Transactions - _3

Related-party Transactions - Number of Shares of Common Stock Purchased (FY) (Details) - USD ($)			9 Months Ended	12 Months Ended
	Nov. 15, 2023	Nov. 13, 2023	Dec. 31, 2022	Dec. 31, 2023
November 2023 Private Placement
Related Party Transaction [Line Items]
Shares issued (in shares)	619.627	149,330.115
Total aggregate purchase price	$ 250,000
Timothy A. Springer, Ph.D. | Related Party | November 2023 Private Placement
Related Party Transaction [Line Items]
Shares issued (in shares)		99,140.326		24,785.081
Total aggregate purchase price		$ 40,000,000		$ 10,000,000
TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) | Related Party | November 2023 Private Placement
Related Party Transaction [Line Items]
Shares issued (in shares)				24,785.081
Total aggregate purchase price				$ 10,000,000
TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) | Related Party | 2022 Offering
Related Party Transaction [Line Items]
Total aggregate purchase price			$ 9,421,056
TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) | Common stock | Related Party | 2022 Offering
Related Party Transaction [Line Items]
Shares issued (in shares)			6,681,600
TAS Partners LLC (affiliate of Timothy A. Springer, Ph.D.) | Warrants to purchase common stock | Related Party | 2022 Offering
Related Party Transaction [Line Items]
Shares issued (in shares)			5,011,200
Seven One Eight Three Four Irrevocable Trust (affiliate of Murat Kalayoglu, MD, Ph.D.) | Related Party | November 2023 Private Placement
Related Party Transaction [Line Items]
Shares issued (in shares)				619.627
Total aggregate purchase price				$ 250,000

Collaboration and License Ag_10

Collaboration and License Agreements - Biogen M.A., Inc. (FY) (Details)	Sep. 08, 2023	Sep. 16, 2019
Biogen M.A., Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Written notice for agreement termination	30 days	30 days

Collaboration and License Ag_11

Collaboration and License Agreements - National Cancer Institute of the National Institutes of Health (FY) (Details) - NCI Agreement $ in Millions	Sep. 16, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
License and option agreement, royalty payment	$ 0.1
License and option agreement, benchmark royalties, maximum	$ 0.8
Written notice for agreement termination	60 days

Collaboration and License Ag_12

Collaboration and License Agreements - Ginkgo Agreement (FY) (Details) - Ginkgo Agreement - USD ($) $ in Millions					12 Months Ended
	Jul. 19, 2023	Jun. 13, 2022	Jan. 03, 2022	Oct. 25, 2021	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Clinical and commercial milestone payment, expected			$ 207	$ 85
Milestone payment	$ 1	$ 0.5			$ 1
Collaborative arrangement, number of shares issued for milestone achievement (in shares)	1,339,285	29,761			44,642
Collaborative arrangement, shares issued for milestone achievement, amount					$ 1.5	$ 1
Previously Reported [Member]
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaborative arrangement, number of shares issued for milestone achievement (in shares)		892,857

Collaboration and License Ag_13

Collaboration and License Agreements - Genovis (FY) (Details) $ in Millions	1 Months Ended
	Feb. 28, 2023 USD ($)
Genovis AB
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 4

Collaboration and License Ag_14

Collaboration and License Agreements - Cyrus Biotechnology, Inc. (FY) (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended
	Sep. 07, 2021	Sep. 30, 2021	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2022
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Equity securities without readily determinable fair value, amount			$ 2,000	$ 2,000	$ 2,000
Cyrus Biotechnology, Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment	$ 1,500,000	$ 1,500,000
Equity securities, stock purchase agreement, shares purchased (in shares)	2,326,934
Equity securities without readily determinable fair value, amount	$ 2,000		$ 2,000	$ 2,000	$ 2,000
Cyrus Biotechnology, Inc. | Series B Preferred Stock Purchase Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Equity securities, stock purchase agreement, par value per share (in dollars per share)	$ 0.0001
Equity securities, stock purchase agreement, purchase price per share (in dollars per share)	$ 0.8595

Collaboration and License Ag_15

Collaboration and License Agreements - AskBio (FY) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration expense	$ 19,688	$ 23,887	$ 38,876	$ 48,206	$ 112,420	$ 96,239	$ 89,674
AskBio License
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration expense	$ 0	$ 100	$ 0	$ 100	$ 100	$ 900
Cost share percentage	50%	50%	50%	50%	50%	50%

Collaboration and License Ag_16

Collaboration and License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (FY) (Details) - 3SBio License - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate amount of upfront and milestone-based payments	$ 7	$ 7
Aggregate amount for future payments upon achievement of clinical and regulatory approval milestones for products containing ImmTOR platform	$ 15	$ 15

Income Taxes - Narrative (FY) (

Income Taxes - Narrative (FY) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Examination [Line Items]
Income tax expense (benefit)	$ (19)	$ (0.6)	$ 16
Penalties and interest			$ 1.3
Valuation allowance, deferred tax asset, increase (decrease), amount	(3.5)	(1.5)
Capitalized research and experimental costs, tax purposes	44.7	$ 29.3
Domestic Tax Authority
Income Tax Examination [Line Items]
Operating loss carryforwards	108.7
Tax credit carryforward, amount	4.9
State and Local Jurisdiction
Income Tax Examination [Line Items]
Operating loss carryforwards	110.3
Tax credit carryforward, amount	$ 0.9

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Income Taxes (FY) (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Statutory U.S. federal rate	21%	21%	21%
State income taxes - net of federal benefit	2.30%	1.60%	(166.00%)
Permanent items	(1.60%)	(18.60%)	8.30%
Research tax credits	0.60%	(3.20%)	55%
Deferred revenue	0%	0%	156.50%
Other	0%	0%	(3.70%)
Change in fair value of forward contract liabilities	(13.20%)	0%	0%
Valuation allowance, net	2.80%	(4.40%)	(230.10%)
Stock-based compensation	(3.90%)	1.80%	(5.20%)
Effective income tax rate	8%	(1.80%)	(164.20%)

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets and Deferred Tax Liabilities (FY) (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred Tax Assets
Net operating loss carryforwards	$ 29,841	$ 17,015
Research and development credits	5,649	2,806
Stock-based compensation expense	8	5,892
Other expenses	705	1,697
Deferred revenue	84,626	83,417
Operating lease liabilities	2,718	3,186
R&E Capitalization	19,778	9,588
Patent and license costs	9,140	7,472
Gross deferred tax assets	152,465	131,073
Deferred Tax Liabilities
Intangible assets	(41,144)	0
Depreciation	(128)	(81)
Operating lease right-of-use assets	(2,751)	(3,174)
Gross deferred tax liabilities	(44,023)	(3,255)
Net deferred tax assets before valuation allowance	108,442	127,818
Valuation allowance	(124,295)	(127,818)
Net deferred tax assets/(liabilities)	$ (15,853)	$ 0

Defined Contribution Plan (FY)

Defined Contribution Plan (FY) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Retirement Benefits [Abstract]
Vesting period			2 years		2 years
Employer contribution made	$ 0.1	$ 0.1	$ 0.1	$ 0.2	$ 0.3	$ 0.3	$ 0.2

Restructuring - Narrative (FY)

Restructuring - Narrative (FY) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended	15 Months Ended
	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2024
Restructuring Cost and Reserve [Line Items]
Employee workforce, termination percentage		79%	89%
Restructuring charges	$ 805	$ 6,431
Cash payments for employee related restructuring charges	4,527	2,535
Research and development
Restructuring Cost and Reserve [Line Items]
Restructuring charges	200	5,600
General and administrative
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 600	$ 800

Restructuring - Schedule of C_2

Restructuring - Schedule of Change in Accrued Restructuring Balance (FY) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Restructuring Reserve [Roll Forward]
Beginning Balance	$ 3,896	$ 0
Charges	805	6,431
Payments	4,527	2,535
Ending Balance	$ 174	$ 3,896

Subsequent Events (FY) (Details

Subsequent Events (FY) (Details) - Frederick lease agreement - Subsequent Event $ in Millions	Feb. 28, 2024 USD ($) ft² extension_option
Subsequent Event [Line Items]
Area of office space leased | ft²	19,199
Lease term	7 years
Number of options to extend lease | extension_option	1
Lease renewal term	5 years
Lease agreement, monthly base rent | $	$ 0.1