Cartesian Therapeutics (RNAC) 10-Q 8 May 24 2024 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2024	May 03, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2024
Document Transition Report	false
Entity File Number	001-37798
Entity Registrant Name	Cartesian Therapeutics, Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1622110
Entity Address, Address Line One	704 Quince Orchard Road
Entity Address, City or Town	Gaithersburg
Entity Address, State or Province	MD
Entity Address, Postal Zip Code	20878
City Area Code	617
Local Phone Number	923-1400
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Trading Symbol	RNAC
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Smaller Reporting Company	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		17,796,053
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001453687

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Current assets:
Cash and cash equivalents	$ 103,418	$ 76,911
Accounts receivable	2,006	5,870
Unbilled receivables	2,370	2,981
Prepaid expenses and other current assets	3,315	4,967
Total current assets	111,109	90,729
Non-current assets:
Property and equipment, net	2,402	2,113
Right-of-use asset, net	9,556	10,068
In-process research and development assets	150,600	150,600
Goodwill	48,163	48,163
Long-term restricted cash	1,377	1,377
Investments	2,000	2,000
Total assets	325,207	305,050
Current liabilities:
Accounts payable	2,517	3,150
Accrued expenses and other current liabilities	9,516	15,572
Lease liability	2,229	2,166
Deferred revenue	412	2,311
Warrant liabilities	597	720
Contingent value right liability	21,383	15,983
Forward contract liabilities	0	28,307
Total current liabilities	36,654	68,209
Non-current liabilities:
Lease liability, net of current portion	8,228	8,789
Deferred revenue, net of current portion	0	3,538
Warrant liabilities, net of current portion	4,755	5,674
Contingent value right liability, net of current portion	376,517	342,617
Deferred tax liabilities, net	15,853	15,853
Total liabilities	442,007	444,680
Commitments and contingencies (Note 18)
Series A Preferred Stock, $0.0001 par value; no and $548,375 shares authorized as of March 31, 2024 and December 31, 2023, respectively; no and $435,120.513 shares issued and outstanding as of March 31, 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 March 31, 2024 and December 31, 2023; 5,515,836 and 5,397,597 shares issued and outstanding as of March 31, 2024 and December 31, 2023, respectively	1	1
Additional paid-in capital	559,275	179,062
Accumulated deficit	(671,471)	(614,647)
Accumulated other comprehensive loss	(4,605)	(4,600)
Total stockholders’ deficit	(116,800)	(440,184)
Total liabilities, convertible preferred stock, and stockholders’ deficit	325,207	305,050
Series A Preferred Stock
Stockholders’ deficit:
Preferred stock, value, issued	$ 0	$ 0

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2024	Dec. 31, 2023
Preferred stock, 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)	9,451,625	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)	5,515,836	5,397,597
Common stock, shares outstanding (in shares)	5,515,836	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)	548,375	0
Preferred stock, shares issued (in shares)	534,260.839	0
Preferred stock, shares outstanding (in shares)	534,260.839	0

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Collaboration and license revenue	$ 5,840	$ 5,938
Operating expenses:
Research and development	9,738	18,624
General and administrative	9,450	5,695
Total operating expenses	19,188	24,319
Operating loss	(13,348)	(18,381)
Investment income	1,164	1,331
Foreign currency transaction, net	0	19
Interest expense	0	(808)
Change in fair value of warrant liabilities	1,042	(4,079)
Change in fair value of contingent value right liability	(39,300)	0
Change in fair value of forward contract liabilities	(6,890)	0
Other income, net	508	255
Net loss	(56,824)	(21,663)
Other comprehensive (loss) income:
Foreign currency translation adjustment	(5)	(22)
Unrealized gain (loss) on marketable securities	0	11
Total comprehensive loss	$ (56,829)	$ (21,674)
Net loss per share:
Basic (in dollars per share)	$ (10.50)	$ (4.24)
Diluted (in dollars per share)	$ (10.50)	$ (4.24)
Weighted-average common shares outstanding:
Basic (in shares)	5,414,020	5,111,518
Diluted (in shares)	5,414,020	5,111,518

Consolidated Statements of Chan

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders’ Equity (Deficit) - 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, 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)			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	(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, 2023	435,120.513
Beginning balance at Dec. 31, 2023	$ 296,851
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series A Preferred Stock 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
Beginning balance at Dec. 31, 2023	3,703
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	5,397,597
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 (in shares)			65,681
Issuance of common stock upon exercise of warrants	2,877			2,877
Stock-based compensation expense	1,431			1,431
Currency translation adjustment	(5)					(5)
Unrealized gain on marketable securities	0
Net loss	$ (56,824)				(56,824)
Ending balance (in shares) at Mar. 31, 2024	5,515,836	534,260.839	5,515,836
Ending balance at Mar. 31, 2024	$ (116,800)	$ 0	$ 1	$ 559,275	$ (671,471)	$ (4,605)

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders’ Equity (Deficit) (Parenthetical)	Apr. 04, 2024
Subsequent Event
Reverse stock split conversion ratio	0.0333

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Cash flows from operating activities
Net loss	$ (56,824)	$ (21,663)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	183	206
Amortization of premiums and discounts on marketable securities	0	(79)
Non-cash lease expense	512	416
Loss on disposal of property and equipment	2	0
Stock-based compensation expense	1,431	2,276
Non-cash interest expense	0	398
Warrant liabilities revaluation	(1,042)	4,079
Contingent value right liability revaluation	39,300	0
Forward contract liabilities revaluation	6,890	0
Changes in operating assets and liabilities:
Accounts receivable	3,864	(243)
Unbilled receivable	611	1,319
Prepaid expenses, deposits and other assets	1,652	(20)
Accounts payable	(633)	747
Deferred revenue	(5,437)	9,158
Accrued expenses and other liabilities	(6,426)	(5,359)
Net cash used in operating activities	(15,917)	(8,765)
Cash flows from investing activities
Proceeds from maturities of marketable securities	0	28,254
Purchases of property and equipment	(602)	(130)
Net cash (used in) provided by investing activities	(602)	28,124
Cash flows from financing activities
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	154	0
Proceeds from issuance of common stock under Employee Stock Purchase Plan	0	149
Net cash provided by financing activities	43,031	149
Effect of exchange rate changes on cash	(5)	(21)
Net change in cash, cash equivalents, and restricted cash	26,507	19,487
Cash, cash equivalents, and restricted cash at beginning of period	78,288	108,038
Cash, cash equivalents, and restricted cash at end of period	104,795	127,525
Supplemental cash flow information
Cash paid for interest	0	625
Noncash investing and financing activities
Purchase of property and equipment not yet paid	$ 0	$ 48

Description of the Business

Description of the Business	3 Months Ended
	Mar. 31, 2024
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 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. 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 months ended March 31, 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 March 31, 2024, the consolidated results of operations for the three months ended March 31, 2024, and cash flows for the three months ended March 31, 2024. Such adjustments are of a normal and recurring nature. The results of operations for the three months ended March 31, 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 March 31, 2024, the Company’s cash, cash equivalents, and restricted cash were $104.8 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 March 31, 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 March 31, 2024, the Company had an accumulated deficit of $671.5 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 March 31, 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.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2024
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 three months ended March 31, 2024. 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.

Merger

Merger	3 Months Ended
	Mar. 31, 2024
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 the 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): Assets acquired: As of November 13, 2023 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 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.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share The following table sets forth the computation of basic and diluted net loss per share for the three months ended March 31, 2024 and 2023 (in thousands, except share and per-share data): Three Months Ended 2024 2023 Numerator: Net loss allocable to shares of common stock - basic and diluted $ (56,824) $ (21,663) Denominator: Weighted-average common shares outstanding - basic and diluted 5,414,020 5,111,518 Net loss per share: Basic and Diluted $ (10.50) $ (4.24) The following table represents the potential dilutive shares of common stock excluded from the computation of the diluted net loss per share for all periods presented, as the effect would have been anti-dilutive: Three Months Ended 2024 2023 Common stock options, restricted stock units and ESPP shares 1,811,636 773,488 Warrants to purchase common stock 975,132 1,040,942 Series A Preferred Stock 17,808,670 — Series A Preferred Stock options 470,403 — Total 21,065,841 1,814,430

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023 (in thousands): March 31, 2024 Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash equivalents) $ 38,672 $ 38,672 $ — $ — Total assets $ 38,672 $ 38,672 $ — $ — Liabilities: Warrant liabilities $ 5,352 $ — $ — $ 5,352 Contingent value right liability $ 397,900 $ — $ — $ 397,900 Total liabilities $ 403,252 $ — $ — $ 403,252 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 three months ended March 31, 2024 or year ended December 31, 2023. Cash, Cash Equivalents, and Restricted Cash As of March 31, 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 March 31, 2024, the Company had a restricted cash balance relating to a secured letter of credit in connection with its lease for the Company’s prior headquarters. 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 March 31, 2024 and 2023 (in thousands): March 31, 2024 2023 Cash and cash equivalents $ 103,418 $ 125,925 Short-term restricted cash — 289 Long-term restricted cash 1,377 1,311 Total cash, cash equivalents, and restricted cash $ 104,795 $ 127,525 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: March 31, December 31, 2024 2023 Risk-free interest rate 5.38 % 4.79 % Dividend yield — — Expected life (in years) 0.73 0.98 Expected volatility 96.03 % 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: March 31, December 31, 2024 2023 Risk-free interest rate 4.40 % 4.01 % Dividend yield — — Expected life (in years) 3.03 3.28 Expected volatility 82.72 % 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 three months ended March 31, 2024 (in thousands): Warrant liabilities Fair value as of December 31, 2023 $ 6,394 Change in fair value (1,042) Fair value as of March 31, 2024 $ 5,352 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. The CVRs represent financial instruments that are accounted for under the fair value option election in ASC 825 , Financial Instruments . 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 March 31, 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: March 31, 2024 Estimated cash flow dates 2024-2038 Estimated probability of success 95.0 % Expected volatility of future revenues 25.0 % 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 three months ended March 31, 2024 (in thousands): CVR liability Fair value as of December 31, 2023 $ 358,600 Change in fair value 39,300 Fair value as of March 31, 2024 $ 397,900 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 March 31, 2024 $ —

Property and Equipment

Property and Equipment	3 Months Ended
	Mar. 31, 2024
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consists of the following (in thousands): March 31, December 31, 2024 2023 Laboratory equipment $ 6,662 $ 6,280 Computer equipment and software 620 702 Leasehold improvements 61 61 Furniture and fixtures 452 452 Office equipment 196 196 Construction in process 241 150 Total property and equipment 8,232 7,841 Less: Accumulated depreciation (5,830) (5,728) Property and equipment, net $ 2,402 $ 2,113 Depreciation expense was $0.2 million for each of the three months ended March 31, 2024 and 2023.

Accrued Expenses

Accrued Expenses	3 Months Ended
	Mar. 31, 2024
Payables and Accruals [Abstract]
Accrued Expenses	Accrued Expenses Accrued expenses consist of the following (in thousands): March 31, December 31, 2024 2023 Payroll and employee related expenses $ 1,701 $ 4,390 Accrued patent fees 1,177 472 Accrued external research and development costs 2,614 4,896 Accrued professional and consulting services 3,055 4,331 Other 380 644 Accrued expenses $ 8,927 $ 14,733

Leases

Leases	3 Months Ended
	Mar. 31, 2024
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 March 31, 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 March 31, 2024 and December 31, 2023, deferred rent of $0.6 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 Lease 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 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, which had not occurred as of March 31, 2024. Upon the Commencement Date, which was determined to be May 1, 2024, the Company will assess the classification of the lease and measure the right-of-use asset and lease liability. The Frederick Lease Agreement will terminate approximately seven 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. The base rent for the initial term is $0.1 million per month. The Company paid first month’s rent of $0.1 million upon execution of the Frederick Lease Agreement and paid a cash security deposit of $0.3 million, both of which are classified as prepaid expenses and other current assets on the consolidated balance sheet. For the three months ended March 31, 2024 and 2023, the components of lease costs were as follows (in thousands): Three Months Ended 2024 2023 Operating lease cost $ 775 $ 696 Variable lease cost 397 142 Short-term lease cost 3 3 Less: Sublease income (510) (255) Total lease cost $ 665 $ 586 The maturity of the Company’s operating lease liabilities as of March 31, 2024 were as follows (in thousands): March 31, 2024 2024 (remainder) $ 2,316 2025 3,164 2026 3,248 2027 3,017 2028 946 Thereafter — Total future minimum lease payments 12,691 Less: Imputed interest 2,234 Total operating lease liabilities $ 10,457 The supplemental disclosure for the statement of cash flows related to operating leases was as follows (in thousands): Three Months Ended 2024 2023 Cash paid for amounts included in the measurement of lease liabilities: $ 761 $ 653 The changes in the Company’s right-of-use assets and lease liabilities for the three months ended March 31, 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: March 31, 2024 2023 Weighted-average remaining lease term 4.0 years 5.1 years Weighted-average discount rate 9.9 % 9.7 %

Debt

Debt	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023, the Company had no outstanding borrowings.

Series A Preferred Stock

Series A Preferred Stock	3 Months Ended
	Mar. 31, 2024
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, will occur eight business days following stockholder approval of the Conversion Proposal. Upon such date, each share of Series A Preferred Stock will automatically convert 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 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. 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. As of March 31, 2024, the Company had 534,260.839 shares of Series A Preferred Stock issued and outstanding. 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; 166,341.592 shares of Series A Preferred Stock did not automatically convert due to beneficial ownership limitations. The 166,341.592 shares of Series A Preferred Stock that did not automatically convert are convertible into 5,544,719 shares of common stock. Based on the 17,779,787 shares of common stock outstanding on April 8, 2024, there would be 23,324,506 shares of common stock outstanding if all shares of Series A Preferred Stock converted into common stock on such date.

Equity

Equity	3 Months Ended
	Mar. 31, 2024
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 three months ended March 31, 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 three months ended March 31, 2024: Number of Warrants Equity Liability classified Total Weighted-average Outstanding at December 31, 2023 74,420 966,393 1,040,813 $ 45.98 Exercises (65,681) — (65,681) 43.80 Outstanding at March 31, 2024 8,739 966,393 975,132 $ 46.12 See Note 14 for further discussion on the exercise of the 65,681 warrants. 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. Reserved Shares The Company has authorized shares of common stock for future issuance as of March 31, 2024 as follows: Exercise of warrants 975,132 Shares available for future stock incentive awards 202,875 Unvested restricted stock units 464,018 Outstanding common stock options 1,347,618 Series A Preferred Stock 17,808,670 Outstanding Series A Preferred Stock options 470,403 Total 21,268,716

Stock Incentive Plans

Stock Incentive Plans	3 Months Ended
	Mar. 31, 2024
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. As of March 31, 2024, 19,446 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. As of March 31, 2024, there are 113,927 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 March 31, 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 $0.4 million was recognized as research and development expense in the consolidated statements of operations and comprehensive income (loss) during the three months ended March 31, 2024. 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) was as follows (in thousands): Three Months Ended 2024 2023 Research and development $ 712 $ 1,192 General and administrative 719 1,084 Total stock-based compensation expense $ 1,431 $ 2,276 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 2024 2023 Risk-free interest rate 3.95 % 3.95 % Dividend yield — — Expected term (in years) 6.20 5.94 Expected volatility 95.37 % 94.64 % Weighted-average fair value of common stock $ 19.78 $ 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 $15.79 and $26.87 during the three months ended March 31, 2024 and 2023, respectively. As of March 31, 2024, total unrecognized compensation expense related to unvested common stock options and Series A Preferred Stock options was $8.3 million and $0.8 million, respectively, which is expected to be recognized over a weighted average period of 3.4 years and 1.4 years, respectively. The following table summarizes the stock option activity under the 2016 Plan, the 2018 Inducement Incentive Award Plan, and Old Cartesian Plan for options for common stock: Weighted-average Number of remaining Aggregate common stock Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Outstanding at December 31, 2023 776,865 $ 2.97 6.50 $ 13,760 Granted 641,089 $ 19.78 Exercised (52,558) $ 2.93 Forfeited (17,778) $ 11.46 Outstanding at March 31, 2024 1,347,618 $ 10.86 7.88 $ 11,827 Vested at March 31, 2024 583,183 $ 2.98 5.90 $ 9,633 Vested and expected to vest at March 31, 2024 1,200,826 $ 9.77 7.72 $ 11,827 The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: Number of Weighted-average Series A remaining Aggregate Preferred Stock Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Outstanding at December 31, 2023 14,112.299 $ 79.94 5.91 $ 8,601 Outstanding at March 31, 2024 14,112.299 $ 79.94 5.65 $ 8,043 Vested at March 31, 2024 11,323.762 $ 73.14 5.05 $ 6,530 Vested and expected to vest at March 31, 2024 14,112.299 $ 79.94 5.65 $ 8,043 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. Restricted Stock Units During the three months ended March 31, 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 $6.5 million as of March 31, 2024, which is expected to be recognized over a weighted-average period of 3.7 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 Unvested at December 31, 2023 — $ — Granted 471,104 19.80 Forfeited (7,086) 19.80 Unvested at March 31, 2024 464,018 $ 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 March 31, 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.

Revenue Arrangements

Revenue Arrangements	3 Months Ended
	Mar. 31, 2024
Revenue from Contract with Customer [Abstract]
Revenue Arrangements	Revenue Arrangements 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 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. 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, effective June 6, 2024. As of March 31, 2024 and December 31, 2023, the Company recorded $0.4 million and $2.3 million as a short-term contract liability, respectively, and no and $3.5 million long-term contract liability, respectively, representing deferred revenue associated with the Astellas Agreement. As of March 31, 2024 and December 31, 2023, the Company recorded a receivable of $0.4 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 $5.8 million and $0.6 million related to the Astellas Agreement was recognized during the three months ended March 31, 2024 and 2023, respectively, 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 three months ended March 31, 2024. 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 both March 31, 2024 and December 31, 2023, the Company recorded no contract liability related to the Takeda Agreement. No revenue related to the Takeda Agreement was recognized during the three months ended March 31, 2024. Revenue of $0.5 million related to the Takeda Agreement was recognized during the three months ended March 31, 2023, all from performance obligations related to prior periods as a result of the change in transaction price. 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. As of March 31, 2024 and December 31, 2023, the Company recorded a total outstanding receivable of $0.6 million and $4.6 million, respectively, representing billings for the Phase 3 DISSOLVE program that are subject to reimbursement by Sobi. Additionally, as of March 31, 2024 and December 31, 2023, the Company recorded a total unbilled receivable of $2.4 million and $3.0 million, respectively, representing revenue earned but not yet billed for the Phase 3 DISSOLVE program. No revenue and revenue of $4.4 million related to the Sobi License was recognized during the three months ended March 31, 2024 and 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 three months ended March 31, 2023. No revenue related to the Sarepta Agreement was recognized during the three months ended March 31, 2024. 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 March 31, 2024, the aggregate amount of the transaction price allocated to remaining performance obligations was $0.4 million. Contract Balances from Contracts with Customers The following table presents changes in the Company’s contract liabilities during the three months ended March 31, 2024 (in thousands): Balance at Balance at beginning of period Additions Deductions end of period Three Months Ended March 31, 2024 Contract liabilities: Deferred revenue $ 5,849 $ — $ (5,437) $ 412 Total contract liabilities $ 5,849 $ — $ (5,437) $ 412

Related-Party Transactions

Related-Party Transactions	3 Months Ended
	Mar. 31, 2024
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 three months ended March 31, 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 months ended March 31, 2023, there were no related party transactions.

Collaboration and License Agree

Collaboration and License Agreements	3 Months Ended
	Mar. 31, 2024
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 Bioworks Holdings, Inc., or 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 March 31, 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 months ended March 31, 2024. For the three months ended March 31, 2023, the Company recognized $0.1 million 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 March 31, 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.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2024
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes As of March 31, 2024, we have not recorded any U.S. federal or state income tax benefits for either the net losses we have incurred or our earned research and orphan drug credits, due to the uncertainty of realizing a benefit from those items in the future.

Defined Contribution Plan

Defined Contribution Plan	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and 2023, respectively.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies As of March 31, 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.

Restructuring

Restructuring	3 Months Ended
	Mar. 31, 2024
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 79% reduction of the Company's existing headcount by March 31, 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 three months ended March 31, 2024, the Company recorded restructuring expenses consisting of one-time cash severance payments and other employee-related costs of $0.3 million, with $0.2 million and $0.1 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 $3.3 million were paid during the three months ended March 31, 2024. The following table summarizes the change in the Company's accrued restructuring balance (in thousands): Beginning Balance Ending Balance December 31, 2023 Charges Payments March 31, 2024 Severance liability $ 3,896 $ 292 $ (3,320) $ 868

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2024
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Reverse Stock Split 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. Series A Preferred Stock Automatic Conversion The Automatic Conversion of the Series A Preferred Stock occurred on April 8, 2024 at 5:00 p.m. Eastern Time pursuant to the terms of the Certificate of Designation, as amended. As a result, 367,919.247 shares of Series A Preferred Stock automatically converted into 12,263,951 shares of common stock; 166,341.592 shares of Series A Preferred Stock did not automatically convert due to beneficial ownership limitations. The 166,341.592 shares of Series A Preferred Stock that did not automatically convert are convertible into 5,544,719 shares of common stock. Based on the 17,779,787 shares of common stock outstanding on April 8, 2024, there would be 23,324,506 shares of common stock outstanding if all shares of Series A Preferred Stock converted into common stock on such date. 7495 New Horizon Way Lease Amendment Effective May 7, 2024, the Company and the Landlord entered into a First Amendment to the Frederick Lease Agreement, or the Amendment, 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 will be 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.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pay vs Performance Disclosure
Net loss	$ (56,824)	$ (21,663)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Mar. 31, 2024
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
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.

Merger (Tables)

Merger (Tables)	3 Months Ended
	Mar. 31, 2024
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
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): Assets acquired: As of November 13, 2023 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 Descartes-08 for MG $ 93,900 Descartes-08 for SLE 56,700 Total in-process research and development assets $ 150,600

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted Net Loss Per Share	The following table sets forth the computation of basic and diluted net loss per share for the three months ended March 31, 2024 and 2023 (in thousands, except share and per-share data): Three Months Ended 2024 2023 Numerator: Net loss allocable to shares of common stock - basic and diluted $ (56,824) $ (21,663) Denominator: Weighted-average common shares outstanding - basic and diluted 5,414,020 5,111,518 Net loss per share: Basic and Diluted $ (10.50) $ (4.24)
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 loss per share for all periods presented, as the effect would have been anti-dilutive: Three Months Ended 2024 2023 Common stock options, restricted stock units and ESPP shares 1,811,636 773,488 Warrants to purchase common stock 975,132 1,040,942 Series A Preferred Stock 17,808,670 — Series A Preferred Stock options 470,403 — Total 21,065,841 1,814,430

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 and December 31, 2023 (in thousands): March 31, 2024 Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash equivalents) $ 38,672 $ 38,672 $ — $ — Total assets $ 38,672 $ 38,672 $ — $ — Liabilities: Warrant liabilities $ 5,352 $ — $ — $ 5,352 Contingent value right liability $ 397,900 $ — $ — $ 397,900 Total liabilities $ 403,252 $ — $ — $ 403,252 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
Schedule of Cash and Cash Equivalents	The Company’s consolidated statements of cash flows include the following as of March 31, 2024 and 2023 (in thousands): March 31, 2024 2023 Cash and cash equivalents $ 103,418 $ 125,925 Short-term restricted cash — 289 Long-term restricted cash 1,377 1,311 Total cash, cash equivalents, and restricted cash $ 104,795 $ 127,525
Schedule of Restricted Cash and Cash Equivalents	The Company’s consolidated statements of cash flows include the following as of March 31, 2024 and 2023 (in thousands): March 31, 2024 2023 Cash and cash equivalents $ 103,418 $ 125,925 Short-term restricted cash — 289 Long-term restricted cash 1,377 1,311 Total cash, cash equivalents, and restricted cash $ 104,795 $ 127,525
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: March 31, December 31, 2024 2023 Risk-free interest rate 5.38 % 4.79 % Dividend yield — — Expected life (in years) 0.73 0.98 Expected volatility 96.03 % 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: March 31, December 31, 2024 2023 Risk-free interest rate 4.40 % 4.01 % Dividend yield — — Expected life (in years) 3.03 3.28 Expected volatility 82.72 % 84.09 % March 31, 2024 Estimated cash flow dates 2024-2038 Estimated probability of success 95.0 % Expected volatility of future revenues 25.0 % December 31, 2023 Estimated cash flow dates 2024 - 2038 Estimated probability of success 95.0 % Risk-adjusted discount rate 13.7 %
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 three months ended March 31, 2024 (in thousands): Warrant liabilities Fair value as of December 31, 2023 $ 6,394 Change in fair value (1,042) Fair value as of March 31, 2024 $ 5,352 The following table reflects a roll-forward of fair value for the Company's Level 3 CVR liability for the three months ended March 31, 2024 (in thousands): CVR liability Fair value as of December 31, 2023 $ 358,600 Change in fair value 39,300 Fair value as of March 31, 2024 $ 397,900 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 March 31, 2024 $ —

Property and Equipment (Tables)

Property and Equipment (Tables)	3 Months Ended
	Mar. 31, 2024
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consists of the following (in thousands): March 31, December 31, 2024 2023 Laboratory equipment $ 6,662 $ 6,280 Computer equipment and software 620 702 Leasehold improvements 61 61 Furniture and fixtures 452 452 Office equipment 196 196 Construction in process 241 150 Total property and equipment 8,232 7,841 Less: Accumulated depreciation (5,830) (5,728) Property and equipment, net $ 2,402 $ 2,113

Accrued Expenses (Tables)

Accrued Expenses (Tables)	3 Months Ended
	Mar. 31, 2024
Payables and Accruals [Abstract]
Schedule of Accrued Expenses	Accrued expenses consist of the following (in thousands): March 31, December 31, 2024 2023 Payroll and employee related expenses $ 1,701 $ 4,390 Accrued patent fees 1,177 472 Accrued external research and development costs 2,614 4,896 Accrued professional and consulting services 3,055 4,331 Other 380 644 Accrued expenses $ 8,927 $ 14,733

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2024
Leases [Abstract]
Schedule of Lease, Cost	For the three months ended March 31, 2024 and 2023, the components of lease costs were as follows (in thousands): Three Months Ended 2024 2023 Operating lease cost $ 775 $ 696 Variable lease cost 397 142 Short-term lease cost 3 3 Less: Sublease income (510) (255) Total lease cost $ 665 $ 586 The following summarizes additional information related to operating leases: March 31, 2024 2023 Weighted-average remaining lease term 4.0 years 5.1 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 March 31, 2024 were as follows (in thousands): March 31, 2024 2024 (remainder) $ 2,316 2025 3,164 2026 3,248 2027 3,017 2028 946 Thereafter — Total future minimum lease payments 12,691 Less: Imputed interest 2,234 Total operating lease liabilities $ 10,457
Schedule of Operating Lease, Lease Income	The supplemental disclosure for the statement of cash flows related to operating leases was as follows (in thousands): Three Months Ended 2024 2023 Cash paid for amounts included in the measurement of lease liabilities: $ 761 $ 653

Equity (Tables)

Equity (Tables)	3 Months Ended
	Mar. 31, 2024
Equity [Abstract]
Schedule of Stockholders' Equity Note, Warrants or Rights	The following is a summary of warrant activity for the three months ended March 31, 2024: Number of Warrants Equity Liability classified Total Weighted-average Outstanding at December 31, 2023 74,420 966,393 1,040,813 $ 45.98 Exercises (65,681) — (65,681) 43.80 Outstanding at March 31, 2024 8,739 966,393 975,132 $ 46.12
Schedule of Authorized Shares of Common Stock for Future Issuance	The Company has authorized shares of common stock for future issuance as of March 31, 2024 as follows: Exercise of warrants 975,132 Shares available for future stock incentive awards 202,875 Unvested restricted stock units 464,018 Outstanding common stock options 1,347,618 Series A Preferred Stock 17,808,670 Outstanding Series A Preferred Stock options 470,403 Total 21,268,716

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	3 Months Ended
	Mar. 31, 2024
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) was as follows (in thousands): Three Months Ended 2024 2023 Research and development $ 712 $ 1,192 General and administrative 719 1,084 Total stock-based compensation expense $ 1,431 $ 2,276
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 2024 2023 Risk-free interest rate 3.95 % 3.95 % Dividend yield — — Expected term (in years) 6.20 5.94 Expected volatility 95.37 % 94.64 % Weighted-average fair value of common stock $ 19.78 $ 34.50
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: Weighted-average Number of remaining Aggregate common stock Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Outstanding at December 31, 2023 776,865 $ 2.97 6.50 $ 13,760 Granted 641,089 $ 19.78 Exercised (52,558) $ 2.93 Forfeited (17,778) $ 11.46 Outstanding at March 31, 2024 1,347,618 $ 10.86 7.88 $ 11,827 Vested at March 31, 2024 583,183 $ 2.98 5.90 $ 9,633 Vested and expected to vest at March 31, 2024 1,200,826 $ 9.77 7.72 $ 11,827 The following table summarizes the stock option activity under the Old Cartesian Plan for options for Series A Preferred Stock: Number of Weighted-average Series A remaining Aggregate Preferred Stock Weighted-average contractual term intrinsic value options exercise price ($) (in years) (in thousands) Outstanding at December 31, 2023 14,112.299 $ 79.94 5.91 $ 8,601 Outstanding at March 31, 2024 14,112.299 $ 79.94 5.65 $ 8,043 Vested at March 31, 2024 11,323.762 $ 73.14 5.05 $ 6,530 Vested and expected to vest at March 31, 2024 14,112.299 $ 79.94 5.65 $ 8,043
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 Unvested at December 31, 2023 — $ — Granted 471,104 19.80 Forfeited (7,086) 19.80 Unvested at March 31, 2024 464,018 $ 19.80

Revenue Arrangements (Tables)

Revenue Arrangements (Tables)	3 Months Ended
	Mar. 31, 2024
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 three months ended March 31, 2024 (in thousands): Balance at Balance at beginning of period Additions Deductions end of period Three Months Ended March 31, 2024 Contract liabilities: Deferred revenue $ 5,849 $ — $ (5,437) $ 412 Total contract liabilities $ 5,849 $ — $ (5,437) $ 412

Related-Party Transactions - (T

Related-Party Transactions - (Tables)	3 Months Ended
	Mar. 31, 2024
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 three months ended March 31, 2024. Name Shares of Series A Preferred Stock purchased Total aggregate purchase price Timothy A. Springer, Ph.D. 99,140.326 $ 40,000,000

Restructuring (Tables)

Restructuring (Tables)	3 Months Ended
	Mar. 31, 2024
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 Ending Balance December 31, 2023 Charges Payments March 31, 2024 Severance liability $ 3,896 $ 292 $ (3,320) $ 868

Description of the Business (De

Description of the Business (Details) $ / shares in Units, $ in Thousands						3 Months Ended
	Apr. 04, 2024	Mar. 27, 2024	Dec. 06, 2023	Dec. 05, 2023 shares	Nov. 13, 2023 USD ($) $ / shares shares	Mar. 31, 2024 USD ($) $ / shares	Dec. 31, 2023 USD ($) $ / shares
Cash and Cash Equivalents [Line Items]
Common stock, par value (in dollars per share) | $ / shares					$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, par value (in dollars per share) | $ / shares					$ 0.0001	$ 0.0001	$ 0.0001
Cash, cash equivalents, restricted cash and marketable securities						$ 104,800
Restricted cash and cash equivalents						1,400
Accumulated deficit						(671,471)	$ (614,647)
Russian subsidiary
Cash and Cash Equivalents [Line Items]
Cash maintained in Russian bank accounts						$ 200
Subsequent Event
Cash and Cash Equivalents [Line Items]
Reverse stock split conversion ratio	0.0333
Minimum
Cash and Cash Equivalents [Line Items]
Reverse stock split conversion ratio		0.05
Maximum
Cash and Cash Equivalents [Line Items]
Reverse stock split conversion ratio		0.0333
Securities Purchase Agreement
Cash and Cash Equivalents [Line Items]
Sale of stock, consideration received on transaction					$ 60,250
Sale of stock, number of shares issued in transaction (in shares) | shares					149,330.115
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					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 (Details)

Merger - Narrative (Details) - Old Cartesian - shares	Dec. 05, 2023	Nov. 13, 2023
Business Acquisition [Line Items]
Percentage acquired		100%
Common stock
Business Acquisition [Line Items]
Issuance of shares (in shares)		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 (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 (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Nov. 13, 2023
Assets acquired:
Goodwill	$ 48,163	$ 48,163
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 (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 Loss Per Share - Computatio

Net Loss Per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Numerator:
Net loss allocable to shares of common stock - basic	$ (56,824)	$ (21,663)
Net loss allocable to shares of common stock - diluted	$ (56,824)	$ (21,663)
Denominator:
Weighted-average common shares outstanding - basic (in shares)	5,414,020	5,111,518
Weighted-average common shares outstanding - diluted (in shares)	5,414,020	5,111,518
Net loss per share:
Basic (in dollars per share)	$ (10.50)	$ (4.24)
Diluted (in dollars per share)	$ (10.50)	$ (4.24)

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details) - shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Potential common shares
Total (in shares)	21,065,841	1,814,430
Common stock options, restricted stock units and ESPP shares
Potential common shares
Total (in shares)	1,811,636	773,488
Warrants to purchase common stock
Potential common shares
Total (in shares)	975,132	1,040,942
Series A Preferred Stock
Potential common shares
Total (in shares)	17,808,670	0
Series A Preferred Stock options
Potential common shares
Total (in shares)	470,403	0

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Liabilities:
Forward contract liabilities	$ 0	$ 28,307
Recurring
Assets:
Total assets	38,672	41,161
Liabilities:
Warrant liabilities	5,352	6,394
Contingent value right liability	397,900	358,600
Forward contract liabilities		28,307
Total liabilities	403,252	393,301
Recurring | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	38,672	41,161
Recurring | Level 1
Assets:
Total assets	38,672	41,161
Liabilities:
Warrant liabilities	0	0
Contingent value right liability	0	0
Forward contract liabilities		0
Total liabilities	0	0
Recurring | Level 1 | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	38,672	41,161
Recurring | Level 2
Assets:
Total assets	0	0
Liabilities:
Warrant liabilities	0	0
Contingent value right liability	0	0
Forward contract liabilities		28,307
Total liabilities	0	28,307
Recurring | Level 2 | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	0	0
Recurring | Level 3
Assets:
Total assets	0	0
Liabilities:
Warrant liabilities	5,352	6,394
Contingent value right liability	397,900	358,600
Forward contract liabilities		0
Total liabilities	403,252	364,994
Recurring | Level 3 | Money market funds (included in cash equivalents)
Assets:
Money market funds (included in cash equivalents)	$ 0	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details)						1 Months Ended	3 Months Ended	12 Months Ended
	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 ($) shares	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Fair Value Disclosures [Abstract]
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, transfers, net							$ 0	$ 0
Fair value, measurement with unobservable inputs reconciliation, recurring basis, liability, transfers, net							$ 0	$ 0
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
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 (in shares)	$ 14,800,000					$ 35,200,000
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
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

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Fair Value Disclosures [Abstract]
Cash and cash equivalents	$ 103,418	$ 76,911	$ 125,925
Short-term restricted cash	0		289
Long-term restricted cash	1,377	1,377	1,311
Total cash, cash equivalents, and restricted cash	$ 104,795	$ 78,288	$ 127,525	$ 108,038

Fair Value Measurements - Assum

Fair Value Measurements - Assumptions Used to Record the Fair Value of the Warrants (Details) - Valuation technique, option pricing model	Mar. 31, 2024	Dec. 31, 2023
Risk-free interest rate | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0538	0.0479
Risk-free interest rate | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0440	0.0401
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0
Dividend yield | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0
Dividend yield | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0
Expected life (in years) | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	8 months 23 days	11 months 23 days
Expected life (in years) | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	3 years 10 days	3 years 3 months 10 days
Expected volatility | 2019 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.9603	0.8367
Expected volatility | 2022 Warrants
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.8272	0.8409

Fair Value Measurements - Chang

Fair Value Measurements - Change in the Warrant Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($)
Warrant liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 6,394
Change in fair value	(1,042)
Ending balance	5,352
Contingent Value Right
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	358,600
Change in fair value	39,300
Ending balance	$ 397,900

Fair Value Measurements - Sch_3

Fair Value Measurements - Schedule of Significant Inputs Used to Estimate the Fair Value of the CVRs (Details) - Level 3	Mar. 31, 2024	Dec. 31, 2023
Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	0.950	0.950
Expected volatility of future revenues
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input	0.250
Risk-adjusted discount rate
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent value right liability, measurement input		0.137

Fair Value Measurements - Cha_2

Fair Value Measurements - Change in the Forward Contract Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair value as of December 31, 2023	$ 28,307
Settlements	(35,197)
Change in fair value	$ 6,890
Change in fair value, location	Change in fair value of forward contract liabilities
Fair value as of March 31, 2024	$ 0

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 8,232	$ 7,841
Less: Accumulated depreciation	(5,830)	(5,728)
Property and equipment, net	2,402	2,113
Laboratory equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	6,662	6,280
Computer equipment and software
Property, Plant and Equipment [Line Items]
Total property and equipment	620	702
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	61	61
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property and equipment	452	452
Office equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	196	196
Construction in process
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 241	$ 150

Property and Equipment - Narrat

Property and Equipment - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 0.2	$ 0.2

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Payroll and employee related expenses	$ 1,701	$ 4,390
Accrued patent fees	1,177	472
Accrued external research and development costs	2,614	4,896
Accrued professional and consulting services	3,055	4,331
Other	380	644
Accrued expenses	$ 8,927	$ 14,733

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Millions

3 Months Ended

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 ($)

May 07, 2024 ft²

Mar. 31, 2024 USD ($)

Jul. 17, 2023 USD ($)

Mar. 31, 2023

Dec. 31, 2022 USD ($)

Oct. 06, 2022 ft²

Sep. 01, 2022 ft²

Jul. 31, 2019 ft²

Operating Leased Assets [Line Items]

Operating lease, weighted average discount rate, percent

9.90%

9.70%

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

$ 1.6

Upfront rental payment

$ 1

Deferred rent

$ 0.8

$ 0.6

Right-of-use asset impairment expense

0.7

65 Grove Street, Watertown, MA | Letter of Credit

Operating Leased Assets [Line Items]

Restricted cash

$ 1.6

Long-term restricted cash

$ 1.4

$ 1.4

704 Quince Orchard Road Leases

Operating Leased Assets [Line Items]

Number of acquired operating leases | operating_lease

2

Operating lease, weighted average discount rate, percent

11.50%

Frederick Lease Agreement

Operating Leased Assets [Line Items]

Area of office space leased | ft²

19,199

Lease agreement, first month's rent paid upon execution of lease agreement

$ 0.1

Security deposit

$ 0.3

Frederick Lease Agreement | Subsequent Event

Operating Leased Assets [Line Items]

Area of office space leased | ft²

7,842

Lease term

7 years

Number of options to extend lease | extension_option

1

Lease renewal term (in years)

5 years

Lease agreement, monthly base rent

$ 0.1

Frederick Lease Agreement | Minimum | Subsequent Event

Operating Leased Assets [Line Items]

Operating lease, cost (as a percent)

100%

Frederick Lease Agreement | Maximum | Subsequent Event

Operating Leased Assets [Line Items]

Operating lease, cost (as a percent)

103%

Leases - Components of Lease Co

Leases - Components of Lease Costs (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Leases [Abstract]
Operating lease cost	$ 775	$ 696
Variable lease cost	397	142
Short-term lease cost	3	3
Less: Sublease income	(510)	(255)
Total lease cost	$ 665	$ 586

Leases - Operating Lease, Liabi

Leases - Operating Lease, Liability, Maturity (Details) $ in Thousands	Mar. 31, 2024 USD ($)
Leases [Abstract]
2024 (remainder)	$ 2,316
2025	3,164
2026	3,248
2027	3,017
2028	946
Thereafter	0
Total future minimum lease payments	12,691
Less: Imputed interest	2,234
Total operating lease liabilities	$ 10,457

Leases - Other Information (Det

Leases - Other Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Leases [Abstract]
Cash paid for amounts included in the measurement of lease liabilities:	$ 761	$ 653

Leases - Lease Term and Discoun

Leases - Lease Term and Discount Rate (Details)	Mar. 31, 2024	Mar. 31, 2023
Leases [Abstract]
Weighted-average remaining lease term	4 years	5 years 1 month 6 days
Weighted-average discount rate	9.90%	9.70%

Debt (Details)

Debt (Details) - 2020 Term Loans - USD ($)		3 Months Ended
	Sep. 11, 2023	Sep. 30, 2023	Mar. 31, 2024	Dec. 31, 2023
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
Accrued interest	$ 100,000
Loss on extinguishment of debt		700,000
Unamortized debt issuance cost		$ 500,000
Outstanding borrowings			$ 0	$ 0

Series A Preferred Stock (Detai

Series A Preferred Stock (Details) $ in Thousands

1 Months Ended

3 Months Ended

Apr. 08, 2024 shares

Feb. 11, 2024 shares

Jan. 12, 2024 shares

Dec. 13, 2023 shares

Dec. 05, 2023 shares

Nov. 15, 2023 USD ($) shares

Nov. 13, 2023 USD ($) shares

Nov. 30, 2023 USD ($)

Mar. 31, 2024 shares

Mar. 26, 2024 business_day shares

Dec. 31, 2023 shares

Mar. 31, 2023 shares

Dec. 31, 2022 shares

Preferred stock

Authorized shares of Preferred Stock (in shares)

548,375

0

548,375

Obligation to issue stock (in shares)

49,570.163

49,570.162

148,710.488

Number of business days | business_day

8

Preferred stock, shares issued (in shares)

0

0

Preferred stock, shares outstanding (in shares)

0

0

Common stock, shares outstanding (in shares)

5,515,836

5,397,597

Subsequent Event

Preferred stock

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)

17,779,787

Common stock, shares outstanding if preferred stock were converted (in shares)

23,324,506

Series A Preferred Stock

Preferred stock

Common stock, shares outstanding (in shares)

534,260.839

0

Series A Preferred Stock | Subsequent Event

Preferred stock

Conversion of securities (in shares)

(367,919.247)

Common stock

Preferred stock

Conversion of securities (in shares)

65,681

Common stock, shares outstanding (in shares)

5,515,836

5,397,597

5,114,269

5,101,459

Common stock | Subsequent Event

Preferred stock

Conversion of securities (in shares)

12,263,951

Series A Preferred Stock

Preferred stock

Convertible preferred stock (in shares)

33.333

Preferred stock, shares issued (in shares)

534,260.839

0

Preferred stock, shares outstanding (in shares)

534,260.839

0

Minimum | Series A Preferred Stock

Preferred stock

Beneficial ownership limitation percentage for conversion of common stock issued and outstanding

0%

Maximum | Series A Preferred Stock

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 common stock | $

$ 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

Equity - Narrative (Details)

Equity - Narrative (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Oct. 25, 2021	Mar. 31, 2024	Dec. 31, 2023
Subsidiary, Sale of Stock [Line Items]
Exercise of warrants (in shares)		65,681
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
Sale of stock, number of shares issued in transaction (in shares)		0	0

Equity - Warranty Activity (Det

Equity - Warranty Activity (Details)	3 Months Ended
	Mar. 31, 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	43.80
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 (Details)	Mar. 31, 2024 shares
Class of Stock [Line Items]
Total (in shares)	21,268,716
Shares available for future stock incentive awards
Class of Stock [Line Items]
Total (in shares)	202,875
Unvested restricted stock units
Class of Stock [Line Items]
Total (in shares)	464,018
Outstanding common stock options
Class of Stock [Line Items]
Total (in shares)	1,347,618
Series A Preferred Stock
Class of Stock [Line Items]
Total (in shares)	17,808,670
Series A Preferred Stock options
Class of Stock [Line Items]
Total (in shares)	470,403
Exercise of warrants
Class of Stock [Line Items]
Total (in shares)	975,132

Stock Incentive Plans - Narrati

Stock Incentive Plans - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2024	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Nov. 13, 2023	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total (in shares)		21,268,716
Stock‑based compensation expense		$ 1,431	$ 2,276
Weighted average grant date fair value of stock options (in dollars per share)		$ 15.79	$ 26.87
Outstanding Series A Preferred Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total (in shares)		1,347,618
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 8 months 12 days
Grants in period (in shares)		471,104
Grants in period (in dollars per share)		$ 19.80
Estimated forfeitures rate		10%
Unrecognized compensation expense		$ 6,500
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		$ 8,300
Weighted average period		3 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		$ 800
Weighted average period		1 year 4 months 24 days
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)	215,903
Total (in shares)		19,446
Employment Inducement Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for grants (in shares)		113,927
Total (in shares)				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
Award vesting term		4 years
Award expiration term		10 years
Share-based compensation arrangement by share-based payment award, options, outstanding, number (in shares)		14,112.299		14,112.299
Unrecognized compensation expense related to unvested employee stock options					$ 2,600
Stock‑based compensation expense		$ 400
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 (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 1,431	$ 2,276
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	712	1,192
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 719	$ 1,084

Stock Incentive Plans - Sched_2

Stock Incentive Plans - Schedule of Assumptions (Details) - Options - $ / shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	3.95%	3.95%
Dividend yield	0%	0%
Expected term (in years)	6 years 2 months 12 days	5 years 11 months 8 days
Expected volatility	95.37%	94.64%
Weighted-average fair value of common stock (in dollars per share)	$ 19.78	$ 34.50

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
2016 Plan, 2018 Inducement Incentive Award Plan and Old Cartesian Plan
Number of Options
Beginning balance (in shares)	776,865
Granted (in shares)	641,089
Exercised (in shares)	(52,558)
Forfeited (in shares)	(17,778)
Ending balance (in shares)	1,347,618	776,865
Vested (in shares)	583,183
Vested and expected to vest (in shares)	1,200,826
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 2.97
Granted (in dollars per share)	19.78
Exercised (in dollars per share)	2.93
Forfeited (in dollars per share)	11.46
Ending balance (in dollars per share)	10.86	$ 2.97
Vested (in dollars per share)	2.98
Vested and expected to vest (in dollars per share)	$ 9.77
Weighted-average remaining contractual term
Outstanding, term	7 years 10 months 17 days	6 years 6 months
Vested, term	5 years 10 months 24 days
Vested and expected to vest, term	7 years 8 months 19 days
Aggregate intrinsic value
Outstanding	$ 11,827	$ 13,760
Vested	9,633
Vested and expected to vest	$ 11,827
Old Cartesian Plan
Number of Options
Beginning balance (in shares)	14,112.299
Ending balance (in shares)	14,112.299	14,112.299
Vested (in shares)	11,323.762
Vested and expected to vest (in shares)	14,112.299
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 79.94
Ending balance (in dollars per share)	79.94	$ 79.94
Vested (in dollars per share)	73.14
Vested and expected to vest (in dollars per share)	$ 79.94
Weighted-average remaining contractual term
Outstanding, term	5 years 7 months 24 days	5 years 10 months 28 days
Vested, term	5 years 18 days
Vested and expected to vest, term	5 years 7 months 24 days
Aggregate intrinsic value
Outstanding	$ 8,043	$ 8,601
Vested	6,530
Vested and expected to vest	$ 8,043

Stock Incentive Plans - Restric

Stock Incentive Plans - Restricted Stock Units (Details) - Restricted Stock Units (RSUs)	3 Months Ended
	Mar. 31, 2024 $ / shares shares
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) | shares	0
Grants in period (in shares) | shares	471,104
Forfeited in period (in shares) | shares	(7,086)
Unvested at end of period (in shares) | shares	464,018
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) | $ / shares	$ 0
Grants in period (in dollars per share) | $ / shares	19.80
Forfeited in period (in dollars per share) | $ / shares	19.80
Unvested at end of period (in dollars per share) | $ / shares	$ 19.80

Stock Incentive Plans - Employe

Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	21,268,716
Stock‑based compensation expense	$ 1,431	$ 2,276
ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized and reserved for future issuance (in shares)	45,795		5,769
Stock‑based compensation expense	$ 0	$ 100

Revenue Arrangements - Narrativ

Revenue Arrangements - Narrative (Details) - USD ($)	1 Months Ended			3 Months Ended
	Jan. 31, 2023	Oct. 31, 2021	Jun. 30, 2020	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Deferred revenue				$ 412,000			$ 2,311,000
Deferred revenue, net of current portion				0			3,538,000
Accounts receivable				2,006,000			5,870,000
Revenue recognized				(5,437,000)
Collaboration and license revenue				5,840,000		$ 5,938,000
Remaining performance obligation				400,000
SOBI Purchase Agreement
Disaggregation of Revenue [Line Items]
Accounts receivable				600,000			4,600,000
Collaboration and license revenue				0		4,400,000
SOBI Purchase Agreement | Billed Revenues
Disaggregation of Revenue [Line Items]
Accounts receivable				2,400,000			3,000,000
Takeda Agreement
Disaggregation of Revenue [Line Items]
Deferred revenue				0			0
Upfront cash payment		$ 3,000,000
Future additional payments, expected		$ 1,124,000,000
Milestone payments					$ 500,000
Customer, liability, revenue recognized, including opening balance				0		500,000
Collaborative Arrangement | SOBI Purchase Agreement
Disaggregation of Revenue [Line Items]
Upfront cash payment			$ 75,000,000
Sales milestone payments			$ 630,000,000
Astellas Gene Therapies
Disaggregation of Revenue [Line Items]
Upfront payment	$ 10,000,000
Development and commercial milestones plus royalties	$ 340,000,000
Deferred revenue				400,000			2,300,000
Deferred revenue, net of current portion				0			3,500,000
Accounts receivable				400,000			$ 300,000
Revenue recognized				5,800,000		600,000
Revenue recognized from performance obligations related to prior periods as a result of the change in transaction price				3,200,000
Sarepta Therapeutics, Inc.
Disaggregation of Revenue [Line Items]
Revenue recognized				$ 0		$ 500,000
Option term			24 months

Revenue Arrangements - Schedule

Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($)
Contract liabilities:
Deferred revenue, beginning of period	$ 5,849
Deferred revenue, additions	0
Deferred revenue, deductions	(5,437)
Deferred revenue, end of period	412
Contract liabilities, beginning of period	5,849
Additions	0
Deductions	(5,437)
Contract liabilities, end of period	$ 412

Related-Party Transactions - Na

Related-Party Transactions - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
	Mar. 26, 2024	Nov. 13, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Related Party Transaction [Line Items]
Proceeds from exercise of common warrants			$ 2,877	$ 0
Weighted average exercise price (in dollars per share)			$ 46.12		$ 45.98
Related Party
Related Party Transaction [Line Items]
Issuance of common stock upon exercise of warrants (in shares)	65,681
Issuance of CVRs upon exercise of warrants (in shares)	1,970,443
2019 Warrants | Related Party
Related Party Transaction [Line Items]
Warrants exercised (in shares)	65,681
Proceeds from exercise of common warrants	$ 2,900
Weighted average exercise price (in dollars per share)	$ 43.80
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
Securities Purchase Agreement | Related Party
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 (Details) - November 2023 Private Placement - USD ($)	Nov. 15, 2023	Nov. 13, 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
Sale of stock, consideration received on transaction		$ 40,000,000

Collaboration and License Agr_2

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

Collaboration and License Agr_3

Collaboration and License Agreements - National Cancer Institute of the National Institutes of Health (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_4

Collaboration and License Agreements - Ginkgo Agreement (Details) - Ginkgo Agreement - USD ($) $ in Millions				12 Months Ended
	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	$ 0.5			$ 1
Collaborative arrangement, number of shares issued for milestone achievement (in shares)	29,761			44,642
Collaborative arrangement, shares issued for milestone achievement, amount				$ 1.5	$ 1

Collaboration and License Agr_5

Collaboration and License Agreements - Genovis (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_6

Collaboration and License Agreements - Cyrus Biotechnology, Inc. (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended
	Sep. 07, 2021	Sep. 30, 2021	Mar. 31, 2024	Dec. 31, 2023
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Equity securities without readily determinable fair value, amount			$ 2,000	$ 2,000
Cyrus Biotechnology, Inc.
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payment		$ 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
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_7

Collaboration and License Agreements - AskBio (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration expense	$ 19,188	$ 24,319
AskBio License
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration expense	$ 0	$ 100
Cost share percentage	50%	50%

Collaboration and License Agr_8

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

Defined Contribution Plan (Deta

Defined Contribution Plan (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Retirement Benefits [Abstract]
Vesting period	2 years
Employer contribution made, net	$ 0.1	$ 0.1

Restructuring - Narrative (Deta

Restructuring - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated, period percent	79%
Restructuring charges	$ 292	$ 6,400
Cash payments for employee related restructuring charges	3,320	2,500
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	$ 100	$ 800

Restructuring - Schedule of Cha

Restructuring - Schedule of Change in Accrued Restructuring Balance (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Restructuring Reserve [Roll Forward]
Beginning Balance	$ 3,896
Charges	292	$ 6,400
Payments	(3,320)	(2,500)
Ending Balance	$ 868	$ 3,896

Subsequent Events (Details)

Subsequent Events (Details) $ in Millions				3 Months Ended
	May 07, 2024 USD ($) ft²	Apr. 08, 2024 shares	Apr. 04, 2024	Mar. 31, 2024 shares	Feb. 28, 2024 ft²	Dec. 31, 2023 shares	Mar. 31, 2023 shares	Dec. 31, 2022 shares
Subsequent Event [Line Items]
Common stock, shares outstanding (in shares)				5,515,836		5,397,597
Frederick Lease Agreement
Subsequent Event [Line Items]
Area of office space leased | ft²					19,199
Series A Preferred Stock
Subsequent Event [Line Items]
Common stock, shares outstanding (in shares)				534,260.839		0
Common stock
Subsequent Event [Line Items]
Conversion of securities (in shares)				65,681
Common stock, shares outstanding (in shares)				5,515,836		5,397,597	5,114,269	5,101,459
Subsequent Event
Subsequent Event [Line Items]
Reverse stock split conversion ratio			0.0333
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)		17,779,787
Common stock, shares outstanding if preferred stock were converted (in shares)		23,324,506
Subsequent Event | Frederick Lease Agreement
Subsequent Event [Line Items]
Area of office space leased | ft²	7,842
Additional annual base rent for first year | $	$ 0.3
Base rent upward annual adjustment percentage after first year	3%
Subsequent Event | Series A Preferred Stock
Subsequent Event [Line Items]
Conversion of securities (in shares)		(367,919.247)
Subsequent Event | Common stock
Subsequent Event [Line Items]
Conversion of securities (in shares)		12,263,951