Spyre Therapeutics (SYRE) 10-Q 9 Nov 23 2023 Q3 Quarterly report Financial data

Cover Page

Cover Page - shares	9 Months Ended
	Sep. 30, 2023	Nov. 03, 2023
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2023
Document Transition Report	false
Entity File Number	001-37722
Entity Registrant Name	AEGLEA BIOTHERAPEUTICS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-4312787
Entity Address, Address Line One	221 Crescent Street
Entity Address, Address Line Two	Building 23
Entity Address, Address Line Three	Suite 105
Entity Address, City or Town	Waltham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02453
City Area Code	617
Local Phone Number	651-5940
Title of 12(b) Security	Common Stock, $0.0001 Par Value Per Share
Trading Symbol	AGLE
Security Exchange Name	NASDAQ
Entity Interactive Data Current	Yes
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		4,048,927
Entity Central Index Key	0001636282
Current Fiscal Year End Date	--12-31
Amendment Flag	false
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
CURRENT ASSETS
Cash and cash equivalents	$ 90,592	$ 34,863
Marketable securities	113,007	20,848
Development receivables	163	375
Prepaid expenses and other current assets	2,187	6,172
Total current assets	205,949	62,258
Restricted cash	1,307	1,553
Property and equipment, net	0	3,220
Operating lease right-of-use assets	0	3,430
Other non-current assets	9	683
TOTAL ASSETS	207,265	71,144
CURRENT LIABILITIES
Accounts payable	1,678	677
CVR liability	7,510	0
Operating lease liabilities	0	625
Deferred revenue	0	517
Accrued and other current liabilities	15,861	12,837
Related party accounts payable	19,823	0
Total current liabilities	44,872	14,656
Non-current CVR liability	20,690	0
Non-current operating lease liabilities	0	4,004
Deferred revenue, net of current portion	0	2,179
TOTAL LIABILITIES	65,562	20,839
Commitments and Contingencies (Note 11)
STOCKHOLDERS’ (DEFICIT) EQUITY
Preferred stock, $0.0001 par value; 8,913,659 shares and 10,000,000 authorized as of September 30, 2023 and December 31, 2022; no shares issued and outstanding as of September 30, 2023 and December 31, 2022	0	0
Common stock, $0.0001 par value; 20,000,000 shares authorized as of September 30, 2023 and December 31, 2022; 4,048,687 shares and 2,614,014 shares issued and outstanding as of September 30, 2023 and December 31, 2022, respectively	7	6
Additional paid-in capital	455,957	475,971
Accumulated other comprehensive income (loss)	(132)	(48)
Accumulated deficit	(701,234)	(425,624)
TOTAL STOCKHOLDERS’ (DEFICIT) EQUITY	(245,402)	50,305
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ (DEFICIT) EQUITY	207,265	71,144
Series A Non Voting Convertible Preferred Stock
CURRENT LIABILITIES
Series A non-voting convertible preferred stock, $0.0001 par value; 1,086,341 and no shares authorized as of September 30, 2023 and December 31, 2022, respectively; 1,086,339 and no shares issued and outstanding as of September 30, 2023 and December 31, 2022, respectively	$ 387,105	$ 0

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2023	Dec. 31, 2022
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	8,913,659	10,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, authorized (in shares)	20,000,000	20,000,000
Common stock, issued (in shares)	4,048,687	2,614,014
Common stock, outstanding (in shares)	4,048,687	2,614,014
Series A Non Voting Convertible Preferred Stock
Series A non-voting convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Series A non-voting convertible preferred stock, authorized (in shares)	1,086,341	0
Series A non-voting convertible preferred stock, issued (in shares)	1,086,339	0
Series A non-voting convertible preferred stock, outstanding (in shares)	1,086,339	0

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenue:
Total revenue		$ 0	$ 174	$ 886	$ 2,161
Revenue from Contract with Customer, Product and Service [Extensible Enumeration]		Development Fee and Royalty [Member]	Development Fee and Royalty [Member]	Development Fee and Royalty [Member]	Development Fee and Royalty [Member]
Operating expenses (income):
Research and development	[1]	$ 24,660	$ 11,977	$ 55,822	$ 44,328
General and administrative		8,584	6,952	25,874	23,452
Acquired in-process research and development		(298)	0	130,188	0
Gain recognized within operating expenses		(14,609)	0	(14,609)	0
Total operating expenses		18,337	18,929	197,275	67,780
Loss from operations		(18,337)	(18,755)	(196,389)	(65,619)
Other (expense) income:
Interest income		1,251	288	2,021	427
Other income, net		2,342	24	2,262	25
Total other (expense) income		(21,767)	312	(79,247)	452
Loss before income tax expense		(40,104)	(18,443)	(275,636)	(65,167)
Income tax (expense) benefit		(3)	209	26	174
Net loss		$ (40,107)	$ (18,234)	$ (275,610)	$ (64,993)
Net loss per share, basic (in dollars per share)		$ (9.34)	$ (4.84)	$ (69.57)	$ (20.17)
Net loss per share, diluted (in dollars per share)		$ (9.34)	$ (4.84)	$ (69.57)	$ (20.17)
Weighted-average common shares outstanding, basic (in dollars per share)		4,293,812	3,767,918	3,961,546	3,222,987
Weighted-average common shares outstanding, diluted (in dollars per share)		4,293,812	3,767,918	3,961,546	3,222,987
Forward contract liability
Other (expense) income:
Change in fair value of forward contract liability		$ (25,360)	$ 0	$ (83,530)	$ 0
[1]Includes $19.4 million and $20.8 million in related party expenses for the three and nine months ended September 30, 2023, respectively and no related party expenses for the three and nine months ended September 30, 2022.

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations (Parenthetical) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Research and development	[1]	$ 24,660	$ 11,977	$ 55,822	$ 44,328
Related Party
Research and development		$ 19,400	$ 0	$ 20,800	$ 0
[1]Includes $19.4 million and $20.8 million in related party expenses for the three and nine months ended September 30, 2023, respectively and no related party expenses for the three and nine months ended September 30, 2022.

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Statement of Comprehensive Income [Abstract]
Net loss	$ (40,107)	$ (18,234)	$ (275,610)	$ (64,993)
Other comprehensive income (loss):
Foreign currency translation adjustment	(29)	(38)	(1)	(87)
Unrealized (loss) gain on marketable securities	(114)	74	(83)	(77)
Total comprehensive loss	$ (40,250)	$ (18,198)	$ (275,694)	$ (65,157)

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' (Deficit) Equity - USD ($)	Total	Series A Non Voting Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2021		0
Beginning balance at Dec. 31, 2021		$ 0
Ending balance (in shares) at Mar. 31, 2022		0
Beginning balance at Mar. 31, 2022		$ 0
Beginning balances (in shares) at Dec. 31, 2021			1,974,000
Beginning balance at Dec. 31, 2021	$ 83,941,000		$ 5,000	$ 425,765,000	$ (20,000)	$ (341,809,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with employee stock purchase plan (in shares)			3,000
Issuance of common stock in connection with employee stock purchase plan	184,000			184,000
Stock-based compensation expense	2,101,000			2,101,000
Foreign currency translation adjustment	(13,000)				(13,000)
Unrealized gain (loss) on marketable securities	(120,000)				(120,000)
Net loss	(24,436,000)					(24,436,000)
Ending balance (in shares) at Mar. 31, 2022			1,977,000
Ending balance at Mar. 31, 2022	61,657,000		$ 5,000	428,050,000	(153,000)	(366,245,000)
Beginning balance (in shares) at Dec. 31, 2021		0
Beginning balance at Dec. 31, 2021		$ 0
Ending balance (in shares) at Sep. 30, 2022		0
Beginning balance at Sep. 30, 2022		$ 0
Beginning balances (in shares) at Dec. 31, 2021			1,974,000
Beginning balance at Dec. 31, 2021	83,941,000		$ 5,000	425,765,000	(20,000)	(341,809,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation adjustment	(87,000)
Unrealized gain (loss) on marketable securities	(77,000)
Net loss	(64,993,000)
Ending balance (in shares) at Sep. 30, 2022			2,460,000
Ending balance at Sep. 30, 2022	67,555,000		$ 6,000	474,535,000	(184,000)	(406,802,000)
Beginning balance (in shares) at Mar. 31, 2022		0
Beginning balance at Mar. 31, 2022		$ 0
Ending balance (in shares) at Jun. 30, 2022		0
Beginning balance at Jun. 30, 2022		$ 0
Beginning balances (in shares) at Mar. 31, 2022			1,977,000
Beginning balance at Mar. 31, 2022	61,657,000		$ 5,000	428,050,000	(153,000)	(366,245,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with exercise of pre-funded warrants (in shares)			40,000
Issuance of common stock and pre-funded warrants in connection with registered direct offering, net of offering costs (in shares)			430,000
Issuance of common stock and pre-funded warrants in connection with registered direct offering, net of offering costs	42,873,000		$ 1,000	42,872,000
Stock-based compensation expense	2,017,000			2,017,000
Foreign currency translation adjustment	(36,000)				(36,000)
Unrealized gain (loss) on marketable securities	(31,000)				(31,000)
Net loss	(22,323,000)					(22,323,000)
Ending balance (in shares) at Jun. 30, 2022			2,447,000
Ending balance at Jun. 30, 2022	84,157,000		$ 6,000	472,939,000	(220,000)	(388,568,000)
Ending balance (in shares) at Sep. 30, 2022		0
Beginning balance at Sep. 30, 2022		$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock and pre-funded warrants in connection with registered direct offering, net of offering costs (in shares)			10,000
Issuance of common stock in connection with exercise of pre-funded warrants	(8,000)			(8,000)
Issuance of common stock in connection with employee stock purchase plan (in shares)			3,000
Issuance of common stock in connection with employee stock purchase plan	38,000			38,000
Stock-based compensation expense	1,566,000			1,566,000
Foreign currency translation adjustment	(38,000)				(38,000)
Unrealized gain (loss) on marketable securities	74,000				74,000
Net loss	(18,234,000)					(18,234,000)
Ending balance (in shares) at Sep. 30, 2022			2,460,000
Ending balance at Sep. 30, 2022	67,555,000		$ 6,000	474,535,000	(184,000)	(406,802,000)
Beginning balance (in shares) at Dec. 31, 2022		0
Beginning balance at Dec. 31, 2022		$ 0
Ending balance (in shares) at Mar. 31, 2023		0
Beginning balance at Mar. 31, 2023		$ 0
Beginning balances (in shares) at Dec. 31, 2022			2,614,000
Beginning balance at Dec. 31, 2022	50,305,000		$ 6,000	475,971,000	(48,000)	(425,624,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with employee stock purchase plan (in shares)			2,000
Issuance of common stock in connection with employee stock purchase plan	18,000			18,000
Stock-based compensation expense	1,709,000			1,709,000
Foreign currency translation adjustment	10,000				10,000
Unrealized gain (loss) on marketable securities	32,000				32,000
Net loss	(18,422,000)					(18,422,000)
Ending balance (in shares) at Mar. 31, 2023			2,616,000
Ending balance at Mar. 31, 2023	33,652,000		$ 6,000	477,698,000	(6,000)	(444,046,000)
Beginning balance (in shares) at Dec. 31, 2022		0
Beginning balance at Dec. 31, 2022		$ 0
Ending balance (in shares) at Sep. 30, 2023		1,086,339
Beginning balance at Sep. 30, 2023		$ 387,105,000
Beginning balances (in shares) at Dec. 31, 2022			2,614,000
Beginning balance at Dec. 31, 2022	50,305,000		$ 6,000	475,971,000	(48,000)	(425,624,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation adjustment	(1,000)
Unrealized gain (loss) on marketable securities	(83,000)
Net loss	(275,610,000)
Ending balance (in shares) at Sep. 30, 2023			4,049,000
Ending balance at Sep. 30, 2023	(245,402,000)		$ 7,000	455,957,000	(132,000)	(701,234,000)
Beginning balance (in shares) at Mar. 31, 2023		0
Beginning balance at Mar. 31, 2023		$ 0
Series A Non-Voting Convertible Preferred Stock
Issuance of Series A non-voting convertible preferred stock (in shares)		721,000
Issuance of Series A non-voting convertible preferred stock		$ 197,323,000
Ending balance (in shares) at Jun. 30, 2023		721,000
Beginning balance at Jun. 30, 2023		$ 197,323,000
Beginning balances (in shares) at Mar. 31, 2023			2,616,000
Beginning balance at Mar. 31, 2023	33,652,000		$ 6,000	477,698,000	(6,000)	(444,046,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock forward in connection with the asset acquisition of Spyre	3,768,000			3,768,000
Issuance of common stock in connection with exercise of pre-funded warrants (in shares)			624,000
CVR distribution to common stockholders	(29,500,000)			(29,500,000)
Stock-based compensation expense	1,775,000			1,775,000
Foreign currency translation adjustment	18,000				18,000
Unrealized gain (loss) on marketable securities	(1,000)				(1,000)
Net loss	(217,081,000)					(217,081,000)
Ending balance (in shares) at Jun. 30, 2023			3,240,000
Ending balance at Jun. 30, 2023	(207,369,000)		$ 6,000	453,741,000	11,000	(661,127,000)
Series A Non-Voting Convertible Preferred Stock
Issuance of Series A non-voting convertible preferred stock (in shares)		365,000
Issuance of Series A non-voting convertible preferred stock		$ 189,741,000
Settlement of financing costs in connection with private placement of Series A non-voting convertible preferred stock		$ 41,000
Ending balance (in shares) at Sep. 30, 2023		1,086,339
Beginning balance at Sep. 30, 2023		$ 387,105,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with exercise of stock options and employee stock purchase plan	105,000			105,000
Issuance of common stock in connection with the asset acquisition of Spyre and settlement of related forward contract (in shares)			518,000
Issuance of common stock in connection with the asset acquisition of Spyre and settlement of related forward contract	0		$ 1,000	(1,000)
Issuance of common stock in connection with exercise of pre-funded warrants (in shares)			281,000
Issuance of common stock in connection with exercise of stock options and employee stock purchase plan (in shares)			10,000
Stock-based compensation expense	2,112,000			2,112,000
Foreign currency translation adjustment	(29,000)				(29,000)
Unrealized gain (loss) on marketable securities	(114,000)				(114,000)
Net loss	(40,107,000)					(40,107,000)
Ending balance (in shares) at Sep. 30, 2023			4,049,000
Ending balance at Sep. 30, 2023	$ (245,402,000)		$ 7,000	$ 455,957,000	$ (132,000)	$ (701,234,000)

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (275,610)	$ (64,993)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	744	1,182
Stock-based compensation	8,405	5,684
Acquired in-process research and development	130,188	0
Gain on sale of in-process research and development asset	(14,609)	0
Lease ROU asset and leasehold improvement impairment loss	2,580	0
Loss on disposal of long-lived assets	915	0
Amortization of operating lease assets	220	292
Net accretion of discount on marketable securities	(612)	(175)
Other	18	351
Changes in operating assets and liabilities:
Prepaid expenses and other assets	3,310	(2,863)
Accounts payable	1,001	859
Deferred revenue	575	(897)
Development receivables	212	146
Operating lease liabilities	(2,326)	(297)
Accrued and other liabilities	(4,000)	(1,293)
Related party payable	(2,115)	0
Net cash used in operating activities	(68,874)	(62,004)
CASH FLOWS FROM INVESTING ACTIVITIES
Cash assumed from asset acquisition of Spyre	3,035	0
Proceeds from sale of in-process research & development asset	15,000	0
Purchases of property and equipment	0	(38)
Proceeds from sale of property and equipment	475	0
Purchases of marketable securities	(112,631)	(35,000)
Proceeds from maturities and sales of marketable securities	21,000	78,046
Net cash (used in) and provided by investing activities	(73,121)	43,008
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of Series A non-voting convertible preferred stock in connection with private placement, net of placement and other offering costs	197,364	0
Proceeds from issuance of common stock and pre-funded warrants in registered direct offering, net of offering costs	0	42,874
Proceeds from employee stock plan purchases and stock option exercises	123	222
Principal payments on finance lease obligation	(16)	(410)
Net cash provided by financing activities	197,471	42,686
Effect of exchange rate on cash, cash equivalents, and restricted cash	7	(152)
NET INCREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH	55,483	23,538
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH
Beginning of period	36,416	16,980
End of period	91,899	40,518
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Settlement of forward contract liability and issuance of Series A non-voting convertible preferred stock in connection with the asset acquisition of Spyre	189,741	0
CVR liability
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of forward contract liability	(1,300)	0
Forward contract liability
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of forward contract liability	$ 83,530	$ 0

The Company and Basis of Presen

The Company and Basis of Presentation	9 Months Ended
	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
The Company and Basis of Presentation	The Company and Basis of Presentation Aeglea BioTherapeutics, Inc. (“Aeglea” or the “Company”) is a preclinical stage biotechnology company focused on developing next generation therapeutics for patients living with inflammatory bowel disease. The Company was formed as a Limited Liability Company ("LLC") in Delaware on December 16, 2013 under the name Aeglea BioTherapeutics Holdings, LLC and was converted from a Delaware LLC to a Delaware corporation on March 10, 2015. The Company operates in one segment and has its principal offices in Waltham, Massachusetts. On April 12, 2023, based on the review of the inconclusive interim results from the Company's Phase 1/2 clinical trial of pegtarviliase for the treatment of classical homocystinuria and other business considerations, the Company announced that it had initiated a process to explore strategic alternatives to maximize stockholder value and engaged an independent exclusive financial advisor to support this process. In April 2023, the Company implemented a restructuring plan resulting in an approximate 83% reduction of the Company’s existing headcount. On September 8, 2023, Aeglea effected a reverse stock split of its common stock at a ratio of 1-for-25 (the “Reverse Split”). All share numbers related to the Company's common stock disclosed in these financial statements have been adjusted on a post-Reverse Split basis. On June 22, 2023, the Company acquired, in accordance with the terms of the Agreement and Plan of Merger (the "Acquisition Agreement"), the assets from Spyre Therapeutics, Inc. ("Spyre"), as disclosed in Notes 7 and 8, a privately held biotechnology company advancing a pipeline of antibody therapeutics with the potential to transform the treatment of inflammatory bowel disease through a research and development option agreement ("Paragon Agreement") with Paragon Therapeutics ("Paragon"). The transaction was structured as a stock-for-stock transaction pursuant to which all of Spyre's outstanding equity interests were exchanged based on a fixed exchange ratio of 0.5494488 to 1 for consideration from Aeglea of 517,809 shares of common stock and 364,887 shares of Series A non-voting convertible preferred stock, par value of $0.0001 per share ("Series A Preferred Stock") (convertible on a 40 to 1 basis) in addition to the assumption of outstanding and unexercised stock options to purchase 2,734 shares of common stock from the Amended and Restated Spyre 2023 Equity Incentive Plan (the "Asset Acquisition"). The Aeglea common stock and Aeglea Series A Preferred Stock related to the Asset Acquisition were issued to the Spyre stockholders on July 7, 2023. For additional information, see Notes 7 and 8. In connection with the Asset Acquisition, on June 26, 2023, the Company completed a private placement of shares of Series A Preferred Stock (the “PIPE”) to a group of investors (the “Investors”). The Company sold an aggregate of 721,452 shares of Series A Preferred Stock for an aggregate purchase price of approximately $210.0 million before deducting approximately $12.7 million in placement agent and other offering expenses (together with the Asset Acquisition, the “Transactions”). For additional information, see Note 9. In connection with the Asset Acquisition and pursuant to a non-transferable contingent value right ("CVR") agreement (the “CVR Agreement”) a CVR was distributed to each Aeglea stockholder of record as of the close of business on July 3, 2023 (the "Legacy Stockholders"), but was not distributed to the holders of shares of common stock or Series A Preferred Stock issued to the former stockholders of Spyre or Investors in the Transactions. Holders of the CVRs will be entitled to receive cash payments from proceeds received by Aeglea for a three-year period, if any, related to the disposition or monetization of its legacy assets for a period of one-year following the closing of the Asset Acquisition. For additional information see Note 3. Liquidity As of September 30, 2023, the Company had an accumulated deficit of $701.2 million, and cash, cash equivalents, and marketable securities of $203.6 million. The Company has not generated any product revenues and has not achieved profitable operations. There can be no assurance that profitable operations will ever be achieved, and, if achieved, whether it can be sustained on a continuing basis. In addition, development activities, clinical and nonclinical testing, and commercialization of the Company’s product candidates will require significant additional financing before a commercial drug can be produced and marketed. The Company is subject to a number of risks similar to other life science companies, including, but not limited to, risks related to the successful discovery, development, and commercialization of product candidates, raising additional capital, development of competing drugs and therapies, protection of proprietary technology and market acceptance of the Company’s product candidates. As a result of these and other factors and uncertainties, there can be no assurance of the Company’s future success. In April 2023, the Board of Directors (the "Board") approved a restructuring of the Company’s workforce pursuant to which the Company’s workforce was reduced by approximately 83% and the Company retained approximately 10 employees. Following a review of the interim results from its ongoing Phase 1/2 clinical trial of pegtarviliase for the treatment of classical homocystinuria and other business considerations, the Company explored strategic alternatives with the goal of maximizing stockholder value, including possible business combinations and/or a divestiture of the Company’s clinical programs. On June 22, 2023, the Company acquired, in accordance with the terms of the Acquisition Agreement, the net assets of Spyre, as disclosed in Notes 7 and 8. Additionally, the Company completed the PIPE. In accordance with Accounting Standards Codification (“ASC") 205-40, Going Concern, the Company has evaluated and determined that there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the accompanying condensed consolidated financial statements included in this Quarterly Report are issued. The Company’s Series A Preferred Stock agreement requires it to seek stockholder approval for the conversion of the Series A Preferred Stock to common stock. The Company has agreed to hold a stockholders' meeting to submit this matter to its stockholders for their consideration. In connection with this, the Company filed with the Securities and Exchange Commission ("SEC") a definitive proxy statement and other relevant materials. The special meeting of stockholders is scheduled for November 21, 2023. If the Company’s stockholders do not timely approve the conversion of its Series A Preferred Stock into common stock, then the holders of its Series A Preferred Stock are entitled to require the Company to settle their shares of Series A Preferred Stock for cash at a price per share equal to the fair value of the Series A Preferred Stock, as described in the Certificate of Designation relating to the Series A Preferred Stock (see Note 9). The cash redemption is not in the Company’s control and raises substantial doubt about the Company’s ability to continue as a going concern. The accompanying condensed consolidated financial statements assume the Company will continue as a going concern through the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business. Unaudited Interim Financial Information The interim condensed consolidated financial statements included in this Quarterly Report on Form 10-Q are unaudited. The unaudited interim financial statements have been prepared on the same basis as the annual financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for a fair statement of the Company’s financial position as of September 30, 2023, and its results of operations for the three and nine months ended September 30, 2023 and 2022, changes in convertible preferred stock and stockholders’ (deficit) equity for the three and nine months ended September 30, 2023 and 2022, and cash flows for the nine months ended September 30, 2023 and 2022. The results of operations for the three and nine months ended September 30, 2023, are not necessarily indicative of the results to be expected for the year ending December 31, 2023 or for any other future annual or interim period. The December 31, 2022 balance sheet was derived from audited financial statements, but does not include all disclosures required by accounting principles generally accepted in the United States (“U.S. GAAP”). These financial statements should be read in conjunction with the audited financial statements included in the Company’s Form 10-K for the year ended December 31, 2022 (the "Annual Report") as filed with the SEC.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Other than policies noted below, there have been no significant changes from the significant accounting policies and estimates disclosed in Note 2 of the “Notes to Consolidated Financial Statements” included in our Annual Report on Form 10-K for the year ended December 31, 2022. These interim condensed consolidated financial statements have been prepared in accordance with U.S. GAAP and SEC instructions for interim financial information, and should be read in conjunction with the Company's Annual Report. Significant accounting policies and other disclosures normally provided have been omitted since such items are disclosed in the Company's Annual Report. The Company uses the same accounting policies in preparing quarterly and annual financial statements. Convertible Preferred Stock Issued through PIPE The Company records shares of convertible preferred stock at their respective fair values on the dates of issuance, net of issuance costs. The Company has applied the guidance in ASC 480-10-S99-3A as well as SEC Staff Announcement, Classification and Measurement of Redeemable Securities, and has therefore classified the Series A Preferred Stock outside of stockholders’ (deficit) equity because, if conversion to common stock is not approved by the stockholders, the Series A Preferred Stock will be redeemable at the option of the holders for cash equal to the closing price of the common stock on the last trading day prior to the holder’s redemption request. The Company has determined that the conversion and redemption are outside of the Company’s control. Additionally, the Company has determined the conversion and redemption features do not require bifurcation as derivatives. Acquisitions The Company evaluates acquisitions of assets and other similar transactions to assess whether or not the transaction should be accounted for as a business combination or asset acquisition by first applying a screen test to determine whether substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or group of similar identifiable assets. If so, the transaction is accounted for as an asset acquisition. If not, further determination is required as to whether or not the Company has acquired inputs and processes that have the ability to create outputs, which would meet the definition of a business. Significant judgment is required in the application of the test to determine whether an acquisition is a business combination or an acquisition of assets. Acquisitions meeting the definition of business combinations are accounted for using the acquisition method of accounting, which requires that the purchase price be allocated to the net assets acquired at their respective fair values. In a business combination, any excess of the purchase price over the estimated fair values of the net assets acquired is recorded as goodwill. The Company measures and recognizes asset acquisitions that are not deemed to be business combinations based on the cost to acquire the assets, which includes pre-acquisition direct costs recorded in accrued professional and consulting fees. Goodwill is not recognized in asset acquisitions. When a transaction accounted for as an asset acquisition includes an in-process research and development (“IPR&D”) asset, the IPR&D asset is only capitalized if it has an alternative future use other than in a particular research and development project. Otherwise, the cost allocated to acquire an IPR&D asset with no alternative future use is charged to expense at the acquisition date. Contingent Value Rights The Company evaluates its contracts to determine if those contracts qualify as derivatives under ASC 815, Derivatives and Hedging (“ASC 815”). For derivative financial instruments that are accounted for as liabilities, the derivative instrument is initially recorded at its fair value and is then re-valued at each reporting date. Any changes in fair value are recorded as other income or expense for each reporting period. Derivative instrument liabilities are classified in the balance sheet as current or non-current based on whether or not net-cash settlement of the derivative instrument is probable within the next 12 months from the balance sheet date. The Company determined that certain contingent payments under the CVR Agreement qualified as derivatives under ASC 815, and as such, were recorded as a liability on the balance sheet. This value is then remeasured for future expected payout as well as the increase in fair value due to the time value of money. These gains or losses, if any, are recognized in the consolidated statements of operations and comprehensive loss within Other (expense) income, net. The Company applies a scenario-based method and weighs them based on the possible achievement of certain milestones. The milestone payments are contingent on formal reimbursement decisions by national authorities in key European markets and pegzilarginase approval by the U.S. Food and Drug Administration ("FDA"), among other events. This fair value measurement is based on significant inputs not observable in the market and thus represents a Level 3 measurement as defined in ASC 820, Fair Value Measurement. The key assumptions used include the discount rate, probability of regulatory success, and reimbursement rates from certain government agencies. The estimated value of the CVR consideration is based upon available information and certain assumptions which the Company's management believes are reasonable under the circumstances. The ultimate payout under the CVRs may differ materially from the assumptions used in determining the fair value of the CVR consideration. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the condensed consolidated financial statements and accompanying notes. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets, liabilities, and equity and the amount of revenues and expenses. Actual results could differ significantly from those estimates. The most significant estimates and assumptions that management considers in the preparation of the Company's financial statements relate to the valuation of consideration transferred in acquiring IPR&D; the discount rate, probabilities of success, and timing of estimated cash flows in the valuation of the CVR liability; inputs used in the Black-Scholes model for stock-based compensation expense; estimated future cash flows used in calculating the impairment of right-of-use lease assets; and estimated cost to complete performance obligations related to revenue recognition. The consideration transferred in acquiring IPR&D in connection with the acquisition of Spyre was comprised of shares of the Company’s common stock and shares of Series A Preferred Stock. To determine the fair value of the equity transferred, the Company considered the per share value of the PIPE, which was an over-subscribed financing event involving a group of accredited investors. Recently Adopted Accounting Pronouncement The Company early adopted the Financial Accounting Standards Board’s Accounting Standards Update 2020-06, Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (“ASU 2020-06”), effective as of January 1, 2023 using the modified retrospective method. Among other amendments, ASU 2020-06 eliminates the cash conversion and beneficial conversion feature models in ASC 470-20 that required an issuer of certain convertible debt and preferred stock to separately account for embedded conversion features as a component of equity, as well as changes the accounting for diluted earnings‑per‑share for convertible instruments and contracts that may be settled in cash or stock. Additionally, ASU 2020-06 requires the if‑converted method, which is more dilutive than the treasury stock method, be used for all convertible instruments. The Company applied ASU 2020-06 to all Series A Preferred Stock during fiscal year 2023, and, accordingly, the Company did not apply the cash conversion or beneficial conversion feature models in its analysis of the Series A Preferred Stock.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company measures and reports certain financial instruments as assets and liabilities at fair value on a recurring basis. The following tables set forth the fair value of the Company’s financial assets and liabilities at fair value on a recurring basis based on the three-tier fair value hierarchy (in thousands): September 30, 2023 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 55,451 $ — $ — $ 55,451 Commercial paper — 107,093 — 107,093 Corporate bonds — 22,828 — 22,828 Total financial assets $ 55,451 $ 129,921 $ — $ 185,372 Liabilities: Parapyre Option Obligation $ — $ 2,952 $ — $ 2,952 CVR liability — — 28,200 28,200 Total liabilities $ — $ 2,952 $ 28,200 $ 31,152 December 31, 2022 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 15,250 $ — $ — $ 15,250 Commercial paper — 23,641 — 23,641 U.S. government securities — 4,230 — 4,230 Corporate bonds — 3,732 — 3,732 Total financial assets $ 15,250 $ 31,603 $ — $ 46,853 The Company measures the fair value of money market funds on quoted prices in active markets for identical assets or liabilities. The Level 2 assets include commercial paper, U.S. government securities and corporate bonds and are valued based on quoted prices for similar assets in active markets and inputs other than quoted prices that are derived from observable market data. The Company evaluates transfers between levels at the end of each reporting period. The Parapyre Option Obligation (as defined in Note 6) is considered a Level 2 liability based on observable market data for substantially the full term of the liability. The Parapyre Option Obligation is measured each period using a Black-Scholes model to estimate the fair value of the option grant. Changes in the fair value of the Parapyre Option Obligation are recorded as stock-based compensation within Research and development expenses for non-employees who provided pre-clinical testing services. The CVR liability value is based on significant inputs not observable in the market such as estimated cash flows, estimated probabilities of success, and risk-adjusted discount rates, which represent a Level 3 liability. As of December 31, 2022, the Company had no financial liabilities outstanding measured at fair value. Forward Contract Liability In connection with the Asset Acquisition, the Company entered into a contract for the issuance of 364,887 shares of Series A Preferred Stock as part of the consideration transferred. This forward contract was classified as a liability because the underlying preferred shares were contingently redeemable. Further, the forward contract liability was considered a Level 2 liability based on observable market data for substantially the full term of the liability and was initially measured at its estimated fair value on the transaction date based on the underlying price per share on an as-converted basis of the Series A Preferred Stock issued in the PIPE. Subsequent remeasurement of the fair value of the forward contract liability through its settlement date was based on the market price of the Company's common stock, which represents the redemption value of the Series A Preferred Stock. The fair value of the forward contract at the transaction date, June 22, 2023, was $106.2 million. The liability was settled with the issuance of the Series A Preferred Stock on July 7, 2023 for $189.7 million. For the three and nine months ended September 30, 2023, $25.4 million and $83.5 million, respectively, was recorded as Other (expense) income in the consolidated statements of operations in connection with the change in fair value of the forward contract liability. The following table presents changes in the forward contract liability for the periods presented (in millions): Forward Contract Liability Beginning balance as of June 22, 2023 $ 106.2 Change in fair value 58.1 Ending balance as of June 30, 2023 164.3 Change in fair value 25.4 Issuance of Series A Preferred Stock on July 7, 2023 (189.7) Ending balance as of September 30, 2023 $ — Parapyre Option Obligation On June 22, 2023, in connection with the Asset Acquisition, the Company assumed the Parapyre Option Obligation which provided for an annual equity grant of options for Parapyre Holding LLC ("Parapyre") to purchase 1% of the then outstanding shares of Spyre's common stock, on a fully diluted basis, on the last business day of each calendar year during the term of the Paragon Agreement, at the fair market value determined by the board of directors of Spyre. On September 29, 2023, the Company amended the Paragon Agreement to amend and restate certain terms of the option grant pertaining to the Parapyre Option Obligation, including but not limited to (i) defining that the annual equity grant of options is based on the outstanding shares of Aeglea’s common stock, (ii) establishing the grant date as the last business day of each applicable calendar year, and (iii) defining the term of the options granted as ten years. The liability related to the Parapyre Option Obligation will be recorded pursuant to the amended Paragon Agreement. As of September 30, 2023, the pro-rated estimated fair value of the options to be granted on December 31, 2023, was approximately $3.0 million, of which $0.1 million was recognized as part of the liabilities assumed with the Asset Acquisition on June 22, 2023. For the three and nine months ended September 30, 2023, $2.7 million and $2.9 million, respectively, was recognized as stock compensation expense related to the Parapyre Option Obligation. CVR Liability In connection with the Asset Acquisition, a non-transferable contingent value right (a “CVR”) was distributed to Aeglea stockholders of record as of the close of business on July 3, 2023, but was not distributed to holders of shares of Common Stock or Series A Preferred Stock issued to the Investors or former stockholders of Spyre in connection with the Transactions. Holders of the CVR will be entitled to receive certain cash payments from proceeds received by the Company for a three-year period, if any, related to the disposition or monetization of Aeglea’s legacy assets for a period of one year following the closing of the Asset Acquisition. The fair value of the CVR liability was determined using the probability weighted discounted cash flow method to estimate future cash flows associated with the sale of the legacy assets. Analogous to a dividend being declared/approved in one period and paid out in another, the liability was recorded at the date of approval, June 22, 2023, as a common stock dividend, returning capital to the Legacy Stockholders. Changes in fair value of the liability will be recognized as a component of Other income (expense) in the condensed consolidated statement of operations and comprehensive loss in each reporting period. The liability value is based on significant inputs not observable in the market such as estimated cash flows, estimated probabilities of success, and risk-adjusted discount rates, which represent a Level 3 measurement within the fair value hierarchy. The significant inputs used to estimate the fair value of the CVR liability were as follows: September 30, 2023 Estimated cash flow dates 11/28/23 - 06/22/26 Estimated probability of success 27% - 100% Risk-adjusted discount rates 7.14% - 7.57% The change in fair value between June 30, 2023 and September 30, 2023 was a $1.3 million decrease, and was primarily driven by changes in the likelihood of a successful disposition of pegtarviliase, changes in the expected timing of achievement of certain milestones, updates to expenses and deductions, partially offset by changes in the likelihood of certain milestones related to the favorable Committee for Medicinal Products Human Use ("CHMP") opinion received by Immedica Pharma AB (“Immedica”). The following table presents changes in the CVR liability for the periods presented (in thousands): CVR Liability Beginning balance as of December 31, 2022 $ — Fair value at CVR issuance 29,500 Changes in the fair value of the CVR liability since issuance $ (1,300) Ending Balance as of September 30, 2023 $ 28,200

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	9 Months Ended
	Sep. 30, 2023
Cash and Cash Equivalents [Abstract]
Cash Equivalents and Marketable Securities	4. Cash Equivalents and Marketable Securities The following tables summarize the estimated fair value of the Company’s cash equivalents and marketable securities and the gross unrealized gains and losses (in thousands): September 30, 2023 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash equivalents: Money market funds $ 55,451 $ — $ — $ 55,451 Commercial paper 16,911 3 — 16,914 Total cash equivalents $ 72,362 $ 3 $ — $ 72,365 Marketable securities: Commercial paper $ 90,272 $ — $ (93) $ 90,179 Corporate bonds 22,849 1 (22) 22,828 U.S. government securities — — — — Total marketable securities $ 113,121 $ 1 $ (115) $ 113,007 December 31, 2022 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash equivalents: Money market funds $ 15,250 $ — $ — $ 15,250 Commercial paper 7,021 1 (2) 7,020 U.S. government securities 3,736 — (1) 3,735 Total cash equivalents $ 26,007 $ 1 $ (3) $ 26,005 Marketable securities: Commercial paper $ 16,644 $ 2 $ (25) $ 16,621 Corporate bonds 3,738 — (6) 3,732 U.S. government securities 495 — — 495 Total marketable securities $ 20,877 $ 2 $ (31) $ 20,848 The following table summarizes the available-for-sale securities in an unrealized loss position for which an allowance for credit losses has not been recorded as of September 30, 2023 and December 31, 2022, aggregated by major security type and length of time in a continuous unrealized loss position: September 30, 2023 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 78,820 $ (93) $ — $ — $ 78,820 $ (93) U.S. government securities — — — — — $ — Corporate bonds 18,373 (22) — — 18,373 (22) Total marketable securities $ 97,193 $ (115) $ — $ — $ 97,193 $ (115) December 31, 2022 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 17,699 $ (27) $ — $ — $ 17,699 $ (27) U.S. government securities 3,735 (1) — — 3,735 $ (1) Corporate bonds 3,732 (6) — — 3,732 (6) Total marketable securities $ 25,166 $ (34) $ — $ — $ 25,166 $ (34) The Company evaluated its securities for credit losses and considered the decline in market value to be primarily attributable to current economic and market conditions and not to a credit loss or other factors. Additionally, the Company does not intend to sell the securities in an unrealized loss position and does not expect it will be required to sell the securities before recovery of the unamortized cost basis. As of September 30, 2023 and December 31, 2022, an allowance for credit losses had not been recognized. Given the Company's intent and ability to hold such securities until recovery, and the lack of significant change in credit risk of these investments, the Company does not consider these marketable securities to be impaired as of September 30, 2023 and December 31, 2022. The financial instruments that potentially subject the Company to a concentration of credit risk consist principally of cash deposits. Accounts at each of our three U.S. banking institutions are insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250,000 per depositor. At September 30, 2023 and December 31, 2022, the Company had $16.9 million and $23.5 million, respectively, of U.S. cash deposits in excess of the FDIC insured limit. Uninsured foreign cash deposits were immaterial for both periods. There were no realized gains or losses on marketable securities for the three and nine months ended September 30, 2023 and 2022. Interest on marketable securities is included in interest income. Accrued interest receivable on available-for-sale debt securities at September 30, 2023 and December 31, 2022 was $0.4 million and $0.1 million, respectively. The following table summarizes the contractual maturities of the Company’s marketable securities at estimated fair value (in thousands): September 30, December 31, Due in one year or less $ 102,518 $ 20,848 Due thereafter 10,489 — Total marketable securities $ 113,007 $ 20,848 The Company may sell investments at any time for use in current operations even if they have not yet reached maturity. As a result, the Company classifies marketable securities, including securities with maturities beyond twelve months as current assets.

Accrued and Other Current Liabi

Accrued and Other Current Liabilities	9 Months Ended
	Sep. 30, 2023
Payables and Accruals [Abstract]
Accrued and Other Current Liabilities	Accrued and Other Current Liabilities Accrued and other current liabilities consist of the following (in thousands): September 30, December 31, Accrued compensation $ 5,368 $ 4,589 Accrued contracted research and development costs 6,669 6,972 Accrued professional and consulting fees 3,484 946 Accrued other 340 330 Total accrued and other current liabilities $ 15,861 $ 12,837

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2023
Related Party Transactions [Abstract]
Related Party Transactions	6. Related Party Transactions Paragon and Parapyre Holding LLC each beneficially owns less than 5% of the Company's capital stock through their respective holdings of the Company's common stock and Series A Preferred Stock. Fairmount Funds Management LLC ("Fairmount") beneficially owns more than 5% of the Company's capital stock, has two seats on the Board and beneficially owns more than 5% of Paragon, which is a joint venture between Fairmount and Fair Journey Biologics. Fairmount appointed Paragon's board of directors and has the contractual right to approve the appointment of any executive officers. Parapyre is an entity formed by Paragon as a vehicle to hold equity in Spyre in order to share profits with certain employees of Paragon. In connection with the Asset Acquisition, the Company assumed the rights and obligations of Spyre under the Paragon Agreement. Under the Paragon Agreement, Spyre is obligated to compensate Paragon for its services performed under each research program based on the actual costs incurred with mark-up costs pursuant to the terms of the Paragon Agreement. As of the date of the Asset Acquisition, Spyre had incurred total expenses of $19.0 million under the Paragon Agreement since inception, inclusive of a $3.0 million research initiation fee that was due upon signing of the Paragon Agreement and $16.0 million of reimbursable expenses under the Paragon Agreement for historical costs owed to Paragon. As of the acquisition date, $19.0 million was unpaid and was assumed by the Company through the Asset Acquisition. For the three and nine months ended September 30, 2023, the Company recognized expenses related to services provided by Paragon subsequent to the Asset Acquisition totaling $19.4 million and $20.8 million, respectively, which were recorded as Research and development expenses in the consolidated statements of operations. As of September 30, 2023, $16.8 million was unpaid and was included in Related party payable on the Company's consolidated balance sheets. For the three and nine months ended September 30, 2023, the Company made payments totaling $20.0 million to Paragon. In July 2023, the Company exercised its option for the SPY001 program with the remaining three options for the SPY002, SPY003, SPY004 programs remaining outstanding. Following the execution of the license agreement with respect to the SPY001 program (the "SPY001 License Agreement"), the Company will be obligated to pay Paragon up to $22.0 million upon the achievement of specific development and clinical milestones for the first product under the SPY001 License Agreement that achieves such specified milestones. The following is the summary of expenses related to the Paragon Agreement, which were ultimately settled in cash (in millions): Three Months Ended Nine Months Ended Financial Statement Line Item 2023 2022 2023 2022 Reimbursable costs under the Paragon Agreement $ 16.7 $ — $ 17.9 $ — Research and development Parapyre Option Obligation The Paragon Agreement provided for an annual equity grant of options to purchase 1% of the then outstanding shares of Spyre’s common stock, on a fully diluted basis, on the last business day of each calendar year, at the fair market value determined by the board of directors of Spyre (the “Parapyre Option Obligation”). In connection with the Asset Acquisition, the Company assumed the rights and obligations of Spyre under the Paragon Agreement, including the Parapyre Option Obligation. As a result, the Parapyre Option Obligation shall continue and Parapyre shall be entitled to receive the equivalent shares of the Company with the same terms. On September 29, 2023, the Company amended the Paragon Agreement to amend and restate certain terms of the option grant pertaining to the Parapyre Option Obligation, including but not limited to (i) defining that the annual equity grant of options is based on the outstanding shares of Aeglea’s common stock, (ii) establishing the grant date as the last business day of each applicable calendar year, and (iii) defining the term of the options granted as ten years. See Notes 3 and 10 for disclosures related to the Parapyre Option Obligation. For the three and nine months ended September 30, 2023, $2.7 million and $2.9 million, respectively, was recognized as stock compensation expense related to the Parapyre Option Obligation. The following is the summary of Related party accounts payable (in millions): September 30, December 31, Reimbursable costs under the Paragon Agreement $ 16.8 $ — Parapyre Option Obligation liability 3.0 — Total related party accounts payable $ 19.8 $ —

Asset Acquisition

Asset Acquisition	9 Months Ended
	Sep. 30, 2023
Business Combination and Asset Acquisition [Abstract]
Asset Acquisition	Asset Acquisition On June 22, 2023, the Company acquired Spyre pursuant to the Acquisition Agreement, by and among the Company, Aspen Merger Sub I, Inc., a Delaware corporation and a wholly owned subsidiary of the Company (“First Merger Sub”), Sequoia Merger Sub II, LLC, a Delaware limited liability company and wholly owned subsidiary of the Company (“Second Merger Sub”), and Spyre. Pursuant to the Acquisition Agreement, First Merger Sub merged with and into Spyre, pursuant to which Spyre was the surviving corporation and became a wholly owned subsidiary of the Company (the “First Merger”). Immediately following the First Merger, Spyre merged with and into Second Merger Sub, pursuant to which Second Merger Sub became the surviving entity. Spyre was a pre-clinical stage biotechnology company that was incorporated on April 28, 2023 under the direction of Peter Harwin, a Managing Member of Fairmount, for the purpose of holding rights to certain intellectual property being developed by Paragon. Fairmount is a founder of Paragon. The Company completed the Asset Acquisition of Spyre, in accordance with the terms of the Acquisition Agreement. Under the terms of the Acquisition Agreement, the Company issued 517,809 shares of common stock and 364,887 shares of Series A Preferred Stock to former Spyre security holders. In addition, outstanding and unexercised stock options to purchase 2,734 shares of common stock were assumed from the Amended and Restated Spyre 2023 Equity Incentive Plan. At the acquisition date, the Company recorded forward contracts to represent the obligation to issue shares of the Company's common stock and shares of Series A Preferred Stock, respectively. 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 stock has a cash redemption feature. On July 7, 2023, both the shares of common stock and Series A Preferred Stock were issued and the forward contract liability associated with the Series A Preferred Stock was settled accordingly. The Company concluded that the arrangement meets the definition of an asset acquisition rather than a business combination, as substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset, Spyre's option (the "Option") to exclusively license IPR&D. The Company determined that the Option to license IPR&D was a single asset as the Company's strategy relies on developing the entire portfolio of individual treatments to create combination treatments that simultaneously address different mechanisms of irritable bowel disease with a single treatment. The Company also determined that the pipeline candidates within the portfolio are similar in nature and risk profile. In addition, the Company did not obtain any substantive processes, assembled workforce, or employees capable of producing outputs in connection with the Asset Acquisition. The Company determined that the cost to acquire the asset was $113.2 million which was recorded as acquired IPR&D. The fair value of the consideration issued consisted of the 364,887 shares of Series A Preferred Stock (14,595,480 shares of common stock on an as-converted basis) and 517,809 shares of common stock, valued at $291.08 per share and $7.277 per share, respectively. The Asset Acquisition Costs are shown on the following table (in millions): June 22, Consideration transferred in Series A Preferred Stock and common stock $ 110.0 Transaction costs incurred by Aeglea 3.2 Total cost to acquire asset $ 113.2 The allocation of the purchase price to net assets acquired is as a follows: June 22, Acquired in-process research and development $ 130.2 Cash acquired 3.0 Assumed liabilities (20.0) Total cost to acquire asset $ 113.2

Paragon Agreement

Paragon Agreement	9 Months Ended
	Sep. 30, 2023
Collaborative Arrangement [Abstract]
Paragon Agreement	Paragon Agreement In May 2023, Spyre entered into the Paragon Agreement with Paragon and Parapyre. In consideration for the Option granted under the Paragon Agreement, Spyre was obligated to pay Paragon an upfront cash amount of $3.0 million in research initiation fees. In addition, Spyre was obligated to compensate Paragon on a quarterly basis for its services performed under each research program based on the actual costs incurred with mark-up costs pursuant to the terms of the Paragon Agreement. As of the date of the Asset Acquisition, Spyre had incurred total expenses of $19.0 million under the Paragon Agreement since inception, which included the $3.0 million research initiation fee and $16.0 million of historical reimbursable expenses owed to Paragon. As of June 22, 2023, $19.0 million was unpaid and was assumed by the Company through the Asset Acquisition. Furthermore, the Paragon Agreement provided for an annual equity grant of options to purchase 1% of the then outstanding shares of Spyre’s common stock, on a fully diluted basis, on the last business day of each calendar year, during the term of the Paragon Agreement, at the fair market value determined by the board of directors of Spyre. As a result of the Asset Acquisition, the Company assumed the rights and obligations of Spyre under the Paragon Agreement, including the Parapyre Option Obligation. Pursuant to the Paragon Agreement, on a research program-by-research program basis following the finalization of the research plan for each respective research program, the Company is required to pay Paragon a nonrefundable fee in cash of $0.8 million. For the three and nine months ended September 30, 2023, the Company incurred $19.4 million and $20.8 million, respectively, in costs reimbursable to Paragon, which were recorded as Research and development expenses in the consolidated statements of operations. For the three and nine months ended September 30, 2023, the Company made payments totaling $20.0 million to Paragon. On July 12, 2023, the Company exercised its Option available under the Paragon Agreement with respect to the SPY001 research program and expects to enter into a SPY001 license agreement (the “SPY001 License Agreement”). The Company's Option available under the Paragon Agreement with respect to the SPY002, SPY003 and SPY004 programs remains unexercised. Following the execution of the SPY001 License Agreement, the Company will be obligated to pay Paragon up to $22.0 million upon the achievement of specific development and clinical milestones for the first product under the SPY001 License Agreement that achieves such specified milestones. Upon execution of the SPY001 License Agreement, the Company expects to pay Paragon a $1.5 million fee for nomination of a development candidate, and the Company expects to be obligated to make a further milestone payment of $2.5 million upon the first dosing of a human patient in a Phase 1 trial. Subject to the execution of the Option with respect to the SPY002, SPY003 or SPY004 research programs, the Company expects to be obligated to make similar payments upon and following the execution of license agreements with respect to the these research programs, respectively.

Series A Non-Voting Convertible

Series A Non-Voting Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2023
Equity [Abstract]
Series A Non-Voting Convertible Preferred Stock	Series A Non-Voting Convertible Preferred Stock On June 22, 2023, the Company filed a Certificate of Designation of Preferences, Rights and Limitations of the Series A Preferred Stock with the Secretary of State of the State of Delaware (the “Certificate of Designation”) in connection with the Asset Acquisition and the PIPE. Pursuant to the Certificate of Designation, holders of Series A Preferred Stock are entitled to receive dividends on shares of Series A Preferred Stock equal to, on an as-if-converted-to-common-stock basis, and in the same form as, dividends actually paid on shares of the Company's common stock. Except as provided in the Certificate of Designation or as otherwise required by law, the Series A Preferred Stock does not have voting rights. However, as long as any shares of Series A Preferred Stock are outstanding, the Company will not, without the affirmative vote of the holders of a majority of the then outstanding shares of the Series A Preferred Stock: (a) alter or change adversely the powers, preferences or rights given to the Series A Preferred Stock, or alter or amend the Certificate of Designation, amend or repeal any provision of, or add any provision to, the Company’s Certificate of Incorporation or its Bylaws, or file any articles of amendment, certificate of designations, preferences, limitations and relative rights of any series of Preferred Stock, if such action would adversely alter or change the preferences, rights, privileges or powers of, or restrictions provided for the benefit of the Series A Preferred Stock, regardless of whether any of the foregoing actions will be by means of amendment to the Certificate of Incorporation or by merger, consolidation, recapitalization, reclassification, conversion or otherwise, (b) issue further shares of Series A Preferred Stock or increase or decrease (other than by conversion) the number of authorized shares of Series A Preferred Stock, (c) prior to the stockholder approval of the conversion of the Series A Preferred Stock into shares of Common Stock in accordance with Nasdaq Stock Market Rules (the “Conversion Proposal”) or at any time while at least 30% of the originally issued Series A Preferred Stock remains issued and outstanding, consummate (x) any Fundamental Transaction (as defined in the Certificate of Designation) or (y) any merger or consolidation of the Company with or into another entity or any stock sale to, or other business combination in which our stockholders immediately before such transaction do not hold at least a majority of our capital stock immediately after such transaction or (d) enter into any agreement with respect to any of the foregoing. The Series A Preferred Stock does not have a preference upon any liquidation, dissolution or winding-up of the Company. The Company has agreed to hold a stockholders’ meeting to submit the following matters to its stockholders for their consideration: (i) the approval of the Conversion Proposal, and (ii) if deemed necessary or appropriate by the Company or as otherwise required by law or contract, the approval of an amendment to the Certificate of Incorporation to authorize sufficient shares of common stock for the conversion of the Series A Preferred Stock issued pursuant to the Acquisition Agreement. In connection with these matters, the Company filed with the SEC a definitive proxy statement and other relevant materials. The stockholder meeting has not occurred as of September 30, 2023. The Series A Preferred Stock is recorded outside of stockholders’ (deficit) equity because, if conversion to common stock is not approved by the stockholders, the Series A Preferred Stock will be redeemable at the option of the holders for cash equal to the closing price of the common stock per share of common stock underlying the Series A Preferred Stock, on the last trading day prior to the holder’s redemption request. As of September 30, 2023, the redemption value of the Company's outstanding Series A Preferred Stock was $532.3 million based on the closing stock price of the Company's common stock on September 30, 2023 of $12.25 per share. The Company has determined that the conversion and redemption features of the Series A Preferred Stock do not require bifurcation as derivatives. Following stockholder approval of the Conversion Proposal, each share of Series A Preferred Stock will automatically convert into 40 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 (established by the holder between 0% and 19.99%) of the total number of shares of common stock issued and outstanding immediately after giving effect to such conversion. On June 26, 2023, the Company completed a private placement of 721,452 shares of Series A Preferred Stock in exchange for gross proceeds of $210.0 million, or net proceeds of $197.3 million, after deducting placement agent and other offering costs. On July 7, 2023, the Company issued 364,887 shares of Series A Preferred Stock as part of its consideration transferred in connection with the Asset Acquisition that closed on June 22, 2023 which settled the related forward contract liability. For additional information, see Note 3.

Liabilities, Convertible Prefer

Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity	9 Months Ended
	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity	Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity Registered Direct Offering In May 2022, the Company issued and sold 430,107 shares of common stock at an offering price of $40.00 per share and pre-funded warrants to purchase up to 694,892 shares of common stock at an offering price of $39.9975 per warrant (representing the price per share of common stock sold in the offering minus the $0.0025 exercise price per warrant) in a registered direct offering pursuant to a shelf registration statement on Form S-3. The net proceeds to the Company from this offering were approximately $42.9 million, after deducting placement agent fees and offering costs of $2.1 million. Pre-Funded Warrants In February 2019, April 2020 and May 2022, the Company issued pre-funded warrants to purchase the Company’s common stock in underwritten public offerings at the offering price of the common stock, less the $0.0025 per share exercise price of each warrant. The warrants were recorded as a component of stockholders’ (deficit) equity within additional paid-in capital and have no expiration date. Per the terms of the warrant agreements, the outstanding warrants to purchase shares of common stock may not be exercised if the holder’s ownership of the Company’s common stock would exceed 4.99% (“Maximum Ownership Percentage”), or 9.99% for certain holders. By written notice to the Company, each holder may increase or decrease the Maximum Ownership Percentage to any other percentage (not in excess of 19.99% for the majority of such warrants). The revised Maximum Ownership Percentage would be effective 61 days after the notice is received by the Company. As of September 30, 2023, the following pre-funded warrants for common stock were issued and outstanding: Issue Date Expiration Date Exercise Price Number of Warrants Outstanding February 8, 2019 None $ 0.0025 — April 30, 2020 None $ 0.0025 — May 20, 2022 None $ 0.0025 250,000 Total pre-funded warrants 250,000 Stock-Based Compensation 2016 Equity Incentive Plan The 2016 Equity Incentive Plan (“2016 Plan”) provides for an automatic increase in the number of shares reserved for issuance thereunder on January 1 of each year for the remaining term of the plan (through 2028) equal to (a) 4.0% of the number of issued and outstanding shares of common stock on December 31 of the immediately preceding year, or (b) a lesser amount as approved by the Board each year. As a result of this provision, on January 1, 2023 and January 1, 2022, an additional 104,560 and 78,968 shares, respectively, became available for issuance under the 2016 Plan. In July 2020, the Company granted 9,128 restricted stock units ("RSUs") to certain employees, with vesting terms subject to regulatory, commercial, and clinical milestones, in addition to a service condition. As of September 30, 2023, none of these RSUs had vested and all RSUs were forfeited since the performance milestones were not met within the required time frame. No stock-based compensation expense was recognized on these awards. On June 22, 2023, concurrent with the closing of the Asset Acquisition, the Board approved an amendment of the 2016 Plan to eliminate the per-participant annual award limits originally intended to comply with the qualified performance-based compensation exception set forth in Section 162(m) of the Internal Revenue Code, in light of the repeal of such exception pursuant to the Tax Cuts and Jobs Act of 2017. In addition, the Company approved 2,720,033 options contingent on stockholder approval to certain members of the Board, legacy Aeglea employees and consultants under the 2016 Plan. These awards are in excess of the shares available for issuance under the 2016 Plan and require stockholder approval before being granted. Accordingly, no expense has been recognized on these contingent awards since they are contingent on stockholder approval. As of September 30, 2023, the 2016 Plan had 293,497 shares available for future issuance. 2018 Equity Inducement Plan During the nine months ended September 30, 2023, the Board approved an increase of 2,800,000 in the number of shares reserved for issuance to the 2018 Equity Inducement Plan and granted 3,583,880 inducement awards to new hires. The grant-date fair value of these inducement awards will be recognized as expense on a pro-rata basis over the vesting period. The awards have an exercise price equal to the grant date closing price of the Company's common stock, vest ratably over four years, and have a ten-year exercise period from the grant date. Spyre 2023 Equity Incentive Plan On June 22, 2023, in connection with the Asset Acquisition, the Company assumed the Amended and Restated Spyre 2023 Equity Incentive Plan (the "Spyre Equity Plan") and its outstanding and unexercised stock options, which were converted to options to purchase 2,734 shares of Aeglea common stock. The acquisition-date fair value of these grants will be recognized as an expense on a pro-rata basis over the vesting period. Parapyre Option Obligation On June 22, 2023, in connection with the Asset Acquisition, the Company assumed the Parapyre Option Obligation which provided for an annual equity grant of options to purchase 1% of the then outstanding shares of Spyre's common stock, on a fully diluted basis, on the last business day of each calendar year during the term of the Paragon Agreement, at the fair market value determined by the board of directors of Spyre. As a result of the Asset Acquisition the Parapyre Option Obligation shall continue and Parapyre shall be entitled to receive the equivalent shares of the Company with the same terms. As of September 30, 2023, the pro-rated estimated fair value of the options to be granted on December 31, 2023, was approximately $3.0 million, of which $0.1 million was recognized as part of the liabilities assumed with the Asset Acquisition. For the three and nine months ended September 30, 2023, $2.7 million and $2.9 million, respectively, was recognized as stock compensation expense related to the Parapyre Option Obligation. As of September 30, 2023, the unamortized expense related to the Parapyre Option Obligation was $2.1 million. The following table summarizes the Company’s stock awards granted under all plans for each of the periods indicated: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Grants Weighted Average Grant Date Fair Value Grants Weighted Average Grant Date Fair Value Grants Weighted Average Grant Date Fair Value Grants Weighted Average Grant Date Fair Value Stock options 1,044,667 $ 14.50 50,806 $ 16.75 3,867,366 $ 9.65 153,686 $ 52.50 2016 Employee Stock Purchase Plan Under the Company’s 2016 Employee Stock Purchase Plan (“2016 ESPP”), the Company issued and sold 2,496 shares for aggregate cash proceeds of less than $0.1 million during the nine months ended September 30, 2023. There were 6,073 shares issued and sold under the 2016 ESPP for aggregate cash proceeds of $0.2 million during the nine months ended September 30, 2022. Stock-based Compensation Expense Total stock-based compensation expense related to all plans was as follows (in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Research and development (1) $ 2,965 $ 639 $ 4,136 $ 2,031 General and administrative 1,820 926 4,269 3,653 Total stock-based compensation expense $ 4,785 $ 1,565 $ 8,405 $ 5,684 (1) For the three and nine months ended September 30, 2023, $2.7 million and $2.9 million, respectively, was recognized as stock compensation expense related to the Parapyre Option Obligation. There were no such expenses for the three and nine months ended September 30, 2022. The following table summarizes the weighted-average Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted under the Company’s 2016 Plan, and the shares purchasable under the 2016 ESPP during the periods presented: Three Months Ended Nine Months Ended 2023 2022 2023 2022 2016 Plan Expected term (in years) 6.08 6.02 6.04 5.96 Expected volatility 101% 85% 111% 83% Risk-free interest 4.28% 3.16% 4.07% 2.43% Dividend yield — — — — 2016 ESPP Expected term (in years) 0.50 0.50 0.49 0.49 Expected volatility 222% 95% 181% 84% Risk-free interest 5.29 3.26 4.99% 1.95% Dividend yield — — — —

Strategic License Agreements

Strategic License Agreements	9 Months Ended
	Sep. 30, 2023
License And Collaboration Agreement [Abstract]
Strategic License Agreements	Strategic License Agreements On March 21, 2021, the Company entered into an exclusive license and supply agreement with Immedica (the "Immedica Agreement"). By entering into this agreement, the Company agreed to provide Immedica the following goods and services: i. Deliver an exclusive, sublicensable, license and know-how (the “License”) to develop and commercialize pegzilarginase (the “Product”) in the territory comprising the members states of the European Economic Area ("EEA"), United Kingdom ("UK"), Switzerland, Andorra, Monaco, San Marino, Vatican City, Turkey, Saudi Arabia, United Arab Emirates, Qatar, Kuwait, Bahrain, and Oman (the “Territory”); ii. Complete the global pivotal PEACE (Pegzilarginase Effect on Arginase 1 Deficiency Clinical Endpoints) Phase 3 trial (“PEACE Trial”) and related Biologics License Application (“BLA”) package to file with the FDA, which will be leveraged by Immedica in obtaining the necessary regulatory approvals in the Territory; and iii. Perform a Pediatric Investigation Plan trial (“PIP Trial”) in order for Immedica to be able to receive certain regulatory approvals within the Territory. In addition, the Company and Immedica formed a Joint Steering Committee (“JSC”) to provide oversight to the activities performed under the agreement; however, the substance of the Company’s participation in the JSC did not represent an additional promised service, but rather, a right of the Company to protect its own interests in the arrangement. Further, the Company agreed to supply to Immedica, and Immedica agreed to purchase from the Company, substantially all commercial requirements of the Product. The terms of the agreement did not provide for either (i) an option to Immedica to purchase the Product from the Company at a discount from the standalone selling price or (ii) minimum purchase quantities. Immedica was expected to bear (i) all costs and expenses for any development or commercialization of the Product in the Territory subject to the License exclusive of the Company’s promised goods and services summarized above and (ii) all costs and fees associated with applying for regulatory approval of the Product in the Territory. The Company received a non-refundable payment of $21.5 million and Immedica agreed to provide payment of 50% of the Company’s costs incurred in performing the PIP Trial up to a maximum of $1.8 million. In addition, the Company had the ability to receive regulatory and commercial milestone payments. The Company was also entitled to receive royalties in the mid-20% range on net sales of the Product in the Territory. In July 2021, the Company modified the agreement with Immedica to provide certain additional services in relation to the PEACE Trial and BLA package performance obligation in exchange for the reimbursement of up to $3.0 million of the actual costs incurred in relation to such incremental services. The Company concluded that Immedica met the definition to be accounted for as a customer because the Company was delivering intellectual property and other services within the Company’s normal course of business, in which the parties are not jointly sharing the risks and rewards. Therefore, the Company concluded that the promises summarized above represent transactions with a customer within the scope of ASC 606. The Company determined that the following promises represent distinct promised services, and therefore, performance obligations: (i) the License, (ii) the PEACE Trial and BLA package, and (iii) the PIP Trial. Specifically, in making these determinations, the Company considered the following factors: – As of inception of the agreement, the Company had completed the Phase 1/2 clinical trial related to the Product and was conducting the PEACE Trial. Accordingly, the Company was not promising, nor expecting, to perform additional research and development activities pursuant to the agreement that would either significantly modify, customize or be considered highly interdependent or interrelated with pegzilarginase. – The License represented functional intellectual property given the functionality of the License was not expected to change substantially as a result of the company’s ongoing activities. – The services necessary to complete the PEACE Trial, BLA package and PIP Trial could have been performed by other parties. Given that Immedica was not obligated to purchase any minimum amount or quantities of Product, the supply of Product for commercial use to Immedica was determined to be an option for Immedica, rather than a performance obligation of the Company at contract inception and will be accounted for if and when exercised. The Company also determined that Immedica’s option to purchase the Product did not create a material right as the expected pricing is not at a discount. The Company determined that the upfront fixed payment amount of $21.5 million should be included in the transaction price. Additionally, the Company determined at inception of the arrangement that 50% of the probable estimated costs to be incurred in relation to the PIP Trial exceeded $1.8 million and included the full reimbursement amount of $1.8 million in the transaction price. Upon subsequent re-evaluation due to changing facts and circumstances, the Company determined the probable estimated costs were less than the maximum allowable reimbursement and a portion of the variable consideration was constrained, which did not materially impact the revenue recognized as of September 30, 2023. Additionally, upon the modification of the agreement in July 2021, the Company determined that the probable estimated costs to perform the additional services related to the PEACE Trial and BLA package exceeded the maximum allowable reimbursement of $3.0 million. Therefore, the Company included an estimated total of $3.6 million that was due in relation to the PIP Trial, PEACE Trial, and BLA package in the transaction price and concluded that it is probable that a significant reversal will not occur in the future. In total, the modified transaction price was determined to be $25.1 million. The Company allocated $9.6 million and $3.5 million of the modified transaction price to the PEACE Trial and BLA package and PIP Trial performance obligations, respectively, based on the stand-alone selling prices ("SSP"), which were based on the estimated costs that a third-party would charge in performing such services on a stand-alone basis. The SSP for the License was established at inception of the arrangement using a residual value approach due to the uniqueness of and lack of observable data related to the License, and without a specific analog from which to make reliable estimates, resulting in an allocation of $12.0 million. The potential regulatory milestone payments that the Company was eligible to receive were excluded from the transaction price, as the milestone amounts were fully constrained based on the probability of achievement, since the milestones related to successful achievement of certain regulatory approvals, which might not have been achieved. The Company determined that the royalties and commercial milestone payments related predominantly to the license of intellectual property and therefore should be excluded from the transaction price under the sales- or usage-based royalty exception under ASC 606. The Company intends to reevaluate the transaction price, including all constrained amounts, at the end of each reporting period and as uncertain events are resolved or other changes in circumstances occur, the Company intends to adjust its estimate of the transaction price as necessary. The Company recognized the royalties and commercial milestone payments as revenue when the associated sales occurred, and relevant sales-based thresholds were met. The Company also assessed the arrangement with Immedica and concluded that a significant financing component does not exist. The Company recognized revenue allocated to the License performance obligation at a point in time and upon transfer of the License. The Company completed the transfer of the know-how necessary for Immedica to benefit from the License in September 2021 and recognized $12.0 million of revenue at that time. The development fee allocated to the PEACE Trial, BLA package and PIP Trial performance obligations is recognized over time using an input method of costs incurred related to the performance obligations. The Company recognized revenue of $0.9 million under the Immedica Agreement for the nine months ended September 30, 2023. There was no such revenue for the three months ended September 30, 2023. The total revenue generated in the nine months ended September 30, 2023 was attributable to the PEACE Phase 3 and PIP trials, drug supply, and royalties from an early access program in France. For the three and nine months ended September 30, 2022, the Company recognized revenue of $0.2 million and $2.2 million, respectively, related to the PEACE Trial and BLA package performance obligation. As of December 31, 2022, the Company recorded deferred revenue of $3.6 million associated with the Immedica Agreement, of which $2.4 million was classified as current. There was no such revenue associated with the Immedica Agreement as of September 30, 2023. On July 27, 2023, the Company announced that it had entered into an agreement to sell the global rights to pegzilarginase to Immedica and concurrently terminated the Immedica Agreement. Remaining deferred revenue was recognized as part of the gain on disposal of the assets. Contract Balances from Customer Contract The timing of revenue recognition, billings and cash collections results in contract assets and contract liabilities on the Company's balance sheets. The Company recognizes license and development receivables based on billed services, which are derecognized upon reimbursement. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded. Contract liabilities are recognized as revenue after control of the goods or services is transferred to the customer and all revenue recognition criteria have been met. The following table presents changes in the Company’s contract liabilities for the periods presented (in thousands): Nine Months Ended September 30, 2023 December 31, Additions Deductions September 30, Contract liabilities: Deferred revenue $ 2,696 $ 575 $ (3,271) $ — The Company had no contract assets during the nine months ended September 30, 2023 and 2022.

Sale of Pegzilarginase to Immed

Sale of Pegzilarginase to Immedica	9 Months Ended
	Sep. 30, 2023
Discontinued Operations and Disposal Groups [Abstract]
Sale of Pegzilarginase to Immedica	Sale of Pegzilarginase to ImmedicaOn July 27, 2023, the Company announced that it had entered into an agreement to sell the global rights to pegzilarginase, an investigational treatment for the rare metabolic disease Arginase 1 Deficiency, to Immedica for $15.0 million in upfront cash proceeds and up to $100.0 million in contingent milestone payments. The sale of pegzilarginase to Immedica superseded and terminated the previous license agreement between the Company and Immedica. On July 27, 2023, the carrying value of the asset was zero as it was internally developed. Accordingly the Company recognized a $14.6 million gain within operating expenses, which is the full $15.0 million in upfront cash proceeds, net of transaction costs and the derecognition of pegzilarginase related nonfinancial assets and liabilities. The milestone payments are contingent on formal reimbursement decisions by national authorities in key European markets and pegzilarginase approval by the FDA, among other events. The upfront payment and contingent milestone payments if paid, net of expenses and adjustments, will be distributed to holders of Aeglea’s CVR pursuant to the CVR Agreement resulting from the Asset Acquisition.

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2023
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share The Company computes net loss attributable per common stockholder using the two-class method required for participating securities. The Company considers convertible. preferred stock to be participating securities. In the event that the Company paid out distributions, holders of convertible preferred stock would participate in the distribution. The two-class method is an earnings (loss) allocation method under which earnings (loss) per share is calculated for common stock and participating security considering a participating security’s rights to undistributed earnings (loss) as if all such earnings (loss) had been distributed during the period. The holders of Series A Preferred Stock do not have an obligation to fund losses and therefore the Series A Preferred Stock was excluded from the calculation of basic net loss per share. The Company included in the calculation of basic net loss per share, contingently issuable common shares related to the Asset Acquisition because they will be issued for no consideration due to the consideration already having been satisfied as of September 30, 2023. Basic and diluted net loss per share is computed by dividing the net loss by the weighted-average number of common stock and pre-funded warrants outstanding during the period, without consideration of potential dilutive securities. The pre-funded warrants are included in the computation of basic net loss per share as the exercise price is negligible and they are fully vested and exercisable. For periods in which the Company generated a net loss, the Company does not include the potential impact of dilutive securities in diluted net loss per share, as the impact of these items is anti-dilutive. The Company has generated a net loss for all periods presented, therefore diluted net loss per share is the same as basic net loss per share since the inclusion of potentially dilutive securities would be anti-dilutive. The following weighted-average equity instruments were excluded from the calculation of diluted net loss per share because their effect would have been anti-dilutive for the periods presented: Three Months Ended Nine Months Ended 2023 2022 2023 2022 Options to purchase common stock 3,135,672 351,533 1,426,224 335,395 Unvested restricted stock units — 6,000 252 7,315 Series A Preferred Stock (on an as-converted basis) 42,501,681 — 14,851,447 —

Restructuring Charges

Restructuring Charges	9 Months Ended
	Sep. 30, 2023
Restructuring and Related Activities [Abstract]
Restructuring Charges	Restructuring Charges Severance and Stock Compensation On April 12, 2023, based on the review of the inconclusive interim results from the Company's Phase 1/2 clinical trial of pegtarviliase for the treatment of classical homocystinuria and other business considerations, the Company announced that it had initiated a process to explore strategic alternatives to maximize stockholder value and engaged an independent exclusive financial advisor to support this process. As a result, the Company implemented a restructuring plan resulting in an approximate 83% reduction of the Company’s existing headcount by June 30, 2023. The Company recognized restructuring expenses consisting of cash severance payments and other employee-related costs of nil and $6.4 million during the three and nine months ended September 30, 2023, respectively. Cash payments for employee related restructuring charges of $4.5 million were paid as of September 30, 2023. In addition, the Company recognized $1.0 million in non-cash stock-based compensation expense related to the accelerated vesting of stock-based awards for certain employees. The Company recorded these restructuring charges based on each employee’s role to the respective research and development and general and administrative operating expense categories on its condensed consolidated statements of operations and comprehensive loss. The following table summarizes the changes in the Company's accrued restructuring balance (in thousands): Beginning Balance Charges Payments Ending Balance Severance liability $ — $ 6,448 $ (4,527) $ 1,921 Sale of Assets During the second quarter of 2023, the Company sold various lab equipment, consumables, and furniture and fixtures for total consideration of $0.5 million. After recording the disposal of all property and equipment net of proceeds, the Company recorded a $0.7 million and $0.2 million loss on disposal of long lived assets which is included in Research and development and General and administrative expenses, respectively. Lease Right-of-use Asset and Leasehold Improvement Impairment Effective June 30, 2023, the Company abandoned its leased office space in Austin, Texas. As a result, the Company recognized an impairment loss of $0.9 million related to the operating lease right-of-use asset and $1.7 million related to leasehold improvements. On August 7, 2023, the Company terminated its building lease in Austin, Texas. The negotiated termination agreement obligated the Company to pay the lessor a $2.0 million termination fee in exchange for releasing the Company of all further obligations under the lease. All charges related to the restructuring activities were recognized during the second quarter of 2023. No further restructuring charges will be incurred under the restructuring plan. A summary of the charges related to the restructuring activities is as follows (in thousands): Severance Related Expenses Stock Compensation Expenses Loss on Disposal of Long Lived Assets Lease Asset Impairment Total Restructuring Costs Research and development $ 3,182 $ 123 $ 749 $ 1,405 $ 5,459 General and administrative 3,266 870 182 1,175 5,493 Total $ 6,448 $ 993 $ 931 $ 2,580 $ 10,952

Novation of Manufacturing Agree

Novation of Manufacturing Agreements	9 Months Ended
	Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Novation of Manufacturing Agreements	Novation of Manufacturing Agreements Pursuant to a Novation Agreement dated September 19, 2023 (the “Novation Agreement”), by and between the Company, Paragon and WuXi Biologics (Hong Kong) Limited (“WuXi Biologics”), the Company novated (i) a Biologics Master Services Agreement (the “WuXi Biologics MSA”) and (ii) a Cell Line License Agreement (the “Cell Line License Agreement”). Biologics Master Services Agreement In April 2023, Paragon and WuXi Biologics entered into the WuXi Biologics MSA, which was subsequently novated to the Company by Paragon on September 19, 2023 pursuant to the Novation Agreement. The WuXi Biologics MSA governs certain development activities and GMP manufacturing and testing for the SPY001 program, as well as potential future programs, on a work order basis. Under the WuXi Biologics MSA, the Company is obligated to pay WuXi Biologics a service fee and all non-cancellable obligations in the amount specified in each work order associated with the agreement for the provision of services. The WuXi Biologics MSA terminates on the later of (i) June 20, 2027 or (ii) the completion of services under all work orders executed by the parties prior to June 20, 2027, unless terminated earlier. The term of each work order terminates upon completion of the services under such work order, unless terminated earlier. The Company can terminate the WuXi Biologics MSA or any work order at any time upon 30 days' prior written notice and immediately upon written notice if WuXi Biologics fails to obtain or maintain required material governmental licenses or approvals. Either party may terminate a work order (i) at any time upon six months’ prior notice with reasonable cause, provided however that if WuXi Biologics terminates a work order in such manner, no termination or cancellation fees shall be paid by the Company and (ii) immediately for cause upon (a) the other party’s material breach that remains uncured for 30 days after notice of such breach, (b) the other party’s bankruptcy or (c) a force majeure event that prevents performance for a period of at least 90 days. Cell Line License Agreement In April 2023, Paragon and WuXi Biologics entered into the Cell Line License Agreement, which was subsequently novated to the Company by Paragon pursuant to the Novation Agreement. Under the Cell Line License Agreement, the Company received a non-exclusive, worldwide, sublicensable license to certain of WuXi Biologics’s know-how, cell line, biological materials (the “WuXi Biologics Licensed Technology”) and media and feeds to make, have made, use, sell and import certain therapeutic products produced through the use of the cell line licensed by WuXi Biologics under the Cell Line License Agreement (the “WuXi Biologics Licensed Products”). Specifically, the WuXi Biologics Licensed Technology is used in certain manufacturing activities in support of the SPY001 program. In consideration for the license, the Company agreed to pay WuXi Biologics a non-refundable license fee of $0.2 million . Additionally, if the Company manufactures all of its commercial supplies of bulk drug product with a manufacturer other than WuXi Biologics or its affiliates, the Company is required to make royalty payments to WuXi Biologics of less than one percent of global net sales of WuXi Biologics Licensed Products manufactured by a third-party manufacturer (the “Royalty”). If the Company manufactures part of its commercial supplies of the WuXi Biologics Licensed Products with WuXi Biologics or its affiliates, then the Royalty will be reduced accordingly on a pro rata basis. The Cell Line License Agreement will continue indefinitely unless terminated (i) by the Company upon six months’ prior written notice and our payment of all undisputed amounts due to WuXi Biologics through the effective date of termination, (ii) by WuXi Biologics for a material breach by the Company that remains uncured for 60 days after written notice, (iii) by WuXi Biologics if the Company fails to make a payment and such failure continues for 30 days after receiving notice of such failure, or (iv) by either party upon the other party’s bankruptcy.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2023
Accounting Policies [Abstract]
Convertible Preferred Stock Issued through PIPE	Convertible Preferred Stock Issued through PIPE The Company records shares of convertible preferred stock at their respective fair values on the dates of issuance, net of issuance costs. The Company has applied the guidance in ASC 480-10-S99-3A as well as SEC Staff Announcement, Classification and Measurement of Redeemable Securities, and has therefore classified the Series A Preferred Stock outside of stockholders’ (deficit) equity because, if conversion to common stock is not approved by the stockholders, the Series A Preferred Stock will be redeemable at the option of the holders for cash equal to the closing price of the common stock on the last trading day prior to the holder’s redemption request. The Company has determined that the conversion and redemption are outside of the Company’s control. Additionally, the Company has determined the conversion and redemption features do not require bifurcation as derivatives.
Acquisitions	Acquisitions The Company evaluates acquisitions of assets and other similar transactions to assess whether or not the transaction should be accounted for as a business combination or asset acquisition by first applying a screen test to determine whether substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or group of similar identifiable assets. If so, the transaction is accounted for as an asset acquisition. If not, further determination is required as to whether or not the Company has acquired inputs and processes that have the ability to create outputs, which would meet the definition of a business. Significant judgment is required in the application of the test to determine whether an acquisition is a business combination or an acquisition of assets. Acquisitions meeting the definition of business combinations are accounted for using the acquisition method of accounting, which requires that the purchase price be allocated to the net assets acquired at their respective fair values. In a business combination, any excess of the purchase price over the estimated fair values of the net assets acquired is recorded as goodwill. The Company measures and recognizes asset acquisitions that are not deemed to be business combinations based on the cost to acquire the assets, which includes pre-acquisition direct costs recorded in accrued professional and consulting fees. Goodwill is not recognized in asset acquisitions. When a transaction accounted for as an asset acquisition includes an in-process research and development (“IPR&D”) asset, the IPR&D asset is only capitalized if it has an alternative future use other than in a particular research and development project. Otherwise, the cost allocated to acquire an IPR&D asset with no alternative future use is charged to expense at the acquisition date.
Contingent Value Rights	Contingent Value Rights The Company evaluates its contracts to determine if those contracts qualify as derivatives under ASC 815, Derivatives and Hedging (“ASC 815”). For derivative financial instruments that are accounted for as liabilities, the derivative instrument is initially recorded at its fair value and is then re-valued at each reporting date. Any changes in fair value are recorded as other income or expense for each reporting period. Derivative instrument liabilities are classified in the balance sheet as current or non-current based on whether or not net-cash settlement of the derivative instrument is probable within the next 12 months from the balance sheet date. The Company determined that certain contingent payments under the CVR Agreement qualified as derivatives under ASC 815, and as such, were recorded as a liability on the balance sheet. This value is then remeasured for future expected payout as well as the increase in fair value due to the time value of money. These gains or losses, if any, are recognized in the consolidated statements of operations and comprehensive loss within Other (expense) income, net.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the condensed consolidated financial statements and accompanying notes. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying value of assets, liabilities, and equity and the amount of revenues and expenses. Actual results could differ significantly from those estimates. The most significant estimates and assumptions that management considers in the preparation of the Company's financial statements relate to the valuation of consideration transferred in acquiring IPR&D; the discount rate, probabilities of success, and timing of estimated cash flows in the valuation of the CVR liability; inputs used in the Black-Scholes model for stock-based compensation expense; estimated future cash flows used in calculating the impairment of right-of-use lease assets; and estimated cost to complete performance obligations related to revenue recognition. The consideration transferred in acquiring IPR&D in connection with the acquisition of Spyre was comprised of shares of the Company’s common stock and shares of Series A Preferred Stock. To determine the fair value of the equity transferred, the Company considered the per share value of the PIPE, which was an over-subscribed financing event involving a group of accredited investors.
Recently Adopted Accounting Pronouncement	Recently Adopted Accounting Pronouncement The Company early adopted the Financial Accounting Standards Board’s Accounting Standards Update 2020-06, Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (“ASU 2020-06”), effective as of January 1, 2023 using the modified retrospective method. Among other amendments, ASU 2020-06 eliminates the cash conversion and beneficial conversion feature models in ASC 470-20 that required an issuer of certain convertible debt and preferred stock to separately account for embedded conversion features as a component of equity, as well as changes the accounting for diluted earnings‑per‑share for convertible instruments and contracts that may be settled in cash or stock. Additionally, ASU 2020-06 requires the if‑converted method, which is more dilutive than the treasury stock method, be used for all convertible instruments. The Company applied ASU 2020-06 to all Series A Preferred Stock during fiscal year 2023, and, accordingly, the Company did not apply the cash conversion or beneficial conversion feature models in its analysis of the Series A Preferred Stock.
Net Loss Per Share	The Company computes net loss attributable per common stockholder using the two-class method required for participating securities. The Company considers convertible. preferred stock to be participating securities. In the event that the Company paid out distributions, holders of convertible preferred stock would participate in the distribution. The two-class method is an earnings (loss) allocation method under which earnings (loss) per share is calculated for common stock and participating security considering a participating security’s rights to undistributed earnings (loss) as if all such earnings (loss) had been distributed during the period. The holders of Series A Preferred Stock do not have an obligation to fund losses and therefore the Series A Preferred Stock was excluded from the calculation of basic net loss per share. The Company included in the calculation of basic net loss per share, contingently issuable common shares related to the Asset Acquisition because they will be issued for no consideration due to the consideration already having been satisfied as of September 30, 2023.

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	9 Months Ended
	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables set forth the fair value of the Company’s financial assets and liabilities at fair value on a recurring basis based on the three-tier fair value hierarchy (in thousands): September 30, 2023 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 55,451 $ — $ — $ 55,451 Commercial paper — 107,093 — 107,093 Corporate bonds — 22,828 — 22,828 Total financial assets $ 55,451 $ 129,921 $ — $ 185,372 Liabilities: Parapyre Option Obligation $ — $ 2,952 $ — $ 2,952 CVR liability — — 28,200 28,200 Total liabilities $ — $ 2,952 $ 28,200 $ 31,152 December 31, 2022 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 15,250 $ — $ — $ 15,250 Commercial paper — 23,641 — 23,641 U.S. government securities — 4,230 — 4,230 Corporate bonds — 3,732 — 3,732 Total financial assets $ 15,250 $ 31,603 $ — $ 46,853
Significant Inputs used to Estimate the Fair Value of Derivative Liabilities	The significant inputs used to estimate the fair value of the CVR liability were as follows: September 30, 2023 Estimated cash flow dates 11/28/23 - 06/22/26 Estimated probability of success 27% - 100% Risk-adjusted discount rates 7.14% - 7.57%
Changes in Derivative Liabilities	The following table presents changes in the forward contract liability for the periods presented (in millions): Forward Contract Liability Beginning balance as of June 22, 2023 $ 106.2 Change in fair value 58.1 Ending balance as of June 30, 2023 164.3 Change in fair value 25.4 Issuance of Series A Preferred Stock on July 7, 2023 (189.7) Ending balance as of September 30, 2023 $ — The following table presents changes in the CVR liability for the periods presented (in thousands): CVR Liability Beginning balance as of December 31, 2022 $ — Fair value at CVR issuance 29,500 Changes in the fair value of the CVR liability since issuance $ (1,300) Ending Balance as of September 30, 2023 $ 28,200

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2023
Cash and Cash Equivalents [Abstract]
Estimated Fair Value of Cash Equivalents and Marketable Securities and the Gross Unrealized Gains and Losses	The following tables summarize the estimated fair value of the Company’s cash equivalents and marketable securities and the gross unrealized gains and losses (in thousands): September 30, 2023 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash equivalents: Money market funds $ 55,451 $ — $ — $ 55,451 Commercial paper 16,911 3 — 16,914 Total cash equivalents $ 72,362 $ 3 $ — $ 72,365 Marketable securities: Commercial paper $ 90,272 $ — $ (93) $ 90,179 Corporate bonds 22,849 1 (22) 22,828 U.S. government securities — — — — Total marketable securities $ 113,121 $ 1 $ (115) $ 113,007 December 31, 2022 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash equivalents: Money market funds $ 15,250 $ — $ — $ 15,250 Commercial paper 7,021 1 (2) 7,020 U.S. government securities 3,736 — (1) 3,735 Total cash equivalents $ 26,007 $ 1 $ (3) $ 26,005 Marketable securities: Commercial paper $ 16,644 $ 2 $ (25) $ 16,621 Corporate bonds 3,738 — (6) 3,732 U.S. government securities 495 — — 495 Total marketable securities $ 20,877 $ 2 $ (31) $ 20,848
Available-for-Sale Securities in an Unrealized Loss Position	The following table summarizes the available-for-sale securities in an unrealized loss position for which an allowance for credit losses has not been recorded as of September 30, 2023 and December 31, 2022, aggregated by major security type and length of time in a continuous unrealized loss position: September 30, 2023 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 78,820 $ (93) $ — $ — $ 78,820 $ (93) U.S. government securities — — — — — $ — Corporate bonds 18,373 (22) — — 18,373 (22) Total marketable securities $ 97,193 $ (115) $ — $ — $ 97,193 $ (115) December 31, 2022 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 17,699 $ (27) $ — $ — $ 17,699 $ (27) U.S. government securities 3,735 (1) — — 3,735 $ (1) Corporate bonds 3,732 (6) — — 3,732 (6) Total marketable securities $ 25,166 $ (34) $ — $ — $ 25,166 $ (34)
Contractual Maturities of Marketable Securities at Estimated Fair Value	The following table summarizes the contractual maturities of the Company’s marketable securities at estimated fair value (in thousands): September 30, December 31, Due in one year or less $ 102,518 $ 20,848 Due thereafter 10,489 — Total marketable securities $ 113,007 $ 20,848

Accrued and Other Current Lia_2

Accrued and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2023
Payables and Accruals [Abstract]
Accrued and Other Current Liabilities	Accrued and other current liabilities consist of the following (in thousands): September 30, December 31, Accrued compensation $ 5,368 $ 4,589 Accrued contracted research and development costs 6,669 6,972 Accrued professional and consulting fees 3,484 946 Accrued other 340 330 Total accrued and other current liabilities $ 15,861 $ 12,837

Related Party Transactions (Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 2023
Related Party Transactions [Abstract]
Expenses related to Related Party which were Settled in Cash	The following is the summary of expenses related to the Paragon Agreement, which were ultimately settled in cash (in millions): Three Months Ended Nine Months Ended Financial Statement Line Item 2023 2022 2023 2022 Reimbursable costs under the Paragon Agreement $ 16.7 $ — $ 17.9 $ — Research and development
Related Party Accounts Payable	The following is the summary of Related party accounts payable (in millions): September 30, December 31, Reimbursable costs under the Paragon Agreement $ 16.8 $ — Parapyre Option Obligation liability 3.0 — Total related party accounts payable $ 19.8 $ —

Asset Acquisition (Tables)

Asset Acquisition (Tables)	9 Months Ended
	Sep. 30, 2023
Business Combination and Asset Acquisition [Abstract]
Asset Acquisition Cost	The Asset Acquisition Costs are shown on the following table (in millions): June 22, Consideration transferred in Series A Preferred Stock and common stock $ 110.0 Transaction costs incurred by Aeglea 3.2 Total cost to acquire asset $ 113.2 The allocation of the purchase price to net assets acquired is as a follows: June 22, Acquired in-process research and development $ 130.2 Cash acquired 3.0 Assumed liabilities (20.0) Total cost to acquire asset $ 113.2

Liabilities, Convertible Pref_2

Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity (Tables)	9 Months Ended
	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Pre-funded Warrants for Common Stock Issued and Outstanding	As of September 30, 2023, the following pre-funded warrants for common stock were issued and outstanding: Issue Date Expiration Date Exercise Price Number of Warrants Outstanding February 8, 2019 None $ 0.0025 — April 30, 2020 None $ 0.0025 — May 20, 2022 None $ 0.0025 250,000 Total pre-funded warrants 250,000
Stock Awards Granted	The following table summarizes the Company’s stock awards granted under all plans for each of the periods indicated: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Grants Weighted Average Grant Date Fair Value Grants Weighted Average Grant Date Fair Value Grants Weighted Average Grant Date Fair Value Grants Weighted Average Grant Date Fair Value Stock options 1,044,667 $ 14.50 50,806 $ 16.75 3,867,366 $ 9.65 153,686 $ 52.50
Stock-Based Compensation Expense	Total stock-based compensation expense related to all plans was as follows (in thousands): Three Months Ended Nine Months Ended 2023 2022 2023 2022 Research and development (1) $ 2,965 $ 639 $ 4,136 $ 2,031 General and administrative 1,820 926 4,269 3,653 Total stock-based compensation expense $ 4,785 $ 1,565 $ 8,405 $ 5,684 (1) For the three and nine months ended September 30, 2023, $2.7 million and $2.9 million, respectively, was recognized as stock compensation expense related to the Parapyre Option Obligation. There were no such expenses for the three and nine months ended September 30, 2022.
Assumptions used to Estimate the Fair Value of Stock Options Granted	The following table summarizes the weighted-average Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted under the Company’s 2016 Plan, and the shares purchasable under the 2016 ESPP during the periods presented: Three Months Ended Nine Months Ended 2023 2022 2023 2022 2016 Plan Expected term (in years) 6.08 6.02 6.04 5.96 Expected volatility 101% 85% 111% 83% Risk-free interest 4.28% 3.16% 4.07% 2.43% Dividend yield — — — — 2016 ESPP Expected term (in years) 0.50 0.50 0.49 0.49 Expected volatility 222% 95% 181% 84% Risk-free interest 5.29 3.26 4.99% 1.95% Dividend yield — — — —

Strategic License Agreements (T

Strategic License Agreements (Tables)	9 Months Ended
	Sep. 30, 2023
License And Collaboration Agreement [Abstract]
Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities for the periods presented (in thousands): Nine Months Ended September 30, 2023 December 31, Additions Deductions September 30, Contract liabilities: Deferred revenue $ 2,696 $ 575 $ (3,271) $ —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2023
Earnings Per Share [Abstract]
Weighted-Average Equity Instruments Excluded from Calculation of Diluted Net Loss Per Share	The following weighted-average equity instruments were excluded from the calculation of diluted net loss per share because their effect would have been anti-dilutive for the periods presented: Three Months Ended Nine Months Ended 2023 2022 2023 2022 Options to purchase common stock 3,135,672 351,533 1,426,224 335,395 Unvested restricted stock units — 6,000 252 7,315 Series A Preferred Stock (on an as-converted basis) 42,501,681 — 14,851,447 —

Restructuring Charges (Tables)

Restructuring Charges (Tables)	9 Months Ended
	Sep. 30, 2023
Restructuring and Related Activities [Abstract]
Changes in Accrued Restructuring Balance	The following table summarizes the changes in the Company's accrued restructuring balance (in thousands): Beginning Balance Charges Payments Ending Balance Severance liability $ — $ 6,448 $ (4,527) $ 1,921
Charges Related to the Restructuring Activities	A summary of the charges related to the restructuring activities is as follows (in thousands): Severance Related Expenses Stock Compensation Expenses Loss on Disposal of Long Lived Assets Lease Asset Impairment Total Restructuring Costs Research and development $ 3,182 $ 123 $ 749 $ 1,405 $ 5,459 General and administrative 3,266 870 182 1,175 5,493 Total $ 6,448 $ 993 $ 931 $ 2,580 $ 10,952

The Company and Basis of Pres_2

The Company and Basis of Presentation - Narrative (Details) $ / shares in Units, $ in Thousands				1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 08, 2023	Jun. 26, 2023 USD ($) shares	Jun. 22, 2023 $ / shares shares	Apr. 30, 2023 employee	Jun. 30, 2023	Sep. 30, 2023 USD ($) segment $ / shares	Dec. 31, 2022 USD ($) $ / shares
Company and Basis of Presentation [Line Items]
Number of operating segments | segment						1
Employee workforce, termination percentage				83%	83%
Preferred stock, par value (in dollars per share) | $ / shares						$ 0.0001	$ 0.0001
Accumulated deficit | $						$ 701,234	$ 425,624
Cash, cash equivalents, and marketable securities | $						$ 203,600
Number of employees retained | employee				10
Asset Acquisition
Company and Basis of Presentation [Line Items]
Fixed exchange ratio			54.94488%
Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Asset acquisition, stockholder payment period			3 years
Asset acquisition, cash payment, threshold period			1 year
Spyre 2023 Equity Incentive Plan | Asset Acquisition
Company and Basis of Presentation [Line Items]
Number of outstanding and unexercised stock options to purchase (in shares)			2,734
Spyre 2023 Equity Incentive Plan | Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Number of outstanding and unexercised stock options to purchase (in shares)			2,734
Series A Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Conversion basis			40
Series A Non Voting Convertible Preferred Stock | Asset Acquisition
Company and Basis of Presentation [Line Items]
Number of shares transferred as equity interest in asset acquisition			364,887
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.0001
Conversion basis			40
Series A Non Voting Convertible Preferred Stock | Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Number of shares transferred as equity interest in asset acquisition			364,887
Private Placement | Series A Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Number of shares sold in private placement		721,452
Gross proceeds received in private placement | $		$ 210,000
Placement agent and other offering expenses in private placement | $		$ 12,700
Common Stock
Company and Basis of Presentation [Line Items]
Reverse stock split, conversion ratio	0.04
Common Stock | Asset Acquisition
Company and Basis of Presentation [Line Items]
Number of shares transferred as equity interest in asset acquisition			517,809

Fair Value Measurements - Finan

Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Financial Assets:
Financial assets, fair value	$ 185,372	$ 46,853
Liabilities:
Liabilities, fair value	31,152
Corporate bonds
Financial Assets:
Financial assets, fair value	22,828	3,732
Commercial paper
Financial Assets:
Financial assets, fair value	107,093	23,641
Level 1
Financial Assets:
Financial assets, fair value	55,451	15,250
Liabilities:
Liabilities, fair value	0
Level 1 | Corporate bonds
Financial Assets:
Financial assets, fair value	0	0
Level 1 | Commercial paper
Financial Assets:
Financial assets, fair value	0	0
Level 2
Financial Assets:
Financial assets, fair value	129,921	31,603
Liabilities:
Liabilities, fair value	2,952
Level 2 | Corporate bonds
Financial Assets:
Financial assets, fair value	22,828	3,732
Level 2 | Commercial paper
Financial Assets:
Financial assets, fair value	107,093	23,641
Level 3
Financial Assets:
Financial assets, fair value	0	0
Liabilities:
Liabilities, fair value	28,200
Level 3 | Corporate bonds
Financial Assets:
Financial assets, fair value	0	0
Level 3 | Commercial paper
Financial Assets:
Financial assets, fair value	0	0
Parapyre Option Obligation
Liabilities:
Liabilities, fair value	2,952
Parapyre Option Obligation | Level 1
Liabilities:
Liabilities, fair value	0
Parapyre Option Obligation | Level 2
Liabilities:
Liabilities, fair value	2,952
Parapyre Option Obligation | Level 3
Liabilities:
Liabilities, fair value	0
CVR liability
Liabilities:
Liabilities, fair value	28,200
CVR liability | Level 1
Liabilities:
Liabilities, fair value	0
CVR liability | Level 2
Liabilities:
Liabilities, fair value	0
CVR liability | Level 3
Liabilities:
Liabilities, fair value	28,200
Money market funds
Financial Assets:
Financial assets, fair value	55,451	15,250
Money market funds | Level 1
Financial Assets:
Financial assets, fair value	55,451	15,250
Money market funds | Level 2
Financial Assets:
Financial assets, fair value	0	0
Money market funds | Level 3
Financial Assets:
Financial assets, fair value	$ 0	0
U.S. government securities
Financial Assets:
Financial assets, fair value		4,230
U.S. government securities | Level 1
Financial Assets:
Financial assets, fair value		0
U.S. government securities | Level 2
Financial Assets:
Financial assets, fair value		4,230
U.S. government securities | Level 3
Financial Assets:
Financial assets, fair value		$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($)

3 Months Ended

9 Months Ended

Dec. 31, 2023

Sep. 29, 2023

Jul. 07, 2023

Jun. 30, 2023

Jun. 26, 2023

Jun. 22, 2023

Sep. 30, 2023

Jun. 30, 2023

Sep. 30, 2022

Sep. 30, 2023

Sep. 30, 2022

May 31, 2023

Dec. 31, 2022

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Financial liabilities outstanding measured at fair value

$ 0

Stock-based compensation

$ 8,405,000

$ 5,684,000

Forward contract liability

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Financial liabilities outstanding measured at fair value

$ 164,300,000

$ 106,200,000

$ 0

$ 164,300,000

0

Liabilities, fair value

$ 106,200,000

Change in fair value of forward contract liability

25,360,000

$ 0

83,530,000

0

Change in fair value of liability

$ (58,100,000)

(25,400,000)

Spyre Therapeutics, Inc.

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Asset acquisition, stockholder payment period

3 years

Asset acquisition, cash payment, threshold period

1 year

Parapyre Option Obligation

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Percentage of annual equity grant of options

1%

1%

Share-based payment award, term

10 years

Fair value of contingent consideration assumed

$ 100,000

Stock-based compensation

2,700,000

$ 0

$ 2,900,000

$ 0

Parapyre Option Obligation | Forecast

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Pro-rated estimated fair value of options

$ 3,000,000

Series A Non Voting Convertible Preferred Stock

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Issuance of Series A non-voting convertible preferred stock

$ 189,700,000

$ 197,300,000

$ 189,741,000

$ 197,323,000

Series A Non Voting Convertible Preferred Stock | Asset Acquisition

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Number of shares transferred as equity interest in asset acquisition

364,887

Series A Non Voting Convertible Preferred Stock | Spyre Therapeutics, Inc.

Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]

Number of shares transferred as equity interest in asset acquisition

364,887

Fair Value Measurements - Chang

Fair Value Measurements - Changes in Derivative Liabilities (Details) - USD ($)		3 Months Ended	9 Months Ended
	Jun. 30, 2023	Sep. 30, 2023	Sep. 30, 2023
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance			$ 0
Forward Contracts
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 106,200,000	$ 164,300,000
Change in fair value	58,100,000	25,400,000
Issuance of Series A Preferred Stock on July 7, 2023		(189,700,000)
Ending balance	$ 164,300,000	0	0
CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance			0
Change in fair value		(1,300,000)	(1,300,000)
Fair value at CVR issuance			29,500,000
Ending balance		$ 28,200,000	$ 28,200,000

Fair Value Measurements - Signi

Fair Value Measurements - Significant Inputs used to Estimate the Fair Value of Derivative Liability (Details) - Level 3	Sep. 30, 2023
Maximum | Risk-adjusted discount rates | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	0.0757
Maximum | CVR liability | Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	1
Minimum | Risk-adjusted discount rates | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	0.0714
Minimum | CVR liability | Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	0.27

Cash Equivalents and Marketab_3

Cash Equivalents and Marketable Securities - Estimated Fair Value of Cash Equivalents and Marketable Securities and the Gross Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]
Cash equivalents, amortized cost	$ 72,362	$ 26,007
Cash equivalents, gross unrealized gains	3	1
Cash equivalents, gross unrealized losses	0	(3)
Cash equivalents, estimated fair value	72,365	26,005
Marketable securities, amortized cost	113,121	20,877
Marketable securities, gross unrealized gains	1	2
Marketable securities, gross unrealized losses	(115)	(31)
Marketable securities, estimated fair value	113,007	20,848
Money market funds
Debt Securities, Available-for-Sale [Line Items]
Cash equivalents, amortized cost	55,451	15,250
Cash equivalents, gross unrealized gains	0	0
Cash equivalents, gross unrealized losses	0	0
Cash equivalents, estimated fair value	55,451	15,250
Commercial paper
Debt Securities, Available-for-Sale [Line Items]
Cash equivalents, amortized cost	16,911	7,021
Cash equivalents, gross unrealized gains	3	1
Cash equivalents, gross unrealized losses	0	(2)
Cash equivalents, estimated fair value	16,914	7,020
Marketable securities, amortized cost	90,272	16,644
Marketable securities, gross unrealized gains	0	2
Marketable securities, gross unrealized losses	(93)	(25)
Marketable securities, estimated fair value	90,179	16,621
Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Marketable securities, amortized cost	22,849	3,738
Marketable securities, gross unrealized gains	1	0
Marketable securities, gross unrealized losses	(22)	(6)
Marketable securities, estimated fair value	22,828	3,732
U.S. government securities
Debt Securities, Available-for-Sale [Line Items]
Cash equivalents, amortized cost		3,736
Cash equivalents, gross unrealized gains		0
Cash equivalents, gross unrealized losses		(1)
Cash equivalents, estimated fair value		3,735
Marketable securities, amortized cost	0	495
Marketable securities, gross unrealized gains	0	0
Marketable securities, gross unrealized losses	0	0
Marketable securities, estimated fair value	$ 0	$ 495

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Available-for-Sale Securities in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair Value
Less Than 12 Months	$ 97,193	$ 25,166
12 Months or Longer	0	0
Unrealized Losses
Less Than 12 Months	(115)	(34)
12 Months or Longer	0	0
Total, fair value	97,193	25,166
Total, unrealized losses	(115)	(34)
Commercial paper
Fair Value
Less Than 12 Months	78,820	17,699
12 Months or Longer	0	0
Unrealized Losses
Less Than 12 Months	(93)	(27)
12 Months or Longer	0	0
Total, fair value	78,820	17,699
Total, unrealized losses	(93)	(27)
U.S. government securities
Fair Value
Less Than 12 Months	0	3,735
12 Months or Longer	0	0
Unrealized Losses
Less Than 12 Months	0	(1)
12 Months or Longer	0	0
Total, fair value	0	3,735
Total, unrealized losses	0	(1)
Corporate bonds
Fair Value
Less Than 12 Months	18,373	3,732
12 Months or Longer	0	0
Unrealized Losses
Less Than 12 Months	(22)	(6)
12 Months or Longer	0	0
Total, fair value	18,373	3,732
Total, unrealized losses	$ (22)	$ (6)

Cash Equivalents and Marketab_5

Cash Equivalents and Marketable Securities - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2023 USD ($) bank	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) bank	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Cash Equivalents And Marketable Securities [Line Items]
Number of domestic banking institutions (in banks) | bank	3		3
US Government Agencies Debt Securities
Cash Equivalents And Marketable Securities [Line Items]
Debt securities, available-for-sale, allowance for credit loss	$ 0		$ 0		$ 0
Impairment of marketable securities			0		0
Realized gains or losses on marketable securities	0	$ 0	0	$ 0
Accrued interest receivable on available-for-sale debt securities	400,000		400,000		100,000
U.S. Banking Institution
Cash Equivalents And Marketable Securities [Line Items]
Cash deposits in excess of FDIC limit	16,900,000		16,900,000		$ 23,500,000
U.S. Banking Institution | Maximum
Cash Equivalents And Marketable Securities [Line Items]
Cash, FDIC insured amount	$ 250,000		$ 250,000

Cash Equivalents and Marketab_6

Cash Equivalents and Marketable Securities - Contractual Maturities of Marketable Securities at Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Cash and Cash Equivalents [Abstract]
Due in one year or less	$ 102,518	$ 20,848
Due thereafter	10,489	0
Total marketable securities	$ 113,007	$ 20,848

Accrued and Other Current Lia_3

Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Accrued compensation	$ 5,368	$ 4,589
Accrued contracted research and development costs	6,669	6,972
Accrued professional and consulting fees	3,484	946
Accrued other	340	330
Total accrued and other current liabilities	$ 15,861	$ 12,837

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) $ in Thousands			1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 29, 2023	Jun. 22, 2023 USD ($)	Jul. 31, 2023 USD ($) programOption	Sep. 30, 2023 USD ($) boardSeat	Sep. 30, 2023 USD ($) boardSeat	Sep. 30, 2022 USD ($)	Jul. 12, 2023 USD ($)	Dec. 31, 2022 USD ($)
Related Party Transaction [Line Items]
Stock compensation expense					$ 8,405	$ 5,684
Related Party
Related Party Transaction [Line Items]
Accounts payable				$ 19,800	$ 19,800			$ 0
Appoint Of Board Members | Related Party
Related Party Transaction [Line Items]
Number of board seats held by related party | boardSeat				2	2
Paragon Agreement | Related Party
Related Party Transaction [Line Items]
Related party expenses incurred prior to asset acquisition		$ 19,000
Research initiation fees		3,000
Reimbursable research costs		16,000
Related party expenses unpaid prior to asset acquisition		$ 19,000
Percentage of annual equity grant of options				1%	1%
Share-based payment award, term	10 years
Stock compensation expense				$ 2,700	$ 2,900
Stock Compensation And Research And Development Expense | Related Party
Related Party Transaction [Line Items]
Amount of related party transaction				19,400	20,800
Reimbursable Costs Under Paragon Agreement | Related Party
Related Party Transaction [Line Items]
Accounts payable				16,800	16,800			$ 0
Milestone Payments | Related Party
Related Party Transaction [Line Items]
Amount of related party transaction				$ 20,000	$ 20,000
Licensing Agreements | Related Party
Related Party Transaction [Line Items]
Number of program options outstanding | programOption			3
SPY001 License Agreement | Maximum | Related Party
Related Party Transaction [Line Items]
Contingent obligation based on milestones							$ 22,000
Paragon Therapeutics Inc | Ownership Interest | Maximum | Related Party
Related Party Transaction [Line Items]
Percentage of ownership by noncontrolling owner				5%	5%
Paragon Therapeutics Inc | SPY001 License Agreement | Maximum
Related Party Transaction [Line Items]
Contingent obligation based on milestones			$ 22,000
Fairmount Funds Management LLC | Ownership Interest | Minimum | Related Party
Related Party Transaction [Line Items]
Percentage of ownership by noncontrolling owner				5%	5%
Percentage of ownership held in third party				5%	5%

Related Party Transactions - Ex

Related Party Transactions - Expenses related to Related Party which were Settled in Cash (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Related Party | Reimbursable Costs Under Paragon Agreement | Research and Development Expense
Related Party Transaction [Line Items]
Reimbursable costs	$ 16.7	$ 0	$ 17.9	$ 0

Related Party Transactions - Re

Related Party Transactions - Related Party Accounts Payable (Details) - Related Party - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Related Party Transaction [Line Items]
Accounts payable	$ 19.8	$ 0
Reimbursable Costs Under Paragon Agreement
Related Party Transaction [Line Items]
Accounts payable	16.8	0
Parapyre Option Obligation
Related Party Transaction [Line Items]
Accounts payable	$ 3	$ 0

Asset Acquisition - Narrative (

Asset Acquisition - Narrative (Details) $ / shares in Units, $ in Millions	Jun. 22, 2023 USD ($) $ / shares shares
Asset Acquisition [Line Items]
Cost to acquire asset | $	$ 113.2
Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Cost to acquire asset | $	$ 113.2
Asset acquisition consideration transferred issuable shares of common stock on an as-converted basis	14,595,480
Spyre 2023 Equity Incentive Plan | Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Number of outstanding and unexercised stock options to purchase (in shares)	2,734
Series A Non Voting Convertible Preferred Stock | Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Number of shares transferred as equity interest in asset acquisition	364,887
Shares issued, price per share (in dollars per share) | $ / shares	$ 291.08
Common Stock | Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Number of shares transferred as equity interest in asset acquisition	517,809
Shares issued, price per share (in dollars per share) | $ / shares	$ 7.277

Asset Acquisition - Asset Acqui

Asset Acquisition - Asset Acquisition Cost (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jun. 22, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Asset Acquisition [Line Items]
Acquired in-process research and development		$ (298)	$ 0	$ 130,188	$ 0
Cash acquired				$ 3,035	$ 0
Cost to acquire asset	$ 113,200
Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Consideration transferred in Series A Preferred Stock and common stock	110,000
Transaction costs incurred by Aeglea	3,200
Acquired in-process research and development	130,200
Cash acquired	3,000
Assumed liabilities	(20,000)
Cost to acquire asset	$ 113,200

Paragon Agreement (Details)

Paragon Agreement (Details) - USD ($) $ in Millions			3 Months Ended	9 Months Ended
	Jul. 12, 2023	Jun. 22, 2023	Sep. 30, 2023	Sep. 30, 2023	May 31, 2023
Paragon Therapeutics Inc
Related Party Transaction [Line Items]
Milestone payments			$ 20	$ 20
Parapyre Option Obligation
Related Party Transaction [Line Items]
Percentage of annual equity grant of options		1%			1%
Related Party
Related Party Transaction [Line Items]
Nonrefundable research initiation fee for one program paid in cash		$ 0.8
Related Party | Paragon Agreement
Related Party Transaction [Line Items]
Research initiation fees		3
Related party expenses incurred prior to asset acquisition		19
Reimbursable research costs		16
Related party expenses unpaid prior to asset acquisition		$ 19
Percentage of annual equity grant of options			1%	1%
Related Party | SPY001 License Agreement
Related Party Transaction [Line Items]
Nomination fee	$ 1.5
First milestone payment	2.5
Related Party | SPY001 License Agreement | Maximum
Related Party Transaction [Line Items]
Contingent obligation based on milestones	$ 22
Related Party | Stock Compensation And Research And Development Expense
Related Party Transaction [Line Items]
Amount of related party transaction			$ 19.4	$ 20.8

Series A Non-Voting Convertib_2

Series A Non-Voting Convertible Preferred Stock (Details) - Series A Non Voting Convertible Preferred Stock - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended
	Jul. 07, 2023	Jun. 26, 2023	Jun. 22, 2023	Sep. 30, 2023	Jun. 30, 2023
Class of Stock [Line Items]
Preferred stock issued and outstanding percentage			30%
Preferred stock, redemption value				$ 532,300
Preferred stock, redemption per share (in dollars per share)				$ 12.25
Conversion basis			40
Issuance of Series A non-voting convertible preferred stock (in shares)		721,452		365,000	721,000
Issuance of Series A non-voting convertible preferred stock	$ 189,700	$ 197,300		$ 189,741	$ 197,323
Asset Acquisition
Class of Stock [Line Items]
Number of shares transferred as equity interest in asset acquisition	364,887
Minimum
Class of Stock [Line Items]
Beneficially holders owned percentage			0%
Maximum
Class of Stock [Line Items]
Beneficially holders owned percentage			19.99%
Private Placement
Class of Stock [Line Items]
Gross proceeds received in private placement		$ 210,000

Liabilities, Convertible Pref_3

Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity - Narrative (Details) - USD ($)

1 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

39 Months Ended

40 Months Ended

Dec. 31, 2023

Sep. 29, 2023

Jun. 22, 2023

May 31, 2022

Jul. 31, 2020

Sep. 30, 2023

Sep. 30, 2022

Sep. 30, 2023

Sep. 30, 2022

Dec. 31, 2016

Sep. 30, 2023

May 31, 2022

Jan. 01, 2023

Jan. 01, 2022

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Warrants to purchase shares (in shares)

694,892

694,892

Offering price of warrant (in dollars per share)

$ 39.9975

$ 39.9975

Exercise price per warrant (in dollars per share)

$ 0.0025

$ 0.0025

Net proceeds from sale of common stock

$ 42,900,000

Placement agent fees and offering costs

$ 2,100,000

Maximum ownership percentage of common stock shares for outstanding warrants to be exercised

4.99%

Ownership percentage for outstanding warrants to purchase shares of common stock to be exercised to certain holders

9.99%

Revised ownership percentage, period to take effect after notice

61 days

Stock-based compensation expense

$ 4,785,000

$ 1,565,000

$ 8,405,000

$ 5,684,000

Options granted (in shares)

1,044,667

50,806

3,867,366

153,686

Stock-based compensation

$ 8,405,000

$ 5,684,000

Maximum

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Maximum ownership percentage of common stock shares for outstanding warrants to be exercised

19.99%

2016 Equity Incentive Plan

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Annual percentage of additional shares

4%

Additional number of shares available for issuance

104,560

78,968

Stock options granted to certain members of board of directors, contingent on stockholder approval (in shares)

2,720,033

Expense on contingent awards

$ 0

Common stock available for future issuance (in shares)

293,497

293,497

293,497

2016 Equity Incentive Plan | Restricted Stock Units (RSUs)

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Equity instruments other than options, granted (in shares)

9,128

Equity instruments other than options, vested (in shares)

0

Stock-based compensation expense

$ 0

2018 Equity Inducement Plan

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Additional number of shares available for grant

2,800,000

Options granted (in shares)

3,583,880

Vest period grant date

4 years

Share-based payment award, term

10 years

Spyre 2023 Equity Incentive Plan | Asset Acquisition

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of outstanding and unexercised stock options to purchase (in shares)

2,734

Parapyre Option Obligation

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Annual percentage of additional shares

1%

Share-based payment award, term

10 years

Fair value of contingent consideration assumed

$ 100,000

Stock-based compensation

$ 2,700,000

$ 0

$ 2,900,000

$ 0

Unamortized expense

$ 2,100,000

$ 2,100,000

$ 2,100,000

Parapyre Option Obligation | Forecast

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Pro-rated estimated fair value of options

$ 3,000,000

2016 Employee Stock Purchase Plan

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of shares issued

2,496

6,073

Aggregate cash proceeds from sale of shares

$ 200,000

2016 Employee Stock Purchase Plan | Maximum

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Aggregate cash proceeds from sale of shares

$ 100,000

Common Stock

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Number of shares issued

430,107

Public offering price (in dollars per share)

$ 40

$ 40

Liabilities, Convertible Pref_4

Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity - Pre-funded Warrants for Common Stock Issued and Outstanding (Details) - $ / shares	Sep. 30, 2023	May 31, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise Price (in dollars per share)		$ 0.0025
Pre-funded Warrants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Warrants Outstanding (in shares)	250,000
Pre-funded Warrants | February 8, 2019
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise Price (in dollars per share)	$ 0.0025
Number of Warrants Outstanding (in shares)	0
Pre-funded Warrants | April 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise Price (in dollars per share)	$ 0.0025
Number of Warrants Outstanding (in shares)	0
Pre-funded Warrants | May 20, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise Price (in dollars per share)	$ 0.0025
Number of Warrants Outstanding (in shares)	250,000

Liabilities, Convertible Pref_5

Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity - Stock Awards Granted (Details) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share-Based Payment Arrangement [Abstract]
Grants (in shares)	1,044,667	50,806	3,867,366	153,686
Weighted Average Grant Date Fair Value (in dollars per share)	$ 14.50	$ 16.75	$ 9.65	$ 52.50

Liabilities, Convertible Pref_6

Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 4,785	$ 1,565	$ 8,405	$ 5,684
Stock-based compensation			8,405	5,684
Parapyre Option Obligation
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation	2,700	0	2,900	0
Research and Development Expense
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	2,965	639	4,136	2,031
General and administrative
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 1,820	$ 926	$ 4,269	$ 3,653

Liabilities, Convertible Pref_7

Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity - Assumptions used to Estimate the Fair Value of Stock Options Granted (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
2016 Plan
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Expected term (in years)	6 years 29 days	6 years 7 days	6 years 14 days	5 years 11 months 15 days
Expected volatility	101%	85%	111%	83%
Risk-free interest	4.28%	3.16%	4.07%	2.43%
Dividend yield	0%	0%	0%	0%
2016 ESPP
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Expected term (in years)	6 months	6 months	5 months 26 days	5 months 26 days
Expected volatility	222%	95%	181%	84%
Risk-free interest	5.29%	3.26%	4.99%	1.95%
Dividend yield	0%	0%	0%	0%

Strategic License Agreements -

Strategic License Agreements - Narrative (Details) - USD ($)		1 Months Ended		3 Months Ended		9 Months Ended
	Mar. 21, 2021	Sep. 30, 2021	Jul. 31, 2021	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Research And Development Arrangement Contract To Perform For Others [Line Items]
Revenue recognized				$ 0	$ 174,000	$ 886,000	$ 2,161,000
Deferred revenue				0		0		$ 2,696,000
Deferred revenue, current				0		0		517,000
Contract assets				0	0	0	0
License [Member] | Immedica Pharma AB
Research And Development Arrangement Contract To Perform For Others [Line Items]
Full reimbursement of costs included in upfront fixed payment received	$ 1,800,000
Immedica Pharma AB
Research And Development Arrangement Contract To Perform For Others [Line Items]
Non refundable payment received	$ 21,500,000
Percentage of payment for cost incurred in trial	50%
Rate of revenue share	25%
Eligible reimbursement (up to)			$ 3,000,000
Upfront payment	$ 21,500,000
Estimated amount due as part of trial			3,600,000
Amount of related party transaction			25,100,000
Allocation for stand-alone selling prices			12,000,000
Deferred revenue				0		0		3,600,000
Deferred revenue, current								$ 2,400,000
Immedica Pharma AB | PEACE Trial and BLA Package
Research And Development Arrangement Contract To Perform For Others [Line Items]
Allocated amount of modified transaction price			9,600,000
Revenue recognized					$ 200,000		$ 2,200,000
Immedica Pharma AB | PIP Trial
Research And Development Arrangement Contract To Perform For Others [Line Items]
Allocated amount of modified transaction price			$ 3,500,000
Immedica Pharma AB | License Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Revenue recognized		$ 12,000,000
Immedica Pharma AB | Peace Phase 3 Trial and Drug Supply
Research And Development Arrangement Contract To Perform For Others [Line Items]
Revenue recognized				$ 0		$ 900,000
Immedica Pharma AB | Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Percentage of payment for cost incurred in trial	50%
Maximum amount of costs to reimburse	$ 1,800,000

Strategic License Agreements _2

Strategic License Agreements - Changes in Contract Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($)
Change in Contract with Customer, Liability [Abstract]
Beginning balance	$ 2,696
Additions	575
Deductions	(3,271)
Ending balance	$ 0

Sale of Pegzilarginase to Imm_2

Sale of Pegzilarginase to Immedica (Details) - USD ($)		3 Months Ended		9 Months Ended
	Jul. 27, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of intangible assets				$ 15,000,000	$ 0
Gain recognized within operating expenses		$ 14,609,000	$ 0	$ 14,609,000	$ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Global Rights To Pegzilarginase
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of intangible assets	$ 15,000,000
Contingent consideration	100,000,000
Carrying value of assets	0
Gain recognized within operating expenses	$ 14,600,000

Net Loss Per Share (Details)

Net Loss Per Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	3,135,672	351,533	1,426,224	335,395
Unvested restricted stock units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	0	6,000	252	7,315
Series A Preferred Stock (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	42,501,681	0	14,851,447	0

Restructuring Charges - Narrati

Restructuring Charges - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Apr. 30, 2023	Jun. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	Aug. 07, 2023
Restructuring Cost and Reserve [Line Items]
Employee workforce, termination percentage	83%	83%
Cash severance payments and other employee-related costs			$ 6,400
Cash payments for employee related restructuring charges			4,500
Non-cash stock-based compensation expense related to accelerated vesting of stock-based awards			1,000
Total consideration from disposal of long lived assets		$ 500
Loss on disposal of long-lived assets			$ 915	$ 0
Termination fee amount					$ 2,000
Lease asset impairment		900
Impairment on leasehold improvements		1,700
Research and Development Expense
Restructuring Cost and Reserve [Line Items]
Loss on disposal of long-lived assets		700
General and administrative
Restructuring Cost and Reserve [Line Items]
Loss on disposal of long-lived assets		$ 200

Restructuring Charges - Changes

Restructuring Charges - Changes in Accrued Restructuring Balance (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($)
Restructuring Cost and Reserve [Line Items]
Payments	$ (4,500)
Severance Liability
Restructuring Cost and Reserve [Line Items]
Beginning balance	0
Charges	6,448
Payments	(4,527)
Ending balance	$ 1,921

Restructuring Charges - Charges

Restructuring Charges - Charges Related to the Restructuring Activities (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restructuring Cost and Reserve [Line Items]
Severance Related Expenses				$ 6,400
Stock-based compensation expense	$ 4,785		$ 1,565	8,405	$ 5,684
Loss on Disposal of Long Lived Assets				915	0
Lease Asset Impairment		$ 900
Research and Development Expense
Restructuring Cost and Reserve [Line Items]
Stock-based compensation expense	2,965		639	4,136	2,031
Loss on Disposal of Long Lived Assets		700
General and administrative
Restructuring Cost and Reserve [Line Items]
Stock-based compensation expense	$ 1,820		$ 926	$ 4,269	$ 3,653
Loss on Disposal of Long Lived Assets		200
Restructuring Activities
Restructuring Cost and Reserve [Line Items]
Severance Related Expenses		6,448
Stock-based compensation expense		993
Loss on Disposal of Long Lived Assets		931
Lease Asset Impairment		2,580
Total Restructuring Costs		10,952
Restructuring Activities | Research and Development Expense
Restructuring Cost and Reserve [Line Items]
Severance Related Expenses		3,182
Stock-based compensation expense		123
Loss on Disposal of Long Lived Assets		749
Lease Asset Impairment		1,405
Total Restructuring Costs		5,459
Restructuring Activities | General and administrative
Restructuring Cost and Reserve [Line Items]
Severance Related Expenses		3,266
Stock-based compensation expense		870
Loss on Disposal of Long Lived Assets		182
Lease Asset Impairment		1,175
Total Restructuring Costs		$ 5,493

Novation of Manufacturing Agr_2

Novation of Manufacturing Agreements (Details)	1 Months Ended
	Apr. 30, 2023 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Right to terminate agreement or work order, prior written notice period	30 days
Right to terminate work order, reasonable cause, prior written notice period	6 months
Right to terminate work order by counterparty, reasonable cause, termination amount payable	$ 0
Right to terminate work order, material breach, uncured period	30 days
Right to terminate work order, unusual or infrequent cause, period	90 days
Non-refundable license fee amount	$ 200,000
Royalty percentage (less than)	1%
Right to terminate agreement, prior written notice period	6 months
Right to terminate agreement by counterparty, material breach, uncured period	60 days
Right to terminate agreement by counterparty, payments not received, period	30 days