Spyre Therapeutics (SYRE) POS AM 14 Mar 24 Prospectus update (post-effective amendment) Financial data

Cover

Cover	12 Months Ended
	Dec. 31, 2023
Document Information [Line Items]
Document Type	POS AM
Entity Registrant Name	Spyre Therapeutics, 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
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Central Index Key	0001636282
Amendment Flag	true
Amendment Description	Spyre Therapeutics, Inc., a Delaware corporation, filed a Registration Statement on Form S-1 on October 6, 2023, which was declared effective on November 20, 2023 (as amended and supplemented, the “registration statement”). This Post-Effective Amendment No. 2 to Form S-1 (the “Post-Effective Amendment”) is being updated to add (i) a summary of key terms for the SPY001 License Agreement and the SPY002 License Agreement (as such terms are defined herein) and (ii) an explanation of the change in our Other expenses, net incurred during the year ended December 31, 2023. The information included in this filing amends the registration statement and the prospectus contained therein (and all amendments thereto). No additional securities are being registered under this Post-Effective Amendment. All applicable registration fees were paid at the time of the original filing of the registration statement.
Business Contact [Member]
Document Information [Line Items]
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
Contact Personnel Name	Heidy King-Jones

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
CURRENT ASSETS
Cash and cash equivalents	$ 188,893	$ 34,863
Marketable securities	150,384	20,848
Development receivables	0	375
Prepaid expenses and other current assets	2,251	6,172
Total current assets	341,528	62,258
Restricted cash	322	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	341,859	71,144
CURRENT LIABILITIES
Accounts payable	896	677
CVR liability	1,390	0
Operating lease liabilities	0	625
Deferred revenue	0	517
Accrued and other current liabilities	13,108	12,837
Related party accounts payable and other current liabilities	16,584	0
Total current liabilities	31,978	14,656
Non-current CVR liability	41,310	0
Non-current operating lease liabilities	0	4,004
Deferred revenue, net of current portion	0	2,179
TOTAL LIABILITIES	73,288	20,839
Commitments and Contingencies (Note 9)
STOCKHOLDERS' EQUITY
Common stock, $0.0001 par value; 400,000,000 and 20,000,000 shares authorized as of December 31, 2023 and December 31, 2022, respectively; 36,057,109 shares and 2,614,014 shares issued and outstanding as of December 31, 2023 and December 31, 2022, respectively.	10	6
Additional paid-in capital	763,191	475,971
Accumulated other comprehensive income (loss)	302	(48)
Accumulated deficit	(764,414)	(425,624)
TOTAL STOCKHOLDERS' EQUITY	184,016	50,305
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY	341,859	71,144
Series A Non Voting Convertible Preferred Stock
STOCKHOLDERS' EQUITY
Preferred stock	184,927	0
Series B Non-Voting Convertible Preferred Stock
CURRENT LIABILITIES
Series B non-voting convertible preferred stock, $0.0001 par value; 150,000 and no shares authorized as of December 31, 2023 and December 31, 2022, respectively; 150,000 and no shares issued and outstanding as of December 31, 2023 and December 31, 2022, respectively.	84,555	0
Preferred Stock
STOCKHOLDERS' EQUITY
Preferred stock	$ 0	$ 0

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Preferred stock, authorized (in shares)	10,000,000
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, authorized (in shares)	400,000,000	20,000,000
Common stock, issued (in shares)	36,057,109	2,614,014
Common stock, outstanding (in shares)	36,057,109	2,614,014
Series B Non-Voting Convertible Preferred Stock
Series B non-voting convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Series B non-voting convertible preferred stock, authorized (in shares)	150,000	0
Series B non-voting convertible preferred stock, issued (in shares)	150,000	0
Series B non-voting convertible preferred stock, outstanding (in shares)	150,000	0
Series A Non Voting Convertible Preferred StockSeries A Non Voting Convertible Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	1,086,341	0
Preferred stock, issued (in shares)	437,037	0
Preferred stock, outstanding (in shares)	437,037	0
Preferred Stock
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	8,763,659	10,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue:
Total revenue		$ 886	$ 2,329	$ 18,739
Operating expenses:
Research and development	[1]	89,504	58,579	57,069
General and administrative		39,946	28,531	27,319
Acquired in-process research and development		130,188	0	0
Gain on sale of in-process research and development asset		(16,449)	0	0
Total operating expenses		243,189	87,110	84,388
Loss from operations		(242,303)	(84,781)	(65,649)
Other (expense) income:
Interest income		6,147	837	111
Other expense, net		(19,130)	(7)	(122)
Total other (expense) income		(96,513)	830	(11)
Loss before income tax expense		(338,816)	(83,951)	(65,660)
Income tax benefit (expense)		26	136	(141)
Net loss		$ (338,790)	$ (83,815)	$ (65,801)
Net loss per share, basic (in dollars per share)		$ (49.12)	$ (24.86)	$ (25.02)
Net loss per share, diluted (in dollars per share)		$ (49.12)	$ (24.86)	$ (25.02)
Weighted-average common shares outstanding, basic (in shares)		6,897,065	3,371,231	2,629,784
Weighted-average common shares outstanding, diluted (in shares)		6,897,065	3,371,231	2,629,784
Forward Contract Liability
Other (expense) income:
Change in fair value of forward contract liability		$ (83,530)	$ 0	$ 0
License
Revenue:
Total revenue		0	0	12,000
Development fee and royalty
Revenue:
Total revenue		$ 886	$ 2,329	$ 6,739
[1]Includes $48.5 million in related party expenses for the year ended December 31, 2023 and no related party expenses for the year ended months ended December 31, 2022 and 2021.

Consolidated Statements of Op_2

Consolidated Statements of Operations (Parenthetical) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Research and development	[1]	$ 89,504	$ 58,579	$ 57,069
Related Party
Research and development		$ 48,500	$ 0	$ 0
[1]Includes $48.5 million in related party expenses for the year ended December 31, 2023 and no related party expenses for the year ended months ended December 31, 2022 and 2021.

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net loss	$ (338,790)	$ (83,815)	$ (65,801)
Other comprehensive income (loss):
Foreign currency translation adjustment	37	(35)	(1)
Unrealized gain (loss) on marketable securities	313	7	(30)
Total comprehensive loss	$ (338,440)	$ (83,843)	$ (65,832)

Consolidated Statements of Chan

Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total	Conversion Of Series A Non-Voting Convertible Preferred Stock	Series B Non-Voting Convertible Preferred Stock	Series A Non Voting Convertible Preferred StockSeries A Non Voting Convertible Preferred Stock	Series A Non-Voting Convertible Preferred Stock Series A Non Voting Convertible Preferred StockSeries A Non Voting Convertible Preferred Stock	Common Stock	Common Stock Conversion Of Series A Non-Voting Convertible Preferred Stock	Common Stock Conversion Of Pre-Funded Warrants	Additional Paid-in Capital	Additional Paid-in Capital Conversion Of Series A Non-Voting Convertible Preferred Stock	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2020			0
Beginning balance at Dec. 31, 2020			$ 0
Ending balance (in shares) at Dec. 31, 2021			0
Ending balance at Dec. 31, 2021			$ 0
Beginning balance (in shares) at Dec. 31, 2020					0
Beginning balance (in shares) at Dec. 31, 2020						1,918,000
Beginning balance at Dec. 31, 2020	$ 139,832				$ 0	$ 5			$ 415,824		$ 11	$ (276,008)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon conversion (in shares)						40,000
Issuance of common stock in connection with exercise of stock options and employee stock purchase plan (in shares)						16,000
Issuance of common stock in connection with exercise of stock options and employee stock purchase plan	1,903								1,903
Stock-based compensation expense	8,038								8,038
Foreign currency translation adjustment	(1)										(1)
Unrealized gain (loss) on marketable securities	(30)										(30)
Net loss	(65,801)											(65,801)
Ending balance (in shares) at Dec. 31, 2021					0
Ending balance (in shares) at Dec. 31, 2021						1,974,000
Ending balance at Dec. 31, 2021	83,941				$ 0	$ 5			425,765		(20)	(341,809)
Ending balance (in shares) at Dec. 31, 2022			0
Ending balance at Dec. 31, 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)						430,000
Issuance of common stock and pre-funded warrants in connection with registered direct offering, net of offering costs	42,874					$ 1			42,873
Issuance of common stock upon conversion (in shares)						204,000
Issuance of common stock in connection with employee stock purchase plan (in shares)						6,000
Issuance of common stock in connection with employee stock purchase plan	222								222
Stock-based compensation expense	7,111								7,111
Foreign currency translation adjustment	(35)										(35)
Unrealized gain (loss) on marketable securities	7										7
Net loss	$ (83,815)											(83,815)
Ending balance (in shares) at Dec. 31, 2022				0	0
Ending balance (in shares) at Dec. 31, 2022	2,614,014					2,614,000
Ending balance at Dec. 31, 2022	$ 50,305				$ 0	$ 6			475,971		(48)	(425,624)
Series B Non-Voting Convertible Preferred Stock
Issuance of Series B non-voting convertible preferred stock in connection with private placement, net of financing costs (in shares)			150,000
Issuance of Series B non-voting convertible preferred stock in connection with private placement, net of financing costs			$ 84,555
Ending balance (in shares) at Dec. 31, 2023			150,000
Ending balance at Dec. 31, 2023			$ 84,555
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon conversion (in shares)					(649,000)		25,972,000	905,000
Issuance of common stock upon conversion		$ 0			$ (202,178)		$ 3			$ 202,175
Issuance of Series A non-voting convertible preferred stock in connection with private placement, net of financing costs (in shares)					721,000
Issuance of Series A non-voting convertible preferred stock in connection with private placement, net of financing costs	197,364				$ 197,364
Issuance of Series A non-voting convertible preferred stock in connection with the asset acquisition of Spyre and settlement of related forward contract (in shares)					365,000
Issuance of Series A non-voting convertible preferred stock in connection with the asset acquisition of Spyre and settlement of related forward contract	189,741				$ 189,741
Issuance of common stock in connection with private placement, net of financing costs (in shares)						6,000,000
Issuance of common stock in connection with private placement, net of financing costs	84,555								84,555
Issuance of common stock in connection with the asset acquisition of Spyre (in shares)						518,000
Issuance of common stock in connection with the asset acquisition of Spyre	3,768					$ 1			3,767
Issuance of common stock in connection with exercise of stock options and employee stock purchase plan (in shares)						48,000
Issuance of common stock in connection with exercise of stock options and employee stock purchase plan	405								405
CVR distribution to common stockholders	(29,500)								(29,500)
Stock-based compensation expense	14,347								14,347
Issuance of Parapyre Option Obligation warrants	11,471								11,471
Foreign currency translation adjustment	37										37
Unrealized gain (loss) on marketable securities	313										313
Net loss	$ (338,790)											(338,790)
Ending balance (in shares) at Dec. 31, 2023				437,037	437,000
Ending balance (in shares) at Dec. 31, 2023	36,057,109					36,057,000
Ending balance at Dec. 31, 2023	$ 184,016				$ 184,927	$ 10			$ 763,191		$ 302	$ (764,414)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (338,790)	$ (83,815)	$ (65,801)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	744	1,567	1,576
Stock-based compensation	25,675	7,111	8,038
Acquired in-process research and development	130,188	0	0
Gain on sale of in-process research and development asset	(16,449)	0	0
Lease ROU asset and leasehold improvement impairment loss	2,580	0	0
Loss on disposal of long-lived assets	915	0	0
Net (accretion of discount) amortization of premium on marketable securities	(2,318)	(327)	548
Amortization of operating lease assets	220	397	425
Other	15	426	(335)
Changes in operating assets and liabilities:
Development receivables	375	440	(815)
Accounts payable	218	(2,641)	1,065
Prepaid expenses and other assets	3,245	(1,144)	(1,216)
Deferred revenue	575	(880)	3,576
Operating lease liabilities	(2,326)	(435)	(404)
Accrued and other liabilities	(4,891)	(843)	(373)
Related party payable	(2,402)	0	0
Net cash used in operating activities	(99,910)	(80,144)	(53,716)
CASH FLOWS FROM INVESTING ACTIVITIES
Cash assumed from asset acquisition of Spyre	3,035	0	0
Proceeds from sale of in-process research & development asset	15,000	0	0
Purchases of property and equipment	0	(38)	(573)
Proceeds from the sale of property plant and equipment	475	0	0
Purchases of marketable securities	(166,803)	(39,500)	(133,079)
Proceeds from maturities and sales of marketable securities	39,900	96,546	111,033
Net cash provided by (used in) investing activities	(108,393)	57,008	(22,619)
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	0
Proceeds from issuance of Series B non-voting convertible preferred stock in connection with private placement, net of placement and other offering costs	84,555	0	0
Proceeds from issuance of common stock in connection with private placement, net of placement and other offering costs	84,555	0	0
Payment of contingent value rights liability	(5,786)	0	0
Proceeds from issuance of common stock and pre-funded warrants in registered direct offering, net of offering costs	0	42,874	0
Proceeds from employee stock plan purchases and stock option exercises	405	222	1,903
Principal payments on finance lease obligation	(16)	(418)	(510)
Net cash provided by financing activities	361,077	42,678	1,393
Effect of exchange rate on cash, cash equivalents, and restricted cash	25	(106)	(15)
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH	152,799	19,436	(74,957)
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH
Beginning of period	36,416	16,980	91,937
End of period	189,215	36,416	16,980
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	0
Conversion of Series A non-voting convertible preferred stock into common stock	202,178	0	0
Leased assets obtained in exchange for lease obligations	0	21	872
CVR liability
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of derivative liability	18,986	0	0
Forward Contract Liability
Adjustments to reconcile net loss to net cash used in operating activities:
Change in fair value of derivative liability	$ 83,530	$ 0	$ 0

Statement of Assets Acquired an

Statement of Assets Acquired and Liabilities Assumed $ in Thousands	Jun. 22, 2023 USD ($)
Current assets:
Cash and cash equivalents	$ 3,035
Total current assets	3,035
Total assets acquired	3,035
Current liabilities:
Accrued liabilities	20,047
Total current liabilities	20,047
Total liabilities assumed	20,047
Net liabilities assumed	$ (17,012)

The Company and Basis of Presen

The Company and Basis of Presentation	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
The Company and Basis of Presentation	1. The Company and Basis of Presentation Spyre Therapeutics, Inc., formerly Aeglea BioTherapeutics, Inc., (“Spyre” 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. On November 27, 2023, the Company completed its corporate rebranding, changing the name of the Company to Spyre Therapeutics, Inc. The Company operates in one segment and has its principal offices in Waltham, Massachusetts. On September 8, 2023, the Company effected a reverse stock split of its Common Stock at a ratio of 1-for-25 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, in April 2023, the Company implemented a restructuring plan resulting in an approximate 83% reduction of the Company’s existing headcount. On June 22, 2023, the Company acquired, in accordance with the terms of the Agreement and Plan of Merger (the “Acquisition Agreement”), the assets of Spyre Therapeutics, Inc. (“Pre-Merger two-step Pre-Merger Pre-Merger Pre-Merger Pre-Merger stock-for-stock Pre-Merger non-voting Pre-Merger In connection with the Asset Acquisition, on June 26, 2023, the Company completed a private placement of shares of Series A Preferred Stock (the “Series A PIPE”) to a group of investors (the “Series A Investors”). The Company sold an aggregate of 721,452 shares of Series A Preferred Stock (the “Series A PIPE Securities”) for an aggregate purchase price of approximately $210.0 million before deducting approximately $12.7 million of placement agent and other offering expenses. For additional information, see Note 11. In connection with the Asset Acquisition, a non-transferable Pre-Merger 3-year one-year On November 21, 2023, the Company’s stockholders approved the conversion of the Company’s Series A non-voting On December 11, 2023, the Company completed a private placement of shares of common stock and Series B non-voting Liquidity The Company is a preclinical stage biotechnology company with a limited operating history, and due to its significant research and development expenditures, the Company has generated operating losses since its inception and has not generated any revenue from the commercial sale of any products. There can be no assurance that profitable operations will ever be achieved, and, if achieved, whether profitability can be sustained on a continuing basis. Since its inception and through December 31, 2023, the Company has funded our operations by raising an aggregate of approximately $896.2 million of gross proceeds from the sale and issuance of convertible preferred stock and common stock, pre-funded Based on current operating plans, the Company has sufficient resources to fund operations for at least one year from the issuance date of these financial statements with existing cash, cash equivalents, and marketable securities. Spyre will need to secure additional financing in the future to fund additional research and development, and before a commercial drug can be produced, marketed and sold. If the Company is unable to obtain additional financing or generate license or product revenue, the lack of liquidity could have a material adverse effect on the Company. Basis of Presentation The consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States (“U.S. GAAP”) as defined by the Financial Accounting Standards Board (“FASB”) and include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.

Description of the Business and

Description of the Business and Summary of Significant Accounting Policies	Jun. 22, 2023
Description of the Business and Summary of Significant Accounting Policies [Abstract]
Description of the Business and Summary of Significant Accounting Policies	1. Description of the Business and Summary of Significant Accounting Policies On June 22, 2023, Aeglea BioTherapeutics, Inc. (“Aeglea”, the “Company”, and “our”) acquired, in accordance with the terms of the Agreement and Plan of Merger (the “Acquisition Agreement”), the assets from Spyre Therapeutics, Inc (“Spyre”), a privately held biotechnology company advancing a pipeline of antibody therapeutics through a research and development option agreement (“Paragon Agreement”) with Paragon Therapeutics (“Paragon”). Spyre was incorporated on April 28, 2023, for the purpose of holding rights to certain intellectual property being developed by Paragon. On September 8, 2023, Aeglea effected a reverse stock split of its Common Stock at a ratio of 1-for-25 The transaction (the “Asset Acquisition”) was structured as a stock-for-stock non-voting 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 certain in-process 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 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 Option, 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 common stock of the Company on the same terms. For additional information, see Note 3. The Asset Acquisition costs are shown on the following table (in thousands): As of June 22, Consideration transferred in Series A Preferred Stock and common stock $ 109,979 Transaction costs incurred by Aeglea 3,197 Total cost to acquire asset $ 113,176 The Company’s allocation of the purchase price to net assets acquired is as follows (in thousands): As of June 22, Acquired in-process $ 130,188 Cash acquired 3,035 Accrued liabilities (20,047 ) Total cost to acquire asset $ 113,176 In accordance with ASC 730-10-25-2(c), In-process Basis of Presentation As a result of acquiring Spyre, and based on the criteria in Rule 3-05 S-X, S-X. 3-05 S-X The Company determined that it was the acquirer of Spyre under ASC 805 due to the relative voting rights in the combined entity, the composition of the governing body of the combined entity, and the composition of the senior management of the combined entity remaining relatively the same before and after the consummation of the transaction. The Company determined that the future conversion of the Series A Preferred Stock, if approved by the Company’s voting shareholders, was an independent event based on it being outside of the control of the Company and therefore substantive. The Company did not succeed to substantially all of Paragon’s business nor acquire from Paragon any separately identifiable line of business, the Company concluded that Paragon did not meet the definition of predecessor. 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 screen test to determine whether an acquisition is a business combination or an acquisition of assets. 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 to exclusively license IPR&D. 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 The Company has not generated any product revenues and has not achieved profitable operations. There is no assurance that profitable operations will ever be achieved, and, if achieved, could 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 the related uncertainties, there can be no assurance of the Company’s future success. In April 2023, the Board of Directors approved a restructuring of the Company’s workforce pursuant to which the Company’s workforce was reduced by approximately 83%, retaining approximately 10 employees. Additionally, the Company announced interim results from its ongoing Phase 1/2 clinical trial of pegtarviliase for the treatment of classical homocystinuria. Following a review of the interim results 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 the net liabilities from Spyre. Additionally, on June 26, 2023, the Company completed a private placement of shares of Series A Preferred Stock and 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 placement agent and other offering expenses. In accordance with ASC 205-40, are entitled to require us 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 our Certificate of Designation relating to the Series A Preferred Stock. The cash redemption is not in our control and raises substantial doubt about our ability to continue as a going concern. The accompanying financial statement assumes 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. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements and certain financial statement disclosures. 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. Key estimates that management considers in the preparation of the Company’s financial statements relate to accrued option costs payable to Paragon and the valuation of consideration transferred. The consideration transferred in acquiring IPR&D in connection with the acquisition of Spyre was comprised 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 its concurrent private financing transaction, which was an over-subscribed financing event involving a group of investors. Cash and Cash Equivalents As of June 22 023, Accrued Liabilities Accrued liabilities primarily consist of research initiation fees, reimbursable expenses under the Paragon Agreement for historical costs incurred by Paragon, professional and consulting fees, and the fair of assumed Parapyre Option Obligation.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies 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 consolidated financial statements and in-process right-of-use Pre-Merger which Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents consist of money market funds and debt securities and are stated at fair value. Marketable Securities All investments have been classified as available-for-sale available-for-sale available-for-sale For available-for-sale available-for-sale available-for-sale Any unrealized losses from declines in fair value below the amortized cost basis as a result of non-credit available-for-sale Restricted Cash Restricted cash consisted of money market accounts held by financial institutions as collateral for the Company’s obligations under a credit agreement and a facility lease for the Company’s corporate headquarters in Austin, Texas. The lease was terminated in August 2023 and the cash was subsequently unrestricted. Remaining restricted cash balances relate to the Company’s operations in the United Kingdom. Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents, marketable securities, and restricted cash. The Company’s investment policy limits investments to high credit quality securities issued by the U.S. government, U.S. government-sponsored agencies, highly rated banks, and corporate issuers, subject to certain concentration limits and restrictions on maturities. The Company’s cash, cash equivalents, marketable securities, and restricted cash are held by financial institutions that management believes are of high credit quality. The financial instruments that potentially subject the Company to a concentration of credit risk consist principally of cash deposits. Accounts at each of the Company’s two U.S. banking institutions are insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250,000 per depositor. As of December 31, 2023 and 2022, balances at the Company’s U.S. banking institutions exceeded the FDIC limits. The Company has not experienced any losses on its deposits of cash, cash equivalents, and restricted cash and its accounts are monitored by management to mitigate risk. The Company is exposed to credit risk in the event of default by the financial institutions holding its cash, cash equivalents, and restricted cash, and bond issuers. Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation and amortization. Depreciation and amortization are computed using the straight-line method over the estimated useful lives of the assets. Repairs and maintenance that do not extend the life or improve an asset are expensed as incurred. Upon retirement or sale, the cost of disposed assets and their related accumulated depreciation and amortization are removed from the balance sheet. Any gain or loss is credited or charged to operations. The useful lives of the property and equipment are as follows: Laboratory equipment 5 years Furniture and office equipment 5 years Computer equipment 3 years Software 3 years Leasehold improvements Shorter of remaining lease term or estimated useful life Impairment of Long-Lived Assets Long-lived assets are reviewed for indications of possible impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amounts to the future undiscounted cash flows attributable to these assets. An impairment loss is recognized to the extent an asset group is not recoverable, and the carrying amount exceeds the fair value. The Company recognized a $2.6 million impairment loss for the year ended December 31, 2023 related to its leased office space in Austin, Texas (see Note 17 for additional information). There were no impairments of long-lived assets for the years ended December 31, 2022 and 2021. Accrued Research and Development Costs The Company records the costs associated with research nonclinical studies, clinical trials, and manufacturing development as incurred. These costs are a significant component of the Company’s research and on-going The Company accrues for expenses resulting from obligations under the Paragon Agreement and agreements with CROs, CMOs, and other outside service providers for which payment flows do not match the periods over which materials or services are provided to the Company. Accruals are recorded based on estimates of services received and efforts expended pursuant to agreements established with Paragon, CROs, CMOs, and other outside service providers. These estimates are typically based on contracted amounts applied to the proportion of work performed and determined through analysis with internal personnel and external service providers as to the progress or stage of completion of the services. The Company makes significant judgments and estimates in determining the accrual balance in each reporting period. In the event advance payments are made to Paragon, a CRO, CMO, or outside service provider, the payments will be recorded as a prepaid asset which will be amortized as the contracted services are performed. As actual costs become known, the Company adjusts its accruals. Inputs, such as the services performed, the number of patients enrolled, or the study duration, may vary from the Company’s estimates, resulting in adjustments to research and development expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Historically, the Company has not experienced any material deviations between accrued and actual research and development expenses. Leases The Company determines if an arrangement is a lease at inception. Right-of-use Prior to the Company’s restructuring, as described in Note 17, the Company had lease agreements with lease and non-lease non-lease non-lease Fair Value of Financial Instruments The Company uses fair value measurements to record fair value adjustments to certain financial and non-financial orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities required to be recorded at fair value, the principal or most advantageous market in which the Company would transact are considered along with assumptions that market participants would use when pricing the asset or liability, such as inherent risk, transfer restrictions, and risk of nonperformance. The accounting standard for fair value establishes a fair value hierarchy based on three levels of inputs, the first two of which are considered observable and the last unobservable, that requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. A financial instrument’s categorization within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The three levels of inputs that may be used to measure fair value are as follows: Level 1: Observable inputs, such as quoted prices in active markets for identical assets or liabilities. Level 2: Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3: Valuations based on unobservable inputs to the valuation methodology and including data about assumptions that market participants would use in pricing the asset or liability based on the best information available under the circumstances. Financial instruments carried at fair value include cash equivalents and marketable securities. The carrying amounts of accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. Revenue Recognition Under ASC Topic 606, “Revenue from Contracts with Customers” (“Topic 606”), an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company assesses its license arrangements within the scope of Topic 606 in accordance with this framework as follows: License revenue The Company assesses whether the goods or services promised within each contract are distinct to identify those that are performance obligations. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. In assessing whether a promised good or service is distinct, and therefore a performance obligation, the Company considers factors such as the research, stage of development of the licensed product, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is The transaction price is determined and allocated to the identified performance obligations in proportion to their stand-alone selling prices (“SSP”) on a relative SSP basis. SSP is based on observable prices of the performance obligations or, when such prices are not observable, are estimated. The estimation of SSP may include factors such as forecasted revenues or costs, development timelines, discount rates, probabilities of technical and regulatory success, and considerations such as market conditions and entity-specific factors. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the SSP is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the amount of estimated variable consideration in the transaction price to the extent that it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates catch-up If an arrangement includes development, regulatory or commercial milestone payments, the Company evaluates whether the milestones are considered likely of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant cumulative revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered likely of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensee and the transfer of the promised goods or services to the licensees will be one year or less. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and if the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time, recognition is based on the use of an output or input method. The Company’s contracts may be modified for changes in the customer’s requirements. If contract modifications are for additional goods and services that are distinct from the existing contract, the modification will be accounted for as either a separate contract or a termination of the existing contract, depending on whether the additional goods or services reflects the SSP. If the additional goods or services in a contract modification are not distinct from the existing contract, they are accounted for as if they were part of the original contract. The effect of the contract modification on the transaction price and the measure of progress for the performance obligation to which it relates is recognized as an adjustment to revenue on a cumulative catch-up catch-up Collaborative arrangements The Company analyzes its license arrangements to assess whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities and therefore within the scope of ASC Topic 808, Collaborative Arrangements (“Topic 808”). This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For arrangements within the scope of Topic 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of Topic 808 and which elements of the collaboration are more reflective of a vendor-customer relationship and therefore within the scope of Topic 606. For elements of collaboration arrangements that are accounted for pursuant to Topic 808, an appropriate recognition method is determined and applied consistently, either by analogy to authoritative accounting literature or by applying a reasonable and rational policy election. For those elements of the arrangement that are accounted for pursuant to Topic 606, the Company applies the five-step model described above. Research and Development Costs Research and development costs are expensed as incurred. Research and development costs include, but are not limited to, salaries, benefits, travel, stock-based compensation, consulting costs, contract research service costs, laboratory supplies and facilities, contract manufacturing costs, and costs paid to other third parties that conduct research and development activities on the Company’s behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Advance payments for goods or services to be rendered in the future for use in research and development activities are recorded as a prepaid asset and expensed as the related goods are delivered or the services are performed. Stock-Based Compensation The Company recognizes the cost of stock-based awards granted to employees and non-employees non-employee 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 classified the Series B Preferred Stock outside of stockholders’ equity because, if conversion to Common Stock is not approved by the stockholders, the Series B 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 determined the Series B Preferred Stock did not contain any embedded derivatives and therefore the conversion and redemption features did not require bifurcation. Contingent Milestone Proceeds The Company recognizes contingent milestone proceeds associated from the sale of in-process 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 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 non-current net-cash 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. Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are recognized for the expected future tax consequences of temporary differences between the financial statements and the tax bases of assets and liabilities. Additionally, any changes in income tax laws are immediately recognized in the year of enactment. A valuation allowance is established against the deferred tax assets to reduce their carrying value to an amount that is more likely than not to be realized. The deferred tax assets and liabilities are classified as noncurrent along with the related valuation allowance. Due to a lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on the technical merits, as the largest amount of benefits that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the unrecognized tax benefits as a component of income tax expense, if applicable. As of December 31, 2023 and 2022, the Company had no unrecognized tax benefits and there were no interest or penalties incurred by the Company in the years ended December 31, 2023, 2022, or 2021. Comprehensive Loss Comprehensive loss is the change in stockholders’ equity from transactions and other events and circumstances other than those resulting from investments by stockholders and distributions to stockholders. The Company’s other comprehensive income (loss) is currently comprised of changes in unrealized losses and gains on available-for-sale 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 and Series B 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 and Series B Preferred Stock. The adoption of ASU 2020-06 did not have a material impact on the Company’s consolidated financial statements. Recently Issued Accounting Pronouncements In November 2023, the FASB issued ASU 2023-07, In December 2023, the FASB issued ASU 2023-09, tax

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 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 sets 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 December 31, 2023 Level 1 Level 2 Level 3 Total Financial Assets Money market funds $ 150,648 $ —  $ —  $ 150,648 U.S. government treasury securities 32,843 —  —  32,843 U.S. government agency securities —  16,257 —  16,257 Commercial paper —  104,141 —  104,141 Corporate bonds —  33,064 —  33,064 Total financial assets $ 183,491 $ 153,462 $ —  $ 336,953 Liabilities: CVR liability $ —  $ —  $ 42,700 $ 42,700 Total liabilities $ —  $ —  $ 42,700 $ 42,700 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 agency 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 asset or liabilities. The Level 2 assets include U.S. government agency securities, commercial paper 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. There were no transfers between Level 1 and Level 2 during the periods presented. 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 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 year ended December 31, 2023, $83.5 million 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. There was no similar expense for the year ended December 31, 2022 and 2021. The following table presents changes in the forward contract liability for the periods presented (in millions): Forward Contract Beginning balance as of June 22, 2023 $ 106.2 Change in fair value 83.5 Issuance of Series A Preferred Stock on July 7, 2023 (189.7 ) Ending balance as of December 31, 2023 $ —  CVR Liability In connection with the Asset Acquisition, a non-transferable Pre-Merger 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 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 regulatory success, and 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: December 31, 2023 Estimated cash flow dates 2/28/24 - 06/22/26 Estimated probability of success 39% - 100% Estimated reimbursement rate compared to reimbursement target 81% - 100% Risk-adjusted discount rates 5.91% - 6.32% The change in fair value between the issuance of the CVR and December 31, 2023 was a $19.0 million increase, and was primarily driven by changes in the expected timing of achievement of certain milestones, changes in the likelihood of certain milestones related to the approval received from the European Medicines Agency by Immedica Pharma AB (“Immedica”), partially offset by a change in the likelihood of a successful disposition of pegtarviliase and updates to expenses and deductions. 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 18,986 Payments (5,786 ) Ending Balance as of December 31, 2023 $ 42,700

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	12 Months Ended
	Dec. 31, 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): December 31, 2023 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash equivalents: Money market funds $ 150,648 $ —  $ —  $ 150,648 Commercial paper 24,950 5 —  24,955 U.S. government treasury securities 10,965 1 —  10,966 Total cash equivalents 186,563 6 —  186,569 Marketable securities: Commercial paper 79,124 62 —  79,186 Corporate bonds 32,984 81 (1 ) 33,064 U.S. government treasury securities 21,846 31 —  21,877 U.S. government agency securities 16,147 110 —  16,257 Total marketable securities $ 150,101 $ 284 $ (1 ) $ 150,384 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 agency 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 agency securities 495 —  —  495 Total marketable securities $  20,877 $ 2 $ (31 ) $  20,848 The following table summarizes the available-for-sale December 31, 2023 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ —  $ —  $ —  $ —  $ —  $ —  Corporate bonds 9,907 (1 ) —  —  9,907 (1 ) U.S. government treasury securities 4,831 —  —  —  4,831 —  Total marketable securities $ 14,738 $ (1 ) $ —  $ —  $ 14,738 $ (1 ) 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 ) Corporate bonds 3,732 (6 ) —  —  3,732 (6 ) U.S. government agency securities 3,735 (1 ) —  —  3,735 (1 ) 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 they will be required to sell the securities before recovery of the unamortized cost basis. As of December 31, 2023 and 2022, an allowance for credit losses had not There were $0.3 million unrealized gains on marketable securities for the year ended December 31, 2023. There were no realized gains on marketable securities for the year ended December 31, 2023, 2022 and 2021. Interest on marketable securities is included in interest income. Accrued interest receivable on available-for-sale securities The following table summarizes the contractual maturities of the Company’s marketable securities at estimated fair value (in thousands): December 31, 2023 2022 Due in one year or less $ 115,784 $ 20,848 Due in 1 – 2 years 34,600 —  Total marketable securities $ 150,384 $ 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.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net consist of the following (in thousands): December 31, 2023 2022 Laboratory equipment $ —  $ 2,257 Furniture and office equipment —  520 Computer equipment —  73 Software —  121 Leasehold improvements —  4,393 Property and equipment, gross —  7,364 Less: Accumulated depreciation and amortization —  (4,144 ) Property and equipment, net $ —  $ 3,220 Depreciation and amortization expense for the years ended December 31, 2023, 2022, and 2021 was $0.7 million, $1.4 million, and $1.4 million, respectively. All of the Company’s long-lived assets were located in the United States. Sale of Assets 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. 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 the Company’s 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.

Accrued and Other Current Liabi

Accrued and Other Current Liabilities	12 Months Ended
	Dec. 31, 2023
Accrued Liabilities and Other Liabilities [Abstract]
Accrued and Other Current Liabilities	6. Accrued and Other Current Liabilities Accrued and other current liabilities consist of the following (in thousands): December 31, 2023 2022 Accrued compensation $ 4,054 $ 4,589 Accrued contracted research and development costs 7,092 6,972 Accrued professional and consulting fees 1,474 946 Other 488 330 Total accrued and other current liabilities $ 13,108 $ 12,837

Accrued Liabilities

Accrued Liabilities	Jun. 22, 2023
Payables and Accruals [Abstract]
Accrued Liabilities	2. Accrued liabilities 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 on a quarterly basis for its services performed under each research program based on the actual costs incurred with mark-up Accrued liabilities consist of the following (in thousands): As of June 22, Accrued option cost payable to Paragon $ 18,987 Accrued professional and consulting fees 917 Fair value of assumed Parapyre Option Obligation (Note 3) 143 Accrued liabilities $ 20,047

Related Party Transactions

Related Party Transactions		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023
Related Party Transactions [Abstract]
Related Party Transactions	4. 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 Common Stock and Series A Preferred Stock of the Company. Fairmount Funds Management LLC (“Fairmount”) beneficially owns more than 5% of our capital stock, has two seats on the Company’s board of directors and beneficially owns more than 5% of Paragon, which is a joint venture between Fairmount and Fair Journey Biologics. Fairmount has 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.	7. Related Party Transactions Paragon and Parapyre Holding LLC (“Parapyre”) each beneficially own less than 5% of the Company’s capital stock through their respective holdings of the Company’s common stock. Fairmount Funds Management LLC (“Fairmount”) beneficially owns more than 5% of the Company’s capital stock on an as-converted In connection with the Asset Acquisition, the Company assumed the rights and obligations of Pre-Merger mark-up Pre-Merger For the year ended December 31, 2023, the Company recognized expenses related to services provided by Paragon subsequent to the Asset Acquisition totaling $48.5 million, which included $11.4 million of stock-based compensation expense, and were recorded as Research and development expenses in the consolidated statements of operations. As of December 31, 2023, $16.6 million was unpaid and was included in Related party accounts payable and other current liabilities on the Company’s consolidated balance sheets. For the year ended December 31, 2023, the Company made payments totaling $39.5 million to Paragon. On July 12, 2023 and December 14, 2023, the Company exercised the Option available under the Paragon Agreement with respect to the SPY001 and SPY002 research programs, respectively, and expects to enter into the SPY001 License Agreement and the SPY002 License Agreement. Following the execution of each of the SPY001 License Agreement and SPY002 License Agreement, the Company will be obligated to pay Paragon up to $22.0 million upon the achievement of specific development, regulatory and clinical milestones for the first product under each agreement, respectively, that achieves such specified milestones. Upon execution of each of the SPY001 License Agreement and the SPY002 License Agreement, we expect to pay Paragon a $1.5 million fee for nomination of a development candidate, as applicable, 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. The following is the summary of expenses related to the Paragon Agreement recognized within research and development expenses, which were ultimately settled in cash (in millions): December 31, 2023 2022 2021 Reimbursable costs under the Paragon Agreement $ 37.1 $ —  $ —  Parapyre Option Obligation As part of the Paragon Agreement, the Company is obligated to issue Parapyre a stock option grant on the last business day of 2023 and 2024 (the “Parapyre Option Obligation”). See Note 15 for additional information. The following is the summary of Related party accounts payable and other current liabilities (in millions): December 31, December 31, Reimbursable costs under the Paragon Agreement $ 16.6 $ —  Related party accounts payable and other current liabilities $ 16.6 $ —  December 2023 PIPE The December 2023 Investors included Fairmount, a related party. Fairmount’s participation in the December 2023 PIPE was approved by the Company’s board of directors. Fairmount’s investment accounted for $10.0 million of the $180.0 million gross proceeds raised in the December 2023 PIPE. Mark McKenna Option Grant On February 1, 2024, the Board appointed Mark McKenna as a Class I director. Mr. McKenna and the Company are parties to a consulting agreement, pursuant to which Mr. McKenna agreed to continue to provide consulting services as an independent contractor to the Company, with an effective date of August 1, 2023 (the “Vesting Commencement Date”). As compensation for Mr. McKenna’s consulting services, on November 22, 2023, he was granted non-qualified

Asset Acquisition

Asset Acquisition		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Asset Acquisition	3. 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 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. The Parapyre Option Obligation is considered a Level 2 liability based on observable market data for substantially the full term of the liabilities. 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 pre-clinical	8. Asset Acquisition On June 22, 2023, the Company acquired Pre-Merger Pre-Merger Pre-Merger Pre-Merger Pre-Merger Pre-Merger pre-clinical With respect to the Asset Acquisition, the Company determined that Aeglea was the acquirer for accounting purposes under ASC 805. The primary factors considered were a) the relative voting rights in the combined entity not resulting in a change of control, b) legacy members of the Company’s Board of Directors maintained control of the Board of Directors, and c) the only change in the composition of senior management was the appointment of a new Chief Operating Officer. Next, the Company considered whether the Asset Acquisition should be defined as a business under ASC 805. ASC 805-10-55-5A 55-5C Pre-Merger The Company completed the Asset Acquisition of Pre-Merger Pre-Merger 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. The forward contract related to the Common Stock was recorded as Additional paid-in 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, 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 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 the Company 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 $ 130.2 Cash acquired 3.0 Assumed liabilities (20.0 ) Total cost to acquire asset $ 113.2

Paragon Agreement

Paragon Agreement	12 Months Ended
	Dec. 31, 2023
Related Party Transactions [Abstract]
Paragon Agreement	9. Paragon Agreement In May 2023, Pre-Merger Pre-Merger Pre-Merger mark-up Pre-Merger As a result of the Asset Acquisition, the Company assumed the rights and obligations of Pre-Merger program-by-research For the year ended December 31, 2023, the Company made payments totaling $39.5 million to Paragon. On July 12, 2023 and December 14, 2023, the Company exercised the Option available under the Paragon Agreement with respect to the SPY001 and SPY002 research programs, respectively, and expects to enter into the SPY001 License Agreement and the SPY002 License Agreement. Our Option available under the Paragon Agreement with respect to the SPY003 and SPY004 programs remains unexercised. Following the execution of each of the SPY001 License Agreement and SPY002 License Agreement, the Company will be obligated to pay Paragon up to $22.0 million upon the achievement of specific development, regulatory and clinical milestones for the first product under each agreement, respectively, that achieves such specified milestones. Upon execution of each of the SPY001 License Agreement and the SPY002 License Agreement, the Company expects to pay Paragon a $1.5 million fee for nomination of a development candidate, as applicable, 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 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 these research programs, respectively.

Leases

Leases	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Leases	10. Leases Prior to the Company’s restructuring, as described in Note 17, the Company leased certain office space, laboratory facilities, and equipment. These leases required monthly lease payments that were subject to annual increases throughout the lease term. Certain of these leases also included renewal options at the election of the Company to renew or extend the lease for an additional three right-of-use In April 2019, the Company entered into a lease agreement (the “Las Cimas Lease”) for its corporate headquarters and laboratory space located in Austin, Texas. The Las Cimas Lease included approximately 30,000 square feet and commenced on April 30, 2019, with an expiration on April 30, 2028. The Company posted a customary letter of credit in the amount of $1.5 million as security, which is subject to automatic reductions per the terms of the Las Cimas Lease. A tenant allowance of up to $1.0 million was provided by the lessor and fully reimbursed to the Company. In August 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 including terminating the associated letter of credit. The following table summarizes the Company’s recognition of its operating and finance leases (in thousands): December 31, Classification 2023 2022 Assets Operating Operating lease right-of-use $ —  $ 3,430 Finance Other non-current —  597 Total leased assets —  4,027 Leases Current Operating Operating lease liabilities —  625 Finance Accrued and other current liabilities —  16 Non-current Operating Non-current —  4,004 Total lease liabilities $ —  $ 4,645 The following table summarizes the weighted-average remaining lease term and discount rates for the Company’s operating and finance leases: December 31, 2023 2022 Lease term (years) Operating leases 0.0 5.3 Finance leases 0.0 0.6 Discount rate Operating leases —  % 10.6 % Finance leases —  % 10.2 % The following table summarizes the lease costs pertaining to the Company’s operating leases (in thousands): Year Ended December 31, 2023 2022 2021 Operating lease cost $ 455 $ 910 $ 991 Variable lease cost 471 472 519 Total lease cost $ 926 $ 1,382 $ 1,510 Cash paid for amounts included in the measurement of operating lease liabilities during the years ended December 31, 2023 and 2022 was $0.5 million and $0.9 million, respectively, and was included within net cash used in operating activities in the cash flows. As of December 31, 2023, the Company had no operating or finance lease obligations.

Convertible Preferred Stock and

Convertible Preferred Stock and Stockholders' Equity	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Convertible Preferred Stock and Stockholders' Equity	11. Convertible Preferred Stock and Stockholders’ Equity The Company is authorized to issue 410,000,000 shares of capital stock of which 400,000,000 shares are designated as Common Stock and 10,000,000 shares are designated as preferred stock, all with a par value of $0.0001 per share. Each holder of Common Stock is entitled to one vote for each share of Common Stock held. The Common Stock is not entitled to preemptive rights, and is not subject to conversion, redemption or sinking As of December 31, 2023 and 2022, no Common Stock dividends had been declared by the board of directors. As of December 31, 2023 there were 437,037 shares of Series A preferred stock and 150,000 shares of Series B preferred stock outstanding. There were no shares of Series A preferred stock or shares of Series B preferred stock outstanding as of December 31, 2022. 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 S-3. June 2023 PIPE In June 2023, in connection with the Asset Acquisition, the Company issued and sold 721,452 shares of Series A Preferred Stock at approximately $291.08 per share through a private placement to a group of accredited investors. The net proceeds from this offering were approximately $197.3 million, after deducting placement agent fees and offering costs of $12.7 million. December 2023 PIPE In December 2023, the Company issued and sold 6,000,000 shares of Common Stock at an offering price of $15.00 per share and 150,000 shares of Series B Preferred Stock at $600 per share through a private placement to a group of accredited investors. The net proceeds from this offering were approximately $169.1 million, after deducting placement agent fees and offering costs of $10.9 million. Parapyre Warrants The Company settled its 2023 obligations under the Parapyre Option Obligation by issuing Parapyre 684,407 warrants to purchase the Company’s common stock, less the $21.52 per share exercise price of each warrant. As of December 31, 2023, none of the warrants issued under the Parapyre Option Obligation have been exercised. See Note 15 for additional information on the Parapyre Option Obligation. Pre-Funded In May 2022, the Company issued pre-funded paid-in As of December 31, 2023, the following pre-funded Issue Date Expiration Date Exercise Price Number of Warrants May 2022 None $ 0.0025 250,000 Total pre-funded 250,000 Series A Non-Voting 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 winding-up The Company held 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. Following stockholder approval of the Conversion Proposal, each share of Series A Preferred Stock automatically converted 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 0.0% and 20.0%) 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 PIPE Securities 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. On November 21, 2023, the Company’s stockholders approved the Conversion Proposal, among other matters, at a special meeting of stockholders. As a result of the approval of the Conversion Proposal, all conditions that could have required cash redemption of the Series A Preferred Stock were satisfied. Since the Series A Preferred Stock is no longer redeemable, the associated balances of the Series A Preferred Stock were reclassified from mezzanine equity to permanent equity during the fourth quarter of 2023. In addition, 649,302 shares of Series A Preferred Stock automatically converted to 25,972,080 shares of Common Stock; 437,037 shares of Series A Preferred Stock did not automatically convert and remain outstanding as of December 31, 2023 due to beneficial ownership limitations. This conversion was recorded as a reclassification between Series A Preferred Stock and Common Stock based on the historical per-share Series B Non-Voting On December 8, 2023, the Company filed a Certificate of Designation of Preferences, Rights and Limitations of Series B Non-Voting Pursuant to the Series B Certificate of Designation, holders of Series B Preferred Stock are entitled to receive dividends on shares of Series B Preferred Stock equal to, on an as-if-converted-to-Common winding-up The Company has agreed to use its best efforts to obtain stockholder approval of the conversion of all issued and outstanding Series B Preferred Stock into shares of Common Stock in accordance with the Nasdaq Stock Market Rules (the “Series B Conversion Proposal”) at its 2024 annual meeting of stockholders, which the Company agreed to hold no later than May 15, 2024. The Series B Preferred Stock is recorded outside of stockholders’ equity because, if conversion to Common Stock is not approved by the stockholders, the Series B 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 B Preferred Stock, on the last trading day prior to the holder’s redemption request. As of December 31, 2023, the redemption value of the Company’s outstanding Series B Preferred Stock was $129.1 million based on the closing stock price of the Company’s Common Stock on December 31, 2023 of $21.52 per share. The Company has determined that the Series B Preferred Stock did not contain any embedded derivatives and therefore the conversion and redemption features did not require bifurcation. Following stockholder approval of the Series B Conversion Proposal, each share of Series B Preferred Stock will automatically convert into 40 shares of the Common Stock, subject to certain limitations, including that a holder of Series B Preferred Stock is prohibited from converting shares of Series B Preferred Stock into shares of Common Stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than a specified percentage (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 December 11, 2023, as part of the December 2023 PIPE, the Company completed a private placement of 150,000 shares of Series B Preferred Stock in exchange for gross proceeds of $90.0 million.

Strategic License Agreements

Strategic License Agreements	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Strategic License Agreements	12. Strategic License Agreements Immedica Pharma AB License and Development Agreement On March 21, 2021, the Company entered into an exclusive license and supply agreement with Immedica Pharma AB (“Immedica”). 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 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 United States Food and Drug Administration (“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 does 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 do 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. Finally, Immedica will 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. In July 2021, the Company modified the agreement with Immedica to provide certain additional services in relation to the PEACE Phase 3 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 received a non-refundable mid-20 percent The Company concluded that Immedica meets the definition to be accounted for as a customer because the Company is 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 were conducting the ongoing PEACE Trial. Accordingly, the Company is 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 represents functional intellectual property given the functionality of the License is 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 be performed by other parties. Given that Immedica was not obligated to purchase any minimum amount or quantities of the Product, the supply of the 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 does 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 must be included in the transaction price. Additionally, the Company determined at inception of the arrangement that 50% of the 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 The Company has 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 was 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 is 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 relate to successful achievement of certain regulatory approvals, which might not be achieved. The Company determined that the royalties and commercial milestone payments relate predominantly to the license of intellectual property and are therefore excluded from the transaction price under the sales- or usage-based royalty exception of ASC 606. The Company will 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 will adjust its estimate of the transaction price as necessary. The Company will recognize the royalties and commercial milestone payments as revenue when the associated sales occur, and relevant sales-based thresholds are met. The Company 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 For the years ended December 31, 2023 and 2022, the Company recognized revenue of $0.9 million and $2.3 million, respectively, related to the PEACE Trial and BLA package performance obligation using a cost to cost model. The Company recognized revenue of $6.7 million related to the PEACE Trial and BLA package performance obligation using a cost to cost model and $12.0 million related to the transfer of the License for the year ended December 31, 2021. As of December 31, 2022, the Company recorded deferred revenue of $2.7 million associated with the license and supply agreement with Immedica, of which $0.5 million was classified as current. On July 27, 2023, the Company announced that it had entered into an agreement to sell the global rights to pegzilarginase 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 $16.4 million gain within Gain on Sale of in-process pre-paid 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 reduce the CVR liability and will be distributed to CVR holders pursuant to the CVR Agreement resulting from the Asset Acquisition. Contract Balances from Customer Contract The timing of revenue recognition, billings and cash collections results in contract assets and contract liabilities on the 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): Year Ended December 31, 2022 December 31, Additions Deductions December 31, Contract liabilities: Deferred revenue $ 2,696 $ 575 $ (3,271 ) $ —  The Company had no contract assets during the years ended December 31, 2023 and 2022.

Sale of Pegzilarginase to Immed

Sale of Pegzilarginase to Immedica	12 Months Ended
	Dec. 31, 2023
Discontinued Operations and Disposal Groups [Abstract]
Sale of Pegzilarginase to Immedica	13. Sale of Pegzilarginase to Immedica On July 27, 2023, the Company announced that it had entered into an agreement to sell the global rights to pegzilarginase 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 $16.4 million gain within Gain on sale of in-process pre-paid 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. Accordingly, the Company will recognize any future milestone payments once the contingency is resolved and payment is contractually required. The upfront payment and contingent milestone payments if paid, net of expenses and adjustments, will be distributed to CVR holders pursuant to the CVR Agreement resulting from the Asset Acquisition.

Novation of Manufacturing Agree

Novation of Manufacturing Agreements	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Novation of Manufacturing Agreements	14. 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 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, know-how, 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 one 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.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Stock-Based Compensation	15. Stock-Based Compensation 2015 Equity Incentive Plan In March 2015, the Company adopted the 2015 Equity Incentive Plan (“2015 Plan”), administered by the board of directors, and provides for the Company to sell or issue share of Common Stock or restricted Common Stock, or to grant incentive stock options or nonqualified stock options for the purchase of Common Stock, to employees, members of the board of directors and consultants of the Company. Under the terms of the 2015 Plan, the exercise prices, vesting and other restrictions may be determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant, the term of stock options may not be greater than ten years for all grants, and for grantees holding more than 10% of the total combined voting power of all classes of stock, the term may not be greater than five years. The Company granted options under the 2015 Plan until April 2016 when it was terminated as to future awards, although it continues to govern the terms of options that remain outstanding under the 2015 Plan. As of December 31, 2023, a total of 3,029 shares of Common Stock are subject to options outstanding under the 2015 Plan and will become available under the 2016 Equity Incentive Plan (“2016 Plan”) to the extent the options are forfeited or lapse unexercised. 2016 Equity Incentive Plan The 2016 Plan became effective in April 2016 and serves as the successor to the 2015 Plan. Under the 2016 Plan, the Company may grant stock options, stock appreciation rights, restricted stock awards, restricted stock units, performance awards, and stock bonuses. The 2016 Plan provides for an initial reserve of 44,000 shares of Common Stock, plus 20,395 shares of Common Stock remaining under the 2015 Plan, and any share awards that subsequently are forfeited or lapse unexercised under the 2015 Plan. The shares reserved exclude shares of Common Stock reserved for issuance under the 2015 Plan. In October 2018, the 2016 Plan was amended to increase the number of shares of Common Stock reserved for issuance thereunder by 70,384 shares, extend the term of the 2016 Plan through August 7, 2028, and provide for an automatic increase in the number of shares reserved for issuance thereunder on January 1 of each year for In November 2023, the 2016 Plan was amended to (i) increase the number of shares of Common Stock reserved for issuance thereunder by 4,481,152 shares, (ii) revise the annual limit on non-employee non-employee pre-funded non-voting As of December 31, 2023, the total number of shares reserved for issuance under the 2016 Plan was 5,019,177, of which 3,294,962 shares were subject to outstanding option awards and restricted unit awards. 2018 Equity Inducement Plan In February 2018, the board of directors approved and adopted the 2018 Equity Inducement Plan (“2018 Plan”), which became effective on the same date. The board of directors approved an initial reserve of 44,000 shares of Common Stock to be used exclusively for individuals who were not previously employees or directors, or following a bona fide period of non-employment, During 2023, the 2018 Plan was amended to increase the number of shares of Common Stock reserved for issuance by 6,000,000. Under the 2016 Plan and 2018 Plan, the Company may grant stock-based awards with service conditions (“service-based” awards), performance conditions (“performance-based” awards), and market conditions (“market-based” awards). Service-based awards granted under the 2018 Plan, 2016 Plan, and 2015 Plan generally vest over four years and expire after ten years, although awards have been granted with vesting terms less than four years. The Company granted 153,865 service-based restricted stock units (“RSUs”) during the year ended December 31, 2023 to certain employees under the 2018 Plan. As of December 31, 2023, the total number of shares reserved for issuance under the 2018 Plan was 6,044,000, of which 5,350,595 shares were subject to outstanding awards. 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 Common Stock. The acquisition-date fair value of these grants will be recognized as an expense on a pro-rata 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 warrants for Parapyre to purchase 1% of the then outstanding shares of Pre-Merger Pre-Merger 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 warrants is based on the outstanding shares of the Company’s Common Stock, (ii) establishing the grant date as the last business day of 2023 and 2024, and (iii) defining the term of the warrants granted as ten years. The Company determined that the 2023 and 2024 grants are two separate grants, as there would be no obligation for the 2024 grant had the Company exercised or terminated all of the options under the Paragon Agreement prior to December 31, 2023. The service inception period for the grant precedes the grant date, with the full award being vested as of the grant date with no post-grant date service requirement. Accordingly, a liability related to the Parapyre Option Obligation was recorded pursuant to the amended Paragon Agreement during 2023 interim periods. The Company determined that the grant date of the award was December 31, 2023, as all terms of the award, including number of shares and exercise price, were known by all parties. Accordingly, the Company measured the grant-date fair value of the warrants granted at approximately $11.5 million as an equity-classified award, of which $0.1 million was recognized as part of the liabilities assumed with the Asset Acquisition on June 22, 2023. For the year ended December 31, 2023, $11.4 million was recognized as stock compensation expense related to the Parapyre Option Obligation. There was no similar expense for the years ended December 31, 2022 and 2021. As of December 31, 2023, the unamortized expense related to the Parapyre Option Obligation was nil. The following table summarizes employee and non-employee Shares Issuable Under Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Outstanding as of December 31, 2022 405,082 $ 113.75 6.72 $ 2 Granted 8,776,245 9.67 Exercised (46,246 ) 8.22 Forfeited (637,686 ) 43.00 Outstanding as of December 31, 2023 8,497,395 $ 12.13 8.40 $ 98,928 Options vested and expected to vest as of December 31, 2023 8,497,395 $ 12.13 8.40 $ 98,928 Options exercisable as of December 31, 2023 1,065,700 $ 24.72 5.62 $ 13,328 The aggregate intrinsic value of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the fair value of the Company’s Common Stock as of the reporting date. For the years ended December 31, 2023, 2022, and 2021, the weighted-average grant date fair value of options granted was $9.67, $1.80, and $4.96, per share, respectively. The total intrinsic value of options exercised during the years ended December 31, 2023, and 2021 was $0.4 million and $0.7 million, respectively. No options were exercised in the year ended December 31, 2022. There were 477,000 stock options issued to non-employees non-employees non-employee 2016 Employee Stock Purchase Plan The 2016 Employee Stock Purchase Plan (“2016 ESPP”) became effective in April 2016. A total of 6,600 shares of Common Stock were In June 2018, the 2016 ESPP was amended to provide for an automatic annual increase in the number of shares reserved for issuance thereunder on January 1 of each year for the remaining term of the year equal to (a) 1.0% 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 of directors each year. As a result of the operation of this provision, on January 1, 2023, 2022 and 2021, an additional 26,140, 19,742, and 19,184 shares, respectively, became available for issuance under the 2016 ESPP. As of December 31, 2023, the reserve remaining and available for future issuance under the 2016 ESPP was 72,404 shares. In February 2023, the 2016 ESPP was amended to increase the maximum shares purchased during any one period from 80 shares to 400 shares or a lesser amount determined by the board of directors. For the year ended December 31, 2023, stock-based compensation expense related to the 2016 ESPP plan was di minimis. For the years ended 2022 and 2021, stock-based compensation expense related to the 2016 ESPP plan was $0.1 million and $0.2 million, respectively. Restricted Common Stock Units In July 2020, the Company granted 9,128 restricted stock units to certain employees, with vesting terms subject to regulatory, commercial, and clinical milestones, in addition to a service condition. As of December 31, 2023 none of these restricted stock units had vested and all restricted stock units were forfeited since the performance milestones were not met within the required time frame. No stock-based compensation expense was recognized on these awards. The Company granted 153,865 service-based restricted stock units during the year ended December 31, 2023. There were no restricted stock units granted during the years ended December 31, 2022 and 2021. The following table summarizes employee restricted stock activity for the year ended December 31, 2023: Shares Weighted Average Grant Date Fair Value Unvested restricted stock units as of December 31, 2022 5,660 $ 203.25 Granted 153,865 18.17 Vested —  —  Forfeited (5,660 ) 203.25 Unvested restricted stock units as of December 31, 2023 153,865 $ 18.17 There were no restricted stock units granted to non-employees Stock-Based Compensation Expense Total stock-based compensation expense recognized from the Company’s equity incentive plans, 2018 Plan, and the 2016 ESPP for the years ended December 31, 2023, 2022, and 2021 was as follows (in thousands): Year Ended December 31, 2023 2022 2021 Employees Non- Employees Employees Non- Employees Employees Non- Employees Research and development $ 2,910 $ 11,328 $ 2,591 $ —  $ 2,723 $ —  General and administrative 11,327 109 4,520 —  5,315 —  Total stock-based compensation expense $ 14,237 $ 11,437 $ 7,111 $ —  $ 8,038 $ —  No related tax benefits were recognized for the years ended December 31, 2023, 2022, and 2021 (see Note 18). The employee and non-employee non-employee non-employee As of December 31, 2023, the Company had an aggregate of $64.4 million of unrecognized stock-based compensation expense for options outstanding, which is expected to be recognized over a weighted average period of 3.5 years. In determining the fair value of the stock-based awards, the Company uses the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment to determine. Expected Term The Company’s expected term represents the period that the Company’s stock-based awards are expected to be outstanding and is determined using the simplified method (based on the midpoint between the vesting date and the end of the contractual term). The Company utilizes this method due to lack of historical exercise data and the plain-vanilla nature of the Company’s stock-based awards. Expected Volatility Since the Company was privately held through April 2016 and transitioned from a clinical stage company to a pre-clinical pre-clinical Risk-Free Interest Rate The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of option. Expected Dividend The Company has never paid dividends on its Common Stock and has no plans to pay dividends on its Common Stock. Therefore, the Company used an expected dividend yield of zero. Valuation of Stock Options and 2016 ESPP The fair value of the stock options granted under the the Company’s equity incentive plans, as well as the shares available for purchase under the 2016 ESPP were determined using the Black-Scholes option-pricing model. The following table summarizes the weighted-average assumptions used in calculating the fair value of the awards: Year Ended December 31, 2023 2022 2021 Stock Options Granted Expected term (in years) 5.88 6.00 5.99 Expected volatility 107 % 84 % 83 % Risk-free interest 4.37 % 2.93 % 0.88 % Dividend yield 0 % 0 % 0 % 2016 ESPP Expected term (in years) 0.49 0.49 0.50 Expected volatility 181 % 84 % 86 % Risk-free interest 4.99 % 1.95 % 0.08 % Dividend yield 0 % 0 % 0 %

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2023
Retirement Benefits [Abstract]
Defined Contribution Plan	16. Defined Contribution Plan The Company sponsors a 401(k) retirement plan in which substantially all of its full-time employees are eligible to participate. Participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. During the years ended December 31, 2023, 2022, 2021, the Company provided $0.2 million, $0.6 million, and $0.6 million, respectively, in contributions to the plan.

Restructuring Charges

Restructuring Charges	12 Months Ended
	Dec. 31, 2023
Restructuring and Related Activities [Abstract]
Restructuring Charges	17. 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 $6.4 million during the year ended December 31, 2023. Cash payments for employee related restructuring charges of $5.3 million were paid as of December 31, 2023. In addition, the Company recognized $1.0 million in non-cash 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 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 December 31, 2023 Severance liability $ —  $ 6,448 $ (5,325 ) $ 1,123 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 the Company’s 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 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 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 Stock Loss on Lease Asset Total 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

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	18. Income Taxes The following table summarizes the (loss) income before income tax expense by jurisdiction for the periods indicated: Year Ended December 31, 2023 2022 2021 Domestic $ (338,942 ) $ (84,113 ) $ (65,940 ) Foreign 126 162 280 Loss before income tax expense $ (338,816 ) $ (83,951 ) $ (65,660 ) For the year ended December 31, 2023, the Company recognized no provision or benefit from income taxes. For both the years ended December 31, 2022 and 2021, the Company recognized an income tax expense of $0.1 million, related to foreign subsidiaries income tax expense and the Texas margins tax. The difference between the Company’s provision for income taxes and the amounts computed by applying the statutory federal income tax rate to income before income taxes is as follows (in thousands): Year Ended December 31, 2023 2022 2021 Tax provision derived by applying the federal statutory rate to income before income taxes $ (71,151 ) $ (17,630 ) $ (13,789 ) Loss on forward contract valuation 17,541 —  —  Acquired IPR&D 27,340 —  —  Loss on CVR revaluation 3,987 —  —  Other permanent differences 4,472 1,042 1,002 Federal tax credits (1 ) (3,559 ) (3,815 ) State tax credits —  (640 ) (152 ) Effect of tax rate on foreign jurisdiction (53 ) 42 (5 ) Change in the valuation allowance 17,839 20,609 16,900 Income tax (benefit) expense $ (26 ) $ (136 ) $ 141 The components of the deferred tax assets and liabilities consist of the following (in thousands): December 31, 2023 2022 Deferred tax assets Net operating loss carryforward $ 74,454 $ 68,917 Capitalized 174 R&D costs 22,532 11,097 Intangible assets 47 52 Deferred revenue —  566 Accrued expense 579 668 Stock-based compensation 4,246 3,293 Federal tax credits 21,914 21,914 State tax credits 1,631 1,631 Other 88 190 Total deferred tax assets 125,491 108,328 Deferred tax liabilities Depreciable assets —  (676 ) Total deferred tax liabilities —  (676 ) Less: Valuation allowance (125,491 ) (107,652 ) Deferred tax assets, net $ —  $ —  The Company has established a full federal and state valuation allowance equal to the net deferred tax assets due to uncertainties regarding the realization of the deferred tax asset based on the Company’s lack of earnings history. The valuation allowance increased by $17.8 million, $20.6 million, and $16.9 million during the years ended December 31, 2023, 2022, and 2021, respectively, primarily due to continuing loss from operations. As of December 31, 2023 and 2022, the Company had U.S. net operating loss carryforwards (“NOL”) of $354.5 million and $328.2 million, respectively. For both the years ended December 31, 2023 and 2022, the Company had U.S. tax credit carryforwards and state tax credit carryforwards of $21.9 million and $1.6 million, respectively. Of the net operating loss and tax credit carryforwards $58.4 million and $21.9 million, respectively, will expire in 2033, if not utilized. Any remaining net operating loss will carry forward indefinitely and can be utilized to offset up to 80% of the taxable income in any tax year. The net operating loss and credit carryforwards are subject to Internal Revenue Service adjustments until the statute closes on the year the net operating loss or tax credits are utilized. The Company has not completed a study to assess whether an ownership change has occurred or whether there have been multiple ownership changes since the Company’s formation due to the complexity and cost associated with such a study, and the fact that there may be additional such ownership changes in the future. If the Company has experienced an ownership change at any time since its formation, utilization of the NOL or research and development credit carryforwards would be subject to an annual limitation under Section 382 or 383 of the Internal Revenue Code, which is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term, tax-exempt The Company is subject to examination by taxing authorities in its significant jurisdictions for the 2019 and subsequent years. However, due to NOL and tax attribute carryovers, the taxing authorities have the ability to adjust the NOLs and other tax attributes related to closed years. As of December 31, 2023 and 2022, there were no amounts recorded for uncertain tax positions. As of December 31, 2023, undistributed earnings of the Company’s incorporated foreign subsidiaries are immaterial. Under the Global Intangible Low-Taxed

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Net Loss Per Share	19. Net Loss Per Share The Company computes net loss attributable per common stockholder using the two-class The two-class 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 pre-funded

Subsequent Events

Subsequent Events	Jun. 22, 2023
Subsequent Events [Abstract]
Subsequent Events	5. Subsequent Events The Company’s management has evaluated subsequent events up to October 6, 2023, the date the Statement of Assets Acquired and Liabilities Assumed from Spyre, Inc. was available to be issued. There have been no subsequent events that require recognition or disclosure in this financial statement except for the following described below. On July 7, 2023, the Company issued 517,809 shares of common stock and 364,887 shares of Series A Preferred Stock as part of its consideration transferred in connection with the Asset Acquisition. On August 8, 2023, the Company filed a preliminary proxy statement with the SEC to solicit approval of the conversion of the Series A Preferred Stock into shares of Common Stock in connection with the Asset acquisition, among other matters, at a special meeting of stockholders. On September 8, 2023, the Company’s Board of Directors approved a reverse stock split of the Company’s common stock, par value $0.0001 per share, at a ratio of 1-for-25 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 is ten years. The liability related to the Parapyre Option Obligation will be recorded pursuant to the amended Paragon Agreement on a go forward basis. The Company determined that these amendments would not have materially impacted the liability as of June 22, 2023.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation As a result of acquiring Spyre, and based on the criteria in Rule 3-05 S-X, S-X. 3-05 S-X The Company determined that it was the acquirer of Spyre under ASC 805 due to the relative voting rights in the combined entity, the composition of the governing body of the combined entity, and the composition of the senior management of the combined entity remaining relatively the same before and after the consummation of the transaction. The Company determined that the future conversion of the Series A Preferred Stock, if approved by the Company’s voting shareholders, was an independent event based on it being outside of the control of the Company and therefore substantive. The Company did not succeed to substantially all of Paragon’s business nor acquire from Paragon any separately identifiable line of business, the Company concluded that Paragon did not meet the definition of predecessor. 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 screen test to determine whether an acquisition is a business combination or an acquisition of assets. 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 to exclusively license IPR&D. 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 The Company has not generated any product revenues and has not achieved profitable operations. There is no assurance that profitable operations will ever be achieved, and, if achieved, could 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 the related uncertainties, there can be no assurance of the Company’s future success. In April 2023, the Board of Directors approved a restructuring of the Company’s workforce pursuant to which the Company’s workforce was reduced by approximately 83%, retaining approximately 10 employees. Additionally, the Company announced interim results from its ongoing Phase 1/2 clinical trial of pegtarviliase for the treatment of classical homocystinuria. Following a review of the interim results 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 the net liabilities from Spyre. Additionally, on June 26, 2023, the Company completed a private placement of shares of Series A Preferred Stock and 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 placement agent and other offering expenses. In accordance with ASC 205-40, are entitled to require us 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 our Certificate of Designation relating to the Series A Preferred Stock. The cash redemption is not in our control and raises substantial doubt about our ability to continue as a going concern. The accompanying financial statement assumes 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.	Basis of Presentation The consolidated financial statements have been prepared in conformity with generally accepted accounting principles in the United States (“U.S. GAAP”) as defined by the Financial Accounting Standards Board (“FASB”) and include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
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 reported amounts of assets and liabilities at the date of the financial statements and certain financial statement disclosures. 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. Key estimates that management considers in the preparation of the Company’s financial statements relate to accrued option costs payable to Paragon and the valuation of consideration transferred. The consideration transferred in acquiring IPR&D in connection with the acquisition of Spyre was comprised 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 its concurrent private financing transaction, which was an over-subscribed financing event involving a group of investors.	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 consolidated financial statements and in-process right-of-use Pre-Merger which
Cash and Cash Equivalents	Cash and Cash Equivalents As of June 22 023,	Cash and Cash Equivalents The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. Cash equivalents consist of money market funds and debt securities and are stated at fair value.
Marketable Securities		Marketable Securities All investments have been classified as available-for-sale available-for-sale available-for-sale For available-for-sale available-for-sale available-for-sale Any unrealized losses from declines in fair value below the amortized cost basis as a result of non-credit available-for-sale
Restricted Cash		Restricted Cash Restricted cash consisted of money market accounts held by financial institutions as collateral for the Company’s obligations under a credit agreement and a facility lease for the Company’s corporate headquarters in Austin, Texas. The lease was terminated in August 2023 and the cash was subsequently unrestricted. Remaining restricted cash balances relate to the Company’s operations in the United Kingdom.
Concentration of Credit Risk		Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents, marketable securities, and restricted cash. The Company’s investment policy limits investments to high credit quality securities issued by the U.S. government, U.S. government-sponsored agencies, highly rated banks, and corporate issuers, subject to certain concentration limits and restrictions on maturities. The Company’s cash, cash equivalents, marketable securities, and restricted cash are held by financial institutions that management believes are of high credit quality. The financial instruments that potentially subject the Company to a concentration of credit risk consist principally of cash deposits. Accounts at each of the Company’s two U.S. banking institutions are insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250,000 per depositor. As of December 31, 2023 and 2022, balances at the Company’s U.S. banking institutions exceeded the FDIC limits. The Company has not experienced any losses on its deposits of cash, cash equivalents, and restricted cash and its accounts are monitored by management to mitigate risk. The Company is exposed to credit risk in the event of default by the financial institutions holding its cash, cash equivalents, and restricted cash, and bond issuers.
Property and Equipment		Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation and amortization. Depreciation and amortization are computed using the straight-line method over the estimated useful lives of the assets. Repairs and maintenance that do not extend the life or improve an asset are expensed as incurred. Upon retirement or sale, the cost of disposed assets and their related accumulated depreciation and amortization are removed from the balance sheet. Any gain or loss is credited or charged to operations. The useful lives of the property and equipment are as follows: Laboratory equipment 5 years Furniture and office equipment 5 years Computer equipment 3 years Software 3 years Leasehold improvements Shorter of remaining lease term or estimated useful life
Impairment of Long-Lived Assets		Impairment of Long-Lived Assets Long-lived assets are reviewed for indications of possible impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amounts to the future undiscounted cash flows attributable to these assets. An impairment loss is recognized to the extent an asset group is not recoverable, and the carrying amount exceeds the fair value. The Company recognized a $2.6 million impairment loss for the year ended December 31, 2023 related to its leased office space in Austin, Texas (see Note 17 for additional information). There were no impairments of long-lived assets for the years ended December 31, 2022 and 2021.
Accrued Research and Development Costs		Accrued Research and Development Costs The Company records the costs associated with research nonclinical studies, clinical trials, and manufacturing development as incurred. These costs are a significant component of the Company’s research and on-going The Company accrues for expenses resulting from obligations under the Paragon Agreement and agreements with CROs, CMOs, and other outside service providers for which payment flows do not match the periods over which materials or services are provided to the Company. Accruals are recorded based on estimates of services received and efforts expended pursuant to agreements established with Paragon, CROs, CMOs, and other outside service providers. These estimates are typically based on contracted amounts applied to the proportion of work performed and determined through analysis with internal personnel and external service providers as to the progress or stage of completion of the services. The Company makes significant judgments and estimates in determining the accrual balance in each reporting period. In the event advance payments are made to Paragon, a CRO, CMO, or outside service provider, the payments will be recorded as a prepaid asset which will be amortized as the contracted services are performed. As actual costs become known, the Company adjusts its accruals. Inputs, such as the services performed, the number of patients enrolled, or the study duration, may vary from the Company’s estimates, resulting in adjustments to research and development expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations. Historically, the Company has not experienced any material deviations between accrued and actual research and development expenses.
Leases		Leases The Company determines if an arrangement is a lease at inception. Right-of-use Prior to the Company’s restructuring, as described in Note 17, the Company had lease agreements with lease and non-lease non-lease non-lease
Fair Value of Financial Instruments		Fair Value of Financial Instruments The Company uses fair value measurements to record fair value adjustments to certain financial and non-financial orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities required to be recorded at fair value, the principal or most advantageous market in which the Company would transact are considered along with assumptions that market participants would use when pricing the asset or liability, such as inherent risk, transfer restrictions, and risk of nonperformance. The accounting standard for fair value establishes a fair value hierarchy based on three levels of inputs, the first two of which are considered observable and the last unobservable, that requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. A financial instrument’s categorization within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement. The three levels of inputs that may be used to measure fair value are as follows: Level 1: Observable inputs, such as quoted prices in active markets for identical assets or liabilities. Level 2: Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3: Valuations based on unobservable inputs to the valuation methodology and including data about assumptions that market participants would use in pricing the asset or liability based on the best information available under the circumstances. Financial instruments carried at fair value include cash equivalents and marketable securities. The carrying amounts of accounts payable and accrued liabilities approximate fair value due to their relatively short maturities.
Revenue Recognition		Revenue Recognition Under ASC Topic 606, “Revenue from Contracts with Customers” (“Topic 606”), an entity recognizes revenue when its customer obtains control of promised goods or services, in an amount that reflects the consideration that the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer; (ii) identify the performance obligations in the contract; (iii) determine the transaction price, including variable consideration, if any; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company assesses its license arrangements within the scope of Topic 606 in accordance with this framework as follows: License revenue The Company assesses whether the goods or services promised within each contract are distinct to identify those that are performance obligations. This assessment involves subjective determinations and requires management to make judgments about the individual promised goods or services and whether such are separable from the other aspects of the contractual relationship. In assessing whether a promised good or service is distinct, and therefore a performance obligation, the Company considers factors such as the research, stage of development of the licensed product, manufacturing and commercialization capabilities of the customer and the availability of the associated expertise in the general marketplace. The Company also considers the intended benefit of the contract in assessing whether a promised good or service is separately identifiable from other promises in the contract. If a promised good or service is not distinct, the Company is required to combine that good or service with other promised goods or services until it identifies a bundle of goods or services that is The transaction price is determined and allocated to the identified performance obligations in proportion to their stand-alone selling prices (“SSP”) on a relative SSP basis. SSP is based on observable prices of the performance obligations or, when such prices are not observable, are estimated. The estimation of SSP may include factors such as forecasted revenues or costs, development timelines, discount rates, probabilities of technical and regulatory success, and considerations such as market conditions and entity-specific factors. In certain circumstances, the Company may apply the residual method to determine the SSP of a good or service if the SSP is considered highly variable or uncertain. The Company validates the SSP for performance obligations by evaluating whether changes in the key assumptions used to determine the SSP will have a significant effect on the allocation of arrangement consideration between multiple performance obligations. If the consideration promised in a contract includes a variable amount, the Company estimates the amount of consideration to which it will be entitled in exchange for transferring the promised goods or services to a customer. The Company determines the amount of variable consideration by using the expected value method or the most likely amount method. The Company includes the amount of estimated variable consideration in the transaction price to the extent that it is probable that a significant reversal of cumulative revenue recognized will not occur. At the end of each subsequent reporting period, the Company re-evaluates catch-up If an arrangement includes development, regulatory or commercial milestone payments, the Company evaluates whether the milestones are considered likely of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant cumulative revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the Company’s control or the licensee’s control, such as regulatory approvals, are generally not considered likely of being achieved until those approvals are received. In determining the transaction price, the Company adjusts consideration for the effects of the time value of money if the timing of payments provides the Company with a significant benefit of financing. The Company does not assess whether a contract has a significant financing component if the expectation at contract inception is such that the period between payment by the licensee and the transfer of the promised goods or services to the licensees will be one year or less. For arrangements with licenses of intellectual property that include sales-based royalties, including milestone payments based on the level of sales, and if the license is deemed to be the predominant item to which the royalties relate, the Company recognizes royalty revenue and sales-based milestones at the later of (i) when the related sales occur, or (ii) when the performance obligation to which the royalty has been allocated has been satisfied. The Company recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) each performance obligation is satisfied at a point in time or over time, and if over time, recognition is based on the use of an output or input method. The Company’s contracts may be modified for changes in the customer’s requirements. If contract modifications are for additional goods and services that are distinct from the existing contract, the modification will be accounted for as either a separate contract or a termination of the existing contract, depending on whether the additional goods or services reflects the SSP. If the additional goods or services in a contract modification are not distinct from the existing contract, they are accounted for as if they were part of the original contract. The effect of the contract modification on the transaction price and the measure of progress for the performance obligation to which it relates is recognized as an adjustment to revenue on a cumulative catch-up catch-up Collaborative arrangements The Company analyzes its license arrangements to assess whether such arrangements involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities and therefore within the scope of ASC Topic 808, Collaborative Arrangements (“Topic 808”). This assessment is performed throughout the life of the arrangement based on changes in the responsibilities of all parties in the arrangement. For arrangements within the scope of Topic 808 that contain multiple elements, the Company first determines which elements of the collaboration are deemed to be within the scope of Topic 808 and which elements of the collaboration are more reflective of a vendor-customer relationship and therefore within the scope of Topic 606. For elements of collaboration arrangements that are accounted for pursuant to Topic 808, an appropriate recognition method is determined and applied consistently, either by analogy to authoritative accounting literature or by applying a reasonable and rational policy election. For those elements of the arrangement that are accounted for pursuant to Topic 606, the Company applies the five-step model described above.
Research and Development Costs		Research and Development Costs Research and development costs are expensed as incurred. Research and development costs include, but are not limited to, salaries, benefits, travel, stock-based compensation, consulting costs, contract research service costs, laboratory supplies and facilities, contract manufacturing costs, and costs paid to other third parties that conduct research and development activities on the Company’s behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Advance payments for goods or services to be rendered in the future for use in research and development activities are recorded as a prepaid asset and expensed as the related goods are delivered or the services are performed.
Stock-Based Compensation		Stock-Based Compensation The Company recognizes the cost of stock-based awards granted to employees and non-employees non-employee
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 classified the Series B Preferred Stock outside of stockholders’ equity because, if conversion to Common Stock is not approved by the stockholders, the Series B 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 determined the Series B Preferred Stock did not contain any embedded derivatives and therefore the conversion and redemption features did not require bifurcation.
Contingent Milestone Proceeds		Contingent Milestone Proceeds The Company recognizes contingent milestone proceeds associated from the sale of in-process
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
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 non-current net-cash 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.
Income Taxes		Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are recognized for the expected future tax consequences of temporary differences between the financial statements and the tax bases of assets and liabilities. Additionally, any changes in income tax laws are immediately recognized in the year of enactment. A valuation allowance is established against the deferred tax assets to reduce their carrying value to an amount that is more likely than not to be realized. The deferred tax assets and liabilities are classified as noncurrent along with the related valuation allowance. Due to a lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on the technical merits, as the largest amount of benefits that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the unrecognized tax benefits as a component of income tax expense, if applicable. As of December 31, 2023 and 2022, the Company had no unrecognized tax benefits and there were no interest or penalties incurred by the Company in the years ended December 31, 2023, 2022, or 2021.
Comprehensive Loss		Comprehensive Loss Comprehensive loss is the change in stockholders’ equity from transactions and other events and circumstances other than those resulting from investments by stockholders and distributions to stockholders. The Company’s other comprehensive income (loss) is currently comprised of changes in unrealized losses and gains on available-for-sale
Net Loss Per Share		The Company computes net loss attributable per common stockholder using the two-class The two-class 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 pre-funded
Recently Adopted Accounting Pronouncement/Recently Issued 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 and Series B 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 and Series B Preferred Stock. The adoption of ASU 2020-06 did not have a material impact on the Company’s consolidated financial statements. Recently Issued Accounting Pronouncements In November 2023, the FASB issued ASU 2023-07, In December 2023, the FASB issued ASU 2023-09, tax

Description of the Business a_2

Description of the Business and Summary of Significant Accounting Policies (Policies)	Jun. 22, 2023
Accounting Policies [Abstract]
Accrued Liabilities	Accrued Liabilities Accrued liabilities primarily consist of research initiation fees, reimbursable expenses under the Paragon Agreement for historical costs incurred by Paragon, professional and consulting fees, and the fair of assumed Parapyre Option Obligation.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Useful Lives of Property and Equipment	The useful lives of the property and equipment are as follows: Laboratory equipment 5 years Furniture and office equipment 5 years Computer equipment 3 years Software 3 years Leasehold improvements Shorter of remaining lease term or estimated useful life

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables sets 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 December 31, 2023 Level 1 Level 2 Level 3 Total Financial Assets Money market funds $ 150,648 $ —  $ —  $ 150,648 U.S. government treasury securities 32,843 —  —  32,843 U.S. government agency securities —  16,257 —  16,257 Commercial paper —  104,141 —  104,141 Corporate bonds —  33,064 —  33,064 Total financial assets $ 183,491 $ 153,462 $ —  $ 336,953 Liabilities: CVR liability $ —  $ —  $ 42,700 $ 42,700 Total liabilities $ —  $ —  $ 42,700 $ 42,700 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 agency securities —  4,230 4,230 Corporate bonds —  3,732 —  3,732 Total financial assets $  15,250 $  31,603 $ —  $  46,853
Changes in Derivative Liabilities	The following table presents changes in the forward contract liability for the periods presented (in millions): Forward Contract Beginning balance as of June 22, 2023 $ 106.2 Change in fair value 83.5 Issuance of Series A Preferred Stock on July 7, 2023 (189.7 ) Ending balance as of December 31, 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 18,986 Payments (5,786 ) Ending Balance as of December 31, 2023 $ 42,700
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: December 31, 2023 Estimated cash flow dates 2/28/24 - 06/22/26 Estimated probability of success 39% - 100% Estimated reimbursement rate compared to reimbursement target 81% - 100% Risk-adjusted discount rates 5.91% - 6.32%

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 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): December 31, 2023 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash equivalents: Money market funds $ 150,648 $ —  $ —  $ 150,648 Commercial paper 24,950 5 —  24,955 U.S. government treasury securities 10,965 1 —  10,966 Total cash equivalents 186,563 6 —  186,569 Marketable securities: Commercial paper 79,124 62 —  79,186 Corporate bonds 32,984 81 (1 ) 33,064 U.S. government treasury securities 21,846 31 —  21,877 U.S. government agency securities 16,147 110 —  16,257 Total marketable securities $ 150,101 $ 284 $ (1 ) $ 150,384 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 agency 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 agency 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 December 31, 2023 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ —  $ —  $ —  $ —  $ —  $ —  Corporate bonds 9,907 (1 ) —  —  9,907 (1 ) U.S. government treasury securities 4,831 —  —  —  4,831 —  Total marketable securities $ 14,738 $ (1 ) $ —  $ —  $ 14,738 $ (1 ) 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 ) Corporate bonds 3,732 (6 ) —  —  3,732 (6 ) U.S. government agency securities 3,735 (1 ) —  —  3,735 (1 ) 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): December 31, 2023 2022 Due in one year or less $ 115,784 $ 20,848 Due in 1 – 2 years 34,600 —  Total marketable securities $ 150,384 $ 20,848

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment Net	Property and equipment, net consist of the following (in thousands): December 31, 2023 2022 Laboratory equipment $ —  $ 2,257 Furniture and office equipment —  520 Computer equipment —  73 Software —  121 Leasehold improvements —  4,393 Property and equipment, gross —  7,364 Less: Accumulated depreciation and amortization —  (4,144 ) Property and equipment, net $ —  $ 3,220

Accrued and Other Current Lia_2

Accrued and Other Current Liabilities (Tables)		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023
Accrued Liabilities and Other Liabilities [Abstract]
Accrued and Other Current Liabilities	Accrued liabilities consist of the following (in thousands): As of June 22, Accrued option cost payable to Paragon $ 18,987 Accrued professional and consulting fees 917 Fair value of assumed Parapyre Option Obligation (Note 3) 143 Accrued liabilities $ 20,047	Accrued and other current liabilities consist of the following (in thousands): December 31, 2023 2022 Accrued compensation $ 4,054 $ 4,589 Accrued contracted research and development costs 7,092 6,972 Accrued professional and consulting fees 1,474 946 Other 488 330 Total accrued and other current liabilities $ 13,108 $ 12,837

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 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 recognized within research and development expenses, which were ultimately settled in cash (in millions): December 31, 2023 2022 2021 Reimbursable costs under the Paragon Agreement $ 37.1 $ —  $ —
Related Party Accounts Payable	The following is the summary of Related party accounts payable and other current liabilities (in millions): December 31, December 31, Reimbursable costs under the Paragon Agreement $ 16.6 $ —  Related party accounts payable and other current liabilities $ 16.6 $ —

Asset Acquisition (Tables)

Asset Acquisition (Tables)		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Asset Acquisition Cost	The Asset Acquisition costs are shown on the following table (in thousands): As of June 22, Consideration transferred in Series A Preferred Stock and common stock $ 109,979 Transaction costs incurred by Aeglea 3,197 Total cost to acquire asset $ 113,176 The Company’s allocation of the purchase price to net assets acquired is as follows (in thousands): As of June 22, Acquired in-process $ 130,188 Cash acquired 3,035 Accrued liabilities (20,047 ) Total cost to acquire asset $ 113,176	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 the Company 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 $ 130.2 Cash acquired 3.0 Assumed liabilities (20.0 ) Total cost to acquire asset $ 113.2

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2023
Leases [Abstract]
Supplemental Balance Sheet Information	The following table summarizes the Company’s recognition of its operating and finance leases (in thousands): December 31, Classification 2023 2022 Assets Operating Operating lease right-of-use $ —  $ 3,430 Finance Other non-current —  597 Total leased assets —  4,027 Leases Current Operating Operating lease liabilities —  625 Finance Accrued and other current liabilities —  16 Non-current Operating Non-current —  4,004 Total lease liabilities $ —  $ 4,645
Weighted-Average Remaining Lease Term/Discount Rates and Lease Cost	The following table summarizes the weighted-average remaining lease term and discount rates for the Company’s operating and finance leases: December 31, 2023 2022 Lease term (years) Operating leases 0.0 5.3 Finance leases 0.0 0.6 Discount rate Operating leases —  % 10.6 % Finance leases —  % 10.2 % The following table summarizes the lease costs pertaining to the Company’s operating leases (in thousands): Year Ended December 31, 2023 2022 2021 Operating lease cost $ 455 $ 910 $ 991 Variable lease cost 471 472 519 Total lease cost $ 926 $ 1,382 $ 1,510

Convertible Preferred Stock a_2

Convertible Preferred Stock and Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Pre-funded Warrants for Common Stock Issued and Outstanding	As of December 31, 2023, the following pre-funded Issue Date Expiration Date Exercise Price Number of Warrants May 2022 None $ 0.0025 250,000 Total pre-funded 250,000

Strategic License Agreement (Ta

Strategic License Agreement (Tables)	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Changes in Contract Liabilities	The following table presents changes in the Company’s contract liabilities for the periods presented (in thousands): Year Ended December 31, 2022 December 31, Additions Deductions December 31, Contract liabilities: Deferred revenue $ 2,696 $ 575 $ (3,271 ) $ —

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2023
Share-Based Payment Arrangement [Abstract]
Employee and Non-Employee Stock Option Activity	The following table summarizes employee and non-employee Shares Issuable Under Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term Aggregate Intrinsic Value (in years) (in thousands) Outstanding as of December 31, 2022 405,082 $ 113.75 6.72 $ 2 Granted 8,776,245 9.67 Exercised (46,246 ) 8.22 Forfeited (637,686 ) 43.00 Outstanding as of December 31, 2023 8,497,395 $ 12.13 8.40 $ 98,928 Options vested and expected to vest as of December 31, 2023 8,497,395 $ 12.13 8.40 $ 98,928 Options exercisable as of December 31, 2023 1,065,700 $ 24.72 5.62 $ 13,328
Employee Restricted Stock Activity	The following table summarizes employee restricted stock activity for the year ended December 31, 2023: Shares Weighted Average Grant Date Fair Value Unvested restricted stock units as of December 31, 2022 5,660 $ 203.25 Granted 153,865 18.17 Vested —  —  Forfeited (5,660 ) 203.25 Unvested restricted stock units as of December 31, 2023 153,865 $ 18.17
Stock-Based Compensation Expense	Total stock-based compensation expense recognized from the Company’s equity incentive plans, 2018 Plan, and the 2016 ESPP for the years ended December 31, 2023, 2022, and 2021 was as follows (in thousands): Year Ended December 31, 2023 2022 2021 Employees Non- Employees Employees Non- Employees Employees Non- Employees Research and development $ 2,910 $ 11,328 $ 2,591 $ —  $ 2,723 $ —  General and administrative 11,327 109 4,520 —  5,315 —  Total stock-based compensation expense $ 14,237 $ 11,437 $ 7,111 $ —  $ 8,038 $ —
Weighted-Average Assumptions Used in Calculating Fair Value of Awards	The following table summarizes the weighted-average assumptions used in calculating the fair value of the awards: Year Ended December 31, 2023 2022 2021 Stock Options Granted Expected term (in years) 5.88 6.00 5.99 Expected volatility 107 % 84 % 83 % Risk-free interest 4.37 % 2.93 % 0.88 % Dividend yield 0 % 0 % 0 % 2016 ESPP Expected term (in years) 0.49 0.49 0.50 Expected volatility 181 % 84 % 86 % Risk-free interest 4.99 % 1.95 % 0.08 % Dividend yield 0 % 0 % 0 %

Restructuring Charges (Tables)

Restructuring Charges (Tables)	12 Months Ended
	Dec. 31, 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 December 31, 2023 Severance liability $ —  $ 6,448 $ (5,325 ) $ 1,123
Charges Related to the Restructuring Activities	A summary of the charges related to the restructuring activities is as follows (in thousands): Severance Stock Loss on Lease Asset Total 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

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
(Loss) Income Before Income Tax Expense by Jurisdiction	The following table summarizes the (loss) income before income tax expense by jurisdiction for the periods indicated: Year Ended December 31, 2023 2022 2021 Domestic $ (338,942 ) $ (84,113 ) $ (65,940 ) Foreign 126 162 280 Loss before income tax expense $ (338,816 ) $ (83,951 ) $ (65,660 )
Effective Income Tax Rate Reconciliation	The difference between the Company’s provision for income taxes and the amounts computed by applying the statutory federal income tax rate to income before income taxes is as follows (in thousands): Year Ended December 31, 2023 2022 2021 Tax provision derived by applying the federal statutory rate to income before income taxes $ (71,151 ) $ (17,630 ) $ (13,789 ) Loss on forward contract valuation 17,541 —  —  Acquired IPR&D 27,340 —  —  Loss on CVR revaluation 3,987 —  —  Other permanent differences 4,472 1,042 1,002 Federal tax credits (1 ) (3,559 ) (3,815 ) State tax credits —  (640 ) (152 ) Effect of tax rate on foreign jurisdiction (53 ) 42 (5 ) Change in the valuation allowance 17,839 20,609 16,900 Income tax (benefit) expense $ (26 ) $ (136 ) $ 141
Components of Deferred Tax Assets and Liabilities	The components of the deferred tax assets and liabilities consist of the following (in thousands): December 31, 2023 2022 Deferred tax assets Net operating loss carryforward $ 74,454 $ 68,917 Capitalized 174 R&D costs 22,532 11,097 Intangible assets 47 52 Deferred revenue —  566 Accrued expense 579 668 Stock-based compensation 4,246 3,293 Federal tax credits 21,914 21,914 State tax credits 1,631 1,631 Other 88 190 Total deferred tax assets 125,491 108,328 Deferred tax liabilities Depreciable assets —  (676 ) Total deferred tax liabilities —  (676 ) Less: Valuation allowance (125,491 ) (107,652 ) Deferred tax assets, net $ —  $ —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 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 Year Ended December 31, 2023 2022 2021 Options to purchase Common Stock 2,583,226   346,331   264,858 Unvested restricted stock units 4,240 6,983 7,975 Outstanding Parapyre Warrants 5,625 —  —
Reconciliation of Basic and Diluted Net Loss Per Share	The following is a reconciliation of the shares used as the denominator for the calculation of basic and diluted net loss per share: Year Ended December 31, 2023 2022 2021 Weighted average Common Shares 6,201,954 2,307,668 1,956,933 Weighted average pre-funded 695,111 1,063,563 672,851 Total basic and diluted weighted average shares 6,897,065 3,371,231 2,629,784

Description of the Business a_3

Description of the Business and Summary of Significant Accounting Policies (Tables)		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023
Business Combination and Asset Acquisition [Abstract]
Summary of Asset Acquisition Cost	The Asset Acquisition costs are shown on the following table (in thousands): As of June 22, Consideration transferred in Series A Preferred Stock and common stock $ 109,979 Transaction costs incurred by Aeglea 3,197 Total cost to acquire asset $ 113,176 The Company’s allocation of the purchase price to net assets acquired is as follows (in thousands): As of June 22, Acquired in-process $ 130,188 Cash acquired 3,035 Accrued liabilities (20,047 ) Total cost to acquire asset $ 113,176	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 the Company 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 $ 130.2 Cash acquired 3.0 Assumed liabilities (20.0 ) Total cost to acquire asset $ 113.2

Accrued Liabilities (Tables)

Accrued Liabilities (Tables)		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023
Payables and Accruals [Abstract]
Accrued and Other Current Liabilities	Accrued liabilities consist of the following (in thousands): As of June 22, Accrued option cost payable to Paragon $ 18,987 Accrued professional and consulting fees 917 Fair value of assumed Parapyre Option Obligation (Note 3) 143 Accrued liabilities $ 20,047	Accrued and other current liabilities consist of the following (in thousands): December 31, 2023 2022 Accrued compensation $ 4,054 $ 4,589 Accrued contracted research and development costs 7,092 6,972 Accrued professional and consulting fees 1,474 946 Other 488 330 Total accrued and other current liabilities $ 13,108 $ 12,837

The Company and Basis of Pres_2

The Company and Basis of Presentation (Details) $ / shares in Units, $ in Thousands				1 Months Ended			3 Months Ended	12 Months Ended			106 Months Ended
	Dec. 11, 2023 USD ($) $ / shares shares	Jun. 26, 2023 USD ($) shares	Jun. 22, 2023 $ / shares shares	Dec. 31, 2023 USD ($) $ / shares shares	Jun. 30, 2023 USD ($) shares	Apr. 30, 2023	Jun. 30, 2023	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($) $ / shares	Dec. 31, 2021 USD ($)	Dec. 31, 2023 USD ($) $ / shares	Dec. 08, 2023
Company and Basis of Presentation [Line Items]
Employee workforce, termination percentage						83%	83%
Proceeds from issuance of private placement								$ 84,555	$ 0	$ 0
Proceeds from raising capital											$ 896,200
Accumulated deficit				$ 764,414				764,414	$ 425,624		764,414
Cash, cash equivalents, and marketable securities				$ 339,300				$ 339,300			$ 339,300
Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Fixed exchange ratio			0.5494488
Asset acquisition, stockholder payment period			3 years
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) | shares			2,734
Series A Non Voting Convertible Preferred StockSeries A Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Preferred stock, par value (in dollars per share) | $ / shares				$ 0.0001				$ 0.0001	$ 0.0001		$ 0.0001
Conversion basis			40
Series A Non Voting Convertible Preferred StockSeries A Non Voting Convertible Preferred Stock | Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Shares transferred as equity interest in asset acquisition (in shares) | shares			364,887
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.0001
Conversion basis			40
Series B Non-Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Conversion basis	40											40
Series B non-voting convertible preferred stock, par value (in dollars per share) | $ / shares	$ 0.0001			$ 0.0001				$ 0.0001	$ 0.0001		$ 0.0001
Private Placement
Company and Basis of Presentation [Line Items]
Sale of stock (in shares) | shares		721,452			721,452
Proceeds from issuance of private placement	$ 180,000	$ 210,000						$ 180,000
Offering costs	$ 10,900	12,700		$ 10,900	$ 12,700
Private Placement | Series A Non Voting Convertible Preferred StockSeries A Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Proceeds from issuance of private placement		$ 210,000
Private Placement | Series B Non-Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Sale of stock (in shares) | shares	150,000			150,000
Proceeds from issuance of private placement	$ 90,000
Common Stock | Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Shares transferred as equity interest in asset acquisition (in shares) | shares			517,809
Common Stock | Private Placement
Company and Basis of Presentation [Line Items]
Sale of stock (in shares) | shares	6,000,000			6,000,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2023 USD ($)
Cash Equivalents And Marketable Securities [Line Items]
Impairments of long lived assets	$ 2,600,000
Maximum | U.S. Banking Institution
Cash Equivalents And Marketable Securities [Line Items]
Cash, FDIC insured amount	$ 250,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Useful Lives of Property and Equipment (Details)	Dec. 31, 2023
Laboratory equipment
Property Plant And Equipment [Line Items]
Useful lives of the property and equipment	5 years
Furniture and office equipment
Property Plant And Equipment [Line Items]
Useful lives of the property and equipment	5 years
Computer equipment
Property Plant And Equipment [Line Items]
Useful lives of the property and equipment	3 years
Software
Property Plant And Equipment [Line Items]
Useful lives of the property and equipment	3 years

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	Dec. 31, 2023	Dec. 31, 2022
Financial Assets
Total financial assets	$ 336,953	$ 46,853
Liabilities:
Total liabilities	42,700
U.S. government treasury securities
Financial Assets
Total financial assets	32,843
Commercial paper
Financial Assets
Total financial assets	104,141	23,641
U.S. government agency securities
Financial Assets
Total financial assets	16,257
Corporate bonds
Financial Assets
Total financial assets	33,064	3,732
CVR liability
Liabilities:
Total liabilities	42,700
Money market funds
Financial Assets
Total financial assets	150,648	15,250
Level 1
Financial Assets
Total financial assets	183,491	15,250
Liabilities:
Total liabilities	0
Level 1 | U.S. government treasury securities
Financial Assets
Total financial assets	32,843
Level 1 | Commercial paper
Financial Assets
Total financial assets	0	0
Level 1 | U.S. government agency securities
Financial Assets
Total financial assets	0	0
Level 1 | Corporate bonds
Financial Assets
Total financial assets	0	0
Level 1 | CVR liability
Liabilities:
Total liabilities	0
Level 1 | Money market funds
Financial Assets
Total financial assets	150,648	15,250
Level 2
Financial Assets
Total financial assets	153,462	31,603
Liabilities:
Total liabilities	0
Level 2 | U.S. government treasury securities
Financial Assets
Total financial assets	0
Level 2 | Commercial paper
Financial Assets
Total financial assets	104,141	23,641
Level 2 | U.S. government agency securities
Financial Assets
Total financial assets	16,257	4,230
Level 2 | Corporate bonds
Financial Assets
Total financial assets	33,064	3,732
Level 2 | CVR liability
Liabilities:
Total liabilities	0
Level 2 | Money market funds
Financial Assets
Total financial assets	0	0
Level 3
Financial Assets
Total financial assets	0	0
Liabilities:
Total liabilities	42,700
Level 3 | U.S. government treasury securities
Financial Assets
Total financial assets	0
Level 3 | Commercial paper
Financial Assets
Total financial assets	0	0
Level 3 | U.S. government agency securities
Financial Assets
Total financial assets	0
Level 3 | Corporate bonds
Financial Assets
Total financial assets	0	0
Level 3 | CVR liability
Liabilities:
Total liabilities	42,700
Level 3 | Money market funds
Financial Assets
Total financial assets	$ 0	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands			6 Months Ended	12 Months Ended
	Jul. 07, 2023	Jun. 22, 2023	Dec. 31, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Issuance of Series A non-voting convertible preferred stock				$ 189,741
Financial liabilities measured at fair value					$ 0
Forward Contract Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of liability		$ 106,200
Change in fair value of derivative liability				83,530	0	$ 0
Change in fair value			$ 83,500
CVR liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Change in fair value of derivative liability				18,986	$ 0	$ 0
Change in fair value				$ 18,986
Series A Non Voting Convertible Preferred StockSeries 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
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
Spyre Therapeutics, Inc. | Series A Non Voting Convertible Preferred StockSeries A Non Voting Convertible Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares transferred as equity interest in asset acquisition (in shares)		364,887

Fair Value Measurements - Chang

Fair Value Measurements - Changes in Forward Contract Liability (Details) - Forward Contracts $ in Millions	6 Months Ended
	Dec. 31, 2023 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 106.2
Change in fair value	83.5
Issuance of Series A Preferred Stock on July 7, 2023	(189.7)
Ending balance	$ 0

Fair Value Measurements - Signi

Fair Value Measurements - Significant Inputs used to Estimate the Fair Value of Derivative Liability (Details) - Level 3	Dec. 31, 2023
Maximum | Risk-adjusted discount rates | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	6.32
Maximum | CVR liability | Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	100
Maximum | CVR liability | Estimated reimbursement rate compared to reimbursement target
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	100
Minimum | Risk-adjusted discount rates | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	5.91
Minimum | CVR liability | Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	39
Minimum | CVR liability | Estimated reimbursement rate compared to reimbursement target
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	81

Fair Value Measurements - Cha_2

Fair Value Measurements - Changes in CVR Liability (Details) - CVR liability $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 0
Fair value at CVR issuance	29,500
Changes in the fair value of the CVR liability since issuance	18,986
Payments	(5,786)
Ending balance	$ 42,700

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	Dec. 31, 2023	Dec. 31, 2022
Cash equivalents:
Amortized Cost	$ 186,563	$ 26,007
Gross Unrealized Gains	6	1
Gross Unrealized Losses	0	(3)
Estimated Fair Value	186,569	26,005
Marketable securities:
Amortized Cost	150,101	20,877
Gross Unrealized Gains	284	2
Gross Unrealized Losses	(1)	(31)
Estimated Fair Value	150,384	20,848
Money market funds
Cash equivalents:
Amortized Cost	150,648	15,250
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	150,648	15,250
Commercial paper
Cash equivalents:
Amortized Cost	24,950	7,021
Gross Unrealized Gains	5	1
Gross Unrealized Losses	0	(2)
Estimated Fair Value	24,955	7,020
Marketable securities:
Amortized Cost	79,124	16,644
Gross Unrealized Gains	62	2
Gross Unrealized Losses	0	(25)
Estimated Fair Value	79,186	16,621
Corporate bonds
Marketable securities:
Amortized Cost	32,984	3,738
Gross Unrealized Gains	81	0
Gross Unrealized Losses	(1)	(6)
Estimated Fair Value	33,064	3,732
U.S. government treasury securities
Cash equivalents:
Amortized Cost	10,965
Gross Unrealized Gains	1
Gross Unrealized Losses	0
Estimated Fair Value	10,966
Marketable securities:
Amortized Cost	21,846
Gross Unrealized Gains	31
Gross Unrealized Losses	0
Estimated Fair Value	21,877
U.S. government agency securities
Cash equivalents:
Amortized Cost		3,736
Gross Unrealized Gains		0
Gross Unrealized Losses		(1)
Estimated Fair Value		3,735
Marketable securities:
Amortized Cost	16,147	495
Gross Unrealized Gains	110	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 16,257	$ 495

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Available-for-Sale Securities in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair Value
Less Than 12 Months	$ 14,738	$ 25,166
12 Months or Longer	0	0
Total, fair value	14,738	25,166
Unrealized Losses
Less Than 12 Months	(1)	(34)
12 Months or Longer	0	0
Total	(1)	(34)
Commercial paper
Fair Value
Less Than 12 Months	0	17,699
12 Months or Longer	0	0
Total, fair value	0	17,699
Unrealized Losses
Less Than 12 Months	0	(27)
12 Months or Longer	0	0
Total	0	(27)
Corporate bonds
Fair Value
Less Than 12 Months	9,907	3,732
12 Months or Longer	0	0
Total, fair value	9,907	3,732
Unrealized Losses
Less Than 12 Months	(1)	(6)
12 Months or Longer	0	0
Total	(1)	(6)
U.S. government treasury securities
Fair Value
Less Than 12 Months	4,831
12 Months or Longer	0
Total, fair value	4,831
Unrealized Losses
Less Than 12 Months	0
12 Months or Longer	0
Total	$ 0
U.S. government agency securities
Fair Value
Less Than 12 Months		3,735
12 Months or Longer		0
Total, fair value		3,735
Unrealized Losses
Less Than 12 Months		(1)
12 Months or Longer		0
Total		$ (1)

Cash Equivalents and Marketab_5

Cash Equivalents and Marketable Securities - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Cash Equivalents And Marketable Securities [Line Items]
Debt Securities, Available-For-Sale, Accrued Interest, After Allowance For Credit Loss, Statement Of Financial Position, Extensible List, Not Disclosed Flag	Accrued interest receivable on available-for-sale debt securities
Unrealized gains or losses on marketable securities	$ 300
Realized gains or losses on marketable securities	0	$ 0
Accrued interest receivable on available-for-sale debt securities	900	100
US Government Agencies Debt Securities
Cash Equivalents And Marketable Securities [Line Items]
Debt securities, available-for-sale, allowance for credit loss, excluding accrued interest	$ 0	$ 0

Cash Equivalents and Marketab_6

Cash Equivalents and Marketable Securities - Contractual Maturities of Marketable Securities at Estimated Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Cash and Cash Equivalents [Abstract]
Due in one year or less	$ 115,784	$ 20,848
Due in 1 - 2 years	34,600	0
Total marketable securities	$ 150,384	$ 20,848

Property and Equipment, Net - S

Property and Equipment, Net - Schedule of Property and Equipment Net (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 0	$ 7,364
Less: Accumulated depreciation and amortization	0	(4,144)
Property and equipment, net	0	3,220
Laboratory equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	0	2,257
Furniture and office equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	0	520
Computer equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	0	73
Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	0	121
Leasehold improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 0	$ 4,393

Property and Equipment, Net - N

Property and Equipment, Net - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2023	Jun. 30, 2023	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property Plant And Equipment [Line Items]
Depreciation and amortization				$ 744	$ 1,567	$ 1,576
Employee workforce, termination percentage	83%	83%
Proceeds from the sale of property plant and equipment			$ 500	475	0	0
Loss on disposal of long-lived assets				915	0	0
Research and development
Property Plant And Equipment [Line Items]
Loss on disposal of long-lived assets			700
General and administrative
Property Plant And Equipment [Line Items]
Loss on disposal of long-lived assets			$ 200
Property, Plant and Equipment
Property Plant And Equipment [Line Items]
Depreciation and amortization				$ 700	$ 1,400	$ 1,400

Accrued and Other Current Lia_3

Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Jun. 22, 2023	Dec. 31, 2022
Accrued Liabilities and Other Liabilities [Abstract]
Accrued compensation	$ 4,054		$ 4,589
Accrued contracted research and development costs	7,092		6,972
Accrued professional and consulting fees	1,474	$ 917	946
Other	488		330
Total accrued and other current liabilities	$ 13,108		$ 12,837

Related Party Transactions - Na

Related Party Transactions - Narrative (Details) $ / shares in Units, $ in Thousands						1 Months Ended	12 Months Ended
	Dec. 14, 2023 USD ($)	Dec. 11, 2023 USD ($)	Nov. 22, 2023 installment $ / shares shares	Jun. 26, 2023 USD ($)	Jun. 22, 2023 USD ($) BOARDSEAT	May 31, 2023 USD ($)	Dec. 31, 2023 USD ($) BOARDSEAT shares	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Related Party Transaction [Line Items]
Number of board seats held by related party | BOARDSEAT					2
Stock compensation expense							$ 25,675	$ 7,111	$ 8,038
Related party accounts payable and other current liabilities							16,584	0
Proceeds from issuance of private placement							$ 84,555	0	$ 0
Granted (in shares) | shares							8,776,245
Private Placement [Member]
Related Party Transaction [Line Items]
Proceeds from issuance of private placement		$ 180,000		$ 210,000			$ 180,000
Related Party
Related Party Transaction [Line Items]
Related party accounts payable and other current liabilities							16,600	0
Paragon Agreement
Related Party Transaction [Line Items]
Related party expenses incurred prior to asset acquisition					$ 19,000
Related party expenses unpaid prior to asset acquisition					19,000
Paragon Agreement | Related Party
Related Party Transaction [Line Items]
Related party expenses incurred prior to asset acquisition					19,000
Research initiation fees					3,000	$ 3,000
Reimbursable research costs					16,000
Related party expenses unpaid prior to asset acquisition					$ 19,000
Stock compensation expense							11,400
Stock Compensation And Research And Development Expense
Related Party Transaction [Line Items]
Related party transaction amount							48,500
Reimbursable Costs Under Paragon Agreement | Related Party
Related Party Transaction [Line Items]
Related party accounts payable and other current liabilities							16,600	$ 0
Milestone Payments
Related Party Transaction [Line Items]
Related party transaction amount							$ 39,500
Appoint Of Board Members
Related Party Transaction [Line Items]
Number of board seats held by related party | BOARDSEAT							2
SPY002 License Agreement | Related Party
Related Party Transaction [Line Items]
Nomination fee	$ 1,500
Milestone payments	2,500
Sale Of Stock, December 2023 PIPE | Related Party
Related Party Transaction [Line Items]
Proceeds from issuance of private placement							$ 10,000
Consulting Agreement | Related Party
Related Party Transaction [Line Items]
Granted (in shares) | shares			477,000
Exercise price (in dollars per share) | $ / shares			$ 10.39
Number of monthly installments | installment			48
Stock-based compensation expense							$ 100
Consulting Agreement | Related Party | Share-Based Payment Arrangement, Tranche One
Related Party Transaction [Line Items]
Vesting percentage			25%
Paragon Therapeutics Inc | Ownership Interest | Maximum | Related Party
Related Party Transaction [Line Items]
Percentage of ownership by noncontrolling owner							5%
Paragon Therapeutics Inc | SPY002 License Agreement | Maximum | Related Party
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%
Paragon and Parapyre Holding LLC | Ownership Interest | Maximum | Related Party
Related Party Transaction [Line Items]
Percentage of ownership by noncontrolling owner					5%

Related Party Transactions - Ex

Related Party Transactions - Expenses related to Related Party which were Settled in Cash (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reimbursable Costs Under Paragon Agreement | Research and development
Related Party Transaction [Line Items]
Reimbursable costs	$ 37.1	$ 0	$ 0

Related Party Transactions - Re

Related Party Transactions - Related Party Accounts Payable (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Related Party Transaction [Line Items]
Related party accounts payable and other current liabilities	$ 16,584	$ 0
Related Party
Related Party Transaction [Line Items]
Related party accounts payable and other current liabilities	16,600	0
Related Party | Reimbursable Costs Under Paragon Agreement
Related Party Transaction [Line Items]
Related party accounts payable and other current liabilities	$ 16,600	$ 0

Asset Acquisition - Narrative (

Asset Acquisition - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	Jun. 22, 2023	May 31, 2023
Asset Acquisition [Line Items]
Asset acquisition, contingent consideration, liability, current	$ 100
Cost to acquire asset	113,200
Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Cost to acquire asset	$ 113,176
Asset acquisition consideration transferred issuable shares of common stock on an as-converted basis	14,595,480
Series A Non Voting Convertible Preferred Stock | Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Shares transferred as equity interest in asset acquisition (in shares)	364,887
Shares issued, price per share (in dollars per share)	$ 291.08
Common Stock | Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Shares transferred as equity interest in asset acquisition (in shares)	517,809
Shares issued, price per share (in dollars per share)	$ 7.277
Parapyre Option Obligation
Asset Acquisition [Line Items]
Percentage of annual equity grant of options	1%	1%
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

Asset Acquisition - Asset Acqui

Asset Acquisition - Asset Acquisition Cost (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Asset Acquisition [Line Items]
Acquired in-process research and development		$ 130,188	$ 0	$ 0
Cash acquired		$ 3,035	$ 0	$ 0
Cost to acquire asset	$ 113,200
Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Consideration transferred in Series A Preferred Stock and Common Stock	109,979
Transaction costs incurred by the Company	3,197
Acquired in-process research and development	130,188
Cash acquired	3,035
Assumed liabilities	(20,000)
Cost to acquire asset	$ 113,176

Paragon Agreement (Details)

Paragon Agreement (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Jun. 22, 2023	May 31, 2023	Dec. 31, 2023
Paragon Therapeutics Inc
Related Party Transaction [Line Items]
Milestone payments			$ 39.5
Parapyre Option Obligation
Related Party Transaction [Line Items]
Percentage of annual equity grant of options	1%	1%
Paragon Agreement
Related Party Transaction [Line Items]
Related party expenses incurred prior to asset acquisition	$ 19
Related party expenses unpaid prior to asset acquisition	19
Stock Compensation And Research And Development Expense
Related Party Transaction [Line Items]
Related party transaction amount			$ 48.5
Related Party | Paragon Agreement
Related Party Transaction [Line Items]
Research initiation fees	3	$ 3
Related party expenses incurred prior to asset acquisition	19
Reimbursable research costs	16
Related party expenses unpaid prior to asset acquisition	19
Nonrefundable research initiation fee for one program paid in cash	$ 0.8

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Aug. 31, 2023 USD ($)	Apr. 30, 2019 USD ($) ft²
Lessee, Lease, Description [Line Items]
Area of land | ft²				30,000
Letter of credit				$ 1.5
Termination fee amount			$ 2
Operating lease, payments	$ 0.5	$ 0.9
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, renewal term	3 years
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, renewal term	5 years
Tenant improvement allowance (up to)				$ 1

Leases - Operating and Financin

Leases - Operating and Financing Leases Presented in Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Assets
Operating	$ 0	$ 3,430
Finance	$ 0	597
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Other non-current assets
Total leased assets	$ 0	4,027
Current
Operating	0	625
Finance	$ 0	16
Finance Lease, Liability, Current, Statement of Financial Position [Extensible Enumeration]	Accrued and other current liabilities
Non-current
Operating	$ 0	4,004
Total lease liabilities	$ 0	$ 4,645

Leases - Weighted-Average Remai

Leases - Weighted-Average Remaining Lease Term and Discount Rates for Operating and Finance Leases (Details)	Dec. 31, 2023	Dec. 31, 2022
Lease term (years)
Operating leases	0 years	5 years 3 months 18 days
Finance leases	0 years	7 months 6 days
Discount rate
Operating leases	0%	10.60%
Finance leases	0%	10.20%

Leases - Lease Cost (Details)

Leases - Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Leases [Abstract]
Operating lease cost	$ 455	$ 910	$ 991
Variable lease cost	471	472	519
Total lease cost	$ 926	$ 1,382	$ 1,510

Convertible Preferred Stock a_3

Convertible Preferred Stock and Stockholders' Equity - Narrative (Details) $ / shares in Units, $ in Thousands						1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 11, 2023 USD ($) shares	Dec. 08, 2023	Jul. 07, 2023 shares	Jun. 26, 2023 USD ($) shares	Jun. 22, 2023	Dec. 31, 2023 USD ($) VOTE $ / shares shares	Jun. 30, 2023 USD ($) $ / shares shares	May 31, 2022 USD ($) $ / shares shares	Dec. 31, 2023 USD ($) VOTE $ / shares shares	Dec. 31, 2023 USD ($) VOTE $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2020 shares
Class Of Stock [Line Items]
Shares authorized (in shares)						410,000,000			410,000,000	410,000,000
Common stock, authorized (in shares)						400,000,000			400,000,000	400,000,000	20,000,000
Preferred stock, authorized (in shares)						10,000,000			10,000,000	10,000,000
Common stock, par value (in dollars per share) | $ / shares						$ 0.0001			$ 0.0001	$ 0.0001	$ 0.0001
Number of votes for common stock holders | VOTE						1			1	1
Common stock dividends declared | $										$ 0
Warrants to purchase shares (in shares)								694,892
Offering price of warrant (in dollars per share) | $ / shares								$ 39.9975
Exercise price per warrant (in dollars per share) | $ / shares								$ 0.0025
Net proceeds from sale of common stock | $								$ 42,900		84,555	$ 0	$ 0
Placement agent fees and offering costs | $								$ 2,100
Maximum ownership percentage of common stock shares for outstanding warrants to be exercised								4.99%
Maximum ownership percentage of common stock shares for outstanding warrants to be exercised upon written notice								19.99%
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
Proceeds from issuance of private placement | $										$ 84,555	$ 0	$ 0
Parapyre Warrants
Class Of Stock [Line Items]
Warrants to purchase shares (in shares)						684,407			684,407	684,407
Exercise price per warrant (in dollars per share) | $ / shares						$ 21.52			$ 21.52	$ 21.52
Series A Non Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Preferred stock, authorized (in shares)						1,086,341			1,086,341	1,086,341	0
Preferred stock, outstanding (in shares)						437,037			437,037	437,037	0
Preferred stock issued and outstanding percentage					30%
Conversion basis					40
Series B Non-Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Series B non-voting convertible preferred stock, outstanding (in shares)						150,000			150,000	150,000	0	0	0
Conversion basis	40	40
Preferred stock, redemption value | $						$ 129,100			$ 129,100	$ 129,100
Preferred stock, redemption per share (in dollars per share) | $ / shares						$ 21.52			$ 21.52	$ 21.52
Maximum | Series A Non Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Beneficially holders owned percentage					20%
Maximum | Series B Non-Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Beneficially holders owned percentage		19.99%
Minimum | Series A Non Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Beneficially holders owned percentage					0%
Minimum | Series B Non-Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Beneficially holders owned percentage		0%
Common Stock
Class Of Stock [Line Items]
Issued (in shares)								430,107
Public offering price (in dollars per share) | $ / shares								$ 40
Issuance of common stock upon conversion (in shares)											204,000	40,000
Common Stock | Conversion Of Series A Non-Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Issuance of common stock upon conversion (in shares)									25,972,080	25,972,000
Series A Non-Voting Convertible Preferred Stock | Series A Non Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Preferred stock, outstanding (in shares)						437,000			437,000	437,000	0	0	0
Issued (in shares)										721,000
Issued as part of consideration transferred in acquisition (in shares)			364,887							365,000
Issuance of common stock upon conversion (in shares)										(649,000)
Series A Non-Voting Convertible Preferred Stock | Series A Non Voting Convertible Preferred Stock | Conversion Of Series A Non-Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Issuance of common stock upon conversion (in shares)									(649,302)
Private Placement
Class Of Stock [Line Items]
Public offering price (in dollars per share) | $ / shares							$ 291.08
Sale of stock (in shares)				721,452			721,452
Aggregate purchase price for stock sold | $				$ 197,300		$ 169,100	$ 197,300
Offering costs | $	$ 10,900			12,700		$ 10,900	$ 12,700
Proceeds from issuance of private placement | $	$ 180,000			210,000						$ 180,000
Private Placement | Series A Non Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Proceeds from issuance of private placement | $				$ 210,000
Private Placement | Series B Non-Voting Convertible Preferred Stock
Class Of Stock [Line Items]
Public offering price (in dollars per share) | $ / shares						$ 600			$ 600	$ 600
Sale of stock (in shares)	150,000					150,000
Proceeds from issuance of private placement | $	$ 90,000
Private Placement | Common Stock
Class Of Stock [Line Items]
Public offering price (in dollars per share) | $ / shares						$ 15			$ 15	$ 15
Sale of stock (in shares)	6,000,000					6,000,000

Convertible Preferred Stock a_4

Convertible Preferred Stock and Stockholders' Equity - Pre-funded Warrants for Common Stock Issued and Outstanding (Details) - $ / shares	Dec. 31, 2023	May 31, 2022
Class of Warrant or Right [Line Items]
Exercise price (in dollars per share)		$ 0.0025
May 2022
Class of Warrant or Right [Line Items]
Exercise price (in dollars per share)	$ 0.0025
Number of warrants outstanding (in shares)	250,000
Pre-Funded Warrants
Class of Warrant or Right [Line Items]
Number of warrants outstanding (in shares)	250,000

Strategic License Agreement - N

Strategic License Agreement - Narrative (Details) - USD ($) $ in Thousands			1 Months Ended		12 Months Ended
	Jul. 27, 2023	Mar. 21, 2021	Jul. 31, 2021	Jun. 30, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognized					$ 886	$ 2,329	$ 18,739
Deferred revenue					0	2,696
Deferred revenue, current					0	517
Contract assets					0	0
Proceeds from sale of intangible assets					15,000	0	0
Gain on sale of in-process research and development asset					16,449	0	0
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Global Rights To Pegzilarginase
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Proceeds from sale of intangible assets	$ 15,000
Contingent consideration	100,000
Carrying value of assets	0
Gain on sale of in-process research and development asset	16,400
Contingent reimbursement of pre-paid manufacturing costs	1,800
Derecognition of nonfinancial assets and liabilities	$ 400
Immedica Pharma AB
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Reimbursement			$ 3,000
Non refundable payment received		$ 21,500
Percentage of payment for cost incurred in trial		50%
Maximum amount of costs to reimburse		$ 1,800
Additional upfront payment to be received		$ 120,800
Milestone payments exchange rate		1.07
Upfront payment		$ 21,500
Estimated amount incurred			3,600
Related party transaction amount			25,100
Allocation for stand-alone selling prices			12,000
Deferred revenue						2,700
Deferred revenue, current						500
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
Revenue recognized					$ 900	$ 2,300	6,700
Immedica Pharma AB | PIP Trial
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Allocated amount of modified transaction price			$ 3,500
Immedica Pharma AB | License Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue recognized				$ 12,000			$ 12,000

Strategic License Agreement - C

Strategic License Agreement - Changes in Contract Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Change In Contract With Customer, Liability [Roll Forward]
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 ($) $ in Thousands		12 Months Ended
	Jul. 27, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of intangible assets		$ 15,000	$ 0	$ 0
Gain on sale of in-process research and development asset		$ 16,449	$ 0	$ 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
Contingent consideration	100,000
Carrying value of assets	0
Gain on sale of in-process research and development asset	16,400
Contingent reimbursement of pre-paid manufacturing costs	1,800
Derecognition of nonfinancial assets and liabilities	$ 400

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	6 months
Right to terminate work order, reasonable cause, prior written notice period	30 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

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($)		1 Months Ended								12 Months Ended
	Jun. 22, 2023	Nov. 30, 2023	Feb. 28, 2023	Jul. 31, 2020	Oct. 31, 2018	Jun. 30, 2018	Feb. 28, 2018	Apr. 30, 2016	Mar. 31, 2015	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Sep. 09, 2023	Jan. 01, 2023	Jan. 01, 2022	Jan. 01, 2021	Jan. 01, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding option awards (in shares)										8,497,395	405,082
Stock compensation expense										$ 25,675,000	$ 7,111,000	$ 8,038,000
Tax benefits recognized										$ 0	$ 0	$ 0
Expected dividend yield										0%	0%	0%
Parapyre Option Obligation
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of annual grant of common stock outstanding for warrants	1%
Warrants term													10 years
Warrant grant date fair value	$ 11,500,000
Liability assumed in asset acquisition related to warrants	$ 100,000
Stock compensation expense										$ 11,400,000	$ 0	$ 0
Warrant unamortized expense										$ 0
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant date fair value (in dollars per share)										$ 9.67	$ 1.8	$ 4.96
Intrinsic value of options exercised										$ 400,000	$ 0	$ 700,000
Unrecognized stock-based compensation expense for options										$ 64,400,000
Weighted average period over which unrecognized compensation is expected to be recognized										3 years 6 months
Expected dividend yield										0%	0%	0%
Stock Options | Non- Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option issued (in shares)										477,000	0	0
Stock option vested (in shares)										0	0	0
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, granted to employees (in shares)										153,865	0	0
Performance Shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expense recognized for unvested employee and non-employee awards										$ 0	$ 0	$ 0
2015 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of market value of common stock									100%
Grantees with more than five years vesting term, percent									10%
Maximum vesting term for more than ten percent grantees									5 years
Shares subject to options outstanding (in shares)										3,029
Available for issuance (in shares)								20,395
2015 Equity Incentive Plan | Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Awards granted, expiration period									10 years
2016 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Initial reserves of common stock (in shares)								44,000
Increase in common stock reserved for issuance (in shares)					70,384
Annual percentage of additional shares		5%			4%
Additional common stock available for issuance (in shares)														104,561	78,968	76,735
Additional shares authorized (in shares)		4,481,152
Annual limit for non-employee director compensation (in shares)								4,000
Annual limit for non-employee director compensation		$ 750,000
Non-employee director compensation		$ 1,000,000
Shares reserved for issuance (in shares)										5,019,177
Outstanding option awards (in shares)										3,294,962
2016 Equity Incentive Plan | Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, granted to employees (in shares)				9,128
Stock-based compensation expense										$ 0
Equity instruments other than options, vested (in shares)										0
2018 Equity Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Initial reserves of common stock (in shares)							44,000
Additional shares authorized (in shares)										6,000,000
Shares reserved for issuance (in shares)										6,044,000
Outstanding option awards (in shares)										5,350,595
2018 Plan, 2016 Plan and 2015 Plan | Service Based Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Awards granted, expiration period							10 years
Awards granted, vesting period							4 years
2016 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Awards granted, expiration period								10 years
Additional common stock available for issuance (in shares)															26,140	19,742	19,184
Shares reserved for issuance (in shares)								6,600		72,404
Percentage of fair market value of common stock								85%
Percentage of discount through payroll deductions to eligible employees to purchase common stock								15%
Maximum purchase value per employee under employee stock purchase plan								$ 25,000
Additional annual percentage increase of common stock						1%
Maximum shares purchased under employee stock purchase plan (in shares)			400					80
Stock-based compensation expense											$ 100,000	$ 200,000
Expected dividend yield										0%	0%	0%

Stock-Based Compensation - Empl

Stock-Based Compensation - Employee and Non-Employee Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Shares Issuable Under Options
Outstanding, beginning balance (in shares)	405,082
Granted (in shares)	8,776,245
Exercised (in shares)	(46,246)
Forfeited (in shares)	(637,686)
Outstanding, ending balance (in shares)	8,497,395	405,082
Options vested and expected to vest (in shares)	8,497,395
Options exercisable (in shares)	1,065,700
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 113.75
Granted (in dollars per share)	9.67
Exercised (in dollars per share)	8.22
Forfeited (in dollars per share)	43
Outstanding, ending balance (in dollars per share)	12.13	$ 113.75
Options vested and expected to vest (in dollars per share)	12.13
Options exercisable (in dollars per share)	$ 24.72
Weighted Average Remaining Contractual Term
Outstanding	8 years 4 months 24 days	6 years 8 months 19 days
Options vested and expected to vest	8 years 4 months 24 days
Options exercisable	5 years 7 months 13 days
Aggregate Intrinsic Value
Outstanding	$ 98,928	$ 2
Options vested and expected to vest	98,928
Options exercisable	$ 13,328

Stock-Based Compensation - Em_2

Stock-Based Compensation - Employee Restricted Stock Activity (Details) - Restricted Common Stock - Employees	12 Months Ended
	Dec. 31, 2023 $ / shares shares
Shares
Unvested (in shares) | shares	5,660
Granted (in shares) | shares	153,865
Vested (in shares) | shares	0
Forfeited (in shares) | shares	(5,660)
Unvested (in shares) | shares	153,865
Weighted Average Grant Date Fair Value
Unvested (in dollars per share) | $ / shares	$ 203.25
Granted (in dollars per share) | $ / shares	18.17
Vested (in dollars per share)) | $ / shares	0
Forfeited (in dollars per share) | $ / shares	203.25
Unvested (in dollars per share) | $ / shares	$ 18.17

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense (Details) - 2018 Equity Inducement Plan and 2016 Employee Stock Purchase Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Employees
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 14,237	$ 7,111	$ 8,038
Non- Employees
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	11,437	0	0
Research and development | Employees
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	2,910	2,591	2,723
Research and development | Non- Employees
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	11,328	0	0
General and administrative | Employees
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	11,327	4,520	5,315
General and administrative | Non- Employees
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 109	$ 0	$ 0

Stock-Based Compensation - Weig

Stock-Based Compensation - Weighted-Average Assumptions Used in Calculating Fair Value of Awards (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Dividend yield	0%	0%	0%
Stock Options
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Expected term (in years)	5 years 10 months 17 days	6 years	5 years 11 months 26 days
Expected volatility	107%	84%	83%
Risk-free interest	4.37%	2.93%	0.88%
Dividend yield	0%	0%	0%
2016 Employee Stock Purchase Plan
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Expected term (in years)	5 months 26 days	5 months 26 days	6 months
Expected volatility	181%	84%	86%
Risk-free interest	4.99%	1.95%	0.08%
Dividend yield	0%	0%	0%

Defined Contribution Plan (Deta

Defined Contribution Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Retirement Benefits [Abstract]
Defined contribution plan, contribution amount	$ 0.2	$ 0.6	$ 0.6

Restructuring Charges - Narrati

Restructuring Charges - Narrative (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	Jun. 30, 2023	Apr. 30, 2023	Jun. 30, 2023	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Aug. 31, 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					5,300
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		$ 500	$ 500
Loss on disposal of long-lived assets					$ 915	$ 0	$ 0
Lease asset impairment	900
Impairment on leasehold improvements	$ 1,700
Termination fee amount								$ 2,000
Research and development
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	12 Months Ended
	Dec. 31, 2023 USD ($)
Restructuring Cost and Reserve [Line Items]
Payments	$ (5,300)
Severance Liability
Restructuring Cost and Reserve [Line Items]
Beginning Balance December 31, 2022	0
Charges	6,448
Payments	(5,325)
Ending Balance December 31, 2023	$ 1,123

Restructuring Charges - Charges

Restructuring Charges - Charges Related to the Restructuring Activities (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Jun. 30, 2023	Jun. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Severance Related Expenses			$ 6,400
Loss on disposal of long-lived assets			$ 915	$ 0	$ 0
Lease Asset Impairment	$ 900
Research and development
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 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
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

Income Taxes - (Loss) Income Be

Income Taxes - (Loss) Income Before Income Tax Expense by Jurisdiction (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Domestic	$ (338,942)	$ (84,113)	$ (65,940)
Foreign	126	162	280
Loss before income tax expense	$ (338,816)	$ (83,951)	$ (65,660)

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) SUBSIDARY	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Income Tax Contingency [Line Items]
Provision or benefit from income taxes	$ (26,000)	$ (136,000)	$ 141,000
Increase in valuation primarily due to operation losses	17,800,000	20,600,000	$ 16,900,000
Net operating loss carryforwards	354,500,000	328,200,000
Net operating loss carryforwards, set to expire if not utilized	58,400,000
Tax credit carryforwards, set to expire if not utilized	$ 21,900,000
Number of domestic subsidiaries | SUBSIDARY	8
Unrecognized tax benefits	$ 0	0
Foreign subsidiaries
Income Tax Contingency [Line Items]
Provision or benefit from income taxes		$ 100,000
U.S
Income Tax Contingency [Line Items]
Tax credit carryforwards	21,900,000
State
Income Tax Contingency [Line Items]
Tax credit carryforwards	$ 1,600,000

Income Taxes - Effective Income

Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Taxes [Line Items]
Tax provision derived by applying the federal statutory rate to income before income taxes	$ (71,151)	$ (17,630)	$ (13,789)
Loss on forward contract valuation	17,541	0	0
Acquired IPR&D	27,340	0	0
Loss on CVR revaluation	3,987	0	0
Other permanent differences	4,472	1,042	1,002
Effect of tax rate on foreign jurisdiction	(53)	42	(5)
Change in the valuation allowance	17,839	20,609	16,900
Income tax (benefit) expense	(26)	(136)	141
Federal tax credits
Income Taxes [Line Items]
Tax credits	(1)	(3,559)	(3,815)
State tax credits
Income Taxes [Line Items]
Tax credits	$ 0	$ (640)	$ (152)

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets
Net operating loss carryforward	$ 74,454	$ 68,917
Capitalized 174 R&D costs	22,532	11,097
Intangible assets	47	52
Deferred revenue	0	566
Accrued expense	579	668
Stock-based compensation	4,246	3,293
Other	88	190
Total deferred tax assets	125,491	108,328
Deferred tax liabilities
Depreciable assets	0	(676)
Total deferred tax liabilities	0	(676)
Less: Valuation allowance	(125,491)	(107,652)
Deferred tax assets, net	0	0
Federal tax credits
Deferred tax assets
Tax credits	21,914	21,914
State tax credits
Deferred tax assets
Tax credits	$ 1,631	$ 1,631

Net Loss Per Share - Weighted-A

Net Loss Per Share - Weighted-Average Equity Instruments Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
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)	2,583,226	346,331	264,858
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)	4,240	6,983	7,975
Outstanding Parapyre Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	5,625	0	0

Net Loss Per Share - Reconcilia

Net Loss Per Share - Reconciliation of Basic and Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Weighted average common shares (in shares)	6,201,954	2,307,668	1,956,933
Weighted average pre-funded warrants (in shares)	695,111	1,063,563	672,851
Total basic weighed average shares (in shares)	6,897,065	3,371,231	2,629,784
Total diluted weighed average shares (in shares)	6,897,065	3,371,231	2,629,784

Description of the Business a_4

Description of the Business and Summary of Significant Accounting Policies - Summary of Asset Acquisition Cost (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 22, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Asset Acquisition [Line Items]
Acquired in-process research and development		$ 130,188	$ 0	$ 0
Cash acquired		$ 3,035	$ 0	$ 0
Cost to acquire asset	$ 113,200
Spyre Therapeutics, Inc.
Asset Acquisition [Line Items]
Consideration transferred in Series A Preferred Stock and common stock	109,979
Transaction costs incurred by Aeglea	3,197
Acquired in-process research and development	130,188
Cash acquired	3,035
Accrued liabilities	(20,047)
Cost to acquire asset	$ 113,176

Description of the Business a_5

Description of the Business and Summary of Significant Accounting Policies - Additional Information (Details) $ / shares in Units, $ in Thousands					1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 11, 2023 USD ($)	Sep. 08, 2023	Jun. 26, 2023 USD ($) shares	Jun. 22, 2023 USD ($) $ / shares shares	Apr. 30, 2023 employee	Jun. 30, 2023	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($) $ / shares	Dec. 31, 2021 USD ($)	May 31, 2023
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Stockholders' equity, reverse stock split		1-for-25
Cost to acquire asset				$ 113,200
Acquired in-process research and development							$ 130,188	$ 0	$ 0
Employee workforce, termination percentage					83%	83%
Number of employees retained | employee					10
Gross proceeds received in private placement							84,555	0	$ 0
cash and cash equivalents				3,000			188,893	$ 34,863
U.S. Banking Institution
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Cash				$ 2,800
Private Placement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Gross proceeds received in private placement	$ 180,000		$ 210,000				$ 180,000
Parapyre Option Obligation
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Percentage of annual equity grant of options				1%						1%
Series A Non Voting Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Preferred stock, par value | $ / shares							$ 0.0001	$ 0.0001
Conversion basis				40
Series A Non Voting Convertible Preferred Stock | Private Placement
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares sold in private placement | shares			721,452
Gross proceeds received in private placement			$ 210,000
Placement agent and other offering expenses in private placement			$ 12,700
Asset Acquisition
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Fixed exchange ratio				0.5494488
Asset Acquisition | Spyre 2023 Equity Incentive Plan
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of outstanding and unexercised stock options to purchase (in shares) | shares				2,734
Asset Acquisition | Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares transferred as equity interest in asset acquisition | shares				517,809
Asset Acquisition | Series A Non Voting Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares transferred as equity interest in asset acquisition | shares				364,887
Preferred stock, par value | $ / shares				$ 0.0001
Conversion basis				40
Spyre Therapeutics, Inc.
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Fixed exchange ratio				0.5494488
Cost to acquire asset				$ 113,176
Asset acquisition consideration transferred issuable shares of common stock on an as-converted basis | shares				14,595,480
Acquired in-process research and development				$ 130,188
Asset Acquisition, net liabilities assumed				$ 17,000
Spyre Therapeutics, Inc. | Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares transferred as equity interest in asset acquisition | shares				517,809
Shares issued, price per share (in dollars per share) | $ / shares				$ 7.277
Spyre Therapeutics, Inc. | Spyre 2023 Equity Incentive Plan
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of outstanding and unexercised stock options to purchase (in shares) | shares				2,734
Spyre Therapeutics, Inc. | Common Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares transferred as equity interest in asset acquisition | shares				517,809
Spyre Therapeutics, Inc. | Series A Non Voting Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Number of shares transferred as equity interest in asset acquisition | shares				364,887
Preferred stock, par value | $ / shares				$ 0.0001
Conversion basis				40
Shares issued, price per share (in dollars per share) | $ / shares				$ 291.08

Accrued Liabilities - Accrued a

Accrued Liabilities - Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Jun. 22, 2023	Dec. 31, 2022
Payables and Accruals [Abstract]
Accrued option cost payable to Paragon		$ 18,987
Accrued professional and consulting fees	$ 1,474	917	$ 946
Fair value of assumed Parapyre Option Obligation (Note 3)		143
Accrued liabilities		$ 20,047

Accrued Liabilities - Additiona

Accrued Liabilities - Additional Information (Details) - Paragon Agreement - USD ($) $ in Millions		1 Months Ended
	Jun. 22, 2023	May 31, 2023
Accrued Liabilities [Line Items]
Related party expenses incurred prior to asset acquisition	$ 19
Related party expenses unpaid prior to asset acquisition	19
Related Party
Accrued Liabilities [Line Items]
Related party expenses incurred prior to asset acquisition	19
Research initiation fees	3	$ 3
Reimbursable research costs	16
Related party expenses unpaid prior to asset acquisition	$ 19

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - $ / shares	Sep. 29, 2023	Sep. 08, 2023	Jul. 07, 2023	Jun. 22, 2023	Dec. 31, 2023	Dec. 31, 2022
Subsequent Event [Line Items]
Common stock, par value					$ 0.0001	$ 0.0001
Stockholders' equity, reverse stock split		1-for-25
Spyre Therapeutics, Inc. | Common Stock
Subsequent Event [Line Items]
Number of shares transferred as equity interest in asset acquisition				517,809
Spyre Therapeutics, Inc. | Series A Non Voting Convertible Preferred Stock
Subsequent Event [Line Items]
Number of shares transferred as equity interest in asset acquisition				364,887
Subsequent Event
Subsequent Event [Line Items]
Common stock, par value		$ 0.0001
Stockholders' equity, reverse stock split		1-for-25
Subsequent Event | Parapyre Option Obligation
Subsequent Event [Line Items]
Share-based payment award, term	10 years
Subsequent Event | Maximum
Subsequent Event [Line Items]
Reduction in the total number of authorized shares		500,000,000
Subsequent Event | Minimum
Subsequent Event [Line Items]
Reduction in the total number of authorized shares		20,000,000
Subsequent Event | Spyre Therapeutics, Inc. | Common Stock
Subsequent Event [Line Items]
Number of shares transferred as equity interest in asset acquisition			517,809
Subsequent Event | Spyre Therapeutics, Inc. | Series A Non Voting Convertible Preferred Stock
Subsequent Event [Line Items]
Number of shares transferred as equity interest in asset acquisition			364,887