Spyre Therapeutics (SYRE) 10-Q 9 May 24 2024 Q1 Quarterly report Financial data

Cover Page

Cover Page - shares	3 Months Ended
	Mar. 31, 2024	May 01, 2024
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2024
Document Transition Report	false
Entity File Number	001-37722
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
Title of 12(b) Security	Common Stock, $0.0001 Par Value Per Share
Trading Symbol	SYRE
Security Exchange Name	NASDAQ
Entity Interactive Data Current	Yes
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		40,283,414
Entity Central Index Key	0001636282
Current Fiscal Year End Date	--12-31
Amendment Flag	false
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
CURRENT ASSETS
Cash and cash equivalents	$ 227,552	$ 188,893
Marketable securities	257,089	150,384
Prepaid expenses and other current assets	2,632	2,251
Total current assets	487,273	341,528
Restricted cash	319	322
Other non-current assets	10	9
TOTAL ASSETS	487,602	341,859
CURRENT LIABILITIES
Accounts payable	3,106	896
CVR liability	2,590	1,390
Accrued and other current liabilities	21,594	13,108
Related party accounts payable and other current liabilities	15,528	16,584
Total current liabilities	42,818	31,978
Non-current CVR liability	39,110	41,310
TOTAL LIABILITIES	81,928	73,288
Commitments and Contingencies (Note 6)
STOCKHOLDERS’ EQUITY
Common stock, $0.0001 par value; 400,000,000 shares authorized as of March 31, 2024 and December 31, 2023; 36,629,680 shares and 36,057,109 shares issued and outstanding as of March 31, 2024 and December 31, 2023, respectively	10	10
Additional paid-in capital	775,966	763,191
Accumulated other comprehensive (loss) income	(363)	302
Accumulated deficit	(808,271)	(764,414)
TOTAL STOCKHOLDERS’ EQUITY	152,269	184,016
TOTAL LIABILITIES, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY	487,602	341,859
Series B Non Voting Convertible Preferred Stock
CURRENT LIABILITIES
Series B non-voting convertible preferred stock, $0.0001 par value; $271,625 and $150,000 shares authorized as of March 31, 2024 and December 31, 2023, respectively; $271,625 and $150,000 shares issued and outstanding as of March 31, 2024 and December 31, 2023, respectively	253,405	84,555
Series A Non Voting Convertible Preferred Stock
STOCKHOLDERS’ EQUITY
Series A non-voting convertible preferred stock & preferred stock shares authorized, issued, and outstanding, value	184,927	184,927
Preferred Stock, Excluding Series A Non Voting Convertible Preferred Stock
STOCKHOLDERS’ EQUITY
Series A non-voting convertible preferred stock & preferred stock shares authorized, issued, and outstanding, value	$ 0	$ 0

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2024	Dec. 31, 2023
Temporary Equity [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	8,642,034	8,763,659
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, authorized (in shares)	400,000,000	400,000,000
Common stock, issued (in shares)	36,629,680	36,057,109
Common stock, outstanding (in shares)	36,629,680	36,057,109
Series B Non Voting Convertible Preferred Stock
Temporary Equity [Abstract]
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)	271,625	150,000
Series B non-voting convertible preferred stock, issued (in shares)	271,625	150,000
Series B non-voting convertible preferred stock, outstanding (in shares)	271,625	150,000
Series A Non Voting Convertible Preferred Stock
Temporary Equity [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	1,086,341	1,086,341
Preferred stock, issued (in shares)	437,037	437,037
Preferred stock, outstanding (in shares)	437,037	437,037

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023
Revenue:
Total revenue		$ 0	$ 198
Operating expenses:
Research and development	[1]	34,928	13,776
General and administrative		12,846	5,228
Total operating expenses		47,774	19,004
Loss from operations		(47,774)	(18,806)
Other income (expense):
Interest income		4,432	420
Other expense		(483)	(72)
Total other income (expense)		3,949	348
Loss before income tax expense		(43,825)	(18,458)
Income tax (expense) benefit		(32)	36
Net loss		$ (43,857)	$ (18,422)
Net loss per share, basic (in dollars per share)		$ (1.20)	$ (4.89)
Net loss per share, diluted (in dollars per share)		$ (1.20)	$ (4.89)
Weighted-average common shares outstanding, basic (in shares)		36,512,662	3,770,506
Weighted-average common shares outstanding, diluted (in shares)		36,512,662	3,770,506
Development fee and royalty
Revenue:
Total revenue		$ 0	$ 198
[1] Includes $17.1 million in related party expenses for the three months ended March 31, 2024 and no related party expenses for the three months ended March 31, 2023.

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Operations (Parenthetical) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023
Research and development	[1]	$ 34,928	$ 13,776
Related Party
Research and development		$ 17,100	$ 0
[1] Includes $17.1 million in related party expenses for the three months ended March 31, 2024 and no related party expenses for the three months ended March 31, 2023.

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Statement of Comprehensive Income [Abstract]
Net loss	$ (43,857)	$ (18,422)
Other comprehensive income (loss):
Foreign currency translation adjustment	16	10
Unrealized (loss) gain on marketable securities	(681)	32
Total comprehensive loss	$ (44,522)	$ (18,380)

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Changes in Convertible Preferred Stock and Stockholders' Equity - USD ($) $ in Thousands	Total	Series B Non Voting Convertible Preferred Stock	Series A Non Voting Convertible Preferred Stock	Preferred Stock Series A Non Voting Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2022		0
Beginning balance at Dec. 31, 2022		$ 0
Ending balance (in shares) at Mar. 31, 2023		0
Ending balance at Mar. 31, 2023		$ 0
Beginning balance, preferred stock (in shares) at Dec. 31, 2022				0
Beginning balance, common stock (in shares) at Dec. 31, 2022					2,614,000
Beginning balance at Dec. 31, 2022	$ 50,305			$ 0	$ 6	$ 475,971	$ (48)	$ (425,624)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with employee stock purchase plan (in shares)					2,000
Issuance of common stock in connection with employee stock purchase plan	18					18
Stock-based compensation expense	1,709					1,709
Foreign currency translation adjustment	10						10
Unrealized gain (loss) on marketable securities	32						32
Net loss	(18,422)							(18,422)
Ending balance, preferred stock (in shares) at Mar. 31, 2023				0
Ending balance, common stock (in shares) at Mar. 31, 2023					2,616,000
Ending balance at Mar. 31, 2023	$ 33,652			$ 0	$ 6	477,698	(6)	(444,046)
Beginning balance (in shares) at Dec. 31, 2023		150,000
Beginning balance at Dec. 31, 2023		$ 84,555
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)		122,000
Issuance of Series B non-voting convertible preferred stock in connection with private placement, net of financing costs		$ 168,850
Ending balance (in shares) at Mar. 31, 2024		271,625
Ending balance at Mar. 31, 2024		$ 253,405
Beginning balance, preferred stock (in shares) at Dec. 31, 2023	0		437,037	437,000
Beginning balance, common stock (in shares) at Dec. 31, 2023					36,057,000
Beginning balance at Dec. 31, 2023	$ 184,016			$ 184,927	$ 10	763,191	302	(764,414)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with exercise of stock options and employee stock purchase plan (in shares)					572,000
Issuance of common stock in connection with exercise of stock options and employee stock purchase plan	4,390					4,390
Stock-based compensation expense	8,385					8,385
Foreign currency translation adjustment	16						16
Unrealized gain (loss) on marketable securities	(681)						(681)
Net loss	$ (43,857)							(43,857)
Ending balance, preferred stock (in shares) at Mar. 31, 2024	0		437,037	437,000
Ending balance, common stock (in shares) at Mar. 31, 2024					36,629,000
Ending balance at Mar. 31, 2024	$ 152,269			$ 184,927	$ 10	$ 775,966	$ (363)	$ (808,271)

Condensed Consolidated Statem_5

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (43,857)	$ (18,422)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	13,835	1,709
Change in fair value of derivative liability	430	0
Net accretion of discount on marketable securities	(2,423)	(107)
Depreciation and amortization	0	384
Amortization of operating lease assets	0	164
Other	0	2
Changes in operating assets and liabilities:
Accounts payable	2,210	1,384
Accrued and other liabilities	8,151	(3,164)
Related party payable	(6,507)	0
Prepaid expenses and other assets	(381)	622
Deferred revenue	0	(53)
Development receivables	0	45
Operating lease liabilities	0	(198)
Net cash used in operating activities	(28,542)	(17,634)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of marketable securities	(152,713)	0
Proceeds from maturities and sales of marketable securities	47,750	17,750
Net cash (used in) and provided by investing activities	(104,963)	17,750
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of Series B non-voting convertible preferred stock in connection with private placement, net of placement and other offering costs	169,205	0
Payments related to contingent value rights liability	(1,430)	0
Proceeds from employee stock plan purchases and stock option exercises	4,390	18
Principal payments on finance lease obligation	0	(8)
Net cash provided by financing activities	172,165	10
Effect of exchange rate on cash, cash equivalents, and restricted cash	(4)	11
NET INCREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH	38,656	137
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH
Beginning of period	189,215	36,416
End of period	227,871	36,553
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Unpaid amounts related to issuance of Series B non-voting convertible preferred stock in connection with private placement	$ 355	$ 0

The Company and Basis of Presen

The Company and Basis of Presentation	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
The Company and Basis of Presentation	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 (the “Reverse Split”). Except as indicated otherwise, all share numbers related to the Company's Common Stock disclosed in these financial statements have been adjusted on a post-Reverse Split basis. On 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 Spyre”), a privately held biotechnology company advancing a pipeline of antibody therapeutics with the potential to transform the treatment of inflammatory bowel disease through a research and development option agreement ("Paragon Agreement") with Paragon Therapeutics ("Paragon"). The asset acquisition was accomplished through a two-step reverse triangular merger whereby a wholly owned subsidiary of the Company merged with and into Pre-Merger Spyre, which existed at the time the Acquisition Agreement was entered into, became a wholly owned subsidiary of the Company in accordance with the terms of the Acquisition Agreement. Immediately following this merger, Pre-Merger Spyre merged with an into a second wholly subsidiary of the Company (“Merger Sub”) in accordance with the terms of the Acquisition Agreement and Pre-Merger Spyre ceased to exist. Subsequently, Aeglea BioTherapeutics, Inc. was renamed Spyre Therapeutics, Inc. and is a different entity than Pre-Merger Spyre, which ceased to exist upon merging with Merger Sub. The transaction was structured as a stock-for-stock transaction pursuant to which all of Pre-Merger Spyre's outstanding equity interests were exchanged based on a fixed exchange ratio of 0.5494488 to 1 for consideration from the Company of 517,809 shares of common stock, par value of $0.0001 per share ("Common Stock"), and 364,887 shares of Series A non-voting convertible preferred stock, par value of $0.0001 per share ("Series A Preferred Stock") (convertible on a 40 to 1 basis), in addition to the assumption of outstanding and unexercised stock options to purchase 2,734 shares of Common Stock from the Amended and Restated Spyre 2023 Equity Incentive Plan (the "Asset Acquisition"). The Common Stock and Series A Preferred Stock related to the Asset Acquisition were issued to the Pre-Merger Spyre stockholders on July 7, 2023. In connection with the Asset Acquisition, on June 26, 2023, the Company completed a private placement of shares of Series A Preferred Stock (the “June 2023 PIPE”) to a group of investors (the “June 2023 Investors”). The Company sold an aggregate of 721,452 shares of Series A Preferred Stock for an aggregate purchase price of approximately $210.0 million before deducting approximately $12.7 million in placement agent and other offering expenses (together with the Asset Acquisition, the “Transactions”). In connection with the Asset Acquisition, a non-transferable contingent value right ("CVR") was distributed to stockholders of record of the Company as of the close of business on July 3, 2023 (the "Legacy Stockholders"), but was not distributed to the holders of shares of Common Stock or Series A Preferred Stock issued to the former stockholders of Pre-Merger Spyre or the June 2023 Investors in the Transactions. Holders of the CVRs will be entitled to receive cash payments from proceeds received by the Company for a three-year period related to the disposition or monetization of its legacy assets for a period of one-year following the closing of the Asset Acquisition. On November 21, 2023, the Company's stockholders approved the conversion of the Company's Series A Preferred Stock to Common Stock. On December 11, 2023, the Company completed a private placement of shares of Common Stock and Series B non-voting convertible preferred stock, par value of $0.0001 per share ("Series B Preferred Stock") (convertible on a 40 to 1 basis) (the “December 2023 PIPE”) to a group of investors. The Company sold an aggregate of 6,000,000 shares of Common Stock and 150,000 shares of Series B Preferred Stock for an aggregate purchase price of approximately $180.0 million before deducting approximately $10.9 million of placement agent and other offering expenses. On March 20, 2024, the Company completed a private placement of Series B Preferred Stock (convertible on a 40 to 1 basis) (the “March 2024 PIPE”) to a group of investors. The Company sold 121,625 shares of Series B Preferred Stock for a purchase price of $180.0 million before deducting approximately $11.2 million of placement agent and other offering costs. 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 March 31, 2024, the Company has funded our operations by raising an aggregate of approximately $1.1 billion of gross proceeds from the sale and issuance of convertible preferred stock and common stock, pre-funded warrants, the collection of grant proceeds, and the licensing of its product rights for commercialization of pegzilarginase in Europe and certain countries in the Middle East. As of March 31, 2024, Spyre had an accumulated deficit of $808.3 million, and cash, cash equivalents, marketable securities and restricted cash of $485.0 million. 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. Unaudited Interim Financial Information The interim condensed consolidated financial statements included in this Quarterly Report on Form 10-Q are unaudited. The unaudited interim financial statements have been prepared on the same basis as the annual financial statements and reflect, in the opinion of management, all adjustments of a normal and recurring nature that are necessary for a fair statement of the Company’s financial position as of March 31, 2024, and its results of operations for the three months ended March 31, 2024 and 2023, changes in convertible preferred stock and stockholders’ equity for the three months ended March 31, 2024 and 2023, and cash flows for the three months ended March 31, 2024 and 2023. The results of operations for the three months ended March 31, 2024, are not necessarily indicative of the results to be expected for the year ending December 31, 2024 or for any other future annual or interim period. The December 31, 2023 balance sheet was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. These financial statements should be read in conjunction with the audited financial statements included in the Company’s Form 10-K for the year ended December 31, 2023 (the "Annual Report") as filed with the SEC on February 29, 2024 and amended on March 1, 2024.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Spyre Therapeutics' significant accounting policies are detailed in the Notes titled “1. The Company and Basis of Presentation” and "2. Summary of Significant Accounting Policies” of the Company's Annual Report. These interim condensed consolidated financial statements have been prepared in accordance with U.S. GAAP and SEC instructions for interim financial information, and should be read in conjunction with the Company's Annual Report. Significant accounting policies and other disclosures normally provided have been omitted since such items are disclosed in the Company's Annual Report. The Company uses the same accounting policies in preparing quarterly and annual financial statements. Recently Adopted Accounting Pronouncement There have been no recent accounting pronouncements or changes in accounting pronouncements during the three months ended March 31, 2024 that are of significance or potential significance to the Company.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The Company measures and reports certain financial instruments as assets and liabilities at fair value on a recurring basis. The following tables set forth the fair value of the Company’s financial assets and liabilities at fair value on a recurring basis based on the three-tier fair value hierarchy (in thousands): March 31, 2024 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 225,797 $ — $ — $ 225,797 U.S. government treasury securities 85,045 — — 85,045 U.S. government agency securities — 55,818 — 55,818 Commercial paper — 74,792 — 74,792 Corporate bonds — 41,434 — 41,434 Total financial assets $ 310,842 $ 172,044 $ — $ 482,886 Liabilities: Parapyre Option Obligation $ — $ 5,449 $ — $ 5,449 CVR liability — — 41,700 41,700 Total liabilities $ — $ 5,449 $ 41,700 $ 47,149 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 The Company measures the fair value of money market funds on quoted prices in active markets for identical assets or liabilities. The Level 2 assets include 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. Parapyre Option Obligation Under the Paragon Agreement, the Company is obligated to issue Parapyre Holding LLC ("Parapyre") an annual equity grant of warrants, on the last business day of each of the years ended December 31, 2023 and December 31, 2024, to purchase 1% of the then outstanding shares of the Company’s Common Stock, on a fully diluted basis, during the term of the Paragon Agreement (the "Parapyre Option Obligation"). 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 is recorded pursuant to the Paragon Agreement during interim periods. On December 31, 2023, the Company settled its 2023 obligation under the Parapyre Option Obligation by issuing Parapyre 684,407 warrants to purchase the Company's Common Stock, with a $21.52 per share exercise price for each warrant. The Parapyre Option Obligation is considered a Level 2 liability based on observable market data for substantially the full term of the liability. The Parapyre Option Obligation is measured each period using a Black-Scholes model to estimate the fair value of the option grant. Changes in the fair value of the Parapyre Option Obligation are recorded as stock-based compensation within Research and development expenses for non-employees who provided pre-clinical development services. CVR Liability In connection with the Asset Acquisition, a non-transferable CVR was distributed to the Legacy Stockholders, but was not distributed to holders of shares of Common Stock or Series A Preferred Stock issued to the June 2023 Investors or former stockholders of Pre-Merger Spyre in connection with the Transactions. Holders of the CVR will be entitled to receive certain cash payments from proceeds received by the Company for a three-year period, if any, related to the disposition or monetization of the Company’s legacy assets for a period of one year following the closing of the Asset Acquisition. The CVR liability value is based on significant inputs not observable in the market such as estimated cash flows, estimated probabilities of success, and risk-adjusted discount rates, which represent a Level 3 liability. 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: March 31, 2024 Estimated cash flow dates 02/28/25 - 06/22/26 Estimated probability of success 39% - 100% Estimated reimbursement rate compared to reimbursement agent 81% - 100% Risk-adjusted discount rates 6.32% - 6.65% The change in fair value between December 31, 2023 and March 31, 2024 was a $0.4 million increase, and was primarily driven by changes in the risk-adjusted discount rates and the time value of money. The following table presents changes in the CVR liability for the periods presented (in thousands): CVR Liability Beginning balance as of December 31, 2023 $ 42,700 Changes in the fair value of the CVR liability 430 Payments (1,430) Ending Balance as of March 31, 2024 $ 41,700

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	3 Months Ended
	Mar. 31, 2024
Cash and Cash Equivalents [Abstract]
Cash Equivalents and Marketable Securities	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): March 31, 2024 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash equivalents: Money market funds $ 225,797 $ — $ — $ 225,797 Total cash equivalents $ 225,797 $ — $ — $ 225,797 Marketable securities: Commercial paper $ 74,803 $ 12 $ (23) $ 74,792 Corporate bonds 41,497 11 (74) 41,434 U.S. government treasury securities 85,250 4 (209) 85,045 U.S. government agency securities 55,937 26 (145) 55,818 Total marketable securities $ 257,487 $ 53 $ (451) $ 257,089 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 The following table summarizes the available-for-sale securities in an unrealized loss position for which an allowance for credit losses has not been recorded as of March 31, 2024 and December 31, 2023, aggregated by major security type and length of time in a continuous unrealized loss position: March 31, 2024 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 30,027 $ (23) $ — $ — $ 30,027 $ (23) Corporate bonds 30,737 (74) — — 30,737 (74) U.S. government treasury securities 77,707 (209) — — 77,707 (209) U.S. government agency securities 44,742 (145) — — 44,742 (145) Total marketable securities $ 183,213 $ (451) $ — $ — $ 183,213 $ (451) December 31, 2023 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses 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) The Company evaluated its securities for credit losses and considered the decline in market value to be primarily attributable to current economic and market conditions and not to a credit loss or other factors. Additionally, the Company does not intend to sell the securities in an unrealized loss position and does not expect it will be required to sell the securities before recovery of the unamortized cost basis. As of March 31, 2024 and December 31, 2023, an allowance for credit losses had not been recognized. Given the Company's intent and ability to hold such securities until recovery, and the lack of significant change in credit risk of these investments, the Company does not consider these marketable securities to be impaired as of March 31, 2024 and December 31, 2023. The financial instruments that potentially subject the Company to a concentration of credit risk consist principally of cash deposits. Accounts at each of our two U.S. banking institutions are insured by the Federal Deposit Insurance Corporation (“FDIC”) up to $250,000 per depositor. As of March 31, 2024 and December 31, 2023, cash deposits at the Company's U.S. banking institutions exceeded the FDIC limits. Uninsured foreign cash deposits were immaterial for both periods. There were no realized gains or losses on marketable securities for the three months ended March 31, 2024 and 2023. Interest on marketable securities is included in interest income. Accrued interest receivable on available-for-sale debt securities as of March 31, 2024 and December 31, 2023, was $1.3 million and $0.9 million, respectively. The following table summarizes the contractual maturities of the Company’s marketable securities at estimated fair value (in thousands): March 31, December 31, Due in one year or less $ 191,090 $ 115,784 Due in 1 - 2 years 65,999 34,600 Total marketable securities $ 257,089 $ 150,384 The Company may sell investments at any time for use in current operations even if they have not yet reached maturity. As a result, the Company classifies marketable securities, including securities with maturities beyond twelve months as current assets.

Accrued and Other Current Liabi

Accrued and Other Current Liabilities	3 Months Ended
	Mar. 31, 2024
Payables and Accruals [Abstract]
Accrued and Other Current Liabilities	Accrued and Other Current Liabilities Accrued and other current liabilities consist of the following (in thousands): March 31, December 31, Accrued compensation $ 2,506 $ 4,054 Accrued contracted research and development costs 18,149 7,092 Accrued professional and consulting fees 720 1,474 Accrued other 219 488 Total accrued and other current liabilities $ 21,594 $ 13,108

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2024
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions Paragon Agreement Paragon and Parapyre each beneficially owns 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 basis, has two seats on the Company's board of directors (the "Board") and beneficially owns more than 5% of Paragon, which is a joint venture between Fairmount and FairJourney Biologics. Fairmount appointed Paragon's board of directors and has the contractual right to approve the appointment of any executive officers. Parapyre is an entity formed by Paragon as a vehicle to hold equity in Spyre in order to share profits with certain employees of Paragon. In connection with the Asset Acquisition, the Company assumed the rights and obligations of Pre-Merger Spyre under the Paragon Agreement. Under the Paragon Agreement, Spyre is obligated to compensate Paragon for its services performed under each research program based on the actual costs incurred with mark-up costs pursuant to the terms of the Paragon Agreement. Spyre is also obligated under the Paragon Agreement to issue Parapyre annual equity grants of warrants in accordance with the Parapyre Option Obligation. For the three months ended March 31, 2024, the Company recognized expenses related to services provided by Paragon subsequent to the Asset Acquisition totaling $17.1 million, which included $5.4 million of stock-based compensation expense, and were recorded as Research and development expenses in the consolidated statements of operations. As of March 31, 2024 and December 31, 2023, $15.5 million and $16.6 million, respectively, was unpaid and was included in Related party accounts payable and other current liabilities on the Company's consolidated balance sheets. For the three months ended March 31, 2024, the Company made payments totaling $18.2 million to Paragon. On July 12, 2023 and December 14, 2023, the Company exercised the option to license certain intellectual property rights (collectively, the "Option") available under the Paragon Agreement with respect to the SPY001 and SPY002 research programs, respectively, and expects to enter into a SPY001 license agreement (the "SPY001 License Agreement") and a SPY002 license agreement (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, 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 subject in a Phase 1 trial. With respect to the SPY002 License Agreement only, on a product by product basis, the Company expects to pay Paragon sublicensing fees of up to approximately $20.0 million upon the achievement of mostly commercial milestones. The following is the summary of expenses related to the Paragon Agreement, which were ultimately settled in cash (in millions): Three Months Ended Financial Statement Line Item 2024 2023 Reimbursable costs under the Paragon Agreement $ 11.7 $ — Research and development Parapyre Option Obligation Pursuant to the Paragon Agreement, the Company agreed to issue Parapyre an annual equity grant of warrants, on the last business day of each of the years ended December 31, 2023 and December 31, 2024, to purchase 1% of the then outstanding shares of the Company's Common Stock, on a fully diluted basis, during the term of the Paragon Agreement The following is the summary of Related party accounts payable and other current liabilities (in millions): March 31, December 31, Reimbursable costs under the Paragon Agreement $ 10.1 $ 16.6 Parapyre warrants liability 5.4 — Total related party accounts payable $ 15.5 $ 16.6 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 stock options to purchase 477,000 shares of the Company’s Common Stock under the 2016 Plan (as defined in Note 8) with an exercise price of $10.39 per share, which vest as to 25% on the one year anniversary of the Vesting Commencement Date and thereafter vest and become exercisable in 36 equal monthly installments, subject to Mr. McKenna’s continued service to the Company through each applicable vesting date. For the three months ended March 31, 2024, the Company recognized $0.3 million in stock-based compensation expense related to Mr. McKenna's consulting agreement. There was no such expense for the three months ended March 31, 2023.

Convertible Preferred Stock and

Convertible Preferred Stock and Stockholders' Equity	3 Months Ended
	Mar. 31, 2024
Equity [Abstract]
Convertible Preferred Stock and Stockholders' Equity	Convertible Preferred Stock and Stockholders’ Equity Pre-Funded Warrants In February 2019, April 2020 and May 2022, the Company issued pre-funded warrants to purchase the Company’s Common Stock in underwritten public offerings at the offering price of the Common Stock, less the $0.0025 per share exercise price of each warrant. The warrants were recorded as a component of stockholders’ (deficit) equity within additional paid-in capital and have no expiration date. Per the terms of the warrant agreements, the outstanding warrants to purchase shares of Common Stock may not be exercised if the holder’s ownership of the Company’s Common Stock would exceed 4.99% (“Maximum Ownership Percentage”), or 9.99% for certain holders. By written notice to the Company, each holder may increase or decrease the Maximum Ownership Percentage to any other percentage (not in excess of 19.99% for the majority of such warrants). The revised Maximum Ownership Percentage would be effective 61 days after the notice is received by the Company. As of March 31, 2024, the following pre-funded warrants for Common Stock were issued and outstanding: Issue Date Expiration Date Exercise Price Number of Warrants Outstanding May 20, 2022 None $ 0.0025 250,000 Total pre-funded warrants 250,000 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, with a $21.52 per share exercise price for each warrant. Pursuant to the terms of the warrant agreement, the outstanding warrants to purchase shares of Common Stock may not be exercised if the holder’s ownership of the Company’s Common Stock would exceed 4.99%. As of March 31, 2024, none of the warrants issued under the Parapyre Option Obligation have been exercised. Series A Non-Voting Convertible Preferred Stock On June 22, 2023, the Company filed a Certificate of Designation of Preferences, Rights and Limitations of the Series A Preferred Stock with the Secretary of State of the State of Delaware (the “Series A Certificate of Designation”) in connection with the Asset Acquisition and the June 2023 PIPE. Pursuant to the Series A Certificate of Designation, holders of Series A Preferred Stock are entitled to receive dividends on shares of Series A Preferred Stock equal to, on an as-if-converted-to-Common Stock basis, and in the same form as, dividends actually paid on shares of Common Stock. Except as provided in the Series A Certificate of Designation or as otherwise required by law, the Series A Preferred Stock does not have voting rights. However, as long as any shares of Series A Preferred Stock are outstanding, the Company will not, without the affirmative vote of the holders of a majority of the then outstanding shares of the Series A Preferred Stock: (a) alter or change adversely the powers, preferences or rights given to the Series A Preferred Stock, or alter or amend the Series A Certificate of Designation, amend or repeal any provision of, or add any provision to, the Company’s Certificate of Incorporation or its Bylaws, or file any articles of amendment, certificate of designations, preferences, limitations and relative rights of any series of preferred stock, if such action would adversely alter or change the preferences, rights, privileges or powers of, or restrictions provided for the benefit of the Series A Preferred Stock, regardless of whether any of the foregoing actions will be by means of amendment to the Certificate of Incorporation or by merger, consolidation, recapitalization, reclassification, conversion or otherwise, (b) issue further shares of Series A Preferred Stock or increase or decrease (other than by conversion) the number of authorized shares of Series A Preferred Stock, (c) prior to the stockholder approval of the conversion of the Series A Preferred Stock into shares of Common Stock in accordance with Nasdaq Stock Market Rules (the “Series A Conversion Proposal”) or at any time while at least 30% of the originally issued Series A Preferred Stock remains issued and outstanding, consummate (x) any Fundamental Transaction (as defined in the Series A Certificate of Designation) or (y) any merger or consolidation of the Company with or into another entity or any stock sale to, or other business combination in which our stockholders immediately before such transaction do not hold at least a majority of our capital stock immediately after such transaction or (d) enter into any agreement with respect to any of the foregoing. The Series A Preferred Stock does not have a preference upon any liquidation, dissolution or winding-up of the Company. The Company held a stockholders’ meeting to submit the following matters to its stockholders for their consideration: (i) the approval of the Series A 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 Series A 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 19.9%) of the total number of shares of Common Stock issued and outstanding immediately after giving effect to such conversion. On June 26, 2023, the Company completed a private placement of 721,452 shares of Series A Preferred Stock in exchange for gross proceeds of approximately $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. On November 21, 2023, the Company's stockholders approved the Series A Conversion Proposal, among other matters, at a special meeting of stockholders. As a result of the approval of the Series A 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 March 31, 2024 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 contributed capital amount, inclusive of any forward-contract valuation adjustments, of the Series A Preferred Stock. Series B Non-Voting Convertible Preferred Stock On December 8, 2023, the Company filed a Certificate of Designation of Preferences, Rights and Limitations of Series B Non-Voting Convertible Preferred Stock with the Secretary of State of the State of Delaware (the “Series B Certificate of Designation”) in connection with the December 2023 PIPE. 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 Stock basis, and in the same form as, dividends actually paid on shares of Common Stock. Except as provided in the Series B Certificate of Designation or as otherwise required by law, the Series B Preferred Stock does not have voting rights. However, as long as any shares of Series B Preferred Stock are outstanding, the Company will not, without the affirmative vote of the holders of a majority of the then outstanding shares of the Series B Preferred Stock, alter or change adversely the powers, preferences or rights given to the Series B Preferred Stock, or alter or amend the Series B Certificate of Designation, amend or repeal any provision of, or add any provision to, the Company’s Certificate of Incorporation or its Bylaws, or file any articles of amendment, certificate of designations, preferences, limitations and relative rights of any series of preferred stock, if such action would adversely alter or change the preferences, rights, privileges or powers of, or restrictions provided for the benefit of the Series B Preferred Stock, regardless of whether any of the foregoing actions will be by means of amendment to the Certificate of Incorporation or by merger, consolidation, recapitalization, reclassification, conversion or otherwise. The Series B Preferred Stock does not have a preference upon any liquidation, dissolution or winding-up of the Company. 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 (the "2024 Annual Meeting"), which the Company expects to hold on May 13, 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 March 31, 2024, the redemption value of the Company's outstanding Series B Preferred Stock was $412.1 million based on the closing stock price of the Company's Common Stock on March 31, 2024 of $37.93 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 0.0% and 19.9%) 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. On March 18, 2024, in connection with the March 2024 PIPE, the Company filed a certificate of amendment to its Series B Certificate of Designation to increase the number of authorized shares of Series B Preferred Stock from 150,000 to 271,625. On March 20, 2024, as part of the March 2024 PIPE, the Company completed a private placement of 121,625 shares of Series B Preferred Stock in exchange for gross proceeds of approximately $180.0 million. On April 1, 2024, the Company filed a definitive proxy statement with the SEC to solicit approval of the Series B Conversion Proposal, among other matters, at the 2024 Annual Meeting.

Stock-Based Compensation

Stock-Based Compensation	3 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Stock-Based Compensation	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. 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 March 31, 2024, 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, as amended, provides for an automatic increase in the number of shares reserved for issuance thereunder on January 1 of each year for the remaining term of the plan equal to (a) 5.0% of the number of issued and outstanding shares of Common Stock (including such shares issuable pursuant to the exercise or conversion, as applicable, of any outstanding pre-funded warrants and nonvoting convertible preferred stock) on December 31 of the immediately preceding year, or (b) a lesser amount as approved by the board each year (the “Evergreen Provision”). As a result of the Evergreen Provision, on January 1, 2024 and 2023, an additional 3,023,650 and 104,561 shares, respectively, became available for issuance under the 2016 Plan. As of March 31, 2024, the 2016 Plan had 7,393,885 shares available for future issuance, of which 2,996,404 shares were subject to outstanding option awards. 2018 Equity Inducement Plan The 2018 Equity Inducement Plan (“2018 Plan”) became effective in February 2018. 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. As of March 31, 2024, the 2018 Plan had 6,029,000 shares available for future issuance, of which 5,384,241 shares were subject to outstanding option awards and restricted unit 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 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 basis over the vesting period. Parapyre Option Obligation As of March 31, 2024, the pro-rated estimated fair value of the options to be granted on December 31, 2024, was approximately $21.9 million. For the three months ended March 31, 2024, $5.4 million was recognized as stock compensation expense related to the Parapyre Option Obligation. There was no similar expense for the three months ended March 31, 2023. As of March 31, 2024, the unamortized expense related to the Parapyre Option Obligation was $16.5 million. The following table summarizes the Company’s stock awards granted under all plans for each of the periods indicated: Three Months Ended March 31, 2024 2023 Grants Weighted Average Grant Date Fair Value Grants Weighted Average Grant Date Fair Value Stock options 1,044,658 $ 26.50 177,620 $ 11.00 2016 Employee Stock Purchase Plan Under the Company’s 2016 Employee Stock Purchase Plan (“2016 ESPP”), the Company issued and sold 2,330 and 1,793 shares during the three months ended March 31, 2024 and March 31, 2023, respectively. The aggregate cash proceeds were di minimis for both periods. Stock-based Compensation Expense Total stock-based compensation expense recognized from the Company’s equity incentive plans, 2018 Plan, 2016 ESPP and Parapyre Option Obligation during the periods presented was as follows (in thousands): Three Months Ended 2024 2023 Research and development (1) $ 6,857 $ 777 General and administrative 6,978 932 Total stock-based compensation expense $ 13,835 $ 1,709 (1) For the three months ended March 31, 2024, $5.4 million, was recognized as stock compensation expense related to the Parapyre Option Obligation. There were no such expenses for the three months ended March 31, 2023. (2) Of the total $13.8 million and $1.7 million of stock-based compensation expense for the three months ended March 31, 2024 and 2023, respectively, $2.9 million and $0.5 million, respectively, is related to legacy Aeglea employees and directors who had been terminated as of the end of the period. The following table summarizes the weighted-average Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted under the Company's equity incentive plans, and the shares purchasable under the 2016 ESPP during the periods presented: Three Months Ended 2024 2023 Stock Options Granted Expected term (in years) 6.03 6.02 Expected volatility 105% 99% Risk-free interest 3.88% 4.06% Dividend yield — — 2016 ESPP Expected term (in years) 0.50 0.49 Expected volatility 98% 181% Risk-free interest 5.31 4.99 Dividend yield — —

Strategic License Agreements

Strategic License Agreements	3 Months Ended
	Mar. 31, 2024
License And Collaboration Agreement [Abstract]
Strategic License Agreements	Strategic License Agreements On March 21, 2021, the Company entered into an exclusive license and supply agreement with Immedica (the "Immedica Agreement"). On July 27, 2023, the Company announced that it had entered into an agreement to sell the global rights to pegzilarginase, an investigational treatment for the rare metabolic disease Arginase 1 Deficiency, to Immedica for $15.0 million in upfront cash proceeds and up to $100.0 million in contingent milestone payments. The sale of pegzilarginase to Immedica superseded and terminated the Immedica Agreement. The milestone payments are contingent on formal reimbursement decisions by national authorities in key European markets and pegzilarginase approval by the FDA, among other events. The upfront payment and contingent milestone payments if paid, net of expenses and adjustments, will be distributed to holders of the Company's CVRs (as defined in Note 1) pursuant to the contingent value rights agreement we entered into with Equiniti Trust Company LLC (f/k/a American Stock Transfer & Trust Company LLC) as rights agent in connection with the Asset Acquisition. The Company did not recognize any revenue under the Immedica Agreement for the three months ended March 31, 2024. For the three months ended March 31, 2023, the Company recognized $0.2 million of development fee revenue in connection with the Immedica Agreement, which was attributable to the PEACE Phase 3 trial and BLA package for pegzilarginase. For more details on the now terminated Immedica Agreement, please refer to the Note under Item 1 of Part I, titled "12. Strategic License Agreements" of the Company's Annual Report. Contract Balances from Customer Contract The timing of revenue recognition, billings and cash collections results in contract assets and contract liabilities on the Company's balance sheets. The Company recognizes license and development receivables based on billed services, which are derecognized upon reimbursement. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded. Contract liabilities are recognized as revenue after control of the goods or services is transferred to the customer and all revenue recognition criteria have been met. The Company did not have any contract assets or liabilities as of March 31, 2024 and December 31, 2023.

Net Loss Per Share

Net Loss Per Share	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share The Company computes net loss attributable per common stockholder using the two-class method required for participating securities. The Company considers convertible. preferred stock to be participating securities. In the event that the Company paid out distributions, holders of convertible preferred stock would participate in the distribution. The two-class method is an earnings (loss) allocation method under which earnings (loss) per share is calculated for Common Stock and participating security considering a participating security’s rights to undistributed earnings (loss) as if all such earnings (loss) had been distributed during the period. The holders of Series A Preferred Stock and Series B Preferred Stock do not have an obligation to fund losses and therefore the Series A Preferred Stock and the Series B Preferred Stock were excluded from the calculation of basic net loss per share. Basic and diluted net loss per share is computed by dividing the net loss by the weighted-average number of Common Stock and pre-funded warrants outstanding during the period, without consideration of potential dilutive securities. The pre-funded warrants are included in the computation of basic net loss per share as the exercise price is negligible and they are fully vested and exercisable. For periods in which the Company generated a net loss, the Company does not include the potential impact of dilutive securities in diluted net loss per share, as the impact of these items is anti-dilutive. The Company has generated a net loss for all periods presented, therefore diluted net loss per share is the same as basic net loss per share since the inclusion of potentially dilutive securities would be anti-dilutive. The following weighted-average equity instruments were excluded from the calculation of diluted net loss per share because their effect would have been anti-dilutive for the periods presented: Three Months Ended 2024 2023 Options to purchase common stock 3,200,918 459,425 Unvested restricted stock units 61,253 766 Outstanding Parapyre warrants 684,407 — The following is a reconciliation of the shares used as the denominator for the calculation of basic and diluted net loss per share: Three Months Ended 2024 2023 Weighted average Common Stock 36,262,662 2,614,843 Weighted average pre-funded warrants 250,000 1,155,663 Total basic and diluted weighted average shares 36,512,662 3,770,506

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2024
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On April 23, 2024, the Company entered into an exchange agreement with Fairmount Healthcare Fund II L.P. (the “Stockholder”), pursuant to which the Stockholder agreed to exchange an aggregate of 90,992 shares of Series A Preferred Stock for an aggregate of 3,639,680 shares of Common Stock (the “April 2024 Exchange”). The Common Stock issued in connection with the April 2024 Exchange was issued without registration under the Securities Act of 1933, as amended (the “Securities Act”) in reliance on the exemption from registration contained in Section 3(a)(9) of the Securities Act. The April 2024 Exchange closed on April 25, 2024.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pay vs Performance Disclosure
Net loss	$ (43,857)	$ (18,422)

Insider Trading Arrangements

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

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Recently Adopted Accounting Pronouncement	Recently Adopted Accounting Pronouncement There have been no recent accounting pronouncements or changes in accounting pronouncements during the three months ended March 31, 2024 that are of significance or potential significance to the Company.
Net Loss Per Share	The Company computes net loss attributable per common stockholder using the two-class method required for participating securities. The Company considers convertible. preferred stock to be participating securities. In the event that the Company paid out distributions, holders of convertible preferred stock would participate in the distribution. The two-class method is an earnings (loss) allocation method under which earnings (loss) per share is calculated for Common Stock and participating security considering a participating security’s rights to undistributed earnings (loss) as if all such earnings (loss) had been distributed during the period. The holders of Series A Preferred Stock and Series B Preferred Stock do not have an obligation to fund losses and therefore the Series A Preferred Stock and the Series B Preferred Stock were excluded from the calculation of basic net loss per share.

Fair Value Measurement (Tables)

Fair Value Measurement (Tables)	3 Months Ended
	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables set forth the fair value of the Company’s financial assets and liabilities at fair value on a recurring basis based on the three-tier fair value hierarchy (in thousands): March 31, 2024 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 225,797 $ — $ — $ 225,797 U.S. government treasury securities 85,045 — — 85,045 U.S. government agency securities — 55,818 — 55,818 Commercial paper — 74,792 — 74,792 Corporate bonds — 41,434 — 41,434 Total financial assets $ 310,842 $ 172,044 $ — $ 482,886 Liabilities: Parapyre Option Obligation $ — $ 5,449 $ — $ 5,449 CVR liability — — 41,700 41,700 Total liabilities $ — $ 5,449 $ 41,700 $ 47,149 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
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: March 31, 2024 Estimated cash flow dates 02/28/25 - 06/22/26 Estimated probability of success 39% - 100% Estimated reimbursement rate compared to reimbursement agent 81% - 100% Risk-adjusted discount rates 6.32% - 6.65%
Changes in Derivative Liabilities	The following table presents changes in the CVR liability for the periods presented (in thousands): CVR Liability Beginning balance as of December 31, 2023 $ 42,700 Changes in the fair value of the CVR liability 430 Payments (1,430) Ending Balance as of March 31, 2024 $ 41,700

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2024
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): March 31, 2024 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash equivalents: Money market funds $ 225,797 $ — $ — $ 225,797 Total cash equivalents $ 225,797 $ — $ — $ 225,797 Marketable securities: Commercial paper $ 74,803 $ 12 $ (23) $ 74,792 Corporate bonds 41,497 11 (74) 41,434 U.S. government treasury securities 85,250 4 (209) 85,045 U.S. government agency securities 55,937 26 (145) 55,818 Total marketable securities $ 257,487 $ 53 $ (451) $ 257,089 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
Available-for-Sale Securities in an Unrealized Loss Position	The following table summarizes the available-for-sale securities in an unrealized loss position for which an allowance for credit losses has not been recorded as of March 31, 2024 and December 31, 2023, aggregated by major security type and length of time in a continuous unrealized loss position: March 31, 2024 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 30,027 $ (23) $ — $ — $ 30,027 $ (23) Corporate bonds 30,737 (74) — — 30,737 (74) U.S. government treasury securities 77,707 (209) — — 77,707 (209) U.S. government agency securities 44,742 (145) — — 44,742 (145) Total marketable securities $ 183,213 $ (451) $ — $ — $ 183,213 $ (451) December 31, 2023 Less Than 12 Months 12 Months or Longer Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses 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)
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): March 31, December 31, Due in one year or less $ 191,090 $ 115,784 Due in 1 - 2 years 65,999 34,600 Total marketable securities $ 257,089 $ 150,384

Accrued and Other Current Lia_2

Accrued and Other Current Liabilities (Tables)	3 Months Ended
	Mar. 31, 2024
Payables and Accruals [Abstract]
Accrued and Other Current Liabilities	Accrued and other current liabilities consist of the following (in thousands): March 31, December 31, Accrued compensation $ 2,506 $ 4,054 Accrued contracted research and development costs 18,149 7,092 Accrued professional and consulting fees 720 1,474 Accrued other 219 488 Total accrued and other current liabilities $ 21,594 $ 13,108

Related Party Transactions (Tab

Related Party Transactions (Tables)	3 Months Ended
	Mar. 31, 2024
Related Party Transactions [Abstract]
Expenses related to Related Party which were Settled in Cash	The following is the summary of expenses related to the Paragon Agreement, which were ultimately settled in cash (in millions): Three Months Ended Financial Statement Line Item 2024 2023 Reimbursable costs under the Paragon Agreement $ 11.7 $ — Research and development
Related Party Accounts Payable	The following is the summary of Related party accounts payable and other current liabilities (in millions): March 31, December 31, Reimbursable costs under the Paragon Agreement $ 10.1 $ 16.6 Parapyre warrants liability 5.4 — Total related party accounts payable $ 15.5 $ 16.6

Convertible Preferred Stock a_2

Convertible Preferred Stock and Stockholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2024
Equity [Abstract]
Pre-funded Warrants for Common Stock Issued and Outstanding	As of March 31, 2024, the following pre-funded warrants for Common Stock were issued and outstanding: Issue Date Expiration Date Exercise Price Number of Warrants Outstanding May 20, 2022 None $ 0.0025 250,000 Total pre-funded warrants 250,000

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Stock Awards Granted	The following table summarizes the Company’s stock awards granted under all plans for each of the periods indicated: Three Months Ended March 31, 2024 2023 Grants Weighted Average Grant Date Fair Value Grants Weighted Average Grant Date Fair Value Stock options 1,044,658 $ 26.50 177,620 $ 11.00
Stock-Based Compensation Expense	Total stock-based compensation expense recognized from the Company’s equity incentive plans, 2018 Plan, 2016 ESPP and Parapyre Option Obligation during the periods presented was as follows (in thousands): Three Months Ended 2024 2023 Research and development (1) $ 6,857 $ 777 General and administrative 6,978 932 Total stock-based compensation expense $ 13,835 $ 1,709 (1) For the three months ended March 31, 2024, $5.4 million, was recognized as stock compensation expense related to the Parapyre Option Obligation. There were no such expenses for the three months ended March 31, 2023. (2) Of the total $13.8 million and $1.7 million of stock-based compensation expense for the three months ended March 31, 2024 and 2023, respectively, $2.9 million and $0.5 million, respectively, is related to legacy Aeglea employees and directors who had been terminated as of the end of the period.
Assumptions used to Estimate the Fair Value of Stock Options Granted	The following table summarizes the weighted-average Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted under the Company's equity incentive plans, and the shares purchasable under the 2016 ESPP during the periods presented: Three Months Ended 2024 2023 Stock Options Granted Expected term (in years) 6.03 6.02 Expected volatility 105% 99% Risk-free interest 3.88% 4.06% Dividend yield — — 2016 ESPP Expected term (in years) 0.50 0.49 Expected volatility 98% 181% Risk-free interest 5.31 4.99 Dividend yield — —

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Weighted-Average Equity Instruments Excluded from Calculation of Diluted Net Loss Per Share	The following weighted-average equity instruments were excluded from the calculation of diluted net loss per share because their effect would have been anti-dilutive for the periods presented: Three Months Ended 2024 2023 Options to purchase common stock 3,200,918 459,425 Unvested restricted stock units 61,253 766 Outstanding Parapyre warrants 684,407 —
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: Three Months Ended 2024 2023 Weighted average Common Stock 36,262,662 2,614,843 Weighted average pre-funded warrants 250,000 1,155,663 Total basic and diluted weighted average shares 36,512,662 3,770,506

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	109 Months Ended
	Mar. 20, 2024 USD ($) shares	Mar. 18, 2024 shares	Dec. 11, 2023 USD ($) $ / shares shares	Sep. 08, 2023	Jun. 26, 2023 USD ($) shares	Jun. 22, 2023 $ / shares shares	Apr. 30, 2023	Mar. 31, 2024 USD ($) segment $ / shares shares	Mar. 31, 2024 USD ($) $ / shares	Dec. 31, 2023 USD ($) $ / shares
Company and Basis of Presentation [Line Items]
Number of operating segments | segment								1
Employee workforce, termination percentage							83%
Common stock, par value (in dollars per share) | $ / shares								$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock, par value (in dollars per share) | $ / shares								$ 0.0001	$ 0.0001	$ 0.0001
Series B non-voting convertible preferred stock, par value (in dollars per share) | $ / shares			$ 0.0001
Proceeds from raising capital									$ 1,100,000
Accumulated deficit								$ 808,271	808,271	$ 764,414
Cash, cash equivalents, and marketable securities								$ 485,000	$ 485,000
Asset Acquisition
Company and Basis of Presentation [Line Items]
Fixed exchange ratio						54.94488%
Spyre Therapeutics, Inc.
Company and Basis of Presentation [Line Items]
Asset acquisition, stockholder payment period						3 years
Asset acquisition, cash payment, threshold period						1 year
Spyre 2023 Equity Incentive Plan | Asset Acquisition
Company and Basis of Presentation [Line Items]
Number of outstanding and unexercised stock options to purchase (in shares) | shares						2,734
Series 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
Conversion basis						40
Issuance of Series B non-voting convertible preferred stock in connection with private placement, net of financing costs (in shares) | shares					721,452
Series A Non Voting Convertible Preferred Stock | Asset Acquisition
Company and Basis of Presentation [Line Items]
Number of shares transferred as equity interest in asset acquisition (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
Series B non-voting convertible preferred stock, par value (in dollars per share) | $ / shares								$ 0.0001	$ 0.0001	$ 0.0001
Issuance of Series B non-voting convertible preferred stock in connection with private placement, net of financing costs (in shares) | shares	121,625	271,625	150,000					122,000
Private Placement
Company and Basis of Presentation [Line Items]
Gross proceeds received in private placement			$ 180,000
Placement agent and other offering costs	$ 11,200		10,900
Private Placement | Series A Non Voting Convertible Preferred Stock
Company and Basis of Presentation [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
Private Placement | Series B Non Voting Convertible Preferred Stock
Company and Basis of Presentation [Line Items]
Gross proceeds received in private placement	$ 180,000		$ 90,000
Common Stock
Company and Basis of Presentation [Line Items]
Reverse stock split, conversion ratio				0.04
Common Stock | Asset Acquisition
Company and Basis of Presentation [Line Items]
Number of shares transferred as equity interest in asset acquisition (in shares) | shares						517,809
Common stock, par value (in dollars per share) | $ / shares						$ 0.0001
Common Stock | Private Placement
Company and Basis of Presentation [Line Items]
Number of shares of common stock sold (in shares) | shares			6,000,000

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	Mar. 31, 2024	Dec. 31, 2023
Financial Assets:
Total financial assets	$ 482,886	$ 336,953
Liabilities:
Total liabilities	47,149	42,700
U.S. government treasury securities
Financial Assets:
Total financial assets	85,045	32,843
U.S. government agency securities
Financial Assets:
Total financial assets	55,818	16,257
Commercial paper
Financial Assets:
Total financial assets	74,792	104,141
Corporate bonds
Financial Assets:
Total financial assets	41,434	33,064
Parapyre Option Obligation
Liabilities:
Total liabilities	5,449
CVR liability
Liabilities:
Total liabilities	41,700	42,700
Level 1
Financial Assets:
Total financial assets	310,842	183,491
Liabilities:
Total liabilities	0	0
Level 1 | U.S. government treasury securities
Financial Assets:
Total financial assets	85,045	32,843
Level 1 | U.S. government agency securities
Financial Assets:
Total financial assets	0	0
Level 1 | Commercial paper
Financial Assets:
Total financial assets	0	0
Level 1 | Corporate bonds
Financial Assets:
Total financial assets	0	0
Level 1 | Parapyre Option Obligation
Liabilities:
Total liabilities	0
Level 1 | CVR liability
Liabilities:
Total liabilities	0	0
Level 2
Financial Assets:
Total financial assets	172,044	153,462
Liabilities:
Total liabilities	5,449	0
Level 2 | U.S. government treasury securities
Financial Assets:
Total financial assets	0	0
Level 2 | U.S. government agency securities
Financial Assets:
Total financial assets	55,818	16,257
Level 2 | Commercial paper
Financial Assets:
Total financial assets	74,792	104,141
Level 2 | Corporate bonds
Financial Assets:
Total financial assets	41,434	33,064
Level 2 | Parapyre Option Obligation
Liabilities:
Total liabilities	5,449
Level 2 | CVR liability
Liabilities:
Total liabilities	0	0
Level 3
Financial Assets:
Total financial assets	0	0
Liabilities:
Total liabilities	41,700	42,700
Level 3 | U.S. government treasury securities
Financial Assets:
Total financial assets	0	0
Level 3 | U.S. government agency securities
Financial Assets:
Total financial assets	0	0
Level 3 | Commercial paper
Financial Assets:
Total financial assets	0	0
Level 3 | Corporate bonds
Financial Assets:
Total financial assets	0	0
Level 3 | Parapyre Option Obligation
Liabilities:
Total liabilities	0
Level 3 | CVR liability
Liabilities:
Total liabilities	41,700	42,700
Money market funds
Financial Assets:
Total financial assets	225,797	150,648
Money market funds | Level 1
Financial Assets:
Total financial assets	225,797	150,648
Money market funds | Level 2
Financial Assets:
Total financial assets	0	0
Money market funds | Level 3
Financial Assets:
Total financial assets	$ 0	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Jun. 22, 2023	Mar. 31, 2024	Dec. 31, 2023	May 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Exercise price per warrant (in dollars per share)				$ 0.0025
Parapyre Option Obligation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of annual equity grant of options		1%
CVR liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Changes in the fair value of the CVR liability		$ 430
Spyre Therapeutics, Inc.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset acquisition, stockholder payment period	3 years
Asset acquisition, cash payment, threshold period	1 year
Parapyre Warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants to purchase shares (in shares)			684,407
Exercise price per warrant (in dollars per share)			$ 21.52

Fair Value Measurements - Signi

Fair Value Measurements - Significant Inputs used to Estimate the Fair Value of Derivative Liability (Details) - Level 3	Mar. 31, 2024
Minimum | Estimated reimbursement rate compared to reimbursement agent | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	0.81
Minimum | Risk-adjusted discount rates | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	0.0632
Minimum | CVR liability | Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	0.39
Maximum | Estimated reimbursement rate compared to reimbursement agent | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	1
Maximum | Risk-adjusted discount rates | CVR liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	0.0665
Maximum | CVR liability | Estimated probability of success
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative liability, measurement input	1

Fair Value Measurements - Chang

Fair Value Measurements - Changes in Derivative Liabilities (Details) - CVR liability $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 42,700
Changes in the fair value of the CVR liability	430
Payments	(1,430)
Ending balance	$ 41,700

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	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-Sale [Line Items]
Cash equivalents, amortized cost	$ 225,797	$ 186,563
Cash equivalents, gross unrealized gains	0	6
Cash equivalents, gross unrealized losses	0	0
Cash equivalents, estimated fair value	225,797	186,569
Marketable securities, amortized cost	257,487	150,101
Marketable securities, gross unrealized gains	53	284
Marketable securities, gross unrealized losses	(451)	(1)
Marketable securities, estimated fair value	257,089	150,384
Money market funds
Debt Securities, Available-for-Sale [Line Items]
Cash equivalents, amortized cost	225,797	150,648
Cash equivalents, gross unrealized gains	0	0
Cash equivalents, gross unrealized losses	0	0
Cash equivalents, estimated fair value	225,797	150,648
Commercial paper
Debt Securities, Available-for-Sale [Line Items]
Cash equivalents, amortized cost		24,950
Cash equivalents, gross unrealized gains		5
Cash equivalents, gross unrealized losses		0
Cash equivalents, estimated fair value		24,955
Marketable securities, amortized cost	74,803	79,124
Marketable securities, gross unrealized gains	12	62
Marketable securities, gross unrealized losses	(23)	0
Marketable securities, estimated fair value	74,792	79,186
Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Marketable securities, amortized cost	41,497	32,984
Marketable securities, gross unrealized gains	11	81
Marketable securities, gross unrealized losses	(74)	(1)
Marketable securities, estimated fair value	41,434	33,064
U.S. government treasury securities
Debt Securities, Available-for-Sale [Line Items]
Cash equivalents, amortized cost		10,965
Cash equivalents, gross unrealized gains		1
Cash equivalents, gross unrealized losses		0
Cash equivalents, estimated fair value		10,966
Marketable securities, amortized cost	85,250	21,846
Marketable securities, gross unrealized gains	4	31
Marketable securities, gross unrealized losses	(209)	0
Marketable securities, estimated fair value	85,045	21,877
U.S. government agency securities
Debt Securities, Available-for-Sale [Line Items]
Marketable securities, amortized cost	55,937	16,147
Marketable securities, gross unrealized gains	26	110
Marketable securities, gross unrealized losses	(145)	0
Marketable securities, estimated fair value	$ 55,818	$ 16,257

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Available-for-Sale Securities in an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value
Less Than 12 Months	$ 183,213	$ 14,738
12 Months or Longer	0	0
Total, fair value	183,213	14,738
Unrealized Losses
Less Than 12 Months	(451)	(1)
12 Months or Longer	0	0
Total, unrealized losses	(451)	(1)
Commercial paper
Fair Value
Less Than 12 Months	30,027
12 Months or Longer	0
Total, fair value	30,027
Unrealized Losses
Less Than 12 Months	(23)
12 Months or Longer	0
Total, unrealized losses	(23)
Corporate bonds
Fair Value
Less Than 12 Months	30,737	9,907
12 Months or Longer	0	0
Total, fair value	30,737	9,907
Unrealized Losses
Less Than 12 Months	(74)	(1)
12 Months or Longer	0	0
Total, unrealized losses	(74)	(1)
U.S. government treasury securities
Fair Value
Less Than 12 Months	77,707	4,831
12 Months or Longer	0	0
Total, fair value	77,707	4,831
Unrealized Losses
Less Than 12 Months	(209)	0
12 Months or Longer	0	0
Total, unrealized losses	(209)	$ 0
U.S. government agency securities
Fair Value
Less Than 12 Months	44,742
12 Months or Longer	0
Total, fair value	44,742
Unrealized Losses
Less Than 12 Months	(145)
12 Months or Longer	0
Total, unrealized losses	$ (145)

Cash Equivalents and Marketab_5

Cash Equivalents and Marketable Securities - Narrative (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2024 USD ($) bank	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)
Cash Equivalents And Marketable Securities [Line Items]
Number of domestic banking institutions (in banks) | bank	2
US Government Agencies Debt Securities
Cash Equivalents And Marketable Securities [Line Items]
Debt securities, available-for-sale, allowance for credit loss	$ 0		$ 0
Impairment of marketable securities	0		0
Realized gains or losses on marketable securities	0	$ 0
Accrued interest receivable on available-for-sale debt securities	1,300,000		$ 900,000
U.S. Banking Institution | Maximum
Cash Equivalents And Marketable Securities [Line Items]
Cash, FDIC insured amount	$ 250,000

Cash Equivalents and Marketab_6

Cash Equivalents and Marketable Securities - Contractual Maturities of Marketable Securities at Estimated Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Cash and Cash Equivalents [Abstract]
Due in one year or less	$ 191,090	$ 115,784
Due in 1 - 2 years	65,999	34,600
Total marketable securities	$ 257,089	$ 150,384

Accrued and Other Current Lia_3

Accrued and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Payables and Accruals [Abstract]
Accrued compensation	$ 2,506	$ 4,054
Accrued contracted research and development costs	18,149	7,092
Accrued professional and consulting fees	720	1,474
Accrued other	219	488
Total accrued and other current liabilities	$ 21,594	$ 13,108

Related Party Transactions - Na

Related Party Transactions - Narrative (Details)				3 Months Ended
	Dec. 14, 2023 USD ($)	Nov. 22, 2023 $ / shares shares	Jul. 12, 2023 USD ($)	Mar. 31, 2024 USD ($) seat shares	Mar. 31, 2023 USD ($) shares	Dec. 31, 2023 USD ($)	Jul. 31, 2023 USD ($)
Related Party Transaction [Line Items]
Stock-based compensation				$ 13,835,000	$ 1,709,000
Related party accounts payable and other current liabilities				$ 15,528,000		$ 16,584,000
Options granted (in shares) | shares				1,044,658	177,620
Stock-based compensation expense				$ 13,835,000	$ 1,709,000
Paragon Agreement
Related Party Transaction [Line Items]
Number of board seats held by related party | seat				2
Related Party
Related Party Transaction [Line Items]
Related party accounts payable and other current liabilities				$ 15,500,000		16,600,000
Related Party | Paragon Agreement
Related Party Transaction [Line Items]
Amount of related party transaction				18,200,000
Stock-based compensation				5,400,000
Related party accounts payable and other current liabilities				$ 10,100,000		$ 16,600,000
Common stock, shares outstanding, percentage available to purchase on a diluted basis				1%
Related Party | Subsequent to Asset Acquisition
Related Party Transaction [Line Items]
Amount of related party transaction				$ 17,100,000
Related Party | SPY001 License Agreement
Related Party Transaction [Line Items]
Nomination fee			$ 1,500,000
First milestone payment			$ 2,500,000
Related Party | SPY002 License Agreement
Related Party Transaction [Line Items]
Nomination fee	$ 1,500,000
First milestone payment	$ 2,500,000
Sublicensing fee, maximum amount				20,000,000
Related Party | Consulting Agreement
Related Party Transaction [Line Items]
Options granted (in shares) | shares		477,000
Exercise price (in dollars per share) | $ / shares		$ 10.39
Vesting period		36 months
Stock-based compensation expense				$ 300,000	$ 0
Related Party | Consulting Agreement | Share-Based Payment Arrangement, Tranche One
Related Party Transaction [Line Items]
Vesting percentage		25%
Vesting period		1 year
Paragon Therapeutics Inc | Maximum | Related Party | Paragon Agreement
Related Party Transaction [Line Items]
Percentage of ownership by noncontrolling owner				5%
Paragon Therapeutics Inc | Maximum | Related Party | SPY001 License Agreement
Related Party Transaction [Line Items]
Contingent obligation based on milestones							$ 22,000,000
Fairmount Funds Management LLC | Minimum | Related Party | Paragon Agreement
Related Party Transaction [Line Items]
Percentage of ownership by noncontrolling owner				5%
Percentage of ownership held in third party				5%

Related Party Transactions - Ex

Related Party Transactions - Expenses related to Related Party which were Settled in Cash (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Paragon Agreement | Research and Development Expense
Related Party Transaction [Line Items]
Reimbursable costs under the Paragon Agreement	$ 11.7	$ 0

Related Party Transactions - Re

Related Party Transactions - Related Party Accounts Payable (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Related Party Transaction [Line Items]
Total related party accounts payable	$ 15,528	$ 16,584
Related Party
Related Party Transaction [Line Items]
Total related party accounts payable	15,500	16,600
Related Party | Paragon Agreement
Related Party Transaction [Line Items]
Total related party accounts payable	10,100	16,600
Related Party | Parapyre Option Obligation
Related Party Transaction [Line Items]
Total related party accounts payable	$ 5,400	$ 0

Convertible Preferred Stock a_3

Convertible Preferred Stock and Stockholders' Equity - Narrative (Details) $ / shares in Units, $ in Thousands							3 Months Ended	40 Months Ended
	Mar. 20, 2024 USD ($) shares	Mar. 18, 2024 shares	Dec. 11, 2023 USD ($) shares	Jul. 07, 2023 shares	Jun. 26, 2023 USD ($) shares	Jun. 22, 2023	Mar. 31, 2024 USD ($) $ / shares shares	May 31, 2022 $ / shares	Dec. 31, 2023 $ / shares shares	Mar. 31, 2023 shares	Dec. 31, 2022 shares
Class of Stock [Line Items]
Exercise price per warrant (in dollars per share) | $ / shares								$ 0.0025
Maximum ownership percentage of common stock shares for outstanding warrants to be exercised								4.99%
Ownership percentage for outstanding warrants to purchase shares of common stock to be exercised to certain holders								9.99%
Revised ownership percentage, period to take effect after notice								61 days
Preferred stock, outstanding (in shares)							0		0
Conversion Of Series A Non-Voting Convertible Preferred Stock | Common Stock
Class of Stock [Line Items]
Shares converted (in shares)							25,972,080
Private Placement
Class of Stock [Line Items]
Gross proceeds received in private placement | $			$ 180,000
Series A Non Voting Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock issued and outstanding percentage						30%
Conversion basis						40
Issuance of non-voting convertible preferred stock (in shares)					721,452
Issuance of Series B non-voting convertible preferred stock in connection with private placement, net of financing costs | $					$ 197,300
Preferred stock, outstanding (in shares)							437,037		437,037
Series A Non Voting Convertible Preferred Stock | Preferred Stock
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)							437,000		437,000	0	0
Series A Non Voting Convertible Preferred Stock | Conversion Of Series A Non-Voting Convertible Preferred Stock | Preferred Stock
Class of Stock [Line Items]
Shares converted (in shares)							649,302
Series A Non Voting Convertible Preferred Stock | Asset Acquisition
Class of Stock [Line Items]
Number of shares transferred as equity interest in asset acquisition (in shares)				364,887
Series A Non Voting Convertible Preferred Stock | Private Placement
Class of Stock [Line Items]
Gross proceeds received in private placement | $					$ 210,000
Series B Non Voting Convertible Preferred Stock
Class of Stock [Line Items]
Conversion basis			40
Issuance of non-voting convertible preferred stock (in shares)	121,625	271,625	150,000				122,000
Issuance of Series B non-voting convertible preferred stock in connection with private placement, net of financing costs | $							$ 168,850
Outstanding shares, redemption value | $							$ 412,100
Redemption price per share (in shares) | $ / shares							$ 37.93
Series B Non Voting Convertible Preferred Stock | Private Placement
Class of Stock [Line Items]
Gross proceeds received in private placement | $	$ 180,000		$ 90,000
Parapyre Warrants
Class of Stock [Line Items]
Exercise price per warrant (in dollars per share) | $ / shares									$ 21.52
Maximum ownership percentage of common stock shares for outstanding warrants to be exercised							4.99%
Warrants to purchase shares (in shares)									684,407
Maximum
Class of Stock [Line Items]
Maximum ownership percentage of common stock shares for outstanding warrants to be exercised								19.99%
Maximum | Series A Non Voting Convertible Preferred Stock
Class of Stock [Line Items]
Beneficially holders owned percentage						19.90%
Maximum | Series B Non Voting Convertible Preferred Stock
Class of Stock [Line Items]
Beneficial holders owned percentage							19.90%
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]
Beneficial holders owned percentage							0%

Convertible Preferred Stock a_4

Convertible Preferred Stock and Stockholders' Equity - Pre-Funded Warrants (Details) (Details) - $ / shares	Mar. 31, 2024	May 31, 2022
Class of Warrant or Right [Line Items]
Exercise price per warrant (in dollars per share)		$ 0.0025
May 20, 2022 | Pre-funded Warrants
Class of Warrant or Right [Line Items]
Exercise price per warrant (in dollars per share)	$ 0.0025
Number of warrants outstanding (in shares)	250,000

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	Jun. 22, 2023	Feb. 28, 2018	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2016	Jan. 01, 2023	Jan. 01, 2022
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation			$ 13,835	$ 1,709
Two Thousand And Fifteen Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares subject to options outstanding (in shares)			3,029
2016 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual percentage of additional shares					5%
Additional number of shares available for issuance (in shares)						3,023,650	104,561
Common stock available for future issuance (in shares)			7,393,885
Outstanding option awards (in shares)			2,996,404
Two Thousand And Fifteen And Two Thousand And Sixteen Equity Incentive Plan And Two Thousand And Eighteen Equity Inducement Plan | Service Based Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vest period grant date		4 years
Share-based payment award, term		10 years
2018 Equity Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for future issuance (in shares)			6,029,000
Outstanding option awards (in shares)			5,384,241
Spyre 2023 Equity Incentive Plan | Asset Acquisition
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of outstanding and unexercised stock options to purchase (in shares)	2,734
Parapyre Option Obligation
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Pro-rated estimated fair value of options			$ 21,900
Stock-based compensation			5,400	$ 0
Unamortized expense			$ 16,500
2016 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares issued (in shares)			2,330	1,793

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock Awards Granted (Details) - $ / shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-Based Payment Arrangement [Abstract]
Grants (in shares)	1,044,658	177,620
Weighted Average Grant Date Fair Value (in dollars per share)	$ 26.50	$ 11

Stock-Based Compensation - St_2

Stock-Based Compensation - Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 13,835	$ 1,709
Stock-based compensation	13,835	1,709
Aeglea Employees and Directors
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation	2,900	500
Parapyre Option Obligation
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Stock-based compensation	5,400	0
Research and Development Expense
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	6,857	777
General and administrative
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 6,978	$ 932

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions used to Estimate the Fair Value of Stock Options Granted (Details)	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Stock Options Granted
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Expected term (in years)	6 years 10 days	6 years 7 days
Expected volatility	105%	99%
Risk-free interest	3.88%	4.06%
Dividend yield	0%	0%
2016 ESPP
Employee Service Share Based Compensation Allocation of Recognized Period Costs [Line Items]
Expected term (in years)	6 months	5 months 26 days
Expected volatility	98%	181%
Risk-free interest	5.31%	4.99%
Dividend yield	0%	0%

Strategic License Agreements -

Strategic License Agreements - Narrative (Details) - USD ($)		3 Months Ended
	Jul. 27, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenue		$ 0	$ 198,000
Contract assets		0		$ 0
Contract liabilities		0		$ 0
Immedica Pharma AB
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenue		$ 0
Immedica Pharma AB | Peace Phase 3 Trial and Drug Supply
Research And Development Arrangement Contract To Perform For Others [Line Items]
Total revenue			$ 200,000
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	$ 15,000,000
Contingent consideration	$ 100,000,000

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	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	3,200,918	459,425
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)	61,253	766
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)	684,407	0

Net Loss Per Share - Reconcilia

Net Loss Per Share - Reconciliation of Basic and Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Earnings Per Share [Abstract]
Weighted average common shares (in shares)	36,262,662	2,614,843
Weighted average pre-funded warrants (in shares)	250,000	1,155,663
Total basic weighed average shares (in shares)	36,512,662	3,770,506
Total diluted weighed average shares (in shares)	36,512,662	3,770,506

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Events	Apr. 23, 2024 shares
Preferred Stock
Subsequent Event [Line Items]
Preferred stock, shares exchanged for common stock (in shares)	90,992
Common Stock
Subsequent Event [Line Items]
Common stock, shares received in exchange (in shares)	3,639,680