Acelyrin (SLRN) 10-Q 14 May 24 2024 Q1 Quarterly report Financial data

Cover

Cover - shares	3 Months Ended
	Mar. 31, 2024	May 08, 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	001-41696
Entity Registrant Name	ACELYRIN, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	85-2406735
Entity Address, Address Line One	4149 Liberty Canyon Road
Entity Address, City or Town	Agoura Hills
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	91301
City Area Code	805
Local Phone Number	730-0360
Title of 12(b) Security	Common Stock ($0.00001 par value)
Trading Symbol	SLRN
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		99,026,983
Entity Central Index Key	0001962918
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Current Fiscal Year End Date	--12-31
Amendment Flag	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Current assets
Cash and cash equivalents	$ 264,501	$ 218,097
Short-term marketable securities	414,037	503,229
Prepaid expenses and other current assets	15,443	15,312
Total current assets	693,981	736,638
Prepaid expenses and other assets, non-current	175	2,678
Operating lease right-of-use asset	1,149	1,195
Property, plant and equipment, net	2,179	2,179
Total assets	697,484	742,690
Current liabilities
Accounts payable	9,994	41,920
Accrued research and development expenses	37,529	35,436
Accrued compensation and other current liabilities	4,182	6,833
Severance liability	578	970
Total current liabilities	52,283	85,159
Operating lease liability, non-current	1,134	1,194
Total liabilities	53,417	86,353
Stockholders’ equity
Common stock, par value of $0.00001 per share; $790,000,000 shares authorized as of March 31, 2024 and December 31, 2023; $98,859,000 and $97,865,890 shares issued and outstanding as of March 31, 2024 and December 31, 2023, respectively	1	1
Additional paid-in capital	1,167,863	1,144,893
Accumulated other comprehensive income (loss)	(105)	162
Accumulated deficit	(523,692)	(488,719)
Total stockholders' equity	644,067	656,337
Total liabilities, redeemable convertible preferred stock and stockholders’ equity	$ 697,484	$ 742,690

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
Common stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Common stock, authorized (in shares)	790,000,000	790,000,000
Common stock, issued (in shares)	98,859,000	97,865,890
Common stock, outstanding (in shares)	98,859,000	97,865,890

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Operating expenses:
Research and development	$ 58,032	$ 167,920
General and administrative	24,742	11,913
Total operating expenses	82,774	179,833
Loss from operations	(82,774)	(179,833)
Change in fair value of derivative tranche liability	0	147
Interest income	9,150	3,299
Other income (expense), net	38,651	(63)
Net loss	(34,973)	(176,450)
Other comprehensive gain (loss)
Unrealized gain (loss) on short-term marketable securities, net	(267)	86
Total other comprehensive gain (loss)	(267)	86
Net loss and other comprehensive loss	$ (35,240)	$ (176,364)
Net loss per share attributable to common stockholder, basic (in shares)	$ (0.36)	$ (8.61)
Net loss per share attributable to common stockholder, diluted (in shares)	$ (0.36)	$ (8.61)
Weighted-average common shares outstanding, basic (in shares)	97,913,660	20,492,101
Weighted-average common shares outstanding, diluted (in shares)	97,913,660	20,492,101

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2022	40,743,552
Beginning balance at Dec. 31, 2022	$ 396,593
Ending balance (in shares) at Mar. 31, 2023	40,743,552
Ending balance at Mar. 31, 2023	$ 396,593
Beginning balance (in shares) at Dec. 31, 2022		2,767,359
Beginning balance at Dec. 31, 2022	(102,862)	$ 0	$ 4,302	$ (107,078)	$ (86)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with ValenzaBio acquisition (in shares)		18,885,731
Issuance of common stock in connection with ValenzaBio acquisition	128,735		128,735
Stock-based compensation expense	7,139		7,139
Net loss	(176,450)			(176,450)
Unrealized gain on short-term marketable securities, net	86				86
Ending balance (in shares) at Mar. 31, 2023		21,653,090
Ending balance at Mar. 31, 2023	$ (143,352)	$ 0	140,176	(283,528)	0
Beginning balance (in shares) at Dec. 31, 2023	97,865,890	97,865,890
Beginning balance at Dec. 31, 2023	$ 656,337	$ 1	1,144,893	(488,719)	162
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon settlement of restricted stock units (in shares)		47,639
Stock-based compensation expense	$ 20,163		20,163
Issuance of common stock upon exercise of options (in shares)	153,916	945,471
Issuance of common stock upon exercise of options	$ 2,807		2,807
Net loss	(34,973)			(34,973)
Unrealized gain on short-term marketable securities, net	$ (267)				(267)
Ending balance (in shares) at Mar. 31, 2024	98,859,000	98,859,000
Ending balance at Mar. 31, 2024	$ 644,067	$ 1	$ 1,167,863	$ (523,692)	$ (105)

Condensed Consolidated Statem_3

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	$ (34,973)	$ (176,450)
Adjustments to reconcile net loss to net cash used in operations:
Stock-based compensation expense	20,163	7,139
Income from sale of asset	(7,000)	0
Expense related to acquired in-process research and development assets	0	133,057
Net amortization of premiums and accretion of discounts on marketable securities	(5,362)	(177)
Change in fair value of derivative tranche liability	0	(147)
Depreciation and amortization expense	55	0
Non-cash lease expense	46	16
Changes in assets and liabilities:
Prepaid expense and other current assets	205	825
Prepaid expenses and other assets, non-current	2,503	2,196
Accounts payable	(31,926)	2,990
Accrued research and development expenses	2,093	3,565
Accrued compensation and other current liabilities	(2,651)	(1,778)
Operating lease liability	(60)	0
Severance liability	(392)	3,435
Net cash used in operating activities	(57,299)	(25,329)
Cash flows from investing activities
Proceeds from sale of asset	7,000	0
ValenzaBio assets acquisition cash acquired, net of acquisition costs	0	10,007
Cash paid to acquire in-process research and development assets	0	(10,000)
Purchase of marketable securities	(198,007)	0
Proceeds from maturities of short-term marketable securities	289,567	47,773
Sales of marketable securities	2,391
Purchase of property, plant and equipment	(55)	(238)
Net cash provided by investing activities	100,896	47,542
Cash flows from financing activities
Proceeds from exercise of common stock options	2,807	0
Payments of initial public offering costs	0	(129)
Net cash (used in) provided by financing activities	2,807	(129)
Net increase in cash and cash equivalents	46,404	22,084
Cash and cash equivalents at beginning of period	218,097	267,110
Cash and cash equivalent at end of period	264,501	289,194
Supplemental disclosure of cash flow information:
Initial public offering costs included in accrued compensation and other current liabilities and accounts payable	0	2,180
Common stock issued in connection with ValenzaBio acquisition	0	128,735
Right-of-use assets obtained in exchange for operating lease liability	$ 0	$ 1,348

Description of Business, Organi

Description of Business, Organization and Liquidity	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business, Organization and Liquidity	Description of Business, Organization and Liquidity Organization and Business ACELYRIN, INC. (the “Company”) is a late-stage clinical biopharma company focused on identifying, acquiring, and accelerating the development and commercialization of transformative medicines. The Company was incorporated in the State of Delaware on July 27, 2020. Since its inception, the Company has devoted substantially all of its resources to organizing the Company, hiring personnel, business planning, acquiring and developing its product candidates, performing research and development, enabling manufacturing activities in support of its product development efforts, establishing and protecting its intellectual property portfolio, raising capital, and providing general and administrative support for these activities. The Company did not have any significant operations from the inception date until August 2021. On August 9, 2021, the Company entered into the License and Collaboration Agreement with Affibody AB, a Swedish company, and licensed worldwide development, manufacturing and commercialization rights to a therapeutic candidate, izokibep, for use in the treatment of inflammatory and autoimmune disorders, excluding rights in certain Asian and Nordic countries. See Note 7 for further details. On January 4, 2023, the Company closed the acquisition of ValenzaBio, Inc. (“ValenzaBio”) and issued as consideration 18,885,731 shares of its Class A common stock (“Class A Common Stock”). ValenzaBio was a privately held company developing therapies for autoimmune and inflammatory diseases. The ValenzaBio acquisition added additional assets to the Company’s portfolio, including lonigutamab and SLRN-517. See Note 3 for further details. Reverse Stock Split In April 2023, the Company effected a reverse split of shares of the Company’s outstanding common stock and redeemable convertible preferred stock at a ratio 1.972-for-1 (the “Reverse Stock Split”). The number of authorized shares and par value per share were not adjusted as a result of the Reverse Stock Split. All references to shares, restricted stock units (“RSUs”) and restricted stock awards (“RSAs”), options to purchase common stock, share data, per share data, and related information contained in the condensed consolidated financial statements have been retrospectively adjusted to reflect the effect of the Reverse Stock Split for all periods presented. Initial Public Offering On May 4, 2023, the Company’s Form S-1 Registration Statement for its initial public offering (the “IPO”) was declared effective, and on May 9, 2023, the Company closed its IPO and issued 34,500,000 shares of common stock at a price to the public of $18.00 per share, including 4,500,000 shares issued upon the exercise of underwriters’ option to purchase additional shares of common stock. The Company received gross proceeds of $621.0 million. Net proceeds were approximately $573.6 million, after deducting underwriting discounts and commissions and offering costs of $47.4 million. The common stock began trading on the Nasdaq Global Select Market on May 5, 2023, under the symbol “SLRN”. Immediately prior to the IPO closing, each share of the Company’s redeemable convertible preferred stock then outstanding converted into an equivalent number of shares of Class A Common Stock, and thereafter each share of Class A Common Stock then issued and outstanding was reclassified and became one share of the Company’s common stock. Liquidity The Company has incurred significant losses and negative cash flows from operations since its inception. During the three months ended March 31, 2024 and 2023, the Company incurred net losses of $35.0 million and $176.5 million, respectively. The net loss of $35.0 million in the three months ended March 31, 2024 includes $37.0 million of other income related to payments in the quarter. The net loss of $176.5 million in the three months ended March 31, 2023 includes $123.1 million of expenses related to acquired in-process research and development assets without alternative future use and $10.0 million license fee payment to Pierre Fabre incurred in connection with the ValenzaBio acquisition. As of March 31, 2024, the Company had an accumulated deficit of $523.7 million. Cash used in operating activities was $57.3 million and $25.3 million for the three months ended March 31, 2024 and 2023, respectively. The Company has historically financed its operations primarily through the sale of shares of its redeemable convertible preferred stock in private placements and the sale of shares of its common stock in its IPO. As of March 31, 2024, the Company had cash and cash equivalents and short-term marketable securities of $678.5 million. The Company does not have any products approved for sale and has not generated any revenue from product sales to date. The Company expects to continue to incur significant and increasing expenses and substantial losses for the foreseeable future as it continues its development of and seeks regulatory approvals for its product candidates and commercializes any approved products, seeks to expand its product pipeline and invests in its organization. The Company’s ability to achieve and sustain profitability will depend on its ability to successfully develop, obtain regulatory approval for and commercialize its product candidates. There can be no assurance that the Company will ever earn revenue or achieve profitability, or if achieved, that the revenue or profitability will be sustained on a continuing basis. Unless and until it does, the Company will need to continue to raise additional capital. Based on its current operating plan, management estimates that its existing cash and cash equivalents, including the proceeds from the IPO, will be sufficient to fund its operating plan and capital expenditure requirements for at least the next 12 months from the date of issuance of these condensed consolidated financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies The accounting policies used by the Company in its presentation of interim financial results are consistent with those presented in Note 2 to the consolidated financial statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2023, filed with the Securities and Exchange Commission (“SEC”) on March 28, 2024, (the “Annual Report on Form 10-K”) except for the updates to the following: Basis of Presentation The condensed consolidated financial statements and accompanying notes are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the SEC regarding interim financial reporting. The accompanying financial statements are consolidated and include the accounts of ACELRYIN, INC. and its wholly owned subsidiary, WH2, LLC. The subsidiary has not had any operations or any balances from its inception. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2023 included in the Company’s Annual Report. The information as of December 31, 2023 included in the condensed consolidated balance sheets was derived from the Company’s audited consolidated financial statements. These unaudited interim condensed consolidated financial statements have been prepared on the same basis as the Company’s annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments necessary for a fair statement of the Company’s consolidated financial statements. The results of operations for the three months ended March 31, 2024 are not necessarily indicative of the results that may be expected for the year ending December 31, 2024 or for any other interim period or for any other future year. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of expenses during the reporting period. On an ongoing basis, the Company evaluates estimates and assumptions, including but not limited to those related to the fair value of its derivative tranche liability, the fair value of its common stock, stock-based compensation expense, accruals for research and development expenses, valuation of deferred tax assets, and uncertain income tax positions. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results may differ materially from those estimates. Concentration of Credit Risk Cash and cash equivalents, and short-term marketable securities are financial instruments that potentially subject the Company to concentrations of credit risk. As of March 31, 2024 and December 31, 2023, cash consists of cash deposited with one financial institution, and account balances exceed federally insured limits. Management believes that the Company is not exposed to significant credit risk due to the financial strength of this institution. The Company also has investments in money market funds, U.S. Treasury obligations, corporate debt obligations, and federal agency obligations, which can be subject to certain credit risks. The Company mitigates the risks by investing in high-grade instruments, limiting its exposure to any one issuer and monitoring the ongoing creditworthiness of the financial institutions and issuers. The Company has not experienced any losses on its financial instruments.

ValenzaBio Acquisition

ValenzaBio Acquisition	3 Months Ended
	Mar. 31, 2024
Business Combination and Asset Acquisition [Abstract]
ValenzaBio Acquisition	ValenzaBio Acquisition On December 20, 2022, the Company entered into the Agreement and the Plan of Merger and Reorganization (the “Merger Agreement”) to acquire ValenzaBio. In connection with the planned ValenzaBio acquisition, the Company formed two wholly owned subsidiaries, WH1, Inc. and WH2 LLC in November 2022. Through the two-step merger and restructuring, WH1 Inc. was merged with and into ValenzaBio with WH1 Inc. ceasing to exist, and ValenzaBio was then merged with and into WH2 LLC, with WH2 LLC continuing as the legal successor to ValenzaBio. (the “Acquisition”). The Acquisition closed on January 4, 2023 (the “Closing Date”). The Company concluded that the Acquisition is an asset acquisition as substantially all of the fair value of the gross assets acquired, excluding cash, was concentrated in a single asset, lonigutamab, and the Company did not acquire a workforce or any substantive process capable of significantly contributing to the ability to create outputs. As consideration, the Company issued 18,885,731 shares of its Class A Common Stock to ValenzaBio stockholders, of which 2,013,673 were being held by Seller LLC for any post-acquisition costs and general indemnities for 12 months from the Closing Date (“Holdback Release Date”), and paid $7,663 in cash to one non-accredited investor. Additionally, $0.1 million is payable in cash to Seller LLC to cover Seller LLC’s fees and expenses related to the Acquisition, with any unused amount to be released to ValenzaBio stockholders as soon as practicable following the completion of Seller LLC’s responsibilities. The Company also incurred $1.2 million of acquisition-related costs that were included in the total consideration and capitalized to assets acquired. The Company assumed options of certain ValenzaBio option holders who entered into consulting agreements with the Company, which became options for the purchase of an aggregate 1,249,811 shares of the Company’s Class A Common Stock upon the closing of the Acquisition on January 4, 2023. The assumed options vested in full on March 31, 2023. Each assumed option is exercisable until the earlier of (i) 12 months following the termination of the option holder’s continuous service with the Company, or (ii) the original expiration date of such assumed option. Outstanding ValenzaBio shares were exchanged into shares of the Company’s Class A Common Stock and the options described above assumed at an exchange ratio of 0.8027010-for-one. The following table represents the total purchase consideration (in thousands): Issued Class A Common Stock (1) $ 128,735 Transaction costs (2) 1,271 Cash (3) 8 Total $ 130,014 (1) Shares were issued for consideration at $6.86 per share, including 2,013,673 shares that were being held by Seller LLC until the Holdback Release Date. The Company used a third party valuation specialist to assist management in determining the fair value of the shares of Class A Common Stock at the Closing Date. (2) Legal and advisory transaction costs of $1.3 million incurred by the Company in connection with the Acquisition, including $0.1 million payable in cash to Seller LLC for the expense fund. (3) Cash payment of $7,663 to one non-accredited investor for settlement of vested ValenzaBio options. The following is the allocation of the purchase consideration to the acquired assets and liabilities (in thousands): Cash $ 11,369 Prepaid expenses and other current assets 2,074 In-process research and development assets 123,057 Accounts payable (1,628) Accrued research and development expenses (4,805) Accrued compensation and other current liabilities (53) Total net asset acquired $ 130,014 In-process research and development (“IPR&D) assets were related to acquired product candidates: lonigutamab in clinical trials and SLRN-517 in preclinical development. The fair value of in-process research and development assets was based on the present value of future discounted cash flows, which was based on significant estimates. These estimates included the number of potential patients and market prices of future product candidates, costs required to conduct clinical trials, future milestones and royalties payable under acquired license agreements, costs to receive regulatory approval and potentially commercialize product candidates, as well as estimates for probability of success and the discount rate. The estimated fair values of lonigutamab and SLRN-517 assets were $114.8 million and $8.2 million, respectively. The Company concluded that acquired assets do not have an alternative future use and recognized the full amount of $123.1 million as research and development expenses in the condensed consolidated statement of operations and comprehensive loss in January 2023. There are a number of additional obligations under the Merger Agreement that are separate from the assets and liabilities acquired, including the following: Assumed options . The assumed options, discussed above, did not have substantive service requirement, and were accounted as a separate transaction from the Acquisition. The fair values of assumed options of $3.1 million and $1.8 million was expensed as research and development and general and administrative expenses, respectively, in the condensed consolidated statement of operations and comprehensive loss for the three months ended March 31, 2023. Settled equity awards . In accordance with the severance obligations of ValenzaBio and per the terms of the Merger Agreement, certain unvested options and restricted stock awards of former ValenzaBio employees, who did not enter into consulting agreements with the Company, were accelerated and net exercised upon the closing of the Acquisition and termination of employment of such ValenzaBio employees. The fair value of unvested equity awards of $0.9 million was expensed as general and administrative expense in the condensed consolidated statement of operations and comprehensive loss for the three months ended March 31, 2023. Payments in cash to one non-accredited investor for settlement of unvested ValenzaBio options and one former ValenzaBio employee to whom options were promised but not granted at the Closing Date of $8,387 and $30,000, respectively, were expensed as general and administrative expenses in the condensed consolidated statement of operations and comprehensive loss for the three months ended March 31, 2023. Severance payment obligation . In accordance with the severance plan of ValenzaBio, the Company is obligated to make severance payments to certain former ValenzaBio employees of approximately $5.1 million, including estimated taxes, for a period of three As of March 31, 2024, and December 31, 2023, severance payments obligations were $0.2 and $0.3 million, respectively, included in the condensed consolidated balance sheets. Amendment to Pierre Fabre Agreement. The Company, ValenzaBio and Pierre Fabre Medicament SAS (“Pierre Fabre”) entered into an amendment to the license and commercialization agreement, which became effective on the

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1 — Quoted prices in active markets for identical assets or liabilities. Level 2 — Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. The Company’s financial instruments measured at fair value on a recurring basis consist of Level 1, Level 2, and Level 3 financial instruments. Usually, short term marketable securities are considered Level 2 when their fair values are determined using inputs that are observable in the market or can be derived principally from or corroborated by observable market data such as pricing for similar securities, recently executed transactions, cash flow models with yield curves, and benchmark securities. In addition, Level 2 financial instruments are valued using comparisons to like-kind financial instruments and models that use readily observable market data as their basis. Corporate debt obligations, commercial paper, government agency obligations and asset-backed securities are valued primarily using market prices of comparable securities, bid/ask quotes, interest rate yields and prepayment spreads and are included in Level 2. Financial assets and liabilities are considered Level 3 when their fair values are determined using pricing models, discounted cash flow methodologies, or similar techniques, and at least one significant model assumption or input is unobservable. The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): Fair Value Measurements as of March 31, 2024 As of March 31, 2024: Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash and cash equivalents) $ 88,994 $ 88,994 $ - $ - U.S. Treasury obligations ($116,279 included in cash and cash equivalents) 398,486 - 398,486 - Corporate debt obligations ($31,015 included in cash and cash equivalents) 148,866 - 148,866 - Federal agency obligations ($4,973 included in cash and cash equivalents) 18,952 - 18,952 - Total fair value of assets $ 655,298 $ 88,994 $ 566,304 $ - Fair Value Measurements as of December 31, 2023 As of December 31, 2023: Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash and cash equivalents) $ 23,205 $ 23,205 $ - $ - U.S. Treasury obligations ($146,497 included in cash and cash equivalents) 525,353 - 525,353 - Corporate debt obligations ($23,313 included in cash and cash equivalents) 135,284 - 135,284 - Federal agency obligations ($15,344 included in cash and cash equivalents) 27,746 - 27,746 - Total fair value of assets $ 711,588 $ 23,205 $ 688,383 $ - Classified as: March 31, 2024 December 31, 2023 Cash and cash equivalents $ 241,261 $ 208,359 Short-term marketable securities $ 414,037 $ 503,229 Total cash equivalents and marketable securities $ 655,298 $ 711,588

Available-For-Sale Marketable S

Available-For-Sale Marketable Securities	3 Months Ended
	Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]
Available-For-Sale Marketable Securities	Available-For-Sale Marketable Securities The following tables summarize the estimated fair value of the Company’s available-for-sale marketable securities as of March 31, 2024 and December 31, 2023 (in thousands): As of March 31, 2024: Total Total Unrealized Gain Total Total Money market funds (included in cash and cash equivalents) $ 88,994 $ — $ - $ 88,994 U.S. Treasury obligations ($116,279 included in cash and cash equivalents) 398,532 7 (53) 398,486 Corporate debt obligations ($31,015 included in cash and cash equivalents) 148,913 14 (61) 148,866 Federal agency obligations ($4,973 included in cash and cash equivalents) 18,963 — (11) 18,952 Total available for sale marketable securities $ 655,402 $ 21 $ (125) $ 655,298 As of December 31, 2023: Total Total Unrealized Gain Total Unrealized Loss Total Money market funds (included in cash and cash equivalents) $ 23,205 $ — $ - $ 23,205 U.S. Treasury obligations ($146,497 included in cash and cash equivalents) 525,198 156 (1) 525,353 Corporate debt obligations ($23,313 included in cash and cash equivalents) 135,288 36 (40) 135,284 Federal agency obligations ($15,344 included in cash and cash equivalents) 27,735 12 (1) 27,746 Total available for sale marketable securities $ 711,426 $ 204 $ (42) $ 711,588 As of March 31, 2024 and December 31, 2023, no significant facts or circumstances were present to indicate a deterioration in the creditworthiness of the issuers of the marketable securities, and the Company has no requirement or intention to sell these securities before maturity or recovery of their amortized cost basis. The Company considered the current and expected future economic and market conditions and determined that its investments were not significantly impacted. For all securities with a fair value less than its amortized cost basis, the Company determined the decline in fair value below amortized cost basis to be immaterial and non-credit related, and therefore no allowance for losses has been recorded. During the three months ended March 31, 2024 and for the year ended December 31, 2023, the Company did not recognize any impairment losses on its investments. The Company presents accrued interest receivable related to the available-for-sale marketable securities in prepaid expenses and other current assets As of March 31, 2024, all available for sale marketable securities mature within one year.

Consolidated Balance Sheet Comp

Consolidated Balance Sheet Components	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Consolidated Balance Sheet Components	Consolidated Balance Sheet Components Prepaid expenses and other current assets Prepaid expenses and other current assets consist of the following (in thousands): March 31, December 31, Prepaid research and development expenses $ 7,247 $ 8,184 Value-Added Tax (VAT) receivable 4,166 3,985 Prepaid insurance and other current assets 1,423 1,712 Interest receivable 1,189 764 Prepaid other services 1,418 667 Total $ 15,443 $ 15,312 Prepaid expenses and other assets, non-current Other non-current assets consist of the following (in thousands): March 31, December 31, Prepaid research and development expenses, non-current $ 141 $ 2,644 Security deposits 34 34 Total $ 175 $ 2,678 Property, plant and equipment, net Property, plant and equipment consisted of the following (in thousands): March 31, 2024 December 31, 2023 Construction in progress $ 1,452 $ 1,460 Computer and other equipment 407 407 Furniture and fixtures 357 306 Leasehold improvements 133 121 Total property, plant and equipment, gross 2,349 2,294 Less: accumulated depreciation and amortization (170) (115) Property, plant and equipment, net $ 2,179 $ 2,179 Accrued research and development expenses Accrued research and development expenses are comprised of the following (in thousands): March 31, 2024 December 31, 2023 Accrued clinical manufacturing expenses $ 26,969 $ 22,232 Accrued clinical expenses 10,560 13,204 Total $ 37,529 $ 35,436 Accrued compensation and other current liabilities Accrued compensation and other current liabilities consist of the following (in thousands): March 31, December 31, Accrued compensation $ 2,643 $ 5,417 Accrued professional service fees 1,020 1,099 Other accrued expenses and current liabilities 519 317 Total $ 4,182 $ 6,833

Significant Agreements

Significant Agreements	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Significant Agreements	Significant Agreements Affibody License and Collaboration Agreement On August 9, 2021, the Company entered into a license agreement with Affibody AB (“Affibody”) (the “Affibody Agreement”) under which Affibody granted the Company exclusive, sublicensable licenses to develop, commercialize and manufacture products containing izokibep for all human therapeutic uses on a worldwide basis, subject to a pre-existing agreement with Inmagene Biopharmaceuticals (“Inmagene”) with respect to certain Asian countries. The Company chairs a global joint steering committee composed of designees from Affibody, Inmagene and the Company and retains final decision-making authority for izokibep global development. In doing so, the Company is obligated to use commercially reasonable efforts (i) to develop products containing izokibep worldwide, excluding certain defined territories, (ii) for the conduct and finalization of certain ongoing clinical trials, and (iii) to commercialize products containing izokibep for all human therapeutic uses worldwide, excluding certain defined territories, after obtaining the applicable marketing authorization. The Company is responsible for manufacturing both the clinical and commercial supply of licensed product globally. In connection with the Affibody Agreement, the Company paid a non-refundable upfront license fee in the aggregate amount of $3.0 million in August 2021 and September 2021, and $22.0 million in October 2021. The Company is also obligated to pay Affibody (i) an aggregate of up to $280.0 million, $30.0 million of which would be due prior to the first approval in the United States, upon the achievement of various development, regulatory and commercialization milestones and (ii) high single-digit to low-teens royalties on net sales of licensed products in the territory where the Company has commercialization rights, subject to certain reductions. Royalties will be due on a licensed product-by-licensed product and country-by-country basis beginning after the first commercial sale of the licensed product, except in Mainland China, Hong Kong, Macau, Taiwan and South Korea, and lasting until the later of (a) the expiration of all valid patent claims or regulatory exclusivity covering the licensed product in that country and (b) ten years after such first commercial sale. In the event the U.S. Food and Drug Administration (“FDA”) grants the Company (or its affiliates or sublicensees) a priority review voucher for a licensed product, the Company will pay Affibody either: (a) if the Company sells or transfer such priority review voucher to a third-party, approximately one third of the proceeds received from the sale, net of taxes, or (b) if the Company uses the priority review voucher for an indication or product outside the scope of the Affibody Agreement, approximately one third of the fair market value of the priority review voucher as determined in accordance with the Affibody Agreement. Unless earlier terminated, the Affibody Agreement will continue on a licensed product-by-licensed product basis and country-by-country basis until there are no more royalty payments owed to Affibody on any licensed product thereunder. The acquisition of the exclusive license was accounted for as an in-process research and development asset acquisition and as the acquired technology did not have an alternative use, the total consideration of $25.0 million was recorded as research and development expense in the consolidated statements of operations and comprehensive loss for the year ended December 31, 2021. Milestone payments are contingent consideration and are accrued when contingent events occur and achievement of milestones is probable. In November 2023, the Company paid a total amount of $15.0 million in relation with attaining one of the development milestones described above and recorded the payment within research and development expenses in the consolidated statement of operations and comprehensive loss for the year ended December 31, 2023. Royalties will be recognized as cost of sales when products are sold and royalties are payable. No royalties or additional milestones were probable and estimable as of March 31, 2024 and December 31, 2023. Pierre Fabre License and Commercialization Agreement Upon the closing of the Acquisition, the Company became the successor to ValenzaBio’s rights under the March 25, 2021 license and commercialization agreement between ValenzaBio and Pierre Fabre, as amended (the “Pierre Fabre Agreement”). The Company received certain exclusive worldwide licenses with the right to sublicense to certain patents, know-how and other intellectual property to develop, manufacture, use and commercialize lonigutamab for non-oncology therapeutic indications. The license from Pierre Fabre extends to any product containing lonigutamab (excluding any fragments or derivatives) as its sole active ingredient (each, a “PF Licensed Product”). The Pierre Fabre Agreement prohibits the Company from using the licensed intellectual property in any antibody drug conjugate, multi-specific antibodies or any other derivatives of lonigutamab. In the event the Company decides to sublicense the rights to develop or commercialize a PF Licensed Product in any territory outside of the United States and Canada, Pierre Fabre retains the right of first negotiation to acquire such development and commercialization rights in one or more countries in such territory. Subject to the validation of certain clinical trial criteria by a joint steering committee, Pierre Fabre has the option to reclaim all exclusive rights to develop, commercialize and exploit the PF Licensed Product in such territories and to obtain an exclusive sublicensable license in such territories for any improvements and trademarks to such PF Licensed Product, and to exploit such PF Licensed Product for non-oncology therapeutic indications, subject to certain payment obligations. If Pierre Fabre exercises such option, and intends to sublicense such rights, then the Company has the right of first negotiation to acquire such development and commercialization rights as to that territory, or Pierre Fabre has the right to require the Company to buy out its right to the option for a one-time payment of $31.0 million or the Company has the right to choose to buy out Pierre Fabre’s option by making the one-time payment of $31.0 million within 30 days from Pierre Fabre’s notice of exercise of such option. If Pierre Fabre does not exercise its option within the option period or if the Company buys out Pierre Fabre’s right to the option, the option will expire or terminate, respectively. The Company is solely responsible for the development, regulatory approvals and commercialization of each PF Licensed Product except to the extent that Pierre Fabre reclaims rights to a PF Licensed Product in the option territory. As consideration for the amendment to the Pierre Fabre Agreement, which became effective upon the closing of the Acquisition (see Note 3), the Company paid Pierre Fabre an aggregate license payment of $10.0 million. The Company is also obligated to (i) make payments of up to $99.5 million upon the achievement of various development and regulatory milestones, (ii) make milestone payments of up to $390.0 million upon the achievement of certain commercial milestones, and (iii) pay tiered royalties in the high single-digit to low-teen percentages to Pierre Fabre on worldwide net sales in a given calendar year. Royalties will be payable for each PF Licensed Product in a given country during a period commencing upon the first commercial sale of such PF Licensed Product in such country and continuing until the latest of (a) 10 years after such first commercial sale, (b) expiration of last-to-expire valid claim in a licensed patent in such country and (c) expiration of regulatory exclusivity for such PF Licensed Product in such country. In the event the Company enters into a sublicense with a third party, the Company must also share with Pierre Fabre a percentage of any revenues from option fees, upfront payments, license maintenance fees, milestone payments or the like generated from the sublicense. Such percentage may be between the high single-digits to the low thirties based on which stage of development of a PF Licensed Product the sublicense relates to. Unless earlier terminated, the Pierre Fabre Agreement will continue on a PF Licensed Product-by-PF Licensed Product and country-by-country basis until there are no more royalty payments owed to Pierre Fabre on any PF Licensed Product thereunder. Either party may terminate the Pierre Fabre Agreement upon an uncured material breach, or upon the bankruptcy or insolvency of the other party. Pierre Fabre may also terminate the agreement if the Company or any of its affiliates institutes a patent challenge against the licensed patents from Pierre Fabre. The Company may also terminate the Pierre Fabre Agreement with or without cause upon nine months’ prior written notice, so long as there is no ongoing clinical trial for any PF Licensed Product. As of March 31, 2024, no milestones were probable and accrued in the condensed consolidated balance sheet. Novelty Nobility License and Commercialization Agreement Upon the closing of the Acquisition, the Company became the successor to an exclusive license agreement between ValenzaBio and Novelty Nobility (the “Novelty License Agreement”) and obtained a worldwide exclusive license for the development and commercialization of SLRN-517, an unmodified IgG1 monoclonal antibody, as a therapeutic treatment. In connection with the arrangement, the Company is obligated to (i) make development and regulatory milestones of up to $44.3 million, (ii) make commercial sales milestone payments of up to $682.0 million and (iii) pay tiered royalties of a low single-digit to high-single-digit percentage on future worldwide net sales. The Novelty License Agreement is effective on a licensed product-by-licensed product and country-by-country basis until the expiration of the latest to expire royalty term, unless early terminated. The royalty term, with respect to a licensed product and a country is the period commencing on the first commercial sale of such product in such country, and ending upon the latest to occur of: a) there being no patent right in such country that had at least one valid claim covering the licensed product in whole or in part, or the manufacture or use thereof; b) 10 years from the first commercial sale of such product worldwide; or c) expiration of regulatory exclusivity for such product in such country. The agreement can be early terminated upon (i) a material breach, (ii) abandonment of development by the Company, in which the Company ceases all development activities for the licensed product, (iii) termination by patent challenge, and (iv) insolvency. The Company may terminate the contract at any point, upon 30 days prior written notice to Novelty Nobility, Inc. As of March 31, 2024, no milestones were probable and accrued in the condensed consolidated balance sheet.

Commitments and Contingent Liab

Commitments and Contingent Liabilities	3 Months Ended
	Mar. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingent Liabilities	Commitments and Contingent Liabilities License Agreements The Company is required to pay certain milestones upon the achievement of specific development and regulatory events, upon products commercialization and products’ royalties under its license agreements, including its agreements with Affibody, Pierre Fabre, Novelty Nobility and other non-exclusive license agreements. None of the milestones, other than the $15.0 million Afiibody milestone in November 2023, were achieved or probable, all products were in development, as such, no milestones or royalties were accrued in the condensed consolidated balance sheets as of March 31, 2024 and December 31, 2023. Research and Development Agreements The Company enters into various agreements in the ordinary course of business, such as those with suppliers, contract research organizations, contract manufacturing organizations, and clinical trial sites. These contracts generally provide for termination on notice or may have a potential termination fee if a purchase order is canceled within a specified time. The total value of non-cancellable obligations under contracts was $117.7 million and $142.3 million as of March 31, 2024, and December 31, 2023, respectively. This presentation of non-cancellable purchase obligations does not include any estimates of potential reduction of such liabilities related to mitigation obligations of the counter-parties in the event of cancellation under the terms of our engagements. During the three months ended March 31, 2024, and year ended December 31, 2023, there were no amounts accrued related to termination and cancellation charges in the consolidated balance sheets, as the Company has not determined cancellation to be probable. Lease In January 2023, the Company entered into a lease agreement to rent approximately 10,012 square feet of office space. The term of the lease is 65 months with an option to extend it for an additional three years. Monthly rent payments are approximately $30,500, subject to an annual 3.0% increase and six months rental abatement during the first year. In addition to the base rent, the Company is obligated to pay variable costs related to its share of operating expenses and taxes. In connection with the lease agreement, the Company made a security deposit of $34,000, that is included in prepaid expenses and other assets, non-current in the condensed consolidated balance sheet as of December 31, 2023 . As of the lease commencement date the Company recorded $1.3 million as right-of-use (“ROU”) asset and operating lease liability, non-current, in the condensed consolidated balance sheet. Operating lease costs were less than $0.1 million for each of the three months ended March 31, 2024 and 2023, and were recorded in general and administrative expenses and research and development expenses in the condensed consolidated statements of operations and comprehensive loss. The following table summarizes a maturity analysis of the Company’s operating lease liabilities showing the aggregate lease payments as of March 31, 2024 (in thousands): 2024 (remainder of the year) $ 283 2025 386 2026 398 2027 409 2028 280 Total future lease payments 1,756 Less imputed interest (392) Total operating lease liability balance 1,364 Less current portion of lease liability (230) Operating lease liability, non-current $ 1,134 The weighted-average remaining lease term was 53 months and the weighted-average discount rate was 12%. Cash paid for amounts included in the measurement of lease liabilities was less than $0.1 million. Legal Contingencies On November 15, 2023, a purported federal securities class action lawsuit was commenced in the United States District Court for the Central District of California. An amended complaint was filed on March 26, 2024 (Boukadoum v. Acelyrin, Inc. et al., No. 2:23-cv-09672-FMO-MAA), naming us and current and former executive officers and directors as defendants. The complaint alleges that the defendants violated the Exchange Act and Securities Act by misleading investors about the Phase 2b trial of izokibep in HS. The original complaint was filed following our announcement of the week 16 results from the Part B portion of such Phase 2b trial. The complaint seeks damages and an award of reasonable costs and expenses, including attorneys' fees, expert fees and other costs, as well as such other and further relief as the court may deem just and proper. It is possible that additional suits will be filed, or allegations made by stockholders, with respect to these same or other matters and also naming the Company and/or its officers and directors as defendants. This lawsuit and any other potential lawsuits are subject to inherent uncertainties, and the actual defense and disposition costs will depend upon many unknown factors. The outcome of this lawsuit is necessarily uncertain. The Company could be forced to expend significant resources in the defense against this and any other related lawsuits and the Company may not prevail. The Company currently is not able to estimate the possible loss to the Company from this lawsuit, as this lawsuit is currently at an early stage, and such amounts could be material to the Company’s financial statements even if the Company prevails in the defense against this lawsuit. The Company cannot be certain how long it may take to resolve this lawsuit or the possible amount of any damages that the Company may be required to pay. The Company does not consider any payment to be probable or reasonably estimable and has not accrued for any potential liability relating to this lawsuit. From time to time, the Company may become involved in additional legal proceedings or be subject to claims arising in the ordinary course of business. The Company records a liability for such matters when it is probable that future losses will be incurred and that such losses can be reasonably estimated. Significant judgment is required to determine both probability and the estimated amount. Guarantees and Indemnifications In the normal course of business, the Company enters into agreements that contain a variety of representations and provide for general indemnification. Its exposure under these agreements is unknown because it involves claims that may be made against the Company in the future. To the extent permitted under Delaware law, the Company has agreed to indemnify its directors and officers for certain events or occurrences while the director or officer is, or was serving, at a request in such capacity. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. As of March 31, 2024, the Company did not have any material indemnification claims that were probable or reasonably possible and consequently has not recorded related liabilities.

Common Stock

Common Stock	3 Months Ended
	Mar. 31, 2024
Equity [Abstract]
Common Stock	Common Stock On May 9, 2023, immediately prior to the IPO closing, each share of the Company’s Class A Common Stock then issued and outstanding was reclassified and became one share of the Company’s common stock. As of December 31, 2023, there were no shares of Class B Common Stock outstanding. As of March 31, 2024 and December 31, 2023, Common Stock reserved for future issuance by the Company was as follows: March 31, December 31, 2024 2023 Shares available for future grants under Equity Incentive Plan 3,787,834 3,526,392 Outstanding stock options 12,915,515 9,630,623 Performance-based restricted stock units 1 2,906,352 2,964,072 Outstanding restricted stock units 3,369,141 2,166,016 Options assumed upon ValenzaBio acquisition 146,885 938,440 ESPP Shares available for future grants 1,854,494 875,836 Total shares reserved for future issuance 24,980,221 20,101,379 Founders’ Common Stock On the IPO closing date, each share of the founders’ Class A common stock issued and outstanding was reclassified and became one share of the Company’s common stock; no vesting or other terms were modified. In July 2020, the Company issued 2,839,749 shares of its common stock to founders at a price of $0.00002 per share. The issuance price was the estimated fair value of the shares as the shares were issued at inception and no intellectual property was contributed by the founders. The founders have voting rights and rights to receive dividends regardless of the vesting of the shares. Issued shares vest monthly over 48 months, as founders continue providing services to the Company. The Company has the right to repurchase unvested shares at the price paid by the founders if services are terminated. Stock-based compensation expense was minimal for these shares. In December 2022, the Company repurchased 591,613 restricted common shares at the original purchase price that were unvested as of the date of repurchase in connection with one founder’s resignation. As of March 31, 2024 and December 31, 2023, 118,320 and 207,060 shares were unvested, respectively. During the three months ended March 31, 2024 and year ended December 31, 2023, 88,743 and 354,972 founders’ shares vested.

Equity Incentive Plan

Equity Incentive Plan	3 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Equity Incentive Plan	Equity Incentive Plan In April 2023, the Company’s board of directors adopted, and stockholders approved, the 2023 Equity Incentive Plan (the “2023 Plan”) that became effective on May 4, 2023. The Company reserved 12,000,000 new shares of common stock for issuance under the 2023 Plan. In addition, 6,920,846 shares issued and outstanding under the Company’s 2020 Equity Incentive Plan, as amended (the “2020 Plan”), have been added to the 2023 Plan as such shares become available from time to time if awards terminate, expire, or lapse for any reason without the delivery of shares, or are reacquired or withheld (or not issued) to satisfy a tax withholding obligation or the purchase or exercise price. The 2023 Plan also provides that the number of shares initially reserved and available for issuance will automatically increase each January 1, beginning on January 1, 2024 and ending on January 1, 2033, by an amount equal to the lesser of (i) 5% of the shares of common stock outstanding on the last day of the immediately preceding fiscal year, and (ii) such smaller number of shares of stock as determined by the Company’s board of directors. On January 1, 2024, 5,230,473 additional shares of common stock became available for issuance under the 2023 Plan pursuant to the provision. No more than 56,762,538 shares of stock may be issued upon the exercise of incentive stock options under the 2023 Plan. The Company may grant incentive stock options, nonstatutory stock options (“NSOs”), restricted stock units (“RSUs”), restricted stock awards (“RSAs”), stock appreciation rights (“SARs”), performance awards and other awards to the Company’s officers, employees, directors and consultants. Options under the 2023 Plan may be granted for periods of up to 10 years at exercise prices no less than the fair market value of the common stock on the date of grant and usually vest over four years. The exercise price of an option granted to a 10% stockholder may not be less than 110% of the fair market value of the shares on the date of grant and such option may not be exercisable after the expiration of five years from the date of grant. The grant date fair market value of all awards made under our 2023 Plan and all cash compensation paid by us to any non-employee director for services as a director in any fiscal year may not exceed $750,000, increased to $1,000,000 in the fiscal year of their initial service as a non-employee director. The 2023 Plan is the successor to and continuation of the 2020 Plan and no additional awards may be granted under the 2020 Plan. All outstanding awards granted under the 2020 Plan will remain subject to the terms of the 2020 Plan. The 2020 Plan provided for the grant of incentive stock options, nonstatutory stock options, RSUs and RSAs to the Company’s officers, employees, directors and consultants. As of March 31, 2024 and December 31, 2023, 3,787,834 and 3,526,392 shares of the Company’s common stock remained available for issuance under the 2023 Plan. In April 2023, the Company’s board of directors and stockholders adopted the 2023 Employee Stock Purchase Plan (the “ESPP”), which became effective on May 4, 2023. The ESPP authorized issuance of up to 900,000 shares of common stock. The ESPP permits participants to purchase common stock through payroll deductions of up to 15% of their eligible compensation. Employees purchase shares of common stock at a price per share equal to 85% of the lower of the fair market value at the start or end of six-month purchase and offering consecutive periods. The aggregate number of shares reserved for sale under the 2023 ESPP will increase automatically on January 1 for a period of up to 10 calendar years, commencing on January 1, 2024, by the number of shares equal to the lesser of 1% of the Company's total outstanding shares of common stock on the immediately preceding December 31st, and 2,700,000 shares or a lesser number of shares as may be determined by the board of directors. On January 1, 2024, the Company registered 978,658 additional shares of its Common Stock under the ESPP pursuant to the provision. There were 1,854,494 and 875,836 ESPP shares available for future grants as of March 31, 2024 and December 31, 2023, respectively. Stock Options Stock options issued under the 2020 and 2023 Plan generally vest over a four-year period and expire ten years from the date of grant. Certain options provide for accelerated vesting if there is a change in control, as defined in the individual award agreements. A summary of option activity under the 2020 and 2023 Plan is as follows: Number of Weighted- Weighted- Aggregate Outstanding at December 31, 2023 9,630,623 $ 10.4619 9.0 $ 12,007 Options granted 3,779,070 7.6815 Options exercised (153,916) 2.7305 Options expired (360) 5.8766 Options forfeited (339,902) 9.6418 Outstanding at March 31, 2024 12,915,515 $ 9.7622 9.1 $ 7,696 Exercisable at March 31, 2024 1,925,317 $ 4.9361 7.8 $ 4,034 Vested and expected to vest at March 31, 2024 12,915,515 $ 9.7622 9.1 $ 7,696 The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock for those stock options that had exercise prices lower than the estimated fair value of the Company’s common stock at March 31, 2024 and December 31, 2023. Fair value of shares vested during the three months ended March 31, 2024 was $2.0 million. The weighted-average grant date fair value of options granted in three months ended March 31, 2024 was $5.3756. ValenzaBio 2020 Stock Option Plan On January 4, 2023, in connection with the Acquisition, the Company assumed the ValenzaBio 2020 Stock Option Plan and options to issue 1,249,811 shares of the Company’s Class A Common Stock to ValenzaBio option holders, who entered into consulting agreements with the Company. The weighted-average exercise price of assumed options was $3.6736 per share. Under the terms of the Merger Agreement, the assumed options vested in full on March 31, 2023. A total of 791,555 options assumed under the ValenzaBio 2020 Stock Option Plan having the weighted-average exercise price of $4.0223 were exercised for the three months ended March 31, 2024. The Company recognized the full amount of stock-based compensation expense of $4.9 million, including $3.1 million as research and development expenses and $1.8 million as general administrative expenses, related to assumed options in the consolidated statement of operations for the three months ended March 31, 2023. Restricted Stock Units A summary of unvested RSU activity is presented in the following table: Number of RSUs Weighted-Average Grant Date Fair Value Unvested at December 31, 2023 2,166,016 $ 22.90 Granted 1,428,000 7.68 Vested (47,639) 8.00 Forfeited (177,236) 12.79 Unvested at March 31, 2024 3,369,141 17.19 Performance-Based Restricted Stock Units In August 2023, the Company granted PSUs to certain employees and officers of the Company. The PSUs may vest over several years subject to the achievement of (i) certain clinical development milestones over a performance period from the grant date to May 2027 (the “Performance Period”) or (ii) market conditions (i.e., stock price hurdle) based on pre-specified volume-weighted average stock price measurements as of each vesting performance measurement date, and continued employment with the Company through the applicable vesting date(s). The target number of shares under the PSUs at grant date was 3,135,104. The ultimate number of PSU shares that may vest, in the aggregate over the Performance Period, could in certain cases be up to 150% of the target number of shares upon the achievement of certain market or performance conditions. A summary of PSU activity based on the target number of shares is presented in the following table: Number of PSUs Weighted-Average Grant Date Fair Value* Outstanding at December 31, 2023 2,964,072 $ 27.43 Granted — — Vested — — Forfeited (57,720) 27.43 Outstanding at March 31, 2024 2,906,352 $ 27.43 *The grant date fair value is based only on the PSUs with market conditions and does not factor in any performance conditions. As the PSUs granted in 2023 are subject to a market condition, the grant date fair value for such PSUs was based on a Monte Carlo simulation model. The Company estimated the fair value of PSUs based on the grant date price of its common stock of $26.97 and the following assumptions: expected volatility of 87.71%, risk-free-rate of 4.47%, and zero expected dividend yield. In 2023, the Company granted PSUs to employees with a weighted-average grant date fair value of $27.43. The unvested awards will expire if it is determined that the vesting conditions have not been met during the applicable three-year performance period. 2023 Employee Stock Purchase Plan The second purchase period commenced on December 15, 2023 and will end on June 14, 2024. The Company recorded less than $0.1 million in accrued liabilities as of March 31, 2024. Stock-Based Compensation Expense The Company used Black-Scholes option pricing model to estimate fair value of each option at the grant date based on the following assumptions for the three months ended March 31, 2024 and 2023: Three Months Ended Three Months Ended 2024 2023 Expected volatility 87.37% - 87.37% 91.36% - 92.20% Expected dividend yield 0 % 0 % Expected term (in years) 6.08 - 6.08 6.01 – 6.08 Risk-free interest rate 3.95% - 3.95% 3.39% - 4.12% The following table presents the classification of stock-based compensation expense related to awards granted under the 2020 Plan and assumed ValenzaBio options (in thousands): Three Months ended March 31, 2024 2023 Research and development expenses $ 4,591 $ 3,765 General and administrative expenses 15,572 3,374 Total stock-based compensation expense $ 20,163 $ 7,139 The stock-based compensation expense relates to the following equity-based awards: Three Months ended March 31, 2024 2023 Restricted stock units $ 4,415 $ — Performance-based restricted stock units 9,900 — Stock options 5,751 7,062 ESPP 97 — Restricted stock awards — 77 Total stock-based compensation expense $ 20,163 $ 7,139 The Company recognized $4.9 million stock-based compensation expense related to assumed ValenzaBio options and $0.9 million related to unvested options and RSAs net-settled at the closing of the Acquisition. As of March 31, 2024 there was $76.9 million of unrecognized stock-based compensation expense related to granted stock options, which is expected to be recognized over a weighted-average period of 3.1 years. As of March 31, 2024, there was $45.8 million of unrecognized stock-based compensation expense related to RSUs which is expected to be recognized over a weighted-average period of 3.0 years. The Company recognized $9.9 million in compensation expense during the three months ended March 31, 2024 related to PSUs. This expense is related to both the market and performance conditions associated with the PSUs. As of March 31, 2024, the Company evaluated the clinical development milestone performance conditions and determined certain conditions to be probable of achievement. As of March 31, 2024, total compensation cost not yet recognized related to unvested PSUs was $60.4 million, which is expected to be recognized over a weighted-average period of 2.0 years. Total compensation cost not recognized related to unvested PSUs can increase up to $80.3 million depending on the future achievement of PSUs performance conditions.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	Net Loss Per Share Attributable to Common Stockholders The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data): Three Months Ended March 31, 2024 2023 Numerator: Net loss $ (34,973) $ (176,450) Denominator: Weighted average common shares outstanding 98,090,167 21,023,566 Less: Weighted-average common shares subject to repurchase (176,507) (531,465) Weighted-average common shares outstanding, basic and diluted 97,913,660 20,492,101 Net loss per share attributable to common stockholders, basic and diluted $ (0.36) $ (8.61) The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: As of March 31, 2024 2023 Outstanding options to purchase common stock 12,915,515 5,554,611 Unvested RSUs outstanding 3,369,141 1,107,213 Unvested PSUs expected to vest 653,929 — Outstanding options to purchase common stock assumed upon acquisition of ValenzaBio 146,885 1,249,811 Common stock subject to repurchase 118,320 473,290 ESPP 96,544 — Redeemable convertible preferred stock — 40,743,522 Total 17,300,334 49,128,447 The PSUs included above represent the expected payout at the reporting date under the current performance vesting conditions assessment (see Note 10).

Other Income

Other Income	3 Months Ended
	Mar. 31, 2024
Other Income and Expenses [Abstract]
Other Income	Other Income Arrangements with Vendors In March 2024, we entered into arrangements with certain vendors where we received a payment of $30.0 million and a $5.0 million service credit. The $30.0 million payment received from these arrangements was recorded as a gain in other income (expense), net in the condensed consolidated statement of operations and comprehensive loss for the three months ended March 31, 2024 and included in the cash flows from operating activities in the condensed consolidated statement of cash flows for the same period. The $5.0 million service credit was recorded as a credit within research and development expenses. Asset sale In January 2024, we entered into an asset purchase agreement (“Purchase Agreement”) with Tenet Medicines, Inc. (“Tenet”). We recorded $7.0 million cash received as other income (expense), net in the condensed consolidated statement of operations and comprehensive loss for the three months ended March 31, 2024. In consideration for the licenses and other rights Tenet received under the Purchase Agreement, the Company is entitled to receive development, regulatory and commercial milestone payments of up to $157.5 million, royalty on worldwide net sales and payments on sublicense income.

Subsequent Events

Subsequent Events	3 Months Ended
	Mar. 31, 2024
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Departure and Appointment of Directors or Certain Officers By mutual agreement of Shao-Lee Lin, M.D., Ph.D. and the Company, Dr. Lin stepped down from her position as Chief Executive Officer and as a member of the Board, effective on May 8, 2024. The Company has entered into a Separation Agreement and Mutual Release with Dr. Lin (the “CEO Agreement”) pursuant to which the Company agreed to provide Dr. Lin with certain benefits , including the following: a lump sum payment equal to approximately 18 months of base salary, approximately 18 months of equity award and restricted stock vesting acceleration, up to 24 months of health insurance premium payments, a lump sum payment equal to a pro-rated portion of her 2024 target bonus, and an extension of the post-termination exercise period of her outstanding stock option awards. Separation expenses including stock-based compensation will be recorded in Q2 2024. In connection with Dr. Lin’s departure, the Company appointed Mina Kim as the Company’s Chief Executive Officer and as a member of the Board , effective as of May 9, 2024.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2024
Accounting Policies [Abstract]
Basis of Presentation	The condensed consolidated financial statements and accompanying notes are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the SEC regarding interim financial reporting. The accompanying financial statements are consolidated and include the accounts of ACELRYIN, INC. and its wholly owned subsidiary, WH2, LLC. The subsidiary has not had any operations or any balances from its inception. Certain information and footnote disclosures normally included in the financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2023 included in the Company’s Annual Report. The information as of December 31, 2023 included in the condensed consolidated balance sheets was derived from the Company’s audited consolidated financial statements. These unaudited interim condensed consolidated financial statements have been prepared on the same basis as the Company’s annual consolidated financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments necessary for a fair statement of the Company’s consolidated financial statements. The results of operations for the three months ended March 31, 2024 are not necessarily indicative of the results that may be expected for the year ending December 31, 2024 or for any other interim period or for any other future year.
Use of Estimates	The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of expenses during the reporting period. On an ongoing basis, the Company evaluates estimates and assumptions, including but not limited to those related to the fair value of its derivative tranche liability, the fair value of its common stock, stock-based compensation expense, accruals for research and development expenses, valuation of deferred tax assets, and uncertain income tax positions. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results may differ materially from those estimates.
Concentration of Credit Risk	Cash and cash equivalents, and short-term marketable securities are financial instruments that potentially subject the Company to concentrations of credit risk. As of March 31, 2024 and December 31, 2023, cash consists of cash deposited with one financial institution, and account balances exceed federally insured limits. Management believes that the Company is not exposed to significant credit risk due to the financial strength of this institution. The Company also has investments in money market funds, U.S. Treasury obligations, corporate debt obligations, and federal agency obligations, which can be subject to certain credit risks. The Company mitigates the risks by investing in high-grade instruments, limiting its exposure to any one issuer and monitoring the ongoing creditworthiness of the financial institutions and issuers. The Company has not experienced any losses on its financial instruments.
Fair Value Measurements	The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1 — Quoted prices in active markets for identical assets or liabilities. Level 2 — Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, quoted prices in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. The Company’s financial instruments measured at fair value on a recurring basis consist of Level 1, Level 2, and Level 3 financial instruments. Usually, short term marketable securities are considered Level 2 when their fair values are determined using inputs that are observable in the market or can be derived principally from or corroborated by observable market data such as pricing for similar securities, recently executed transactions, cash flow models with yield curves, and benchmark securities. In addition, Level 2 financial instruments are valued using comparisons to like-kind financial instruments and models that use readily observable market data as their basis. Corporate debt obligations, commercial paper, government agency obligations and asset-backed securities are valued primarily using market prices of comparable securities, bid/ask quotes, interest rate yields and prepayment spreads and are included in Level 2.

ValenzaBio Acquisition (Tables)

ValenzaBio Acquisition (Tables)	3 Months Ended
	Mar. 31, 2024
Business Combination and Asset Acquisition [Abstract]
Asset Acquisition	The following table represents the total purchase consideration (in thousands): Issued Class A Common Stock (1) $ 128,735 Transaction costs (2) 1,271 Cash (3) 8 Total $ 130,014 (1) Shares were issued for consideration at $6.86 per share, including 2,013,673 shares that were being held by Seller LLC until the Holdback Release Date. The Company used a third party valuation specialist to assist management in determining the fair value of the shares of Class A Common Stock at the Closing Date. (2) Legal and advisory transaction costs of $1.3 million incurred by the Company in connection with the Acquisition, including $0.1 million payable in cash to Seller LLC for the expense fund. (3) Cash payment of $7,663 to one non-accredited investor for settlement of vested ValenzaBio options. The following is the allocation of the purchase consideration to the acquired assets and liabilities (in thousands): Cash $ 11,369 Prepaid expenses and other current assets 2,074 In-process research and development assets 123,057 Accounts payable (1,628) Accrued research and development expenses (4,805) Accrued compensation and other current liabilities (53) Total net asset acquired $ 130,014

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2024
Fair Value Disclosures [Abstract]
Schedule of Financial Instruments Measured on Recurring Basis	The following table sets forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): Fair Value Measurements as of March 31, 2024 As of March 31, 2024: Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash and cash equivalents) $ 88,994 $ 88,994 $ - $ - U.S. Treasury obligations ($116,279 included in cash and cash equivalents) 398,486 - 398,486 - Corporate debt obligations ($31,015 included in cash and cash equivalents) 148,866 - 148,866 - Federal agency obligations ($4,973 included in cash and cash equivalents) 18,952 - 18,952 - Total fair value of assets $ 655,298 $ 88,994 $ 566,304 $ - Fair Value Measurements as of December 31, 2023 As of December 31, 2023: Total Level 1 Level 2 Level 3 Assets: Money market funds (included in cash and cash equivalents) $ 23,205 $ 23,205 $ - $ - U.S. Treasury obligations ($146,497 included in cash and cash equivalents) 525,353 - 525,353 - Corporate debt obligations ($23,313 included in cash and cash equivalents) 135,284 - 135,284 - Federal agency obligations ($15,344 included in cash and cash equivalents) 27,746 - 27,746 - Total fair value of assets $ 711,588 $ 23,205 $ 688,383 $ - Classified as: March 31, 2024 December 31, 2023 Cash and cash equivalents $ 241,261 $ 208,359 Short-term marketable securities $ 414,037 $ 503,229 Total cash equivalents and marketable securities $ 655,298 $ 711,588

Available-For-Sale Marketable_2

Available-For-Sale Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]
Schedule of Available-for-Sale Marketable Securities	The following tables summarize the estimated fair value of the Company’s available-for-sale marketable securities as of March 31, 2024 and December 31, 2023 (in thousands): As of March 31, 2024: Total Total Unrealized Gain Total Total Money market funds (included in cash and cash equivalents) $ 88,994 $ — $ - $ 88,994 U.S. Treasury obligations ($116,279 included in cash and cash equivalents) 398,532 7 (53) 398,486 Corporate debt obligations ($31,015 included in cash and cash equivalents) 148,913 14 (61) 148,866 Federal agency obligations ($4,973 included in cash and cash equivalents) 18,963 — (11) 18,952 Total available for sale marketable securities $ 655,402 $ 21 $ (125) $ 655,298 As of December 31, 2023: Total Total Unrealized Gain Total Unrealized Loss Total Money market funds (included in cash and cash equivalents) $ 23,205 $ — $ - $ 23,205 U.S. Treasury obligations ($146,497 included in cash and cash equivalents) 525,198 156 (1) 525,353 Corporate debt obligations ($23,313 included in cash and cash equivalents) 135,288 36 (40) 135,284 Federal agency obligations ($15,344 included in cash and cash equivalents) 27,735 12 (1) 27,746 Total available for sale marketable securities $ 711,426 $ 204 $ (42) $ 711,588

Consolidated Balance Sheet Co_2

Consolidated Balance Sheet Components (Tables)	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure	Prepaid expenses and other current assets consist of the following (in thousands): March 31, December 31, Prepaid research and development expenses $ 7,247 $ 8,184 Value-Added Tax (VAT) receivable 4,166 3,985 Prepaid insurance and other current assets 1,423 1,712 Interest receivable 1,189 764 Prepaid other services 1,418 667 Total $ 15,443 $ 15,312 Other non-current assets consist of the following (in thousands): March 31, December 31, Prepaid research and development expenses, non-current $ 141 $ 2,644 Security deposits 34 34 Total $ 175 $ 2,678
Property, Plant and Equipment	Property, plant and equipment consisted of the following (in thousands): March 31, 2024 December 31, 2023 Construction in progress $ 1,452 $ 1,460 Computer and other equipment 407 407 Furniture and fixtures 357 306 Leasehold improvements 133 121 Total property, plant and equipment, gross 2,349 2,294 Less: accumulated depreciation and amortization (170) (115) Property, plant and equipment, net $ 2,179 $ 2,179
Accrued Research and Development Expense	Accrued research and development expenses are comprised of the following (in thousands): March 31, 2024 December 31, 2023 Accrued clinical manufacturing expenses $ 26,969 $ 22,232 Accrued clinical expenses 10,560 13,204 Total $ 37,529 $ 35,436
Other Current Liabilities	Accrued compensation and other current liabilities consist of the following (in thousands): March 31, December 31, Accrued compensation $ 2,643 $ 5,417 Accrued professional service fees 1,020 1,099 Other accrued expenses and current liabilities 519 317 Total $ 4,182 $ 6,833

Commitments and Contingent Li_2

Commitments and Contingent Liabilities (Tables)	3 Months Ended
	Mar. 31, 2024
Commitments and Contingencies Disclosure [Abstract]
Lessee, Operating Lease, Liability, to be Paid, Maturity	The following table summarizes a maturity analysis of the Company’s operating lease liabilities showing the aggregate lease payments as of March 31, 2024 (in thousands): 2024 (remainder of the year) $ 283 2025 386 2026 398 2027 409 2028 280 Total future lease payments 1,756 Less imputed interest (392) Total operating lease liability balance 1,364 Less current portion of lease liability (230) Operating lease liability, non-current $ 1,134

Common Stock (Tables)

Common Stock (Tables)	3 Months Ended
	Mar. 31, 2024
Equity [Abstract]
Schedule of Common Stock Reserved for Future Issuance	As of March 31, 2024 and December 31, 2023, Common Stock reserved for future issuance by the Company was as follows: March 31, December 31, 2024 2023 Shares available for future grants under Equity Incentive Plan 3,787,834 3,526,392 Outstanding stock options 12,915,515 9,630,623 Performance-based restricted stock units 1 2,906,352 2,964,072 Outstanding restricted stock units 3,369,141 2,166,016 Options assumed upon ValenzaBio acquisition 146,885 938,440 ESPP Shares available for future grants 1,854,494 875,836 Total shares reserved for future issuance 24,980,221 20,101,379

Equity Incentive Plan (Tables)

Equity Incentive Plan (Tables)	3 Months Ended
	Mar. 31, 2024
Share-Based Payment Arrangement [Abstract]
Summary of Stock Option Activity	A summary of option activity under the 2020 and 2023 Plan is as follows: Number of Weighted- Weighted- Aggregate Outstanding at December 31, 2023 9,630,623 $ 10.4619 9.0 $ 12,007 Options granted 3,779,070 7.6815 Options exercised (153,916) 2.7305 Options expired (360) 5.8766 Options forfeited (339,902) 9.6418 Outstanding at March 31, 2024 12,915,515 $ 9.7622 9.1 $ 7,696 Exercisable at March 31, 2024 1,925,317 $ 4.9361 7.8 $ 4,034 Vested and expected to vest at March 31, 2024 12,915,515 $ 9.7622 9.1 $ 7,696
Summary of Restricted Stock Unit Activity	A summary of unvested RSU activity is presented in the following table: Number of RSUs Weighted-Average Grant Date Fair Value Unvested at December 31, 2023 2,166,016 $ 22.90 Granted 1,428,000 7.68 Vested (47,639) 8.00 Forfeited (177,236) 12.79 Unvested at March 31, 2024 3,369,141 17.19
Summary of Performance Share Activity	A summary of PSU activity based on the target number of shares is presented in the following table: Number of PSUs Weighted-Average Grant Date Fair Value* Outstanding at December 31, 2023 2,964,072 $ 27.43 Granted — — Vested — — Forfeited (57,720) 27.43 Outstanding at March 31, 2024 2,906,352 $ 27.43 *The grant date fair value is based only on the PSUs with market conditions and does not factor in any performance conditions.
Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions	The Company used Black-Scholes option pricing model to estimate fair value of each option at the grant date based on the following assumptions for the three months ended March 31, 2024 and 2023: Three Months Ended Three Months Ended 2024 2023 Expected volatility 87.37% - 87.37% 91.36% - 92.20% Expected dividend yield 0 % 0 % Expected term (in years) 6.08 - 6.08 6.01 – 6.08 Risk-free interest rate 3.95% - 3.95% 3.39% - 4.12%
Share-Based Payment Arrangement, Expensed and Capitalized, Amount	The following table presents the classification of stock-based compensation expense related to awards granted under the 2020 Plan and assumed ValenzaBio options (in thousands): Three Months ended March 31, 2024 2023 Research and development expenses $ 4,591 $ 3,765 General and administrative expenses 15,572 3,374 Total stock-based compensation expense $ 20,163 $ 7,139 The stock-based compensation expense relates to the following equity-based awards: Three Months ended March 31, 2024 2023 Restricted stock units $ 4,415 $ — Performance-based restricted stock units 9,900 — Stock options 5,751 7,062 ESPP 97 — Restricted stock awards — 77 Total stock-based compensation expense $ 20,163 $ 7,139

Net Loss Per Share Attributab_2

Net Loss Per Share Attributable to Common Stockholders (Tables)	3 Months Ended
	Mar. 31, 2024
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data): Three Months Ended March 31, 2024 2023 Numerator: Net loss $ (34,973) $ (176,450) Denominator: Weighted average common shares outstanding 98,090,167 21,023,566 Less: Weighted-average common shares subject to repurchase (176,507) (531,465) Weighted-average common shares outstanding, basic and diluted 97,913,660 20,492,101 Net loss per share attributable to common stockholders, basic and diluted $ (0.36) $ (8.61)
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive: As of March 31, 2024 2023 Outstanding options to purchase common stock 12,915,515 5,554,611 Unvested RSUs outstanding 3,369,141 1,107,213 Unvested PSUs expected to vest 653,929 — Outstanding options to purchase common stock assumed upon acquisition of ValenzaBio 146,885 1,249,811 Common stock subject to repurchase 118,320 473,290 ESPP 96,544 — Redeemable convertible preferred stock — 40,743,522 Total 17,300,334 49,128,447

Description of Business, Orga_2

Description of Business, Organization and Liquidity (Details) $ / shares in Units, $ in Thousands				3 Months Ended
	May 09, 2023 USD ($) $ / shares shares	Apr. 25, 2023	Jan. 04, 2023 USD ($) shares	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Jul. 31, 2020 $ / shares
Subsidiary, Sale of Stock [Line Items]
Reverse stock split, conversion ratio		0.5071
Sale of stock, price (in dollars per share) | $ / shares							$ 0.00002
Temporary equity, convertible, conversion ratio				100%
Net losses				$ 34,973	$ 176,450
Other nonoperating income				37,000
Research and development				58,032	167,920
Payments to acquire in-process research and development				0	10,000
Accumulated deficit				523,692		$ 488,719
Cash used in operating activities				57,299	25,329
Cash, cash equivalents, and marketable securities				$ 678,500
Pierre Fabre
Subsidiary, Sale of Stock [Line Items]
Payments to acquire in-process research and development			$ 10,000		10,000
ValenzaBio Asset Acquisition
Subsidiary, Sale of Stock [Line Items]
Shares issued (in shares) | shares			2,013,673
Research and development			$ 123,100		$ 123,100
ValenzaBio Asset Acquisition | Common Class A
Subsidiary, Sale of Stock [Line Items]
Shares issued (in shares) | shares			18,885,731
IPO
Subsidiary, Sale of Stock [Line Items]
Number of shares issued in transaction (in shares) | shares	34,500,000
Sale of stock, price (in dollars per share) | $ / shares	$ 18
Gross proceeds received on transaction	$ 621,000
Net proceeds received on transaction	573,600
Underwriting discounts, commissions and offering costs	$ 47,400
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Number of shares issued in transaction (in shares) | shares	4,500,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Details)	Mar. 31, 2024 institution
Accounting Policies [Abstract]
Number of financial institutions with cash deposits	1

ValenzaBio Acquisition - Narrat

ValenzaBio Acquisition - Narrative (Details)		3 Months Ended
	Jan. 04, 2023 USD ($) shares	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Oct. 20, 2022 subsidiary
Asset Acquisition [Line Items]
Number of subsidiaries formed | subsidiary					2
Expense related to acquired in-process research and development assets		$ 0	$ 133,057,000
Research and development		58,032,000	167,920,000
Severance payable, current		200,000		$ 300,000
Payments to acquire in-process research and development		0	10,000,000
Minimum
Asset Acquisition [Line Items]
Severance payment obligation period	3 months
Maximum
Asset Acquisition [Line Items]
Severance payment obligation period	18 months
Unvested Equity Awards
Asset Acquisition [Line Items]
Separately recognized expenses		$ 900,000
Severance Payment Obligation
Asset Acquisition [Line Items]
Liabilities recognized	$ 5,100,000
Severance Payment Obligation | Measurement Input, Discount Rate [Member]
Asset Acquisition [Line Items]
Measurement input	8%
Research and Development Expense | Assumed Options
Asset Acquisition [Line Items]
Separately recognized expenses			3,100,000
Research and Development Expense | Severance Payment Obligation
Asset Acquisition [Line Items]
Separately recognized expenses			2,500,000
General and Administrative Expense | Assumed Options
Asset Acquisition [Line Items]
Separately recognized expenses			1,800,000
General and Administrative Expense | Severance Payment Obligation
Asset Acquisition [Line Items]
Separately recognized expenses			2,400,000
Non-Accredited Investor | General and Administrative Expense
Asset Acquisition [Line Items]
Separately recognized expenses			8,387
Former ValenzaBio Employee | General and Administrative Expense
Asset Acquisition [Line Items]
Separately recognized expenses			30,000
Pierre Fabre
Asset Acquisition [Line Items]
Payments to acquire in-process research and development	$ 10,000,000		10,000,000
ValenzaBio Asset Acquisition
Asset Acquisition [Line Items]
Shares issued (in shares) | shares	2,013,673
Share holdback (in shares) | shares	2,013,673
Shares issued, withholding period	12 months
Asset acquisition payment	$ 7,663
Contingent consideration	100,000
Transaction cost, net	$ 1,200,000
Options exercisable (in shares) | shares	1,249,811
Option exercisable period	12 months
Options exchange ratio	0.008027010
Research and development	$ 123,100,000		$ 123,100,000
ValenzaBio Asset Acquisition | Common Class A
Asset Acquisition [Line Items]
Shares issued (in shares) | shares	18,885,731
ValenzaBio Asset Acquisition | lonigutamab
Asset Acquisition [Line Items]
Expense related to acquired in-process research and development assets	$ 114,800,000
ValenzaBio Asset Acquisition | SLRN-517
Asset Acquisition [Line Items]
Expense related to acquired in-process research and development assets	$ 8,200,000

ValenzaBio Acquisition - Schedu

ValenzaBio Acquisition - Schedule of Total Purchase Consideration (Details) - ValenzaBio Asset Acquisition	Jan. 04, 2023 USD ($) $ / shares shares
Asset Acquisition [Line Items]
Issued common stock	$ 128,735,000
Transaction costs	1,271,000
Cash	7,663
Total	$ 130,014,000
Shares issued (in dollars per share) | $ / shares	$ 6.86
Shares issued (in shares) | shares	2,013,673
Contingent consideration	$ 100,000

ValenzaBio Acquisition - Sche_2

ValenzaBio Acquisition - Schedule of Allocation of Purchase Consideration (Details) - ValenzaBio Asset Acquisition $ in Thousands	Jan. 04, 2023 USD ($)
Asset Acquisition [Line Items]
Cash	$ 11,369
Prepaid expenses and other current assets	2,074
In-process research and development assets	123,057
Accounts payable	(1,628)
Accrued research and development expenses	(4,805)
Accrued compensation and other current liabilities	(53)
Total net asset acquired	$ 130,014

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Financial Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Assets:
Money market funds (included in cash and cash equivalents)	$ 655,298	$ 711,588
Total fair value of assets	655,298	711,588
Cash and cash equivalents
Assets:
Money market funds (included in cash and cash equivalents)	241,261	208,359
Money market funds (included in cash and cash equivalents)
Assets:
Money market funds (included in cash and cash equivalents)	88,994	23,205
U.S. Treasury obligations
Assets:
Money market funds (included in cash and cash equivalents)	398,486	525,353
U.S. Treasury obligations | Cash and cash equivalents
Assets:
Money market funds (included in cash and cash equivalents)	116,279	146,497
Corporate debt obligations
Assets:
Money market funds (included in cash and cash equivalents)	148,866	135,284
Corporate debt obligations | Cash and cash equivalents
Assets:
Money market funds (included in cash and cash equivalents)	31,015	23,313
Federal agency obligations
Assets:
Money market funds (included in cash and cash equivalents)	18,952	27,746
Federal agency obligations | Cash and cash equivalents
Assets:
Money market funds (included in cash and cash equivalents)	4,973	15,344
Level 1
Assets:
Total fair value of assets	88,994	23,205
Level 1 | Money market funds (included in cash and cash equivalents)
Assets:
Money market funds (included in cash and cash equivalents)	88,994	23,205
Level 1 | U.S. Treasury obligations
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Level 1 | Corporate debt obligations
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Level 1 | Federal agency obligations
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Level 2
Assets:
Total fair value of assets	566,304	688,383
Level 2 | Money market funds (included in cash and cash equivalents)
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Level 2 | U.S. Treasury obligations
Assets:
Money market funds (included in cash and cash equivalents)	398,486	525,353
Level 2 | Corporate debt obligations
Assets:
Money market funds (included in cash and cash equivalents)	148,866	135,284
Level 2 | Federal agency obligations
Assets:
Money market funds (included in cash and cash equivalents)	18,952	27,746
Level 3
Assets:
Total fair value of assets	0	0
Level 3 | Money market funds (included in cash and cash equivalents)
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Level 3 | U.S. Treasury obligations
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Level 3 | Corporate debt obligations
Assets:
Money market funds (included in cash and cash equivalents)	0	0
Level 3 | Federal agency obligations
Assets:
Money market funds (included in cash and cash equivalents)	$ 0	$ 0

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Cash and Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	$ 655,298	$ 711,588
Cash and cash equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	241,261	208,359
Short-term marketable securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	$ 414,037	$ 503,229

Available-For-Sale Marketable_3

Available-For-Sale Marketable Securities - Schedule of Available-for-Sale Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-Sale [Line Items]
Total Amortized Cost	$ 655,402	$ 711,426
Total Unrealized Gain	21	204
Total Unrealized Loss	(125)	(42)
Total Estimated Fair Value	655,298	711,588
Cash and cash equivalents
Debt Securities, Available-for-Sale [Line Items]
Total Estimated Fair Value	241,261	208,359
Money market funds (included in cash and cash equivalents)
Debt Securities, Available-for-Sale [Line Items]
Total Amortized Cost	88,994	23,205
Total Unrealized Gain	0	0
Total Unrealized Loss	0	0
Total Estimated Fair Value	88,994	23,205
U.S. Treasury obligations
Debt Securities, Available-for-Sale [Line Items]
Total Amortized Cost	398,532	525,198
Total Unrealized Gain	7	156
Total Unrealized Loss	(53)	(1)
Total Estimated Fair Value	398,486	525,353
U.S. Treasury obligations | Cash and cash equivalents
Debt Securities, Available-for-Sale [Line Items]
Total Estimated Fair Value	116,279	146,497
Corporate debt obligations
Debt Securities, Available-for-Sale [Line Items]
Total Amortized Cost	148,913	135,288
Total Unrealized Gain	14	36
Total Unrealized Loss	(61)	(40)
Total Estimated Fair Value	148,866	135,284
Corporate debt obligations | Cash and cash equivalents
Debt Securities, Available-for-Sale [Line Items]
Total Estimated Fair Value	31,015	23,313
Federal agency obligations
Debt Securities, Available-for-Sale [Line Items]
Total Amortized Cost	18,963	27,735
Total Unrealized Gain	0	12
Total Unrealized Loss	(11)	(1)
Total Estimated Fair Value	18,952	27,746
Federal agency obligations | Cash and cash equivalents
Debt Securities, Available-for-Sale [Line Items]
Total Estimated Fair Value	$ 4,973	$ 15,344

Available-For-Sale Marketable_4

Available-For-Sale Marketable Securities - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Allowance for credit loss	$ 0
Accrued interest receivable	$ 1,200,000	$ 800,000
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss, Current, Statement of Financial Position [Extensible Enumeration]	Prepaid expenses and other current assets	Prepaid expenses and other current assets
Accrued interest writeoff	$ 0

Consolidated Balance Sheet Co_3

Consolidated Balance Sheet Components - Schedule of Prepaid and Other Current Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid research and development expenses	$ 7,247	$ 8,184
Value-Added Tax (VAT) receivable	4,166	3,985
Prepaid insurance and other current assets	1,423	1,712
Interest receivable	1,189	764
Prepaid other services	1,418	667
Total	$ 15,443	$ 15,312

Consolidated Balance Sheet Co_4

Consolidated Balance Sheet Components - Schedule of Prepaid and Other Noncurrent Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid research and development expenses, non-current	$ 141	$ 2,644
Security deposits	34	34
Total	$ 175	$ 2,678

Consolidated Balance Sheet Co_5

Consolidated Balance Sheet Components - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 2,349	$ 2,294
Less: accumulated depreciation and amortization	(170)	(115)
Property, plant and equipment, net	2,179	2,179
Construction in progress
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	1,452	1,460
Computer and other equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	407	407
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	357	306
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, gross	$ 133	$ 121

Consolidated Balance Sheet Co_6

Consolidated Balance Sheet Components - Accrued Research and Development Expense (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued clinical manufacturing expenses	$ 26,969	$ 22,232
Accrued clinical expenses	10,560	13,204
Total	$ 37,529	$ 35,436

Consolidated Balance Sheet Co_7

Consolidated Balance Sheet Components - Schedule of Accrued Compensation and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued compensation	$ 2,643	$ 5,417
Accrued professional service fees	1,020	1,099
Other accrued expenses and current liabilities	519	317
Total	$ 4,182	$ 6,833

Significant Agreements (Details

Significant Agreements (Details) - USD ($)			1 Months Ended	3 Months Ended		12 Months Ended
	Jan. 04, 2023	Mar. 25, 2021	Nov. 30, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Oct. 31, 2021	Sep. 30, 2021	Aug. 09, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Research and development				$ 58,032,000	$ 167,920,000
Affibody | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate milestone payments								$ 22,000,000	$ 3,000,000
Maximum aggregate milestone payments										$ 280,000,000
Payment due prior to milestone										$ 30,000,000
Aggregate milestone payments, period		10 years
Contingent payments, percentage										33.33%
Fair value threshold percentage										33.33%
Research and development			$ 15,000,000			$ 25,000,000
Probable milestone payments				0			$ 0
Pierre Fabre | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate milestone payments		$ 10,000,000
Aggregate milestone payments, period		10 years
Probable milestone payments				0
One-time payment		$ 31,000,000
One-time payment, period		30 days
Termination period		9 months
Pierre Fabre | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | Development and Regulatory Milestone Payments
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum aggregate milestone payments		$ 99,500,000
Pierre Fabre | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | Commercial Sales Milestone Payments
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum aggregate milestone payments		$ 390,000,000
Novelty Nobility | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Aggregate milestone payments, period	10 years
Probable milestone payments				$ 0
Contract termination, period	30 days
Novelty Nobility | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | Development and Regulatory Milestone Payments
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum aggregate milestone payments	$ 44,300,000
Novelty Nobility | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement | Commercial Sales Milestone Payments
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum aggregate milestone payments	$ 682,000,000

Commitments and Contingent Li_3

Commitments and Contingent Liabilities - Narrative (Details)	1 Months Ended		3 Months Ended		12 Months Ended
	Nov. 30, 2023 USD ($)	Jan. 31, 2023 USD ($) ft²	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2023 USD ($)
Other Commitments [Line Items]
Research and development			$ 58,032,000	$ 167,920,000
Unrecorded unconditional purchase obligation			117,700,000			$ 142,300,000
Office space leased (in square feet) | ft²		10,012
Term of contract				65 months
Renewal term		3 years
Monthly payments		$ 30,500
Annual rent increase		3%
Rent abatement, term		6 months
Security deposit			34,000
Operating lease right-of-use asset		$ 1,300,000	1,149,000			1,195,000
Operating lease liability, non-current		$ 1,300,000	1,134,000			$ 1,194,000
Operating lease costs (less than)			$ 100,000	$ 100,000
Weighted average remaining lease term			53 months
Discount rate			12%
Lease liability measurement (less than)			$ 100,000
Affibody | Collaborative Arrangement, Transaction with Party to Collaborative Arrangement
Other Commitments [Line Items]
Research and development	$ 15,000,000				$ 25,000,000

Commitments and Contingent Li_4

Commitments and Contingent Liabilities - Summary of Maturity Analysis of Operating Lease Liability (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Jan. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
2024 (remainder of the year)	$ 283
2025	386
2026	398
2027	409
2028	280
Total future lease payments	1,756
Less imputed interest	(392)
Total operating lease liability balance	1,364
Less current portion of lease liability	(230)
Operating lease liability, non-current	$ 1,134	$ 1,194	$ 1,300

Common Stock - Narrative (Detai

Common Stock - Narrative (Details)	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2022 shares	Jul. 31, 2020 $ / shares shares	Mar. 31, 2024 shares	Dec. 31, 2023 shares	May 09, 2023
Class of Stock [Line Items]
Conversion of stock, ratio					1
Common stock, outstanding (in shares)			98,859,000	97,865,890
Stock issued during period, issued for services (in shares)		2,839,749
Sale of stock, price (in dollars per share) | $ / shares		$ 0.00002
Common Class B
Class of Stock [Line Items]
Common stock, outstanding (in shares)				0
Restricted stock awards
Class of Stock [Line Items]
Vesting period		48 months
Forfeited in period (in shares)	591,613
Shares unvested (in shares)			118,320	207,060
Vested (in shares)			88,743	354,972

Common Stock - Schedule of Comm

Common Stock - Schedule of Common Stock Reserved for Future Issuance (Details) - shares	Mar. 31, 2024	Dec. 31, 2023
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	24,980,221	20,101,379
Outstanding stock options
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	12,915,515	9,630,623
Performance Shares
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	2,906,352	2,964,072
Outstanding restricted stock units
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	3,369,141	2,166,016
Employee Stock
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	1,854,494	875,836
Shares available for future grants under Equity Incentive Plan
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	3,787,834	3,526,392
Options assumed upon ValenzaBio acquisition | Outstanding stock options
Class of Stock [Line Items]
Total shares reserved for future issuance (in shares)	146,885	938,440

Equity Incentive Plan - Narrati

Equity Incentive Plan - Narrative (Details) - USD ($)					1 Months Ended			3 Months Ended		12 Months Ended
	Jan. 01, 2024	Aug. 16, 2023	May 04, 2023	Jan. 04, 2023	Aug. 31, 2023	Apr. 30, 2023	Jul. 31, 2020	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)								24,980,221		20,101,379
Common stock, issued (in shares)								98,859,000		97,865,890
Common stock, outstanding (in shares)								98,859,000		97,865,890
Share-based and cash-based compensation, maximum			$ 750,000
Fair value of shares vested								$ 2,000,000
Weighted-average grant date fair value, grants in period (in dollars per share)								$ 5.3756
Options granted (in shares)								3,779,070
Exercisable (in dollars per share)								$ 4.9361
Exercises in period (in shares)								153,916
Options expired (in dollars per share)								$ 5.8766
Total stock-based compensation expense								$ 20,163,000	$ 7,139,000
Expected dividend yield								0%	0%
Accrued compensation and other current liabilities								$ 4,182,000		$ 6,833,000
Research and Development Expense
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense								4,591,000	$ 3,765,000
General and Administrative Expense
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense								$ 15,572,000	3,374,000
Share-Based Payment Arrangement, Nonemployee
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Share-based and cash-based compensation, maximum			$ 1,000,000
Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)								12,915,515		9,630,623
Expiration period										10 years
Total stock-based compensation expense								$ 5,751,000	7,062,000
Employee Stock
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)								1,854,494		875,836
Additional shares authorized (in shares)	978,658
Purchase price of common stock						85%
Number of shares authorized (in shares)						900,000
Percentage of eligible compensation for payroll deductions to purchase stock						15%
Purchase period						6 months
Annual increase, period						10 years
Annual increase as percentage of shares outstanding						1%
Maximum number of shares available over award term						2,700,000
Total stock-based compensation expense								$ 97,000	0
Accrued compensation and other current liabilities								$ 100,000
Restricted stock units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)								3,369,141		2,166,016
Total stock-based compensation expense								$ 4,415,000	0
Granted (in dollars per share)								$ 7.68
Restricted stock units | Tranche One
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting percentage		25%
Restricted stock units | Tranche Two
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting percentage		25%
Restricted stock units | Tranche Three
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting percentage		25%
Restricted stock units | Tranche Four
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting percentage		25%
Performance Shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)								2,906,352		2,964,072
Expiration period								3 years
Total stock-based compensation expense								$ 9,900,000	0
Target number of shares (in shares)					3,135,104
Target number of shares, percentage					150%
Share price (in usd per share)								$ 26.97
Expected volatility								87.71%
Risk-free interest rate								4.47%
Expected dividend yield								0%
Granted (in dollars per share)								$ 0		$ 27.43
Weighted-average recognition period								2 years
Unrecognized stock-based compensation expense								$ 60,400,000
Unrecognized stock-based compensation expense, maximum								80,300,000
Restricted stock awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Vesting period							48 months
Total stock-based compensation expense								$ 0	$ 77,000
Equity Incentive Plan 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)			12,000,000
Percentage of outstanding stock maximum			5%
Vesting period			4 years
Equity Incentive Plan 2023 | Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Additional shares authorized (in shares)	5,230,473
Shares issuable (in shares)			56,762,538
Equity Incentive Plan 2023 | Stock options | Stockholder, 10% Or More
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expiration period			5 years
Equity Incentive Plan 2023 | Stock options | Minimum | Stockholder, 10% Or More
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Purchase price of common stock			110%
2020 Equity Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock, issued (in shares)			6,920,846
Common stock, outstanding (in shares)			6,920,846
2020 Stock Option Plan | Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expiration period								10 years
Vesting period								4 years
ValenaBio 2020 Stock Option Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Options granted (in shares)				1,249,811
Exercisable (in dollars per share)				$ 3.6736
Exercises in period (in shares)								791,555
Options expired (in dollars per share)								$ 4.0223
Unrecognized stock-based compensation expense								$ 76,900,000
ValenaBio 2020 Stock Option Plan | ValenzaBio Asset Acquisition
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense										$ 4,900,000
ValenaBio 2020 Stock Option Plan | ValenzaBio Asset Acquisition | Research and Development Expense
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense										3,100,000
ValenaBio 2020 Stock Option Plan | ValenzaBio Asset Acquisition | General and Administrative Expense
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense										$ 1,800,000
ValenaBio 2020 Stock Option Plan | Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total shares reserved for future issuance (in shares)								146,885		938,440
Weighted-average recognition period								3 years 1 month 6 days
ValenaBio 2020 Stock Option Plan | Stock options | ValenzaBio Asset Acquisition
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense				$ 4,900,000
ValenaBio 2020 Stock Option Plan | Restricted stock units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted-average recognition period								3 years
Unrecognized stock-based compensation expense								$ 45,800,000
ValenaBio 2020 Stock Option Plan | Restricted stock awards | ValenzaBio Asset Acquisition
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense				$ 900,000

Equity Incentive Plan - Summary

Equity Incentive Plan - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Number of Options
Outstanding, beginning balance (in shares)	9,630,623
Options granted (in shares)	3,779,070
Options exercised (in shares)	(153,916)
Options expired (in shares)	(360)
Options forfeited (in shares)	(339,902)
Outstanding, ending balance (in shares)	12,915,515	9,630,623
Exercisable (in shares)	1,925,317
Vested and expected to vest (in shares)	12,915,515
Weighted-Average Exercise Price Per Share
Outstanding, beginning balance (in dollars per share)	$ 10.4619
Granted (in dollars per share)	7.6815
Options exercised (in dollars per share)	2.7305
Options expired (in dollars per share)	5.8766
Options forfeited (in dollars per share)	9.6418
Outstanding, ending balance (in dollars per share)	9.7622	$ 10.4619
Exercisable (in dollars per share)	4.9361
Vested and expected to vest (in dollars per share)	$ 9.7622
Weighted-Average Remaining Contractual Term
Outstanding	9 years 1 month 6 days	9 years
Exercisable	7 years 9 months 18 days
Vested and expected to vest	9 years 1 month 6 days
Aggregate Intrinsic Value
Outstanding	$ 7,696	$ 12,007
Exercisable	4,034
Vested and expected to vest	$ 7,696

Equity Incentive Plan - Summa_2

Equity Incentive Plan - Summary of Restricted Stock Unit Activity (Details) - Restricted stock units	3 Months Ended
	Mar. 31, 2024 $ / shares shares
Number of Shares
Outstanding at beginning of period (in shares) | shares	2,166,016
Shares granted (in shares) | shares	1,428,000
Vested (in shares) | shares	(47,639)
Forfeited (in shares) | shares	(177,236)
Outstanding at end of period (in shares) | shares	3,369,141
Weighted-Average Grant Date Fair Value
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 22.90
Granted (in dollars per share) | $ / shares	7.68
Vested (in dollars per share) | $ / shares	8
Forfeited (in dollars per share) | $ / shares	12.79
Outstanding at end of period (in dollars per share) | $ / shares	$ 17.19

Equity Incentive Plan - Summa_3

Equity Incentive Plan - Summary of Performance Share Activity (Details) - Performance Shares - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Number of Shares
Outstanding at beginning of period (in shares)	2,964,072
Shares granted (in shares)	0
Vested (in shares)	0
Forfeited (in shares)	(57,720)
Outstanding at end of period (in shares)	2,906,352	2,964,072
Weighted-Average Grant Date Fair Value
Outstanding at beginning of period (in dollars per share)	$ 27.43
Granted (in dollars per share)	0	$ 27.43
Vested (in dollars per share)	0
Forfeited (in dollars per share)	27.43
Outstanding at end of period (in dollars per share)	$ 27.43	$ 27.43

Equity Incentive Plan - Schedul

Equity Incentive Plan - Schedule of Valuation Assumptions (Details)	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected volatility minimum	87.37%	91.36%
Expected volatility maximum	87.37%	92.20%
Expected dividend yield	0%	0%
Risk-free interest rate minimum	3.95%	3.39%
Risk-free interest rate maximum	3.95%	4.12%
Minimum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 3 days
Maximum
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days

Equity Incentive Plan - Sched_2

Equity Incentive Plan - Schedule of Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense	$ 20,163	$ 7,139
Restricted stock units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense	4,415	0
Performance-based restricted stock units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense	9,900	0
Stock options
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense	5,751	7,062
ESPP
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense	97	0
Restricted stock awards
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense	0	77
Research and development expenses
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense	4,591	3,765
General and administrative expenses
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Total stock-based compensation expense	$ 15,572	$ 3,374

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Schedule of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Numerator:
Net loss	$ (34,973)	$ (176,450)
Denominator:
Weighted average common shares outstanding (in shares)	98,090,167	21,023,566
Less: Weighted-average common shares subject to repurchase (in shares)	(176,507)	(531,465)
Weighted-average common shares outstanding, basic (in shares)	97,913,660	20,492,101
Weighted-average common shares outstanding, diluted (in shares)	97,913,660	20,492,101
Net loss per share attributable to common stockholder, basic (in shares)	$ (0.36)	$ (8.61)
Net loss per share attributable to common stockholder, diluted (in shares)	$ (0.36)	$ (8.61)

Net Loss Per Share Attributab_4

Net Loss Per Share Attributable to Common Stockholders - Schedule of Outstanding Shares of Potentially Dilutive Securities (Details) - shares	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	17,300,334	49,128,447
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	12,915,515	5,554,611
Stock options | ValenzaBio Asset Acquisition
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	146,885	1,249,811
Unvested RSUs outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	3,369,141	1,107,213
Unvested PSUs expected to vest
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	653,929	0
Common stock subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	118,320	473,290
ESPP
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	96,544	0
Redeemable convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	0	40,743,522

Other Income (Details)

Other Income (Details) - USD ($)	1 Months Ended		3 Months Ended
	Mar. 31, 2024	Jan. 31, 2024	Mar. 31, 2024	Mar. 31, 2023
Other Income and Expenses [Abstract]
Other nonrecurring income	$ 30,000,000
Research and development expense, vendor service credit	5,000,000
Income from sale of asset		$ 7,000,000	$ 7,000,000	$ 0
Disposal group, including discontinued operation, consideration	$ 157,500,000		$ 157,500,000

Subsequent Events (Details)

Subsequent Events (Details) - Subsequent Event - Shao-Lee Lin, M.D., Ph.D.	May 08, 2024
Subsequent Event [Line Items]
Severance, base salary period	18 months
Accelerated vesting period	18 months
Health insurance premium payment period	24 months