Alpine Immune Sciences (ALPN) 10-K 19 Mar 24 2023 FY Annual report Financial data

Cover

Cover - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Mar. 07, 2024	Jun. 30, 2023
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2023
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Entity File Number	001-37449
Entity Registrant Name	ALPINE IMMUNE SCIENCES, INC.
Entity Incorporation, Date of Incorporation	DE
Entity Tax Identification Number	20-8969493
Entity Address, Address Line One	188 East Blaine Street
Entity Address, Address Line Two	Suite 200
Entity Address, City or Town	Seattle,
Entity Address, State or Province	WA
Entity Address, Postal Zip Code	98102
City Area Code	206
Local Phone Number	788-4545
Title of 12(b) Security	Common Stock, par value $0.001 per share
Trading Symbol	ALPN
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	false
Document Financial Statement Error Correction	false
Entity Shell Company	false
Entity Public Float			$ 349.1
Entity Common Stock, Shares Outstanding		65,546,136
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to the 2024 Annual Meeting of Stockholders, which will be filed with the Securities and Exchange Commission subsequent to the date hereof, are incorporated by reference into Part III of this Report. Such Proxy Statement will be filed with the Securities and Exchange Commission not later than 120 days following the end of the Registrant’s fiscal year ended December 31, 2023.
Entity Central Index Key	0001626199
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	FY
Amendment Flag	false

Audit Information

Audit Information	12 Months Ended
	Dec. 31, 2023
Audit Information [Abstract]
Auditor Name	Ernst & Young LLP
Auditor Location	Seattle, Washington
Auditor Firm ID	42

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 43,921	$ 13,376
Short-term investments	283,491	224,265
Accounts receivable	167	392
Prepaid expenses and other current assets	2,455	2,960
Total current assets	330,034	240,993
Restricted cash, noncurrent	268	254
Property and equipment, net	1,484	1,584
Operating lease, right-of-use asset	7,510	8,219
Long-term investments	40,556	35,481
Deferred tax asset	0	155
Total assets	379,852	286,686
Current liabilities:
Accounts payable	3,593	4,286
Accrued liabilities	21,187	14,003
Deferred revenue, current	16,288	35,571
Operating lease liability, current	912	756
Current portion of long-term debt	0	3,380
Total current liabilities	41,980	57,996
Deferred revenue, noncurrent	929	39,185
Operating lease liability, noncurrent	9,002	10,085
Total liabilities	51,911	107,266
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value per share; 10,000,000 shares authorized at December 31, 2023 and 2022; zero shares issued and outstanding at December 31, 2023 and 2022	0	0
Common stock, $0.001 par value per share; 200,000,000 shares authorized at December 31, 2023 and 2022; 60,347,457 shares issued and outstanding at December 31, 2023; 47,234,900 shares issued and 45,984,433 shares outstanding at December 31, 2022	60	46
Treasury stock, at cost; zero and 1,250,467 shares at December 31, 2023 and 2022, respectively	0	0
Additional paid-in capital	583,629	404,456
Accumulated other comprehensive gain (loss)	397	(1,121)
Accumulated deficit	(256,145)	(223,961)
Total stockholders’ equity	327,941	179,420
Total liabilities and stockholders’ equity	$ 379,852	$ 286,686

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares outstanding (in shares)	0	0
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	60,347,457	47,234,900
Common stock, shares outstanding (in shares)	60,347,457	45,984,433
Treasury stock, shares (in shares)	0	1,250,467

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement [Abstract]
Revenue from Contract with Customer, Product and Service [Extensible Enumeration]	Collaboration [Member]	Collaboration [Member]	Collaboration [Member]
Collaboration revenue	$ 58,876	$ 30,064	$ 23,443
Operating expenses:
Research and development	80,904	70,243	58,742
General and administrative	22,222	17,968	14,560
Total operating expenses	103,126	88,211	73,302
Loss from operations	(44,250)	(58,147)	(49,859)
Other income (expense):
Interest income	11,852	3,288	259
Interest expense	(98)	(476)	(816)
Other, net	415	(97)	(4)
Loss before taxes	(32,081)	(55,432)	(50,420)
Income tax (expense) benefit	(103)	(2,330)	87
Net loss	(32,184)	(57,762)	(50,333)
Comprehensive income (loss):
Unrealized gain (loss) on investments	1,493	(901)	(238)
Unrealized gain (loss) on foreign currency translation	25	53	(88)
Comprehensive loss	$ (30,666)	$ (58,610)	$ (50,659)
Weighted-average shares used to compute basic net loss per share (in shares)	50,000,339	33,435,280	25,476,889
Weighted-average shares used to compute diluted net loss per share (in shares)	50,000,339	33,435,280	25,476,889
Basic net loss per share (in dollars per share)	$ (0.64)	$ (1.73)	$ (1.98)
Diluted net loss per share (in dollars per share)	$ (0.64)	$ (1.73)	$ (1.98)

CONSOLIDATED STATEMENT OF STOCK

CONSOLIDATED STATEMENT OF STOCKHOLDERS’ EQUITY - USD ($) $ in Thousands	Total	Common Stock	Treasury	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2020		23,803,183
Balance at beginning of period (in shares) at Dec. 31, 2020			50,467
Balance at beginning of period at Dec. 31, 2020	$ 62,158	$ 24		$ 177,947	$ 53	$ (115,866)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in Private Placement/public, net of offering costs (in shares)		6,489,357
Issuance of common stock in Private Placement/public, net of offering costs	90,733	$ 6		90,727
Issuance of common stock to Amgen (in shares)		951,980
Issuance of common stock to Amgen	11,929	$ 1		11,928
Exchange of common stock for prefunded warrants (in shares)		(1,200,000)	1,200,000
Exchange of common stock for prefunded warrants	0	$ (1)		1
Issuance of common stock under equity incentive plans (in shares)		149,759
Issuance of common stock under equity incentive plans	502			502
Stock-based compensation	6,240			6,240
Unrealized (loss) gain on investments	(238)				(238)
Unrealized (loss) gain on foreign currency translation	(88)				(88)
Net loss	(50,333)					(50,333)
Balance at end of period (in shares) at Dec. 31, 2021		30,194,279
Balance at end of period (in shares) at Dec. 31, 2021			1,250,467
Balance at end of period at Dec. 31, 2021	120,903	$ 30		287,345	(273)	(166,199)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in Private Placement/public, net of offering costs (in shares)		15,509,282
Issuance of common stock in Private Placement/public, net of offering costs	106,706	$ 16		106,690
Issuance of common stock under equity incentive plans (in shares)		280,872
Issuance of common stock under equity incentive plans	773			773
Stock-based compensation	9,648			9,648
Unrealized (loss) gain on investments	(901)				(901)
Unrealized (loss) gain on foreign currency translation	53				53
Net loss	$ (57,762)					(57,762)
Balance at end of period (in shares) at Dec. 31, 2022	45,984,433	45,984,433
Balance at end of period (in shares) at Dec. 31, 2022	1,250,467		1,250,467
Balance at end of period at Dec. 31, 2022	$ 179,420	$ 46		404,456	(1,121)	(223,961)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in Private Placement/public, net of offering costs (in shares)		9,791,832
Issuance of common stock in Private Placement/public, net of offering costs	152,221	$ 10		152,211
Issuance of common stock under at the market offering, net of issuance cost (in shares)		919,413
Issuance of common stock under at-the-market offering, net of offering costs	9,694	$ 1		9,693
Issuance of common stock under equity incentive plans (in shares)		940,390
Issuance of common stock under equity incentive plans	4,791	$ 1		4,790
Exercise of warrants, net of shares withheld (in shares)		2,711,389
Exercise of warrants, net of shares withheld	1,851	$ 2		1,849
Stock-based compensation	10,630			10,630
Retirement of treasury stock			(1,250,467)
Unrealized (loss) gain on investments	1,493				1,493
Unrealized (loss) gain on foreign currency translation	25				25
Net loss	$ (32,184)					(32,184)
Balance at end of period (in shares) at Dec. 31, 2023	60,347,457	60,347,457
Balance at end of period (in shares) at Dec. 31, 2023	0		0
Balance at end of period at Dec. 31, 2023	$ 327,941	$ 60		$ 583,629	$ 397	$ (256,145)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Operating activities
Net loss	$ (32,184)	$ (57,762)	$ (50,333)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	10,630	9,648	6,240
Amortization of premium/discount on investments	(8,051)	(700)	906
Depreciation expense	576	597	620
Non-cash interest expense	45	178	274
Realized gain on investments	(33)	0	0
Loss on sale of property and equipment	11	93	4
Deferred income tax	155	46	(214)
Changes in operating assets and liabilities:
Accounts receivable	225	24,602	0
Prepaid expenses and other current assets	853	2,001	(3,063)
Operating lease, right-of-use asset	709	618	564
Accounts payable and accrued liabilities	6,519	5,557	5,970
Deferred revenue	(57,539)	(27,847)	24,628
Operating lease liability	(928)	(785)	(844)
Net cash used in operating activities	(79,012)	(43,754)	(15,248)
Investing activities
Purchases of property and equipment	(487)	(412)	(118)
Purchase of investments	(351,347)	(266,984)	(133,518)
Maturities of investments	296,275	154,048	81,156
Net cash used in investing activities	(55,559)	(113,348)	(52,480)
Financing activities
Proceeds from sale of common stock and warrants, net of offering costs	161,905	106,716	90,733
Proceeds from issuance of common stock to Amgen	0	0	11,929
Proceeds from exercise of stock options, net of taxes paid on vested RSUs	4,791	773	502
Proceeds from exercise of warrants	1,851	0	0
Repayment of debt	(3,425)	(4,800)	(2,400)
Net cash provided by financing activities	165,122	102,689	100,764
Effect of exchange rate on cash and cash equivalents	8	(118)	(88)
Net increase (decrease) in cash and cash equivalents and restricted cash	30,559	(54,531)	32,948
Cash and cash equivalents and restricted cash, beginning of period	13,630	68,161	35,213
Cash and cash equivalents and restricted cash, end of period	44,189	13,630	68,161
Supplemental Information
Cash paid for income taxes	1,535	543	0
Cash paid for interest	$ 66	$ 322	$ 554

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of BusinessAlpine Immune Sciences, Inc. (the “Company”, “Alpine”, “we”, “us”, or “our”), together with its consolidated subsidiaries, is a clinical-stage biopharmaceutical company dedicated to discovering and developing innovative, protein-based immunotherapies to treat autoimmune and inflammatory diseases. Our approach includes a proprietary scientific platform that converts native immune system proteins into differentiated, multi-targeted therapeutics. We are seeking to create first- or best-in-class multifunctional immunotherapies via our unique protein engineering technologies to improve outcomes in patients with serious diseases. We were incorporated under the laws of the State of Delaware and are headquartered in Seattle, Washington.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Basis of Presentation and Use of Estimates The accompanying consolidated financial statements have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (“SEC”) and generally accepted accounting principles in the United States of America (“GAAP”). The preparation of financial statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Significant estimates inherent in the preparation of the accompanying consolidated financial statements include those used for revenue recognition, accruals for clinical trial activities and other accruals, the potential outcome of uncertain tax positions that have been recognized in our consolidated financial statements or tax returns, and the estimated fair value of equity-based awards. We base our estimates and assumptions on historical experience when available and on various factors we believe to be reasonable under the circumstances. Actual results could differ materially from those estimates. Principles of Consolidation Our consolidated financial statements include the financial position and results of operations of Alpine Immune Sciences, Inc. and our wholly owned operating company and subsidiary, AIS Operating Co., Inc., and our wholly owned subsidiary, Alpine Immune Sciences Australia PTY LTD. All inter-company balances and transactions have been eliminated in consolidation. Segments We operate as one operating segment and use cash flow as the primary financial measure to manage our business. Cash and Cash Equivalents and Restricted Cash We consider all highly liquid investments with an original maturity of 90 days or less at the time of purchase to be cash equivalents. Cash and cash equivalents consist of deposits with commercial banks in checking and interest-bearing accounts, highly liquid money market funds, and U.S. Treasury bills. Restricted cash represents cash reserved under our letter of credit, which serves as a security deposit for our operating lease to rent office and laboratory space in Seattle, Washington. Periodically, we maintain deposits in financial institutions in excess of government insured limits. We believe we are not exposed to significant credit risk as our deposits, which are held at financial institutions, are high credit quality securities such as money market funds, U.S. Treasury bills, and commercial paper. To date, we have not realized any losses on these deposits. Investments Our investments include funds invested in highly liquid money market funds, U.S. Treasury bills, U.S. agency securities, non-U.S. government securities, and corporate debt securities and commercial paper with contractual maturities of less than two years. These investments are classified as available-for-sale debt securities, which are recorded at fair value based on quoted prices in active markets for Level 1 assets, and based on market pricing and other observable market inputs for similar securities for Level 2 assets. We classify our investments maturing within one year of the reporting date as short-term investments. If the estimated fair value of a debt security is below its amortized cost basis, we evaluate whether it is more likely than not that we will sell the security before its anticipated recovery in market value and whether credit losses exist for the related securities. A credit loss exists if the present value of expected cash flows is less than the amortized cost basis of the security. Credit-related losses are recognized as an allowance for credit losses on the balance sheet with a corresponding adjustment to earnings. Unrealized gains and losses that are unrelated to credit deterioration are reported in other comprehensive income (loss). Purchase premiums and discounts are recognized within interest income using the interest method over the terms of the securities. Realized gains and losses and declines in fair value for these investments deemed to be expected credit losses are reflected in our Consolidated Statements of Operations and Comprehensive Income (Loss) using the specific-identification method. Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Depreciation is recorded using the straight-line method over the estimated useful lives of the assets, generally three Impairment of Long-lived Assets We evaluate our long-lived tangible assets for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets may not be fully recoverable. When such events occur, we assess recoverability by comparing the asset's carrying value to the total undiscounted future cash flows the asset is expected to generate. If the carrying value exceeds the amount of undiscounted cash flows, we then measure impairment by reducing the carrying value to the assets estimated fair value. We did not record any impairments in the years ended December 31, 2023, 2022 or 2021. Accrued Liabilities As part of the process of preparing our consolidated financial statements, we are required to estimate accruals for professional services and research and development expenses. This process involves reviewing contracts and vendor agreements and communicating with applicable personnel to identify services that have been performed on our behalf. We estimate the level of service performed and the associated cost incurred for the service when we have not yet been invoiced or otherwise notified of the actual cost. We estimate accrued liabilities as of each balance sheet date based on known facts and circumstances. Although we do not expect our estimates to be materially different from amounts actually incurred, if our estimates of the status and timing of services performed differ from the actual status and timing of services performed, we may report amounts that are too high or too low in any particular period. To date, we have not experienced any significant adjustments to our estimates. Leases We account for our leases under Accounting Standards Codification (“ASC”) 842, Leases. Under this guidance, we applied the practical expedients regarding the identification of leases, lease classification, indirect costs, and the combination of lease and non-lease components. Arrangements meeting the definition of a lease are classified as operating or financing leases, and are recorded on the consolidated balance sheet as both a right-of-use asset and lease liability, calculated by discounting fixed lease payments over the lease term at the rate implicit in the lease or our incremental borrowing rate. As we do not know the lessor’s implicit rate, we use our incremental borrowing rate at the commencement date of the lease in determining the present value of lease payments. Lease liabilities are increased by interest and reduced by payments each period, and the right-of-use asset is amortized over the lease term. For operating leases, interest on the lease liability and the amortization of the right-of-use asset result in straight-line rent expense over the lease term. For finance leases, interest on the lease liability and the amortization of the right-of-use asset results is front-loaded expense over the lease term. Variable lease expenses are recorded when incurred. We exclude short-term leases having initial terms of 12 months or less as an accounting policy election, and recognize rent expense on a straight-line basis over the lease term. Derivative Financial Instruments We evaluate all of our financial instruments, including prefunded warrants and warrants to purchase common stock, to determine if such instruments are derivatives or contain features qualifying as embedded derivatives. For derivative financial instruments accounted for as liabilities, the derivative instrument is initially recorded at its fair value and is then re-valued at each reporting date, with changes in the fair value reported in the Consolidated Statements of Operations and Comprehensive Income (Loss) . We use the Black-Scholes option-pricing model to value the derivative instruments at inception and subsequent valuation dates. The classification of derivative instruments, including whether such instruments should be recorded as liabilities or as equity, is reassessed at the end of each reporting period. We do not use derivative instruments to hedge exposures to cash flow, market or foreign currency risks. Revenue Recognition Revenue is recognized when control of the promised goods or services is transferred to our customers, in an amount that reflects the consideration we expect to be entitled to in exchange for those goods or services. Our steps for recognizing revenue consist of (1) identifying the contract; (2) identifying the performance obligations as either distinct or bundled goods and services; (3) determining the transaction price associated with each performance obligation for which we expect to be entitled in exchange for transferring such goods and services; (4) allocating the transaction price to the performance obligations in the contract; and (5) recognizing revenue upon satisfaction of performance obligations. Our collaboration agreements principally contain multiple performance obligations, which may include (1) grants of, or options to obtain, intellectual property licenses; (2) research and development services; and/or (3) manufacturing or supply services. Payments received under these arrangements typically include one or more of the following: non-refundable upfront license fees, option exercise fees, payment for research and/or development efforts, amounts due upon the achievement of specified objectives, and/or royalties on future product sales. Our revenue is primarily derived from our collaboration agreements with Adaptimmune Therapeutics plc (“Adaptimmune”), AbbVie Ireland Unlimited Company (“AbbVie”), and Horizon Therapeutics Ireland DAC (acquired by Amgen Inc., or “Amgen,” in October 2023). See further discussion of our collaboration agreements in Note 10 . We allocate revenue to each performance obligation based on its relative stand-alone selling price. We generally determine stand-alone selling prices at the inception of the contract based on our best estimate of what the selling price would be if the deliverable was regularly sold by us on a stand-alone basis. Payments received prior to satisfying the relevant revenue recognition criteria are recorded as deferred revenue in the accompanying Consolidated Balance Sheets and recognized as revenue when the related revenue recognition criteria are met. We recognize revenue under our collaboration agreements by using a cost-based input method to measure progress toward completion of the performance obligation, including employee hours contributed to each performance obligation, and to calculate the corresponding revenue to recognize each period. Our collaboration agreements provide for non-refundable milestone payments. We recognize revenue that is contingent upon the achievement of a substantive milestone in its entirety in the period in which the milestone is achieved. A milestone is considered substantive when the consideration payable to us for such milestone (1) is consistent with our performance necessary to achieve the milestone or the increase in value to the collaboration resulting from our performance; (2) relates solely to our past performance; and (3) is reasonable relative to all of the other deliverables and payments within the arrangement. In making this assessment, we consider all facts and circumstances relevant to the arrangement, including factors such as the scientific, regulatory, commercial, and other risks that must be overcome to achieve the milestone, the level of effort and investment required to achieve the milestone and whether any portion of the milestone consideration is related to future performance or deliverables. We review the contributed employee hours and progress towards completion for each performance obligation under our collaboration agreements, and adjust the revenue recognized to reflect changes in assumptions relating to the estimated satisfaction of the performance obligation. Revenue recognition may be accelerated in the event of early termination of programs or if our expectations change. Alternatively, revenue recognition may be decelerated if programs are extended or delayed. While such changes to our estimates have no impact on our reported cash flows, the timing of revenue recorded in future periods could be materially impacted. Research and Development Research and development costs are expensed as incurred. Research and development costs include personnel costs, clinical trials, external contract research and development expenses, raw materials, drug product manufacturing costs and allocated overhead – including depreciation, rent and utilities. Research and development costs that are paid in advance of performance are capitalized as a prepaid expense and amortized over the service period as the services are provided. Stock-based Compensation Stock-based compensation is recognized for all share-based payments based on the estimated fair value as of the date of grant. The fair value of our stock options is calculated using the Black-Scholes option pricing model, which requires judgmental assumptions including volatility, risk-free interest rate, expected term and dividend yield. The fair value of restricted stock units (“RSUs”) is based on the closing price of our common stock on the award date. Stock-based compensation expense is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis. For performance-based awards where the vesting of the options may be accelerated upon the achievement of certain milestones, the related stock-based compensation is recognized as expense when it is probable the milestone will be met. We recognize forfeiture of awards as they occur rather than estimating the expected forfeiture rate. Income Taxes Income taxes are accounted for using an asset and liability approach that requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the consolidated financial statement and tax bases of assets and liabilities at the applicable enacted tax rates. We establish a valuation allowance for deferred tax assets if it is more likely than not that these items will expire before we are able to realize their benefits or that future deductibility is uncertain. We recognize the tax benefit from uncertain tax positions only if it is more likely than not that the tax position will be sustained on examination by the tax authorities, based on the technical merits of the position. The tax position is measured based on the largest benefit that has a greater than 50% likelihood of being realized upon ultimate settlement. We recognize interest and penalties related to income tax matters in income tax expense, if incurred. Comprehensive Income (Loss) Comprehensive income (loss) is comprised of net loss and certain changes in equity excluded from net loss. For the years ended December 31, 2023, 2022 and 2021, comprehensive income (loss) consisted of unrealized gains and losses on our investments and unrealized gains and losses on foreign currency translation. Foreign Currency Translation Our wholly-owned Australian subsidiary uses the Australian dollar as its functional currency. All assets and liabilities related to this subsidiary are translated using period-end exchange rates, and revenues and expenses are translated at average exchange rates for the year. Translation adjustments are included as components of comprehensive income (loss) in the Consolidated Statements of Operations and Comprehensive Income (Loss) . Recently Issued Accounting Pronouncements In December 2023, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update ("ASU”) 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures , which requires public entities to disclose disaggregated information about their effective tax rate reconciliation as well as expanded information on income taxes paid by jurisdiction. The disclosure requirements will be applied on a prospective basis, with the option to apply them retrospectively. The standard is effective for annual periods beginning after December 15, 2024, with early adoption permitted. We plan to adopt this guidance for the fiscal year ending December 31, 2025. We are currently evaluating the impact of this new guidance on our disclosure requirements related to the new standard, but do not expect it to have a material impact on our financial condition, results of operations, or cash flows. In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures , which requires enhanced segment disclosures. Public entities with a single reporting segment are required to provide all disclosures required by Topic 280. This standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024, with early adoption permitted. The enhanced disclosures are required to be applied retrospectively to all prior periods presented in the financial statements. We plan to adopt this guidance for the fiscal year ending December 31, 2024. We are currently evaluating the impact of this new guidance on our disclosure requirements related to the new standard, but do not expect it to have a material impact on our financial condition, results of operations, or cash flows.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding during the period. For each period presented, the increase in the number of shares outstanding impacts the comparability of our net loss per share. The net loss per share for the year ended December 31, 2023 reflects the November 2023 issuance of 9,791,832 shares of common stock in an underwritten public offering, including the subsequent partial exercise of the underwriters’ over-allotment option, 919,413 shares of common stock sold under our at-the-market offering in June 2023, and 2,711,389 shares of common stock issued throughout 2023 related to the exercise of warrants. The net loss per share for the year ended December 31, 2022, reflects the issuance of 15,509,282 shares of common stock sold in an underwritten public offering in September 2022, including the subsequent partial exercise of the underwriters’ over-allotment option in October 2022. The net loss per share for the year ended December 31, 2021, reflects 6,489,357 shares of our common stock issued pursuant to a private placement financing completed in September 2021 and 951,980 shares of our common stock issued in December 2021 pursuant to a private placement financing with Amgen. The common stock issuable upon the conversion or exercise of the following dilutive securities has been excluded from the diluted net loss per share calculation because their effect would have been anti-dilutive. Diluted net loss per share, therefore, is the same as basic net loss per share for the periods presented. December 31, 2023 2022 2021 Common stock warrants 2,369,351 3,656,497 3,666,435 Prefunded warrants to purchase common stock 6,102,127 5,182,197 5,182,197 Stock options and RSUs outstanding 8,013,824 7,236,758 5,877,309 Total 16,485,302 16,075,452 14,725,941

Cash Equivalents and Investment

Cash Equivalents and Investments	12 Months Ended
	Dec. 31, 2023
Cash, Cash Equivalents, and Short-Term Investments [Abstract]
Cash Equivalents and Investments	Cash Equivalents and Investments The amortized cost and fair value of our cash equivalents and investments are as follows (in thousands): December 31, 2023 Amortized Cost Gross unrealized gains Gross unrealized losses Fair market value Money market funds $ 31,530 $ — $ — $ 31,530 U.S. treasury bills 217,857 377 (47) 218,187 U.S. agency securities 30,412 14 (57) 30,369 Corporate debt securities and commercial paper 84,425 91 (24) 84,492 Total $ 364,224 $ 482 $ (128) $ 364,578 Classified as: Cash equivalents $ 40,531 Short-term investments 283,491 Long-term investments 40,556 Total $ 364,578 December 31, 2022 Amortized Cost Gross unrealized gains Gross unrealized losses Fair market value Money market funds $ 11,004 $ — $ — $ 11,004 U.S. treasury bills 113,964 5 (721) 113,248 U.S. agency securities 10,921 5 (11) 10,915 Non-U.S. government securities 6,060 — (22) 6,038 Corporate debt securities and commercial paper 129,940 4 (399) 129,545 Total $ 271,889 $ 14 $ (1,153) $ 270,750 Classified as: Cash equivalents $ 11,004 Short-term investments 224,265 Long-term investments 35,481 Total $ 270,750 All investments held as of December 31, 2023 and 2022 were classified as available-for-sale debt securities and consist of highly liquid funds with high credit ratings that, on their date of purchase, had a contractual maturity of two years or less. Realized gains on investments were $33,000 for the year ended December 31, 2023, and zero for the years ended December 31, 2022 and 2021,. There were no realized losses on investments for the years ended December 31, 2023, 2022, or 2021. For investments in an unrealized loss position as of the respective balance sheet dates, the following table summarizes the fair value and gross unrealized losses by category, disaggregated by the length of time that individual debt securities have been in a continuous unrealized loss position (in thousands): December 31, 2023 Less Than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses U.S. treasury bills $ 12,779 $ (25) $ 6,994 $ (22) $ 19,773 $ (47) U.S. agency securities 26,248 (57) — — 26,248 (57) Corporate debt securities and commercial paper 27,477 (24) — — 27,477 (24) Total $ 66,504 $ (106) $ 6,994 $ (22) $ 73,498 $ (128) December 31, 2022 Less Than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses U.S. treasury bills $ 82,694 $ (368) $ 17,648 $ (353) $ 100,342 $ (721) U.S. agency securities 7,552 (11) — — 7,552 (11) Non-U.S. government securities 3,012 (16) 3,026 (6) 6,038 (22) Corporate debt securities and commercial paper 27,839 (195) 22,171 (204) 50,010 (399) Total $ 121,097 $ (590) $ 42,845 $ (563) $ 163,942 $ (1,153)

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Cash and cash equivalents, restricted cash, receivables, accounts payable and accrued liabilities, which are recorded at invoiced amount or cost, approximate fair value based on the short-term nature of these financial instruments. Fair value is defined as the exchange price received for an asset or paid to transfer a liability, or an exit price, in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value, is 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 inactive markets, 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 supported by little or no market activity and significant to the fair value of the assets or liabilities. As of December 31, 2023 and 2022, cash of $3.4 million and $2.4 million, respectively, is excluded from the fair value table below. We had no financial liabilities measured at fair value for the periods presented and the following tables summarize our financial assets measured at fair value on a recurring basis (in thousands): Assets: December 31, 2023 Level 1 Level 2 Level 3 Total Money market funds $ 31,530 $ — $ — $ 31,530 U.S. treasury bills 218,187 — — 218,187 U.S. agency securities — 30,369 — 30,369 Corporate debt securities and commercial paper — 84,492 — 84,492 Total $ 249,717 $ 114,861 $ — $ 364,578 Assets: December 31, 2022 Level 1 Level 2 Level 3 Total Money market funds $ 11,004 $ — $ — $ 11,004 U.S. treasury bills 113,248 — — 113,248 U.S. agency securities — 10,915 — 10,915 Non-U.S. government securities — 6,038 — 6,038 Corporate debt securities and commercial paper — 129,545 — 129,545 Total $ 124,252 $ 146,498 $ — $ 270,750 Our Level 2 assets consist of U.S. agency securities, non-U.S government securities, corporate debt securities, and commercial paper. We review trading activity and pricing for our available-for-sale securities as of the measurement date. When sufficient quoted pricing for identical securities is not available, we use market pricing and other observable market inputs for similar securities obtained from various third-party data providers. These inputs either represent quoted prices for similar assets in active markets or have been derived from observable market data.

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment, net, consists of the following (in thousands): December 31, 2023 2022 Laboratory equipment $ 3,472 $ 3,191 General equipment and furniture 459 459 Computer equipment and software 268 217 Leasehold improvements 118 118 Property and equipment, at cost 4,317 3,985 Less accumulated depreciation and amortization (2,833) (2,401) Property and equipment, net $ 1,484 $ 1,584

Additional Balance Sheet Inform

Additional Balance Sheet Information	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Additional Balance Sheet Information	Additional Balance Sheet Information Prepaid expenses and other current assets consist of the following (in thousands): December 31, 2023 2022 Prepaid research and development $ 1,025 $ 1,881 Prepaid insurance 521 368 Prepaid other 327 394 Other receivables 582 317 Prepaid expenses and other current assets $ 2,455 $ 2,960 Accrued liabilities consist of the following (in thousands): December 31, 2023 2022 Research and development services $ 12,838 $ 7,657 Employee compensation 7,970 4,110 Legal and professional fees 283 334 Accrued taxes 4 1,847 Accrued other 92 55 Accrued liabilities $ 21,187 $ 14,003

Long-term Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2023
Debt Disclosure [Abstract]
Long-term Debt	Long-term Debt In August 2019, we entered into an Amended and Restated Loan and Security Agreement (the “Loan Agreement”) with Silicon Valley Bank (“SVB”), pursuant to which SVB agreed to extend term loans to us with an aggregate principal amount of up to $15.0 million (the “Term Loans”). Borrowings under the Loan Agreement consisted of up to three separate tranches. The initial tranche of $5.0 million was funded in August 2019, the second tranche of $5.0 million was funded in March 2020, and we did not draw down the final tranche of $5.0 million, which expired in July 2020. The Term Loans accrued interest at a floating per annum rate of 0.25% above the prime rate, subject to a floor of 5.75%, payable monthly commencing in September 2019. The Term Loans were interest only until September 30, 2020, however, under the Loan Agreement our interest only period automatically extended to June 30, 2021, if we received aggregate new capital of at least $40.0 million no later than June 30, 2020. We met this milestone in June 2020 in conjunction with the execution of our agreement with AbbVie, discussed in detail in Note 10 . In connection with the Loan Agreement, we issued SVB warrants to purchase up to 52,083 shares of our common stock at a price of $4.32 per share, 17,361 shares of which became exercisable in August 2019 after we drew down the initial tranche. In March 2020, after we drew down the second tranche of our Term Loans, an additional 17,361 shares became exercisable. The remaining warrants did not vest and expired upon the expiration of the third tranche of our Term Loans. The fair value of the warrants on the date of issuance for the initial tranche and second tranche was $60,000 and $60,000, respectively, determined using the Black-Scholes option-pricing model, and was recorded as a component of equity and as a debt discount in our accompanying Consolidated Balance Sheets . In May 2023, we voluntarily repaid in full the remaining outstanding carrying value of $1.4 million of our Term Loans, which consisted of $800,000 in principal and final payments of $625,000, plus prorated accrued interest. In connection with the debt payoff, we wrote-off the remaining unamortized debt discount. As of December 31, 2023, we had no remaining balance outstanding under our Loan Agreement with SVB.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Operating Leases In March 2019, we entered into a lease for office and laboratory space located at 188 East Blaine Street, Seattle, Washington. The term of the lease is 10.8 years with one option to extend the term by 5.0 years. Our option to extend the rental term of our lease was not considered reasonably certain as of December 31, 2023. The lease term commenced in June 2019. The “Rent Commencement Date” began in March 2020, nine months after the commencement date. The annual base rent under the lease is $1.7 million for the first year and will increase by 3.0% each year thereafter. We were not required to pay base rent from the Rent Commencement Date through November 2020, the last day of the ninth month following the Rent Commencement Date. We received a tenant improvement allowance of $5.4 million, which is included in our base rent, and a maximum additional tenant improvement allowance of $1.8 million, which will result in additional rent amortized over the term of the lease at an annual rate of 8.0%. The lease also requires us to pay additional amounts for operating and maintenance expenses. In connection with the lease, we have provided to our landlord a $254,000 letter of credit as a security deposit, which is recorded as noncurrent restricted cash in our accompanying Consolidated Balance Sheets . As of December 31, 2023 , our operating lease right-of-use assets and operating lease liability associated with our lease were $7.5 million and $9.9 million, respectively. Supplemental operating lease information is as follows (in thousands): Years Ended December 31, 2023 2022 2021 Operating lease cost $ 1,843 $ 1,839 $ 1,837 Variable lease cost 917 823 603 Total lease cost $ 2,760 $ 2,662 $ 2,440 Other information: Cash paid for amounts included in the measurement of lease liabilities $ 2,062 $ 2,009 $ 2,117 Weighted-average remaining lease term (years) 6.2 7.2 8.2 Weighted-average discount rate 10.7% 10.7% 10.7% Variable lease costs represent our share of the landlord’s operating expenses. We do not act as a lessor or have any leases classified as financing leases. Maturities of our operating lease liabilities as of December 31, 2023, are as follows (in thousands): Minimum Lease Payments 2024 $ 1,939 2025 2,167 2026 2,224 2027 2,283 2028 2,344 Thereafter 2,811 Total future minimum lease payments 13,768 Less: imputed interest (3,854) Operating lease liabilities $ 9,914 Contingencies Certain credits received related to our research and development expenditures, which were previously recorded within other income within our Consolidated Statements of Operations and Comprehensive Income (Loss) , were subject to review by foreign taxing authorities. We reached constructive agreement with the Australian Taxation Office (the “ATO”) during 2022 and recorded an estimated current foreign income tax provision of $1.3 million for the expected repayments, of which we paid $590,000 and $543,000 during the years December 31, 2023, and 2022, respectively. In 2023, the remaining balance of $136,000 was fully offset by research and development tax credits by the ATO, which were recorded within other income in our Consolidated Statements of Operations and Comprehensive Income (Loss) . As of December 31, 2023, we have no further liabilities associated with the ATO.

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License and Collaboration Agreements	License and Collaboration Agreements AbbVie In June 2020, we entered into an option and license agreement with AbbVie (as amended, the “AbbVie Agreement”) for the development of ALPN-101 (“acazicolcept”). which grants AbbVie an exclusive option to take an exclusive license to acazicolcept (the “License Option”). In December 2023, we and AbbVie amended the AbbVie Agreement (the “AbbVie Amendment”) and agreed to stop enrollment in our global, randomized, double-blind, placebo-controlled Phase 2 study of acazicolcept in adults with moderate-to-severe SLE (“Synergy”) to allow for an early assessment of the study’s clinical trial data. Currently enrolled patients will be allowed to complete their study treatment protocol. Amended payment terms are discussed below. Under the AbbVie Agreement, the License Option is exercisable by AbbVie at any time and will expire 90 days from the delivery of an agreed-upon data package by us to AbbVie. If AbbVie exercises the License Option, AbbVie will take over the future development and commercialization. Prior to the exercise of the License Option, we will perform research and development services, including completing our remaining deliverables under the Synergy study, based on an agreed-upon development plan (the “Development Plan”). We are fully responsible for all costs incurred to conduct the activities under the Development Plan, provided that, AbbVie may be responsible for increased costs under the Development Plan in connection with certain material amendments, if proposed by AbbVie. We are also responsible, at our sole cost and expense, for manufacturing and regulatory filings for acazicolcept necessary to complete activities under the Development Plan. In June 2020, in connection with the execution of the AbbVie Agreement, AbbVie paid us a nonrefundable upfront payment of $60.0 million. Prior to the exercise of the License Option, AbbVie also agreed to make cash payments upon our achievement of certain development milestones (the “Alpine Development Milestones”) up to an aggregate amount of $75.0 million, of which $45.0 million was achieved in 2021, and the remaining $30.0 million was removed under the AbbVie Amendment. If AbbVie exercises the License Option, they will pay a one-time cash payment of $10.0 million (reduced from $75.0 million under the AbbVie Agreement). Following the exercise of the License Option, AbbVie has furthermore agreed to make aggregate cash payments per the AbbVie Amendment of up to $153.8 million upon AbbVie’s achievement of certain development and commercial milestones and additional aggregate cash payments of up to $337.5 million upon AbbVie’s achievement of certain sales-based cash milestones, collectively referred to as the “AbbVie Milestones”. Lastly, subsequent to commercialization, we are also eligible to receive mid-single-digit percentage royalties on worldwide net sales of licensed products. For revenue recognition purposes, we determined that our contractual promises in the AbbVie Agreement are not distinct and are interdependent with our performance obligation to provide research and development services under the Development Plan. Thus, all contractual promises related to the upfront payment and Alpine’s Development Milestones were combined into a single performance obligation. We determined the Alpine Development Milestone payments are probable of significant revenue reversal as the achievement is highly dependent on factors outside our control. Therefore, these milestone payments were fully constrained and were not initially included in the transaction price. In June 2021, we re-evaluated and updated the transaction price to include the achieved portion of the Alpine Development Milestones. We will continue to re-evaluate the transaction price each reporting period and update as uncertain events are resolved or other changes in circumstances occur. We expect to recognize the remaining deferred revenue over the remaining life of the Development Plan, which began in June 2020 and ends upon the later of the exercise or expiration of the option. The License Option and the AbbVie Milestones were determined not to be performance obligations at the inception of the contract as they did not represent material rights. If exercised, the License Option and AbbVie Milestones will be accounted for as a separate contract and will be recognized as revenue if and when triggered. Any consideration related to sales-based royalties and profit-sharing payments will be recognized when the related sales occur. Amgen In December 2021, we entered into an exclusive license and collaboration agreement with Amgen (the “Amgen Agreement”) for the development, manufacture, and commercialization of up to four preclinical candidates generated from our unique discovery platform. The agreement includes licensing of one of our existing preclinical biologic therapeutic programs (the “Existing Program”), as well as a research partnership to jointly develop candidates for up to three additional autoimmune and inflammatory disease programs for other designated biological targets (the “Research Programs”). These candidates include previously undisclosed multi-specific fusion protein-based therapeutic candidates for autoimmune and inflammatory diseases. We will advance candidate molecules to predefined preclinical milestones while Amgen will be responsible for the respective costs and, ultimately, Amgen will assume responsibility for development and commercialization activities and costs. In connection with the execution of the Amgen Agreement in December 2021, we entered into a stock purchase agreement under which Amgen purchased 951,980 shares of our common stock in a private placement for approximately $15.76 per share and aggregate proceeds of $15.0 million. The shares were sold at a 25% premium to the volume-weighted average share price of our common stock for a specified 30-day period prior to entering into the agreement. The fair value of the common stock issued to Amgen of $11.9 million was recorded to equity, based on the closing price of common stock on the effective date of the Amgen Agreement. For accounting purposes, the $3.1 million difference between the cash proceeds and the fair value of the common stock was treated as additional consideration attributable to the Amgen Agreement. Under the terms of the agreement, Amgen also paid us a non-refundable upfront payment of $25.0 million in the first quarter of 2022. As of December 31, 2023, we have completed our activities under the Existing Program. We continue to support activities related to the First Research Program. In January 2024, we received a termination notification from Amgen for the non-exclusive license to use the Second Research Program. A Third Research Program was not selected by the respective option deadline. In addition, we are eligible to receive up to $381.0 million per program, or a total of approximately $762.0 million for the remaining programs, in future success-based payments related to development, regulatory and commercial milestones. Furthermore, we are eligible to receive tiered royalties from a mid-single digit percentage to a low double-digit percentage on global net sales. In addition to proceeds from the non-refundable upfront payment, we have recognized $2.1 million in research and development support provided by us through December 31, 2023. For revenue recognition purposes, we determined the transaction price at inception was $28.1 million, which consists of the upfront payment of $25.0 million and the $3.1 million premium on the stock purchase, and that the Existing Program and each Research Program are distinct performance obligations. We allocated revenue to each performance obligation based on its relative stand-alone selling price. The future success-based payments related to development and regulatory milestones are probable of significant revenue reversal as the achievement is highly dependent on factors outside our control. Therefore, these milestone payments are fully constrained and are not initially included in the transaction price. We will continue to re-evaluate the transaction price each reporting period and update as uncertain events are resolved or other changes in circumstances occur. Any consideration related to commercial milestones and royalties will be recognized when the related sales occur. Adaptimmune In May 2019, we entered into a collaboration and licensing agreement with Adaptimmune (the “Adaptimmune Agreement”) to develop next-generation SPEAR T cell products. Under this agreement, we performed certain research services and granted Adaptimmune a worldwide exclusive license to programs from our secreted immunomodulatory protein and transmembrane immunomodulatory protein technologies. Through December 31, 2023, we have recorded a total of $3.0 million in license payments under the terms of the Adaptimmune Agreement, consisting of a $2.0 million upfront license payment received in June 2019, and an additional $1.0 million license fee upon Adaptimmune’s selection of an additional research program in June 2022. Furthermore, through December 31, 2023, we have recorded $2.1 million in research support payments received in connection with research services completed by us. If respective pre-specified milestones for each of the two active research programs are achieved, we are eligible for downstream development and commercialization milestones of up to an aggregate amount of $105.0 million, and we are also eligible to receive low-single digit percentage royalties on worldwide net sales of the applicable products. Contract balances and revenue recognition We report contract assets resulting from unconditional rights to consideration related to upfront payments, and for completed but unpaid research and development services within accounts receivable in our accompanying Consolidated Balance Sheets . Contract liabilities, representing advance consideration for licensing rights bundled with research and development services and other promises for which the underlying performance obligations have not yet been satisfied, are reported as deferred revenue in our accompanying Consolidated Balance Sheets . Contract liabilities are presented as current and noncurrent based on estimated timing of when the underlying performance obligations will be met. Respective balances are as follows (in thousands): December 31, 2023 2022 Contract Assets $ 167 $ 392 Contract Liabilities $ 17,217 $ 74,756 We use cost-based input methods to measure progress towards completion of our performance obligations and to calculate the corresponding revenue to recognize under our contracts with customers each period. In applying the cost-based input, we use actual costs incurred relative to budgeted costs for each combined performance obligation. Actual costs consist primarily of labor related to internal personnel, and third-party contracts. Revenue is recognized based on the percentage of costs incurred relative to the total estimated costs for the performance obligation. A cost-based input method of revenue recognition requires management to estimate the costs to complete our performance obligations. In making such estimates, significant judgment is required to evaluate assumptions related to cost estimates. The cumulative effect of revisions to estimated costs to complete our performance obligations will be recorded in the period in which changes are identified and amounts can be reasonably estimated. A significant change in these assumptions and estimates could have a material impact on the timing and amount of revenue recognized in future periods. Collaboration revenue recognized in our accompanying Consolidated Statements of Operations and Comprehensive Income (Loss) , disaggregated by customer, is as follows (in thousands): Years Ended December 31, 2023 2022 2021 AbbVie $ 41,925 $ 17,064 $ 23,443 Amgen 16,487 11,964 — Adaptimmune 464 1,036 — Total collaboration revenue $ 58,876 $ 30,064 $ 23,443 Revenue recognized during the period that was included in the opening contract liab ility balance was $57.5 million, $28.2 million and $23.4 million for the years ended December 31, 2023, 2022 and 2021, respectively. For 2023, the decrease in the contract liability balance includes a cumulative catch-up adjustment of $20.4 million recognized upon the contract modification related to the AbbVie Amendment, which resulted in down ward revision of the projected program duration and a corresponding decrease in total direct and indirect development costs to complete the program, and $6.7 million in revenue recognized under the Amgen Agreement upon expiration of Amgen’s option to select an additional Third Research Program. For 2022, the change in the contract liability balance for AbbVie was reduced by a cumulative catch-up adjustment related to upward revisions to the projected program duration and corresponding increase in estimated total direct and indirect development costs to complete the program.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Stockholders’ Equity	Stockholders’ Equity Common Stock Shares of common stock reserved for future issuance were as follows: December 31, 2023 2022 Shares to be issued upon exercise of outstanding stock options 7,968,360 7,058,140 Shares to be issued upon release of RSUs 45,464 178,618 Shares to be issued upon conversion of common stock warrants 2,369,351 3,656,497 Shares to be issued upon conversion of prefunded warrants 6,102,127 5,182,197 Shares available for future stock grants 281,984 314,523 Shares to be issued under employee stock purchase plan 45,211 45,211 Shares of common stock reserved for future issuance 16,812,497 16,435,186 Securities Offerings In November 2023, we sold an aggregate of 9,791,832 shares of our common stock (the “2023 Shares”) and prefunded warrants to purchase 3,200,000 shares of common stock (the “2023 Prefunded Warrants”) in an underwritten public offering, including the subsequent partial exercise of the underwriters’ over-allotment option, pursuant to our effective shelf registration statement on Form S-3 (File No. 333-271517). The purchase price for each 2023 Share and 2023 Prefunded Warrant was $12.50 and $12.499 per share, respectively, for an aggregate purchase price of $162.4 million. The 2023 Prefunded Warrants became fully exercisable upon the closing date and have an exercise price of $0.001 per share. In connection with this offering, we received net proceeds of $152.2 million after deducting underwriting discounts, commissions and offering costs. Offering costs of $440,000 were netted against the proceeds within additional-paid-in-capital on our accompanying Consolidated Balance Sheets . In September 2022, we sold an aggregate of 15,509,282 shares of our common stock in an underwritten public offering, including the subsequent partial exercise of the underwriters’ over-allotment option in October 2022, pursuant to our effective shelf registration statement on Form S-3 (File No. 333-256107). We received resulting net proceeds of $106.7 million after deducting underwriting discounts, commissions and offering costs. Offering costs of $438,000 were netted against the proceeds within additional-paid-in-capital on our accompanying Consolidated Balance Sheets . In September 2021, we entered into a securities purchase agreement (the “2021 Securities Purchase Agreement”) for a private placement with a select group of institutional investors, pursuant to which we sold 6,489,357 shares of our common stock (the “2021 Shares”) and prefunded warrants to purchase 3,191,487 shares of common stock (the “2021 Prefunded Warrants”). The purchase price for each 2021 Share and for each 2021 Prefunded Warrant was $9.40 per share, for an aggregate purchase price of approximately $91.0 million. The 2021 Prefunded Warrants became fully exercisable upon the closing date and have an exercise price of $0.001 per share. We incurred $266,000 in offering costs associated with the 2021 Securities Purchase Agreement, which was netted against the proceeds within additional-paid-in-capital on our accompanying Consolidated Balance Sheets . The securities sold and issued under the 2021 Securities Purchase Agreement have not been registered under the Securities Act of 1933, as amended, or state securities laws and may not be offered or sold in the United States absent registration with the SEC or an applicable exemption from such registration requirements. We filed a registration statement for the 2021 Securities Purchase Agreement with the SEC, which was declared effective by the SEC in November 2021 and covers the resale of the shares of common stock issuable in connection with the private placements and upon exercise of the warrants. In September 2021, we entered into an exchange agreement (the “Exchange Agreement”) with Frazier Life Sciences VIII, L.P. (the “Exchanging Stockholder”), which Exchanging Stockholder is affiliated with a member of our board of directors, pursuant to which we exchanged an aggregate of 1,200,000 shares of common stock held by the Exchanging Stockholder for prefunded warrants (the “Exchange Warrants”) to purchase an aggregate of 1,200,000 shares of common stock. Upon the closing of the exchange, we reclassified 1,200,000 shares of common stock into treasury stock on our accompanying Consolidated Balance Sheets . Financing Agreements In April 2023, we entered into a sales agreement (the “2023 Sales Agreement”) with Cowen and Company, LLC (“TD Cowen”) to sell shares of our common stock, from time to time, through an “at-the-market” equity offering for up to $100.0 million in aggregate gross proceeds. TD Cowen will act as the sales agent and will be entitled to compensation for services of up to 3.0% of the gross sales proceeds from shares sold through TD Cowen under the 2023 Sales Agreement. The shares will be issued pursuant to our shelf registration statement on Form S-3 (File No. 333-271517), which was filed with the SEC on April 28, 2023, and declared effective on May 9, 2023. On May 11, 2023, we filed a final prospectus supplement with the SEC relating to the offer and sale of the shares pursuant to the 2023 Sales Agreement. As of December 31, 2023, we have sold 919,413 shares of common stock under the 2023 Sales Agreement for a price of $10.93 per share. The associated gross proceeds of $10.0 million were recorded net of directly related offering costs of $355,000 within additional-paid-in-capital on our accompanying Consolidated Balance Sheets . In September 2022, we and TD Cowen mutually terminated our prior sales agreement (the “2021 Sales Agreement”), which we had entered into in July 2021. The 2021 Sales Agreement provided that we may sell shares of our common stock from time to time through an “at-the-market” equity offering for up to $75.0 million in gross cash proceeds, with TD Cowen acting as the sales agent. No common stock was sold under the 2021 Sales Agreement. In connection with the termination of the 2021 Sales Agreement, we wrote off $488,000 of deferred offering costs to expense, which were previously included within prepaid expenses and other current assets on our accompanying Consolidated Balance Sheets . Treasury Shares In June 2023, we retired 1,250,467 shares of treasury stock, and returned the shares of common stock to the status of authorized but unissued shares. Warrants We have issued warrants in connection with our securities offerings, SVB loans, and to certain non-employee professional advisers. Excluding the prefunded warrants we issued in connection with our securities offerings discussed above, the table below summarizes our common stock warrant activity: Warrants Weighted- Weighted- Outstanding at December 31, 2022 3,656,497 $ 12.66 1.12 Exercised (1,287,146) 12.74 Outstanding and Exercisable at December 31, 2023 2,369,351 $ 12.62 0.15 In January 2024, we issued 1,694,282 shares of common stock and received $17.6 million in proceeds in connection with exercises of 2,309,404 of our common stock warrants. In addition, in February 2024, we issued 3,199,879 shares of common stock in connection with the exercise of 3,200,000 of our prefunded warrants. Equity Incentive Plans Our board of directors has approved “inducement” grants, pursuant to Rule 5635(c)(4) of the Nasdaq Listing Rules, which are exempt from registration under the Securities Act of 1933, as amended, pursuant to Section 4(a)(2). In August 2023, we issued an “inducement” option grant to purchase 185,000 shares of our common stock at a per-share exercise price of $13.44 to our Chief Technology Officer. In August 2022, we issued an “inducement” option grant to purchase 160,000 shares of our common stock at a per-share exercise price of $8.38 to our Chief Medical Officer. In June 2018, our stockholders approved the 2018 Equity Incentive Plan (the “2018 Plan”). Upon adoption, we ceased granting stock awards under the Nivalis Therapeutics, Inc. 2015 Equity Incentive Plan (the “2015 EIP”) and the Amended and Restated 2015 Stock Plan, collectively, the “Legacy Plans”. All shares of common stock subject to awards under the Legacy Plans that expire or terminate without having been exercised in full, or are forfeited to or repurchased by the company, will be added to the 2018 Plan, up to a maximum of 1,972,784 shares. Under our 2018 Plan we may issue stock options, stock appreciation rights, restricted stock, RSUs or performance shares. As of December 31, 2023, we have only issued stock options and RSUs. Our 2018 Plan provides for an annual increase in the number of shares reserved for issuance equal to the lesser of (1) 5% of the number of shares of common stock outstanding as of the last day of the preceding calendar year or (2) 1,500,000. However, our board of directors may act prior to January 1 of a given year to provide that there will be no January 1 increase for such year or that the increase for such year will be a lesser number of shares. On January 1, 2024, a total of 1,500,000 additional shares was automatically added to the shares authorized under the 2018 Plan. In July 2017, in connection with the merger with Nivalis, we assumed Nivalis’ Employee Stock Purchase Plan (the “ESPP”) and the 2015 EIP. Upon assumption of the ESPP, there were 45,211 shares available for issuance under the ESPP. As of December 31, 2023, we have not activated the ESPP. Stock options granted to our employees under our equity plans generally vest within four years, and vested options are exercisable from the vest date until ten years after the date of grant. Vesting of certain employee options may be accelerated in the event of a change in control of the Company. We grant stock options to employees with exercise prices equal to the fair value of our common stock on the date of grant. The term of incentive stock options may not exceed ten years from the date of grant. As of December 31, 2023, a total of 9,616,862 shares of common stock were authorized for issuance under our 2018 Plan and Legacy Plans. A summary of stock option activity under our plans is presented below: Options Weighted- Weighted- Aggregate Outstanding at December 31, 2022 7,058,140 $ 8.37 Granted 2,816,197 $ 9.50 Exercised (858,511) $ 5.91 Forfeited (1,047,466) $ 10.40 Outstanding at December 31, 2023 7,968,360 $ 8.77 6.96 $ 82,254 Vested and expected to vest after December 31, 2023 7,790,360 $ 8.79 6.95 $ 80,307 Exercisable at December 31, 2023 4,462,145 $ 7.79 5.62 $ 50,542 The aggregate intrinsic value of stock options exercised during the years ended December 31, 2023, 2022 and 2021 was $7.6 million, $1.1 million and $546,000, respectively. The fair value of stock options vested during the years ended December 31, 2023, 2022 and 2021 was $9.6 million, $8.6 million and $3.4 million, respectively. A summary of our RSU activity under our plans is presented below: Number of Shares Weighted- Weighted- Aggregate Non-vested at December 31, 2022 178,618 $ 12.00 Vested (114,651) $ 12.00 Forfeited (18,503) $ 12.00 Non-vested at December 31, 2023 45,464 $ 12.00 0.0 $ 867 The aggregate intrinsic value of RSUs vested during the years ended December 31, 2023 and 2022 was $1.0 million and $527,000, respectively. The fair value of RSUs vested during the years ended December 31, 2023 and 2022 was $1.4 million and $735,000, respectively. No RSUs vested during the year ended December 31, 2021. We utilize newly issued shares to satisfy option exercises and RSU releases. As of December 31, 2023, the re was $22.4 million of unrecognized stock-based compensation expense related to approximately 3.6 million nonvested stock options and RSU awards that is expected to be recognized over a weighted-average remaining period of 2.7 years. Stock-Based Compensation Expense We use the Black-Scholes option pricing model to estimate the fair value of stock options at the grant date. The fair value of RSUs is equal to the closing stock price on the date of grant. The Black-Scholes option pricing model requires us to make certain estimates and assumptions, including assumptions related to the expected price volatility of our stock, the period during which the options will be outstanding, the rate of return on risk-free investments, and the expected dividend yield of our stock. The fair values of stock options granted to employees were calculated using the following assumptions: Years Ended December 31, 2023 2022 2021 Weighted-average estimated fair value at grant $6.94 $7.56 $8.44 Risk-free interest rate (1) 3.36% - 4.67% 1.39% - 4.04% 0.25% - 1.3% Expected term of options (in years) (2) 5.27 - 6.08 5.27 – 6.08 3.49 – 6.08 Expected stock price volatility (3) 81% - 85% 80% - 83% 79% - 83% Expected dividend yield (4) —% —% —% (1) The risk-free interest rate assumption was based on zero-coupon U.S. Treasury instruments that had terms consistent with the expected term of our stock option grants. (2) We used the “simplified method” for options to determine the expected term of stock options granted, since we do not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term due to the limited time our shares have been publicly traded. Under this approach, the weighted-average expected life is presumed to be the average of the vesting term and the contractual term of the option. (3) Volatility is a measure of the amount by which a financial variable, such as share price, has fluctuated or is expected to fluctuate during a period. We analyzed the historical stock price volatility of companies at a similar stage of development to estimate the expected volatility of our stock price. (4) We have never declared or paid any cash dividends and do not presently plan to pay cash dividends in the foreseeable future. Stock-based compensation expense is classified in the Consolidated Statements of Operations and Comprehensive Income (Loss) as follows (in thousands): Years Ended December 31, 2023 2022 2021 Research and development $ 6,251 $ 5,713 $ 3,347 General and administrative 4,379 3,935 2,893 Total stock-based compensation expense $ 10,630 $ 9,648 $ 6,240

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Our loss before taxes is derived from domestic (United States) and foreign (Australian) sources as follows (in thousands): Years Ended December 31, 2023 2022 2021 Domestic $ (32,450) $ (55,475) $ (48,932) Foreign 369 43 (1,488) Total $ (32,081) $ (55,432) $ (50,420) The provision for income taxes is composed of the following (in thousands): Years Ended December 31, 2023 2022 2021 Current: U.S. - Federal $ 47 $ (981) $ — U.S. - State 4 (16) — Foreign — (1,285) (129) Total current 51 (2,282) (129) Deferred: Foreign (154) (48) 216 Total deferred (154) (48) 216 Total income tax (expense) benefit $ (103) $ (2,330) $ 87 The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Years Ended December 31, 2023 2022 2021 U.S. Statutory rate 21.0 % 21.0 % 21.0 % Effect of: State taxes (net of federal benefit) 0.3 % 0.1 % — % Permanent differences 0.3 % — % (0.4) % Federal research and development credit 9.2 % 5.0 % 4.5 % Change in valuation allowance (29.1) % (28.6) % (21.4) % Global intangible low-taxed income recapture — % (0.6) % (1.4) % Stock-based compensation (1.6) % (1.9) % (1.8) % Foreign rate differential (0.1) % — % 0.2 % Other (0.3) % — % (0.5) % Voluntary disclosure net change in prior years’ tax liabilities — % 0.8 % — % Effective income tax rate (0.3) % (4.2) % 0.2 % For the year ended December 31, 2023, tax expense of $103,000 primarily represents deferred tax expense from the application of a full valuation allowance against our remaining foreign deferred tax assets, partially offset by the recognition of current income tax benefits resulting from the finalization of our prior period tax returns. For the year ended December 31, 2022, $1.0 million of our tax expense represents the recognition of current domestic federal and state income tax liabilities, which was a result of the mandatory capitalization of research and development expenses under Internal Revenue Code (the “IRC”) Section 174. As part of the Tax Cuts and Jobs Act of 2017, beginning with the 2022 tax year, expenses that are incurred for research and development in the U.S. will be capitalized and amortized over five years, and expenses that are incurred for research and experimentation outside the U.S. will be capitalized and amortized over 15 years. For the year ended December 31, 2022, $1.3 million of our tax expense represents the recognition of current tax liabilities, which were the result of a cumulative change to our foreign income tax provision for the activities of Alpine Immune Sciences Australia PTY LTD. In June 2022, Alpine Immune Sciences Australia PTY LTD’s was notified that the tax return filed under a revised transfer pricing model was accepted for the six-month period ended December 31, 2019. Following the acceptance notification, the tax provisions for each open tax year have been recalculated using the same transfer pricing methodology underlying the accepted return. Additionally, consistent with the change to our transfer pricing methodology, we recorded $48,000 of deferred tax expense resulting from a reduction to our deferred tax assets for the year ended December 31, 2022. The $87,000 tax benefit for the year ended December 31, 2021, represents deferred tax benefits resulting from the finalization of our prior-period tax returns and the removal of the valuation allowance in place against our foreign deferred tax assets, partially offset by income tax expense for current year activity. For the years ended December 31, 2023, 2022, and 2021, the difference between the U.S. federal statutory tax rate of 21% and our effective tax rates in all periods is primarily due to changes in our valuation allowance related to our deferred tax assets and the generation and consumption of federal research and development tax credits. We have elected to treat taxes due on future U.S. inclusions in taxable income under the global intangible low-taxed income (the “GILTI”) provision as a current-period expense when incurred. As such, expected future GILTI inclusions have not been factored into the measurement of our deferred taxes. Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. The following table represents the significant components of our deferred tax assets and liabilities for the periods presented (in thousands): December 31, 2023 2022 Deferred tax assets: Net operating loss $ 23,205 $ 18,590 Research and development credits 9,259 6,240 Intangible asset basis 4 2 Lease liability 2,126 2,315 Deferred revenue 3,600 14,752 Stock based compensation 2,428 2,343 Capitalized research and experimental costs 24,147 12,681 Other 1,088 156 Gross deferred tax assets 65,857 57,079 Valuation allowance (63,897) (54,841) Total deferred tax assets, net of valuation allowance 1,960 2,238 Deferred tax liabilities: Prepaid expenses (251) (173) Fixed asset basis (126) (182) Right-of-use asset basis (1,583) (1,728) Total deferred tax liability (1,960) (2,083) Net deferred tax assets and liabilities $ — $ 155 A valuation allowance is provided for deferred tax assets where the recoverability of the assets is uncertain. The determination to provide a valuation allowance is dependent upon the assessment of whether it is more likely than not that sufficient taxable income will be generated to utilize the deferred tax assets. For the year ended December 31, 2023, we determined that based on an evaluation of our sources of income and all available evidence, both positive and negative, including our latest forecasts and cumulative losses in recent years, it was more likely than not that none of our domestic deferred tax assets would be realized and therefore we continued to record a full valuation allowance. For the year ended December 31, 2023, we determined that it was more likely than not that none of our foreign deferred tax assets would be realized. As such, we have recorded a full valuation allowance against all foreign deferred tax assets. The valuation allowance increased by $9.1 million and $16.1 million during the years ended December 31, 2023 and 2022, respectively. We have net operating loss (“NOL”) carryforwards as follows (in thousands): December 31, 2023 2022 Federal (after January 1, 2018) $ 108,393 $ 85,012 State $ 6,778 $ 6,433 Federal NOL carryforwards created after January 1, 2018, carry forward indefinitely, and our state NOL carryforwards begin to expire in 2038. We have net research and development tax credit carryforwards as follows (in thousands): December 31, 2023 2022 Federal $ 11,584 $ 7,892 Foreign $ 578 $ 527 Federal research and development tax credit carryforwards begin to expire in 2035, and our foreign research and development tax credits carry forward indefinitely. Current tax laws impose substantial restrictions on the utilization of research and development credit and NOL carryforwards in the event of an ownership change, as defined by the IRC Section 382 and 383. Such an event may limit our ability to utilize NOLs and research and development tax credit carryforwards. We have commissioned Section 382 studies, focused on AIS Operating Co., Inc., evaluating changes in ownership for periods from inception through December 31, 2023. We concluded that we experienced ownership changes on June 10, 2016, and September 17, 2021, and as a result, our ability to use research and development tax credit and NOL carryforwards created prior to September 17, 2021, has been limited. We have incorporated the resulting limitations into our provision for income taxes for the years ended December 31, 2023 and 2022. Though limited in the amount which can be utilized for the year ended December 31, 2023, and annually thereafter, the restrictions do not definitively prevent the impacted research and development tax credit carryforwards from utilization in future years and before expiration. As such, we have not reduced our research and development tax credit carryforwards as of December 31, 2023. Additionally, under IRC Section 382 and 383, the 2017 merger with Nivalis is likely considered an ownership change with respect to the potential limitation of the Nivalis federal tax credits and NOLs. As such, it is likely that any future utilization of Nivalis federal tax credits and NOLs is substantially limited. Therefore, as of December 31, 2018, all Nivalis tax credit and NOL carryforwards have been reduced to zero. We account for uncertainty in income taxes in accordance with ASC 740, Income Taxes. Tax positions are evaluated in a two-step process, whereby we first determine whether it is more likely than not that a tax position will be sustained upon examination by the tax authority, including resolutions of any related appeals or litigation processes, based on technical merit. If a tax position meets the more-likely-than-not recognition threshold it is then measured to determine the amount of benefit to recognize in the financial statements. The tax position is measured as the largest amount of benefit that is greater than 50% likely of being realized upon ultimate settlement. The following table summarizes the activity related to unrecognized tax benefits (in thousands): Years Ended December 31, 2023 2022 2021 Unrecognized benefits – beginning of year $ 2,149 $ 1,553 $ 990 Gross increases (decreases) – prior year tax positions (2) — — Gross increases – current year tax positions 727 596 563 Unrecognized benefit – end of year $ 2,874 $ 2,149 $ 1,553 All of the unrecognized tax benefits as of December 31, 2023, are accounted for as a reduction in our deferred tax assets. Due to our valuation allowance, none of the $2.9 million of unrecognized tax benefits would affect our effective tax rate, if recognized. We do not expect any significant change in our unrecognized tax benefits during the next twelve months. We recognize interest and penalties related to unrecognized tax benefits as income tax expense. There were no accrued interest or penalties related to unrecognized tax benefits for the years ended December 31, 2023, 2022, or 2021. Our material income tax jurisdictions are the United States (federal), California (state), and Australia (foreign). We are subject to audit for tax years 2012 and forward for federal purposes, 2017 and forward for California purposes, and 2019 and forward for foreign purposes.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2023
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions In February 2024, certain of our stockholders whose beneficial ownership exceeded 5%, exercised 3,200,000 prefunded warrants pursuant to a net exercise mechanism under the warrants. In January 2024, certain of our affiliates or stockholders whose beneficial ownership exceeded 5%, exercised 1,379,887 common stock warrants for proceeds of $17.6 million. In December 2023, certain of our affiliates or stockholders whose beneficial ownership exceeded 5% exercised 219,692 common stock warrants for proceeds of $1.9 million.

401(k) Retirement Plan

401(k) Retirement Plan	12 Months Ended
	Dec. 31, 2023
Retirement Benefits [Abstract]
401(k) Retirement Plan	401(k) Retirement Plan We have adopted a 401(k) defined contribution retirement savings plan. All employees are eligible to participate, provided they meet the requirements of the plan. Beginning in 2022, we started offering employer 401(k) matching contributions at our discretion subject to annually determined limits. The expense recognized related to the employer matching

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pay vs Performance Disclosure
Net loss	$ (32,184)	$ (57,762)	$ (50,333)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended	12 Months Ended
	Dec. 31, 2023 shares	Dec. 31, 2023 shares
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Terminated	false
Mitchell H. Gold, M.D [Member]
Trading Arrangements, by Individual
Material Terms of Trading Arrangement		On December 28, 2023, Mitchell H. Gold, M.D., our Chief Executive Officer, terminated a Rule 10b5-1 trading arrangement providing for the sale from time to time of an aggregate of 438,000 shares of our common stock. The trading arrangement was adopted on March 30, 2023 and was intended to satisfy the affirmative defense in Rule 10b5-1(c). The duration of the trading arrangement was from June 30, 2023 until June 30, 2024, or earlier if all transactions under the trading arrangement were completed. Dr. Gold sold 213,302 shares under the trading arrangement prior to the date of termination.
Name	Mitchell H. Gold, M.D
Rule 10b5-1 Arrangement Adopted	true
Rule 10b5-1 Arrangement Terminated	true
Termination Date	December 28, 2023
Aggregate Available	438,000	438,000

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2023
Accounting Policies [Abstract]
Basis of Presentation and Use of Estimates	Basis of Presentation and Use of Estimates The accompanying consolidated financial statements have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (“SEC”) and generally accepted accounting principles in the United States of America (“GAAP”). The preparation of financial statements in conformity with GAAP requires management to make judgments, estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Significant estimates inherent in the preparation of the accompanying consolidated financial statements include those used for revenue recognition, accruals for clinical trial activities and other accruals, the potential outcome of uncertain tax positions that have been recognized in our consolidated financial statements or tax returns, and the estimated fair value of equity-based awards. We base our estimates and assumptions on historical experience when available and on various factors we believe to be reasonable under the circumstances. Actual results could differ materially from those estimates.
Principles of Consolidation	Principles of Consolidation Our consolidated financial statements include the financial position and results of operations of Alpine Immune Sciences, Inc. and our wholly owned operating company and subsidiary, AIS Operating Co., Inc., and our wholly owned subsidiary, Alpine Immune Sciences Australia PTY LTD. All inter-company balances and transactions have been eliminated in consolidation.
Segments	Segments We operate as one operating segment and use cash flow as the primary financial measure to manage our business.
Cash and Cash Equivalents and Restricted Cash	Cash and Cash Equivalents and Restricted Cash We consider all highly liquid investments with an original maturity of 90 days or less at the time of purchase to be cash equivalents. Cash and cash equivalents consist of deposits with commercial banks in checking and interest-bearing accounts, highly liquid money market funds, and U.S. Treasury bills. Restricted cash represents cash reserved under our letter of credit, which serves as a security deposit for our operating lease to rent office and laboratory space in Seattle, Washington.
Concentration of Credit Risk	Periodically, we maintain deposits in financial institutions in excess of government insured limits. We believe we are not exposed to significant credit risk as our deposits, which are held at financial institutions, are high credit quality securities such as money market funds, U.S. Treasury bills, and commercial paper. To date, we have not realized any losses on these deposits.
Investments	Investments Our investments include funds invested in highly liquid money market funds, U.S. Treasury bills, U.S. agency securities, non-U.S. government securities, and corporate debt securities and commercial paper with contractual maturities of less than two years. These investments are classified as available-for-sale debt securities, which are recorded at fair value based on quoted prices in active markets for Level 1 assets, and based on market pricing and other observable market inputs for similar securities for Level 2 assets. We classify our investments maturing within one year of the reporting date as short-term investments. If the estimated fair value of a debt security is below its amortized cost basis, we evaluate whether it is more likely than not that we will sell the security before its anticipated recovery in market value and whether credit losses exist for the related securities. A credit loss exists if the present value of expected cash flows is less than the amortized cost basis of the security. Credit-related losses are recognized as an allowance for credit losses on the balance sheet with a corresponding adjustment to earnings. Unrealized gains and losses that are unrelated to credit deterioration are reported in other comprehensive income (loss). Purchase premiums and discounts are recognized within interest income using the interest method over the terms of the securities. Realized gains and losses and declines in fair value for these investments deemed to be expected credit losses are reflected in our Consolidated Statements of Operations and Comprehensive Income (Loss) using the specific-identification method.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, net of accumulated depreciation. Depreciation is recorded using the straight-line method over the estimated useful lives of the assets, generally three
Impairment of Long-lived Assets	Impairment of Long-lived Assets We evaluate our long-lived tangible assets for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets may not be fully recoverable. When such events occur, we assess recoverability by comparing the asset's carrying value to the total undiscounted future cash flows the asset is expected to generate. If the carrying value exceeds the amount of undiscounted cash flows, we then measure impairment by reducing the carrying value to the assets estimated fair value. We did not record any impairments in the years ended December 31, 2023, 2022 or 2021.
Accrued Liabilities	Accrued Liabilities As part of the process of preparing our consolidated financial statements, we are required to estimate accruals for professional services and research and development expenses. This process involves reviewing contracts and vendor agreements and communicating with applicable personnel to identify services that have been performed on our behalf. We estimate the level of service performed and the associated cost incurred for the service when we have not yet been invoiced or otherwise notified of the actual cost. We estimate accrued liabilities as of each balance sheet date based on known facts and circumstances. Although we do not expect our estimates to be materially different from amounts actually incurred, if our estimates of the status and timing of services performed differ from the actual status and timing of services performed, we may report amounts that are too high or too low in any particular period. To date, we have not experienced any significant adjustments to our estimates.
Leases	Leases
Derivative Financial Instruments	Derivative Financial Instruments We evaluate all of our financial instruments, including prefunded warrants and warrants to purchase common stock, to determine if such instruments are derivatives or contain features qualifying as embedded derivatives. For derivative financial instruments accounted for as liabilities, the derivative instrument is initially recorded at its fair value and is then re-valued at each reporting date, with changes in the fair value reported in the Consolidated Statements of Operations and Comprehensive Income (Loss) . We use the Black-Scholes option-pricing model to value the derivative instruments at inception and subsequent valuation dates. The classification of derivative instruments, including whether such instruments should be recorded as liabilities or as equity, is reassessed at the end of each reporting period. We do not use derivative instruments to hedge exposures to cash flow, market or foreign currency risks.
Revenue Recognition	Revenue Recognition Revenue is recognized when control of the promised goods or services is transferred to our customers, in an amount that reflects the consideration we expect to be entitled to in exchange for those goods or services. Our steps for recognizing revenue consist of (1) identifying the contract; (2) identifying the performance obligations as either distinct or bundled goods and services; (3) determining the transaction price associated with each performance obligation for which we expect to be entitled in exchange for transferring such goods and services; (4) allocating the transaction price to the performance obligations in the contract; and (5) recognizing revenue upon satisfaction of performance obligations. Our collaboration agreements principally contain multiple performance obligations, which may include (1) grants of, or options to obtain, intellectual property licenses; (2) research and development services; and/or (3) manufacturing or supply services. Payments received under these arrangements typically include one or more of the following: non-refundable upfront license fees, option exercise fees, payment for research and/or development efforts, amounts due upon the achievement of specified objectives, and/or royalties on future product sales. Our revenue is primarily derived from our collaboration agreements with Adaptimmune Therapeutics plc (“Adaptimmune”), AbbVie Ireland Unlimited Company (“AbbVie”), and Horizon Therapeutics Ireland DAC (acquired by Amgen Inc., or “Amgen,” in October 2023). See further discussion of our collaboration agreements in Note 10 . We allocate revenue to each performance obligation based on its relative stand-alone selling price. We generally determine stand-alone selling prices at the inception of the contract based on our best estimate of what the selling price would be if the deliverable was regularly sold by us on a stand-alone basis. Payments received prior to satisfying the relevant revenue recognition criteria are recorded as deferred revenue in the accompanying Consolidated Balance Sheets and recognized as revenue when the related revenue recognition criteria are met. We recognize revenue under our collaboration agreements by using a cost-based input method to measure progress toward completion of the performance obligation, including employee hours contributed to each performance obligation, and to calculate the corresponding revenue to recognize each period. Our collaboration agreements provide for non-refundable milestone payments. We recognize revenue that is contingent upon the achievement of a substantive milestone in its entirety in the period in which the milestone is achieved. A milestone is considered substantive when the consideration payable to us for such milestone (1) is consistent with our performance necessary to achieve the milestone or the increase in value to the collaboration resulting from our performance; (2) relates solely to our past performance; and (3) is reasonable relative to all of the other deliverables and payments within the arrangement. In making this assessment, we consider all facts and circumstances relevant to the arrangement, including factors such as the scientific, regulatory, commercial, and other risks that must be overcome to achieve the milestone, the level of effort and investment required to achieve the milestone and whether any portion of the milestone consideration is related to future performance or deliverables. We review the contributed employee hours and progress towards completion for each performance obligation under our collaboration agreements, and adjust the revenue recognized to reflect changes in assumptions relating to the estimated satisfaction of the performance obligation. Revenue recognition may be accelerated in the event of early termination of programs or if our expectations change. Alternatively, revenue recognition may be decelerated if programs are extended or delayed. While such changes to our estimates have no impact on our reported cash flows, the timing of revenue recorded in future periods could be materially impacted.
Research and Development	Research and Development Research and development costs are expensed as incurred. Research and development costs include personnel costs, clinical trials, external contract research and development expenses, raw materials, drug product manufacturing costs and allocated overhead – including depreciation, rent and utilities. Research and development costs that are paid in advance of performance are capitalized as a prepaid expense and amortized over the service period as the services are provided.
Stock-based Compensation	Stock-based Compensation Stock-based compensation is recognized for all share-based payments based on the estimated fair value as of the date of grant. The fair value of our stock options is calculated using the Black-Scholes option pricing model, which requires judgmental assumptions including volatility, risk-free interest rate, expected term and dividend yield. The fair value of restricted stock units (“RSUs”) is based on the closing price of our common stock on the award date. Stock-based compensation expense is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis. For performance-based awards where the vesting of the options may be accelerated upon the achievement of certain milestones, the related stock-based compensation is recognized as expense when it is probable the milestone will be met. We recognize forfeiture of awards as they occur rather than estimating the expected forfeiture rate.
Income Taxes	Income Taxes Income taxes are accounted for using an asset and liability approach that requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the consolidated financial statement and tax bases of assets and liabilities at the applicable enacted tax rates. We establish a valuation allowance for deferred tax assets if it is more likely than not that these items will expire before we are able to realize their benefits or that future deductibility is uncertain.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is comprised of net loss and certain changes in equity excluded from net loss. For the years ended December 31, 2023, 2022 and 2021, comprehensive income (loss) consisted of unrealized gains and losses on our investments and unrealized gains and losses on foreign currency translation.
Foreign Currency Translation	Foreign Currency Translation Our wholly-owned Australian subsidiary uses the Australian dollar as its functional currency. All assets and liabilities related to this subsidiary are translated using period-end exchange rates, and revenues and expenses are translated at average exchange rates for the year. Translation adjustments are included as components of comprehensive income (loss) in the Consolidated Statements of Operations and Comprehensive Income (Loss) .
Net Loss Per Share	Net Loss Per ShareBasic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding during the period.
Fair Value Measurements	Fair Value Measurements Cash and cash equivalents, restricted cash, receivables, accounts payable and accrued liabilities, which are recorded at invoiced amount or cost, approximate fair value based on the short-term nature of these financial instruments. Fair value is defined as the exchange price received for an asset or paid to transfer a liability, or an exit price, in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value, is 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 inactive markets, 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 supported by little or no market activity and significant to the fair value of the assets or liabilities.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In December 2023, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update ("ASU”) 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures , which requires public entities to disclose disaggregated information about their effective tax rate reconciliation as well as expanded information on income taxes paid by jurisdiction. The disclosure requirements will be applied on a prospective basis, with the option to apply them retrospectively. The standard is effective for annual periods beginning after December 15, 2024, with early adoption permitted. We plan to adopt this guidance for the fiscal year ending December 31, 2025. We are currently evaluating the impact of this new guidance on our disclosure requirements related to the new standard, but do not expect it to have a material impact on our financial condition, results of operations, or cash flows. In November 2023, the FASB issued ASU 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures , which requires enhanced segment disclosures. Public entities with a single reporting segment are required to provide all disclosures required by Topic 280. This standard is effective for fiscal years beginning after December 15, 2023, and interim periods within fiscal years beginning after December 15, 2024, with early adoption permitted. The enhanced disclosures are required to be applied retrospectively to all prior periods presented in the financial statements. We plan to adopt this guidance for the fiscal year ending December 31, 2024. We are currently evaluating the impact of this new guidance on our disclosure requirements related to the new standard, but do not expect it to have a material impact on our financial condition, results of operations, or cash flows.

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2023
Earnings Per Share [Abstract]
Schedule of Anti-dilutive Securities Excluded from Diluted Net Loss Per Share Attributable to Common Stockholders	The common stock issuable upon the conversion or exercise of the following dilutive securities has been excluded from the diluted net loss per share calculation because their effect would have been anti-dilutive. Diluted net loss per share, therefore, is the same as basic net loss per share for the periods presented. December 31, 2023 2022 2021 Common stock warrants 2,369,351 3,656,497 3,666,435 Prefunded warrants to purchase common stock 6,102,127 5,182,197 5,182,197 Stock options and RSUs outstanding 8,013,824 7,236,758 5,877,309 Total 16,485,302 16,075,452 14,725,941

Cash Equivalents and Investme_2

Cash Equivalents and Investments (Tables)	12 Months Ended
	Dec. 31, 2023
Cash, Cash Equivalents, and Short-Term Investments [Abstract]
Schedule of Amortized Cost and Fair Value of Cash Equivalents and Investments	The amortized cost and fair value of our cash equivalents and investments are as follows (in thousands): December 31, 2023 Amortized Cost Gross unrealized gains Gross unrealized losses Fair market value Money market funds $ 31,530 $ — $ — $ 31,530 U.S. treasury bills 217,857 377 (47) 218,187 U.S. agency securities 30,412 14 (57) 30,369 Corporate debt securities and commercial paper 84,425 91 (24) 84,492 Total $ 364,224 $ 482 $ (128) $ 364,578 Classified as: Cash equivalents $ 40,531 Short-term investments 283,491 Long-term investments 40,556 Total $ 364,578 December 31, 2022 Amortized Cost Gross unrealized gains Gross unrealized losses Fair market value Money market funds $ 11,004 $ — $ — $ 11,004 U.S. treasury bills 113,964 5 (721) 113,248 U.S. agency securities 10,921 5 (11) 10,915 Non-U.S. government securities 6,060 — (22) 6,038 Corporate debt securities and commercial paper 129,940 4 (399) 129,545 Total $ 271,889 $ 14 $ (1,153) $ 270,750 Classified as: Cash equivalents $ 11,004 Short-term investments 224,265 Long-term investments 35,481 Total $ 270,750
Schedule of Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value	For investments in an unrealized loss position as of the respective balance sheet dates, the following table summarizes the fair value and gross unrealized losses by category, disaggregated by the length of time that individual debt securities have been in a continuous unrealized loss position (in thousands): December 31, 2023 Less Than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses U.S. treasury bills $ 12,779 $ (25) $ 6,994 $ (22) $ 19,773 $ (47) U.S. agency securities 26,248 (57) — — 26,248 (57) Corporate debt securities and commercial paper 27,477 (24) — — 27,477 (24) Total $ 66,504 $ (106) $ 6,994 $ (22) $ 73,498 $ (128) December 31, 2022 Less Than 12 Months 12 Months or Greater Total Fair Value Unrealized Losses Fair Value Unrealized Losses Fair Value Unrealized Losses U.S. treasury bills $ 82,694 $ (368) $ 17,648 $ (353) $ 100,342 $ (721) U.S. agency securities 7,552 (11) — — 7,552 (11) Non-U.S. government securities 3,012 (16) 3,026 (6) 6,038 (22) Corporate debt securities and commercial paper 27,839 (195) 22,171 (204) 50,010 (399) Total $ 121,097 $ (590) $ 42,845 $ (563) $ 163,942 $ (1,153)

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2023
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	We had no financial liabilities measured at fair value for the periods presented and the following tables summarize our financial assets measured at fair value on a recurring basis (in thousands): Assets: December 31, 2023 Level 1 Level 2 Level 3 Total Money market funds $ 31,530 $ — $ — $ 31,530 U.S. treasury bills 218,187 — — 218,187 U.S. agency securities — 30,369 — 30,369 Corporate debt securities and commercial paper — 84,492 — 84,492 Total $ 249,717 $ 114,861 $ — $ 364,578 Assets: December 31, 2022 Level 1 Level 2 Level 3 Total Money market funds $ 11,004 $ — $ — $ 11,004 U.S. treasury bills 113,248 — — 113,248 U.S. agency securities — 10,915 — 10,915 Non-U.S. government securities — 6,038 — 6,038 Corporate debt securities and commercial paper — 129,545 — 129,545 Total $ 124,252 $ 146,498 $ — $ 270,750

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2023
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net, consists of the following (in thousands): December 31, 2023 2022 Laboratory equipment $ 3,472 $ 3,191 General equipment and furniture 459 459 Computer equipment and software 268 217 Leasehold improvements 118 118 Property and equipment, at cost 4,317 3,985 Less accumulated depreciation and amortization (2,833) (2,401) Property and equipment, net $ 1,484 $ 1,584

Additional Balance Sheet Info_2

Additional Balance Sheet Information (Tables)	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following (in thousands): December 31, 2023 2022 Prepaid research and development $ 1,025 $ 1,881 Prepaid insurance 521 368 Prepaid other 327 394 Other receivables 582 317 Prepaid expenses and other current assets $ 2,455 $ 2,960
Schedule of Accrued Liabilities Disclosure	Accrued liabilities consist of the following (in thousands): December 31, 2023 2022 Research and development services $ 12,838 $ 7,657 Employee compensation 7,970 4,110 Legal and professional fees 283 334 Accrued taxes 4 1,847 Accrued other 92 55 Accrued liabilities $ 21,187 $ 14,003

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2023
Commitments and Contingencies Disclosure [Abstract]
Schedule of Components of Lease Cost	Supplemental operating lease information is as follows (in thousands): Years Ended December 31, 2023 2022 2021 Operating lease cost $ 1,843 $ 1,839 $ 1,837 Variable lease cost 917 823 603 Total lease cost $ 2,760 $ 2,662 $ 2,440 Other information: Cash paid for amounts included in the measurement of lease liabilities $ 2,062 $ 2,009 $ 2,117 Weighted-average remaining lease term (years) 6.2 7.2 8.2 Weighted-average discount rate 10.7% 10.7% 10.7%
Schedule of Future Minimum Lease Payments for Non-cancelable Operating Leases	Maturities of our operating lease liabilities as of December 31, 2023, are as follows (in thousands): Minimum Lease Payments 2024 $ 1,939 2025 2,167 2026 2,224 2027 2,283 2028 2,344 Thereafter 2,811 Total future minimum lease payments 13,768 Less: imputed interest (3,854) Operating lease liabilities $ 9,914

License and Collaboration Agr_2

License and Collaboration Agreements (Tables)	12 Months Ended
	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Contract Balances	Respective balances are as follows (in thousands): December 31, 2023 2022 Contract Assets $ 167 $ 392 Contract Liabilities $ 17,217 $ 74,756
Schedule of Disaggregation of Revenue	Collaboration revenue recognized in our accompanying Consolidated Statements of Operations and Comprehensive Income (Loss) , disaggregated by customer, is as follows (in thousands): Years Ended December 31, 2023 2022 2021 AbbVie $ 41,925 $ 17,064 $ 23,443 Amgen 16,487 11,964 — Adaptimmune 464 1,036 — Total collaboration revenue $ 58,876 $ 30,064 $ 23,443

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2023
Equity [Abstract]
Schedule of Shares of Common Stock Reserved for Future Issuance	Shares of common stock reserved for future issuance were as follows: December 31, 2023 2022 Shares to be issued upon exercise of outstanding stock options 7,968,360 7,058,140 Shares to be issued upon release of RSUs 45,464 178,618 Shares to be issued upon conversion of common stock warrants 2,369,351 3,656,497 Shares to be issued upon conversion of prefunded warrants 6,102,127 5,182,197 Shares available for future stock grants 281,984 314,523 Shares to be issued under employee stock purchase plan 45,211 45,211 Shares of common stock reserved for future issuance 16,812,497 16,435,186
Schedule of Stockholders' Equity Note, Warrants or Rights	Excluding the prefunded warrants we issued in connection with our securities offerings discussed above, the table below summarizes our common stock warrant activity: Warrants Weighted- Weighted- Outstanding at December 31, 2022 3,656,497 $ 12.66 1.12 Exercised (1,287,146) 12.74 Outstanding and Exercisable at December 31, 2023 2,369,351 $ 12.62 0.15
Schedule of Stock Option Activity	A summary of stock option activity under our plans is presented below: Options Weighted- Weighted- Aggregate Outstanding at December 31, 2022 7,058,140 $ 8.37 Granted 2,816,197 $ 9.50 Exercised (858,511) $ 5.91 Forfeited (1,047,466) $ 10.40 Outstanding at December 31, 2023 7,968,360 $ 8.77 6.96 $ 82,254 Vested and expected to vest after December 31, 2023 7,790,360 $ 8.79 6.95 $ 80,307 Exercisable at December 31, 2023 4,462,145 $ 7.79 5.62 $ 50,542
Schedule of RSU Activity	A summary of our RSU activity under our plans is presented below: Number of Shares Weighted- Weighted- Aggregate Non-vested at December 31, 2022 178,618 $ 12.00 Vested (114,651) $ 12.00 Forfeited (18,503) $ 12.00 Non-vested at December 31, 2023 45,464 $ 12.00 0.0 $ 867
Schedule of Stock Option Assumptions	The fair values of stock options granted to employees were calculated using the following assumptions: Years Ended December 31, 2023 2022 2021 Weighted-average estimated fair value at grant $6.94 $7.56 $8.44 Risk-free interest rate (1) 3.36% - 4.67% 1.39% - 4.04% 0.25% - 1.3% Expected term of options (in years) (2) 5.27 - 6.08 5.27 – 6.08 3.49 – 6.08 Expected stock price volatility (3) 81% - 85% 80% - 83% 79% - 83% Expected dividend yield (4) —% —% —% (1) The risk-free interest rate assumption was based on zero-coupon U.S. Treasury instruments that had terms consistent with the expected term of our stock option grants. (2) We used the “simplified method” for options to determine the expected term of stock options granted, since we do not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term due to the limited time our shares have been publicly traded. Under this approach, the weighted-average expected life is presumed to be the average of the vesting term and the contractual term of the option. (3) Volatility is a measure of the amount by which a financial variable, such as share price, has fluctuated or is expected to fluctuate during a period. We analyzed the historical stock price volatility of companies at a similar stage of development to estimate the expected volatility of our stock price. (4) We have never declared or paid any cash dividends and do not presently plan to pay cash dividends in the foreseeable future.
Schedule of Stock-based Compensation Expense	Stock-based compensation expense is classified in the Consolidated Statements of Operations and Comprehensive Income (Loss) as follows (in thousands): Years Ended December 31, 2023 2022 2021 Research and development $ 6,251 $ 5,713 $ 3,347 General and administrative 4,379 3,935 2,893 Total stock-based compensation expense $ 10,630 $ 9,648 $ 6,240

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Schedule of Loss Before Taxes, Domestic and Foreign	Our loss before taxes is derived from domestic (United States) and foreign (Australian) sources as follows (in thousands): Years Ended December 31, 2023 2022 2021 Domestic $ (32,450) $ (55,475) $ (48,932) Foreign 369 43 (1,488) Total $ (32,081) $ (55,432) $ (50,420)
Schedule of Provision for Income Taxes	The provision for income taxes is composed of the following (in thousands): Years Ended December 31, 2023 2022 2021 Current: U.S. - Federal $ 47 $ (981) $ — U.S. - State 4 (16) — Foreign — (1,285) (129) Total current 51 (2,282) (129) Deferred: Foreign (154) (48) 216 Total deferred (154) (48) 216 Total income tax (expense) benefit $ (103) $ (2,330) $ 87
Schedule of Effective Tax Rate of Provision for Income Taxes Differs from Federal Statutory Rate	The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Years Ended December 31, 2023 2022 2021 U.S. Statutory rate 21.0 % 21.0 % 21.0 % Effect of: State taxes (net of federal benefit) 0.3 % 0.1 % — % Permanent differences 0.3 % — % (0.4) % Federal research and development credit 9.2 % 5.0 % 4.5 % Change in valuation allowance (29.1) % (28.6) % (21.4) % Global intangible low-taxed income recapture — % (0.6) % (1.4) % Stock-based compensation (1.6) % (1.9) % (1.8) % Foreign rate differential (0.1) % — % 0.2 % Other (0.3) % — % (0.5) % Voluntary disclosure net change in prior years’ tax liabilities — % 0.8 % — % Effective income tax rate (0.3) % (4.2) % 0.2 %
Schedule of Significant Components of Deferred Tax Assets and Liabilities	The following table represents the significant components of our deferred tax assets and liabilities for the periods presented (in thousands): December 31, 2023 2022 Deferred tax assets: Net operating loss $ 23,205 $ 18,590 Research and development credits 9,259 6,240 Intangible asset basis 4 2 Lease liability 2,126 2,315 Deferred revenue 3,600 14,752 Stock based compensation 2,428 2,343 Capitalized research and experimental costs 24,147 12,681 Other 1,088 156 Gross deferred tax assets 65,857 57,079 Valuation allowance (63,897) (54,841) Total deferred tax assets, net of valuation allowance 1,960 2,238 Deferred tax liabilities: Prepaid expenses (251) (173) Fixed asset basis (126) (182) Right-of-use asset basis (1,583) (1,728) Total deferred tax liability (1,960) (2,083) Net deferred tax assets and liabilities $ — $ 155
Schedule of Net Operating Loss Carryforwards	We have net operating loss (“NOL”) carryforwards as follows (in thousands): December 31, 2023 2022 Federal (after January 1, 2018) $ 108,393 $ 85,012 State $ 6,778 $ 6,433
Schedule of Net Research and Development Tax Credit Carryforwards	We have net research and development tax credit carryforwards as follows (in thousands): December 31, 2023 2022 Federal $ 11,584 $ 7,892 Foreign $ 578 $ 527
Schedule of Activity Related to Unrecognized Tax benefits	The following table summarizes the activity related to unrecognized tax benefits (in thousands): Years Ended December 31, 2023 2022 2021 Unrecognized benefits – beginning of year $ 2,149 $ 1,553 $ 990 Gross increases (decreases) – prior year tax positions (2) — — Gross increases – current year tax positions 727 596 563 Unrecognized benefit – end of year $ 2,874 $ 2,149 $ 1,553

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Details) - segment	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments	1
Debt securities, available-for-sale, term (less than) (in years)	2 years	2 years
Minimum percentage of income tax benefit realized upon ultimate settlement (greater than) (as a percent)	50%
Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets (in years)	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets (in years)	5 years

Net Loss Per Share - Narrative

Net Loss Per Share - Narrative (Details) - shares	1 Months Ended					12 Months Ended
	Nov. 30, 2023	Jun. 30, 2023	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issued (in shares)	9,791,832	919,413	15,509,282			9,791,832	15,509,282	6,489,357
Exercise of warrants, net of shares withheld (in shares)						2,711,389
2021 PIPE
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issued (in shares)					6,489,357
Amgen
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issued (in shares)				951,980

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Anti-dilutive Securities Excluded from Diluted Net Loss Per Share Attributable to Common Stockholders Calculation (Details) - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	16,485,302	16,075,452	14,725,941
Common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	2,369,351	3,656,497	3,666,435
Prefunded warrants to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	6,102,127	5,182,197	5,182,197
Stock options and RSUs outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	8,013,824	7,236,758	5,877,309

Cash Equivalents and Investme_3

Cash Equivalents and Investments - Amortized Cost and Fair Value of Cash Equivalents and Short Term Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 364,224	$ 271,889
Gross unrealized gains	482	14
Gross unrealized losses	(128)	(1,153)
Fair market value	364,578	270,750
Cash equivalents
Debt Securities, Available-for-sale [Line Items]
Fair market value	40,531	11,004
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	283,491	224,265
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	40,556	35,481
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	31,530	11,004
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair market value	31,530	11,004
U.S. treasury bills
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	217,857	113,964
Gross unrealized gains	377	5
Gross unrealized losses	(47)	(721)
Fair market value	218,187	113,248
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	30,412	10,921
Gross unrealized gains	14	5
Gross unrealized losses	(57)	(11)
Fair market value	30,369	10,915
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		6,060
Gross unrealized gains		0
Gross unrealized losses		(22)
Fair market value		6,038
Corporate debt securities and commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	84,425	129,940
Gross unrealized gains	91	4
Gross unrealized losses	(24)	(399)
Fair market value	$ 84,492	$ 129,545

Cash Equivalents and Investme_4

Cash Equivalents and Investments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash, Cash Equivalents, and Short-Term Investments [Abstract]
Debt securities, available-for-sale, term (less than) (in years)	2 years	2 years
Realized gains on investments	$ 33	$ 0	$ 0
Realized losses on securities	$ 0	$ 0	$ 0

Cash Equivalents and Investme_5

Cash Equivalents and Investments - Schedule of Debt Securities, Available-for-Sale, Unrealized Loss Position, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
Debt securities, less than 12 months, fair value	$ 66,504	$ 121,097
Debt securities, less than 12 months, unrealized losses	(106)	(590)
Debt securities, 12 months or greater, fair value	6,994	42,845
Debt securities, 12 months or greater, unrealized losses	(22)	(563)
Debt securities, total, fair value	73,498	163,942
Debt securities, total, unrealized losses	(128)	(1,153)
U.S. treasury bills
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
Debt securities, less than 12 months, fair value	12,779	82,694
Debt securities, less than 12 months, unrealized losses	(25)	(368)
Debt securities, 12 months or greater, fair value	6,994	17,648
Debt securities, 12 months or greater, unrealized losses	(22)	(353)
Debt securities, total, fair value	19,773	100,342
Debt securities, total, unrealized losses	(47)	(721)
U.S. agency securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
Debt securities, less than 12 months, fair value	26,248	7,552
Debt securities, less than 12 months, unrealized losses	(57)	(11)
Debt securities, 12 months or greater, fair value	0	0
Debt securities, 12 months or greater, unrealized losses	0	0
Debt securities, total, fair value	26,248	7,552
Debt securities, total, unrealized losses	(57)	(11)
Non-U.S. government securities
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
Debt securities, less than 12 months, fair value		3,012
Debt securities, less than 12 months, unrealized losses		(16)
Debt securities, 12 months or greater, fair value		3,026
Debt securities, 12 months or greater, unrealized losses		(6)
Debt securities, total, fair value		6,038
Debt securities, total, unrealized losses		(22)
Corporate debt securities and commercial paper
Debt Securities, Available-for-Sale, Unrealized Loss Position [Line Items]
Debt securities, less than 12 months, fair value	27,477	27,839
Debt securities, less than 12 months, unrealized losses	(24)	(195)
Debt securities, 12 months or greater, fair value	0	22,171
Debt securities, 12 months or greater, unrealized losses	0	(204)
Debt securities, total, fair value	27,477	50,010
Debt securities, total, unrealized losses	$ (24)	$ (399)

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Fair Value Disclosures [Abstract]
Cash	$ 3.4	$ 2.4

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	$ 364,578	$ 270,750
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	31,530	11,004
U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	218,187	113,248
U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	30,369	10,915
Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure		6,038
Corporate debt securities and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	84,492	129,545
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	249,717	124,252
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	31,530	11,004
Level 1 | U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	218,187	113,248
Level 1 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	0	0
Level 1 | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure		0
Level 1 | Corporate debt securities and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	114,861	146,498
Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	0	0
Level 2 | U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	0	0
Level 2 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	30,369	10,915
Level 2 | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure		6,038
Level 2 | Corporate debt securities and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	84,492	129,545
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	0	0
Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	0	0
Level 3 | U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	0	0
Level 3 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	0	0
Level 3 | Non-U.S. government securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure		0
Level 3 | Corporate debt securities and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	$ 0	$ 0

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	$ 4,317	$ 3,985
Less accumulated depreciation and amortization	(2,833)	(2,401)
Property and equipment, net	1,484	1,584
Laboratory equipment
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	3,472	3,191
General equipment and furniture
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	459	459
Computer equipment and software
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	268	217
Leasehold improvements
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	$ 118	$ 118

Property and Equipment - Narrat

Property and Equipment - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Depreciation expense	$ 576	$ 597	$ 620

Additional Balance Sheet Info_3

Additional Balance Sheet Information - Prepaid Expense and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid research and development	$ 1,025	$ 1,881
Prepaid insurance	521	368
Prepaid other	327	394
Other receivables	582	317
Prepaid expenses and other current assets	$ 2,455	$ 2,960

Additional Balance Sheet Info_4

Additional Balance Sheet Information - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Research and development services	$ 12,838	$ 7,657
Employee compensation	7,970	4,110
Legal and professional fees	283	334
Accrued taxes	4	1,847
Accrued other	92	55
Accrued liabilities	$ 21,187	$ 14,003

Long-term Debt (Details)

Long-term Debt (Details)	1 Months Ended
	Mar. 31, 2020 USD ($)	Aug. 31, 2019 USD ($) tranche $ / shares shares	Dec. 31, 2023 USD ($)	May 31, 2023 USD ($)	Jul. 31, 2020 USD ($)	Mar. 30, 2020 USD ($) shares
SVB Term Loan
Debt Instrument [Line Items]
Long-term debt, gross			$ 0	$ 1,400,000
Debt instrument, face amount				800,000
Prepayment fee				$ 625,000
Term Loans | Term Loans, Tranche One | Common Stock Warrant
Debt Instrument [Line Items]
Fair value of warrant liability		$ 60,000
Term Loans | Term Loans, Tranche Two | Common Stock Warrant
Debt Instrument [Line Items]
Fair value of warrant liability						$ 60,000
Silicon Valley Bank | Term Loans
Debt Instrument [Line Items]
Maximum borrowing capacity, amount		$ 15,000,000
Number of tranches | tranche		3
Variable interest rate (as a percent)		0.25%
Aggregate new capital milestone for continuation of Interest only payment period		$ 40,000,000
Silicon Valley Bank | Term Loans | Common Stock Warrant
Debt Instrument [Line Items]
Warrants exercisable (in shares) | shares		52,083
Silicon Valley Bank | Term Loans | Base Rate
Debt Instrument [Line Items]
Interest rate floor (as a percent)		5.75%
Silicon Valley Bank | Term Loans | Term Loans, Tranche One
Debt Instrument [Line Items]
Proceeds from borrowings, net of issuance costs		$ 5,000,000
Silicon Valley Bank | Term Loans | Term Loans, Tranche One | Common Stock Warrant
Debt Instrument [Line Items]
Warrants issued, exercise price (in dollars per share) | $ / shares		$ 4.32
Number of shares exercisable (in shares) | shares		17,361
Silicon Valley Bank | Term Loans | Term Loans, Tranche Two
Debt Instrument [Line Items]
Proceeds from borrowings, net of issuance costs	$ 5,000,000
Silicon Valley Bank | Term Loans | Term Loans, Tranche Two | Common Stock Warrant
Debt Instrument [Line Items]
Number of shares exercisable (in shares) | shares						17,361
Silicon Valley Bank | Term Loans | Term Loans, Tranche Three
Debt Instrument [Line Items]
Unused borrowing capacity, expired amount					$ 5,000,000

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) lease_term_type	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Loss Contingencies [Line Items]
Operating lease, term of contract (in years)	10 years 9 months 18 days
Number of option to extend term lease | lease_term_type	1
Operating lease, renewal term (in years)	5 years
Operating lease, annual base rent	$ 1,700
Operating lease, annual base rent increasing (as a percent)	3%
Maximum tenant improvement allowance	$ 5,400
Maximum additional tenant improvement allowance	$ 1,800
Amortization percent, additional tenant improvement allowance (as a percent)	8%
Operating lease, right-of-use asset		$ 7,510	$ 8,219
Operating lease, liability		9,914
Current tax expense		0	1,285	$ 129
Cash paid for income taxes		1,535	543	$ 0
Foreign | Australian Taxation Office
Loss Contingencies [Line Items]
Current tax expense			1,300
Cash paid for income taxes		590	$ 543
Accrued liabilities		0
Foreign | Australian Taxation Office | Research Tax Credit Carryforward
Loss Contingencies [Line Items]
Foreign tax credit		$ 136
Line of Credit
Loss Contingencies [Line Items]
Security deposit	$ 254

Commitments and Contingencies_2

Commitments and Contingencies - Components of Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Lease cost:
Operating lease cost	$ 1,843	$ 1,839	$ 1,837
Variable lease cost	917	823	603
Total lease cost	2,760	2,662	2,440
Other information:
Cash paid for amounts included in the measurement of lease liabilities	$ 2,062	$ 2,009	$ 2,117
Weighted-average remaining lease term (years)	6 years 2 months 12 days	7 years 2 months 12 days	8 years 2 months 12 days
Weighted-average discount rate	10.70%	10.70%	10.70%

Commitments and Contingencies_3

Commitments and Contingencies - Schedule of Future Minimum Lease Payments for Non-cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2024	$ 1,939
2025	2,167
2026	2,224
2027	2,283
2028	2,344
Thereafter	2,811
Total future minimum lease payments	13,768
Less: imputed interest	(3,854)
Operating lease liabilities	$ 9,914

License and Collaboration Agr_3

License and Collaboration Agreements - Narrative (Details) $ / shares in Units, $ in Thousands

1 Months Ended

12 Months Ended

24 Months Ended

55 Months Ended

Dec. 31, 2023 USD ($)

Dec. 31, 2021 USD ($) program candidate $ / shares shares

Sep. 30, 2021 shares

Dec. 31, 2023 USD ($)

Dec. 31, 2022 USD ($)

Dec. 31, 2021 USD ($) $ / shares

Dec. 31, 2023 USD ($)

Dec. 31, 2023 USD ($)

Nov. 30, 2023 $ / shares

Jun. 30, 2022 USD ($)

Mar. 31, 2022 USD ($)

Jun. 30, 2020 USD ($) payment

Jun. 30, 2019 USD ($)

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

$ 17,217

$ 17,217

$ 74,756

$ 17,217

$ 17,217

Revenue recognized from contract with customer

57,500

$ 28,200

$ 23,400

Common Stock

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Purchase price (in dollars per share) | $ / shares

$ 12.50

Private Placement | Common Stock

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Stock issued in transaction (in shares) | shares

6,489,357

AbbVie

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Expiration period

90 days

Potential cash payment upon license option exercise

$ 75,000

Cumulative catch-up adjustment to revenue

$ (20,400)

AbbVie | Product

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Potential future revenue recognition milestones, number of payments | payment

1

AbbVie | Product | Maximum

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Potential future milestones receivable (up to)

$ 75,000

AbbVie | Product | Upfront License Payment

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

$ 60,000

AbbVie | Product | Achieved Milestone

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

$ 45,000

$ 45,000

AbbVie Amendment | Product

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Potential cash payment upon license option exercise

10,000

10,000

10,000

10,000

Potential cash payment upon achievement of certain development and commercial milestones

153,800

153,800

153,800

153,800

Potential cash payment upon achievement of certain sales-based cash milestones

337,500

337,500

337,500

337,500

AbbVie Amendment | Product | Removed from Milestone

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

30,000

30,000

30,000

30,000

Amgen

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Number of preclinical candidates | candidate

4

Number of existing programs | program

1

Number of additional programs | program

3

Cumulative catch-up adjustment to revenue

6,700

Amgen | Common Stock

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Stock price period for premium on sale of stock

30 days

Amgen | Private Placement

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Stock issued in transaction (in shares) | shares

951,980

Purchase price (in dollars per share) | $ / shares

$ 15.76

$ 15.76

Aggregate proceeds from sale of stock

$ 15,000

Purchase price as premium to volume-weighted average share price (as a percent)

25%

Fair value portion of aggregate proceeds from sale of stock

$ 11,900

Additional consideration portion of aggregate proceeds from sale of stock

3,100

Amgen | Product

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

28,100

$ 28,100

Potential future milestones receivable (up to)

762,000

762,000

762,000

762,000

Potential future milestones receivable per program (up to)

381,000

381,000

Amgen | Product | Upfront Payment

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

25,000

25,000

$ 25,000

Amgen | Product | Research and Development

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Revenue recognized from contract with customer

2,100

Amgen | Product | Premium Payment

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

$ 3,100

$ 3,100

Adaptimmune | Product | Maximum

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Potential future milestones receivable (up to)

$ 105,000

$ 105,000

$ 105,000

105,000

Adaptimmune | Product | License Payment

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Revenue recognized from contract with customer

3,000

Adaptimmune | Product | Initial License Payment

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

$ 2,000

Adaptimmune | Product | Additional License Payment

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Contract Liabilities

$ 1,000

Adaptimmune | Product | Research Support Payment

Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]

Revenue recognized from contract with customer

$ 2,100

License and Collaboration Agr_4

License and Collaboration Agreements - Contract Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Contract Assets	$ 167	$ 392
Contract Liabilities	$ 17,217	$ 74,756

License and Collaboration Agr_5

License and Collaboration Agreements - Collaboration Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total collaboration revenue	$ 58,876	$ 30,064	$ 23,443
AbbVie
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total collaboration revenue	41,925	17,064	23,443
Amgen
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total collaboration revenue	16,487	11,964	0
Adaptimmune
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total collaboration revenue	$ 464	$ 1,036	$ 0

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2023	Dec. 31, 2022
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	16,812,497	16,435,186
Shares to be issued upon exercise of outstanding stock options
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	7,968,360	7,058,140
Shares to be issued upon release of RSUs
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	45,464	178,618
Shares to be issued upon conversion of common stock warrants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	2,369,351	3,656,497
Shares to be issued upon conversion of prefunded warrants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	6,102,127	5,182,197
Shares available for future stock grants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	281,984	314,523
Shares to be issued under employee stock purchase plan
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	45,211	45,211

Stockholders' Equity - Narrativ

Stockholders' Equity - Narrative (Details) - USD ($)

1 Months Ended

2 Months Ended

9 Months Ended

12 Months Ended

Aug. 01, 2023

Feb. 29, 2024

Jan. 31, 2024

Nov. 30, 2023

Jun. 30, 2023

Apr. 30, 2023

Sep. 30, 2022

Aug. 31, 2022

Sep. 30, 2021

Jun. 30, 2018

Oct. 31, 2022

Sep. 30, 2022

Dec. 31, 2023

Dec. 31, 2022

Dec. 31, 2021

Jul. 31, 2017

Class of Stock [Line Items]

Proceeds from issuance or sale of equity, gross

$ 162,400,000

Stock issuance costs incurred

$ 266,000

Proceeds from sale of common stock and warrants, net of offering costs

$ 91,000,000

Proceeds from sale of common stock and warrants, net of offering costs

$ 161,905,000

$ 106,716,000

$ 90,733,000

Retirement of treasury stock (in shares)

1,250,467

Common stock, shares issued (in shares)

60,347,457

47,234,900

Proceeds from exercise of warrants

$ 1,851,000

$ 0

0

Shares of common stock reserved for future issuance (in shares)

16,812,497

16,435,186

Vesting period (in years)

4 years

Term (in years)

10 years

Unrecognized compensation cost related to nonvested stock options

$ 22,400,000

Weighted-average period over which unrecognized compensation expense related to nonvested stock options is expected to be recognized (in years)

2 years 8 months 12 days

Subsequent Event

Class of Stock [Line Items]

Proceeds from exercise of warrants

$ 17,600,000

Sales Agreement | Cowen & Co.

Class of Stock [Line Items]

Stock issuance costs incurred

$ 488,000

Maximum amount of common stock eligible to be sold (up to)

$ 75,000,000

2018 Plan

Class of Stock [Line Items]

Percentage of outstanding stock maximum (as a percent)

5%

2018 Plan | Maximum

Class of Stock [Line Items]

Additional number of shares authorized (in shares)

1,500,000

2018 Plan | Subsequent Event

Class of Stock [Line Items]

Additional number of shares authorized (in shares)

1,500,000

Incentive Stock Option

Class of Stock [Line Items]

Term (in years)

10 years

Stock Plan 2018, Stock Plan 2015 And Equity Incentive Plan 2015

Class of Stock [Line Items]

Number of shares authorized (in shares)

9,616,862

Prefunded warrants

Class of Stock [Line Items]

Shares of common stock reserved for future issuance (in shares)

6,102,127

5,182,197

Inducement Grant | Common Stock

Class of Stock [Line Items]

Options granted (in shares)

185,000

160,000

Exercise price (in dollars per share)

$ 13.44

$ 8.38

Legacy Plan Cancellations | 2018 Plan

Class of Stock [Line Items]

Number of shares authorized (in shares)

1,972,784

Stock options

Class of Stock [Line Items]

Shares of common stock reserved for future issuance (in shares)

7,968,360

7,058,140

Aggregate intrinsic value of stock options exercised

$ 7,600,000

$ 1,100,000

546,000

Fair value of shares vested

$ 9,600,000

$ 8,600,000

3,400,000

Restricted stock units

Class of Stock [Line Items]

Shares of common stock reserved for future issuance (in shares)

45,464

178,618

Aggregate intrinsic value of RSU's released

$ 1,000,000

$ 527,000

Fair value of RSUs vesting

$ 1,400,000

$ 735,000

$ 0

Stock options and Restricted stock units

Class of Stock [Line Items]

Nonvested awards (in shares)

3,600,000

Common Stock

Class of Stock [Line Items]

Purchase price (in dollars per share)

$ 12.50

Exchange of common stock for prefunded warrants (in shares)

1,200,000

(1,200,000)

Common Stock | Subsequent Event

Class of Stock [Line Items]

Common stock, shares issued (in shares)

3,199,879

1,694,282

Treasury

Class of Stock [Line Items]

Exchange of common stock for prefunded warrants (in shares)

1,200,000

1,200,000

Retirement of treasury stock (in shares)

1,250,467

Underwritten Public Offering

Class of Stock [Line Items]

Proceeds from issuance or sale of equity, net

$ 152,200,000

Stock issuance costs incurred

$ 440,000

Underwritten Public Offering | Common Stock

Class of Stock [Line Items]

Stock issued in transaction (in shares)

9,791,832

Public Stock Offering

Class of Stock [Line Items]

Stock issued in transaction (in shares)

15,509,282

Stock issuance costs incurred

$ 438,000

Proceeds from sale of equity

$ 106,700,000

Maximum amount of common stock eligible to be sold (up to)

$ 100,000,000

Public Stock Offering | Sales Agreement

Class of Stock [Line Items]

Stock issued in transaction (in shares)

919,413

Purchase price (in dollars per share)

$ 10.93

Stock issuance costs incurred

$ 355,000

Proceeds from sale of common stock and warrants, net of offering costs

$ 10,000,000

Public Stock Offering | Sales Agreement | TD Cowen

Class of Stock [Line Items]

Percentage of compensation for services (as a percent)

3%

Private Placement | Common Stock

Class of Stock [Line Items]

Stock issued in transaction (in shares)

6,489,357

Employee stock purchase plan

Class of Stock [Line Items]

Shares of common stock reserved for future issuance (in shares)

45,211

Prefunded warrants

Class of Stock [Line Items]

Warrants exercisable (in shares)

3,191,487

Warrants issued, exercise price (in dollars per share)

$ 0.001

$ 0.001

Prefunded warrants | Subsequent Event

Class of Stock [Line Items]

Warrants exercised (in shares)

3,200,000

Prefunded warrants | Common Stock

Class of Stock [Line Items]

Purchase price (in dollars per share)

$ 12.499

$ 9.40

Prefunded warrants | Underwritten Public Offering

Class of Stock [Line Items]

Warrants exercisable (in shares)

3,200,000

Exchange Warrants | Prefunded warrants

Class of Stock [Line Items]

Warrants exercisable (in shares)

1,200,000

Common stock warrants | Subsequent Event

Class of Stock [Line Items]

Warrants exercised (in shares)

2,309,404

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Warrants (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Warrants Outstanding
Warrants outstanding (in shares)	3,656,497
Exercised (in shares)	(1,287,146)
Warrants outstanding and exercisable (in shares)	2,369,351	3,656,497
Weighted- average Exercise Price
Weighted average, exercise price, outstanding (in dollars per share)	$ 12.66
Exercise (in dollars per share)	12.74
Weighted average, exercise price, outstanding and exercisable (in dollars per share)	$ 12.62	$ 12.66
Weighted- average Remaining Contract Term (in years)
Weighted-average remaining contractual term, outstanding	1 month 24 days	1 year 1 month 13 days

Stockholders' Equity - Schedu_3

Stockholders' Equity - Schedule of Option Activity (Details) - Stock options $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares
Options Outstanding
Outstanding at beginning of period (in shares) | shares	7,058,140
Granted (in shares) | shares	2,816,197
Exercised (in shares) | shares	(858,511)
Forfeited (in shares) | shares	(1,047,466)
Outstanding at ending of period (in shares) | shares	7,968,360
Vested and expected to vest (in shares) | shares	7,790,360
Exercisable (in shares) | shares	4,462,145
Weighted- average Exercise Price (per share)
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 8.37
Granted (in dollars per share) | $ / shares	9.50
Exercised (in dollars per share) | $ / shares	5.91
Forfeited (in dollars per share) | $ / shares	10.40
Outstanding at ending of period (in dollars per share) | $ / shares	8.77
Vested and expected to vest (in dollars per share) | $ / shares	8.79
Exercisable (in dollars per share) | $ / shares	$ 7.79
Weighted- average Remaining Contract Term (in years)
Outstanding (in years)	6 years 11 months 15 days
Vested and expected to vest (in years)	6 years 11 months 12 days
Exercisable (in years)	5 years 7 months 13 days
Aggregate Intrinsic Value (in thousands)
Outstanding | $	$ 82,254
Vested and expected to vest | $	80,307
Exercisable | $	$ 50,542

Stockholders' Equity - Schedu_4

Stockholders' Equity - Schedule of RSU Activity (Details) - Restricted stock units $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares shares
Number of Shares
Non-vested at beginning of period (in shares) | shares	178,618
Vested (in shares) | shares	(114,651)
Forfeited (in shares) | shares	(18,503)
Non-vested at ending of period (in shares) | shares	45,464
Weighted- Average Grant Date Fair Value (per share)
Non-vested at beginning of period (in dollars per share) | $ / shares	$ 12
Vested (in dollars per share) | $ / shares	12
Forfeited (in dollars per share) | $ / shares	12
Non-vested at ending of period (in dollars per share) | $ / shares	$ 12
Weighted- average Remaining Contract Life (in years)
Non-vested (in years)	0 years
Aggregate Intrinsic Value (in thousands)
Non-vested | $	$ 867

Stockholders' Equity - Schedu_5

Stockholders' Equity - Schedule of Stock Option Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average estimated fair value at grant (in dollars per share)	$ 6.94	$ 7.56	$ 8.44
Risk-free interest rate, minimum (as a percent)	3.36%	1.39%	0.25%
Risk-free interest rate, maximum (as a percent)	4.67%	4.04%	1.30%
Expected stock price volatility, minimum (as a percent)	81%	80%	79%
Expected stock price volatility, maximum (as a percent)	85%	83%	83%
Expected dividend yield (as a percent)	0%	0%	0%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of options (in years)	5 years 3 months 7 days	5 years 3 months 7 days	3 years 5 months 26 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of options (in years)	6 years 29 days	6 years 29 days	6 years 29 days

Stockholders' Equity - Schedu_6

Stockholders' Equity - Schedule of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 10,630	$ 9,648	$ 6,240
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	6,251	5,713	3,347
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 4,379	$ 3,935	$ 2,893

Income Taxes - Schedule of Dome

Income Taxes - Schedule of Domestic and Foreign Components of Loss Before Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
Domestic	$ (32,450)	$ (55,475)	$ (48,932)
Foreign	369	43	(1,488)
Loss before taxes	$ (32,081)	$ (55,432)	$ (50,420)

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Current:
U.S. - Federal	$ 47	$ (981)	$ 0
U.S. - State	4	(16)	0
Foreign	0	(1,285)	(129)
Total current	51	(2,282)	(129)
Deferred:
Foreign	(154)	(48)	216
Total deferred	(154)	(48)	216
Total income tax (expense) benefit	$ (103)	$ (2,330)	$ 87

Income Taxes - Schedule of Effe

Income Taxes - Schedule of Effective Tax Rate of Provision for Income Taxes Differs from Federal Statutory Rate (Details)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax Disclosure [Abstract]
U.S. Statutory rate	21%	21%	21%
Effect of:
State taxes (net of federal benefit)	0.30%	0.10%	0%
Permanent differences	0.30%	0%	(0.40%)
Federal research and development credit	9.20%	5%	4.50%
Change in valuation allowance	(29.10%)	(28.60%)	(21.40%)
Global intangible low-taxed income recapture	0%	(0.60%)	(1.40%)
Stock-based compensation	(1.60%)	(1.90%)	(1.80%)
Foreign rate differential	(0.10%)	0%	0.20%
Other	(0.30%)	0%	(0.50%)
Voluntary disclosure net change in prior years’ tax liabilities	0%	0.80%	0%
Effective income tax rate	(0.30%)	(4.20%)	0.20%

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2018
Income Taxes [Line Items]
Income tax expense	$ 103,000	$ 2,330,000	$ (87,000)
Current federal, state and local, tax expense		1,000,000
Current tax expense	0	1,285,000	129,000
Deferred tax expense	154,000	48,000	(216,000)
Increase in valuation allowance	9,100,000	16,100,000
Unrecognized tax benefits that would impact effective tax rate	0
Unrecognized tax benefits	2,874,000	2,149,000	1,553,000	$ 990,000
Accrued interest or penalties related to unrecognized tax benefits	$ 0	$ 0	$ 0
US
Income Taxes [Line Items]
Research and development, capitalized and amortized period (in years)		5 years
Outline United States
Income Taxes [Line Items]
Research and development, capitalized and amortized period (in years)		15 years
Nivalis Therapeutics, Inc.
Income Taxes [Line Items]
Tax credit and net operating loss carryforwards					$ 0

Income Taxes - Schedule of Sign

Income Taxes - Schedule of Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Net operating loss	$ 23,205	$ 18,590
Research and development credits	9,259	6,240
Intangible asset basis	4	2
Lease liability	2,126	2,315
Deferred revenue	3,600	14,752
Stock based compensation	2,428	2,343
Capitalized research and experimental costs	24,147	12,681
Other	1,088	156
Gross deferred tax assets	65,857	57,079
Valuation allowance	(63,897)	(54,841)
Total deferred tax assets, net of valuation allowance	1,960	2,238
Deferred tax liabilities:
Prepaid expenses	(251)	(173)
Fixed asset basis	(126)	(182)
Right-of-use asset basis	(1,583)	(1,728)
Total deferred tax liability	(1,960)	(2,083)
Net deferred tax assets and liabilities	$ 0	$ 155

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Operating Loss Carryforwards (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
State
Income Taxes [Line Items]
Net operating loss carryforwards	$ 6,778	$ 6,433
After January 1 2018 | Federal
Income Taxes [Line Items]
Net operating loss carryforwards	$ 108,393	$ 85,012

Income Taxes - Schedule of Tax

Income Taxes - Schedule of Tax Credit Carryforwards (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Income Taxes [Line Items]
Research and development credits	$ 9,259	$ 6,240
Federal
Income Taxes [Line Items]
Research and development credits	11,584	7,892
Foreign
Income Taxes [Line Items]
Research and development credits	$ 578	$ 527

Income Taxes - Schedule of Acti

Income Taxes - Schedule of Activity Related to Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized benefits – beginning of year	$ 2,149	$ 1,553	$ 990
Gross increases (decreases) – prior year tax positions	(2)	0	0
Gross increases – current year tax positions	727	596	563
Unrecognized benefit – end of year	$ 2,874	$ 2,149	$ 1,553

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Jan. 31, 2024	Dec. 31, 2023	Sep. 30, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Feb. 29, 2024
Related Party Transaction [Line Items]
Proceeds from exercise of warrants				$ 1,851	$ 0	$ 0
Proceeds from sale of common stock and warrants, net of offering costs			$ 91,000
Subsequent Event
Related Party Transaction [Line Items]
Proceeds from exercise of warrants	$ 17,600
Exchange Warrants | Prefunded warrants
Related Party Transaction [Line Items]
Warrants exercisable (in shares)			1,200,000
Affiliated Entity | Prefunded warrants | Subsequent Event
Related Party Transaction [Line Items]
Warrants exercisable (in shares)							3,200,000
Affiliated Entity | 2021 Security Purchases Agreement | Prefunded warrants
Related Party Transaction [Line Items]
Warrants exercisable (in shares)			2,340,424
Affiliated Entity | Exchange of Stock | Exchange Warrants | Prefunded warrants
Related Party Transaction [Line Items]
Warrants exercisable (in shares)			1,200,000
Common Stock and Common Stock Warrants | Affiliated Entity
Related Party Transaction [Line Items]
Proceeds from exercise of warrants		$ 1,900
Common Stock and Common Stock Warrants | Affiliated Entity | Subsequent Event
Related Party Transaction [Line Items]
Proceeds from exercise of warrants	$ 17,600
Common Stock and Common Stock Warrants | Affiliated Entity | 2021 Security Purchases Agreement
Related Party Transaction [Line Items]
Proceeds from sale of common stock and warrants, net of offering costs			$ 57,000
Common Stock
Related Party Transaction [Line Items]
Exchange of common stock for prefunded warrants (in shares)			(1,200,000)			1,200,000
Common Stock | Affiliated Entity | Alpine Immune Sciences
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners (as a percent)		5%		5%
Common Stock | Affiliated Entity | Alpine Immune Sciences | Subsequent Event
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners (as a percent)	5%						5%
Common Stock | Affiliated Entity | 2021 Security Purchases Agreement
Related Party Transaction [Line Items]
Stock issued in transaction (in shares)			3,723,402
Common Stock | Affiliated Entity | 2021 Security Purchases Agreement | Alpine Immune Sciences
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners (as a percent)			5%
Common Stock | Affiliated Entity | Exchange of Stock
Related Party Transaction [Line Items]
Exchange of common stock for prefunded warrants (in shares)			1,200,000
Common Stock Warrant | Affiliated Entity
Related Party Transaction [Line Items]
Warrants exercisable (in shares)		219,692		219,692
Common Stock Warrant | Affiliated Entity | Subsequent Event
Related Party Transaction [Line Items]
Warrants exercisable (in shares)	1,379,887

401(k) Retirement Plan (Details

401(k) Retirement Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Retirement Benefits [Abstract]
Employee contributions plan, cost	$ 493	$ 287