Alpine Immune Sciences (ALPN) 10-K 23 Mar 23 2022 FY Annual report Financial data

Cover

Cover - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Mar. 14, 2023	Jun. 30, 2022
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2022
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	No
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
Entity Shell Company	false
Entity Public Float			$ 132.3
Entity Common Stock, Shares Outstanding		47,756,907
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to the 2023 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, 2022.
Entity Central Index Key	0001626199
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	FY
Amendment Flag	false

Audit Information

Audit Information	12 Months Ended
	Dec. 31, 2022
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, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 13,376	$ 67,907
Short-term investments	224,265	94,396
Accounts receivable	392	25,000
Prepaid expenses and other current assets	2,960	4,710
Total current assets	240,993	192,013
Restricted cash, noncurrent	254	254
Property and equipment, net	1,584	1,716
Operating lease, right-of-use asset	8,219	8,837
Long-term investments	35,481	52,866
Deferred tax asset	155	214
Total assets	286,686	255,900
Current liabilities:
Accounts payable	4,286	3,349
Accrued liabilities	14,003	9,417
Deferred revenue, current	35,571	51,773
Operating lease liability, current	756	617
Current portion of long-term debt	3,380	4,622
Total current liabilities	57,996	69,778
Deferred revenue, noncurrent	39,185	50,830
Operating lease liability, noncurrent	10,085	11,009
Long-term debt	0	3,380
Total liabilities	107,266	134,997
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value per share; 10,000,000 shares authorized at December 31, 2022 and 2021; zero shares issued and outstanding at December 31, 2022 and 2021	0	0
Common stock, $0.001 par value per share; 200,000,000 shares authorized at December 31, 2022 and 2021; 47,234,900 shares issued and 45,984,433 shares outstanding at December 31, 2022; 31,444,746 shares issued and 30,194,279 shares outstanding at December 31, 2021	46	30
Treasury stock, at cost; 1,250,467 shares at December 31, 2022 and 2021	0	0
Additional paid-in capital	404,456	287,345
Accumulated other comprehensive loss	(1,121)	(273)
Accumulated deficit	(223,961)	(166,199)
Total stockholders’ equity	179,420	120,903
Total liabilities and stockholders’ equity	$ 286,686	$ 255,900

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2022	Dec. 31, 2021
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 issued (in shares)	0	0
Preferred stock, shares outstanding (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)	47,234,900	31,444,746
Common stock, shares outstanding (in shares)	45,984,433	30,194,279
Treasury stock, shares (in shares)	1,250,467	1,250,467

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Statement [Abstract]
Revenue from Contract with Customer, Product and Service [Extensible Enumeration]	Collaboration [Member]	Collaboration [Member]	Collaboration [Member]
Collaboration revenue	$ 30,064	$ 23,443	$ 9,335
Operating expenses:
Research and development	70,243	58,742	27,185
General and administrative	17,968	14,560	10,899
Total operating expenses	88,211	73,302	38,084
Loss from operations	(58,147)	(49,859)	(28,749)
Other income (expense):
Interest income	3,288	259	245
Interest expense	(476)	(816)	(775)
Other, net	(97)	(4)	1,333
Loss before taxes	(55,432)	(50,420)	(27,946)
Income tax benefit (expense)	(2,330)	87	6
Net loss	(57,762)	(50,333)	(27,940)
Comprehensive income (loss):
Unrealized loss on investments	(901)	(238)	(15)
Unrealized gain (loss) on foreign currency translation	53	(88)	58
Comprehensive loss	$ (58,610)	$ (50,659)	$ (27,897)
Weighted-average shares used to compute basic net loss per share (in shares)	33,435,280	25,476,889	20,826,466
Weighted-average shares used to compute diluted net loss per share (in shares)	33,435,280	25,476,889	20,826,466
Basic net loss per share (in dollars per share)	$ (1.73)	$ (1.98)	$ (1.34)
Diluted net loss per share (in dollars per share)	$ (1.73)	$ (1.98)	$ (1.34)

CONSOLIDATED STATEMENT OF STOCK

CONSOLIDATED STATEMENT OF STOCKHOLDERS’ EQUITY - USD ($) $ in Thousands	Total	Common Stock	Treasury	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Gain	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2019		18,587,892
Balance at beginning of period (in shares) at Dec. 31, 2019			50,467
Balance at beginning of period at Dec. 31, 2019	$ 29,474	$ 19		$ 117,371	$ 10	$ (87,926)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in public offering/Private Placement, net of offering costs/financing costs (in shares)		5,139,610
Issuance of common stock in public offering/Private Placement, net of offering costs/financing costs	56,258	$ 5		56,253
Issuance of warrants	60			60
Issuance of common stock under equity incentive plans (in shares)		75,681
Issuance of common stock under equity incentive plans	123			123
Stock-based compensation	4,140			4,140
Unrealized gain (loss) on investments	(15)				(15)
Unrealized gain (loss) on foreign currency translation	58				58
Net loss	(27,940)					(27,940)
Balance at end of period (in shares) at Dec. 31, 2020		23,803,183
Balance at end of period (in shares) at Dec. 31, 2020			50,467
Balance at end 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 public offering/Private Placement, net of offering costs/financing costs (in shares)		6,489,357
Issuance of common stock in public offering/Private Placement, net of offering costs/financing costs	90,733	$ 6		90,727
Issuance of common stock to Horizon (in shares)		951,980
Issuance of common stock to Horizon	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 gain (loss) on investments	(238)				(238)
Unrealized gain (loss) 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	30,194,279
Balance at end of period (in shares) at Dec. 31, 2021	1,250,467		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 public offering/Private Placement, net of offering costs/financing costs (in shares)		15,509,282
Issuance of common stock in public offering/Private Placement, net of offering costs/financing 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 gain (loss) on investments	(901)				(901)
Unrealized gain (loss) 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)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Operating activities
Net loss	$ (57,762)	$ (50,333)	$ (27,940)
Adjustments to reconcile net loss to net cash used in operating activities:
Loss on sale of property and equipment	93	4	5
Depreciation expense	597	620	578
Amortization of premium/discount on investments	(700)	906	103
Non-cash interest expense	178	274	261
Deferred income tax	46	(214)	0
Stock-based compensation expense	9,648	6,240	4,140
Changes in operating assets and liabilities:
Accounts receivable	24,602	0	0
Prepaid expenses and other current assets	2,001	(3,063)	255
Operating lease, right-of-use asset	618	564	592
Accounts payable and accrued liabilities	5,557	5,970	(491)
Deferred revenue	(27,847)	24,628	51,540
Operating lease liability	(785)	(844)	1,041
Net cash (used in) provided by operating activities	(43,754)	(15,248)	30,084
Investing activities
Purchases of property and equipment	(412)	(118)	(802)
Purchase of investments	(266,984)	(133,518)	(101,328)
Maturities of investments	154,048	81,156	29,311
Net cash used in investing activities	(113,348)	(52,480)	(72,819)
Financing activities
Proceeds from issuance of common stock	106,716	90,733	56,258
Proceeds from borrowings, net of issuance costs	0	0	5,000
Repayment of debt	(4,800)	(2,400)	0
Proceeds from exercise of stock options	773	502	123
Net cash provided by financing activities	102,689	100,764	61,381
Effect of exchange rate on cash and cash equivalents	(118)	(88)	58
Net (decrease) increase in cash and cash equivalents and restricted cash	(54,531)	32,948	18,704
Cash and cash equivalents and restricted cash, beginning of period	68,161	35,213	16,509
Cash and cash equivalents and restricted cash, end of period	13,630	68,161	35,213
Supplemental Information
Cash paid for interest	322	554	490
Discount in connection with issuance of debt	0	0	334
Cash paid for income taxes	543	0	0
Horizon
Financing activities
Proceeds from issuance of common stock	$ 0	$ 11,929	$ 0

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2022
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, 2022
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, and highly liquid money market funds. Restricted cash represents cash drawn on our line of credit used to establish collateral to support the security deposit on 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 securities, 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 securities, U.S. agency securities, non-U.S. government securities, and corporate debt securities and commercial paper with a contractual maturity of each security 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. 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 as interest income using the interest method over the terms of the securities. Realized gains and losses and declines in fair value deemed to be other than temporary 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, 2022, 2021 or 2020. 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 re-assessed 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 (“Horizon”). 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, 2022, 2021 and 2020, other comprehensive 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) .

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2022
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. The net loss per share for the year ended December 31, 2022 reflects the impact to weighted-average shares from issuance of 15,509,282 shares of common stock that were sold in September 2022 in an underwritten public offering, 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 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 Horizon. The net loss per share for the year ended December 31, 2020 reflects 5,139,610 shares of our common stock issued pursuant to the securities offering completed in July 2020. The number of shares issued during the periods presented impacts the comparability of our net loss per share for each period. 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, 2022 2021 2020 Common stock warrants 3,656,497 3,666,435 3,673,551 Prefunded warrants to purchase common stock 5,182,197 5,182,197 790,710 Stock options and RSUs outstanding 7,236,758 5,877,309 4,175,345 Total 16,075,452 14,725,941 8,639,606

Cash Equivalents and Investment

Cash Equivalents and Investments	12 Months Ended
	Dec. 31, 2022
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, 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 December 31, 2021 Amortized Cost Gross unrealized gains Gross unrealized losses Fair market value Money market funds $ 50,277 $ — $ — $ 50,277 U.S. treasury bills 30,006 1 (100) 29,907 Non-U.S. government securities 3,090 — (10) 3,080 Corporate debt securities and commercial paper 114,402 2 (129) 114,275 Total $ 197,775 $ 3 $ (239) $ 197,539 Classified as: Cash equivalents $ 50,277 Short-term investments 94,396 Long-term investments 52,866 Total $ 197,539

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2022
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value MeasurementsCash 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, 2022 and 2021, cash of $2.4 million and $17.6 million, respectively, is excluded from the fair value table below. The following tables summarize our financial assets and liabilities measured at fair value on a recurring basis (in thousands): 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 Assets: December 31, 2021 Level 1 Level 2 Level 3 Total Money market funds $ 50,277 $ — $ — $ 50,277 U.S. treasury bills 29,907 — — 29,907 Non-U.S. government securities — 3,080 — 3,080 Corporate debt securities and commercial paper — 114,275 — 114,275 Total $ 80,184 $ 117,355 $ — $ 197,539 Our Level 2 assets consist of U.S. agency securities, non-U.S government securities, and 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, 2022
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment, net, consists of the following (in thousands): December 31, 2022 2021 Laboratory equipment $ 3,191 $ 3,132 General equipment and furniture 459 479 Computer equipment and software 217 211 Leasehold improvements 118 85 Property and equipment, at cost 3,985 3,907 Less accumulated depreciation and amortization (2,401) (2,191) Property and equipment, net $ 1,584 $ 1,716

Additional Balance Sheet Inform

Additional Balance Sheet Information	12 Months Ended
	Dec. 31, 2022
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, 2022 2021 Prepaid research and development $ 1,881 $ 3,315 Prepaid insurance 368 454 Prepaid other 394 266 Deferred financing — 352 Other receivables 317 323 Prepaid expenses and other current assets $ 2,960 $ 4,710 Accrued liabilities consist of the following (in thousands): December 31, 2022 2021 Research and development services $ 7,657 $ 5,536 Employee compensation 4,110 3,084 Accrued taxes 1,847 127 Legal and professional fees 334 394 Accrued other 55 276 Accrued liabilities $ 14,003 $ 9,417

Long-term Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2022
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, $3.0 million of which was used to repay amounts owing under our prior loan and security agreement with SVB. In March 2020, the second tranche of $5.0 million was funded to us. We did not draw down the final tranche of $5.0 million, which expired in July 2020. The Term Loans accrue interest at a floating per annum rate of 0.25% above the prime rate, subject to a floor of 5.75%, which interest is payable monthly commencing in September 2019. Upon the occurrence and during the continuance of an event of default, a default interest rate will apply that is 4.0% above the otherwise applicable interest rate. 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 the agreement with AbbVie Ireland Unlimited Company (“AbbVie”), discussed in detail in Note 10 . As a result of the interest only extension, the Term Loans are payable in 25 equal monthly installments of principal plus interest, with the final installment due and payable on July 1, 2023. We may prepay all, but not less than all, of the Term Loans subject to a prepayment fee equal to $75,000. Additionally, a final payment in the amount of 5.5% of the funded Term Loans is payable to SVB on the date on which the Term Loans are prepaid, paid or become due and payable in full. The final payment fees are recorded in long-term debt in our accompanying Consolidated Balance Sheets . The Loan Agreement contains customary representations and warranties, events of default and affirmative and negative covenants, including, among others, covenants that limit or restrict our ability to, among other things, incur additional indebtedness, grant liens, merge or consolidate, make acquisitions, pay dividends or other distributions or repurchase equity, make investments, dispose of assets, engage in any new lines of business, and enter into certain transactions with affiliates, in each case subject to certain exceptions. We were in compliance with our covenants as of December 31, 2022. As security for our obligations under the Loan Agreement, we granted SVB a first priority security interest on substantially all of our assets, except intellectual property, and subject to certain other exceptions. In connection with the Loan Agreement, we issued a warrant to SVB 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 . We recorded a total debt discount of $812,000 in connection with the Loan Agreement, which is being amortized to interest expense using the effective interest method over the repayment term of the loan. The unamortized discount included in the current portion of long-term debt in our accompanying Consolidated Balance Sheets was $45,000 as of December 31, 2022. Interest expense associated with the amortization of the discount is recorded as non-cash interest expense on our accompanying Consolidated Statements of Cash Flows . As of December 31, 2022, $2.8 million in remaining principle and $625,000 in final payment fees due under our Loan Agreement are classified within current portion of long-term debt in our accompanying Consolidated Balance Sheets

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2022
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, 2022. 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 March 2019, in connection with the lease, we provided 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, 2022 , our operating lease right-of-use assets and operating lease liability associated with our leases were $8.2 million and $10.8 million, respectively. Supplemental operating lease information is as follows (in thousands): Years Ended December 31, 2022 2021 2020 Operating lease cost $ 1,839 $ 1,837 $ 1,872 Variable lease cost 823 603 475 Total lease cost $ 2,662 $ 2,440 $ 2,347 Other information: Cash paid for amounts included in the measurement of lease liabilities $ 2,009 $ 2,117 $ 213 Weighted-average remaining lease term (years) 7.2 8.2 9.2 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, 2022 are as follows (in thousands): Minimum Lease Payments 2023 $ 1,889 2024 2,111 2025 2,167 2026 2,224 2027 2,283 Thereafter 5,155 Total future minimum lease payments 15,829 Less: imputed interest (4,988) Operating lease liabilities $ 10,841 Contingencies Certain credits received related to our research and development expenditures 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 during 2022 and recorded an estimated current foreign income tax provision of $1.3 million for the expected repayments, of which we paid $543,000. The remaining $849,000 is included as accrued taxes within accrued liabilities on our accompanying Consolidated Balance Sheets .

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2022
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 (the “AbbVie Agreement”) for the development of ALPN-101, (“acazicolcept”). The AbbVie Agreement grants AbbVie the exclusive option to purchase an exclusive worldwide license to acazicolcept (the “License Option”). The License Option is exercisable by AbbVie at any time and will expire 90 days from the achievement of certain development milestones. 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 conducting our Phase 2 study in systemic lupus erythematosus, based on an agreed-upon development plan (the “Development Plan”). We will be 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 proposed by AbbVie. We will also be solely 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 has agreed to make cash payments upon our achievement of certain predefined pre-option development milestones (the “Alpine Development Milestones”) up to an aggregate amount of $75.0 million. In 2021, we received $45.0 million of the Alpine Development Milestones. If AbbVie exercises the License Option, they will pay a one-time cash payment of $75.0 million. Following the exercise of the License Option, AbbVie has also agreed to make aggregate cash payments of up to $205.0 million upon AbbVie’s achievement of certain development and commercial milestones and additional aggregate cash payments of up to $450.0 million upon AbbVie’s achievement of certain sales-based cash milestones, collectively referred to as (the “AbbVie Milestones”). Subsequent to commercialization, we are also eligible to receive high single-digit to low double-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. Horizon In December 2021, we entered into an exclusive license and collaboration agreement with Horizon (the “Horizon Agreement”) for the development 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 Horizon will be responsible for the respective costs and, ultimately, Horizon will assume responsibility for development and commercialization activities and costs. In connection with the execution of the Horizon Agreement in December 2021, we entered into a stock purchase agreement under which Horizon 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 Horizon of $11.9 million was recorded to equity, based on the closing price of common stock on the effective date of the Horizon 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 Horizon Agreement. Under the terms of the agreements, Horizon also paid us a non-refundable upfront payment of $25.0 million in the first quarter of 2022. In addition, we are eligible to receive up to $381.0 million per program, or up to approximately $1.5 billion in total, 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 $839,000 in research and development support provided by us during the year ended December 31, 2022. 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 Therapeutics plc (“Adaptimmune”) to develop next-generation SPEAR T cell products (the “Adaptimmune Agreement”). Under the Adaptimmune Agreement, we are to perform certain research services and grant Adaptimmune an exclusive license to programs from our secreted immunomodulatory protein (“SIP”) and transmembrane immunomodulatory protein (“TIP”) technologies. Through December 31, 2022, 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. We have also recorded $2.0 million in research support payments to fund ongoing programs through December 31, 2022. In addition, we are eligible for research support payments, one-time payments and downstream development and commercialization milestones of up to $288.0 million, if respective pre-specified milestones for each program are achieved. We are also eligible to receive low-single digit percentage royalties on worldwide net sales of the applicable products. For revenue recognition purposes, licensing and research support fees billed under the agreement are being recorded as deferred revenue and recognized to revenue based on employee hours contributed to each performance obligation. 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, 2022 2021 Contract Assets $ 392 $ 25,000 Contract Liabilities $ 74,756 $ 102,603 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 proportion 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, 2022 2021 2020 AbbVie $ 17,064 $ 23,443 $ 7,025 Horizon 11,964 — — Adaptimmune 1,036 — 2,310 Total collaboration revenue $ 30,064 $ 23,443 $ 9,335 Revenue recognized during the period that was included in the opening contract liab ility balance was $28.2 million, $23.4 million and $2.3 million for the years ended December 31, 2022, 2021 and 2020, respectively. 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, 2022
Equity [Abstract]
Stockholders’ Equity	Stockholders’ Equity Common Stock Shares of common stock reserved for future issuance were as follows: December 31, 2022 2021 Shares to be issued upon exercise of outstanding stock options 7,058,140 5,611,743 Shares to be issued upon release of RSUs 178,618 265,566 Shares to be issued upon conversion of common stock warrants 3,656,497 3,666,435 Shares to be issued upon conversion of prefunded warrants 5,182,197 5,182,197 Shares available for future stock grants 314,523 284,906 Shares to be issued under employee stock purchase plan 45,211 45,211 Shares of common stock reserved for future issuance 16,435,186 15,056,058 In January 2023, in connection with the exercise of certain prefunded warrants, we issued 1,708,773 shares of common stock. Securities Offerings 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 $448,000 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 “Shares”) and prefunded warrants to purchase 3,191,487 Shares (the “Prefunded Warrants”). The purchase price for each Share and for each Prefunded Warrant was $9.40 per share, for an aggregate purchase price of approximately $91.0 million. The Prefunded Warrants became fully exercisable upon the closing date and have an exercise price of $0.001 per share. We incurred $266,000 in financing 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 . 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 . In July 2020, we entered into a securities purchase agreement (the “2020 Securities Purchase Agreement”) for a private placement with a select group of institutional investors, pursuant to which we sold 5,139,610 units (the “Common Units”) and 790,710 units (the “Prefunded Warrant Units”), for an aggregate purchase price of $60.0 million. Each Common Unit consists of one share of our common stock plus a warrant to purchase 0.3 shares of common stock (the “Common Stock Warrants”), and each Prefunded Warrant Unit consists of one prefunded warrant to purchase one share of common stock plus 0.3 Common Stock Warrants. The Prefunded Warrant Units and the Common Units are collectively referred to as the “Units” and each Unit has a purchase price of $10.1175. Pursuant to the terms of the 2020 Securities Purchase Agreement, we issued warrants to purchase 1,779,096 shares of common stock with an exercise price of $12.74 and a term of 3.5 years. Additionally, we issued 790,710 prefunded warrants, which became fully exercisable upon the closing date and have an exercise price of $0.001 per share. We incurred $3.7 million in financing costs associated with the 2020 Securities Purchase Agreement, which was netted against the proceeds within additional-paid-in-capital on our accompanying Consolidated Balance Sheets . The issuance of the securities sold under the 2021 and 2020 Securities Purchase Agreements 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 registration statements for the 2021 and 2020 Securities Purchase Agreements with the SEC, which were declared effective by the SEC in November 2021 and August 2020, respectively, which cover the resale of the shares of common stock issuable in connection with the private placements and upon exercise of the warrants. In May 2021, the registration statement for the 2019 Securities Purchase Agreement was deactivated following the expiration of our obligation to maintain its effectiveness under the related registration rights agreement. Financing Agreements In September 2022, we and Cowen and Company, LLC (“Cowen”) mutually terminated the sales agreement (the “Sales Agreement”) dated July 2, 2021. The 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, under which Cowen acted as the sales agent. No sales of our common stock were sold under the Sales Agreement. In connection with the termination of the Sales Agreement, we wrote off $488,000 of deferred financing costs to expense, which were previously included within prepaid expenses and other current assets on our accompanying Consolidated Balance Sheets . Common Stock 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, 2021 3,666,435 $ 12.64 2.12 Exercised (9,938) 5.02 Outstanding and Exercisable at December 31, 2022 3,656,497 $ 12.66 1.12 Equity Incentive Plans Our board of directors approved an “inducement” grant, pursuant to Rule 5635(c)(4) of the Nasdaq Listing Rules, to our Chief Medical Officer for options to purchase 160,000 shares of our common stock at a per-share exercise price of $8.38, which was set based on our stock price as of the grant date in August 2022. The option grant is exempt from registration under the Securities Act of 1933, as amended, pursuant to Section 4(a)(2) thereof as a transaction by an issuer not involving a public offering. Options granted vest within four years, subject to continued employment, and expire ten years after the date of grant. In June 2018, our stockholders approved the 2018 Equity Incentive Plan (“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 (the “2015 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. In June 2020, in conjunction with our annual meeting of stockholders, our stockholders approved an additional increase of 743,515 shares authorized under our 2018 Plan. Under our 2018 Plan we may issue stock options, stock appreciation rights, restricted stock, RSUs or performance shares. As of December 31, 2022, we have only issued stock options and RSUs. Our 2018 Plan provides for an annual increase in the number of shares reserved for insurance 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, 2023, a total of 1,500,000 additional shares were automatically added to the shares authorized under the 2018 Plan. In July 2017, in connection with the merger, 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, 2022, we have not activated the ESPP. Stock options granted under our equity plans generally vest within four years and vested options are exercisable from the grant 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, 2022, a total of 8,116,862 shares of common stock were authorized for issuance under our 2018 Plan, 2015 Plan and 2015 EIP. A summary of stock option activity under our plans is presented below: Options Weighted- Weighted- Aggregate Outstanding at December 31, 2021 5,611,743 $ 7.41 Granted 1,904,650 $ 10.95 Exercised (226,480) $ 3.20 Forfeited/Expired (231,773) $ 11.20 Outstanding at December 31, 2022 7,058,140 $ 8.37 7.12 $ 9,833 Vested and expected to vest after December 31, 2022 6,918,140 $ 8.38 7.11 $ 9,833 Exercisable at December 31, 2022 4,035,424 $ 6.75 5.97 $ 8,865 The aggregate intrinsic value of stock options exercised during the years ended December 31, 2022, 2021 and 2020 was $1.1 million, $546,000 and $104,000, respectively. The fair value of stock options vested during the years ended December 31, 2022, 2021 and 2020 was $8.6 million, $3.4 million and $4.2 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, 2021 265,566 $ 12.00 Granted 3,750 $ 9.00 Released (61,227) $ 12.00 Forfeited (29,471) $ 11.62 Non-vested at December 31, 2022 178,618 $ 12.00 8.90 $ 1,313 The aggregate intrinsic value of RSUs released during the years ended December 31, 2022, 2021 and 2020 was $527,000, $0 and $1.6 million, respectively. The fair value of RSUs vested during the years ended December 31, 2022, 2021 and 2020 was $735,000, $0 and $461,000, respectively. We utilize newly issued shares to satisfy option exercises and RSU releases. As of December 31, 2022, there was $19.6 million of unrecognized stock-based compensation expense related to approximately 3.2 million nonvested stock options and RSU awards that are expected to be recognized over a weighted-average period of 2.5 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, 2022 2021 2020 Weighted-average estimated fair value at grant $7.56 $8.44 $3.03 Risk-free interest rate (1) 1.39% - 4.04% 0.25% - 1.3% 0.38% - 1.68% Expected term of options (in years) (2) 5.27 - 6.08 3.49 – 6.08 5.27 – 6.90 Expected stock price volatility (3) 80% - 83% 79% - 83% 73% - 82% 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 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, 2022 2021 2020 Research and development $ 5,713 $ 3,347 $ 2,179 General and administrative 3,935 2,893 1,961 Total stock-based compensation expense $ 9,648 $ 6,240 $ 4,140

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2022
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, 2022 2021 2020 Domestic $ (55,475) $ (48,932) $ (28,356) Foreign 43 (1,488) 410 Total $ (55,432) $ (50,420) $ (27,946) The provision for income taxes is composed of the following (in thousands): Years Ended December 31, 2022 2021 2020 Current: U.S. - Federal $ (981) $ — $ — U.S. - State (16) — 6 Foreign (1,285) (129) — Total current (2,282) (129) 6 Deferred: U.S. - Federal — — — U.S. - State — — — Foreign (48) 216 — Total deferred (48) 216 — Total income tax benefit (expense) $ (2,330) $ 87 $ 6 The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Years Ended December 31, 2022 2021 2020 U.S. Statutory rate 21.0 % 21.0 % 21.0 % Effect of: State taxes (net of federal benefit) 0.1 % — % — % Permanent differences — % (0.4) % 0.1 % Federal research and development credit 5.0 % 4.5 % 2.6 % Change in valuation allowance (28.6) % (21.4) % (20.7) % Global intangible low-taxed income recapture (0.6) % (1.4) % — % Stock-based compensation (1.9) % (1.8) % (2.8) % Foreign rate differential — % 0.2 % (0.1) % Other — % (0.5) % (0.1) % Voluntary disclosure net change in prior years’ tax liabilities 0.8 % — % — % Effective income tax rate (4.2) % 0.2 % — % For the year ended December 31, 2022, $1.0 million of the tax expense represents 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 (“IRC”) Section 174. As part of the Tax Cuts and Jobs Act of 2017 (TCJA), 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 the tax expense represents recognition of current tax liabilities, which were the result of a cumulative change to our foreign income tax provision for the activities of our wholly owned subsidiary, 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-months 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 from true-ups 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. In addition to tax rate impacts attributable to our revised transfer pricing model and the mandatory capitalization of research and development expenses under IRC Section 174 for the year ended December 31, 2022, the difference between the U.S. federal statutory tax rates of 21% and our effective tax rate 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 (“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, 2022 2021 Deferred tax assets: Net operating loss $ 18,590 $ 22,054 Research and development credits 6,240 6,297 Intangible asset basis 2 13 Lease liability 2,315 2,476 Deferred revenue 14,752 8,333 Stock based compensation 2,343 1,672 Capitalized research and experimental costs 12,681 — Other 156 217 Gross deferred tax assets 57,079 41,062 Valuation allowance (54,841) (38,752) Total deferred tax assets, net of valuation allowance 2,238 2,310 Deferred tax liabilities: Prepaid expenses (173) (147) Fixed asset basis (182) (93) Right-of-use asset basis (1,728) (1,856) Total deferred tax liability (2,083) (2,096) Net deferred tax assets and liabilities $ 155 $ 214 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, 2022, 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, 2022, we determined that it was more likely than not that our foreign deferred tax assets, with the exception of our foreign research and development income tax credit carryforward deferred tax assets, would be realized. As such, we have recorded a full valuation allowance against our foreign research and development income tax credit carryforward deferred tax assets. The valuation allowance increased by $16.1 million and $10.9 million during the years ended December 31, 2022 and 2021, respectively. We have net operating loss (“NOL”) carryforwards as follows (in thousands): December 31, 2022 2021 Federal (before January 1, 2018) $ — $ 11,094 Federal (after January 1, 2018) $ 85,012 $ 91,592 State $ 6,433 $ 6,433 Foreign $ — $ — 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, 2022 2021 Federal $ 7,892 $ 7,880 Foreign $ 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 are currently reviewing the preliminary results of a Section 382 study that was commissioned during the fourth quarter of 2022, focused on our wholly owned operating company and subsidiary, AIS Operating Co., Inc. It is likely we experienced ownership changes on June 10, 2016 and September 17, 2021, and as a result, our ability to utilize 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 year ended December 31, 2022 and will continue to evaluate the results throughout the first quarter of 2023. Though limited in the amount which can be utilized for the year ended December 31, 2022 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, 2022. Under IRC Section 382 and 383, the 2017 merger with Nivalis Therapeutics Inc., or 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, 2022 2021 2020 Unrecognized benefits – beginning of year $ 1,553 $ 990 $ 775 Gross increases (decreases) – prior year tax positions — — (7) Gross increases – current year tax positions 596 563 222 Unrecognized benefit – end of year $ 2,149 $ 1,553 $ 990 All of the unrecognized tax benefits as of December 31, 2022 are accounted for as a reduction in our deferred tax assets. Due to our valuation allowance, none of the $2.1 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 2022, 2021 or 2020. 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, 2022
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions In September 2021, in connection with our 2021 Securities Purchase Agreement, we issued 3,723,402 shares of common stock and 2,340,424 prefunded warrants to purchase shares of common stock for gross proceeds of approximately $57.0 million to certain of our stockholders whose beneficial ownership exceeded 5% prior to the completion of the 2021 Securities Purchases Agreement. Also in September 2021, we exchanged an aggregate of 1,200,000 shares of common stock held Frazier Life Sciences VIII, L.P. for Prefunded Warrants to purchase an aggregate of 1,200,000 shares of common stock. None of the purchasers in the July 2020 private placement was a greater than 5% holder of our outstanding capital stock prior to the July 2020 private placement.

401(k) Retirement Plan

401(k) Retirement Plan	12 Months Ended
	Dec. 31, 2022
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 related expense recognized was $287,000 for the year ended December 31, 2022. Prior to 2022, we did not match employee contributions to the plan.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2022
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, and highly liquid money market funds. Restricted cash represents cash drawn on our line of credit used to establish collateral to support the security deposit on 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 securities, 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 securities, U.S. agency securities, non-U.S. government securities, and corporate debt securities and commercial paper with a contractual maturity of each security 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. 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 as interest income using the interest method over the terms of the securities. Realized gains and losses and declines in fair value deemed to be other than temporary 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, 2022, 2021 or 2020.
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 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	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
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 (“Horizon”). 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
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, 2022, 2021 and 2020, other comprehensive 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 MeasurementsCash 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.

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2022
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, 2022 2021 2020 Common stock warrants 3,656,497 3,666,435 3,673,551 Prefunded warrants to purchase common stock 5,182,197 5,182,197 790,710 Stock options and RSUs outstanding 7,236,758 5,877,309 4,175,345 Total 16,075,452 14,725,941 8,639,606

Cash Equivalents and Investme_2

Cash Equivalents and Investments (Tables)	12 Months Ended
	Dec. 31, 2022
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, 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 December 31, 2021 Amortized Cost Gross unrealized gains Gross unrealized losses Fair market value Money market funds $ 50,277 $ — $ — $ 50,277 U.S. treasury bills 30,006 1 (100) 29,907 Non-U.S. government securities 3,090 — (10) 3,080 Corporate debt securities and commercial paper 114,402 2 (129) 114,275 Total $ 197,775 $ 3 $ (239) $ 197,539 Classified as: Cash equivalents $ 50,277 Short-term investments 94,396 Long-term investments 52,866 Total $ 197,539

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2022
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables summarize our financial assets and liabilities measured at fair value on a recurring basis (in thousands): 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 Assets: December 31, 2021 Level 1 Level 2 Level 3 Total Money market funds $ 50,277 $ — $ — $ 50,277 U.S. treasury bills 29,907 — — 29,907 Non-U.S. government securities — 3,080 — 3,080 Corporate debt securities and commercial paper — 114,275 — 114,275 Total $ 80,184 $ 117,355 $ — $ 197,539

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2022
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net, consists of the following (in thousands): December 31, 2022 2021 Laboratory equipment $ 3,191 $ 3,132 General equipment and furniture 459 479 Computer equipment and software 217 211 Leasehold improvements 118 85 Property and equipment, at cost 3,985 3,907 Less accumulated depreciation and amortization (2,401) (2,191) Property and equipment, net $ 1,584 $ 1,716

Additional Balance Sheet Info_2

Additional Balance Sheet Information (Tables)	12 Months Ended
	Dec. 31, 2022
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, 2022 2021 Prepaid research and development $ 1,881 $ 3,315 Prepaid insurance 368 454 Prepaid other 394 266 Deferred financing — 352 Other receivables 317 323 Prepaid expenses and other current assets $ 2,960 $ 4,710
Schedule of Accrued Liabilities Disclosure	Accrued liabilities consist of the following (in thousands): December 31, 2022 2021 Research and development services $ 7,657 $ 5,536 Employee compensation 4,110 3,084 Accrued taxes 1,847 127 Legal and professional fees 334 394 Accrued other 55 276 Accrued liabilities $ 14,003 $ 9,417

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2022
Commitments and Contingencies Disclosure [Abstract]
Schedule of Components of Lease Cost	Supplemental operating lease information is as follows (in thousands): Years Ended December 31, 2022 2021 2020 Operating lease cost $ 1,839 $ 1,837 $ 1,872 Variable lease cost 823 603 475 Total lease cost $ 2,662 $ 2,440 $ 2,347 Other information: Cash paid for amounts included in the measurement of lease liabilities $ 2,009 $ 2,117 $ 213 Weighted-average remaining lease term (years) 7.2 8.2 9.2 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, 2022 are as follows (in thousands): Minimum Lease Payments 2023 $ 1,889 2024 2,111 2025 2,167 2026 2,224 2027 2,283 Thereafter 5,155 Total future minimum lease payments 15,829 Less: imputed interest (4,988) Operating lease liabilities $ 10,841

License and Collaboration Agr_2

License and Collaboration Agreements (Tables)	12 Months Ended
	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Contract Balances	Respective balances are as follows (in thousands): December 31, 2022 2021 Contract Assets $ 392 $ 25,000 Contract Liabilities $ 74,756 $ 102,603
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, 2022 2021 2020 AbbVie $ 17,064 $ 23,443 $ 7,025 Horizon 11,964 — — Adaptimmune 1,036 — 2,310 Total collaboration revenue $ 30,064 $ 23,443 $ 9,335

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2022
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, 2022 2021 Shares to be issued upon exercise of outstanding stock options 7,058,140 5,611,743 Shares to be issued upon release of RSUs 178,618 265,566 Shares to be issued upon conversion of common stock warrants 3,656,497 3,666,435 Shares to be issued upon conversion of prefunded warrants 5,182,197 5,182,197 Shares available for future stock grants 314,523 284,906 Shares to be issued under employee stock purchase plan 45,211 45,211 Shares of common stock reserved for future issuance 16,435,186 15,056,058
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, 2021 3,666,435 $ 12.64 2.12 Exercised (9,938) 5.02 Outstanding and Exercisable at December 31, 2022 3,656,497 $ 12.66 1.12
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, 2021 5,611,743 $ 7.41 Granted 1,904,650 $ 10.95 Exercised (226,480) $ 3.20 Forfeited/Expired (231,773) $ 11.20 Outstanding at December 31, 2022 7,058,140 $ 8.37 7.12 $ 9,833 Vested and expected to vest after December 31, 2022 6,918,140 $ 8.38 7.11 $ 9,833 Exercisable at December 31, 2022 4,035,424 $ 6.75 5.97 $ 8,865
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, 2021 265,566 $ 12.00 Granted 3,750 $ 9.00 Released (61,227) $ 12.00 Forfeited (29,471) $ 11.62 Non-vested at December 31, 2022 178,618 $ 12.00 8.90 $ 1,313
Schedule of Stock Option Assumptions	The fair values of stock options granted to employees were calculated using the following assumptions: Years Ended December 31, 2022 2021 2020 Weighted-average estimated fair value at grant $7.56 $8.44 $3.03 Risk-free interest rate (1) 1.39% - 4.04% 0.25% - 1.3% 0.38% - 1.68% Expected term of options (in years) (2) 5.27 - 6.08 3.49 – 6.08 5.27 – 6.90 Expected stock price volatility (3) 80% - 83% 79% - 83% 73% - 82% 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 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, 2022 2021 2020 Research and development $ 5,713 $ 3,347 $ 2,179 General and administrative 3,935 2,893 1,961 Total stock-based compensation expense $ 9,648 $ 6,240 $ 4,140

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2022
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, 2022 2021 2020 Domestic $ (55,475) $ (48,932) $ (28,356) Foreign 43 (1,488) 410 Total $ (55,432) $ (50,420) $ (27,946)
Schedule of Provision for Income Taxes	The provision for income taxes is composed of the following (in thousands): Years Ended December 31, 2022 2021 2020 Current: U.S. - Federal $ (981) $ — $ — U.S. - State (16) — 6 Foreign (1,285) (129) — Total current (2,282) (129) 6 Deferred: U.S. - Federal — — — U.S. - State — — — Foreign (48) 216 — Total deferred (48) 216 — Total income tax benefit (expense) $ (2,330) $ 87 $ 6
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, 2022 2021 2020 U.S. Statutory rate 21.0 % 21.0 % 21.0 % Effect of: State taxes (net of federal benefit) 0.1 % — % — % Permanent differences — % (0.4) % 0.1 % Federal research and development credit 5.0 % 4.5 % 2.6 % Change in valuation allowance (28.6) % (21.4) % (20.7) % Global intangible low-taxed income recapture (0.6) % (1.4) % — % Stock-based compensation (1.9) % (1.8) % (2.8) % Foreign rate differential — % 0.2 % (0.1) % Other — % (0.5) % (0.1) % Voluntary disclosure net change in prior years’ tax liabilities 0.8 % — % — % Effective income tax rate (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, 2022 2021 Deferred tax assets: Net operating loss $ 18,590 $ 22,054 Research and development credits 6,240 6,297 Intangible asset basis 2 13 Lease liability 2,315 2,476 Deferred revenue 14,752 8,333 Stock based compensation 2,343 1,672 Capitalized research and experimental costs 12,681 — Other 156 217 Gross deferred tax assets 57,079 41,062 Valuation allowance (54,841) (38,752) Total deferred tax assets, net of valuation allowance 2,238 2,310 Deferred tax liabilities: Prepaid expenses (173) (147) Fixed asset basis (182) (93) Right-of-use asset basis (1,728) (1,856) Total deferred tax liability (2,083) (2,096) Net deferred tax assets and liabilities $ 155 $ 214
Schedule of Net Operating Loss Carryforwards	We have net operating loss (“NOL”) carryforwards as follows (in thousands): December 31, 2022 2021 Federal (before January 1, 2018) $ — $ 11,094 Federal (after January 1, 2018) $ 85,012 $ 91,592 State $ 6,433 $ 6,433 Foreign $ — $ —
Schedule of Net Research and Development Tax Credit Carryforwards	We have net research and development tax credit carryforwards as follows (in thousands): December 31, 2022 2021 Federal $ 7,892 $ 7,880 Foreign $ 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, 2022 2021 2020 Unrecognized benefits – beginning of year $ 1,553 $ 990 $ 775 Gross increases (decreases) – prior year tax positions — — (7) Gross increases – current year tax positions 596 563 222 Unrecognized benefit – end of year $ 2,149 $ 1,553 $ 990

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Details) - segment	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments	1
Debt securities, available-for-sale, term (less than)	2 years	2 years
Minimum percentage of income tax benefit realized upon ultimate settlement (greater than)	50%
Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of the assets	5 years

Net Loss Per Share - Narrative

Net Loss Per Share - Narrative (Details) - shares	1 Months Ended			12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jul. 31, 2020	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issued (in shares)			5,139,610
Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issued (in shares)				15,509,282	6,489,357	5,139,610
2021 PIPE
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issued (in shares)		6,489,357
Horizon
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, 2022	Dec. 31, 2021	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	16,075,452	14,725,941	8,639,606
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)	3,656,497	3,666,435	3,673,551
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)	5,182,197	5,182,197	790,710
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)	7,236,758	5,877,309	4,175,345

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, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 271,889	$ 197,775
Gross unrealized gains	14	3
Gross unrealized losses	(1,153)	(239)
Fair market value	270,750	197,539
Cash equivalents
Debt Securities, Available-for-sale [Line Items]
Fair market value	11,004	50,277
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	224,265	94,396
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	35,481	52,866
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,004	50,277
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair market value	11,004	50,277
U.S. treasury bills
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	113,964	30,006
Gross unrealized gains	5	1
Gross unrealized losses	(721)	(100)
Fair market value	113,248	29,907
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,921
Gross unrealized gains	5
Gross unrealized losses	(11)
Fair market value	10,915
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	6,060	3,090
Gross unrealized gains	0	0
Gross unrealized losses	(22)	(10)
Fair market value	6,038	3,080
Corporate debt securities and commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	129,940	114,402
Gross unrealized gains	4	2
Gross unrealized losses	(399)	(129)
Fair market value	$ 129,545	$ 114,275

Cash Equivalents and Investme_4

Cash Equivalents and Investments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Cash, Cash Equivalents, and Short-Term Investments [Abstract]
Realized gains and losses on securities	$ 0
Available-for-sale debt securities in unrealized loss position	163,900,000	$ 83,000,000
Security of carrying amount	42,800,000
Gross unrealized loss	$ 600,000	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Cash	$ 2.4	$ 17.6

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, 2022	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	$ 270,750	$ 197,539
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	11,004	50,277
U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	113,248	29,907
U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	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	3,080
Corporate debt securities and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	129,545	114,275
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	124,252	80,184
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	11,004	50,277
Level 1 | U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	113,248	29,907
Level 1 | U.S. agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	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	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	146,498	117,355
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	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	3,080
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	129,545	114,275
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
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	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	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	$ 3,985	$ 3,907
Less accumulated depreciation and amortization	(2,401)	(2,191)
Property and equipment, net	1,584	1,716
Depreciation expenses	597	620	$ 578
Laboratory equipment
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	3,191	3,132
General equipment and furniture
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	459	479
Computer equipment and software
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	217	211
Leasehold improvements
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	$ 118	$ 85

Additional Balance Sheet Info_3

Additional Balance Sheet Information - Prepaid Expense and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid research and development	$ 1,881	$ 3,315
Prepaid insurance	368	454
Prepaid other	394	266
Deferred financing	0	352
Other receivables	317	323
Prepaid expenses and other current assets	$ 2,960	$ 4,710

Additional Balance Sheet Info_4

Additional Balance Sheet Information - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Research and development services	$ 7,657	$ 5,536
Employee compensation	4,110	3,084
Accrued taxes	1,847	127
Legal and professional fees	334	394
Accrued other	55	276
Accrued liabilities	$ 14,003	$ 9,417

Long-term Debt (Details)

Long-term Debt (Details) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($)	Aug. 31, 2019 USD ($) installment tranche $ / shares shares	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jul. 31, 2020 USD ($) $ / shares shares	Mar. 30, 2020 USD ($) shares
Debt Instrument [Line Items]
Term loan drew down			$ 0	$ 0	$ 5,000
Unamortized debt discount			45
Long-term debt			2,800
Prepayment fee			$ 625
Common Stock Warrant
Debt Instrument [Line Items]
Warrants exercisable for shares of common stock (in shares) | shares						1,779,096
Warrants issued, exercise price (in dollars per share) | $ / shares						$ 12.74
Term Loans
Debt Instrument [Line Items]
Unamortized debt discount		$ 812
Term Loans | Term Loans, Tranche One | Common Stock Warrant
Debt Instrument [Line Items]
Fair value of warrant liability		60
Term Loans | Term Loans, Tranche Two | Common Stock Warrant
Debt Instrument [Line Items]
Fair value of warrant liability							$ 60
Silicon Valley Bank | Original Agreement
Debt Instrument [Line Items]
Prepayment fee representing deferred portion of final payment		75
Silicon Valley Bank | Term Loans
Debt Instrument [Line Items]
Maximum borrowing capacity, amount		$ 15,000
Number of tranches | tranche		3
Variable interest rate		0.25%
Percentage of default interest rate		4%
Aggregate new capital milestone for continuation of Interest only payment period		$ 40,000
Number of equal monthly installments payable | installment		25
Long-term debt final payment fee percentage		5.50%
Silicon Valley Bank | Term Loans | Common Stock Warrant
Debt Instrument [Line Items]
Warrants exercisable for shares of common stock (in shares) | shares		52,083
Silicon Valley Bank | Term Loans | Base Rate
Debt Instrument [Line Items]
Interest rate floor		5.75%
Silicon Valley Bank | Term Loans | Term Loans, Tranche One
Debt Instrument [Line Items]
Term loan drew down		$ 5,000
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 | Original Agreement
Debt Instrument [Line Items]
Repayments of debt		$ 3,000
Silicon Valley Bank | Term Loans | Term Loans, Tranche Two
Debt Instrument [Line Items]
Term loan drew down	$ 5,000
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

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) lease_term_type	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Loss Contingencies [Line Items]
Operating lease, term of contract	10 years 9 months 18 days
Number of option to extend term lease | lease_term_type	1
Operating lease, renewal term	5 years
Operating lease, annual base rent	$ 1,700
Operating lease, annual base rent increasing percentage	3%
Maximum tenant improvement allowance	$ 5,400
Maximum additional tenant improvement allowance	$ 1,800
Amortization percent, additional tenant improvement allowance	8%
Operating lease, right-of-use asset		$ 8,219	$ 8,837
Operating lease, liability		10,841
Current foreign tax expense (benefit)		1,285	129	$ 0
Amount of income tax paid		543	$ 0	$ 0
Accrued liabilities		$ 849
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, 2022	Dec. 31, 2021	Dec. 31, 2020
Lease cost:
Operating lease cost	$ 1,839	$ 1,837	$ 1,872
Variable lease cost	823	603	475
Total lease cost	2,662	2,440	2,347
Other information:
Cash paid for amounts included in the measurement of lease liabilities	$ 2,009	$ 2,117	$ 213
Weighted-average remaining lease term (years)	7 years 2 months 12 days	8 years 2 months 12 days	9 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, 2022 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2023	$ 1,889
2024	2,111
2025	2,167
2026	2,224
2027	2,283
Thereafter	5,155
Total future minimum lease payments	15,829
Less: imputed interest	(4,988)
Operating lease liabilities	$ 10,841

License and Collaboration Agr_3

License and Collaboration Agreements-Narrative (Details) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Dec. 31, 2021 USD ($) program candidate $ / shares shares	Sep. 30, 2021 shares	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($) $ / shares	Dec. 31, 2020 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2020 USD ($) payment	Jun. 30, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	$ 102,603		$ 74,756	$ 102,603
Revenue recognized from contract with customer			28,200	23,400	$ 2,300
Private Placement | Common Stock
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock issued in transaction (in shares) | shares		6,489,357
AbbVie | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential future revenue recognition milestones, number of payments | payment							1
Potential cash payment upon license option exercise							$ 75,000
Potential cash payment upon achievement of certain development and commercial milestones							205,000
Potential cash payment upon achievement of certain sales-based cash milestones							450,000
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 with customer, liability							$ 60,000
AbbVie | Product | Achieved Milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	$ 45,000			$ 45,000
Horizon
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
Horizon | Common Stock
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock price period for premium on sale of stock	30 days
Horizon | 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
Horizon | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	28,100			$ 28,100
Potential future milestones receivable (up to)	1,500,000			1,500,000
Potential future milestones receivable per program (up to)	381,000			381,000
Horizon | Product | Upfront Payment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	25,000		25,000	25,000
Horizon | Product | Research and Development
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized from contract with customer			839
Horizon | Product | Premium Payment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	$ 3,100			$ 3,100
Adaptimmune | Product | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential future milestones receivable (up to)			288,000
Adaptimmune | Product | Upfront License Payment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability								$ 2,000
Adaptimmune | Product | License Payment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized from contract with customer			3,000
Adaptimmune | Product | License Fee And Research Support Payment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability						$ 1,000
Adaptimmune | Product | Research Support Payment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized from contract with customer			$ 2,000

License and Collaboration Agr_4

License and Collaboration Agreements - Contract Balances (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Contract Assets	$ 392	$ 25,000
Contract Liabilities	$ 74,756	$ 102,603

License and Collaboration Agr_5

License and Collaboration Agreements - Collaboration Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenue	$ 30,064	$ 23,443	$ 9,335
AbbVie
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenue	17,064	23,443	7,025
Horizon
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenue	11,964	0	0
Adaptimmune
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenue	$ 1,036	$ 0	$ 2,310

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2022	Dec. 31, 2021
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	16,435,186	15,056,058
Stock options
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	7,058,140	5,611,743
Restricted stock units
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	178,618	265,566
Common stock warrants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	3,656,497	3,666,435
Prefunded warrants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	5,182,197	5,182,197
Future stock grants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	314,523	284,906
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) $ / shares in Units, $ in Thousands

1 Months Ended

2 Months Ended

12 Months Ended

Jan. 01, 2022 shares

Sep. 30, 2022 USD ($)

Aug. 31, 2022 $ / shares shares

Sep. 30, 2021 USD ($) $ / shares shares

Jul. 31, 2020 USD ($) shares warrant $ / shares

Jun. 30, 2020 shares

Jun. 30, 2018 shares

Oct. 31, 2022 USD ($) shares

Dec. 31, 2022 USD ($) shares

Dec. 31, 2021 USD ($) shares

Dec. 31, 2020 USD ($)

Jan. 31, 2023 shares

Jul. 31, 2017 shares

Class of Stock [Line Items]

Stock issuance costs incurred | $

$ 266

$ 3,700

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

$ 91,000

$ 60,000

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

$ 10.1175

Deferred financing | $

$ 0

$ 352

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

16,435,186

15,056,058

Vesting period (in years)

4 years

Term (in years)

10 years

Unrecognized compensation cost related to nonvested stock options | $

$ 19,600

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

2 years 6 months

Sales Agreement | Cowen & Co.

Class of Stock [Line Items]

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

$ 75,000

Deferred financing | $

$ 488

2018 Plan

Class of Stock [Line Items]

Additional number of shares authorized (in shares)

1,500,000

743,515

Percentage of outstanding stock maximum

5%

2018 Plan | Maximum

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)

8,116,862

Prefunded warrants

Class of Stock [Line Items]

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

5,182,197

5,182,197

Inducement Grant | Common Stock

Class of Stock [Line Items]

Options granted (in shares)

160,000

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

$ 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)

7,058,140

5,611,743

Aggregate intrinsic value of stock options exercised | $

$ 1,100

$ 546

$ 104

Fair value of shares vested | $

$ 8,600

$ 3,400

4,200

Restricted stock units

Class of Stock [Line Items]

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

178,618

265,566

Aggregate intrinsic value of RSU's released | $

$ 527

$ 0

1,600

Fair value of RSUs vesting | $

$ 735

$ 0

$ 461

Stock options and Restricted stock units

Class of Stock [Line Items]

Nonvested awards (in shares)

3,200,000

Common Stock

Class of Stock [Line Items]

Exchange of common stock for prefunded warrants (in shares)

(1,200,000)

(1,200,000)

Treasury

Class of Stock [Line Items]

Exchange of common stock for prefunded warrants (in shares)

1,200,000

1,200,000

Public Stock Offering

Class of Stock [Line Items]

Stock issued in transaction (in shares)

15,509,282

Proceeds from sale of equity | $

$ 106,700

Stock issuance costs incurred | $

$ 448

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

790,710

Warrants issued, exercise price (in dollars per share) | $ / shares

$ 0.001

$ 0.001

Prefunded warrants | Common Stock

Class of Stock [Line Items]

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

$ 9.40

Prefunded warrants | Subsequent Event

Class of Stock [Line Items]

Warrants exercisable (in shares)

1,708,773

Exchange Warrants | Prefunded warrants

Class of Stock [Line Items]

Warrants exercisable (in shares)

1,200,000

Common Units

Class of Stock [Line Items]

Stock issued in transaction (in shares)

5,139,610

Common Units | Common Stock

Class of Stock [Line Items]

Common stock per unit sold in purchase agreement (in shares)

1

Common Units | Common Stock Warrant

Class of Stock [Line Items]

Purchase agreement ratio of common stock to common stock warrants (in shares)

0.3

Common Stock Warrant

Class of Stock [Line Items]

Warrants exercisable (in shares)

1,779,096

Warrants issued, exercise price (in dollars per share) | $ / shares

$ 12.74

Term of warrants

3 years 6 months

Prefunded Warrant Unit

Class of Stock [Line Items]

Number of prefunded warrants per unit sold in purchase agreement | warrant

1

Prefunded Warrant Unit | Common Stock

Class of Stock [Line Items]

Number of securities called by each warrant or right (in shares)

1

Prefunded Warrant Unit | Common Stock Warrant

Class of Stock [Line Items]

Purchase agreement ratio of prefunded warrants to common stock warrants (percent of warrants) (in shares)

0.3

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Warrants (Details) - $ / shares	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Warrants Outstanding
Warrants outstanding (in shares)	3,666,435
Exercised (in shares)	(9,938)
Warrants outstanding and exercisable (in shares)	3,656,497	3,666,435
Weighted- average Exercise Price
Weighted average, exercise price, outstanding (in dollars per share)	$ 12.64
Exercise (in dollars per share)	5.02
Weighted average, exercise price, outstanding and exercisable (in dollars per share)	$ 12.66	$ 12.64
Weighted- average Remaining Contract Term (in years)
Weighted-average remaining contractual term, outstanding	1 year 1 month 13 days	2 years 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, 2022 USD ($) $ / shares shares
Options Outstanding
Outstanding at beginning of period (in shares) | shares	5,611,743
Granted (in shares) | shares	1,904,650
Exercised (in shares) | shares	(226,480)
Forfeited/Expired (in shares) | shares	(231,773)
Outstanding at ending of period (in shares) | shares	7,058,140
Vested and expected to vest (in shares) | shares	6,918,140
Exercisable (in shares) | shares	4,035,424
Weighted- average Exercise Price (per share)
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 7.41
Granted (in dollars per share) | $ / shares	10.95
Exercised (in dollars per share) | $ / shares	3.20
Forfeited/Expired (in dollars per share) | $ / shares	11.20
Outstanding at ending of period (in dollars per share) | $ / shares	8.37
Vested and expected to vest (in dollars per share) | $ / shares	8.38
Exercisable (in dollars per share) | $ / shares	$ 6.75
Weighted- average Remaining Contract Term (in years)
Outstanding (in years)	7 years 1 month 13 days
Vested and expected to vest (in years)	7 years 1 month 9 days
Exercisable (in years)	5 years 11 months 19 days
Aggregate Intrinsic Value (in thousands)
Outstanding | $	$ 9,833
Vested and expected to vest | $	9,833
Exercisable | $	$ 8,865

Stockholders' Equity - Schedu_4

Stockholders' Equity - Schedule of RSU Activity (Details) - RSUs $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2022 USD ($) $ / shares shares
Number of Shares
Non-vested at beginning of period (in shares) | shares	265,566
Granted (in shares) | shares	3,750
Released (in shares) | shares	(61,227)
Forfeited (in shares) | shares	(29,471)
Non-vested at ending of period (in shares) | shares	178,618
Weighted- Average Grant Date Fair Value (per share)
Non-vested at beginning of period (in dollars per share) | $ / shares	$ 12
Granted (in dollars per share) | $ / shares	9
Released (in dollars per share) | $ / shares	12
Forfeited (in dollars per share) | $ / shares	11.62
Non-vested at ending of period (in dollars per share) | $ / shares	$ 12
Weighted- average Remaining Contract Life (in years)
Non-vested (in years)	8 years 10 months 24 days
Aggregate Intrinsic Value (in thousands)
Non-vested | $	$ 1,313

Stockholders' Equity - Schedu_5

Stockholders' Equity - Schedule of Stock Option Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average estimated fair value at grant (in dollars per share)	$ 7.56	$ 8.44	$ 3.03
Risk-free interest rate, minimum	1.39%	0.25%	0.38%
Risk-free interest rate, maximum	4.04%	1.30%	1.68%
Expected stock price volatility, minimum	80%	79%	73%
Expected stock price volatility, maximum	83%	83%	82%
Expected dividend yield	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	3 years 5 months 26 days	5 years 3 months 7 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 10 months 24 days

Stockholders' Equity - Schedu_6

Stockholders' Equity - Schedule of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 9,648	$ 6,240	$ 4,140
Employee | Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	5,713	3,347	2,179
Employee | General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 3,935	$ 2,893	$ 1,961

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, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Domestic	$ (55,475)	$ (48,932)	$ (28,356)
Foreign	43	(1,488)	410
Loss before taxes	$ (55,432)	$ (50,420)	$ (27,946)

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Current:
U.S. - Federal	$ (981)	$ 0	$ 0
U.S. - State	(16)	0	6
Current tax expense	(1,285)	(129)	0
Total current	(2,282)	(129)	6
Deferred:
U.S. - Federal	0	0	0
U.S. - State	0	0	0
Foreign	(48)	216	0
Total deferred	(48)	216	0
Total income tax benefit (expense)	$ (2,330)	$ 87	$ 6

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, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Tax Disclosure [Abstract]
U.S. Statutory rate	21%	21%	21%
Effect of:
State taxes (net of federal benefit)	0.10%	0%	0%
Permanent differences	0%	(0.40%)	0.10%
Federal research and development credit	5%	4.50%	2.60%
Change in valuation allowance	(28.60%)	(21.40%)	(20.70%)
Global intangible low-taxed income recapture	(0.60%)	(1.40%)	0%
Stock-based compensation	(1.90%)	(1.80%)	(2.80%)
Foreign rate differential	0%	0.20%	(0.10%)
Other	0%	(0.50%)	(0.10%)
Voluntary disclosure net change in prior years’ tax liabilities	0.80%	0%	0%
Effective income tax rate	(4.20%)	0.20%	0%

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Current federal, state and local, tax expense	$ 1,000,000
Current tax expense	1,285,000	$ 129,000	$ 0
Deferred tax expense	48,000	(216,000)	0
Income tax benefit	(2,330,000)	87,000	6,000
Increase in valuation allowance	16,100,000	10,900,000
Unrecognized tax benefits that would impact effective tax rate	0
Unrecognized tax benefits	2,149,000	1,553,000	990,000	$ 775,000
Accrued interest or penalties related to unrecognized tax benefits	$ 0	$ 0	$ 0
UNITED STATES
Income Taxes [Line Items]
Research and development, capitalized and amortized period	5 years
Outline United States
Income Taxes [Line Items]
Research and development, capitalized and amortized period	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, 2022	Dec. 31, 2021
Deferred tax assets:
Net operating loss	$ 18,590	$ 22,054
Research and development credits	6,240	6,297
Intangible asset basis	2	13
Lease liability	2,315	2,476
Deferred revenue	14,752	8,333
Stock based compensation	2,343	1,672
Capitalized research and experimental costs	12,681	0
Other	156	217
Gross deferred tax assets	57,079	41,062
Valuation allowance	(54,841)	(38,752)
Total deferred tax assets, net of valuation allowance	2,238	2,310
Deferred tax liabilities:
Prepaid expenses	(173)	(147)
Fixed asset basis	(182)	(93)
Right-of-use asset basis	(1,728)	(1,856)
Total deferred tax liability	(2,083)	(2,096)
Net deferred tax assets and liabilities	$ 155	$ 214

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Operating Loss Carryforwards (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
State
Income Taxes [Line Items]
Net operating loss carryforwards	$ 6,433	$ 6,433
Foreign
Income Taxes [Line Items]
Net operating loss carryforwards	0	0
Before January 1 2018 | Federal
Income Taxes [Line Items]
Net operating loss carryforwards	0	11,094
After January 1 2018 | Federal
Income Taxes [Line Items]
Net operating loss carryforwards	$ 85,012	$ 91,592

Income Taxes - Schedule of Tax

Income Taxes - Schedule of Tax Credit Carryforwards (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Income Taxes [Line Items]
Research and development credits	$ 6,240	$ 6,297
Federal
Income Taxes [Line Items]
Research and development credits	7,892	7,880
Foreign
Income Taxes [Line Items]
Research and development credits	$ 527	$ 0

Income Taxes - Schedule of Acti

Income Taxes - Schedule of Activity Related to Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized benefits – beginning of year	$ 1,553	$ 990	$ 775
Gross increases (decreases) – prior year tax positions	0	0	(7)
Gross increases – current year tax positions	596	563	222
Unrecognized benefit – end of year	$ 2,149	$ 1,553	$ 990

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Sep. 30, 2021	Jul. 31, 2020	Dec. 31, 2021
Related Party Transaction [Line Items]
Proceeds from sale of common stock and warrants, net of offering costs	$ 91	$ 60
Exchange Warrants | Prefunded warrants
Related Party Transaction [Line Items]
Warrants exercisable (in shares)	1,200,000
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 | Alpine Immune Sciences
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners		5%
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
Common Stock and Common Stock Warrants | Affiliated Entity | 2021 Security Purchases Agreement | Alpine Immune Sciences
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners	5%
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 | 2021 Security Purchases Agreement
Related Party Transaction [Line Items]
Stock issued in transaction (in shares)	3,723,402
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 | 2021 Security Purchases Agreement
Related Party Transaction [Line Items]
Warrants exercisable (in shares)	2,340,424

401(k) Retirement Plan (Details

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