Alpine Immune Sciences (ALPN) 10-K 17 Mar 22 2021 FY Annual report Financial data

Cover

Cover - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Mar. 07, 2022	Jun. 30, 2021
Cover [Abstract]
Entity Central Index Key	0001626199
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Amendment Flag	false
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
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			$ 104.8
Entity Common Stock, Shares Outstanding		30,294,434
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to the 2022 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, 2021.

Audit Information

Audit Information	12 Months Ended
	Dec. 31, 2021
Audit Information [Abstract]
Auditor Name	Ernst & Young LLP
Auditor Location	Seattle, WA
Auditor Firm ID	42

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 67,907	$ 34,959
Short-term investments	94,396	70,622
Accounts receivable	25,000	0
Prepaid expenses and other current assets	4,710	1,520
Total current assets	192,013	107,101
Restricted cash, noncurrent	254	254
Property and equipment, net	1,716	1,785
Operating lease, right-of-use asset	8,837	9,401
Long-term investments	52,866	25,549
Deferred tax asset	214	0
Total assets	255,900	144,090
Current liabilities:
Accounts payable	3,349	582
Accrued liabilities	9,417	5,777
Deferred revenue, current	51,773	31,627
Operating lease liability, current	617	655
Current portion of long-term debt	4,622	2,526
Total current liabilities	69,778	41,167
Deferred revenue, noncurrent	50,830	21,348
Operating lease liability, noncurrent	11,009	11,815
Long-term debt	3,380	7,602
Total liabilities	134,997	81,932
Commitments and contingencies
Preferred stock, $0.001 par value per share; 10,000,000 shares authorized at December 31, 2021 and 2020; zero shares issued and outstanding at December 31, 2021 and 2020	0	0
Stockholders’ equity:
Common stock, $0.001 par value per share; 200,000,000 shares authorized at December 31, 2021 and 2020; 31,444,746 shares issued and 30,194,279 shares outstanding at December 31, 2021; 23,853,650 shares issued and 23,803,183 shares outstanding at December 31, 2020	30	24
Treasury stock, at cost; 1,250,467 shares and 50,467 shares at December 31, 2021 and 2020, respectively	0	0
Additional paid-in capital	287,345	177,947
Accumulated other comprehensive (loss) gain	(273)	53
Accumulated deficit	(166,199)	(115,866)
Total stockholders’ equity	120,903	62,158
Total liabilities and stockholders’ equity	$ 255,900	$ 144,090

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Convertible preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Convertible preferred stock, shares issued (in shares)	0	0
Convertible 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)	31,444,746	23,853,650
Common stock, shares outstanding (in shares)	30,194,279	23,803,183
Treasury stock, shares (in shares)	1,250,467	50,467

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Collaboration revenue	$ 23,443	$ 9,335	$ 1,740
Revenue from Contract with Customer, Product and Service [Extensible Enumeration]	Collaboration [Member]	Collaboration [Member]	Collaboration [Member]
Operating expenses:
Research and development	$ 58,742	$ 27,185	$ 35,847
General and administrative	14,560	10,899	9,467
Total operating expenses	73,302	38,084	45,314
Loss from operations	(49,859)	(28,749)	(43,574)
Other income (expense):
Interest expense	(816)	(775)	(338)
Interest income	259	245	1,248
Other (expense) income	(4)	1,333	812
Loss before taxes	(50,420)	(27,946)	(41,852)
Income tax (expense) benefit	87	6	0
Net loss	(50,333)	(27,940)	(41,852)
Comprehensive income (loss):
Unrealized loss on investments	(238)	(15)	29
Unrealized (loss) gain on foreign currency translation	(88)	58	(6)
Comprehensive loss	$ (50,659)	$ (27,897)	$ (41,829)
Weighted-average shares used to compute basic net loss per share (in shares)	25,476,889	20,826,466	18,358,864
Weighted-average shares used to compute diluted net loss per share (in shares)	25,476,889	20,826,466	18,358,864
Basic net loss per share (in dollars per share)	$ (1.98)	$ (1.34)	$ (2.28)
Diluted net loss per share (in dollars per share)	$ (1.98)	$ (1.34)	$ (2.28)

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, 2018		13,854,205	50,467
Balance at beginning of period at Dec. 31, 2018	$ 44,591	$ 14	$ 0	$ 90,664	$ (13)	$ (46,074)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Units/common stock in Private Placement, net of offering costs (in shares)		4,706,700
Issuance of Units/common stock in Private Placement, net of offering costs	23,598	$ 5		23,593
Exercise of stock options (in shares)		26,987
Exercise of stock options	13			13
Stock-based compensation	3,041			3,041
Issuance of warrants	60			60
Unrealized gain (loss) on investments	29				29
Unrealized gain (loss) on foreign currency translation	(6)				(6)
Net loss	(41,852)					(41,852)
Balance at end of period (in shares) at Dec. 31, 2019		18,587,892	50,467
Balance at end of period at Dec. 31, 2019	29,474	$ 19	$ 0	117,371	10	(87,926)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Units/common stock in Private Placement, net of offering costs (in shares)		5,139,610
Issuance of Units/common stock in Private Placement, net of offering costs	56,258	$ 5		56,253
Stock-based compensation	4,140			4,140
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
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	50,467
Balance at end of period at Dec. 31, 2020	62,158	$ 24	$ 0	177,947	53	(115,866)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Units/common stock in Private Placement, net of offering costs (in shares)		6,489,357
Issuance of Units/common stock in Private Placement, net of offering 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
Stock-based compensation	6,240			6,240
Issuance of common stock under equity incentive plans (in shares)		149,759
Issuance of common stock under equity incentive plans	502			502
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	1,250,467
Balance at end of period at Dec. 31, 2021	$ 120,903	$ 30	$ 0	$ 287,345	$ (273)	$ (166,199)

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating activities
Net loss	$ (50,333)	$ (27,940)	$ (41,852)
Adjustments to reconcile net loss to net cash used in operating activities:
Loss on sale of property and equipment	4	5	16
Depreciation expense	620	578	468
Amortization of premium/discount on investments	906	103	(360)
Non-cash interest expense	274	261	140
Deferred income tax	(214)	0	0
Stock-based compensation expense	6,240	4,140	3,041
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(3,063)	255	(408)
Right-of-use asset	564	592	1,553
Accounts payable and accrued liabilities	5,970	(491)	816
Deferred revenue	24,628	51,540	1,435
Lease liabilities	(844)	1,041	(195)
Net cash (used in) provided by operating activities	(15,248)	30,084	(35,346)
Investing activities
Purchases of property and equipment	(118)	(802)	(821)
Purchase of investments	(133,518)	(101,328)	(59,382)
Maturities of investments	81,156	29,311	75,575
Proceeds from the sale of investments	0	0	1,391
Net cash (used in) provided by investing activities	(52,480)	(72,819)	16,763
Financing activities
Proceeds from sale of common stock and warrants, net of offering costs	90,733	56,258	23,598
Proceeds from issuance of common stock to Horizon	11,929	0	0
Proceeds from borrowings, net of issuance costs	0	5,000	1,977
Repayment of debt	(2,400)	0	(1,333)
Proceeds from exercise of stock options	502	123	13
Net cash provided by financing activities	100,764	61,381	24,255
Effect of exchange rate on cash, cash equivalents and restricted cash	(88)	58	(6)
Net increase in cash and cash equivalents and restricted cash	32,948	18,704	5,666
Cash and cash equivalents and restricted cash, beginning of period	35,213	16,509	10,843
Cash and cash equivalents and restricted cash, end of period	68,161	35,213	16,509
Supplemental Information
Recognition of right-of-use asset	0	0	11,173
Cash paid for interest	554	490	170
Discount in connection with issuance of debt	$ 0	$ 334	$ 0

Description of Business

Description of Business	12 Months Ended
	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of Business	Description of Business Alpine 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 cancer and autoimmune and inflammatory diseases. Our approach includes a proprietary scientific platform that converts native immune system proteins into differentiated, multi-targeted therapeutics. We believe our strategies are capable of meaningfully modulating the human immune system and significantly improving outcomes in patients with serious diseases. We were incorporated under the laws of the State of Delaware and are headquartered in Seattle, Washington. A novel strain of coronavirus, SARS-CoV-2 (“COVID-19”), was first reported in December 2019, and subsequently declared a global pandemic by the World Health Organization in March 2020. As a result of the COVID-19 outbreak, many companies have experienced disruptions in their operations and in markets served. We have implemented some and may take additional temporary precautionary measures intended to help ensure the well-being of our employees and minimize business disruption. We considered the impact of COVID-19 on the assumptions and estimates used and determined that there were no material adverse impacts to our results of operations and financial position at December 31, 2021. The full extent of the future impacts of the continuing COVID-19 outbreak on our operations is uncertain and may adversely impact our business, including our clinical trials.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2021
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, 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, commercial paper, and corporate debt securities with a final 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 the 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 recoverable. If the carrying value exceeds the undiscounted future cash flows estimated to result from the use and eventual disposition of the asset, we write down the asset to its estimated fair value. Impairment is assessed by comparing the undiscounted cash flows expected to be generated by the asset to its carrying value. We did not record any impairments in the years ended December 31, 2021, 2020 or 2019. 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. In calculating the right-of-use asset and lease liability, we elected to combine lease and non-lease components. We exclude short-term leases having initial terms of 12 months or less from the new guidance 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 typically received under these arrangements 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 11 . 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 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 will 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, 2021, 2020, and 2019, 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) . Recently Adopted Accounting Pronouncements

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2021
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, 2021 reflects 6,489,357 shares of our common stock issued pursuant to a private placement financing completed in September 2021 and 951,980 shares of our common stock issued in December 2021 pursuant to a private placement financing with 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 net loss per share for the year ended December 31, 2019 reflects 4,706,700 shares of our common stock issued pursuant to a private placement financing completed in January 2019. The increased number of shares issued in these periods has affected the year-over-year comparability of our net loss per share calculations. 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, does not differ from basic net loss per share for the periods presented. December 31, 2021 2020 2019 Common stock warrants 3,666,435 3,673,551 1,877,094 Prefunded warrants to purchase common stock 5,182,197 790,710 — Stock options and RSUs outstanding 5,877,309 4,175,345 3,252,144 Total 14,725,941 8,639,606 5,129,238

Cash Equivalents and Investment

Cash Equivalents and Investments	12 Months Ended
	Dec. 31, 2021
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, 2021 Amortized Gross Gross Fair market Money market funds $ 50,277 $ — $ — $ 50,277 U.S. treasury bills 30,006 1 (100) 29,907 Corporate debt securities and commercial paper 117,492 2 (139) 117,355 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 December 31, 2020 Amortized Gross Gross Fair market Money market funds $ 28,424 $ — $ — $ 28,424 U.S. treasury bills 18,122 8 — 18,130 Corporate debt securities and commercial paper 82,047 2 (9) 82,040 Total $ 128,593 $ 10 $ (9) $ 128,594 Classified as: Cash equivalents $ 32,423 Short-term investments 70,622 Long-term investments 25,549 Total $ 128,594

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2021
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Cash and cash equivalents, restricted cash, receivables, accounts payable and accrued liabilities, which are recorded at invoiced amount or cost, approximate fair value based on the short-term nature of these financial instruments. Fair value is defined as the exchange price received for an asset or paid to transfer a liability, or an exit price, in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value, is as follows: Level 1 : Quoted prices in active markets for identical assets or liabilities. Level 2 : Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, quoted prices in inactive markets, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 : Unobservable inputs supported by little or no market activity and significant to the fair value of the assets or liabilities. As of December 31, 2021 and 2020, cash of $17.6 million and $2.5 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, 2021 Level 1 Level 2 Level 3 Total Money market funds $ 50,277 $ — $ — $ 50,277 U.S. treasury bills 29,907 — — 29,907 Corporate debt securities and commercial paper — 117,355 — 117,355 Total $ 80,184 $ 117,355 $ — $ 197,539 Assets: December 31, 2020 Level 1 Level 2 Level 3 Total Money market funds $ 28,424 $ — $ — $ 28,424 U.S. treasury bills 18,130 — — 18,130 Corporate debt securities and commercial paper — 82,040 — 82,040 Total $ 46,554 $ 82,040 $ — $ 128,594 Our Level 2 assets consist of commercial paper and corporate debt securities. 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, 2021
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment, net, consists of the following (in thousands): December 31, 2021 2020 Laboratory equipment $ 3,132 $ 2,604 General equipment and furniture 479 479 Computer equipment and software 211 211 Leasehold improvements 85 85 Property and equipment, at cost 3,907 3,379 Less accumulated depreciation and amortization (2,191) (1,594) Property and equipment, net $ 1,716 $ 1,785

Sale of Intangible Asset

Sale of Intangible Asset	12 Months Ended
	Dec. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Sale of Intangible Assets	Sale of Intangible Asset In February 2018, we entered into an Option License Agreement (“Option Agreement”) with Laurel Venture Capital Ltd. (“Laurel”), which granted Laurel a limited license to evaluate the indefinite-life GSNOR inhibitor IPR&D asset acquired as part of the merger with Nivalis in 2017. The IPR&D represents the processes, expertise, and technology employed in the development of GSNOR inhibitors and Nivalis’ lead product candidate, cavosonstat. Under the Option Agreement, we received an upfront non-refundable payment of $75,000, which was recognized as revenue in our accompanying Consolidated Statements of Operations and Comprehensive Income (Loss) . In June 2018, we entered into an Asset Purchase Agreement (“Purchase Agreement”) with Laurel and completed the sale of global rights to the GSNOR asset. Upon the sale of the GSNOR assets, we derecognized the full carrying value of the intangible asset. As consideration under the Purchase Agreement, we received a non-refundable closing payment of $250,000, which was accounted for as a purchase of our intangible asset. In June 2019, we recognized as revenue an additional payment of $425,000, related to the asset purchase. In addition, we are eligible to receive milestone payments of up to $20.0 million, in the aggregate upon satisfaction by Laurel of certain regulatory approval milestones. We will also be eligible to receive royalty payments equal to a low single-digit percentage rate of worldwide net sales of any approved products.

Additional Balance Sheet Inform

Additional Balance Sheet Information	12 Months Ended
	Dec. 31, 2021
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, 2021 2020 Prepaid research and development $ 3,315 $ 517 Prepaid insurance 454 447 Deferred financing 352 — Tenant improvement allowance receivable — 84 Prepaid other 266 168 Other receivables 323 304 Prepaid expenses and other current assets $ 4,710 $ 1,520 Accrued liabilities consist of the following (in thousands): December 31, 2021 2020 Research and development services $ 5,536 $ 2,571 Employee compensation 3,084 2,619 Legal and professional fees 394 482 Accrued taxes 127 — Accrued other 276 105 Accrued Liabilities $ 9,417 $ 5,777

Long-term Debt

Long-term Debt	12 Months Ended
	Dec. 31, 2021
Debt Disclosure [Abstract]
Long-term Debt	Long-term Debt In December 2016, we entered into a Loan and Security Agreement (the “Original Agreement”), with Silicon Valley Bank (“SVB”), under which we borrowed $5.0 million. The Original Agreement accrued interest at a floating per annum rate equal to the lender’s prime rate minus 1.75%. The Original Agreement had an interest-only period through July 2018. In August 2019 (the “Effective Date”), we entered into an Amended and Restated Loan and Security Agreement (the “Loan Agreement”) with 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 Original Agreement. 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 on July 31, 2020. We intend to use the debt proceeds for working capital and other general corporate purposes, including the advancement of our development programs. 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 AbbVie agreement, discussed in detail in Note 11 . As a result of the interest only extension, the Term Loans will be 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 of the Term Loans subject to a prepayment fee equal to $75,000, which represents the deferred portion of the final payment due under the Original Agreement, plus the outstanding principal balance under the Term Loans at the time of such prepayment multiplied by a prepayment fee of 2.0% in the first year, 1.0% in the second year, and 0% in the third year and thereafter. 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 with an offsetting reduction to debt discount on 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 assessed the likelihood of the lender accelerating payment of the loan due to a material adverse change in our business, operations, financial, or other condition as remote. We were in compliance with our covenants as of December 31, 2021. As such, as of December 31, 2021, the classification of the loan is split between current and noncurrent based on the timing of payment obligations. As security for its 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 Loan, an additional 17,361 shares became exercisable. The remaining warrants did not vest and expired on July 31, 2020, upon the expiration of the third tranche of our Term Loan. 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 on our accompanying Consolidated Balance Sheets . In connection with Original Agreement, SVB also holds 7,069 fully vested common stock warrants at an exercise price of $12.38 per share. The Term Loan was accounted for as a debt modification in a non-troubled debt restructuring, rather than a debt extinguishment, based on a comparison of the present value of the cash flows under the terms of the debt immediately before and after the Effective Date of the Term Loan, which resulted in a change of less than 10%. As a result, the remaining unamortized debt discount recorded in connection with the Original Agreement will be amortized to interest expense over the repayment term of Loan Agreement. In connection with the initial and second tranches of the Loan Agreement, we recorded a total debt discount of $812,000, which is being amortized to interest expense using the effective interest method over the repayment term of the loan. Non-cash interest expense associated with the amortization of the discount was $274,000, $261,000, and $140,000, for years ended December 31, 2021, 2020, and 2019, respectively. The unamortized discount was $223,000 as of December 31, 2021. Scheduled principal payments on our outstanding debt as of December 31, 2021 under our Loan Agreement, excluding final fee amounts, are as follows (in thousands): Year Ending December 31, Total 2022 $ 4,800 2023 2,800 Total future principal payments $ 7,600

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Operating Leases We leased office and laboratory space located at 201 Elliott Avenue West, in Seattle, Washington, under an agreement classified as an operating lease that expired on December 31, 2019. In May 2017, as required by the terms of the lease, we entered into a line of credit to establish collateral to support the security deposit in an amount of $132,000, which was recorded as current restricted cash in our Consolidated Balance Sheets for the year ended December 31, 2019. 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, 2021. 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, 2021 , our operating lease right-of-use assets and operating lease liability associated with our leases were $8.8 million and $11.6 million, respectively. Supplemental operating lease information for the year ended December 31, 2021 was as follows (in thousands): For the Year Ended December 31, 2021 2020 Operating lease cost $ 1,837 $ 1,872 Variable lease cost 603 475 Total lease cost $ 2,440 $ 2,347 Other information: Cash paid for amounts included in the measurement of lease liabilities $ 2,117 $ 213 Weighted-average remaining lease term (years) 8.2 9.2 Weighted-average discount rate 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, 2021 are as follows (in thousands): Minimum Lease Payments 2022 $ 1,841 2023 2,057 2024 2,111 2025 2,167 2026 2,224 Thereafter 7,438 Total future minimum lease payments 17,838 Less: imputed interest (6,212) Operating lease liabilities $ 11,626 Contingencies Certain credits received related to our research and development expenditures and recorded within other income in our accompanying Consolidated Statements of Operations and Comprehensive Income (Loss) are subject to review by foreign taxing authorities. It is reasonably possible we may incur losses upon the completion of these reviews ranging from $0 to $1.8 million, which we could be required to repay to certain tax authorities.

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
License and Collaboration Agreements	License and Collaboration Agreements 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. As of December 31, 2021, we recorded on our accompanying Consolidated Balance Sheets $28.1 million in current and noncurrent deferred revenue, of which $25.0 million relates to accounts receivable for the up-front payment and $3.1 million relates to the premium paid by Horizon on the stock issuance. As work under the Horizon license and collaboration agreement did not begin until 2022, we did not recognize any related revenue for the year ended December 31, 2021. For revenue recognition purposes, we determined 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. 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 a 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 the second quarter of 2021, we achieved $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. The License Option and the AbbVie Milestones were not determined 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. We use a cost-based input method to measure progress toward completion of the performance obligation and to calculate the corresponding revenue to recognize each period. In applying the cost-based input, we use actual costs incurred relative to budgeted costs for the combined performance obligation. These costs consist primarily of internal personnel efforts and third-party contract costs relative to the level of patient enrollment in the study. Revenue will be recognized based on the level of costs incurred relative to the total budgeted costs for the performance obligation. A cost-based input method of revenue recognition requires management to make estimates of costs to complete our performance obligation. 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 obligation 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. We recognized revenue from the AbbVie Agreement of $23.4 million and $7.0 million for the years ended December 31, 2021 and 2020, respectively. As of December 31, 2021 the remaining balance of the transaction price is $74.5 million and is recorded as current and noncurrent deferred revenue on our accompanying Consolidated Balance Sheets . We expect to recognize the remaining deferred revenue over the remainder of our Development Plan, which began in June 2020 and ends upon the later of the exercise or expiration of the option. Adaptimmune In May 2019, we entered into a collaboration and licensing agreement with Adaptimmune (the “Adaptimmune Agreement”) to develop next-generation SPEAR T cell products. Under 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. In June 2019, under the terms of the Adaptimmune Agreement, we received an upfront license payment of $2.0 million, and through December 31, 2021 we have received an additional $1.6 million in research support payments to fund ongoing programs. These payments were recorded as deferred revenue upon receipt and were recognized to revenue based on employee hours contributed to each performance obligation. In the fourth quarter of 2020, based on the completion of our initial research and development efforts in connection with our performance obligations, we recognized the remaining balance in deferred revenue associated with Adaptimmune on our accompanying Consolidated Balance Sheets . Under the Adaptimmune Agreement we recognized no revenue for the year ended December 31, 2021, and $2.3 million and $1.3 million for the years ended December 31, 2020, and 2019, respectively. In addition, we are eligible for additional research support payments, one-time payments and downstream development and commercialization milestones of up to $288.0 million, if all pre-specified milestones for each program are achieved. We are also eligible to receive low-single digit royalties on worldwide net sales of the applicable products. In February 2022, Adaptimmune selected an additional research program, triggering a $1.0 million upfront payment, which will be recorded as deferred revenue upon receipt and recognized to revenue based on employee hours contributed.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2021
Equity [Abstract]
Stockholders’ Equity	Stockholders’ Equity Common Stock Shares of common stock reserved for future issuance were as follows: December 31, 2021 2020 Shares to be issued upon exercise of outstanding stock options 5,611,743 4,175,345 Shares to be issued upon release of RSUs 265,566 — Shares to be issued upon conversion of common stock warrants 3,666,435 3,673,551 Shares to be issued upon conversion of prefunded warrants 5,182,197 790,710 Shares available for future stock grants 284,906 887,901 Shares to be issued under employee stock purchase plan 45,211 45,211 Shares of common stock reserved for future issuance 15,056,058 9,572,718 Securities Offerings 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 . In January 2019, we entered into a securities purchase agreement (the “2019 Securities Purchase Agreement”) with a limited number of accredited investors, pursuant to which we sold 4,706,700 units (the “2019 Units”) for an aggregate purchase price of $25.3 million in a private placement (the “Private Placement”). Each 2019 Unit has a purchase price of $5.37 and consists of one share of our common stock and a warrant to purchase 0.39 shares of common stock. Pursuant to the terms of the 2019 Securities Purchase Agreement, we issued 4,706,700 shares of common stock and warrants to purchase an aggregate of 1,835,610 shares of common stock. The warrants have an exercise price of $12.74 and have a term of five years. We incurred $1.7 million in financing costs associated with the 2019 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, 2020, and 2019 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, 2020 and 2019 Securities Purchase Agreements with the SEC, which were declared effective by the SEC in November 2021, August 2020 and April 2019, 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 July 2021, we entered into a sales agreement (the “Sales Agreement”) with Cowen and Company, LLC (“Cowen”) pursuant to which 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. Cowen will act as the sales agent and will be entitled to compensation for services of up to 3.0% of the gross sales price per share of all shares sold through Cowen under the Sales Agreement. The shares would be issued pursuant to our effective shelf registration statement on Form S-3 (File No. 333-256107). We filed a prospectus supplement, dated July 2, 2021, with the SEC in connection with the offer and sale of the shares pursuant to the Sales Agreement. As of December 31, 2021, no sales have been made under the Sales Agreement. 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 warrant activity: Warrants Weighted- Weighted- Outstanding at December 31, 2020 3,673,551 $ 12.74 3.12 Exercised (2,484) 5.02 Expired (4,632) 97.12 Outstanding at December 31, 2021 3,666,435 $ 12.64 2.12 Exercisable at December 31, 2021 3,666,435 $ 12.64 2.12 Equity Incentive Plans 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, 2021 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, 2022, 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, 2021, 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, 2021, a total of 6,616,862 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, 2020 4,175,345 $ 5.30 Granted 1,940,518 $ 12.38 Exercised (91,190) $ 5.37 Forfeited (412,930) $ 9.88 Outstanding at December 31, 2021 5,611,743 $ 7.41 7.41 $ 36,465 Vested and expected to vest after December 31, 2021 5,471,743 $ 7.38 7.37 $ 35,685 Exercisable at December 31, 2021 2,944,273 $ 5.25 6.31 $ 26,053 The aggregate intrinsic value of stock options exercised during the years ended December 31, 2021, 2020 and 2019 was $546,000, $104,000 and $155,000, respectively. The fair value of stock options vested during the years ended December 31, 2021, 2020 and 2019 was $3.4 million, $4.2 million and $3.0 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, 2020 — $ — Granted 265,566 $ 12.00 Non-vested at December 31, 2021 265,566 $ 12.00 1.95 $ 491 The aggregate intrinsic value of RSUs released during the years ended December 31, 2021 and 2020 was $0 and $1.6 million, respectively. The fair value of RSUs vested during the years ended December 31, 2021 and 2020 was $0 and $461,000, respectively. We had no RSU activity for the year ended December 31, 2019. We utilize newly issued shares to satisfy option exercises and RSU releases. As of December 31, 2021, there was $16.4 million of unrecognized stock-based compensation expense related to approximately 2.9 million nonvested stock options and RSU awards that are expected to be recognized over a weighted-average period of 2.7 years. Stock-Based Compensation Expense The fair value of RSUs is equal to the closing stock price on the date of grant. We use the Black-Scholes option pricing model to estimate the fair value of stock options at the grant date. 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, 2021 2020 2019 Weighted-average estimated fair value at grant $8.44 $3.03 $4.14 Risk-free interest rate (1) 0.25% - 1.3% 0.38% - 1.68% 1.42% - 2.63% Expected term of options (in years) (2) 3.49 - 6.08 5.27 – 6.90 5.27 – 6.08 Expected stock price volatility (3) 79% - 83% 73% - 82% 70% - 77% 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 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, 2021 2020 2019 Employee: Research and development $ 3,323 $ 2,145 $ 1,608 General and administrative 2,891 1,955 1,359 Non-Employee: Research and development 24 34 68 General and administrative 2 6 6 Total stock-based compensation expense $ 6,240 $ 4,140 $ 3,041

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2021
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, 2021 2020 2019 Domestic $ (48,932) $ (28,356) $ (38,234) Foreign (1,488) 410 (3,618) Total $ (50,420) $ (27,946) $ (41,852) The provision for income taxes is composed of the following (in thousands): Years Ended December 31, 2021 2020 2019 Current: U.S. - Federal $ — $ — $ — U.S. - State — (6) — Foreign 129 — — Total current 129 (6) — Deferred: U.S. - Federal — — — U.S. - State — — — Foreign (216) — — Total deferred (216) — — Total income tax benefit $ (87) $ (6) $ — The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Years Ended December 31, 2021 2020 2019 U.S. Statutory rate 21.0 % 21.0 % 21.0 % Effect of: Permanent differences (0.4) % 0.1 % (0.1) % Federal research and development credit 4.5 % 2.6 % 1.1 % Change in valuation allowance (21.4) % (20.7) % (19.8) % Global intangible low-taxed income recapture (1.4) % — % — % Stock-based compensation (1.8) % (2.8) % (0.5) % Foreign rate differential 0.2 % (0.1) % 0.6 % Other (0.5) % (0.1) % (2.3) % Effective income tax rate 0.2 % — % — % We recorded tax benefits of $87,000, $6,000, and $0, representing effective tax rates of 0.2% for the year ended December 31, 2021, and 0.0%, for the years ended December 31, 2020 and 2019, respectively. 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. 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, 2021 2020 Deferred tax assets: Net operating loss $ 22,054 $ 21,992 Research and development credits 6,297 4,044 Intangible asset basis 13 24 Lease liability 2,476 2,619 Deferred revenue 8,333 — Stock based compensation 1,672 1,337 Other 217 72 Gross deferred tax assets 41,062 30,088 Valuation allowance (38,752) (27,851) Total deferred tax assets, net of valuation allowance 2,310 2,237 Deferred tax liabilities: Prepaid expenses (147) (125) Fixed asset basis (93) (137) Right-of-use asset basis (1,856) (1,975) Total deferred tax liability (2,096) (2,237) Net deferred tax assets and liabilities $ 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, 2021, we determined that based on an evaluation of the four 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, 2021, we determined that it was more likely than not that our foreign deferred tax assets would be realized. As such, we removed the valuation allowance previously in place against our foreign deferred tax assets as we have generated sufficient foreign taxable income to utilize all historical operating losses and are expecting future foreign taxable income. The valuation allowance increased by $10.9 million and $5.8 million during the year ended December 31, 2021 and 2020, respectively. We have net operating loss (“NOL”) carryforwards as follows (in thousands): December 31, 2021 2020 2019 Federal (before January 1, 2018) $ 11,094 $ 11,094 $ 11,094 Federal (after January 1, 2018) $ 91,592 $ 91,868 $ 67,500 State $ 6,433 $ 6,433 $ 6,433 Foreign $ — $ — $ 787 Federal NOL carryforwards created before January 1, 2018 begin to expire in 2037. Federal NOL carryforwards created after January 1, 2018 carryforward indefinitely. State NOL carryforwards begin to expire in 2038. Foreign NOLs carryforward indefinitely. We have net research and development tax credit carryforwards as follows (in thousands): December 31, 2021 2020 2019 Federal $ 7,880 $ 5,063 $ 3,986 Federal research and development tax credit carryforwards begin to expire in 2035. 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 Internal Revenue Code Section 382 and 383. Such an event may limit our ability to utilize NOLs and research and development tax credit carryforwards. Under Internal Revenue Code Section 382 and 383, the 2017 merger with Nivalis is likely considered an ownership change with respect to the potential limitation of the Nivalis federal tax credits and NOLs. As such, it is likely that any future utilization of Nivalis federal tax credits and NOLs is substantially limited. Therefore, as of December 31, 2018, all Nivalis tax credit and NOL carryforwards have been reduced to zero. We account for uncertainty in income taxes in accordance with ASC 740. 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): December 31, 2021 2020 2019 Unrecognized benefits – beginning of year $ 990 $ 775 $ 469 Gross increases (decreases) – prior year tax positions — (7) — Gross increases – current year tax positions 563 222 306 Unrecognized benefit – end of year $ 1,553 $ 990 $ 775 All of the unrecognized tax benefits as of December 31, 2021 are accounted for as a reduction in our deferred tax assets. Due to our valuation allowance, none of the $1.6 million of unrecognized tax benefits would affect our effective tax rate, if recognized. We do not believe it is reasonably possible that our unrecognized tax benefits will significantly change in 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 2021, 2020 and 2019. We do not expect any significant change in our unrecognized tax benefits during the next twelve months.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2021
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, 2021
Retirement Benefits [Abstract]
401(k) Retirement Plan	401(k) Retirement PlanWe have adopted a 401(k) plan. All employees are eligible to participate, provided they meet the requirements of the plan. Through December 31, 2021, we have not matched employee contributions to the plan. Beginning in 2022, we will match the employees’ 401(k) contributions, at our discretion and not to exceed a prescribed annual limit.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2021
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, 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, commercial paper, and corporate debt securities with a final 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 the 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 recoverable. If the carrying value exceeds the undiscounted future cash flows estimated to result from the use and eventual disposition of the asset, we write down the asset to its estimated fair value. Impairment is assessed by comparing the undiscounted cash flows expected to be generated by the asset to its carrying value. We did not record any impairments in the years ended December 31, 2021, 2020 or 2019.
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.
Derivative Financial Instruments	Derivative Financial Instruments We evaluate all of our financial instruments, including prefunded warrants and warrants to purchase common stock, to determine if such instruments are derivatives or contain features qualifying as embedded derivatives. For derivative financial instruments accounted for as liabilities, the derivative instrument is initially recorded at its fair value and is then re-valued at each reporting date, with changes in the fair value reported in the Consolidated Statements of Operations and Comprehensive Income (Loss) . We use the Black-Scholes option-pricing model to value the derivative instruments at inception and subsequent valuation dates. The classification of derivative instruments, including whether such instruments should be recorded as liabilities or as equity, is 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 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 typically received under these arrangements 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 11 . 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
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 is recognized over the requisite service period of the awards, usually the vesting period, on a straight-line basis. For performance-based awards where the vesting of the options may be accelerated upon the achievement of certain milestones, the related stock-based compensation is recognized as expense when it is probable the milestone will be met. We recognize forfeiture of awards as they occur rather than estimating the expected forfeiture rate.
Income Taxes	Income Taxes Income taxes are accounted for using an asset and liability approach that requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of temporary differences between the consolidated financial statement and tax bases of assets and liabilities at the applicable enacted tax rates. We will 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, 2021, 2020, and 2019, 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) .
Recently Issued Accounting Pronouncements and Recently Adopted Accounting Pronouncements	Recently Adopted Accounting PronouncementsIn December 2019, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2019-12, Income Taxes (“Topic 740”): Simplifying the Accounting for Income Taxes. This ASU simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740 and also improves consistent application by clarifying and amending existing guidance. The standard is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2020. We adopted this standard on January 1, 2021, and concluded that the adoption of the standard did not have a material impact on our consolidated financial statements and related disclosures.
Net Loss Per Share	Net Loss Per ShareBasic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding during the period.
Fair Value Measurements	Fair Value Measurements Cash and cash equivalents, restricted cash, receivables, accounts payable and accrued liabilities, which are recorded at invoiced amount or cost, approximate fair value based on the short-term nature of these financial instruments. Fair value is defined as the exchange price received for an asset or paid to transfer a liability, or an exit price, in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. The three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value, is as follows: Level 1 : Quoted prices in active markets for identical assets or liabilities. Level 2 : Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities, quoted prices in inactive markets, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 : Unobservable inputs supported by little or no market activity and significant to the fair value of the assets or liabilities.

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2021
Earnings Per Share [Abstract]
Schedule of Anti-dilutive Securities Excluded from Diluted Net Loss Per Share Attributable to Common Stockholders Calculation	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, does not differ from basic net loss per share for the periods presented. December 31, 2021 2020 2019 Common stock warrants 3,666,435 3,673,551 1,877,094 Prefunded warrants to purchase common stock 5,182,197 790,710 — Stock options and RSUs outstanding 5,877,309 4,175,345 3,252,144 Total 14,725,941 8,639,606 5,129,238

Cash Equivalents and Investme_2

Cash Equivalents and Investments (Tables)	12 Months Ended
	Dec. 31, 2021
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Schedule of Amortized Cost and Fair Value of Cash Equivalents and Short Term Investments	The amortized cost and fair value of our cash equivalents and investments are as follows (in thousands): December 31, 2021 Amortized Gross Gross Fair market Money market funds $ 50,277 $ — $ — $ 50,277 U.S. treasury bills 30,006 1 (100) 29,907 Corporate debt securities and commercial paper 117,492 2 (139) 117,355 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 December 31, 2020 Amortized Gross Gross Fair market Money market funds $ 28,424 $ — $ — $ 28,424 U.S. treasury bills 18,122 8 — 18,130 Corporate debt securities and commercial paper 82,047 2 (9) 82,040 Total $ 128,593 $ 10 $ (9) $ 128,594 Classified as: Cash equivalents $ 32,423 Short-term investments 70,622 Long-term investments 25,549 Total $ 128,594

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2021
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, 2021 Level 1 Level 2 Level 3 Total Money market funds $ 50,277 $ — $ — $ 50,277 U.S. treasury bills 29,907 — — 29,907 Corporate debt securities and commercial paper — 117,355 — 117,355 Total $ 80,184 $ 117,355 $ — $ 197,539 Assets: December 31, 2020 Level 1 Level 2 Level 3 Total Money market funds $ 28,424 $ — $ — $ 28,424 U.S. treasury bills 18,130 — — 18,130 Corporate debt securities and commercial paper — 82,040 — 82,040 Total $ 46,554 $ 82,040 $ — $ 128,594

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2021
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment, net, consists of the following (in thousands): December 31, 2021 2020 Laboratory equipment $ 3,132 $ 2,604 General equipment and furniture 479 479 Computer equipment and software 211 211 Leasehold improvements 85 85 Property and equipment, at cost 3,907 3,379 Less accumulated depreciation and amortization (2,191) (1,594) Property and equipment, net $ 1,716 $ 1,785

Additional Balance Sheet Info_2

Additional Balance Sheet Information (Tables)	12 Months Ended
	Dec. 31, 2021
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, 2021 2020 Prepaid research and development $ 3,315 $ 517 Prepaid insurance 454 447 Deferred financing 352 — Tenant improvement allowance receivable — 84 Prepaid other 266 168 Other receivables 323 304 Prepaid expenses and other current assets $ 4,710 $ 1,520
Schedule of Accrued Liabilities Disclosure	Accrued liabilities consist of the following (in thousands): December 31, 2021 2020 Research and development services $ 5,536 $ 2,571 Employee compensation 3,084 2,619 Legal and professional fees 394 482 Accrued taxes 127 — Accrued other 276 105 Accrued Liabilities $ 9,417 $ 5,777

Long-term Debt (Tables)

Long-term Debt (Tables)	12 Months Ended
	Dec. 31, 2021
Debt Disclosure [Abstract]
Schedule of Maturities of Long-term Debt	Scheduled principal payments on our outstanding debt as of December 31, 2021 under our Loan Agreement, excluding final fee amounts, are as follows (in thousands): Year Ending December 31, Total 2022 $ 4,800 2023 2,800 Total future principal payments $ 7,600

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Schedule of Components of Lease Cost	Supplemental operating lease information for the year ended December 31, 2021 was as follows (in thousands): For the Year Ended December 31, 2021 2020 Operating lease cost $ 1,837 $ 1,872 Variable lease cost 603 475 Total lease cost $ 2,440 $ 2,347 Other information: Cash paid for amounts included in the measurement of lease liabilities $ 2,117 $ 213 Weighted-average remaining lease term (years) 8.2 9.2 Weighted-average discount rate 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, 2021 are as follows (in thousands): Minimum Lease Payments 2022 $ 1,841 2023 2,057 2024 2,111 2025 2,167 2026 2,224 Thereafter 7,438 Total future minimum lease payments 17,838 Less: imputed interest (6,212) Operating lease liabilities $ 11,626

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2021
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, 2021 2020 Shares to be issued upon exercise of outstanding stock options 5,611,743 4,175,345 Shares to be issued upon release of RSUs 265,566 — Shares to be issued upon conversion of common stock warrants 3,666,435 3,673,551 Shares to be issued upon conversion of prefunded warrants 5,182,197 790,710 Shares available for future stock grants 284,906 887,901 Shares to be issued under employee stock purchase plan 45,211 45,211 Shares of common stock reserved for future issuance 15,056,058 9,572,718
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 warrant activity: Warrants Weighted- Weighted- Outstanding at December 31, 2020 3,673,551 $ 12.74 3.12 Exercised (2,484) 5.02 Expired (4,632) 97.12 Outstanding at December 31, 2021 3,666,435 $ 12.64 2.12 Exercisable at December 31, 2021 3,666,435 $ 12.64 2.12
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, 2020 4,175,345 $ 5.30 Granted 1,940,518 $ 12.38 Exercised (91,190) $ 5.37 Forfeited (412,930) $ 9.88 Outstanding at December 31, 2021 5,611,743 $ 7.41 7.41 $ 36,465 Vested and expected to vest after December 31, 2021 5,471,743 $ 7.38 7.37 $ 35,685 Exercisable at December 31, 2021 2,944,273 $ 5.25 6.31 $ 26,053
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, 2020 — $ — Granted 265,566 $ 12.00 Non-vested at December 31, 2021 265,566 $ 12.00 1.95 $ 491
Schedule of Stock Option Assumptions	The fair values of stock options granted to employees were calculated using the following assumptions: Years Ended December 31, 2021 2020 2019 Weighted-average estimated fair value at grant $8.44 $3.03 $4.14 Risk-free interest rate (1) 0.25% - 1.3% 0.38% - 1.68% 1.42% - 2.63% Expected term of options (in years) (2) 3.49 - 6.08 5.27 – 6.90 5.27 – 6.08 Expected stock price volatility (3) 79% - 83% 73% - 82% 70% - 77% 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 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, 2021 2020 2019 Employee: Research and development $ 3,323 $ 2,145 $ 1,608 General and administrative 2,891 1,955 1,359 Non-Employee: Research and development 24 34 68 General and administrative 2 6 6 Total stock-based compensation expense $ 6,240 $ 4,140 $ 3,041

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2021
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, 2021 2020 2019 Domestic $ (48,932) $ (28,356) $ (38,234) Foreign (1,488) 410 (3,618) Total $ (50,420) $ (27,946) $ (41,852)
Schedule of Provision for Income Taxes	The provision for income taxes is composed of the following (in thousands): Years Ended December 31, 2021 2020 2019 Current: U.S. - Federal $ — $ — $ — U.S. - State — (6) — Foreign 129 — — Total current 129 (6) — Deferred: U.S. - Federal — — — U.S. - State — — — Foreign (216) — — Total deferred (216) — — Total income tax benefit $ (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, 2021 2020 2019 U.S. Statutory rate 21.0 % 21.0 % 21.0 % Effect of: Permanent differences (0.4) % 0.1 % (0.1) % Federal research and development credit 4.5 % 2.6 % 1.1 % Change in valuation allowance (21.4) % (20.7) % (19.8) % Global intangible low-taxed income recapture (1.4) % — % — % Stock-based compensation (1.8) % (2.8) % (0.5) % Foreign rate differential 0.2 % (0.1) % 0.6 % Other (0.5) % (0.1) % (2.3) % Effective income tax rate 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, 2021 2020 Deferred tax assets: Net operating loss $ 22,054 $ 21,992 Research and development credits 6,297 4,044 Intangible asset basis 13 24 Lease liability 2,476 2,619 Deferred revenue 8,333 — Stock based compensation 1,672 1,337 Other 217 72 Gross deferred tax assets 41,062 30,088 Valuation allowance (38,752) (27,851) Total deferred tax assets, net of valuation allowance 2,310 2,237 Deferred tax liabilities: Prepaid expenses (147) (125) Fixed asset basis (93) (137) Right-of-use asset basis (1,856) (1,975) Total deferred tax liability (2,096) (2,237) Net deferred tax assets and liabilities $ 214 $ —
Schedule of Net Operating Loss Carryforwards	We have net operating loss (“NOL”) carryforwards as follows (in thousands): December 31, 2021 2020 2019 Federal (before January 1, 2018) $ 11,094 $ 11,094 $ 11,094 Federal (after January 1, 2018) $ 91,592 $ 91,868 $ 67,500 State $ 6,433 $ 6,433 $ 6,433 Foreign $ — $ — $ 787
Schedule of Net Research and Development Tax Credit Carryforwards	We have net research and development tax credit carryforwards as follows (in thousands): December 31, 2021 2020 2019 Federal $ 7,880 $ 5,063 $ 3,986
Schedule of Activity Related to Unrecognized Tax benefits	The following table summarizes the activity related to unrecognized tax benefits (in thousands): December 31, 2021 2020 2019 Unrecognized benefits – beginning of year $ 990 $ 775 $ 469 Gross increases (decreases) – prior year tax positions — (7) — Gross increases – current year tax positions 563 222 306 Unrecognized benefit – end of year $ 1,553 $ 990 $ 775

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Narrative (Details) - segment	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
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)	50.00%
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
	Dec. 31, 2021	Sep. 30, 2021	Jul. 31, 2020	Jan. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issued (in shares)			5,139,610	4,706,700
Horizon
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock issued (in shares)	951,980	6,489,357

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, 2021	Dec. 31, 2020	Dec. 31, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	14,725,941	8,639,606	5,129,238
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)	3,666,435	3,673,551	1,877,094
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	790,710	0
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)	5,877,309	4,175,345	3,252,144

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, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 197,775	$ 128,593
Gross unrealized gains	3	10
Gross unrealized losses	(239)	(9)
Fair market value	197,539	128,594
Cash equivalents
Debt Securities, Available-for-sale [Line Items]
Fair market value	50,277	32,423
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	94,396	70,622
Long-term investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	52,866	25,549
Money market funds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,277	28,424
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair market value	50,277	28,424
U.S. treasury bills
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	30,006	18,122
Gross unrealized gains	1	8
Gross unrealized losses	(100)	0
Fair market value	29,907	18,130
Corporate debt securities and commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	117,492	82,047
Gross unrealized gains	2	2
Gross unrealized losses	(139)	(9)
Fair market value	$ 117,355	$ 82,040

Cash Equivalents and Investme_4

Cash Equivalents and Investments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Debt securities, available-for-sale, term (less than)	2 years	2 years
Realized gains and losses on securities	$ 0	$ 0
Available-for-sale debt securities in unrealized loss position	83,000,000
Available-for-sale debt securities in unrealized loss position for more than twelve months	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Cash	$ 17.6	$ 2.5

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, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	$ 197,539	$ 128,594
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	50,277	28,424
U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	29,907	18,130
Corporate debt securities and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	117,355	82,040
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	80,184	46,554
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	50,277	28,424
Level 1 | U.S. treasury bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	29,907	18,130
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	117,355	82,040
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 | Corporate debt securities and commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets disclosure	117,355	82,040
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 | 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, 2021	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	$ 3,907	$ 3,379
Less accumulated depreciation and amortization	(2,191)	(1,594)
Property and equipment, net	1,716	1,785
Depreciation expenses	620	578	$ 468
Laboratory equipment
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	3,132	2,604
General equipment and furniture
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	479	479
Computer equipment and software
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	211	211
Leasehold improvements
Property, Plant and Equipment, Gross [Abstract]
Property and equipment, at cost	$ 85	$ 85

Sale of Intangible Asset (Detai

Sale of Intangible Asset (Details) - USD ($) $ in Thousands	1 Months Ended				12 Months Ended
	Dec. 31, 2021	Jun. 30, 2019	Jun. 30, 2018	Feb. 28, 2018	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Revenue					$ 23,443	$ 9,335	$ 1,740
GSNOR | Laurel Venture Capital Ltd.
Finite-Lived Intangible Assets [Line Items]
Revenue				$ 75
Proceeds from sale of intangible asset			$ 250
Potential milestone payments eligible to be received	$ 20,000
Laurel Venture Capital Ltd. | GSNOR
Finite-Lived Intangible Assets [Line Items]
Asset purchase agreement, milestone payment		$ 425

Additional Balance Sheet Info_3

Additional Balance Sheet Information - Prepaid Expense and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Prepaid research and development	$ 3,315	$ 517
Prepaid insurance	454	447
Deferred financing	352	0
Tenant improvement allowance receivable	0	84
Prepaid other	266	168
Other receivables	323	304
Prepaid expenses and other current assets	$ 4,710	$ 1,520

Additional Balance Sheet Info_4

Additional Balance Sheet Information - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Research and development services	$ 5,536	$ 2,571
Employee compensation	3,084	2,619
Legal and professional fees	394	482
Accrued taxes	127	0
Accrued other	276	105
Accrued liabilities	$ 9,417	$ 5,777

Long-term Debt - Narrative (Det

Long-term Debt - Narrative (Details)	1 Months Ended			12 Months Ended
	Mar. 31, 2020USD ($)	Aug. 31, 2019USD ($)installmenttranche$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2021USD ($)	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Jul. 31, 2020USD ($)$ / sharesshares	Mar. 30, 2020USD ($)shares	Jan. 31, 2019$ / sharesshares
Debt Instrument [Line Items]
Term loan drew down				$ 0	$ 5,000,000	$ 1,977,000
Warrants exercisable for shares of common stock (in shares) | shares									1,835,610
Warrants issued, exercise price (in dollars per share) | $ / shares									$ 12.74
Unamortized debt discount				223,000
Amortization of debt discount				$ 274,000	$ 261,000	$ 140,000
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								$ 812,000
Term Loans | Term Loans, Tranche One | Common Stock Warrant
Debt Instrument [Line Items]
Fair value of warrant liability		$ 60,000
Term Loans | Term Loans, Tranche Two | Common Stock Warrant
Debt Instrument [Line Items]
Fair value of warrant liability								$ 60,000
Silicon Valley Bank | Common Stock Warrant
Debt Instrument [Line Items]
Warrants exercisable for shares of common stock (in shares) | shares			7,069
Warrants issued, exercise price (in dollars per share) | $ / shares			$ 12.38
Silicon Valley Bank | Original Agreement
Debt Instrument [Line Items]
Term loan drew down			$ 5,000,000
Prepayment fee representing deferred portion of final payment		$ 75,000
Silicon Valley Bank | Original Agreement | Prime Rate
Debt Instrument [Line Items]
Variable interest rate			1.75%
Silicon Valley Bank | Term Loans
Debt Instrument [Line Items]
Variable interest rate		0.25%
Maximum borrowing capacity, amount		$ 15,000,000
Number of tranches | tranche		3
Percentage of default interest rate		4.00%
Aggregate new capital milestone for continuation of Interest only payment period		$ 40,000,000
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		$ 5,000,000
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		$ 3,000,000
Silicon Valley Bank | Term Loans | Term Loans, Tranche Two
Debt Instrument [Line Items]
Term loan drew down	$ 5,000,000
Silicon Valley Bank | Term Loans | Term Loans, Tranche Two | Common Stock Warrant
Debt Instrument [Line Items]
Number of shares exercisable (in shares) | shares								17,361
Silicon Valley Bank | Term Loans | Term Loans, Tranche Three
Debt Instrument [Line Items]
Unused borrowing capacity, expired amount							$ 5,000,000
Debt Instrument, Redemption, Period One | Silicon Valley Bank | Term Loans
Debt Instrument [Line Items]
Prepayment fee, percentage		2.00%
Debt Instrument, Redemption, Period Two | Silicon Valley Bank | Term Loans
Debt Instrument [Line Items]
Prepayment fee, percentage		1.00%
Debt Instrument, Redemption, Period Three | Silicon Valley Bank | Term Loans
Debt Instrument [Line Items]
Prepayment fee, percentage		0.00%
Maximum
Debt Instrument [Line Items]
Percentage change in non-troubled debt		0.10

Long-term Debt - Schedule of Ma

Long-term Debt - Schedule of Maturities of Long-term Debt (Details) $ in Thousands	Dec. 31, 2021USD ($)
Debt Disclosure [Abstract]
2022	$ 4,800
2023	2,800
Total future principal payments	$ 7,600

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2019USD ($)lease_term_type	Dec. 31, 2021USD ($)	Dec. 31, 2020USD ($)	May 31, 2017USD ($)
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	3.00%
Maximum tenant improvement allowance	$ 5,400
Maximum additional tenant improvement allowance	$ 1,800
Amortization percent, additional tenant improvement allowance	8.00%
Operating lease, right-of-use asset		$ 8,837	$ 9,401
Operating lease, liability		11,626
Line of Credit
Loss Contingencies [Line Items]
Security deposit				$ 132
Line of Credit
Loss Contingencies [Line Items]
Security deposit	$ 254
Minimum
Loss Contingencies [Line Items]
Income tax review, range of possible losses		0
Foreign | Maximum
Loss Contingencies [Line Items]
Income tax review, range of possible losses		$ 1,800

Commitments and Contingencies_2

Commitments and Contingencies - Components of Lease Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Lease cost:
Operating lease cost	$ 1,837	$ 1,872
Variable lease cost	603	475
Total lease cost	2,440	2,347
Other information:
Cash paid for amounts included in the measurement of lease liabilities	$ 2,117	$ 213
Weighted-average remaining lease term (years)	8 years 2 months 12 days	9 years 2 months 12 days
Weighted-average discount rate	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, 2021USD ($)
Commitments and Contingencies Disclosure [Abstract]
2022	$ 1,841
2023	2,057
2024	2,111
2025	2,167
2026	2,224
Thereafter	7,438
Total future minimum lease payments	17,838
Less: imputed interest	(6,212)
Operating lease liabilities	$ 11,626

License and Collaboration Agr_2

License and Collaboration Agreements (Details)	1 Months Ended		12 Months Ended			30 Months Ended
	Dec. 31, 2021USD ($)candidateprogram$ / sharesshares	Sep. 30, 2021shares	Dec. 31, 2021USD ($)$ / shares	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2021USD ($)$ / shares	Feb. 28, 2022USD ($)	Jan. 31, 2022USD ($)	Jun. 30, 2021USD ($)	Jun. 30, 2020USD ($)payment	Jun. 30, 2019USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue			$ 23,443,000	$ 9,335,000	$ 1,740,000
Common Stock | Private Placement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock issued in transaction (in shares) | shares		6,489,357
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 | 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			$ 15.76
Aggregate proceeds from sale of stock			$ 15,000,000
Purchase price as premium to volume-weighted average share price (as a percent)	25.00%
Fair value portion of aggregate proceeds from sale of stock			11,900,000
Additional consideration portion of aggregate proceeds from sale of stock			$ 3,100,000
Horizon | Common Stock
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Stock price period for premium on sale of stock			30 days
Horizon | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	$ 28,100,000		$ 28,100,000			$ 28,100,000
Potential future milestones receivable per program (up to)	381,000,000		381,000,000			381,000,000
Revenue			0
Potential future milestones receivable (up to)	1,500,000,000		1,500,000,000			1,500,000,000
AbbVie | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	74,500,000		74,500,000			74,500,000
Revenue			23,400,000	7,000,000
Potential future revenue recognition milestones, number of payments | payment										1
Potential cash payment upon license option exercise										$ 75,000,000
Potential cash payment upon achievement of certain development and commercial milestones										205,000,000
Potential cash payment upon achievement of certain sales-based cash milestones										450,000,000
Adaptimmune | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue			0	$ 2,300,000	$ 1,300,000
Adaptimmune | Product | Forecast
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability							$ 1,000,000
Maximum | AbbVie | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential future milestones receivable (up to)									$ 45,000,000	75,000,000
Maximum | Adaptimmune | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential future milestones receivable (up to)	288,000,000		288,000,000			288,000,000
Upfront Payment | Horizon | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	25,000,000		25,000,000			25,000,000
Upfront Payment | Horizon | Product | Subsequent Event
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability								$ 25,000,000
Upfront License Payment | AbbVie | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability										$ 60,000,000
Upfront License Payment | Adaptimmune | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability											$ 2,000,000
Revenue recognized from contract with customer						2,000,000
Premium Payment | Horizon | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability	$ 3,100,000		$ 3,100,000			3,100,000
Research Support Payment | Adaptimmune | Product
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contract with customer, liability											$ 1,600,000
Revenue recognized from contract with customer						$ 1,600,000

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Shares of Common Stock Reserved for Future Issuance (Details) - shares	Dec. 31, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	15,056,058	9,572,718
Stock options
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	5,611,743	4,175,345
Restricted stock units
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	265,566	0
Common stock warrants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	3,666,435	3,673,551
Prefunded warrants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	5,182,197	790,710
Future stock grants
Class of Stock [Line Items]
Shares of common stock reserved for future issuance (in shares)	284,906	887,901
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)	Jan. 01, 2022shares	Sep. 30, 2021USD ($)$ / sharesshares	Jul. 31, 2021USD ($)	Jul. 31, 2020USD ($)shareswarrant$ / shares	Jun. 30, 2020shares	Jan. 31, 2019USD ($)shares$ / shares	Jun. 30, 2018shares	Dec. 31, 2021USD ($)shares	Dec. 31, 2020USD ($)shares	Dec. 31, 2019USD ($)	Jul. 31, 2017shares
Class of Stock [Line Items]
Warrants exercisable (in shares)						1,835,610
Gross proceeds from issuance of private placement | $		$ 91,000,000		$ 60,000,000		$ 25,300,000		$ 90,733,000	$ 56,258,000	$ 23,598,000
Warrants issued, exercise price (in dollars per share) | $ / shares						$ 12.74
Stock issuance costs incurred | $		$ 266,000		$ 3,700,000		$ 1,700,000
Treasury stock, shares (in shares)								1,250,467	50,467
Purchase price per unit (in dollars per share) | $ / shares				$ 10.1175		$ 5.37
Term of warrants						5 years
Shares available for future issuance (in shares)								15,056,058	9,572,718
Vesting period (in years)								4 years
Term (in years)								10 years
Unrecognized compensation cost related to nonvested stock options | $								$ 16,400,000
Weighted-average period over which unrecognized compensation expense related to nonvested stock options is expected to be recognized								2 years 8 months 12 days
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
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
Common Units
Class of Stock [Line Items]
Stock issued in transaction (in shares)				5,139,610		4,706,700
Common Units | Common Stock
Class of Stock [Line Items]
Common stock per unit sold in purchase agreement (in shares)				1		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		0.39
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)				0.3
2019 Units
Class of Stock [Line Items]
Stock issued in transaction (in shares)						4,706,700
Stock options and Restricted stock units
Class of Stock [Line Items]
Nonvested awards (in shares)								2,900,000
Stock options
Class of Stock [Line Items]
Shares available for future issuance (in shares)								5,611,743	4,175,345
Aggregate intrinsic value of stock options exercised | $								$ 546,000	$ 104,000	155,000
Fair value of shares vested | $								$ 3,400,000	$ 4,200,000	3,000,000
Restricted stock units
Class of Stock [Line Items]
Shares available for future issuance (in shares)								265,566	0
Aggregate intrinsic value of RSU's released | $								$ 0	$ 1,600,000	0
Fair value of RSUs vesting | $								$ 0	$ 461,000	$ 0
Prefunded Warrants
Class of Stock [Line Items]
Shares available for future issuance (in shares)								5,182,197	790,710
Prefunded Warrants | Exchange Warrants
Class of Stock [Line Items]
Warrants exercisable (in shares)		1,200,000
2018 Plan
Class of Stock [Line Items]
Additional number of shares authorized (in shares)					743,515
Percentage of outstanding stock maximum							5.00%
2018 Plan | Subsequent Event
Class of Stock [Line Items]
Additional number of shares authorized (in shares)	1,500,000
2018 Plan | Legacy Plan Cancellations
Class of Stock [Line Items]
Number of shares authorized (in shares)							1,972,784
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)								6,616,862
Employee stock purchase plan
Class of Stock [Line Items]
Shares available for future issuance (in shares)											45,211
Private Placement | Common Stock
Class of Stock [Line Items]
Stock issued in transaction (in shares)		6,489,357
Cowen & Co. | Sales Agreement
Class of Stock [Line Items]
Maximum amount of common stock eligible to be sold (up to) | $			$ 75,000,000
Percentage of compensation for services			3.00%
Maximum | 2018 Plan
Class of Stock [Line Items]
Additional number of shares authorized (in shares)							1,500,000

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Warrants (Details) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Warrants Outstanding
Warrants outstanding (in shares)	3,673,551
Warrants exercised (in shares)	(2,484)
Warrants expired (un shares)	(4,632)
Warrants outstanding (in shares)	3,666,435	3,673,551
Warrants exercisable (in shares)	3,666,435
Weighted- average Exercise Price
Weighted average, exercise price, outstanding (in dollars per share)	$ 12.74
Weighted average, exercise (in dollars per share)	5.02
Weighted average, expired (in dollars per share)	97.12
Weighted average, exercise price, outstanding (in dollars per share)	12.64	$ 12.74
Weighted average, exercisable (in dollars per share)	$ 12.64
Weighted- average Remaining Contract Term (in years)
Weighted-average remaining contractual term, outstanding	2 years 1 month 13 days	3 years 1 month 13 days
Weighted-average remaining contractual term, exercisable	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, 2021USD ($)$ / sharesshares
Options Outstanding
Outstanding at beginning of period (in shares) | shares	4,175,345
Granted (in shares) | shares	1,940,518
Exercised (in shares) | shares	(91,190)
Forfeited (in shares) | shares	(412,930)
Outstanding at ending of period (in shares) | shares	5,611,743
Vested and expected to vest (in shares) | shares	5,471,743
Exercisable (in shares) | shares	2,944,273
Weighted- average Exercise Price
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 5.30
Granted (in dollars per share) | $ / shares	12.38
Exercised (in dollars per share) | $ / shares	5.37
Forfeited (in dollars per share) | $ / shares	9.88
Outstanding at ending of period (in dollars per share) | $ / shares	7.41
Vested and expected to vest (in dollars per share) | $ / shares	7.38
Exercisable (in dollars per share) | $ / shares	$ 5.25
Weighted- average Remaining Contract Term (in years)
Outstanding (in years)	7 years 4 months 28 days
Vested and expected to vest (in years)	7 years 4 months 13 days
Exercisable (in years)	6 years 3 months 21 days
Aggregate Intrinsic Value (in thousands)
Outstanding | $	$ 36,465
Vested and expected to vest | $	35,685
Exercisable | $	$ 26,053

Stockholders' Equity - Schedu_4

Stockholders' Equity - Schedule of RSU Activity (Details) - RSUs $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021USD ($)$ / sharesshares
Number of Shares
Outstanding at beginning of period (in shares) | shares	0
Granted (in shares) | shares	265,566
Outstanding at ending of period (in shares) | shares	265,566
Weighted- Average Grant Date Fair Value (per share)
Outstanding at beginning of period (in dollars per share) | $ / shares	$ 0
Granted (in dollars per share) | $ / shares	12
Outstanding at ending of period (in dollars per share) | $ / shares	$ 12
Weighted- average Remaining Contract Life (in years)
Outstanding (in years)	1 year 11 months 12 days
Aggregate Intrinsic Value (in thousands)
Outstanding | $	$ 491

Stockholders' Equity - Schedu_5

Stockholders' Equity - Schedule of Stock Option Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average estimated fair value at grant (in dollars per share)	$ 8.44	$ 3.03	$ 4.14
Risk-free interest rate, minimum	0.25%	0.38%	1.42%
Risk-free interest rate, maximum	1.30%	1.68%	2.63%
Expected stock price volatility, minimum	79.00%	73.00%	70.00%
Expected stock price volatility, maximum	83.00%	82.00%	77.00%
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of options (in years)	3 years 5 months 26 days	5 years 3 months 7 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 10 months 24 days	6 years 29 days

Stockholders' Equity - Schedu_6

Stockholders' Equity - Schedule of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 6,240	$ 4,140	$ 3,041
Employee | Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	3,323	2,145	1,608
Employee | General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	2,891	1,955	1,359
Non-Employee | Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	24	34	68
Non-Employee | General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Total stock-based compensation expense	$ 2	$ 6	$ 6

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, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Domestic	$ (48,932)	$ (28,356)	$ (38,234)
Foreign	(1,488)	410	(3,618)
Loss before taxes	$ (50,420)	$ (27,946)	$ (41,852)

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Current:
U.S. - Federal	$ 0	$ 0	$ 0
U.S. - State	0	(6)	0
Foreign	129	0	0
Total current	129	(6)	0
Deferred:
U.S. - Federal	0	0	0
U.S. - State	0	0	0
Foreign	(216)	0	0
Total deferred	(216)	0	0
Total income tax benefit	$ (87)	$ (6)	$ 0

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, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
U.S. Statutory rate	21.00%	21.00%	21.00%
Effect of:
Permanent differences	(0.40%)	0.10%	(0.10%)
Federal research and development credit	4.50%	2.60%	1.10%
Change in valuation allowance	(21.40%)	(20.70%)	(19.80%)
Global intangible low-taxed income recapture	(1.40%)	0.00%	0.00%
Stock-based compensation	(1.80%)	(2.80%)	(0.50%)
Foreign rate differential	0.20%	(0.10%)	0.60%
Other	(0.50%)	(0.10%)	(2.30%)
Effective income tax rate	0.20%	0.00%	0.00%

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Income tax expense	$ (87,000)	$ (6,000)	$ 0
Effective income tax rate (as a percent)	0.20%	0.00%	0.00%
Increase in valuation allowance	$ 10,900,000	$ 5,800,000
Unrecognized tax benefits that would impact effective tax rate	0
Unrecognized tax benefits	1,553,000	990,000	$ 775,000	$ 469,000
Accrued interest or penalties related to unrecognized tax benefits	$ 0	$ 0	$ 0
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, 2021	Dec. 31, 2020
Deferred tax assets:
Net operating loss	$ 22,054	$ 21,992
Research and development credits	6,297	4,044
Intangible asset basis	13	24
Lease liability	2,476	2,619
Deferred revenue	8,333	0
Stock based compensation	1,672	1,337
Other	217	72
Gross deferred tax assets	41,062	30,088
Valuation allowance	(38,752)	(27,851)
Total deferred tax assets, net of valuation allowance	2,310	2,237
Deferred tax liabilities:
Prepaid expenses	(147)	(125)
Fixed asset basis	(93)	(137)
Right-of-use asset basis	(1,856)	(1,975)
Total deferred tax liability	(2,096)	(2,237)
Net deferred tax assets and liabilities	$ 214	$ 0

Income Taxes - Schedule of Net

Income Taxes - Schedule of Net Operating Loss Carryforwards (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
State
Income Taxes [Line Items]
Net operating loss carryforwards	$ 6,433	$ 6,433	$ 6,433
Foreign
Income Taxes [Line Items]
Net operating loss carryforwards	0	0	787
Before January 1 2018 | Federal
Income Taxes [Line Items]
Net operating loss carryforwards	11,094	11,094	11,094
After January 1 2018 | Federal
Income Taxes [Line Items]
Net operating loss carryforwards	$ 91,592	$ 91,868	$ 67,500

Income Taxes - Schedule of Tax

Income Taxes - Schedule of Tax Credit Carryforwards (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Line Items]
Research and development credits	$ 6,297	$ 4,044
Federal
Income Taxes [Line Items]
Research and development credits	$ 7,880	$ 5,063	$ 3,986

Income Taxes - Schedule of Acti

Income Taxes - Schedule of Activity Related to Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized benefits – beginning of year	$ 990	$ 775	$ 469
Gross increases (decreases) – prior year tax positions	0	(7)	0
Gross increases – current year tax positions	563	222	306
Unrecognized benefit – end of year	$ 1,553	$ 990	$ 775

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Sep. 30, 2021	Jul. 31, 2020	Jan. 31, 2019	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Related Party Transaction [Line Items]
Warrants exercisable (in shares)			1,835,610
Gross proceeds from issuance of private placement	$ 91,000	$ 60,000	$ 25,300	$ 90,733	$ 56,258	$ 23,598
Exchange Warrants | Prefunded warrants
Related Party Transaction [Line Items]
Warrants exercisable (in shares)	1,200,000
Exchange Warrants | Affiliated Entity | Exchange of Stock | Prefunded warrants
Related Party Transaction [Line Items]
Warrants exercisable (in shares)	1,200,000
Common Stock and Common Stock Warrants | Affiliated Entity | 2021 Security Purchases Agreement
Related Party Transaction [Line Items]
Gross proceeds from issuance of private placement	$ 57,000
Common Stock and Common Stock Warrants | Affiliated Entity | 2019 Securities Purchase Agreement
Related Party Transaction [Line Items]
Gross proceeds from issuance of private placement			$ 5,000
Common Stock and Common Stock Warrants | Alpine Immune Sciences
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners		5.00%
Common Stock and Common Stock Warrants | Alpine Immune Sciences | Affiliated Entity | 2021 Security Purchases Agreement
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners	5.00%
Common Stock and Common Stock Warrants | Alpine Immune Sciences | Affiliated Entity | 2019 Securities Purchase Agreement
Related Party Transaction [Line Items]
Ownership percentage by noncontrolling owners			5.00%
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 | Affiliated Entity | 2019 Securities Purchase Agreement
Related Party Transaction [Line Items]
Stock issued in transaction (in shares)			935,753
Common Stock Warrant | Affiliated Entity | 2021 Security Purchases Agreement
Related Party Transaction [Line Items]
Warrants exercisable (in shares)	2,340,424
Common Stock Warrant | Affiliated Entity | 2019 Securities Purchase Agreement
Related Party Transaction [Line Items]
Warrants exercisable (in shares)			364,943