Sierra Oncology (SRRA) 10-K 11 Mar 21 2020 FY Annual report Financial data

Cover Page

Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Mar. 08, 2021	Jun. 30, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Registrant Name	Sierra Oncology, Inc.
Entity Central Index Key	0001290149
Trading Symbol	SRRA
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	true
Entity Common Stock, Shares Outstanding		11,628,479
Entity Interactive Data Current	Yes
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, $0.001 par value
Entity Address, Address Line One	1820 Gateway Drive, Suite 110
Entity Address, City or Town	San Mateo,
Entity Address, State or Province	CA
Entity Public Float			$ 56.1
Document Annual Report	true
Document Transition Report	false
Entity File Number	001-37490
Entity Tax Identification Number	20-0138994
Entity Address, Postal Zip Code	94404
City Area Code	650
Local Phone Number	376-8679
ICFR Auditor Attestation Flag	false
Entity Incorporation, State or Country Code	DE
Document Information [Text Block]	Portions of the Registrant’s Definitive Proxy Statement (“Proxy Statement”) relating to the 2021 Annual Meeting of Stockholders will be filed with the Commission within 120 days after the end of the Registrant’s 2020 fiscal year and is incorporated by reference into Part III of this Report.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
CURRENT ASSETS:
Cash and cash equivalents	$ 104,055	$ 147,528
Prepaid expenses and other current assets	2,415	2,369
Total current assets	106,470	149,897
Property and equipment, net	52	113
Operating lease right-of-use asset	318	589
Other assets	647	729
TOTAL ASSETS	107,487	151,328
CURRENT LIABILITIES:
Accrued and other liabilities	7,148	7,170
Accounts payable	2,205	1,019
Warrant liabilities		45,935
Securities issuance obligation		10,485
Total current liabilities	9,353	64,609
Operating lease liability	175	374
TOTAL LIABILITIES	9,528	64,983
Commitments and Contingencies (Note 8)
STOCKHOLDERS’ EQUITY:
Preferred stock, $0.001 par value; 10,000,000 shares authorized as of December 31, 2020 and December 31, 2019; nil shares issued and outstanding as of December 31, 2020 and 103,000 shares of Series A convertible voting preferred stock issued and outstanding as of December 31, 2019		1
Common stock, $0.001 par value; 500,000,000 shares authorized as of December 31, 2020 and December 31, 2019; 11,128,484 and 1,867,176 shares issued and outstanding as of December 31, 2020 and 2019	11	74
Additional paid-in capital	944,537	851,957
Accumulated deficit	(846,589)	(765,687)
TOTAL STOCKHOLDERS’ EQUITY	97,959	86,345
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 107,487	$ 151,328

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	11,128,484	1,867,176
Common stock, shares outstanding	11,128,484	1,867,176
Series A Convertible Preferred Stock [Member]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	103,000
Preferred stock, shares outstanding	0	103,000

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 300
Operating expenses:
Research and development	45,118	$ 53,249	$ 41,078
General and administrative	20,123	13,743	14,339
Total operating expenses	65,241	66,992	55,417
Loss from operations	(64,941)	(66,992)	(55,417)
Other income (expense), net:
Changes in fair value of warrant liabilities	(16,240)	(20,926)
Other income (expense), net	421	(517)	1,780
Total other income (expense), net	(15,819)	(21,443)	1,780
Loss before provision for (benefit from) income taxes, net	(80,760)	(88,435)	(53,637)
Provision for (benefit from) income taxes, net	142	(160)	(302)
Net loss and comprehensive loss	$ (80,902)	$ (88,275)	$ (53,335)
Net loss per common share, basic and diluted	$ (7.70)	$ (30.30)	$ (30.16)
Weighted-average shares used in computing net loss per common share, basic and diluted	10,506,739	2,913,487	1,768,480

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]
Stockholders Equity, Beginning Balance at Dec. 31, 2017	$ 94,726		$ 52	$ 718,751	$ (624,077)
Stockholders Equity, Beginning Balance, Shares at Dec. 31, 2017			1,309,856
Issuance of common stock for exercise of stock options	180			180
Issuance of common stock for exercise of stock options, Shares			3,014
Stock-based compensation	6,796			6,796
Issuance of common stock	45,996		$ 22	45,974
Issuance of common stock, Shares			546,250
Issuance of common stock warrant	116			116
Net loss	(53,335)				(53,335)
Stockholders Equity, Ending Balance at Dec. 31, 2018	94,479		$ 74	771,817	(677,412)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2018			1,859,120
Issuance of common stock for exercise of stock options	445			445
Issuance of common stock for exercise of stock options, Shares			8,056
Stock-based compensation	5,695			5,695
Issuance of convertible voting preferredstock, net of offering costs	74,001	$ 1		74,000
Issuance of convertible voting preferredstock, net of offering costs, Share		103,000
Net loss	(88,275)				(88,275)
Stockholders Equity, Ending Balance at Dec. 31, 2019	86,345	$ 1	$ 74	851,957	(765,687)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2019		103,000	1,867,176
Conversion of Series A convertible voting preferred stock to common stock		$ (1)	$ 8	(7)
Conversion of Series A convertible voting preferred stock to common stock, Shares		(103,000)	7,803,273
Reclassification of warrant liabilities to equity	62,175			62,175
Issuance of common stock in connection with an amendment to the asset purchase agreement	8,782		$ 1	8,781
Number of shares issued in connection with an amendment to asset purchase agreement			725,283
Reverse stock split adjustment			$ (73)	73
Stock-based compensation	9,470			9,470
Issuance of common stock	8,901		$ 1	8,900
Issuance of common stock, Shares			732,752
Issuance of common stock warrant	3,188			3,188
Net loss	(80,902)				(80,902)
Stockholders Equity, Ending Balance at Dec. 31, 2020	$ 97,959		$ 11	$ 944,537	$ (846,589)
Stockholders Equity, Ending Balance, Shares at Dec. 31, 2020			11,128,484

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net of offering costs			$ 3.2
At The Market Offering [Member]
Net of offering costs	$ 0.4
Series A Convertible Preferred Stock [Member]
Net of offering costs		$ 4

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (80,902)	$ (88,275)	$ (53,335)
Adjustments to reconcile net loss to net cash used in operating activities:
Changes in fair value of warrant liabilities	16,240	20,926
Securities issuance obligation	1,485	10,485
Stock-based compensation	9,470	5,695	6,796
Depreciation and amortization	238	83	111
Asset impairment	106
Other	201	161	(68)
Warrant issuance costs		1,279
Term loan repayment fee		438
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(33)	336	(1,341)
Accrued, other and operating lease liabilities	(366)	(2,034)	2,770
Accounts payable	1,194	(277)	(48)
Net cash used in operating activities	(52,367)	(51,183)	(45,115)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(12)	(39)	(118)
Net cash used in investing activities	(12)	(39)	(118)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from public offering, net of offering costs		97,731
Proceeds of term loan and repayment fee		(5,438)
Proceeds from exercise of common stock options		445	180
Proceeds from issuance of term loan, net of issuance costs			4,955
Net cash provided by financing activities	8,901	92,738	51,131
Effect of foreign exchange rate changes on cash, cash equivalents and restricted cash	5	(34)	(88)
NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(43,473)	41,482	5,810
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — Beginning of period	147,828	106,346	100,536
CASH, CASH EQUIVALENTS AND RESTRICTED CASH — End of period	104,355	147,828	106,346
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Cash paid for (refund of) income taxes, net	28	(69)	15
Cash paid for interest		336	87
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Issuance of common stock and common stock warrant in connection with an amendment to the asset purchase agreement	11,970
Reclassification of warrant liabilities to equity	62,175
Unpaid deferred financing costs in accrued and other liabilities	10
Right-of-use asset obtained in exchange for operating lease obligation		771
Issuance costs of convertible voting preferred stock and warrants included in accrued and other liabilities		$ 268
Issuance of common stock warrant	3,188		116
At-The-Market Offering [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of offering costs	$ 8,901
Follow-on Offering [Member]
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of offering costs			$ 45,996

The Company and Basis of Presen

The Company and Basis of Presentation	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
The Company and Basis of Presentation	1. The Company and Basis of Presentation Organization and Description of Business Sierra Oncology, Inc. (together with its subsidiaries, collectively referred to as the “Company”), a Delaware corporation, is a late-stage biopharmaceutical company on a quest to deliver targeted therapies that treat rare forms of cancer. The Company’s main focus is the development of momelotinib, an investigational agent for the treatment of myelofibrosis. Momelotinib is a selective and orally-bioavailable JAK1 (Janus kinase 1), JAK2 (Janus kinase 2) and ACVR1 (Activin A receptor type 1) inhibitor with a differentiated mechanism of action that enables it to potentially address all three key manifestations of myelofibrosis: anemia of inflammation, constitutional symptoms and enlarged spleen. Currently, momelotinib is in a global Phase 3 clinical trial for patients with myelofibrosis, called the MOMENTUM study, that if successful will be registration enabling. At its completion, approximately 1,000 myelofibrosis patients will have received momelotinib, and several of our clinical trial patients remain on treatment more than 10 years later. The Company’s portfolio also includes SRA737, a selective, orally bioavailable small molecule inhibitor of Checkpoint kinase 1 (Chk1), an emerging target for the treatment of cancer which has a key role in the DNA Damage Response (DDR). The Company’s primary activities since inception have been conducting research and development activities, conducting preclinical and clinical testing, recruiting personnel, performing business and financial planning, identifying and evaluating additional drug candidates for potential in-licensing or acquisition, and raising capital to support development activities. The Company has not generated any product revenue related to its primary business purpose to date, nor has it generated any net income, and is subject to a number of risks and uncertainties, which include dependence on key individuals, the need to identify and successfully develop commercially viable products, the need to obtain regulatory approval for its products and commercialize them, and the need to obtain adequate additional financing to fund the development of momelotinib. As of December 31, 2020, the Company had $104.1 million of cash and cash equivalents. The Company believes that its balance of cash and cash equivalents as of the date of the issuance of these consolidated financial statements is sufficient to fund its current operational plan for at least the next twelve months though it may pursue raising additional capital through equity financings or other arrangements. Follow-On Offerings On March 6, 2018, the Company completed an underwritten public offering of an aggregate of 546,250 shares of common stock, including the underwriters’ exercise of their overallotment option, at a price to the public of $90.00 per share. The aggregate net proceeds received by the Company from the offering were $46.0 million, net of underwriting discounts and commissions and offering expenses of $3.2 million. On November 13, 2019, the Company completed an underwritten public offering of an aggregate of (i) 103,000 shares of Series A convertible voting preferred stock (Series A Preferred Stock), (ii) Series A warrants to purchase up to an aggregate of 7,802,241 shares of common stock at an exercise price equal to $13.20, and (iii) Series B warrants to purchase up to an aggregate of 2,574,727 shares of common stock at an exercise price equal to $13.20. Each share of Series A Preferred Stock and the accompanying Series A and Series B warrants were issued at a combined purchase price to the public of $1,000. The aggregate net proceeds received by the Company from the offering were $97.7 million, net of underwriting discounts and commissions and offering expenses of $5.3 million. On January 29, 2020, all shares of Series A Preferred Stock converted into 7,803,273 shares of common stock. At-The-Market Common Stock Offerings In August 2020, the Company entered into an open market sales agreement, pursuant to which it can issue and sell an aggregate of up to $20.0 million of its common stock from time to time in At-The-Market (ATM) offerings. During the year ended December 31, 2020, the Company sold 732,752 shares under the ATM program for net proceeds of $8.9 million, net of commissions and offering expenses. In February 2021, the Company filed a prospectus supplement pursuant to which it can issue and sell an aggregate of up to $30.0 million of its common stock from time to time in ATM offerings. Reverse Stock Split On January 21, 2020, the Company’s shareholders approved an amendment to the Company’s certificate of incorporation to effect a reverse split of the Company’s common stock (Reverse Stock Split). On January 21, 2020, the Company’s board of directors approved the specific ratio for the Reverse Stock Split, which became effective on January 22, 2020, at 1-for-40. The authorized shares and par value of the common and preferred stock were not adjusted as a result of the Reverse Stock Split. All issued and outstanding common stock, warrants for common stock, options for common stock and per share amounts contained in the consolidated financial statements have been retroactively adjusted to reflect this Reverse Stock Split for all periods presented.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (U.S. GAAP). The accompanying consolidated financial statements include the accounts of Sierra Oncology, Inc. and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. Use of Estimates The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying consolidated financial statements include, but are not limited to the fair values of stock options and warrants issued, the convertible voting preferred stock and securities issuance obligation, the probability of achieving performance-based milestones of stock options, accruals such as research and development costs, and recoverability of the Company’s net deferred tax assets, and related valuation allowance. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates. Foreign Currency The functional currency of the Company’s foreign subsidiaries is the U.S. Dollar. Transactions denominated in currencies other than the functional currency are recorded at prevailing exchange rates during the period. At the end of each reporting period, monetary assets and liabilities are remeasured to the functional currency using exchange rates in effect at the balance sheet date. Non-monetary assets and liabilities are recorded at historical exchange rates. Gains and losses related to remeasurement are recorded in other income (expense), net in the consolidated statements of operations. The net foreign exchange transaction gains (losses) included in other income (expense), net in the accompanying consolidated statements of operations were insignificant for the years ended December 31, 2020, 2019 and 2018. Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents consist primarily of funds invested in readily available checking and savings accounts and highly liquid investments in money market funds. Restricted Cash Restricted cash, which consists of funds invested in a money market fund, represents collateral for a corporate credit card facility and is included in other assets in the accompanying consolidated balance sheets. Concentrations of Credit Risk Financial instruments that subject the Company to significant concentrations of credit risk consist of cash, cash equivalents and restricted cash. All of the Company’s cash, cash equivalents and restricted cash are held at financial institutions in the United States and Canada that management believes to be of high credit quality. Deposits held in the United States and Canada with these financial institutions exceed federally insured limits. The primary focus of the Company’s investment strategy is to preserve capital and meet liquidity requirements. The Company’s investment policy addresses the level of credit exposure by limiting the concentration in any one corporate issuer and establishing a minimum allowable credit rating. Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, other current assets, accounts payable and accrued liabilities approximate their fair values at December 31, 2020 and 2019, due to their short duration. The warrant liabilities and securities issuance obligation contained unobservable inputs that reflected the Company’s own assumptions in which there was little, if any, market activity at the measurement date, thus the Company’s warrant liabilities and securities issuance obligation were measured at their fair values on a recurring basis using unobservable inputs until such time the warrants were no longer considered derivative instruments and the securities issuance obligation was settled. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 —Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2 —Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 —Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. Property and Equipment, Net Property and equipment, net are stated at cost, less accumulated depreciation. Depreciation on property and equipment, excluding leasehold improvements, is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized on a straight-line basis over the shorter of the estimated useful lives of the assets or the remaining lease term. Depreciation begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statement of operations. Other Assets Other assets consist primarily of restricted cash pledged as collateral for a corporate credit card facility and deferred income tax assets in foreign jurisdictions. Operating Lease Right-of-Use Asset and Lease Liability The Company recognizes an operating lease with terms greater than one year as right-of-use (ROU) asset and lease liability on its consolidated balance sheet using the portfolio approach. Lease liability and ROU asset are recorded based on the present value of future lease payments over the contractual term of the operating lease. The Company utilized its incremental borrowing rate from information available as at the date of initial adoption in determining the present value of the future lease payments. The lease liability and ROU asset are amortized over the term of the lease. Warrant Liabilities The Company accounted for its warrants issued in connection with its November 2019 financing based upon the characteristics and provisions of the instruments. Warrants classified as derivative liabilities were recorded on the Company’s consolidated balance sheets at their fair value on the date of issuance and remeasured to fair value on each subsequent reporting period, with the changes in fair value recognized as a component of other income (expense), net in the accompanying consolidated statements of operations. The Company revalued the warrant liabilities until they ceased to be derivative instruments, at which time they were reclassified to additional paid-in capital at their fair value. The Company estimated the fair value of these liabilities using the Black-Scholes option-pricing model and the fair value of the underlying stock, as well as assumptions for expected volatility, expected term and risk-free interest rate. An estimated non-marketable discount was also applied if applicable. Offering expenses arising from the issuance of warrants are expensed as incurred. Securities Issuance Obligation The Company recognized its securities issuance obligation pursuant to the amendment to the Asset Purchase Agreement with Gilead Sciences, Inc. (Gilead) at their fair values on the date such obligations arose. At the end of each reporting period, changes in estimated fair values during the period were recognized as research and development costs in the accompanying consolidated statements of operations. The Company adjusted the fair values of the obligations until the securities issuance obligation was settled, at which time the liabilities were reclassified to common stock and additional paid-in capital at their fair values. The Company estimated the fair value of its obligation to issue common stock using the fair value of the underlying stock and a non-marketable discount. The Company estimated the fair value of its obligation to issue warrant using the Black-Scholes option-pricing model and the fair value of the underlying stock, as well as assumptions for expected volatility, expected term and risk-free interest rate. An estimated non-marketable discount was also applied if applicable. Research and Development Costs Research and development costs are expensed as incurred. The Company accounts for non-refundable advance payments for goods and services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made. Depending on the timing of payments to service providers of research and development costs, the Company recognizes prepaid expenses or accrued expenses related to these costs. These prepaid or accrued expenses are based on management’s estimates of the work performed under service agreements and milestones achieved. In the event that a clinical trial is terminated early, the Company records an accrual for the estimated remaining costs to complete the trial in the period of termination. Upfront payments made in connection with license and asset purchase agreements are expensed as research and developments costs, as the assets acquired do not have alternative future use. Contingent milestone payment obligations due to third parties under license and asset purchase agreements are expensed when the milestones are considered probable of occurring. To the extent an obligation is to be settled by future issuance of securities, the fair value of these instruments is recorded in research and development expense until the securities are issued. Research and development costs include fees incurred in connection with license and asset purchase agreements and their related amendments, compensation and other related costs for employees engaged in research and development, costs associated with research and preclinical studies, clinical trials, regulatory activities, manufacturing activities to support clinical activities, fees paid to external service providers that conduct certain research and development, clinical, and manufacturing activities on behalf of the Company and an allocation of overhead expenses. Stock-Based Compensation The Company accounts for stock-based payments at fair value, which is measured using the Black-Scholes option-pricing model. For stock-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis over the vesting period, which is generally the service period. For stock-based awards that vest subject to the satisfaction of a service requirement and a performance component, the fair value measurement date is the date of grant and the expense is recognized over the requisite service period as achievement of the performance objective becomes probable. The Company accounts for forfeitures as they occur. Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net U.S. deferred tax assets have been offset by a full valuation allowance. The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company recognizes interest and penalties related to the underpayment of income taxes as a component of provision for (benefit from) income taxes, net. Segment Information Operating segments are components of an enterprise for which separate financial information is available and is evaluated regularly by the Company’s chief operating decision maker in deciding how to allocate resources and assessing performance. The Company’s chief operating decision maker is its Chief Executive Officer. The Company’s Chief Executive Officer views the Company’s operations and manages its business in one operating segment, which is the business of researching, developing and commercializing therapies for the treatment of patients with hematology and oncology needs. Accordingly, the Company has a single reporting segment.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	3. Net Loss Per Share Basic net loss per share is calculated by dividing net loss by the weighted-average number of common stock outstanding during the period without consideration for common stock equivalents. In 2019, preferred stock with characteristics of common stock was also included in the determination of the weighted average. Diluted net loss per share is computed by dividing net loss by the weighted-average number of common stock equivalents outstanding for the period determined using the treasury-stock method. For purposes of this calculation, stock options and warrants for common stock are considered to be common stock equivalents and are only included in the calculation of diluted net loss per share when their effect is dilutive. The following shares of common stock equivalents were excluded from the calculation of diluted net loss per share for the periods presented because including them would have been antidilutive: As of December 31, 2020 2019 2018 Series A warrants for common stock 7,802,241 7,802,241 — Series B warrants for common stock 2,574,727 2,574,727 — Options to purchase common stock 4,146,928 326,023 262,539 Warrants for common stock 727,122 1,839 1,839 Total potential dilutive shares 15,251,018 10,704,830 264,378 Also excluded from the calculation of diluted net loss per share are 450,000 shares of common stock issued by the Company in January 2021, under its ATM program for proceeds of $7.1 million, net of commissions.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The Company measures and reports its cash equivalents, restricted cash, warrant liabilities and securities issuance obligation at fair value. The following table sets forth the fair value of the Company’s financial assets and liabilities measured on a recurring basis by level within the fair value hierarchy: December 31, 2020 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 101,919 $ — $ — $ 101,919 Restricted money market funds 300 — — 300 Total financial assets $ 102,219 $ — $ — $ 102,219 December 31, 2019 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 146,240 $ — $ — $ 146,240 Restricted money market funds 300 — — 300 Total financial assets $ 146,540 $ — $ — $ 146,540 Financial Liabilities Warrant liabilities $ — $ — $ 45,935 $ 45,935 Securities issuance obligation — — 10,485 $ 10,485 Total financial liabilities $ — $ — $ 56,420 $ 56,420 Money market funds and restricted money market funds are measured at fair value on a recurring basis using quoted prices and are classified as a Level 1 input. The Company’s warrant liabilities and securities issuance obligation contained unobservable inputs that reflected the Company’s own assumptions in which there was little, if any, market activity at the measurement date. Accordingly, the Company’s warrant liabilities and securities issuance obligation were measured at fair value on a recurring basis using unobservable inputs at each reporting period. The warrant liabilities and securities issuance obligation were classified as Level 3 inputs. These liabilities were shown as current liabilities on the balance sheet until such time the warrants were no longer considered derivative instruments and the securities issuance obligation was settled . The fair values of the Series A and Series B warrants were estimated using the Black-Scholes option-pricing model. The expected terms represented the periods that the warrants are expected to be outstanding. The risk-free interest rates were based on the U.S. Constant Maturity treasury curve commensurate with the time to expiry. The expected dividend was zero as the Company has not paid nor does it anticipate paying any dividends on its common stock in the foreseeable future. The expected volatilities were estimated by backsolving to volatility implied in the transaction price. Discount for lack of marketability was dependent on the restriction period and the estimated volatility during the period. The fair value of the warrant issuance obligation was estimated using the Black-Scholes option-pricing model. The expected term represented the period that the underlying warrant is expected to be outstanding from the time the issuance obligation arose. The risk-free interest rate was based on the U.S. Constant Maturity treasury curve commensurate with the time to expiry. The expected dividend was zero as the Company has not paid nor does it anticipate paying any dividends on its common stock in the foreseeable future. The expected volatility was estimated by backsolving to volatility implied in the transaction price. The fair value of the common stock issuance obligation was estimated based on the fair value of the underlying common stock. Discount for lack of marketability was dependent on the restriction period and the estimated volatility during the period. The assumptions used in calculating the estimated fair values at the end of the reporting period represent the Company’s best estimate. However, inherent uncertainties are involved. If factors or assumptions change, the estimated fair values could be materially different. At November 13, 2019, upon the issuance of Series A and Series B warrants and when the securities issuance obligation arose, the Company estimated the fair values of the financial liabilities using the following assumptions: Series A Warrant Series B Warrant Warrant Issuance Obligation Common Stock Issuance Obligation Expected term (in years) 5.2 2.3 5.2 N/A Expected volatility 43 % 88 % 43 % N/A Risk-free interest rate 1.70 % 1.64 % 1.70 % N/A Expected dividend yield — % — % — % N/A Discount for lack of marketability 30 % 30 % 32 % 32 % At December 31, 2019, the Company remeasured these liabilities to their fair values using the following assumptions: Series A Warrant Series B Warrant Warrant Issuance Obligation Common Stock Issuance Obligation Expected term (in years) 5.1 2.2 5.1 N/A Expected volatility 43 % 88 % 43 % N/A Risk-free interest rate 1.69 % 1.59 % 1.70 % N/A Expected dividend yield — % — % — % N/A Discount for lack of marketability 25 % 25 % 25 % 25 % At January 22, 2020, Series A and Series B warrants were no longer considered to be derivative instruments. The Company remeasured the fair value of the warrant liabilities at the time of reclassification to equity using the following assumptions: Series A Warrant Series B Warrant Expected term (in years) 5.0 2.1 Expected volatility 43 % 88 % Risk-free interest rate 1.57 % 1.53 % Expected dividend yield — % — % At January 31, 2020, the securities issuance obligation was settled by the issuance of common stock and a common stock warrant. The Company remeasured the fair value of its common stock issuance obligation based on the value of the common stock at the time of issuance. The warrant issuance obligation was remeasured using the following assumptions: Warrant Issuance Obligation Expected term (in years) 5.0 Expected volatility 43 % Risk-free interest rate 1.57 % Expected dividend yield — % The following table provides a summary of changes in the estimated fair values of the Company’s Level 3 financial liabilities, which were measured at fair value on a recurring basis using unobservable inputs: Series A Warrant Liability Series B Warrant Liability Warrant Issuance Obligation Common Stock Issuance Obligation Total (in thousands) Balance, December 31, 2018 $ — $ — $ — $ — $ — Issuance of warrants 17,133 7,876 — — 25,009 Securities issuance obligation — — 1,543 4,903 6,446 Changes in fair value 15,483 5,443 1,493 2,546 24,965 Balance, December 31, 2019 32,616 13,319 3,036 7,449 56,420 Changes in fair value 11,597 4,643 152 1,333 17,725 Settlement of financial liabilities by securities issuance — — (3,188 ) (8,782 ) (11,970 ) Reclassification to equity (44,213 ) (17,962 ) — — (62,175 ) Balance, December 31, 2020 $ — $ — $ — $ — $ — Fluctuations in fair values of the financial liabilities were attributable to changes in the fair value of the underlying stock and non-marketable discount. There were no transfers between Levels 1, 2 or 3 during the years ended December 31, 2020 and 2019.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance Sheet Components	5. Balance Sheet Components Cash and Cash Equivalents Cash and cash equivalents consist of the following: December 31, 2020 December 31, 2019 (in thousands) Cash $ 2,136 $ 1,288 Cash equivalents: Money market accounts 101,919 146,240 Total cash and cash equivalents $ 104,055 $ 147,528 The following table provides a reconciliation of cash, cash equivalents and restricted cash reported in the consolidated balance sheets to the amounts shown in the consolidated statements of cash flows. December 31, 2020 December 31, 2019 (in thousands) Cash and cash equivalents $ 104,055 $ 147,528 Restricted cash included in other assets 300 300 Total cash, cash equivalents and restricted cash shown in the condensed consolidated statement of cash flows $ 104,355 $ 147,828 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consist of the following: December 31, 2020 December 31, 2019 (in thousands) Prepaid insurance $ 991 $ 918 Prepaid research and development project costs 321 853 Other receivables 311 190 Other 792 408 Total prepaid expenses and other current assets $ 2,415 $ 2,369 Property and Equipment, net Property and equipment, net consists of the following: December 31, 2020 December 31, 2019 (in thousands) Software $ 361 $ 352 Leasehold improvements — 112 Computer equipment — 89 Furniture and fixtures — 3 Property and equipment, gross 361 556 Less: accumulated depreciation (309 ) (443 ) Total property and equipment, net $ 52 $ 113 Depreciation related to the Company’s property and equipment for each of the years ended December 31, 2020, 2019 and 2018 was $0.1 million. Accrued and Other Liabilities Accrued and other liabilities consist of the following: December 31, 2020 December 31, 2019 (in thousands) Accrued employee related costs $ 4,359 $ 3,420 Accrued research and development costs 1,715 2,668 Accrued professional fees 774 817 Operating lease liability 207 187 Other 93 78 Total accrued and other liabilities $ 7,148 $ 7,170

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	6. Leases The Company entered into an operating lease agreement to lease the office space in Vancouver, Canada commencing March 1, 2018. The lease expires on February 28, 2023. In December 2020, the Company entered into an agreement to sublet the entire office premises to a third party until February 27, 2023. Pursuant to the sublease agreement, the subtenant will pay base rent of $0.2 million per annum and all operating costs to the Company. The Company recorded an impairment charge of $0.1 million during the year ended December 31, 2020. The components of lease expense, which are recorded in general and administrative expense, and related cash flows for the year ended December 31, 2020 and 2019 were as follows: Year ended December 31, 2020 2019 (in thousands) Operating lease cost $ 195 $ 203 Short-term lease cost 42 117 237 320 Operating cash flows used for operating leases $ 212 $ 198 The total rent expense was $0.5 million for the year ended December 31, 2018. As of December 31, 2020, the weighted average remaining lease term and discount rate for the operating lease are 2.2 years and 6.5%, respectively. As of December 31, 2020, maturities of lease liability due under the lease agreement are as follows: Years Ending December 31: Operating Leases (in thousands) 2021 226 2022 180 Total lease payments 406 Less imputed interest (24 ) Total $ 382 In addition to base rent, this lease requires payment of operating costs. These costs are not included in the table above. In December 2020, the Company entered into a 48-month operating lease agreement to lease office space in San Mateo, California. The lease has not commenced as of December 31, 2020. The Company’s obligation under this office lease is $0.1 million for 2021, $0.2 million for each of 2022, 2023 and 2024, and $0.1 million for 2025.

Term Loan

Term Loan	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Term Loan	7 . Term Loan In August 2018, the Company entered into a Loan and Security Agreement (Loan Agreement) with Silicon Valley Bank (SVB). Contemporaneously with executing the Loan Agreement, the Company drew down the first $5.0 million tranche. In December 2019, the Company repaid the $5.0 million term loan and paid the prepayment and final payment fees of $0.1 million and $0.3 million, respectively. In connection with the Loan Agreement, the Company issued a warrant to SVB to purchase 1,839 of the Company’s common stock at a price per share of $74.80. The warrant was immediately exercisable, will expire on August 21, 2028, contains a cashless exercise provision and is classified as equity No interest expense was recognized for the year ended December 31, 2020.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8 . Commitments and Contingencies Asset Purchase Agreement In August 2018, the Company entered into an Asset Purchase Agreement with Gilead whereby the Company acquired worldwide rights to the pharmaceutical product momelotinib, an investigational orally-bioavailable JAK1, JAK2 and ACVT1 inhibitor together with all related intellectual property rights and certain other related assets. Pursuant to the agreement, the Company made a one-time upfront payment of $3.0 million in August 2018. The related expense was included in research and development for the year ended December 31, 2018 in the accompanying consolidated statement of operations. I n October 2019, the Company entered into an amendment to the Asset Purchase Agreement in which the Company agreed to issue, subject to certain conditions, shares of common stock and a warrant to purchase common stock to Gilead in consideration for meaningfully reduced royalty rates and elimination of a near term milestone payment in the Asset Purchase Agreement. Pursuant to the amended agreement, In connection with obligations under the amendment, the Company recognized a $10.5 million non-cash research and development expense for the year ended December 31, 2019 based on the fair value of the securities to be issued. On January 31, 2020, the Company fulfilled its obligation to issue securities by entering into a securities purchase agreement with Gilead, pursuant to which the Company issued to Gilead 725,283 shares of the Company’s common stock and a warrant to purchase 725,283 shares of common stock at a price per share of $13.20. The warrant is immediately exercisable, will expire on January 31, 2025 and contains a cash and/or cashless exercise provision. Upon remeasurement of the securities issuance obligation immediately prior to the issuance, an additional $1.5 million of non-cash research and development expense, representing changes in fair value of the securities since December 31, 2019, was recognized in the consolidated statement of operations for the year ended December 31, 2020. See Note 4, Fair Value Measurement for further discussions in valuation techniques. License Agreements In September 2016, the Company entered into an exclusive license agreement with CRT Pioneer Fund LP (CPF) for worldwide rights, know-how and materials to develop SRA737, a small molecule inhibitor targeting Chk1, a promising therapeutic target to treat cancer. Pursuant to the agreement, the Company made a one-time upfront payment of $7.0 million to CPF in October 2016 and paid $2.0 million to CPF in January 2017 for the successful transfer of two ongoing Phase 1 clinical trials. Pursuant to the original license agreement, additional milestone payments of up to an aggregate of $319.5 million may have become payable to CPF upon the achievement of certain milestones. In November 2020, the Company entered into an amendment to the license agreement with CPF, which amended the terms and reduced the amounts of certain future milestones. Pursuant to the amended agreement, future milestone payments of up to an aggregate of $290.0 million may become payable to CPF upon the achievement of certain developmental, regulatory and commercial milestones, including a milestone payment of $2.0 million upon the dosing of the first patient of the first trial of SRA737 following the effective date of the amendment. These milestones will be accrued once they are considered probable of occurring. In addition, the Company is required to pay CPF, on a product-by-product and country-by-country basis, tiered high single-digit to low double-digit royalties on the net sales of any product successfully developed. In May 2016, the Company entered into an exclusive license agreement (Carna License Agreement) with Carna Biosciences, Inc. (Carna) for worldwide rights to develop and commercialize SRA141, a small molecule kinase inhibitor targeting Cdc7. In exchange for this exclusive right, the Company paid Carna an upfront payment of $0.9 million in June 2016. In June 2020, the Company entered into a collaboration agreement (Carna Collaboration Agreement) with Carna effectively terminating the Carna License Agreement. Pursuant to the Carna Collaboration Agreement, Carna paid an upfront fee of $0.3 million, which was recognized as collaboration revenue during the year ended December 31, 2020 by the Company, for the exclusive worldwide rights for SRA141 and other transition services. In addition, the Company may be entitled to single-digit royalties on product sales, on a product-by-product basis, and low to mid-teen profit share on royalty and non-royalty income. Legal From time to time, the Company may become subject to other legal proceedings, claims and litigation arising in the ordinary course of business. In addition, the Company may receive letters alleging infringement of patent or other intellectual property rights. The Company is not currently a party to any other material legal proceedings, nor is it aware of any pending or threatened litigation that, in the Company’s opinion, would have a material adverse effect on the business, operating results, cash flows or financial condition should such litigation be resolved unfavorably. COVID-19 The full extent of the impact of the COVID-19 pandemic on financial markets, economies worldwide and our business is highly uncertain. As of December 31, 2020, the Company was not aware of any contingencies and no related estimates were recorded on its financial statements as a result of COVID-19.

Common Stock Reserved for Issua

Common Stock Reserved for Issuance	12 Months Ended
	Dec. 31, 2020
Common Stock Reserved for Issuance [Member]
Stockholders' Equity Note Disclosure	9 . Common Stock The Company is required to reserve and keep available out of its authorized but unissued shares of common stock a number of shares sufficient to effect the conversion of all outstanding preferred stock, options granted and available for grant under the incentive plans, shares reserved for issuance under the employee stock purchase plan and issued warrant. December 31, 2020 December 31, 2019 Shares reserved under Series A warrant 7,802,241 7,802,241 Shares reserved under Series B warrant 2,574,727 2,574,727 Shares reserved for future option grants under equity plans 1,117,796 51,514 Outstanding stock options under equity incentive plans 4,146,928 326,023 Outstanding warrants 727,122 1,839 Shares reserved under the 2015 employee stock purchase plan 17,500 17,500 Shares reserved for conversion of Series A Preferred Stock — 7,803,273 Total common stock reserved for issuance 16,386,314 18,577,117

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2020
Preferred Stock [Member]
Stockholders' Equity Note Disclosure	10 . Preferred Stock As of December 31, 2020 and 2019, the Company had 10,000,000 shares of preferred stock authorized with a par value of $0.001. There were no shares of preferred stock issued and outstanding as of December 31, 2020 and 103,000 shares of Series A Preferred Stock outstanding as of December 31, 2019. Series A Convertible Voting Preferred Stock On November 13, 2019, the Company completed an underwritten public offering whereby it issued 103,000 shares of Series A Preferred Stock together with Series A warrants and Series B warrants for a combined purchase price of $1,000. The aggregate proceeds received by the Company was $97.7 million, net of underwriting discounts and commissions and offering expenses. Each share of Series A Preferred Stock was convertible into shares of the Company’s common stock equal to the stated value of the Series A Preferred Stock of $1,000 divided by the voting conversion price of $13.20. On January 29, 2020, all shares of Series A Preferred Stock converted into 7,803,273 shares of the Company’s common stock.

Warrant Liabilities

Warrant Liabilities	12 Months Ended
	Dec. 31, 2020
Warrants And Rights Note Disclosure [Abstract]
Warrant Liabilities	11 . Warrant Liabilities In connection with the Company’s November 2019 public offering of the Series A Preferred Stock, the Company issued Series A warrants to purchase up to 7,802,241 shares of common stock at an exercise price equal to $13.20, and Series B warrants to purchase up to 2,574,727 shares of common stock at an exercise price equal to $13.20. Both Series A and Series B warrants are exercisable following stockholder approval in January 2020 of an increase in authorized common stock sufficient to allow for the exercise of the warrants, subject to certain beneficial ownership limitations. The Series A warrants will expire five years from the date they first became exercisable or on January 22, 2025 and contain a cash and/or cashless exercise provision. The Series B warrants will expire on the 75 th The Company revalued the warrant liabilities using the Black-Scholes option pricing model instruments, following stockholder approval of an increase in authorized common stock sufficient to allow for the exercise of the warrants, at which time they were reclassified to equity. The fair values of the Series A and Series B warrants at the time of issuance in November 2019, at December 31, 2019 and at the time they ceased to be derivative instruments in January 2020 were estimated to be $25.0 million, $45.9 million and $62.1 million, respectively. The Company recorded a $20.9 million non-cash expense relating to the change in fair value of warrant liabilities in other income (expense), net in the accompanying consolidated statement of operations for the periods ended December 31, 2020 and 2019, respectively.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	1 2 . Stock-Based Compensation In the accompanying consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows: Year ended December 31, 2020 2019 2018 (in thousands) Research and development $ 4,316 $ 3,873 $ 4,499 General and administrative 5,154 1,822 2,297 Total stock-based compensation $ 9,470 $ 5,695 $ 6,796 Determination of Fair Value The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions: Year Ended December 31, 2020 2019 2018 Expected term (in years) 5.3 – 7.0 5.3 – 6.9 5.3 – 7.0 Expected volatility 87 – 90 % 89 – 94 % 88 – 91 % Risk-free interest rate 0.3 – 1.2 % 1.6 – 2.6 % 2.6 – 3.1 % Expected dividend rate — % — % — % The fair value of each stock option grant was determined by the Company on the date of grant using the methods and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment and estimation by management. Expected Term— The expected term represents the period that stock-based awards are expected to be outstanding. As the Company’s historical share option exercise is limited due to a lack of sufficient data points, and does not provide a reasonable basis upon which to estimate an expected term, the expected term is derived by using the midpoint between the weighted-average vesting term and the contractual expiration period of the stock-based award. Expected Volatility— The expected volatility is derived from a weighted volatility using both the Company’s trading history for its common stock and the historical stock volatilities of peer public companies within its industry that are considered to be comparable to the Company’s business over a period equivalent to the expected term of the stock-based awards . Risk-Free Interest Rate— The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the date of grant for zero-coupon U.S. Treasury notes with maturities approximately equal to the stock-based awards’ expected term. Expected Dividend Rate— The expected dividend is zero as the Company has not paid nor anticipate paying any dividends on its common stock in the foreseeable future. Forfeiture Rate— The Company accounts for forfeitures when they occur. Equity Incentive Plans 2018 Equity Inducement Plan In September 2018, the Company’s Compensation Committee approved the 2018 Equity Inducement Plan (2018 Plan). The number of shares available for awards under the 2018 Plan was set to 37,500. On June 30, 2020, the Company’s Board of Directors approved an amendment to the 2018 Plan to increase the authorized number of shares available for issuance by 500,000 shares. As of December 31, 2020, 537,500 shares were reserved for issuance under the 2018 Plan. On February 3, 2021, the Company’s Compensation Committee approved an amendment to the 2018 Plan to increase the authorized number of shares available for issuance by 500,000. The exercise price of each stock-based award issued under the 2018 Plan is required to be no less than the fair value of the Company’s common stock. The vesting and exercise provisions of options or restricted awards granted are determined individually with each grant. Stock options have a 10-year life and expire if not exercised within that period or if not exercised within three months of cessation of employment with the Company or such longer period of time as specified in the option agreement. 2015 Plan The 2015 Equity Incentive Plan (2015 Plan) became effective on July 14, 2015. On January 21, 2020, the Company’s stockholders approved the following amendments to the 2015 Plan: (i) increase to the authorized number of shares available for issuance by 4,312,500 shares and proportionately increase the share limit related to incentive stock options, (ii) provide limits on the total value of compensation that may be granted to any non-employee director in each calendar year, and (iii) eliminate the annual individual grant limit to reflect changes to the tax law in 2017 tax legislation. As of December 31, 2020, 4,678,597 shares were reserved for issuance under the 2015 Plan. The number of shares reserved for issuance under the 2015 Plan will increase automatically on January 1 of each calendar year 2016 through 2025 by the number of shares equal to 4% of the total outstanding shares of the Company’s common stock as of the immediately preceding December 31. The Company’s Board of Directors or Compensation Committee may reduce the amount of the increase in any particular year. The exercise price of each stock-based award issued under the 2015 Plan is required to be no less than the fair value of the Company’s capital stock. The vesting and exercise provisions of options or restricted awards granted are determined individually with each grant. Stock options have a 10-year life and expire if not exercised within that period or if not exercised within three months of cessation of employment with the Company or such longer period of time as specified in the option agreement, unless modified. 2008 Plan The Company granted options under the 2008 Stock Plan (2008 Plan) until July 2015 when it was terminated as to future awards, although it continues to govern the terms of options that remain outstanding under the 2008 Plan. The 2008 Plan provided for the granting of Incentive Stock Options (ISO), nonqualified stock options and stock purchase rights. In connection with the Board of Director’s approval of the 2015 Plan, all remaining shares available for future award under the 2008 Plan were transferred to the 2015 Plan, and the 2008 Plan was terminated. Options Outstanding Shares Available for Grant Number of Shares Outstanding Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value of Outstanding Options (in thousands) Outstanding — December 31, 2019 51,514 326,023 $ 102.56 7.45 $ 11 Awards authorized 4,887,187 Options granted (4,074,939 ) 4,074,939 12.88 Options forfeited/cancelled 254,034 (254,034 ) 20.73 Outstanding — December 31, 2020 1,117,796 4,146,928 $ 19.45 8.84 $ 12,227 Exercisable — December 31, 2020 466,879 $ 65.48 3.90 $ 555 Vested and expected to vest — December 31, 2020 3,594,690 $ 20.57 8.71 $ 10,084 The weighted-average grant date fair values of options granted during the years ended December 31, 2020, 2019 and 2018 was $8.81, $50.40 and $70.00 per share. There were no options exercised for the year ended December 31, 2020. The aggregate intrinsic value of options exercised was $0.1 million In May 2020, the Company entered into a separation agreement with Dr. Glover, the Company’s former President and Chief Executive Officer, in connection with his resignation. Pursuant to the separation agreement, Dr. Glover’s unvested options that would have vested during the one-year period from the date of separation accelerated and vested immediately. The vesting date of all remaining unvested options accelerated by one year, and vested in accordance with the accelerated vesting schedule through December 31, 2020. All unvested options were cancelled on December 31, 2020. Furthermore, Dr. Glover received an extension of the expiration date of his vested stock options to 75 days following the Company’s announcement of the top-line data results from its MOMENTUM clinical trial. Compensation costs relating to the vesting acceleration and the modifications to option terms was $2.2 million for the year ended December 31, 2020. In August 2020, the Company granted executives and employees 1,107,250 stock options with performance-based conditions. Vesting is achieved based upon the satisfaction of pre-determined milestones. As of December 31, 2020, all of the performance-based options remain unvested. For the year ended December 31, 2020, the Company has recognized approximately $0.9 million in stock-based compensation expense relating to certain performance-based criteria that are considered probable of occurring. As of December 31, 2020, total unrecognized stock-based compensation related to unvested stock options was $23.1 million, which the Company expects to recognize over a remaining weighted-average period of 3.1 years. In addition, as of December 31, 2020, total unrecognized stock-based compensation related to unvested stock options with performance-based conditions was $8.9 million. 2015 Employee Stock Purchase Plan The Company adopted the 2015 Employee Stock Purchase Plan (ESPP) and initially reserved 17,500 shares of common stock as of its effective date of July 15, 2015. The aggregate number of shares issued over the term of the 2015 Employee Stock Purchase Plan will not exceed 85,000 shares of common stock. The ESPP will not become effective until such time as the Compensation Committee determines in the future, and as of December 31, 2020, the initial offering periods had not commenced. As of December 31, 2020, no shares of common stock have been issued to employees participating in the ESPP and 17,500 shares were available for issuance under the ESPP.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	1 3 . Income Taxes The geographical breakdown of loss before provision for income taxes is as follows: Year Ended December 31, 2020 2019 2018 (in thousands) United States $ (81,670 ) $ (89,459 ) $ (54,395 ) International $ 910 1,024 758 Loss before provision for (benefit from) income taxes, net $ (80,760 ) $ (88,435 ) $ (53,637 ) The components of the provision for (benefit from) income taxes are as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Current tax provision (benefit): Federal $ — $ — $ — State — — — Foreign 59 85 (180 ) Total current tax provision (benefit) $ 59 $ 85 $ (180 ) Deferred tax provision (benefit): Foreign 83 (245 ) (122 ) Total deferred tax provision (benefit) $ 83 $ (245 ) $ (122 ) Total provision for (benefit from) income taxes $ 142 $ (160 ) $ (302 ) The reconciliation between income taxes computed at the federal statutory income tax rate and the provision for (benefit from) income taxes is as follows: Year Ended December 31, 2020 2019 2018 Federal statutory rate 21.0 % 21.0 % 21.0 % Effect of: Change in valuation allowance (23.6 ) 12.4 (22.2 ) Federal tax credit 5.8 1.5 2.4 Warrant issuance and remeasurement (4.2 ) (5.3 ) — Effect of ownership change on deferred tax assets 1.3 (29.0 ) — State income tax benefit, net of federal benefit 0.2 0.2 0.3 Other permanent items (0.7 ) (0.6 ) (1.0 ) Total provision for (benefit from) income taxes% (0.2 )% 0.2 % 0.5 % The components of the deferred tax assets are as follows: December 31, 2020 2019 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 14,011 $ 2,649 Stock based compensation 6,459 4,663 59 (e) expenditures and amortization 6,222 5,902 Federal R&D and orphan drug credits 5,107 486 License fee 3,224 2,538 Other 1,024 967 Gross deferred tax assets 36,047 17,205 Valuation allowance (35,513 ) (16,441 ) Total deferred tax assets 534 764 Deferred tax liabilities: Lease Asset 114 244 Other 73 90 Total deferred tax liabilities 187 334 Total net deferred tax assets $ 347 $ 430 Recognition of deferred tax assets is appropriate when realization of these assets is more likely than not. Based upon the weight of available evidence, which includes historical operating performance and the recorded cumulative net losses in prior fiscal periods, the Company recorded a full valuation allowance of $35.2 million and $16.4 million against the net U.S. deferred tax assets as of December 31, 2020 and 2019. The U.S. net valuation allowance increased by $18.8 million for the year ended December 31, 2020. The U.S. net valuation allowance decreased by $10.9 million for the year ended December 31, 2019. Management assesses the available positive and negative evidence to estimate if sufficient future taxable income will be generated to use the existing U.S. deferred tax assets. Based on the weight of all evidence, including a history of operating losses and the Company’s ability to generate future taxable income to realize the assets, management has determined that it is more likely than not that the U.S. deferred tax assets will not be realized. Utilization of the Company’s net operating loss and U.S. research and development credit carryforwards to offset taxable income are subject to an annual limitation, pursuant to Internal Revenue Code (IRC) Sections 382 and 383. As a result of ownership changes that have occurred, the most recent which occurred during 2019, certain of the Company’s tax attributes existing as of the date of the ownership change are not be available for future use. The loss of these attributes did not have any impact on the financial statements since the net U.S. deferred tax assets are offset by a full valuation allowance. As of December 31, 2020, the Company had gross U.S. federal tax net operating loss carryforwards of $54.8 million, that are eligible for an indefinite carryforward, and gross state operating loss carryforwards of $52.4 million expiring in years ranging from 2022 to 2040. The Company also has U.S. net tax credit carryforwards of $4.6 million which begin to expire in 2039 and net tax credit carryforwards in a foreign jurisdiction of $0.5 million which begin to expire in 2038. Uncertain Tax Positions The activity related to the gross amount of unrecognized tax benefits is as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Beginning balance $ 314 $ 264 $ 43 Increases based on tax positions related to prior years — — 109 Decreases based on tax positions related to prior years (54 ) (103 ) — Decreases due to ownership change (207 ) — — Increases based on tax positions in current year 568 153 112 Settlement — — — Lapse of statute of limitations — — — Ending balance $ 621 $ 314 $ 264 If recognized, gross unrecognized tax benefits would not have a material impact on the Company’s effective tax rate due to the Company’s full valuation allowance position on the U.S. deferred tax assets. From time to time, the Company is subject to review by tax authorities. It is not possible to estimate the impact of changes, if any, to previously recorded uncertain tax positions. However, the Company does not expect the changes, if any, to be materially different from what is recorded and will adjust its estimate and liability as necessary. The Company recognizes interest and penalties related to unrecognized tax benefits in the provision for income taxes in the accompanying consolidated statement of operations. Accrued interest and penalties, if applicable, are included in accrued liabilities in the consolidated balance sheet. For the years ended December 31, 2020 and 2019, the Company did not recognize any accrued interest and penalties. The Company is subject to taxation in the United States, various states, Canada and Australia. Tax years 2017 through 2019 remain open to examination by the United States, various state jurisdictions and Canada. The tax year ended December 31, 2019 remains open to examination in Australia. The Company is not under examination in any tax jurisdiction for any year.

Selected Quarterly Financial Da

Selected Quarterly Financial Data	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	1 4 . Selected Quarterly Financial Data (Unaudited) The following tables present certain selected unaudited consolidated quarterly financial information for each of the eight quarters ended December 31, 2020. This consolidated quarterly information has been prepared on the same basis as the consolidated financial statements and includes all adjustments necessary to state fairly the information for the periods presented. The selected consolidated quarterly financial results from operations for the years ended December 31, 2020 and 2019 are set forth therein. Fiscal 2020 Quarter Ended March 31, 2020 (2) June 30, 2020 September 30, 2020 December 31, 2020 (in thousands, except per share amounts) Collaboration revenue $ — $ — $ 100 $ 200 Operating expenses $ 16,135 $ 16,449 $ 14,544 $ 18,113 Net loss (1) $ (31,912 ) $ (16,462 ) $ (14,505 ) $ (18,023 ) Basic and diluted net loss per share $ (3.14 ) $ (1.58 ) $ (1.39 ) $ (1.63 ) Fiscal 2019 Quarter Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, 2019 (2) (in thousands, except per share amounts) Operating expenses $ 13,502 $ 15,207 $ 13,264 $ 25,019 Net loss (1) $ (13,032 ) $ (14,878 ) $ (12,903 ) $ (47,462 ) Basic and diluted net loss per share $ (7.00 ) $ (7.97 ) $ (6.91 ) $ (7.88 ) (1) (2) Net loss for the quarters ended March 31, 2020 and December 31, 2019 included a $16.2 million and $20.9 million non-cash charge, respectively, relating to changes in fair value of warrant liabilities (see Note 11) and a $1.5 million and $10.5 million non-cash charge, respectively, relating to the Company’s obligation to issue securities pursuant to an amendment to the Asset Purchase Agreement with Gilead (see Note 8).

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (U.S. GAAP). The accompanying consolidated financial statements include the accounts of Sierra Oncology, Inc. and its wholly-owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of consolidated financial statements and the reported amounts of expense during the reporting period. Significant estimates and assumptions made in the accompanying consolidated financial statements include, but are not limited to the fair values of stock options and warrants issued, the convertible voting preferred stock and securities issuance obligation, the probability of achieving performance-based milestones of stock options, accruals such as research and development costs, and recoverability of the Company’s net deferred tax assets, and related valuation allowance. The Company evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors and adjusts those estimates and assumptions when facts and circumstances dictate. Actual results could materially differ from those estimates.
Foreign Currency	Foreign Currency The functional currency of the Company’s foreign subsidiaries is the U.S. Dollar. Transactions denominated in currencies other than the functional currency are recorded at prevailing exchange rates during the period. At the end of each reporting period, monetary assets and liabilities are remeasured to the functional currency using exchange rates in effect at the balance sheet date. Non-monetary assets and liabilities are recorded at historical exchange rates. Gains and losses related to remeasurement are recorded in other income (expense), net in the consolidated statements of operations. The net foreign exchange transaction gains (losses) included in other income (expense), net in the accompanying consolidated statements of operations were insignificant for the years ended December 31, 2020, 2019 and 2018.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with an original maturity of three months or less from the date of purchase to be cash equivalents. Cash and cash equivalents consist primarily of funds invested in readily available checking and savings accounts and highly liquid investments in money market funds.
Restricted Cash	Restricted Cash Restricted cash, which consists of funds invested in a money market fund, represents collateral for a corporate credit card facility and is included in other assets in the accompanying consolidated balance sheets.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that subject the Company to significant concentrations of credit risk consist of cash, cash equivalents and restricted cash. All of the Company’s cash, cash equivalents and restricted cash are held at financial institutions in the United States and Canada that management believes to be of high credit quality. Deposits held in the United States and Canada with these financial institutions exceed federally insured limits. The primary focus of the Company’s investment strategy is to preserve capital and meet liquidity requirements. The Company’s investment policy addresses the level of credit exposure by limiting the concentration in any one corporate issuer and establishing a minimum allowable credit rating.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s cash and cash equivalents, restricted cash, other current assets, accounts payable and accrued liabilities approximate their fair values at December 31, 2020 and 2019, due to their short duration. The warrant liabilities and securities issuance obligation contained unobservable inputs that reflected the Company’s own assumptions in which there was little, if any, market activity at the measurement date, thus the Company’s warrant liabilities and securities issuance obligation were measured at their fair values on a recurring basis using unobservable inputs until such time the warrants were no longer considered derivative instruments and the securities issuance obligation was settled. The Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible. The Company determines the fair value of its financial instruments based on assumptions that market participants would use in pricing an asset or liability in the principal or most advantageous market. When considering market participant assumptions in fair value measurements, the following fair value hierarchy distinguishes between observable and unobservable inputs, which are categorized in one of the following levels: Level 1 —Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2 —Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 —Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.
Property and Equipment, Net	Property and Equipment, Net Property and equipment, net are stated at cost, less accumulated depreciation. Depreciation on property and equipment, excluding leasehold improvements, is computed using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized on a straight-line basis over the shorter of the estimated useful lives of the assets or the remaining lease term. Depreciation begins at the time the asset is placed in service. Maintenance and repairs are charged to operations as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is reflected in the consolidated statement of operations.
Other Assets	Other Assets Other assets consist primarily of restricted cash pledged as collateral for a corporate credit card facility and deferred income tax assets in foreign jurisdictions.
Operating Lease Right-of-Use Asset and Lease Liabilities	Operating Lease Right-of-Use Asset and Lease Liability The Company recognizes an operating lease with terms greater than one year as right-of-use (ROU) asset and lease liability on its consolidated balance sheet using the portfolio approach. Lease liability and ROU asset are recorded based on the present value of future lease payments over the contractual term of the operating lease. The Company utilized its incremental borrowing rate from information available as at the date of initial adoption in determining the present value of the future lease payments. The lease liability and ROU asset are amortized over the term of the lease.
Warrant Liabilities	Warrant Liabilities The Company accounted for its warrants issued in connection with its November 2019 financing based upon the characteristics and provisions of the instruments. Warrants classified as derivative liabilities were recorded on the Company’s consolidated balance sheets at their fair value on the date of issuance and remeasured to fair value on each subsequent reporting period, with the changes in fair value recognized as a component of other income (expense), net in the accompanying consolidated statements of operations. The Company revalued the warrant liabilities until they ceased to be derivative instruments, at which time they were reclassified to additional paid-in capital at their fair value. The Company estimated the fair value of these liabilities using the Black-Scholes option-pricing model and the fair value of the underlying stock, as well as assumptions for expected volatility, expected term and risk-free interest rate. An estimated non-marketable discount was also applied if applicable. Offering expenses arising from the issuance of warrants are expensed as incurred. Securities Issuance Obligation The Company recognized its securities issuance obligation pursuant to the amendment to the Asset Purchase Agreement with Gilead Sciences, Inc. (Gilead) at their fair values on the date such obligations arose. At the end of each reporting period, changes in estimated fair values during the period were recognized as research and development costs in the accompanying consolidated statements of operations. The Company adjusted the fair values of the obligations until the securities issuance obligation was settled, at which time the liabilities were reclassified to common stock and additional paid-in capital at their fair values. The Company estimated the fair value of its obligation to issue common stock using the fair value of the underlying stock and a non-marketable discount. The Company estimated the fair value of its obligation to issue warrant using the Black-Scholes option-pricing model and the fair value of the underlying stock, as well as assumptions for expected volatility, expected term and risk-free interest rate. An estimated non-marketable discount was also applied if applicable.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. The Company accounts for non-refundable advance payments for goods and services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made. Depending on the timing of payments to service providers of research and development costs, the Company recognizes prepaid expenses or accrued expenses related to these costs. These prepaid or accrued expenses are based on management’s estimates of the work performed under service agreements and milestones achieved. In the event that a clinical trial is terminated early, the Company records an accrual for the estimated remaining costs to complete the trial in the period of termination. Upfront payments made in connection with license and asset purchase agreements are expensed as research and developments costs, as the assets acquired do not have alternative future use. Contingent milestone payment obligations due to third parties under license and asset purchase agreements are expensed when the milestones are considered probable of occurring. To the extent an obligation is to be settled by future issuance of securities, the fair value of these instruments is recorded in research and development expense until the securities are issued. Research and development costs include fees incurred in connection with license and asset purchase agreements and their related amendments, compensation and other related costs for employees engaged in research and development, costs associated with research and preclinical studies, clinical trials, regulatory activities, manufacturing activities to support clinical activities, fees paid to external service providers that conduct certain research and development, clinical, and manufacturing activities on behalf of the Company and an allocation of overhead expenses.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based payments at fair value, which is measured using the Black-Scholes option-pricing model. For stock-based awards that vest subject to the satisfaction of a service requirement, the fair value measurement date for stock-based compensation awards is the date of grant and the expense is recognized on a straight-line basis over the vesting period, which is generally the service period. For stock-based awards that vest subject to the satisfaction of a service requirement and a performance component, the fair value measurement date is the date of grant and the expense is recognized over the requisite service period as achievement of the performance objective becomes probable. The Company accounts for forfeitures as they occur.
Income Taxes	Income Taxes The Company uses the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Management makes an assessment of the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s historical operating performance and the recorded cumulative net losses in prior fiscal periods, the net U.S. deferred tax assets have been offset by a full valuation allowance. The Company recognizes uncertain income tax positions at the largest amount that is more likely than not to be sustained upon audit by the relevant taxing authority. An uncertain income tax position will not be recognized if it has less than a 50% likelihood of being sustained. Changes in recognition or measurement are reflected in the period in which judgment occurs. The Company recognizes interest and penalties related to the underpayment of income taxes as a component of provision for (benefit from) income taxes, net.
Segment Information	Segment Information Operating segments are components of an enterprise for which separate financial information is available and is evaluated regularly by the Company’s chief operating decision maker in deciding how to allocate resources and assessing performance. The Company’s chief operating decision maker is its Chief Executive Officer. The Company’s Chief Executive Officer views the Company’s operations and manages its business in one operating segment, which is the business of researching, developing and commercializing therapies for the treatment of patients with hematology and oncology needs. Accordingly, the Company has a single reporting segment.

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Schedule of Outstanding Shares of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share	The following shares of common stock equivalents were excluded from the calculation of diluted net loss per share for the periods presented because including them would have been antidilutive: As of December 31, 2020 2019 2018 Series A warrants for common stock 7,802,241 7,802,241 — Series B warrants for common stock 2,574,727 2,574,727 — Options to purchase common stock 4,146,928 326,023 262,539 Warrants for common stock 727,122 1,839 1,839 Total potential dilutive shares 15,251,018 10,704,830 264,378

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of Fair Value Assets And Liabilities Measured On Recurring Basis	The following table sets forth the fair value of the Company’s financial assets and liabilities measured on a recurring basis by level within the fair value hierarchy: December 31, 2020 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 101,919 $ — $ — $ 101,919 Restricted money market funds 300 — — 300 Total financial assets $ 102,219 $ — $ — $ 102,219 December 31, 2019 Level 1 Level 2 Level 3 Total (in thousands) Financial Assets Money market funds $ 146,240 $ — $ — $ 146,240 Restricted money market funds 300 — — 300 Total financial assets $ 146,540 $ — $ — $ 146,540 Financial Liabilities Warrant liabilities $ — $ — $ 45,935 $ 45,935 Securities issuance obligation — — 10,485 $ 10,485 Total financial liabilities $ — $ — $ 56,420 $ 56,420
Schedule of Fair Value Measurement Inputs and Valuation Techniques	At November 13, 2019, upon the issuance of Series A and Series B warrants and when the securities issuance obligation arose, the Company estimated the fair values of the financial liabilities using the following assumptions: Series A Warrant Series B Warrant Warrant Issuance Obligation Common Stock Issuance Obligation Expected term (in years) 5.2 2.3 5.2 N/A Expected volatility 43 % 88 % 43 % N/A Risk-free interest rate 1.70 % 1.64 % 1.70 % N/A Expected dividend yield — % — % — % N/A Discount for lack of marketability 30 % 30 % 32 % 32 % At December 31, 2019, the Company remeasured these liabilities to their fair values using the following assumptions: Series A Warrant Series B Warrant Warrant Issuance Obligation Common Stock Issuance Obligation Expected term (in years) 5.1 2.2 5.1 N/A Expected volatility 43 % 88 % 43 % N/A Risk-free interest rate 1.69 % 1.59 % 1.70 % N/A Expected dividend yield — % — % — % N/A Discount for lack of marketability 25 % 25 % 25 % 25 % At January 22, 2020, Series A and Series B warrants were no longer considered to be derivative instruments. The Company remeasured the fair value of the warrant liabilities at the time of reclassification to equity using the following assumptions: Series A Warrant Series B Warrant Expected term (in years) 5.0 2.1 Expected volatility 43 % 88 % Risk-free interest rate 1.57 % 1.53 % Expected dividend yield — % — % At January 31, 2020, the securities issuance obligation was settled by the issuance of common stock and a common stock warrant. The Company remeasured the fair value of its common stock issuance obligation based on the value of the common stock at the time of issuance. The warrant issuance obligation was remeasured using the following assumptions: Warrant Issuance Obligation Expected term (in years) 5.0 Expected volatility 43 % Risk-free interest rate 1.57 % Expected dividend yield — %
Schedule of Financial Liabilities Fair Value of Recurring Basis Using Unobservable Inputs	The following table provides a summary of changes in the estimated fair values of the Company’s Level 3 financial liabilities, which were measured at fair value on a recurring basis using unobservable inputs: Series A Warrant Liability Series B Warrant Liability Warrant Issuance Obligation Common Stock Issuance Obligation Total (in thousands) Balance, December 31, 2018 $ — $ — $ — $ — $ — Issuance of warrants 17,133 7,876 — — 25,009 Securities issuance obligation — — 1,543 4,903 6,446 Changes in fair value 15,483 5,443 1,493 2,546 24,965 Balance, December 31, 2019 32,616 13,319 3,036 7,449 56,420 Changes in fair value 11,597 4,643 152 1,333 17,725 Settlement of financial liabilities by securities issuance — — (3,188 ) (8,782 ) (11,970 ) Reclassification to equity (44,213 ) (17,962 ) — — (62,175 ) Balance, December 31, 2020 $ — $ — $ — $ — $ —

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Summary of Cash and Cash Equivalents	Cash and cash equivalents consist of the following: December 31, 2020 December 31, 2019 (in thousands) Cash $ 2,136 $ 1,288 Cash equivalents: Money market accounts 101,919 146,240 Total cash and cash equivalents $ 104,055 $ 147,528
Summary of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported in the consolidated balance sheets to the amounts shown in the consolidated statements of cash flows. December 31, 2020 December 31, 2019 (in thousands) Cash and cash equivalents $ 104,055 $ 147,528 Restricted cash included in other assets 300 300 Total cash, cash equivalents and restricted cash shown in the condensed consolidated statement of cash flows $ 104,355 $ 147,828
Summary of Prepaid Expenses and Other Current Assets	Prepaid expenses and other current assets consist of the following: December 31, 2020 December 31, 2019 (in thousands) Prepaid insurance $ 991 $ 918 Prepaid research and development project costs 321 853 Other receivables 311 190 Other 792 408 Total prepaid expenses and other current assets $ 2,415 $ 2,369
Summary of Property and Equipment, Net	Property and equipment, net consists of the following: December 31, 2020 December 31, 2019 (in thousands) Software $ 361 $ 352 Leasehold improvements — 112 Computer equipment — 89 Furniture and fixtures — 3 Property and equipment, gross 361 556 Less: accumulated depreciation (309 ) (443 ) Total property and equipment, net $ 52 $ 113
Schedule of Accrued and Other Liabilities	Accrued and other liabilities consist of the following: December 31, 2020 December 31, 2019 (in thousands) Accrued employee related costs $ 4,359 $ 3,420 Accrued research and development costs 1,715 2,668 Accrued professional fees 774 817 Operating lease liability 207 187 Other 93 78 Total accrued and other liabilities $ 7,148 $ 7,170

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Schedule of Components of Lease Expense and Related Cash Flows	The components of lease expense, which are recorded in general and administrative expense, and related cash flows for the year ended December 31, 2020 and 2019 were as follows: Year ended December 31, 2020 2019 (in thousands) Operating lease cost $ 195 $ 203 Short-term lease cost 42 117 237 320 Operating cash flows used for operating leases $ 212 $ 198
Schedule of Maturities Lease Liabilities under Leases Agreement	As of December 31, 2020, maturities of lease liability due under the lease agreement are as follows: Years Ending December 31: Operating Leases (in thousands) 2021 226 2022 180 Total lease payments 406 Less imputed interest (24 ) Total $ 382

Common Stock Reserved for Iss_2

Common Stock Reserved for Issuance (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of Common Stock Reserved for Issuance	The Company is required to reserve and keep available out of its authorized but unissued shares of common stock a number of shares sufficient to effect the conversion of all outstanding preferred stock, options granted and available for grant under the incentive plans, shares reserved for issuance under the employee stock purchase plan and issued warrant. December 31, 2020 December 31, 2019 Shares reserved under Series A warrant 7,802,241 7,802,241 Shares reserved under Series B warrant 2,574,727 2,574,727 Shares reserved for future option grants under equity plans 1,117,796 51,514 Outstanding stock options under equity incentive plans 4,146,928 326,023 Outstanding warrants 727,122 1,839 Shares reserved under the 2015 employee stock purchase plan 17,500 17,500 Shares reserved for conversion of Series A Preferred Stock — 7,803,273 Total common stock reserved for issuance 16,386,314 18,577,117

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Stock-based Compensation Expense for Employees and Non-employees	In the accompanying consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows: Year ended December 31, 2020 2019 2018 (in thousands) Research and development $ 4,316 $ 3,873 $ 4,499 General and administrative 5,154 1,822 2,297 Total stock-based compensation $ 9,470 $ 5,695 $ 6,796
Schedule of Estimated Grant-date Fair Value of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions	The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions: Year Ended December 31, 2020 2019 2018 Expected term (in years) 5.3 – 7.0 5.3 – 6.9 5.3 – 7.0 Expected volatility 87 – 90 % 89 – 94 % 88 – 91 % Risk-free interest rate 0.3 – 1.2 % 1.6 – 2.6 % 2.6 – 3.1 % Expected dividend rate — % — % — %
Summary of Stock-Based Compensation Activity	. Options Outstanding Shares Available for Grant Number of Shares Outstanding Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value of Outstanding Options (in thousands) Outstanding — December 31, 2019 51,514 326,023 $ 102.56 7.45 $ 11 Awards authorized 4,887,187 Options granted (4,074,939 ) 4,074,939 12.88 Options forfeited/cancelled 254,034 (254,034 ) 20.73 Outstanding — December 31, 2020 1,117,796 4,146,928 $ 19.45 8.84 $ 12,227 Exercisable — December 31, 2020 466,879 $ 65.48 3.90 $ 555 Vested and expected to vest — December 31, 2020 3,594,690 $ 20.57 8.71 $ 10,084

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Geographical Breakdown of Loss Before Provision for Income Taxes	The geographical breakdown of loss before provision for income taxes is as follows: Year Ended December 31, 2020 2019 2018 (in thousands) United States $ (81,670 ) $ (89,459 ) $ (54,395 ) International $ 910 1,024 758 Loss before provision for (benefit from) income taxes, net $ (80,760 ) $ (88,435 ) $ (53,637 )
Components of Provision (Benefit from) for Income Taxes	The components of the provision for (benefit from) income taxes are as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Current tax provision (benefit): Federal $ — $ — $ — State — — — Foreign 59 85 (180 ) Total current tax provision (benefit) $ 59 $ 85 $ (180 ) Deferred tax provision (benefit): Foreign 83 (245 ) (122 ) Total deferred tax provision (benefit) $ 83 $ (245 ) $ (122 ) Total provision for (benefit from) income taxes $ 142 $ (160 ) $ (302 )
Reconciliation Between Income Taxes Computed at the Federal Statutory Income Tax Rate and the Provision for Income Taxes	The reconciliation between income taxes computed at the federal statutory income tax rate and the provision for (benefit from) income taxes is as follows: Year Ended December 31, 2020 2019 2018 Federal statutory rate 21.0 % 21.0 % 21.0 % Effect of: Change in valuation allowance (23.6 ) 12.4 (22.2 ) Federal tax credit 5.8 1.5 2.4 Warrant issuance and remeasurement (4.2 ) (5.3 ) — Effect of ownership change on deferred tax assets 1.3 (29.0 ) — State income tax benefit, net of federal benefit 0.2 0.2 0.3 Other permanent items (0.7 ) (0.6 ) (1.0 ) Total provision for (benefit from) income taxes% (0.2 )% 0.2 % 0.5 %
Components of the Deferred Tax Assets	The components of the deferred tax assets are as follows: December 31, 2020 2019 (in thousands) Deferred tax assets: Net operating loss carryforwards $ 14,011 $ 2,649 Stock based compensation 6,459 4,663 59 (e) expenditures and amortization 6,222 5,902 Federal R&D and orphan drug credits 5,107 486 License fee 3,224 2,538 Other 1,024 967 Gross deferred tax assets 36,047 17,205 Valuation allowance (35,513 ) (16,441 ) Total deferred tax assets 534 764 Deferred tax liabilities: Lease Asset 114 244 Other 73 90 Total deferred tax liabilities 187 334 Total net deferred tax assets $ 347 $ 430
Unrecognized Tax Benefits	The activity related to the gross amount of unrecognized tax benefits is as follows: Year Ended December 31, 2020 2019 2018 (in thousands) Beginning balance $ 314 $ 264 $ 43 Increases based on tax positions related to prior years — — 109 Decreases based on tax positions related to prior years (54 ) (103 ) — Decreases due to ownership change (207 ) — — Increases based on tax positions in current year 568 153 112 Settlement — — — Lapse of statute of limitations — — — Ending balance $ 621 $ 314 $ 264

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Summary of Selected Unaudited Consolidated Quarterly Financial Information	Fiscal 2020 Quarter Ended March 31, 2020 (2) June 30, 2020 September 30, 2020 December 31, 2020 (in thousands, except per share amounts) Collaboration revenue $ — $ — $ 100 $ 200 Operating expenses $ 16,135 $ 16,449 $ 14,544 $ 18,113 Net loss (1) $ (31,912 ) $ (16,462 ) $ (14,505 ) $ (18,023 ) Basic and diluted net loss per share $ (3.14 ) $ (1.58 ) $ (1.39 ) $ (1.63 ) Fiscal 2019 Quarter Ended March 31, 2019 June 30, 2019 September 30, 2019 December 31, 2019 (2) (in thousands, except per share amounts) Operating expenses $ 13,502 $ 15,207 $ 13,264 $ 25,019 Net loss (1) $ (13,032 ) $ (14,878 ) $ (12,903 ) $ (47,462 ) Basic and diluted net loss per share $ (7.00 ) $ (7.97 ) $ (6.91 ) $ (7.88 ) (1) (2) Net loss for the quarters ended March 31, 2020 and December 31, 2019 included a $16.2 million and $20.9 million non-cash charge, respectively, relating to changes in fair value of warrant liabilities (see Note 11) and a $1.5 million and $10.5 million non-cash charge, respectively, relating to the Company’s obligation to issue securities pursuant to an amendment to the Asset Purchase Agreement with Gilead (see Note 8).

The Company and Basis of Pres_2

The Company and Basis of Presentation - Additional Information (Detail) - USD ($)	Jan. 22, 2020	Nov. 13, 2019	Mar. 06, 2018	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2018	Feb. 28, 2021	Aug. 31, 2020	Jan. 29, 2020	Dec. 31, 2019
Cash and cash equivalents					$ 104,055,000					$ 147,528,000
Description of reverse stock split	1-for-40
Issuance of common stock					$ 8,901,000	$ 45,996,000
Follow on Offering [Member]
Issuance of common stock, Shares			546,250
Share issued price per share			$ 90
Proceeds from issuance of common stock, net of deferred financing costs			$ 46,000,000
Underwriting discounts and commissions and offering expenses			$ 3,200,000
ATM Program [Member]
Issuance of common stock, Shares					732,752
Issuance of common stock					$ 8,900,000
ATM Program [Member] | Subsequent Event [Member]
Issuance of common stock, Shares				450,000
ATM Program [Member] | Maximum [Member]
Common stock reserved for issuance								$ 20,000,000
ATM Program [Member] | Maximum [Member] | Subsequent Event [Member]
Common stock reserved for issuance							$ 30,000,000
Series A Warrants [Member] | Underwritten Public Offering [Member]
Number of securitites called by warrants and rights		7,802,241
Excercise price of warrants or rights		$ 13.20
Series B Warrants [Member] | Underwritten Public Offering [Member]
Number of securitites called by warrants and rights		2,574,727
Excercise price of warrants or rights		$ 13.20
Series A Convertible Voting Preferred Stock and Warrants [Member] | Underwritten Public Offering [Member]
Issuance of common stock, Shares		103,000
Share issued price per share		$ 1,000
Proceeds from issuance of convertible preferred stock and warrants net of issuance cost		$ 97,700,000
Stock issuance costs		$ 5,300,000
Preferred Stock converted into of the Company's common stock									7,803,273

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020Segment
Summary Of Significant Accounting Policy [Line Items]
Number of operating segment	1
Minimum [Member] | Property Plant and Equipment Other than Leasehold Improvement [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	3 years
Maximum [Member] | Property Plant and Equipment Other than Leasehold Improvement [Member]
Summary Of Significant Accounting Policy [Line Items]
Estimated useful lives of assets	5 years

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Outstanding Shares of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	15,251,018	10,704,830	264,378
Series A Warrants For Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	7,802,241	7,802,241
Series B Warrants For Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	2,574,727	2,574,727
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	4,146,928	326,023	262,539
Warrants for Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total potential dilutive shares	727,122	1,839	1,839

Net Loss Per Share - Additional

Net Loss Per Share - Additional Information (Detail) - ATM Program [Member] - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2021	Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Issuance of common stock, Shares		732,752
Subsequent Event [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Issuance of common stock, Shares	450,000
Proceeds from issuance of common stock	$ 7.1

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Fair Value of Financial Assets Measured on Recurring Basis (Detail) - Fair Value Measurements Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financial Assets
Total financial assets	$ 102,219	$ 146,540
Financial Liabilities
Total financial liabilities		56,420
Warrant Liabilities [Member]
Financial Liabilities
Total financial liabilities		45,935
Securities Issuance Obligation [Member]
Financial Liabilities
Total financial liabilities		10,485
Level 1 [Member]
Financial Assets
Total financial assets	102,219	146,540
Level 3 [Member]
Financial Liabilities
Total financial liabilities		56,420
Level 3 [Member] | Warrant Liabilities [Member]
Financial Liabilities
Total financial liabilities		45,935
Level 3 [Member] | Securities Issuance Obligation [Member]
Financial Liabilities
Total financial liabilities		10,485
Money Market Funds [Member]
Financial Assets
Total financial assets	101,919	146,240
Money Market Funds [Member] | Level 1 [Member]
Financial Assets
Total financial assets	101,919	146,240
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	300	300
Restricted Money Market Funds [Member] | Level 1 [Member]
Financial Assets
Total financial assets	$ 300	$ 300

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Fair Value of Financial Liabilities Assumptions (Detail)	Jan. 31, 2020yr	Jan. 22, 2020yr	Dec. 31, 2019yr	Nov. 13, 2019yr
Expected term [Member] | Warrant Issuance Obligation [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input	5		5.1	5.2
Expected term [Member] | Series A Warrants [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input		5	5.1	5.2
Expected term [Member] | Series B Warrants [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input		2.1	2.2	2.3
Expected volatility [Member] | Warrant Issuance Obligation [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input	0.43		0.43	0.43
Expected volatility [Member] | Series A Warrants [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input		0.43	0.43	0.43
Expected volatility [Member] | Series B Warrants [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input		0.88	0.88	0.88
Risk-free interest rate [Member] | Warrant Issuance Obligation [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input	0.0157		0.0170	0.0170
Risk-free interest rate [Member] | Series A Warrants [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input		0.0157	0.0169	0.0170
Risk-free interest rate [Member] | Series B Warrants [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input		0.0153	0.0159	0.0164
Discount for lack of marketability [Member] | Warrant Issuance Obligation [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input			0.25	0.32
Discount for lack of marketability [Member] | Common stock Issuance Obligation [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input			0.25	0.32
Discount for lack of marketability [Member] | Series A Warrants [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input			0.25	0.30
Discount for lack of marketability [Member] | Series B Warrants [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Warrants and Rights Outstanding, Measurement Input			0.25	0.30

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Changes in Fair Values Level Financial Liabilities Recurring Basis Using Unobservable Inputs (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Reclassification of warrant liabilities to equity			$ 62,175
Changes in fair value	$ 16,200	$ 20,900	16,240	$ 20,926
Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance, beginning	56,420		56,420
Issuance of warrants				25,009
Securities issuance obligation				6,446
Changes in fair value			17,725	24,965
Balance, at end		56,420		56,420
Settlement of financial liabilities by securities issuance			(11,970)
Reclassification of warrant liabilities to equity			(62,175)
Series A Warrant [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance, beginning	32,616		32,616
Issuance of warrants				17,133
Balance, at end		32,616		32,616
Changes in fair value			11,597	15,483
Reclassification to equity			(44,213)
Series B Warrant [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance, beginning	13,319		13,319
Issuance of warrants				7,876
Balance, at end		13,319		13,319
Changes in fair value			4,643	5,443
Reclassification to equity			(17,962)
Warrant Issuance Obligation [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance, beginning	3,036		3,036
Securities issuance obligation				1,543
Balance, at end		3,036		3,036
Settlement of financial liabilities by securities issuance			(3,188)
Changes in fair value			152	1,493
Common stock Issuance Obligation [Member] | Fair Value Measurements Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance, beginning	$ 7,449		7,449
Securities issuance obligation				4,903
Balance, at end		$ 7,449		7,449
Settlement of financial liabilities by securities issuance			(8,782)
Changes in fair value			$ 1,333	$ 2,546

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Cash And Cash Equivalents [Abstract]
Cash	$ 2,136	$ 1,288
Cash equivalents:
Money market accounts	101,919	146,240
Total cash and cash equivalents	$ 104,055	$ 147,528

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 104,055	$ 147,528
Restricted cash included in other assets	300	300
Total cash, cash equivalents and restricted cash shown in the condensed consolidated statement of cash flows	$ 104,355	$ 147,828	$ 106,346	$ 100,536

Balance Sheet Components - Su_3

Balance Sheet Components - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid insurance	$ 991	$ 918
Prepaid research and development project costs	321	853
Other receivables	311	190
Other	792	408
Total prepaid expenses and other current assets	$ 2,415	$ 2,369

Balance Sheet Components - Su_4

Balance Sheet Components - Summary of Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 361	$ 556
Less: accumulated depreciation	(309)	(443)
Total property and equipment, net	52	113
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 361	352
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		112
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		89
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 3

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation	$ 0.1	$ 0.1	$ 0.1

Balance Sheet Components - Su_5

Balance Sheet Components - Summary of Accrued and Other Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Payables And Accruals [Abstract]
Accrued employee related costs	$ 4,359	$ 3,420
Accrued research and development costs	1,715	2,668
Accrued professional fees	774	817
Operating lease liability	$ 207	$ 187
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherCurrentLiabilitiesMember	us-gaap:OtherCurrentLiabilitiesMember
Other	$ 93	$ 78
Total accrued and other liabilities	$ 7,148	$ 7,170

Leases - Additional Information

Leases - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2018	Nov. 30, 2020
Lease Description [Line Items]
Operating lease impairment charge	$ 106
Vancouver [Member]
Lease Description [Line Items]
Operating lease agreement expiration date	Feb. 28, 2023
Sublease base rent			$ 200
Operating lease impairment charge	$ 100
Rent expense		$ 500
Weighted-average remaining lease term, operating lease (Year)	2 years 2 months 12 days
Weighted-average discount rate, Operating lease	6.50%
California [Member]
Lease Description [Line Items]
Operating lease agreement term	48 months
2021	$ 100
2022	200
2023	200
2024	200
2025	$ 100

Leases - Schedule of Components

Leases - Schedule of Components of Lease Expense and Related Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 195	$ 203
Short-term lease cost	42	117
Total Lease Cost	237	320
Operating cash flows used for operating leases	$ 212	$ 198

Leases - Schedule of Maturities

Leases - Schedule of Maturities Lease Liabilities under Leases Agreement (Detail) $ in Thousands	Dec. 31, 2020USD ($)
Leases [Abstract]
2021	$ 226
2022	180
Total lease payments	406
Less imputed interest	(24)
Total	$ 382

Term Loan - Additional Informat

Term Loan - Additional Information (Detail) - Silicon Valley Bank [Member] - Term Loans [Member] - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 31, 2018
Debt Instrument [Line Items]
Loan agreement, description		Contemporaneously with executing the Loan Agreement, the Company drew down the first $5.0 million tranche.
Interest expense		$ 0	$ 0.9	$ 0.2
Prepayment Fee [Member]
Debt Instrument [Line Items]
Payment of fees	$ 0.1
Final Payment Fee [Member]
Debt Instrument [Line Items]
Payment of fees	0.3
Warrants to Purchase Common Stock [Member]
Debt Instrument [Line Items]
Warrants issued in connection with loan		1,839
Excercise price of warrants or rights		$ 74.80
Warrant expiration date					Aug. 21, 2028
Tranche One [Member]
Debt Instrument [Line Items]
Aggregate principal amount of loans	$ 5

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended				12 Months Ended
	Jan. 31, 2020	Aug. 31, 2018	Oct. 31, 2016	Jun. 30, 2016	Dec. 31, 2020	Dec. 31, 2019	Jan. 31, 2017
Carna Collaboration Agreement [Member]
Loss Contingencies [Line Items]
Upfront License Fee					$ 0.3
Agreement Amendment [Member]
Loss Contingencies [Line Items]
Securities issuance obligation						$ 10.5
Changes in fair value					1.5
Asset Purchase Agreement [Member]
Loss Contingencies [Line Items]
Upfront payment paid		$ 3
Aggregate milestone payment					190
Asset Purchase Agreement Amendment [Member] | Gilead [Member]
Loss Contingencies [Line Items]
Stock issued, shares	725,283
Number of securities called by warrants and rights	725,283
Excercise price of warrants or rights	$ 13.20
CPF License Agreements [Member]
Loss Contingencies [Line Items]
Aggregate milestone payment					290
Milestone payment							$ 2
Upfront payment paid			$ 7
Additional milestone payment payable					319.5
CPF License Agreements [Member] | First patient first trial post amendment [Member]
Loss Contingencies [Line Items]
Milestone payment					$ 2
Carna License Agreement [Member]
Loss Contingencies [Line Items]
Upfront payment paid				$ 0.9

Common Stock Reserved for Iss_3

Common Stock Reserved for Issuance - Schedule of Common Stock Reserved for Issuance (Detail) - shares	Dec. 31, 2020	Dec. 31, 2019	Jul. 15, 2015
Class of Stock [Line Items]
Total common stock reserved for future issuance	16,386,314	18,577,117
Series A Warrant [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	7,802,241	7,802,241
Series B Warrant [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	2,574,727	2,574,727
Equity Incentive Plans [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	1,117,796	51,514
Awards outstanding	4,146,928	326,023
2015 Employee Stock Purchase Plan [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance	17,500	17,500	17,500
Warrants to Purchase Common Stock [Member]
Class of Stock [Line Items]
Outstanding warrants	727,122	1,839
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Total common stock reserved for future issuance		7,803,273

Preferred Stock - Additional in

Preferred Stock - Additional information (Detail) - USD ($) $ / shares in Units, $ in Millions	Nov. 13, 2019	Dec. 31, 2020	Jan. 29, 2020	Dec. 31, 2019
Series A Convertible Preferred Stock [Member]
Preferred stock, shares authorized		10,000,000		10,000,000
Preferred stock, par value		$ 0.001		$ 0.001
Preferred stock, shares issued		0		103,000
Preferred stock, shares outstanding		0		103,000
Stock issued, shares	103,000
Preferred Stock converted into of the Company's common stock			7,803,273
Convertible Preferred Stock And Warrants And Underwritten Follow On Offering [Member]
Share issued price per share	$ 1,000	$ 1,000
Proceeds from issuance of convertible preferred stock and warrants net of issuance cost	$ 97.7
Series A Preferred Stock [Member]
Share issued price per share		1,000
Preferred stock conversion price per share		$ 13.20

Warrant Liabilities - Additiona

Warrant Liabilities - Additional information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Jan. 31, 2020	Nov. 30, 2019
Class Of Warrant Or Right [Line Items]
Fair value of warrants estimate		$ 45,900		$ 45,900	$ 62,100	$ 25,000
Changes in fair value	$ 16,200	$ 20,900	$ 16,240	$ 20,926
Series A Warrants [Member]
Class Of Warrant Or Right [Line Items]
Number of securitites called by warrants and rights			7,802,241
Excercise price of warrants or rights			$ 13.20
Warrant expiration period after specified event			5 years
Series B Warrants [Member]
Class Of Warrant Or Right [Line Items]
Number of securitites called by warrants and rights			2,574,727
Excercise price of warrants or rights			$ 13.20
Warrant expiration period after specified event			75 days
Exercise of Future Warrants Value			$ 34,000

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-based Compensation Expense for Employees and Non-employees (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 9,470	$ 5,695	$ 6,796
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	4,316	3,873	4,499
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 5,154	$ 1,822	$ 2,297

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Estimated Grant-date Fair Values of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 3 months 18 days	5 years 3 months 18 days	5 years 3 months 18 days
Expected volatility, Minimum	87.00%	89.00%	88.00%
Expected volatility, Maximum	90.00%	94.00%	91.00%
Risk-free interest rate, Minimum	0.30%	1.60%	2.60%
Risk-free interest rate, Maximum	1.20%	2.60%	3.10%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	7 years	6 years 10 months 24 days	7 years

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Feb. 03, 2021	Jan. 21, 2020	Jul. 15, 2015	Aug. 31, 2020	May 31, 2020	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance							16,386,314	18,577,117
Stock option grants description							The aggregate number of shares issued over the term of the 2015 Employee Stock Purchase Plan will not exceed 85,000 shares of common stock. The ESPP will not become effective until such time as the Compensation Committee determines in the future, and as of December 31, 2020, the initial offering periods had not commenced.
Total stock-based compensation							$ 9,470	$ 5,695	$ 6,796
2018 Equity Inducement Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance							537,500			37,500
Common shares authorized to be issued during the period						500,000
Stock option life in years							10 years
2018 Equity Inducement Plan [Member] | Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares authorized to be issued during the period	500,000
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance							4,678,597
Common shares authorized to be issued during the period		4,312,500
Stock option life in years							10 years
Percentage threshold of outstanding shares increased annually under the plan							4.00%
Stock option grants description							the number of shares equal to 4% of the total outstanding shares of the Company’s common stock as of the immediately preceding December 31.
2008 Plan, 2015 Plan and 2018 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares authorized to be issued during the period							4,887,187
Weighted-average grant date fair values of options granted							$ 8.81	$ 50.40	$ 70
Issuance of common stock for exercise of stock options, Shares							0
Aggregate intrinsic value of options exercised								$ 100	$ 200
Total grant date fair value of options vested							$ 5,200	$ 6,000	$ 5,900
Total unrecognized stock-based compensation related to unvested stock options							$ 23,100
Weighted-average period							3 years 1 month 6 days
2008 Plan, 2015 Plan and 2018 Plan [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Executives and employees stock options granted				1,107,250
Total stock-based compensation							$ 900
Total unrecognized stock-based compensation related to unvested stock options							8,900
2008 Plan, 2015 Plan and 2018 Plan [Member] | Share Based Compensation Arrangement Modification [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrant expiration period after specified event					75 days
Compensation costs relating to vesting acceleration and modification							$ 2,200
2015 Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common stock shares reserved for issuance			17,500				17,500	17,500
Common shares authorized to be issued during the period			85,000
Number of common stock issued							0

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - 2008 Plan, 2015 Plan and 2018 Plan [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance	51,514
Shares Available for Grant, Awards authorized	4,887,187
Shares Available for Grant, Granted	(4,074,939)
Shares Available for Grant, forfeited/cancelled	254,034
Shares Available for Grant, Ending balance	1,117,796	51,514
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	326,023
Number of Shares Outstanding, Options granted	4,074,939
Number of Shares Outstanding, Options forfeited/cancelled	(254,034)
Number of Shares Outstanding, Ending balance	4,146,928	326,023
Number of Shares Outstanding, Exercisable	466,879
Number of Shares Outstanding, Vested and expected to vest	3,594,690
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance	$ 102.56
Weighted-Average Exercise Price Per Share, Options granted	12.88
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled	20.73
Weighted-Average Exercise Price Per Share, Ending balance	19.45	$ 102.56
Weighted-Average Exercise Price Per Share, Exercisable	65.48
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 20.57
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	8 years 10 months 2 days	7 years 5 months 12 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	3 years 10 months 24 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 8 months 15 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 12,227	$ 11
Aggregate Intrinsic Value of Outstanding Options, Exercisable	555
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 10,084

Income Taxes - Schedule of Geog

Income Taxes - Schedule of Geographical Breakdown of Loss Before Provision for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States	$ (81,670)	$ (89,459)	$ (54,395)
International	910	1,024	758
Loss before provision for (benefit from) income taxes, net	$ (80,760)	$ (88,435)	$ (53,637)

Income Taxes - Components of Pr

Income Taxes - Components of Provision for (Benefit from) Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current tax provision (benefit):
Foreign	$ 59	$ 85	$ (180)
Total current tax provision (benefit)	59	85	(180)
Deferred tax provision (benefit):
Foreign	83	(245)	(122)
Total deferred tax provision (benefit)	83	(245)	(122)
Total provision for (benefit from) income taxes	$ 142	$ (160)	$ (302)

Income Taxes - Reconciliation B

Income Taxes - Reconciliation Between Income Taxes Computed at the Federal Statutory Income Tax Rate and the Provision for Income Taxes (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	21.00%	21.00%
Change in valuation allowance	(23.60%)	12.40%	(22.20%)
Federal tax credit	5.80%	1.50%	2.40%
Warrant issuance and remeasurement	(4.20%)	(5.30%)
Effect of ownership change on deferred tax assets	1.30%	(29.00%)
State income tax benefit, net of federal benefit	0.20%	0.20%	0.30%
Other permanent items	(0.70%)	(0.60%)	(1.00%)
Total provision for (benefit from) income taxes%	(0.20%)	0.20%	0.50%

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 14,011	$ 2,649
Stock based compensation	6,459	4,663
59 (e) expenditures and amortization	6,222	5,902
Federal R&D and orphan drug credits	5,107	486
License fee	3,224	2,538
Other	1,024	967
Gross deferred tax assets	36,047	17,205
Valuation allowance	(35,513)	(16,441)
Total deferred tax assets	534	764
Deferred tax liabilities:
Lease Asset	114	244
Other	73	90
Total deferred tax liabilities	187	334
Total net deferred tax assets	$ 347	$ 430

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Tax Contingency [Line Items]
Deferred tax assets, valuation allowance	$ 35,200	$ 16,400
Deferred tax assets, valuation allowance, change in amount	18,800	10,900
Operating loss carry forwards	54,800
Tax credit carry forwards	$ 4,600
Tax credit carry forwards, year begin to expiration	2039
Unrecognized tax benefits, accrued interest and penalties	$ 0	$ 0
Earliest Tax Year [Member]
Income Tax Contingency [Line Items]
Open Tax Year	2017
Latest Tax Year [Member]
Income Tax Contingency [Line Items]
Open Tax Year	2019
Domestic Tax Authority [Member]
Income Tax Contingency [Line Items]
Operating loss carry forwards	$ 52,400
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Operating loss carry forwards, expiry year start	2022
Operating loss carry forwards, expiry year end	2040
Foreign Tax Authority [Member]
Income Tax Contingency [Line Items]
Tax credit carry forwards	$ 500
Tax credit carry forwards, year begin to expiration	2038

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Beginning balance	$ 314	$ 264	$ 43
Increases based on tax positions related to prior years			109
Decreases based on tax positions related to prior years	(54)	(103)
Decreases due to ownership change	(207)
Increases based on tax positions in current year	568	153	112
Ending balance	$ 621	$ 314	$ 264

Selected Quarterly Financial _3

Selected Quarterly Financial Data - Summary of Selected Unaudited Consolidated Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 200	$ 100							$ 300
Operating expenses	18,113	14,544	$ 16,449	$ 16,135	$ 25,019	$ 13,264	$ 15,207	$ 13,502	65,241	$ 66,992	$ 55,417
Net loss	$ (18,023)	$ (14,505)	$ (16,462)	$ (31,912)	$ (47,462)	$ (12,903)	$ (14,878)	$ (13,032)	$ (80,902)	$ (88,275)	$ (53,335)
Basic and diluted net loss per share	$ (1.63)	$ (1.39)	$ (1.58)	$ (3.14)	$ (7.88)	$ (6.91)	$ (7.97)	$ (7)	$ (7.70)	$ (30.30)	$ (30.16)

Selected Quarterly Financial _4

Selected Quarterly Financial Data - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Changes in fair value of warrant liabilities	$ 16,200	$ 20,900	$ 16,240	$ 20,926
Exepense included Research and development cost	$ 1,500	$ 10,500	$ 1,485	$ 10,485