Allakos (ALLK) 10-K 25 Feb 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 20, 2020	Jun. 28, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	ALLK
Entity Registrant Name	Allakos Inc.
Entity Central Index Key	0001564824
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 959.2
Entity Common Stock, Shares Outstanding		48,708,305
Entity File Number	001-38582
Entity Tax Identification Number	45-4798831
Entity Address, Address Line One	975 Island Drive
Entity Address, Address Line Two	Suite 201
Entity Address, City or Town	Redwood City
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94065
City Area Code	650
Local Phone Number	597-5002
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, par value $0.001
Security Exchange Name	NASDAQ
Document Annual Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	DE
Documents Incorporated by Reference	Portions of the Registrant’s Definitive Proxy Statement relating to the registrant’s 2020 Annual Meeting of Shareholders are incorporated by reference into Part III of this Annual Report on Form 10-K where indicated. Such Definitive Proxy Statement will be filed with the Securities and Exchange Commission within 120 days after the end of the registrant’s 2019 fiscal year ended December 31, 2019.

BALANCE SHEETS

BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 38,367	$ 33,660
Investments in marketable securities	457,534	145,246
Prepaid expenses and other current assets	3,969	2,703
Total current assets	499,870	181,609
Property and equipment, net	8,410	8,848
Operating lease right-of-use assets	5,775	0
Other long-term assets	2,839	802
Total assets	516,894	191,259
Current liabilities:
Accounts payable	5,963	2,092
Accrued expenses and other current liabilities	7,098	3,164
Total current liabilities	13,061	5,256
Other long-term liabilities	8,112	2,009
Total liabilities	21,173	7,265
Commitments and contingencies (Note 6)	0	0
Stockholders' equity:
Preferred stock, $0.001 par value per share; 20,000 shares authorized as of December 31, 2019 and 2018; no shares issued and outstanding as of December 31, 2019 and 2018	0	0
Common stock, $0.001 par value per share; 200,000 shares authorized as of December 31, 2019 and 2018; 48,668 and 42,117 shares issued and outstanding as of December 31, 2019 and 2018, respectively	48	42
Additional paid-in capital	685,020	288,079
Accumulated other comprehensive gain (loss)	137	(15)
Accumulated deficit	(189,484)	(104,112)
Total stockholders’ equity	495,721	183,994
Total liabilities and stockholders’ equity	$ 516,894	$ 191,259

BALANCE SHEETS (Parenthetical)

BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018	Jul. 23, 2018
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares authorized	20,000,000	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000	200,000,000
Common stock, shares issued	48,667,809	42,117,000
Common stock, shares outstanding	48,667,809	42,117,000

STATEMENTS OF OPERATIONS AND CO

STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses
Research and development	$ 61,858	$ 33,287	$ 18,506
General and administrative	29,560	12,434	3,748
Total operating expenses	91,418	45,721	22,254
Loss from operations	(91,418)	(45,721)	(22,254)
Interest income (expense), net	6,201	2,375	(1,302)
Other expense, net	(155)	(192)	(287)
Loss before benefit from income taxes	(85,372)	(43,538)	(23,843)
Provision for (benefit from) income taxes	0	0	(291)
Net loss	(85,372)	(43,538)	(23,552)
Unrealized gain (loss) on marketable securities, net of tax	152	(15)	0
Comprehensive loss	$ (85,220)	$ (43,553)	$ (23,552)
Net loss per common share:
Basic and diluted	$ (1.89)	$ (2.20)	$ (14.54)
Weighted-average number of common shares outstanding:
Basic and diluted	45,191	19,833	1,620

STATEMENTS OF CONVERTIBLE PREFE

STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Total	Series B Convertible Preferred Stock	Preferred Stock	Preferred StockSeries B Convertible Preferred Stock	Common Stock	Common StockSeries B Convertible Preferred Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Other Comprehensive Gain (Loss)Series B Convertible Preferred Stock	Accumulated Deficit
Balance, Shares at Dec. 31, 2016					1,605
Balance at Dec. 31, 2016	$ (36,436)				$ 2		$ 584	$ 0		$ (37,022)
Temporary Equity, Balance, Shares at Dec. 31, 2016			20,866
Temporary Equity, Balance at Dec. 31, 2016			$ 42,996
Issuance of Series B convertible preferred stock for cash, net of issuance costs of $168		$ 0		$ 92,331		$ 0			$ 0
Issuance of Series B convertible preferred stock for cash, net of issuance costs of $168, Shares				9,334
Issuance of Series B convertible preferred stock upon conversion of convertible promissory notes		$ 0		$ 7,642		$ 0			$ 0
Issuance of Series B convertible preferred stock upon conversion of convertible promissory notes, Shares				771
Stock-based compensation expense	402		0		0		402	0		0
Repurchase of unvested restricted common stock	0		0		$ 0		0	0		0
Repurchase of unvested restricted common stock, Shares					(34)
Issuance of common stock upon exercise of stock options	228		0		$ 1		227	0		0
Issuance of common stock upon exercise of stock options, Shares					543
Vesting of restricted common stock	28		0		$ 0		28	0		0
Recognition of beneficial conversion feature related to convertible promissory notes payable to related parties, net of $966 tax benefit	1,867		0		0		1,867	0		0
Reclassification of beneficial conversion feature related to convertible promissory notes payable to related parties, net of $675 tax expense	(1,305)		0		0		(1,305)	0		0
Unrealized gain (loss) on marketable securities, net of tax	0
Net loss	(23,552)		$ 0		$ 0		0	0		(23,552)
Balance, Shares at Dec. 31, 2017					2,114
Balance at Dec. 31, 2017	(58,768)				$ 3		1,803	0		(60,574)
Temporary Equity, Balance, Shares at Dec. 31, 2017			30,971
Temporary Equity, Balance at Dec. 31, 2017			$ 142,969
Proceeds from repayment of recourse promissory note	50		0		0		50	0		0
Conversion of preferred stock upon initial public offering	142,969		$ (142,969)		$ 30		142,939	0		0
Conversion of preferred stock upon initial public offering, Shares			(30,971)		30,972
Issuance of common stock upon follow-on offering/Initial public offering, net of offering costs	138,357		$ 0		$ 8		138,349	0		0
Issuance of common stock upon follow-on offering/Initial public offering, net of offering costs, Shares					8,453
Stock-based compensation expense	4,570		0		$ 0		4,570	0		0
Issuance of common stock upon exercise of stock options	345		0		$ 1		344	0		0
Issuance of common stock upon exercise of stock options, Shares					531
Issuance of common stock upon exercise of warrants	0		0		$ 0		0	0		0
Issuance of common stock upon exercise of warrants, Shares					47
Vesting of restricted common stock	24		0		$ 0		24	0		0
Unrealized gain (loss) on marketable securities, net of tax	(15)		0		0		0	(15)		0
Net loss	(43,538)		$ 0		$ 0		0	0		(43,538)
Balance, Shares at Dec. 31, 2018					42,117
Balance at Dec. 31, 2018	183,994				$ 42		288,079	(15)		(104,112)
Temporary Equity, Balance, Shares at Dec. 31, 2018			0
Temporary Equity, Balance at Dec. 31, 2018			$ 0
Issuance of common stock upon follow-on offering/Initial public offering, net of offering costs	377,525		0		$ 5		377,520	0		0
Issuance of common stock upon follow-on offering/Initial public offering, net of offering costs, Shares					5,227
Stock-based compensation expense	15,764		0		$ 0		15,764	0		0
Issuance of common stock upon exercise of stock options	2,448		0		$ 1		2,447	0		0
Issuance of common stock upon exercise of stock options, Shares					1,250
Issuance of common stock upon 2018 ESPP purchase	1,190		0		$ 0		1,190	0		0
Issuance of common stock upon 2018 ESPP purchase, Shares					74
Vesting of restricted common stock	20		0		$ 0		20	0		0
Unrealized gain (loss) on marketable securities, net of tax	152		0		0		0	152		0
Net loss	(85,372)		$ 0		$ 0		0	0		(85,372)
Balance, Shares at Dec. 31, 2019					48,668
Balance at Dec. 31, 2019	$ 495,721				$ 48		$ 685,020	$ 137		$ (189,484)
Temporary Equity, Balance, Shares at Dec. 31, 2019			0
Temporary Equity, Balance at Dec. 31, 2019			$ 0

STATEMENTS OF CONVERTIBLE PRE_2

STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Recognition of beneficial conversion feature related to convertible promissory notes payable to related parties, tax benefit			$ 966
Reclassification of beneficial conversion feature related to convertible promissory notes payable to related parties, tax expense			675
Common Stock
Stock issuance costs	$ 24,975	$ 3,466
Series B Convertible Preferred Stock
Stock issuance costs			$ 168

STATEMENTS OF CASH FLOWS

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (85,372)	$ (43,538)	$ (23,552)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	15,764	4,570	402
Net amortization of premiums and discounts on marketable securities	(2,664)	(1,310)	0
Amortization of beneficial conversion feature related to convertible promissory notes payable to related parties	0	0	853
Benefit from deferred income taxes	0	0	(291)
Depreciation and amortization	1,508	242	241
Noncash lease expense	275	0	0
Noncash interest related to convertible promissory notes payable to related parties	0	0	228
Loss on extinguishment of debt facility	0	0	159
Noncash interest related to debt facility	0	0	101
Accretion of tenant improvement allowance	0	(82)	0
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	463	(1,489)	(637)
Other long-term assets	(564)	313	(150)
Accounts payable	3,571	76	510
Accrued expenses and other current liabilities	4,077	2,063	(432)
Other long-term liabilities	(70)	705	0
Net cash used in operating activities	(63,012)	(38,450)	(22,568)
Cash flows from investing activities
Purchases of marketable securities	(541,701)	(236,601)	0
Proceeds from maturities of marketable securities	230,500	92,500	0
Purchases of property and equipment	(770)	(6,946)	(264)
Net cash used in investing activities	(311,971)	(151,047)	(264)
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	377,525	138,357	0
Proceeds from issuance of convertible preferred stock, net of issuance costs	0	0	92,331
Proceeds from issuance of convertible promissory notes, net of issuance costs	0	0	7,414
Repayment of debt facility	0	0	(5,250)
Proceeds from exercise of stock options, net of repurchases	2,448	345	228
Proceeds from issuance of common stock under 2018 ESPP	1,190	0	0
Payments for deferred financing costs	0	0	(100)
Proceeds from repayment of recourse promissory note	0	50	0
Net cash provided by financing activities	381,163	138,752	94,623
Net increase (decrease) in cash, cash equivalents and restricted cash	6,180	(50,745)	71,791
Cash, cash equivalents and restricted cash, beginning of period	34,462	85,207	13,416
Cash, cash equivalents and restricted cash, end of period	40,642	34,462	85,207
Supplemental disclosures
Cash paid for interest	0	0	228
Noncash investing and financing items:
Right-of-use assets obtained in exchange for lease obligations	6,050	0	0
Conversion of convertible promissory notes payable to related parties	0	0	7,642
Recognition of beneficial conversion feature related to convertible promissory notes to related parties, net of benefit for income taxes	0	0	1,867
Lessor funded lease incentives included in property and equipment	0	1,386	0
Reclassification of beneficial conversion feature related to convertible promissory notes payable to related parties, net of tax expense	0	0	1,305
Property and equipment purchased in accounts payable	0	313	89
Deferred initial public offering costs in accounts payable	0	0	63
Vesting of restricted common stock subject to repurchase	$ 20	$ 24	$ 28

Organization and Business

Organization and Business	12 Months Ended
	Dec. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization and Business	1. Organization and Business Allakos Inc. (“Allakos” or the “Company”) was incorporated in the state of Delaware in March 2012. Allakos is a clinical stage biopharmaceutical company focused on the development of antolimab (AK002) for the treatment of eosinophil and mast cell related diseases. The Company’s primary activities to date have included establishing its facilities, recruiting personnel, conducting research and development of its product candidates and raising capital. The Company’s operations are located in Redwood City, California. Liquidity Matters Since inception, the Company has incurred net losses and negative cash flows from operations. During the year ended December 31, 2019, the Company incurred a net loss of $85.4 million and used $63.0 million of cash in operations. As of December 31, 2019, the Company had an accumulated deficit of $189.5 million and does not expect to experience positive cash flows from operating activities in the foreseeable future. The Company has financed its operations to date primarily through the sale of common stock and issuance of convertible preferred stock. Management expects to incur additional operating losses in the future as the Company continues to further develop, seek regulatory approval for and, if approved, commence commercialization of its product candidates. The Company had $495.9 million of cash, cash equivalents and marketable securities at December 31, 2019. Management believes that this amount is sufficient to fund the Company’s operations for at least the next 12 months from the issuance date of these financial statements. July 2018 Initial Public Offering and Related Transactions On July 23, 2018, the Company completed an initial public offering (“IPO”), selling 8,203,332 shares of common stock at an offering price of $18.00 per share (the “July 2018 IPO”). Proceeds from the IPO, net of underwriting discounts and commissions, were $137.3 million. Concurrently with the July 2018 IPO, the Company completed a private placement of 250,000 shares of common stock at the IPO offering price of $18.00 per share to an existing stockholder. Proceeds from this private placement were $4.5 million. In connection with the completion of the July 2018 IPO on July 23, 2018, all then outstanding shares of convertible preferred stock were converted into 30,971,627 shares of common stock. Upon the completion of the July 2018 IPO, the Company’s certificate of incorporation was amended and restated. Under the amended and restated certificate of incorporation, the Company’s authorized capital stock consists of 200,000,000 shares of common stock with a par value $0.001 per share and 20,000,000 shares of convertible preferred stock with a par value $0.001 per share. August 2019 Follow-On Offering On August 9, 2019, the Company closed an underwritten public offering (the “August 2019 Offering”) under its shelf registration statement on Form S-3 at a public offering price of $77.00 per share. The Company received aggregate net proceeds of $377.5 million, after deducting the underwriting discounts and commissions and offering expenses. Reverse Stock Split On July 6, 2018, the Company amended its certificate of incorporation to effect a 1-for-1.25

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The financial statements have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”). The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements and accompanying notes. Use of Estimates Management uses significant judgment when making estimates related to common stock valuation and related stock-based compensation expense, accrued expenses related to clinical trials and deferred tax valuation allowances. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results could differ from those estimates under different assumptions or conditions, and those differences could be material to the financial position and results of operations. Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject the Company to credit risk principally consist of cash, cash equivalents and marketable securities. These financial instruments are held in accounts at a single financial institution that management believes possesses high credit quality. Amounts on deposit with this financial institution have and will continue to exceed federally-insured limits. The Company has not experienced any losses on its cash deposits. Additionally, the Company’s investment policy limits its investments to certain types of securities issued by the United States government and its agencies. The Company is subject to a number of risks similar to that of other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future clinical trials, its reliance on third-parties to conduct its clinical trials, the need to obtain regulatory and marketing approvals for its product candidates, competitive developments, the need to successfully commercialize and gain market acceptance of the Company’s product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, protection of proprietary technology, the ability to make milestone, royalty or other payments due under licensing agreements, and the need to secure and maintain adequate manufacturing arrangements with third-parties. If the Company does not successfully commercialize or partner its product candidates, it will be unable to generate product revenue or achieve profitability. Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the Company’s balance sheets and which, in aggregate, represent the amounts reported in the statements of cash flows (in thousands): December 31, 2019 2018 2017 Cash and cash equivalents $ 38,367 $ 33,660 $ 85,207 Restricted cash 2,275 802 — Total $ 40,642 $ 34,462 $ 85,207 Restricted cash at December 31, 2019 represents $2.3 million of security deposits for the lease of the Company’s facilities in Redwood City, California and San Carlos, California. Both security deposits are in the form of letters of credit secured by restricted cash. Restricted cash amounts are included within other long-term assets on the Company’s balance sheets. Marketable Securities The Company invests in marketable securities, primarily securities issued by the United States government and its agencies. The Company’s marketable securities are considered available-for-sale and are classified as current assets even when the stated maturities of the underlying securities exceed one year from the date of the current balance sheet being reported. This classification reflects management’s ability and intent to utilize proceeds from the sale of such investments to fund ongoing operations. Unrealized gains and losses are excluded from earnings and are reported as a component of accumulated comprehensive income (loss). The cost of securities sold is determined using the specific-identification method. Interest earned and adjustments for the amortization of premiums and discounts on investments are included in interest income, net, on the statements of operations and comprehensive loss. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on investments in marketable securities are included in other expense, net, on the statements of operations and comprehensive loss. Fair Value Measurements The Company accounts for fair value of its financial instruments in accordance with Financial Accounting Standards Board (the “FASB”) Accounting Standards Codification (“ASC”) Topic No. 820, Fair Value Measurements The Company measures fair value based on a three-level hierarchy of inputs, of which the first two are considered observable and the last unobservable. Unobservable inputs reflect the Company’s own assumptions about current market conditions. The three-level hierarchy of inputs is as follows: Level 1 – Quoted prices in active markets for identical assets or liabilities. Level 2 – Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of assets or liabilities. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying amounts reflected in the Company’s balance sheets for cash and cash equivalents, prepaid expenses and other current assets, other long-term assets, accounts payable, and accrued expenses and other current liabilities approximate fair value, due to their short-term nature. The Company’s investments in marketable securities are measured at fair value in accordance with the levels above. Property and Equipment, Net Property and equipment are stated at cost, net of accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful lives of the assets. The useful lives of property and equipment are as follows: Laboratory equipment – 3 to 5 years Leasehold improvements – Shorter of remaining lease term or estimated life of the assets Upon retirement or sale, the cost of disposed assets and their related accumulated depreciation are removed from the balance sheet. Any resulting gains or losses on dispositions of property and equipment are included as a component of other income (expense), net, within the Company’s statements of operations and comprehensive loss. Repair and maintenance costs that do not significantly add value to the property and equipment, or prolong its life, are charged to operating expense as incurred. Leases Effective January 1, 2019, the Company accounts for its leases in accordance with Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 842, “Leases” (“ASC 842”). Prior period amounts continue to be reported in accordance with the Company’s historic accounting under previous lease guidance. Additionally, the Company elected a number of optional practical expedients made available under the ASC 842 transition guidance. Such elections include (i) carrying forward the Company’s historical lease classifications, (ii) foregoing a re-evaluation of historical contracts to identify embedded leases, (iii) foregoing a re-assessment of initial direct costs related to leases that existed prior to adoption, (iv) combining lease and non-lease components, and (v) recognizing lease expense for all contracts with an initial term of 12 months or less within the statements of operations and comprehensive loss on a straight-line basis over the requisite lease term. The Company accounts for its leases by recording right-of-use assets and lease liabilities on the Balance Sheet. Right-of-use assets represent the Company’s right to use an underlying asset over the lease term and include any lease payments made prior to the lease commencement date and exclude lease incentives. Lease liabilities represent the present value of the total lease payments over the lease term, calculated using the Company’s incremental borrowing rate. In determining the Company’s incremental borrowing rate, consideration is given to the term of the lease and the Company’s credit risk. The Company’s recognizes options to extend or terminate a lease when it is reasonably certain that the Company will exercise any such options. Lease expense is recognized on a straight-line basis over the expected lease term. Accrued Contract Research and Development Expense Costs associated with research and development services performed on behalf of the Company by third-party CROs and CDMOs comprise a significant component of total research and development expense included on the statements of operations and comprehensive loss. Services performed by these CROs and CDMOs include various research and development activities supporting the Company’s preclinical studies, clinical trials and drug manufacturing activities, which are governed by executed service agreements. Underlying amounts included in these service agreements with CROs and CDMOs are expensed as incurred. The Company accrues for expenses related to services performed by Accrued contract research and development expense requires certain estimates by management surrounding the extent of unbilled services received and the extent and duration of remaining services still to be performed. Management’s estimates are based on the evaluation of data obtained from multiple internal and external sources including, but not limited to, clinical site activity logs, subject visit reports, and project management timelines. Results from these evaluations are reviewed by internal personnel from the Company’s clinical and technical operations departments. Research and Development Expense Research and development costs are expensed as incurred. Research and development costs include, among others, consulting fees, salaries, benefits, travel, stock-based compensation, laboratory supplies and other non-capital equipment utilized for in-house research, allocations of facilities and overhead costs, amounts owed under in-licensing agreements, and amounts paid to CROs and CDMOs that conduct research and development activities on the Company’s behalf. Goods or services incurred for research and development activities that have not yet been invoiced are recorded as liabilities within accrued expenses and other current liabilities on the Company’s balance sheets. Amounts recorded for unbilled services often represent estimates, which are typically based on contracted amounts for the proportion of work performed and determined through analysis with internal personnel and external service providers as to the progress or stage of completion of the associated services. Non-refundable a dvance payments for goods or services to be rendered as part of future research and development activities are capitalized on the Company’s balance sheets. Classification between current and long-term assets is based on an evaluation of when the goods will be delivered and/or services will be performed, with such amounts subsequently amortized to expense once realized. Segments Operating segments are defined as components of an entity about which separate discrete information is available for evaluation by the chief operating decision maker or decision-making group, in deciding how to allocate resources and in assessing performance. The Company’s chief operating decision maker, its Chief Executive Officer, views its operations and manages its business in one operating segment operating exclusively in the United States. Patent Costs The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the statements of operations and comprehensive loss Stock-Based Compensation The Company accounts for its stock-based compensation in accordance with FASB ASC Topic 718, Compensation—Stock Compensation For purposes of determining the estimated fair value of stock options granted to employees and nonemployees, the Company uses the Black-Scholes option pricing model. The Black-Scholes option pricing model requires the input of certain assumptions that involve judgment, for which changes can materially affect the resulting estimates of fair value. The assumptions used to determine the fair value of stock options granted were as follows: Expected volatility – As there is insufficient trading history for the Company’s common stock, the Company has based its computation of expected volatility on the historical volatility of a representative group of public companies with similar characteristics to the Company, including stage of product development and life science industry focus. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. Expected term – The Company determines the expected term in accordance with the “simplified method” described by SEC Staff Accounting Bulletin No. 107, , as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. Risk-free interest rate – The Company bases the risk-free interest rate on United States Treasury securities with terms consistent to the expected term of the stock option being valued. Expected dividends – The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The fair value of restricted stock units (“RSUs”) is determined using the quoted market price of the Company’s common stock on the date of grant. The Company uses historical data to estimate pre-vesting forfeitures and records stock-based compensation expense only for those awards expected to vest. To the extent that actual forfeitures differ from estimates, the difference is recorded as a cumulative adjustment in the period the estimate are revised. The Company expenses the fair value of its stock-based compensation awards to employees and nonemployees on a straight-line basis over the requisite service period, which is generally the vesting period. Income Taxes In December 2017, the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) was signed into law. The Tax Act, among other changes, lowered the Company’s federal tax rate from 34% to 21%. Based on provisions of the Tax Act, the Company remeasured its deferred tax assets and liabilities at December 31, 2017 to reflect the lower statutory tax rate, however, since the Company established a full valuation allowance to offset its deferred tax assets, there was no impact to the effective tax rate. The deferred tax remeasurement was provisional and represented our reasonable estimate within the meaning of Staff Accounting Board 118, which provided a measurement period that should not extend beyond one year from the Tax Act’s enactment date for companies to complete the accounting under ASC 740. As of December 31, 2018, the Company has completed its analysis of the income tax effects of the Tax Act. The results of this analysis have been reflected in the Company’s financial statements and related footnotes. Comprehensive Loss Comprehensive loss is defined as the change in stockholders’ equity (deficit) during a period from transactions and other events and circumstances from non-owner sources. The difference between net loss and comprehensive loss for the years ended December 31, 2019 and 2018 are a result of unrealized gains and losses on the Company’s investments in marketable securities included in current assets on the balance sheets. Net Loss per Share The Company calculates basic net loss per share by dividing the net loss attributable to common stockholders by the weighted-average shares of common stock outstanding during the period. The Company calculates diluted net loss per share after giving consideration to all potentially dilutive securities outstanding during the period using the treasury-stock and if-converted methods, except where the effect of including such securities would be anti-dilutive. Because the Company has reported net losses since inception, the effect from potentially dilutive securities would have been anti-dilutive and therefore has been excluded from the calculation of diluted net loss per share. Basic and diluted net loss per share was calculated as follows (in thousands, except per share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss $ (85,372 ) $ (43,538 ) $ (23,552 ) Denominator: Weighted-average shares of common stock outstanding, basic and diluted 45,191 19,833 1,620 Net loss per share, basic and diluted $ (1.89 ) $ (2.20 ) $ (14.54 ) The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect for the periods indicated (in thousands): Year Ended December 31, 2019 2018 2017 Series A convertible preferred stock — — 20,866 Series B convertible preferred stock — — 10,105 Options to purchase common stock 7,148 7,811 4,884 Unvested restricted stock units 542 — — Warrants to purchase common stock — — 48 Unvested restricted common stock — 47 104 Shares issuable under employee stock purchase plans 31 29 — Total 7,721 7,887 36,007 Foreign Currency Transactions The Company is party to multiple contract manufacturing and clinical research agreements for which services to be performed are denominated in foreign currencies other than the United States Dollar. The Company records gains and losses attributable to fluctuations in foreign currencies as a component of other income (expense), net, on the statements of operations and comprehensive loss. Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Codification (“ASC”) 842 which became effective for fiscal years beginning after December 15, 2018. ASC 842 requires an entity to recognize a right-of-use asset and lease liability for all leases with terms of more than 12 months. The recognition, measurement and presentation of expenses will depend on the lease’s classification as a finance or operating lease. ASC 842 also requires certain quantitative and qualitative disclosures about leasing arrangements. The Company adopted ASC 842 using a modified retrospective approach effective January 1, 2019, recording a right-of-use asset of $6.1 million and a long-term lease liability of $8.2 million. Adoption of ASC 842 did not result in a cumulative effect adjustment to accumulated deficit. See Note 6 for further disclosure. On January 1, 2019, the Company adopted Accounting Standards Update (“ASU”) No. 2018-07, Compensation-Stock Compensation Recently Issued Accounting Pronouncements Not Yet Adopted In December 2019, the FASB issued ASU No. 2019-12, Income Taxes In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company measures and reports certain financial instruments as assets and liabilities at fair value on a recurring basis. The Company’s financial assets measured at fair value on a recurring basis were as follows (in thousands): December 31, 2019 Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 35,935 $ — $ — $ 35,935 Total cash equivalents 35,935 — — 35,935 Marketable securities U.S. treasuries 457,534 — — 457,534 Total marketable securities 457,534 — — 457,534 Total cash equivalents and marketable securities $ 493,469 $ — $ — $ 493,469 December 31, 2018 Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 31,555 $ — $ — $ 31,555 Total cash equivalents 31,555 — — 31,555 Marketable securities U.S. treasuries 145,246 — — 145,246 Total marketable securities 145,246 — — 145,246 Total cash equivalents and marketable securities $ 176,801 $ — $ — $ 176,801 The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between levels during the years ended December 31, 2019 and 2018.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities All marketable securities were considered available-for-sale at December 31, 2019 and 2018. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type are summarized in the table below (in thousands): December 31, 2019 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Available-for-sale securities U.S. treasuries classified as investments $ 457,397 $ 161 $ (24 ) $ 457,534 Total $ 457,397 $ 161 $ (24 ) $ 457,534 December 31, 2018 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Available-for-sale securities U.S. treasuries classified as investments $ 145,261 $ — $ (15 ) $ 145,246 Total $ 145,261 $ — $ (15 ) $ 145,246 The amortized cost of available-for-sale securities is adjusted for amortization of premiums and accretion of discounts to maturity. As of December 31, 2019 and 2018, the aggregate fair value of securities held by the Company in an unrealized loss position for less than twelve months was $187.4 million and $132.2 million, respectively. All of these securities had remaining maturities of less than one year. The Company has the intent and ability to hold such securities until recovery and has determined that there has been no material change to their credit risk. As a result, the Company determined it did not hold any investments with any other-than-temporary impairment as of December 31, 2019 and 2018. There were no material realized gains or losses recognized on the sale or maturity of available-for-sale securities during the years ended December 31, 2019 or 2018, and as a result, there were no material reclassifications out of accumulated other comprehensive gain (loss) for the same periods.

Balance Sheet Components and Su

Balance Sheet Components and Supplemental Disclosures	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components and Supplemental Disclosures	5. Balance Sheet Components and Supplemental Disclosures Property and Equipment, Net Property and equipment, net, consisted of the following (in thousands): December 31, 2019 2018 Laboratory equipment $ 4,170 $ 3,272 Furniture and office equipment 1,695 1,666 Leasehold improvements 4,581 4,545 10,446 9,483 Less accumulated depreciation (2,036 ) (635 ) Property and equipment, net $ 8,410 $ 8,848 Depreciation and amortization expense for the years ended December 31, 2019, 2018 and 2017 was $1.5 million, $0.2 million and $0.2 million, respectively. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2019 2018 Accrued contract research and development expense $ 4,990 $ 1,866 Accrued compensation and benefits expense 1,608 1,041 Lease liability, current 410 — Lease incentive obligation, current — 123 Other current liabilities 90 134 Total $ 7,098 $ 3,164

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Lease Obligations The Company’s lease obligations primarily relate to leased office and laboratory space under noncancelable operating leases. In January 2018, the Company entered into a lease agreement for approximately 25,000 square feet of office and laboratory space in Redwood City, California (the “2018 Redwood City Lease”). The 2018 Redwood City Lease includes a contractual lease term commencing upon substantial completion and delivery of the premises, which occurred in November 2018. The Company subsequently terminated its previous lease agreement for office and laboratory space in San Carlos, California (the “2015 San Carlos Lease”). The base term of the 2018 Redwood City Lease is 10.75 years and contains a one-time option to extend the lease term for five years. This option to extend the lease term has not been included in the Company’s calculations under ASC 842 as the exercise of the option is highly uncertain and therefore deemed not probable The 2018 Redwood City Lease also included a $1.4 million tenant improvement allowance that has been applied to the total cost of tenant improvements made to the leased premises. Tenant improvement allowances received under the 2018 Redwood City Lease were recorded as leasehold improvements with an offsetting adjustment included in the Company’s calculation of its right-of-use asset under ASC 842. Leasehold improvements are depreciated over the term of the lease. In December 2019, the Company entered into an additional lease agreement for approximately 98,000 The 2018 Redwood City Lease and the 2019 San Carlos Lease required security deposits of $0.8 million and $1.5 million, respectively, which the Company satisfied by establishing letters of credit secured by restricted cash. Restricted cash related to the Company’s lease agreements are recorded in other long-term assets on the Company’s balance sheets. As described in Note 2, the Company adopted ASC 842 effective January 1, 2019. In accordance with ASC 842, the Company has performed an evaluation of its other contracts with vendors and has determined that, except for the leases described above, none of its other contracts contain a lease. Classification of the Company’s lease liabilities included on the Balance Sheet at December 31, 2019 was as follows (in thousands): Operating lease liabilities Current portion included in accrued expenses and other current liabilities $ 410 Non-current portion included in other long-term liabilities 8,112 Total operating lease liabilities $ 8,522 The components of lease costs, which are included in operating expenses in the Company’s statements of operations and comprehensive loss were as follows (in thousands): Year ended December 31, 2019 Operating lease cost $ 1,118 Variable cost 346 Total lease costs $ 1,464 In addition to the minimum future lease commitments presented below, the lease requires the Company to pay property taxes, insurance, maintenance and repair costs. Rent expense is recognized using the straight-line method over the respective terms. The Company records a deferred rent liability calculated as the difference between rent expense and cash rental payments. The current portion of the liability is included within accrued expenses and other current liabilities on the Company’s balance sheets. The remaining non-current portion is classified in other long-term liabilities. Future minimum lease payments required under operating leases included on the Company’s Balance Sheet at December 31, 2019 are as follows (in thousands): Fiscal Year Ending December 31, 2020 $ 1,233 2021 1,270 2022 1,308 2023 1,348 2024 1,388 Thereafter 6,916 Total minimum future lease payments (1) 13,463 Less: Present value adjustment 4,941 Operating lease liabilities $ 8,522 (1) Net rent expense was $1.1 million, $1.0 million and $0.5 million for the years ended December 31, 2019, 2018 and 2017, respectively. Operating lease liabilities are based on the net present value of the remaining lease payments over the remaining lease term. In determining the present value of lease payments, the Company used its incremental borrowing rate based on the information available at the lease commencement date. As of December 31, 2019, the remaining lease term is 9.6 years and the discount rate used to determine the operating lease liability was 10.0%. As of December 31, 2019, the Company is not party to any lease agreements containing material residual value guarantees or material restrictive covenants. Purchase Obligations The Company has entered into contractual agreements with various research and development organizations and suppliers in the normal course of its business. All contracts are terminable, with varying provisions regarding termination. If a contract were to be terminated, the Company would only be obligated for the products or services that the Company had received through the time of termination as well as any noncancelable minimum payments contractually agreed upon prior to the effective date of termination. In the case of terminating a clinical trial agreement with an investigational site conducting clinical activities on behalf of the Company, the Company would also be obligated to provide continued support for appropriate safety procedures through completion or termination of the associated study. As of December 31, 2019, the Company had $8.7 million of noncancelable purchase obligations under these agreements. In-Licensing Agreements The Company has entered into exclusive and non-exclusive, royalty bearing license agreements with third-parties for certain intellectual property. Under the terms of the license agreements, the Company is obligated to pay milestone payments upon the achievement of specified clinical, regulatory and commercial milestones. Actual amounts due under the license agreements will vary depending on factors including, but not limited to, the number of products developed and the Company’s ability to further develop and commercialize the licensed products. The Company is also subject to future royalty payments based on sales of the licensed products. In-licensing payments to third-parties for milestones are recognized as research and development expense in the period of achievement. The Company did not incur any milestone expense for the years ended December 31, 2019 and 2017. The Company recognized $0.3 million in milestone expense for the years ended December 31, 2018. Milestone payments are not creditable against royalties. As of December 31, 2019, the Company has not incurred any royalty liabilities related to its license agreements, as product sales have not yet commenced. Exclusive License Agreement with The Johns Hopkins University In December 2013, the Company entered into a license agreement with The Johns Hopkins University (“JHU”) for a worldwide exclusive license to develop, use, manufacture and commercialize covered product candidates including antolimab (AK002), which was amended in September 30, 2016. Under the terms of the agreement, the Company has made upfront and milestone payments of $0.3 million through December 31, 2019 and may be required to make aggregate additional milestone payments of up to $4.0 million. The Company also issued 88,887 shares of common stock as consideration under the JHU license agreement. In addition to milestone payments, the Company is also subject to single-digit royalties to JHU based on future net sales of each licensed therapeutic product candidate by the Company and its affiliates and sublicensees, with up to a low six-digit dollar minimum annual royalty payment. Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG In October 2013, the Company entered into a tripartite agreement with BioWa Inc. (“BioWa”), and Lonza Sales AG (“Lonza”), for the non-exclusive worldwide license to develop and commercialize product candidates including antolimab (AK002) that are manufactured using a technology jointly developed and owned by BioWa and Lonza. Under the terms of the agreement, the Company has made milestone payments of $0.4 million through December 31, 2019 and may be required to make aggregate additional milestone payments of up to $41.0 million. In addition to milestone payments, the Company is also subject to minimum annual commercial license fees of $40,000 per year to BioWa until such time as BioWa receives royalty payments, as well as low single-digit royalties to BioWa and to Lonza. Royalties are based on future net sales by the Company and its affiliates and sublicensees and vary dependent on Lonza’s participation as sole manufacturer for commercial production. Indemnification Agreements The Company has entered into indemnification agreements with certain directors and officers that require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. To date, no such matters have arisen and the Company does not believe that the outcome of any claims under indemnification arrangements will have a material adverse effect on its financial positions, results of operations or cash flows. Accordingly, the Company has not recorded a liability related to such indemnifications at December 31, 2019.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2019
Stockholders Equity Note [Abstract]
Stockholders' Equity	7. Stockholders’ Equity The Company’s amended and restated certificate of incorporation filed on July 23, 2018 authorizes the issuance of a total of 220,000,000 shares of stock. Of these shares, 200,000,000 are designated as common stock and 20,000,000 are designated as preferred stock. Common Stock There were 48,667,809 shares of common stock issued and outstanding at December 31, 2019. Common shares reserved for future issuance upon the exercise, issuance or conversion of the respective equity instruments are as follows (in thousands): December 31, 2019 2018 Exercise of common stock options outstanding 7,148 7,811 Shares reserved for issuance under equity incentive plans 3,762 2,785 Vesting of restricted stock units 542 — Shares reserved for issuance under employee stock purchase plans 848 500 Total 12,300 11,096 Common stockholders are entitled to dividends if and when declared by the Board of Directors subject to the prior rights of preferred stockholders. As of December 31, 2019, no dividends on common stock had been declared by the Board of Directors. Preferred Stock There were no shares of preferred stock issued and outstanding at December 31, 2019.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Total stock-based compensation expense recognized is as follows (in thousands): Year Ended December 31, 2019 2018 2017 Research and development $ 5,351 $ 1,792 $ 175 General and administrative 10,413 2,778 227 Total $ 15,764 $ 4,570 $ 402 No income tax benefits for stock-based compensation expense have been recognized for the years ended December 31, 2019, 2018 and 2017 as a result of the Company’s full valuation allowance applied to net deferred tax assets and net operating loss carryforwards. Equity Incentive Plans In July 2018, the Board of Directors adopted the 2018 Equity Incentive Plan (the “2018 Plan”). The 2018 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock units (“RSUs”), stock appreciation rights and other stock-based awards. The Company initially reserved 4,000,000 shares of common stock for issuance under the 2018 Plan. The number of shares of common stock that may be issued under the 2018 Plan will automatically increase on each January 1, beginning with the fiscal year ending December 31, 2019, equal to the least of (i) 5,000,000 shares, (ii) 5% of the outstanding shares of common stock as of the last day of the preceding fiscal year and (iii) such other amount as the Board of Directors may determine. Stock options and RSUs granted under the 2018 Plan generally vest over four years and expire no more than 10 years from the date of grant. Following the IPO and upon the effectiveness of the 2018 Plan, the Company’s 2012 Equity Incentive Plan, as amended, (the “2012 Plan”), terminated and no further awards will be granted thereunder. All outstanding awards under the 2012 Plan will continue to be governed by their existing terms. Any shares subject to awards granted under the 2012 Plan that, on or after the termination of the 2012 Plan, expire or terminate and shares previously issued pursuant to awards granted under the 2012 Plan that, on or after the termination of the 2012 Plan, are forfeited or repurchased by the Company will be transferred into the 2018 Plan. As of December 31, 2019, the maximum number of shares that may be added to the 2018 Plan pursuant to the preceding clause is 5,276,050 shares. Prior to its termination, the 2012 Plan provided for the grant of stock options, stock appreciation rights, restricted stock and restricted stock units to employees, directors and consultants. Stock options granted under the 2012 Plan generally vest over four years and expire no more than 10 years from the date of grant. Stock Options Stock option activity under the 2018 Plan and the 2012 Plan is summarized as follows (in thousands, except per share data): Weighted- Weighted- Average Average Aggregate Options Exercise Remaining Intrinsic Outstanding Price Years Value Balance at December 31, 2018 7,811 $ 8.60 8.2 $ 342,292 Granted 645 $ 46.75 Exercised (1,251 ) $ 2.11 Forfeited (57 ) $ 35.60 Balance at December 31, 2019 7,148 $ 12.96 8.0 $ 589,114 Options exercisable 3,982 $ 5.10 7.6 $ 359,399 Options vested and expected to vest 7,119 $ 12.91 8.0 $ 587,065 The following weighted-average assumptions were used to calculate the fair value of stock options granted during the periods indicated: Year Ended December 31, 2019 2018 2017 Risk-free interest rate 1.91 % 2.79 % 1.83 % Expected volatility 67.22 % 73.47 % 77.59 % Expected dividend yield — — — Expected term (in years) 6.01 6.01 6.08 The weighted-average fair value of options granted during the years ended December 31, 2019, 2018 and 2017 was $28.66, $11.05 and $0.54 per share, respectively. The aggregate fair value of stock options that vested during the years ended December 31, 2019, 2018 and 2017 was $12.6 million, $1.8 million and $0.2 million, respectively. The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock. Following the IPO, the aggregate intrinsic value represents the value of the Company’s closing stock price on the last trading day of the year in excess of the weighted-average exercise price multiplied by the number of options outstanding or exercisable. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2019, 2018 and 2017 was $55.8 million, $ 2.0 million and $0.1 million, respectively. During the years ended December 31, 2019 and 2018, the Company did not grant any stock options with performance-based or market-based vesting conditions. As of December 31, 2019, total unrecognized stock-based compensation expense relating to unvested stock options was $41.6 million. This amount is expected to be recognized over a weighted-average period of 2.9 years. Restricted Stock Awards The 2012 Plan allows for the issuance of restricted common stock and early exercise of unvested stock options in exchange for restricted common stock. Unvested shares of restricted common stock are subject to repurchase by the Company at the original issuance price in the event of the employee’s termination, either voluntarily or involuntarily. Consideration received for unvested stock-based awards is initially recorded as a liability and subsequently reclassified into stockholders’ deficit as the related awards vest. A summary of the restricted common stock activity during the year ended December 31, 2019 is as follows (in thousands, except per share data): Weighted- Average Number Grant Date of Shares Fair Value Balance at December 31, 2018 47 $ 0.43 Vested (47 ) $ 0.43 Balance at December 31, 2019 — $ — The fair value of restricted common stock that vested during the years ended December 31, 2019 and 2018 was $20,000 and $24,000, respectively. There were no unvested shares of restricted common stock at December 31, 2019. Restricted Stock Units RSU activity under the 2018 Plan is summarized as follows (in thousands, except per share data): Weighted- Average Number Grant Date of Shares Fair Value Balance at December 31, 2018 — $ — Granted 542 $ 93.67 Balance at December 31, 2019 542 $ 93.67 The weighted-average fair value of RSUs granted during the year ended December 31, 2019 was $93.67. No RSUs were granted during the years ended December 31, 2018 and 2017. As of December 31, 2019, total unrecognized stock-based compensation expense relating to unvested RSUs was $48.7 million and the weighted-average remaining vesting period was 3.9 years. The aggregate intrinsic value of RSUs is calculated as the closing price per share of the Company’s common stock on the last trading day of the fiscal period, multiplied by the number of RSUs expected to vest as of December 31, 2019. As of December 31, 2019, the aggregate intrinsic value of RSUs was $51.7 million Employee Stock Purchase Plan In July 2018, the Company’s Board of Directors and stockholders approved the 2018 Employee Stock Purchase Plan (the “2018 ESPP”). There were 500,000 shares of common stock initially reserved for issuance under the 2018 ESPP. The number of shares of common stock that may be issued under the 2018 ESPP shall automatically increase on each January 1, beginning with the fiscal year ending December 31, 2019, equal to the least of (i) 1,000,000 shares, (ii) 1% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year and (iii) such other amount determined by the 2018 ESPP administrator. Under the 2018 ESPP, employees may purchase shares of the Company’s common stock at a price per share equal to 85% of the lower of the fair market value of the common stock on the first trading day of the offering period or on the exercise date. The 2018 ESPP provides for consecutive, overlapping 24-month offering periods, each of which will include purchase periods. The first offering period commenced on July 18, 2018 and will end on the first trading day on or before August 15, 2020. The second and third offering periods commenced on February 15, 2019 and August 16, 2019, respectively. During the year ended December 31, 2019 and 2018, stock-based compensation related to the 2018 ESPP was $0.7 million and $0.2 million, respectively. The following weighted-average assumptions were used to calculate the fair value of ESPP shares during the periods indicated: Year Ended December 31, 2019 2018 Risk-free interest rate 2.37 % 2.42 % Expected volatility 64.26 % 65.92 % Expected dividend yield — — Expected term (in years) 1.22 1.24 As of December 31, 2019, total unrecognized compensation expense relating to shares to be purchased under ESPP was $0.6 million over a weighted-average period of 1.0 years.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes The Company’s deferred income tax assets include operating losses and tax credit carryforwards, as well as certain temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Total deferred income tax assets, net of valuation allowance, at December 31, 2019 and 2018 were as follows (in thousands): December 31, 2019 2018 Deferred tax assets Net operating loss carryforwards $ 47,003 $ 24,091 Research and development credits 8,644 3,995 Accruals and reserves 1,199 260 Stock-based compensation 2,870 611 Lease liability 1,798 — Gross deferred tax assets 61,514 28,957 Less: valuation allowance (59,901 ) (28,892 ) Deferred tax assets, net of valuation allowance 1,613 65 Deferred tax liabilities Fixed and intangible assets 395 65 Right-of-use asset 1,218 — Gross deferred tax liabilities 1,613 65 Net deferred tax assets $ — $ — Management has evaluated the positive and negative evidence surrounding the realizability of its deferred tax assets and has determined that it is more likely than not that the Company will not recognize the benefits of its federal and state deferred tax assets, and as a result, a valuation allowance of $59.9 million and $28.9 million has been established at December 31, 2019 and 2018, respectively. The change in the valuation allowance was $31.0 million and $10.8 million for the years ended December 31, 2019 and 2018, respectively. The Company has incurred net operating losses (“NOL”) since inception. As of December 31, 2019, the Company had federal and state NOL carryforwards of $210.2 million and $42.4 million, respectively. Federal NOL carryforwards of $148.3 million, which were generated after December 31, 2017, do not expire. The remaining $61.9 million of Federal NOL carryforwards expire beginning in 2032. As of December 31, 2019, the Company had federal and California research and other tax credit carryforwards of $8.8 million and $4.0 million, respectively. The federal tax credits expire beginning in 2033. The California tax credits can be carried forward indefinitely. The Internal Revenue Code of 1986, as amended (the “Code”), provides for a limitation of the annual use of net operating losses and other tax attributes (such as research and development tax credit carryforwards) following certain ownership changes defined by the Code that could limit the Company’s ability to utilize these carryforwards in the future. At this time, the Company has not completed a study to assess whether an ownership change under Section 382 of the Code has occurred, or whether there have been multiple ownership changes since the Company’s formation. The Company may have experienced ownership changes, as defined by the Code, as a result of past financing transactions and may not be able to take full advantage of these carryforwards for federal or state income tax purposes. The effective tax rate for the years ended December 31, 2019 and 2018 is different from the federal statutory rate primarily due to the valuation allowance against deferred tax assets as a result of insufficient income. The Company’s effective tax rate differs from the federal statutory tax rate as follows: Year Ended December 31, 2019 2018 Federal statutory tax rate 21.0 % 21.0 % Change in deferred tax asset valuation allowance (36.4 )% (24.8 )% State taxes, net of federal benefit 1.1 % 0.9 % Research and development tax credits 4.4 % 3.3 % Stock-based compensation 9.9 % (0.2 )% Other — % (0.2 )% Effective tax rate — % — % Uncertain Tax Positions The Company accounts for its uncertain tax positions in accordance with FASB ASC Topic No. 740-10, Accounting for Uncertainty in Income Taxes A reconciliation of the beginning and ending amount of unrecognized benefits is as follows (in thousands): Year Ended December 31, 2019 2018 Balance at the beginning of the year $ 1,827 $ 1,149 Increase related to current year tax positions 1,718 678 Balance at the end of the year $ 3,545 $ 1,827 The entire amount of the unrecognized tax benefits would not impact the Company’s effective tax rate if recognized. During the years ended December 31, 2019 and 2018, the Company did not recognize accrued interest and penalties related to unrecognized tax benefits. The Company does not anticipate that the amount of existing unrecognized tax benefits will significantly increase or decrease during the next twelve months. The Company files income tax returns in the U.S. federal and California tax jurisdictions. The federal and state income tax returns from inception to December 31, 2019 remain subject to examination. It is the Company’s policy to include penalties and interest expense related to income taxes as a component of the income tax provision as necessary. Management determined that no accrual for interest and penalties was required at December 31, 2019 and 2018. Since the Company is in a loss carryforward position, it is generally subject to examination by the U.S. federal, state and local income tax authorities for all tax years in which a loss carryforward is available. Recent Changes to U.S. Tax Law In December 2017, the 2017 Tax Cuts and Jobs Act (the “2017 Tax Act”) was enacted and includes a broad range of provisions, many of which differ significantly from those contained in previous U.S. tax law. The Company accounts for changes in tax law in accordance with ASC 740 which requires companies to recognize the effect of such changes in the period of enactment. However, the SEC staff issued Staff Accounting Bulletin 118 which will allow companies to record provisional amounts during a measurement period that is similar to the measurement period used when accounting for business combinations. Accordingly, the Company adjusted its deferred taxes and related valuation allowances on a provisional basis to reflect the reduction in U.S. federal corporate tax rate from 35% to 21%, based on current understanding of the new law. As of December 31, 2018, the Company has completed its analysis of the income effects of the 2017 Tax Act. There was no material impact on the Company’s financial statements as a result of the analysis.

Defined Contribution Plans

Defined Contribution Plans	12 Months Ended
	Dec. 31, 2019
Defined Contribution Pension And Other Postretirement Plans Disclosure [Abstract]
Defined Contribution Plans	10. Defined Contribution Plans In July 2013, the Company established a Savings Incentive Match Plan (the “SIMPLE IRA plan”) for its employees, allowing for both employee and employer contributions for those employees who meet defined minimum age and service requirements. The SIMPLE IRA plan allows participants to defer a portion of their annual compensation on a pretax basis. During the year ended December 31, 2017, the Company made contributions to the SIMPLE IRA plan of $0.1 million. In January 2018, the Company terminated and replaced the SIMPLE IRA with a defined contribution plan under Section 401(k) of the Internal Revenue Code (the “401(k) plan”). The 401(k) plan covers all employees who meet defined minimum age and service requirements. Employee contributions are voluntary and are determined on an individual basis, limited to the maximum amount allowable under U.S. federal tax regulations. The Company makes matching contributions of up to 4% of the eligible employees’ compensation to the 401(k) plan. During the years ended December 31, 2019 and 2018, the Company made contributions to the 401(k) plan of $0.5 million and $0.3 million, respectively.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	11. Selected Quarterly Financial Data (Unaudited) The following tables summarize the Company’s quarterly results for the years ended December 31, 2019 and 2018 (in thousands, except per share data): Quarter Ended March 31, June 30, September 30, December 31, 2019 2019 2019 2019 2019 Loss from operations $ (20,927 ) $ (20,057 ) $ (23,584 ) $ (26,850 ) Net loss $ (19,953 ) $ (19,072 ) $ (21,732 ) $ (24,615 ) Net loss per common share, basic and diluted $ (0.47 ) $ (0.44 ) $ (0.47 ) $ (0.51 ) Quarter Ended March 31, June 30, September 30, December 31, 2018 2018 2018 2018 2018 Loss from operations $ (8,709 ) $ (9,524 ) $ (11,975 ) $ (15,513 ) Net loss $ (8,485 ) $ (9,377 ) $ (11,148 ) $ (14,528 ) Net loss per common share, basic and diluted $ (4.19 ) $ (4.17 ) $ (0.34 ) $ (0.35 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The financial statements have been prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”). The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements and accompanying notes.
Use of Estimates	Use of Estimates Management uses significant judgment when making estimates related to common stock valuation and related stock-based compensation expense, accrued expenses related to clinical trials and deferred tax valuation allowances. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results could differ from those estimates under different assumptions or conditions, and those differences could be material to the financial position and results of operations.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties Financial instruments that potentially subject the Company to credit risk principally consist of cash, cash equivalents and marketable securities. These financial instruments are held in accounts at a single financial institution that management believes possesses high credit quality. Amounts on deposit with this financial institution have and will continue to exceed federally-insured limits. The Company has not experienced any losses on its cash deposits. Additionally, the Company’s investment policy limits its investments to certain types of securities issued by the United States government and its agencies. The Company is subject to a number of risks similar to that of other early-stage biopharmaceutical companies, including, but not limited to, the need to obtain adequate additional funding, possible failure of current or future clinical trials, its reliance on third-parties to conduct its clinical trials, the need to obtain regulatory and marketing approvals for its product candidates, competitive developments, the need to successfully commercialize and gain market acceptance of the Company’s product candidates, its right to develop and commercialize its product candidates pursuant to the terms and conditions of the licenses granted to the Company, protection of proprietary technology, the ability to make milestone, royalty or other payments due under licensing agreements, and the need to secure and maintain adequate manufacturing arrangements with third-parties. If the Company does not successfully commercialize or partner its product candidates, it will be unable to generate product revenue or achieve profitability.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with original maturities of three months or less from the date of purchase to be cash equivalents. The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the Company’s balance sheets and which, in aggregate, represent the amounts reported in the statements of cash flows (in thousands): December 31, 2019 2018 2017 Cash and cash equivalents $ 38,367 $ 33,660 $ 85,207 Restricted cash 2,275 802 — Total $ 40,642 $ 34,462 $ 85,207 Restricted cash at December 31, 2019 represents $2.3 million of security deposits for the lease of the Company’s facilities in Redwood City, California and San Carlos, California. Both security deposits are in the form of letters of credit secured by restricted cash. Restricted cash amounts are included within other long-term assets on the Company’s balance sheets.
Marketable Securities	Marketable Securities The Company invests in marketable securities, primarily securities issued by the United States government and its agencies. The Company’s marketable securities are considered available-for-sale and are classified as current assets even when the stated maturities of the underlying securities exceed one year from the date of the current balance sheet being reported. This classification reflects management’s ability and intent to utilize proceeds from the sale of such investments to fund ongoing operations. Unrealized gains and losses are excluded from earnings and are reported as a component of accumulated comprehensive income (loss). The cost of securities sold is determined using the specific-identification method. Interest earned and adjustments for the amortization of premiums and discounts on investments are included in interest income, net, on the statements of operations and comprehensive loss. Realized gains and losses and declines in fair value judged to be other than temporary, if any, on investments in marketable securities are included in other expense, net, on the statements of operations and comprehensive loss.
Fair Value Measurements	Fair Value Measurements The Company accounts for fair value of its financial instruments in accordance with Financial Accounting Standards Board (the “FASB”) Accounting Standards Codification (“ASC”) Topic No. 820, Fair Value Measurements The Company measures fair value based on a three-level hierarchy of inputs, of which the first two are considered observable and the last unobservable. Unobservable inputs reflect the Company’s own assumptions about current market conditions. The three-level hierarchy of inputs is as follows: Level 1 – Quoted prices in active markets for identical assets or liabilities. Level 2 – Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of assets or liabilities. To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. The carrying amounts reflected in the Company’s balance sheets for cash and cash equivalents, prepaid expenses and other current assets, other long-term assets, accounts payable, and accrued expenses and other current liabilities approximate fair value, due to their short-term nature. The Company’s investments in marketable securities are measured at fair value in accordance with the levels above.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost, net of accumulated depreciation. Depreciation is computed using the straight-line method over the estimated useful lives of the assets. The useful lives of property and equipment are as follows: Laboratory equipment – 3 to 5 years Leasehold improvements – Shorter of remaining lease term or estimated life of the assets Upon retirement or sale, the cost of disposed assets and their related accumulated depreciation are removed from the balance sheet. Any resulting gains or losses on dispositions of property and equipment are included as a component of other income (expense), net, within the Company’s statements of operations and comprehensive loss. Repair and maintenance costs that do not significantly add value to the property and equipment, or prolong its life, are charged to operating expense as incurred.
Leases	Leases Effective January 1, 2019, the Company accounts for its leases in accordance with Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 842, “Leases” (“ASC 842”). Prior period amounts continue to be reported in accordance with the Company’s historic accounting under previous lease guidance. Additionally, the Company elected a number of optional practical expedients made available under the ASC 842 transition guidance. Such elections include (i) carrying forward the Company’s historical lease classifications, (ii) foregoing a re-evaluation of historical contracts to identify embedded leases, (iii) foregoing a re-assessment of initial direct costs related to leases that existed prior to adoption, (iv) combining lease and non-lease components, and (v) recognizing lease expense for all contracts with an initial term of 12 months or less within the statements of operations and comprehensive loss on a straight-line basis over the requisite lease term. The Company accounts for its leases by recording right-of-use assets and lease liabilities on the Balance Sheet. Right-of-use assets represent the Company’s right to use an underlying asset over the lease term and include any lease payments made prior to the lease commencement date and exclude lease incentives. Lease liabilities represent the present value of the total lease payments over the lease term, calculated using the Company’s incremental borrowing rate. In determining the Company’s incremental borrowing rate, consideration is given to the term of the lease and the Company’s credit risk. The Company’s recognizes options to extend or terminate a lease when it is reasonably certain that the Company will exercise any such options. Lease expense is recognized on a straight-line basis over the expected lease term.
Accrued Contract Research and Development Expense	Accrued Contract Research and Development Expense Costs associated with research and development services performed on behalf of the Company by third-party CROs and CDMOs comprise a significant component of total research and development expense included on the statements of operations and comprehensive loss. Services performed by these CROs and CDMOs include various research and development activities supporting the Company’s preclinical studies, clinical trials and drug manufacturing activities, which are governed by executed service agreements. Underlying amounts included in these service agreements with CROs and CDMOs are expensed as incurred. The Company accrues for expenses related to services performed by Accrued contract research and development expense requires certain estimates by management surrounding the extent of unbilled services received and the extent and duration of remaining services still to be performed. Management’s estimates are based on the evaluation of data obtained from multiple internal and external sources including, but not limited to, clinical site activity logs, subject visit reports, and project management timelines. Results from these evaluations are reviewed by internal personnel from the Company’s clinical and technical operations departments.
Research and Development Expense	Research and Development Expense Research and development costs are expensed as incurred. Research and development costs include, among others, consulting fees, salaries, benefits, travel, stock-based compensation, laboratory supplies and other non-capital equipment utilized for in-house research, allocations of facilities and overhead costs, amounts owed under in-licensing agreements, and amounts paid to CROs and CDMOs that conduct research and development activities on the Company’s behalf. Goods or services incurred for research and development activities that have not yet been invoiced are recorded as liabilities within accrued expenses and other current liabilities on the Company’s balance sheets. Amounts recorded for unbilled services often represent estimates, which are typically based on contracted amounts for the proportion of work performed and determined through analysis with internal personnel and external service providers as to the progress or stage of completion of the associated services. Non-refundable a dvance payments for goods or services to be rendered as part of future research and development activities are capitalized on the Company’s balance sheets. Classification between current and long-term assets is based on an evaluation of when the goods will be delivered and/or services will be performed, with such amounts subsequently amortized to expense once realized.
Segments	Segments Operating segments are defined as components of an entity about which separate discrete information is available for evaluation by the chief operating decision maker or decision-making group, in deciding how to allocate resources and in assessing performance. The Company’s chief operating decision maker, its Chief Executive Officer, views its operations and manages its business in one operating segment operating exclusively in the United States.
Patent Costs	Patent Costs The Company expenses patent application and related legal costs as incurred and classifies such costs as general and administrative expenses in the statements of operations and comprehensive loss
Stock-Based Compensation	Stock-Based Compensation The Company accounts for its stock-based compensation in accordance with FASB ASC Topic 718, Compensation—Stock Compensation For purposes of determining the estimated fair value of stock options granted to employees and nonemployees, the Company uses the Black-Scholes option pricing model. The Black-Scholes option pricing model requires the input of certain assumptions that involve judgment, for which changes can materially affect the resulting estimates of fair value. The assumptions used to determine the fair value of stock options granted were as follows: Expected volatility – As there is insufficient trading history for the Company’s common stock, the Company has based its computation of expected volatility on the historical volatility of a representative group of public companies with similar characteristics to the Company, including stage of product development and life science industry focus. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. Expected term – The Company determines the expected term in accordance with the “simplified method” described by SEC Staff Accounting Bulletin No. 107, , as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. Risk-free interest rate – The Company bases the risk-free interest rate on United States Treasury securities with terms consistent to the expected term of the stock option being valued. Expected dividends – The expected dividend yield is assumed to be zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The fair value of restricted stock units (“RSUs”) is determined using the quoted market price of the Company’s common stock on the date of grant. The Company uses historical data to estimate pre-vesting forfeitures and records stock-based compensation expense only for those awards expected to vest. To the extent that actual forfeitures differ from estimates, the difference is recorded as a cumulative adjustment in the period the estimate are revised. The Company expenses the fair value of its stock-based compensation awards to employees and nonemployees on a straight-line basis over the requisite service period, which is generally the vesting period.
Income Taxes	Income Taxes In December 2017, the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) was signed into law. The Tax Act, among other changes, lowered the Company’s federal tax rate from 34% to 21%. Based on provisions of the Tax Act, the Company remeasured its deferred tax assets and liabilities at December 31, 2017 to reflect the lower statutory tax rate, however, since the Company established a full valuation allowance to offset its deferred tax assets, there was no impact to the effective tax rate. The deferred tax remeasurement was provisional and represented our reasonable estimate within the meaning of Staff Accounting Board 118, which provided a measurement period that should not extend beyond one year from the Tax Act’s enactment date for companies to complete the accounting under ASC 740. As of December 31, 2018, the Company has completed its analysis of the income tax effects of the Tax Act. The results of this analysis have been reflected in the Company’s financial statements and related footnotes.
Comprehensive Loss	Comprehensive Loss Comprehensive loss is defined as the change in stockholders’ equity (deficit) during a period from transactions and other events and circumstances from non-owner sources. The difference between net loss and comprehensive loss for the years ended December 31, 2019 and 2018 are a result of unrealized gains and losses on the Company’s investments in marketable securities included in current assets on the balance sheets.
Net Loss per Share	Net Loss per Share The Company calculates basic net loss per share by dividing the net loss attributable to common stockholders by the weighted-average shares of common stock outstanding during the period. The Company calculates diluted net loss per share after giving consideration to all potentially dilutive securities outstanding during the period using the treasury-stock and if-converted methods, except where the effect of including such securities would be anti-dilutive. Because the Company has reported net losses since inception, the effect from potentially dilutive securities would have been anti-dilutive and therefore has been excluded from the calculation of diluted net loss per share. Basic and diluted net loss per share was calculated as follows (in thousands, except per share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss $ (85,372 ) $ (43,538 ) $ (23,552 ) Denominator: Weighted-average shares of common stock outstanding, basic and diluted 45,191 19,833 1,620 Net loss per share, basic and diluted $ (1.89 ) $ (2.20 ) $ (14.54 ) The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect for the periods indicated (in thousands): Year Ended December 31, 2019 2018 2017 Series A convertible preferred stock — — 20,866 Series B convertible preferred stock — — 10,105 Options to purchase common stock 7,148 7,811 4,884 Unvested restricted stock units 542 — — Warrants to purchase common stock — — 48 Unvested restricted common stock — 47 104 Shares issuable under employee stock purchase plans 31 29 — Total 7,721 7,887 36,007
Foreign Currency Transactions	Foreign Currency Transactions The Company is party to multiple contract manufacturing and clinical research agreements for which services to be performed are denominated in foreign currencies other than the United States Dollar. The Company records gains and losses attributable to fluctuations in foreign currencies as a component of other income (expense), net, on the statements of operations and comprehensive loss.
Recent Accounting Pronouncements	Recently Adopted Accounting Pronouncements In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Codification (“ASC”) 842 which became effective for fiscal years beginning after December 15, 2018. ASC 842 requires an entity to recognize a right-of-use asset and lease liability for all leases with terms of more than 12 months. The recognition, measurement and presentation of expenses will depend on the lease’s classification as a finance or operating lease. ASC 842 also requires certain quantitative and qualitative disclosures about leasing arrangements. The Company adopted ASC 842 using a modified retrospective approach effective January 1, 2019, recording a right-of-use asset of $6.1 million and a long-term lease liability of $8.2 million. Adoption of ASC 842 did not result in a cumulative effect adjustment to accumulated deficit. See Note 6 for further disclosure. On January 1, 2019, the Company adopted Accounting Standards Update (“ASU”) No. 2018-07, Compensation-Stock Compensation Recently Issued Accounting Pronouncements Not Yet Adopted In December 2019, the FASB issued ASU No. 2019-12, Income Taxes In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses
Uncertain Tax Positions	Uncertain Tax Positions The Company accounts for its uncertain tax positions in accordance with FASB ASC Topic No. 740-10, Accounting for Uncertainty in Income Taxes

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Accounting Policies [Abstract]
Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the Company’s balance sheets and which, in aggregate, represent the amounts reported in the statements of cash flows (in thousands): December 31, 2019 2018 2017 Cash and cash equivalents $ 38,367 $ 33,660 $ 85,207 Restricted cash 2,275 802 — Total $ 40,642 $ 34,462 $ 85,207
Calculation of Basic and Diluted Net Loss per Share	Basic and diluted net loss per share was calculated as follows (in thousands, except per share data): Year Ended December 31, 2019 2018 2017 Numerator: Net loss $ (85,372 ) $ (43,538 ) $ (23,552 ) Denominator: Weighted-average shares of common stock outstanding, basic and diluted 45,191 19,833 1,620 Net loss per share, basic and diluted $ (1.89 ) $ (2.20 ) $ (14.54 )
Summary of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share	The following table sets forth the potentially dilutive securities that have been excluded from the calculation of diluted net loss per share due to their anti-dilutive effect for the periods indicated (in thousands): Year Ended December 31, 2019 2018 2017 Series A convertible preferred stock — — 20,866 Series B convertible preferred stock — — 10,105 Options to purchase common stock 7,148 7,811 4,884 Unvested restricted stock units 542 — — Warrants to purchase common stock — — 48 Unvested restricted common stock — 47 104 Shares issuable under employee stock purchase plans 31 29 — Total 7,721 7,887 36,007

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured at Fair Value on Recurring Basis	The Company measures and reports certain financial instruments as assets and liabilities at fair value on a recurring basis. The Company’s financial assets measured at fair value on a recurring basis were as follows (in thousands): December 31, 2019 Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 35,935 $ — $ — $ 35,935 Total cash equivalents 35,935 — — 35,935 Marketable securities U.S. treasuries 457,534 — — 457,534 Total marketable securities 457,534 — — 457,534 Total cash equivalents and marketable securities $ 493,469 $ — $ — $ 493,469 December 31, 2018 Level 1 Level 2 Level 3 Total Cash equivalents Money market funds $ 31,555 $ — $ — $ 31,555 Total cash equivalents 31,555 — — 31,555 Marketable securities U.S. treasuries 145,246 — — 145,246 Total marketable securities 145,246 — — 145,246 Total cash equivalents and marketable securities $ 176,801 $ — $ — $ 176,801

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Marketable Securities	All marketable securities were considered available-for-sale at December 31, 2019 and 2018. The amortized cost, gross unrealized holding gains or losses, and fair value of the Company’s marketable securities by major security type are summarized in the table below (in thousands): December 31, 2019 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Available-for-sale securities U.S. treasuries classified as investments $ 457,397 $ 161 $ (24 ) $ 457,534 Total $ 457,397 $ 161 $ (24 ) $ 457,534 December 31, 2018 Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Available-for-sale securities U.S. treasuries classified as investments $ 145,261 $ — $ (15 ) $ 145,246 Total $ 145,261 $ — $ (15 ) $ 145,246

Balance Sheet Components and _2

Balance Sheet Components and Supplemental Disclosures (Tables)	12 Months Ended
	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Schedule of Property and Equipment, Net	Property and equipment, net, consisted of the following (in thousands): December 31, 2019 2018 Laboratory equipment $ 4,170 $ 3,272 Furniture and office equipment 1,695 1,666 Leasehold improvements 4,581 4,545 10,446 9,483 Less accumulated depreciation (2,036 ) (635 ) Property and equipment, net $ 8,410 $ 8,848
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following (in thousands): December 31, 2019 2018 Accrued contract research and development expense $ 4,990 $ 1,866 Accrued compensation and benefits expense 1,608 1,041 Lease liability, current 410 — Lease incentive obligation, current — 123 Other current liabilities 90 134 Total $ 7,098 $ 3,164

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Summary of Classification of Company's Lease Liabilities	Classification of the Company’s lease liabilities included on the Balance Sheet at December 31, 2019 was as follows (in thousands): Operating lease liabilities Current portion included in accrued expenses and other current liabilities $ 410 Non-current portion included in other long-term liabilities 8,112 Total operating lease liabilities $ 8,522
Summary of Components of Lease Costs	The components of lease costs, which are included in operating expenses in the Company’s statements of operations and comprehensive loss were as follows (in thousands): Year ended December 31, 2019 Operating lease cost $ 1,118 Variable cost 346 Total lease costs $ 1,464
Summary of Future Minimum Lease Payments Required Under Operating Leases	Future minimum lease payments required under operating leases included on the Company’s Balance Sheet at December 31, 2019 are as follows (in thousands): Fiscal Year Ending December 31, 2020 $ 1,233 2021 1,270 2022 1,308 2023 1,348 2024 1,388 Thereafter 6,916 Total minimum future lease payments (1) 13,463 Less: Present value adjustment 4,941 Operating lease liabilities $ 8,522 (1)

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Stockholders Equity Note [Abstract]
Summary of Common Stock Shares Reserved for Future Issuance	Common shares reserved for future issuance upon the exercise, issuance or conversion of the respective equity instruments are as follows (in thousands): December 31, 2019 2018 Exercise of common stock options outstanding 7,148 7,811 Shares reserved for issuance under equity incentive plans 3,762 2,785 Vesting of restricted stock units 542 — Shares reserved for issuance under employee stock purchase plans 848 500 Total 12,300 11,096

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Total Stock-Based Compensation Expense Recognized	Total stock-based compensation expense recognized is as follows (in thousands): Year Ended December 31, 2019 2018 2017 Research and development $ 5,351 $ 1,792 $ 175 General and administrative 10,413 2,778 227 Total $ 15,764 $ 4,570 $ 402
Summary of Stock Option Activity	Stock option activity under the 2018 Plan and the 2012 Plan is summarized as follows (in thousands, except per share data): Weighted- Weighted- Average Average Aggregate Options Exercise Remaining Intrinsic Outstanding Price Years Value Balance at December 31, 2018 7,811 $ 8.60 8.2 $ 342,292 Granted 645 $ 46.75 Exercised (1,251 ) $ 2.11 Forfeited (57 ) $ 35.60 Balance at December 31, 2019 7,148 $ 12.96 8.0 $ 589,114 Options exercisable 3,982 $ 5.10 7.6 $ 359,399 Options vested and expected to vest 7,119 $ 12.91 8.0 $ 587,065
Summary of Weighted-Average Assumptions Used to Calculate Fair Value of Stock Options Granted	The following weighted-average assumptions were used to calculate the fair value of stock options granted during the periods indicated: Year Ended December 31, 2019 2018 2017 Risk-free interest rate 1.91 % 2.79 % 1.83 % Expected volatility 67.22 % 73.47 % 77.59 % Expected dividend yield — — — Expected term (in years) 6.01 6.01 6.08
Summary of Restricted Stock Awards and Restricted Stock Units	A summary of the restricted common stock activity during the year ended December 31, 2019 is as follows (in thousands, except per share data): Weighted- Average Number Grant Date of Shares Fair Value Balance at December 31, 2018 47 $ 0.43 Vested (47 ) $ 0.43 Balance at December 31, 2019 — $ — RSU activity under the 2018 Plan is summarized as follows (in thousands, except per share data): Weighted- Average Number Grant Date of Shares Fair Value Balance at December 31, 2018 — $ — Granted 542 $ 93.67 Balance at December 31, 2019 542 $ 93.67
Summary of Weighted-Average Assumptions Used to Calculate Fair Value of ESPP Shares	The following weighted-average assumptions were used to calculate the fair value of ESPP shares during the periods indicated: Year Ended December 31, 2019 2018 Risk-free interest rate 2.37 % 2.42 % Expected volatility 64.26 % 65.92 % Expected dividend yield — — Expected term (in years) 1.22 1.24

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Summary of Total Deferred Income Tax Assets, Net of Valuation Allowance	Total deferred income tax assets, net of valuation allowance, at December 31, 2019 and 2018 were as follows (in thousands): December 31, 2019 2018 Deferred tax assets Net operating loss carryforwards $ 47,003 $ 24,091 Research and development credits 8,644 3,995 Accruals and reserves 1,199 260 Stock-based compensation 2,870 611 Lease liability 1,798 — Gross deferred tax assets 61,514 28,957 Less: valuation allowance (59,901 ) (28,892 ) Deferred tax assets, net of valuation allowance 1,613 65 Deferred tax liabilities Fixed and intangible assets 395 65 Right-of-use asset 1,218 — Gross deferred tax liabilities 1,613 65 Net deferred tax assets $ — $ —
Summary of Effective Tax Rate Differs From Federal Statutory Tax Rate	The Company’s effective tax rate differs from the federal statutory tax rate as follows: Year Ended December 31, 2019 2018 Federal statutory tax rate 21.0 % 21.0 % Change in deferred tax asset valuation allowance (36.4 )% (24.8 )% State taxes, net of federal benefit 1.1 % 0.9 % Research and development tax credits 4.4 % 3.3 % Stock-based compensation 9.9 % (0.2 )% Other — % (0.2 )% Effective tax rate — % — %
Reconciliation of Beginning and Ending Amount of Unrecognized Benefits	A reconciliation of the beginning and ending amount of unrecognized benefits is as follows (in thousands): Year Ended December 31, 2019 2018 Balance at the beginning of the year $ 1,827 $ 1,149 Increase related to current year tax positions 1,718 678 Balance at the end of the year $ 3,545 $ 1,827

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Results	The following tables summarize the Company’s quarterly results for the years ended December 31, 2019 and 2018 (in thousands, except per share data): Quarter Ended March 31, June 30, September 30, December 31, 2019 2019 2019 2019 2019 Loss from operations $ (20,927 ) $ (20,057 ) $ (23,584 ) $ (26,850 ) Net loss $ (19,953 ) $ (19,072 ) $ (21,732 ) $ (24,615 ) Net loss per common share, basic and diluted $ (0.47 ) $ (0.44 ) $ (0.47 ) $ (0.51 ) Quarter Ended March 31, June 30, September 30, December 31, 2018 2018 2018 2018 2018 Loss from operations $ (8,709 ) $ (9,524 ) $ (11,975 ) $ (15,513 ) Net loss $ (8,485 ) $ (9,377 ) $ (11,148 ) $ (14,528 ) Net loss per common share, basic and diluted $ (4.19 ) $ (4.17 ) $ (0.34 ) $ (0.35 )

Organization and Business - Add

Organization and Business - Additional Information (Details) $ / shares in Units, $ in Thousands	Aug. 09, 2019USD ($)$ / sharesshares	Jul. 23, 2018USD ($)$ / sharesshares	Jul. 06, 2018	Dec. 31, 2019USD ($)$ / sharesshares	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)$ / sharesshares	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)
Organization and Business [Line Items]
Net loss				$ 24,615	$ 21,732	$ 19,072	$ 19,953	$ 14,528	$ 11,148	$ 9,377	$ 8,485	$ 85,372	$ 43,538	$ 23,552
Cash used in operations												63,012	38,450	22,568
Accumulated deficit				189,484				$ 104,112				189,484	$ 104,112
Cash, cash equivalents and marketable securities				$ 495,900								$ 495,900
Common stock, shares authorized | shares		200,000,000		200,000,000				200,000,000				200,000,000	200,000,000
Common stock, par value | $ / shares		$ 0.001		$ 0.001				$ 0.001				$ 0.001	$ 0.001
Preferred stock, shares authorized | shares		20,000,000		20,000,000				20,000,000				20,000,000	20,000,000
Preferred stock, par value | $ / shares		$ 0.001		$ 0.001				$ 0.001				$ 0.001	$ 0.001
Proceeds from issuance of common stock, net of issuance costs												$ 377,525	$ 138,357	0
Reverse stock split			1-for-1.25
Reverse stock split ratio			0.8
Common Stock
Organization and Business [Line Items]
Net loss												$ 0	$ 0	$ 0
Common stock shares sold | shares												5,227,000	8,453,000
Proceeds from IPO, net of underwriting discounts and commissions		$ 137,300
Proceeds from private placement		$ 4,500
Conversion of preferred stock upon initial public offering, Shares | shares													30,972,000
Initial Public Offering | Common Stock
Organization and Business [Line Items]
Common stock shares sold | shares		8,203,332
Common stock offering price per share | $ / shares		$ 18
Conversion of preferred stock upon initial public offering, Shares | shares		30,971,627
Private Placement | Common Stock
Organization and Business [Line Items]
Common stock shares sold | shares		250,000
Common stock offering price per share | $ / shares		$ 18
August 2019 Follow-On Offering | Common Stock
Organization and Business [Line Items]
Common stock shares sold | shares	5,227,272
Common stock offering price per share | $ / shares	$ 77
Proceeds from issuance of common stock, net of issuance costs	$ 377,500

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019USD ($)Segment	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Jan. 01, 2019USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash in other long term assets, security deposit for lease facility	$ 2,275,000	$ 802,000	$ 0
Number of operating segments | Segment	1
Expected dividend yield	0.00%	0.00%	0.00%
Income tax rate	21.00%	21.00%	34.00%
Change in federal tax rate, amount	$ 0
Right-of-use asset	5,775,000	$ 0
Lease liability	$ 8,522,000
ASC 842
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset				$ 6,100,000
Lease liability				$ 8,200,000
Tax Cut And Jobs Act
Summary Of Significant Accounting Policies [Line Items]
Income tax rate	21.00%
Maximum
Summary Of Significant Accounting Policies [Line Items]
Income tax rate			35.00%
Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Useful life	Shorter of remaining lease term or estimated life of the assets
Laboratory Equipment | Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Useful life	3 years
Laboratory Equipment | Maximum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, Useful life	5 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 38,367	$ 33,660	$ 85,207
Restricted cash	2,275	802	0
Total	$ 40,642	$ 34,462	$ 85,207	$ 13,416

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Calculation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (24,615)	$ (21,732)	$ (19,072)	$ (19,953)	$ (14,528)	$ (11,148)	$ (9,377)	$ (8,485)	$ (85,372)	$ (43,538)	$ (23,552)
Weighted-average number of common shares outstanding:
Weighted-average shares of common stock outstanding, basic and diluted									45,191	19,833	1,620
Net loss per share, basic and diluted	$ (0.51)	$ (0.47)	$ (0.44)	$ (0.47)	$ (0.35)	$ (0.34)	$ (4.17)	$ (4.19)	$ (1.89)	$ (2.20)	$ (14.54)

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Summary of Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss per Share (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	7,721	7,887	36,007
Series A Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	0	0	20,866
Series B Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	0	0	10,105
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	7,148	7,811	4,884
Unvested Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	542	0	0
Warrants to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	0	0	48
Unvested Restricted Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	0	47	104
Shares Issuable under Employee Stock Purchase Plans
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted net loss per share due to anti-dilutive effect	31	29	0

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 457,534	$ 145,246
Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	35,935	31,555
Marketable securities	457,534	145,246
Total cash equivalents and marketable securities	493,469	176,801
Fair Value, Measurements, Recurring Basis | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	35,935	31,555
Marketable securities	457,534	145,246
Total cash equivalents and marketable securities	493,469	176,801
Fair Value, Measurements, Recurring Basis | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable securities	0	0
Total cash equivalents and marketable securities	0	0
Fair Value, Measurements, Recurring Basis | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Marketable securities	0	0
Total cash equivalents and marketable securities	0	0
Money Market Funds | Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	35,935	31,555
Money Market Funds | Fair Value, Measurements, Recurring Basis | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	35,935	31,555
Money Market Funds | Fair Value, Measurements, Recurring Basis | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Money Market Funds | Fair Value, Measurements, Recurring Basis | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
U.S. Treasuries | Fair Value, Measurements, Recurring Basis
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	457,534	145,246
U.S. Treasuries | Fair Value, Measurements, Recurring Basis | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	457,534	145,246
U.S. Treasuries | Fair Value, Measurements, Recurring Basis | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	0	0
U.S. Treasuries | Fair Value, Measurements, Recurring Basis | Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities	$ 0	$ 0

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Transfers of assets between level 1 to level 2	$ 0	$ 0
Transfers of assets between level 2 to level 1	0	0
Transfers of liabilities between level 1 to level 2	0	0
Transfers of liabilities between level 2 to level 1	0	0
Transfers of assets into level 3	0	0
Transfers of assets out of level 3	0	0
Transfers of liabilities into level 3	0	0
Transfers of liabilities out of level 3	$ 0	$ 0

Marketable Securities - Marketa

Marketable Securities - Marketable Securities - Summary of Amortized Cost, Gross Unrealized Holding Gains or Losses, and Fair Value of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 457,397	$ 145,261
Unrealized Gains	161	0
Unrealized Losses	(24)	(15)
Fair Value	457,534	145,246
U.S. Treasuries | Investments
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	457,397	145,261
Unrealized Gains	161	0
Unrealized Losses	(24)	(15)
Fair Value	$ 457,534	$ 145,246

Marketable Securities - Additio

Marketable Securities - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, fair value	$ 187,400,000	$ 132,200,000
Available-for-sale Securities, realized gains	0	0
Available-for-sale Securities, realized losses	$ 0	$ 0
Maximum
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, remaining maturity period	1 year

Balance Sheet Components and _3

Balance Sheet Components and Supplemental Disclosures - Schedule of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 10,446	$ 9,483
Less accumulated depreciation	(2,036)	(635)
Property and equipment, net	8,410	8,848
Laboratory Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	4,170	3,272
Furniture And Office Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,695	1,666
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 4,581	$ 4,545

Balance Sheet Components and _4

Balance Sheet Components and Supplemental Disclosures - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization	$ 1,508	$ 242	$ 241

Balance Sheet Components and _5

Balance Sheet Components and Supplemental Disclosures - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued contract research and development expense	$ 4,990	$ 1,866
Accrued compensation and benefits expense	1,608	1,041
Lease liability, current	410	0
Lease incentive obligation, current	0	123
Other current liabilities	90	134
Total	$ 7,098	$ 3,164

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details)	1 Months Ended		12 Months Ended			73 Months Ended	75 Months Ended
	Dec. 31, 2019USD ($)ft²shares	Jan. 31, 2018USD ($)ft²	Dec. 31, 2019USD ($)ft²shares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2019USD ($)ft²shares	Dec. 31, 2019USD ($)ft²shares	Dec. 31, 2013shares
Commitments And Contingencies [Line Items]
Operating lease right-of-use assets	$ 5,775,000		$ 5,775,000	$ 0		$ 5,775,000	$ 5,775,000
Operating lease liabilities	$ 8,522,000		8,522,000			$ 8,522,000	$ 8,522,000
Net rent expense			$ 1,100,000	$ 1,000,000	$ 500,000
Remaining lease term			9 years 7 months 6 days
Operating lease liability discount rate	10.00%		10.00%			10.00%	10.00%
Noncancelable purchase obligations	$ 8,700,000		$ 8,700,000			$ 8,700,000	$ 8,700,000
Common stock, shares issued | shares	48,667,809		48,667,809	42,117,000		48,667,809	48,667,809
In-Licensing Agreements
Commitments And Contingencies [Line Items]
Milestone expense			$ 0	$ 300,000	$ 0
Accrued Royalties	$ 0		$ 0			$ 0	$ 0
Exclusive License Agreement with The Johns Hopkins University
Commitments And Contingencies [Line Items]
Upfront and milestone payments						300,000
Common stock, shares issued | shares								88,887
Exclusive License Agreement with The Johns Hopkins University | Maximum
Commitments And Contingencies [Line Items]
Aggregate additional milestone payments						$ 4,000,000
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG
Commitments And Contingencies [Line Items]
Milestone payments							400,000
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG | Minimum
Commitments And Contingencies [Line Items]
Annual commercial license fees							40,000
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG | Maximum
Commitments And Contingencies [Line Items]
Aggregate additional milestone payments							$ 41,000,000
2019 San Carlos Lease Agreement
Commitments And Contingencies [Line Items]
Lease agreement initiation period	2019-12
Lease term extension period	5 years		5 years			5 years	5 years
Area of office and laboratory space | ft²	98,000		98,000			98,000	98,000
Lessee operating lease contractual lease expected commencement period	2020-11
Lessee operating lease expected termination Period	2031-08
Operating lease, future minimum rental payments	$ 77,600,000		$ 77,600,000			$ 77,600,000	$ 77,600,000
Operating lease right-of-use assets	0		0			0	0
Operating lease liabilities	0		0			0	0
Security deposit in the form of letter of credit secured by restricted cash	1,500,000		1,500,000			1,500,000	1,500,000
2018 Redwood City Lease Agreement
Commitments And Contingencies [Line Items]
Lease agreement initiation period		2018-01
Base term of lease		10 years 9 months
Lease term extension period		5 years
Area of office and laboratory space | ft²		25,000
Tenant improvement allowance		$ 1,400,000
Security deposit in the form of letter of credit secured by restricted cash	$ 800,000		$ 800,000			$ 800,000	$ 800,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Classification of Company's Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Operating lease liabilities
Current portion included in accrued expenses and other current liabilities	$ 410	$ 0
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccruedLiabilitiesCurrent
Non-current portion included in other long-term liabilities	$ 8,112
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent
Total operating lease liabilities	$ 8,522

Commitments and Contingencies_3

Commitments and Contingencies - Summary of Components of Lease Costs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Leases [Abstract]
Operating lease cost	$ 1,118
Variable cost	346
Total lease costs	$ 1,464

Commitments and Contingencies_4

Commitments and Contingencies - Summary of Future Minimum Lease Payments Required Under Operating Leases (Details) $ in Thousands	Dec. 31, 2019USD ($)
Operating Lease Liabilities Payments Due [Abstract]
2020	$ 1,233
2021	1,270
2022	1,308
2023	1,348
2024	1,388
Thereafter	6,916
Total minimum future lease payments	13,463
Present value adjustment	4,941
Operating lease liabilities	$ 8,522

Commitments and Contingencies_5

Commitments and Contingencies - Summary of Future Minimum Lease Payments Required Under Operating Leases (Parenthetical) (Details) $ in Thousands	Dec. 31, 2019USD ($)
Commitments And Contingencies [Line Items]
Minimum future lease payments	$ 13,463
2019 San Carlos Lease Agreement
Commitments And Contingencies [Line Items]
Minimum future lease payments	$ 77,600

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Jul. 23, 2018
Stockholders Equity Note [Abstract]
Total number of shares authorized			220,000,000
Common stock, shares authorized	200,000,000	200,000,000	200,000,000
Preferred stock, shares authorized	20,000,000	20,000,000	20,000,000
Common stock, shares issued	48,667,809	42,117,000
Common stock, shares outstanding	48,667,809	42,117,000
Common stock dividends declared	$ 0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Stockholders' Equity - Summary

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Details) - shares shares in Thousands	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Common shares reserved for future issuance	12,300	11,096
Exercise of Common Stock Options Outstanding
Class Of Stock [Line Items]
Common shares reserved for future issuance	7,148	7,811
Shares Reserved for Issuance Under Equity Incentive Plans
Class Of Stock [Line Items]
Common shares reserved for future issuance	3,762	2,785
Vesting of Restricted Stock Units
Class Of Stock [Line Items]
Common shares reserved for future issuance	542	0
Shares Reserved for Issuance Under Employee Stock Purchase Plans
Class Of Stock [Line Items]
Common shares reserved for future issuance	848	500

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Total Stock-Based Compensation Expense Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 15,764	$ 4,570	$ 402
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	5,351	1,792	175
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 10,413	$ 2,778	$ 227

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jul. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Tax benefits for stock-based compensation expense recognized		$ 0	$ 0	$ 0
Common stock shares reserved for issuance		12,300,000	11,096,000
Weighted-average fair value of options granted		$ 28.66	$ 11.05	$ 0.54
Aggregate fair value of stock options vested		$ 12,600,000	$ 1,800,000	$ 200,000
Aggregate intrinsic value of stock options exercised		55,800,000	2,000,000	100,000
Total unrecognized stock-based compensation expense relating to unvested stock options		41,600,000
Total stock-based compensation expense		$ 15,764,000	4,570,000	$ 402,000
ESPP Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance	500,000
Increase in number of common stock shares reserved for issuance per year	1,000,000
Percentage increase in shares reserved for issuance per year	1.00%
Weighted-average recognition period		1 year
Total unrecognized stock-based compensation expense		$ 600,000
Price as a percentage of fair market value of common stock	85.00%
Consecutive overlapping offering period	24 months
First offering period commence date	Jul. 18, 2018
First offering period end date	Aug. 15, 2020
Second offering period commence date	Feb. 15, 2019
Third offering period commence date	Aug. 16, 2019
Total stock-based compensation expense		$ 700,000	$ 200,000
Performance-Based Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted		0	0
Market-Based Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted		0	0
Options to Purchase Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		7,148,000	7,811,000
Weighted-average recognition period		2 years 10 months 24 days
Restricted Stock Awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of restricted common stock vested		$ 20,000	$ 24,000
Shares of restricted common stock unvested		0	47,000
Unvested Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance		542,000	0
Weighted-average recognition period		3 years 10 months 24 days
Shares of restricted common stock unvested		542,000	0
Weighted-average fair value of RSUs granted		$ 93.67
Number of RSUs granted		542,000	0	0
Total unrecognized stock-based compensation expense		$ 48,700,000
Aggregate intrinsic value of RSUs		$ 51,700,000
2018 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock shares reserved for issuance	4,000,000
Increase in number of common stock shares reserved for issuance per year	5,000,000
Percentage increase in shares reserved for issuance per year	5.00%
Award vesting period	4 years
Maximum number of additional shares authorized to be added to the Plan		5,276,050
2018 Equity Incentive Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option expiration period	10 years
2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	4 years
2012 Equity Incentive Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option expiration period	10 years

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate Intrinsic Value, Exercised	$ 55,800	$ 2,000	$ 100
2018 and 2012 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options Outstanding, Beginning balance	7,811
Options Outstanding, Granted	645
Options Outstanding, Exercised	(1,251)
Options Outstanding, forfeited	(57)
Options Outstanding, Ending balance	7,148	7,811
Options Outstanding, Options exercisable	3,982
Options Outstanding, Options vested and expected to vest	7,119
Weighted Average Exercise Price, Beginning balance	$ 8.60
Weighted Average Exercise Price, Granted	46.75
Weighted Average Exercise Price, Exercised	2.11
Weighted Average Exercise Price, Forfeited	35.60
Weighted Average Exercise Price, Ending balance	12.96	$ 8.60
Weighted Average Exercise Price, Options exercisable	5.10
Weighted Average Exercise Price, Options vested and expected to vest	$ 12.91
Weighted Average Remaining Years	8 years	8 years 2 months 12 days
Weighted Average Remaining Years, Options exercisable	7 years 7 months 6 days
Weighted Average Remaining Years, Options vested and expected to vest	8 years
Aggregate Intrinsic Value, Beginning balance	$ 342,292
Aggregate Intrinsic Value, Ending balance	589,114	$ 342,292
Aggregate Intrinsic Value, Options exercisable	359,399
Aggregate Intrinsic Value, Options vested and expected to vest	$ 587,065

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Weighted-Average Assumptions Used to Calculate Fair Value of Stock Options Granted and ESPP Shares (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	1.91%	2.79%	1.83%
Expected volatility	67.22%	73.47%	77.59%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 years 3 days	6 years 3 days	6 years 29 days
ESPP Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate	2.37%	2.42%
Expected volatility	64.26%	65.92%
Expected dividend yield	0.00%	0.00%
Expected term (in years)	1 year 2 months 19 days	1 year 2 months 26 days

Stock-Based Compensation - Su_4

Stock-Based Compensation - Summary of Restricted Common Stock Activity (Details) - Restricted Stock Awards	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Number of Shares
Balance at December 31, 2018 | shares	47,000
Vested | shares	(47,000)
Balance at December 31, 2019 | shares	0
Weighted-Average Grant Date Fair Value
Balance at December 31, 2018 | $ / shares	$ 0.43
Vested | $ / shares	0.43
Balance at December 31, 2019 | $ / shares	$ 0

Stock-Based Compensation - Su_5

Stock-Based Compensation - Summary of RSU Activity Under 2018 Plan (Details) - Unvested Restricted Stock Units - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Balance at December 31, 2018	0
Granted	542,000	0	0
Balance at December 31, 2019	542,000	0
Weighted-Average Grant Date Fair Value
Balance at December 31, 2018	$ 0
Granted	93.67
Balance at December 31, 2019	$ 93.67	$ 0

Income Taxes - Summary of Total

Income Taxes - Summary of Total Deferred Income Tax Assets, Net of Valuation Allowance (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets
Net operating loss carryforwards	$ 47,003	$ 24,091
Research and development credits	8,644	3,995
Accruals and reserves	1,199	260
Stock-based compensation	2,870	611
Lease liability	1,798	0
Gross deferred tax assets	61,514	28,957
Less: valuation allowance	(59,901)	(28,892)
Deferred tax assets, net of valuation allowance	1,613	65
Deferred tax liabilities
Fixed and intangible assets	395	65
Right-of-use asset	1,218	0
Gross deferred tax liabilities	1,613	65
Net deferred tax assets	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Deferred tax assets, valuation allowance	$ 59,901,000	$ 28,892,000
Decrease in valuation allowance	31,000,000	10,800,000
Federal NOL carryforwards	210,200,000
State NOL carryforwards	42,400,000
Research and development credits	8,644,000	3,995,000
Accrual for interest and penalties	$ 0	$ 0
Federal statutory tax rate	21.00%	21.00%	34.00%
Maximum
Income Tax Disclosure [Line Items]
Federal statutory tax rate			35.00%
Federal
Income Tax Disclosure [Line Items]
NOL carryforwards, expiration year	2032
Research and development credits	$ 8,800,000
Research and development tax credit carryforwards, expiration year	2033
Federal | Generated After December 31, 2017
Income Tax Disclosure [Line Items]
Federal NOL carryforwards	$ 148,300,000
Federal | 2032 Expiration Period
Income Tax Disclosure [Line Items]
Federal NOL carryforwards	$ 61,900,000
State
Income Tax Disclosure [Line Items]
NOL carryforwards, expiration year	2032
Research and development credits	$ 4,000,000
Tax Cut And Jobs Act
Income Tax Disclosure [Line Items]
Federal statutory tax rate	21.00%

Income Taxes - Summary of Effec

Income Taxes - Summary of Effective Tax Rate Differs From Federal Statutory Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	34.00%
Change in deferred tax asset valuation allowance	(36.40%)	(24.80%)
State taxes, net of federal benefit	1.10%	0.90%
Research and development tax credits	4.40%	3.30%
Stock-based compensation	9.90%	(0.20%)
Other	0.00%	(0.20%)
Effective tax rate	0.00%	0.00%

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance at the beginning of the year	$ 1,827	$ 1,149
Increase related to current year tax positions	1,718	678
Balance at the end of the year	$ 3,545	$ 1,827

Defined Contribution Plans - Ad

Defined Contribution Plans - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
401(K) Plan
Defined Contribution Plan Disclosure [Line Items]
Contributions made by employer	$ 0.5	$ 0.3
Employer contribution matching percentage	4.00%
SIMPLE IRA Plan
Defined Contribution Plan Disclosure [Line Items]
Contributions made by employer			$ 0.1

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Results (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Loss from operations	$ (26,850)	$ (23,584)	$ (20,057)	$ (20,927)	$ (15,513)	$ (11,975)	$ (9,524)	$ (8,709)	$ (91,418)	$ (45,721)	$ (22,254)
Net loss	$ (24,615)	$ (21,732)	$ (19,072)	$ (19,953)	$ (14,528)	$ (11,148)	$ (9,377)	$ (8,485)	$ (85,372)	$ (43,538)	$ (23,552)
Net loss per share, basic and diluted	$ (0.51)	$ (0.47)	$ (0.44)	$ (0.47)	$ (0.35)	$ (0.34)	$ (4.17)	$ (4.19)	$ (1.89)	$ (2.20)	$ (14.54)