IDEAYA Biosciences (IDYA) 10-Q 13 Nov 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Nov. 01, 2019
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Trading Symbol	IDYA
Entity Registrant Name	IDEAYA Biosciences, Inc.
Entity Central Index Key	0001676725
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Shell Company	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		20,331,095
Title of 12(b) Security	Common Stock, $0.0001 par value per share
Security Exchange Name	NASDAQ
Entity File Number	001-38915
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4268251
Entity Address, Address Line One	7000 Shoreline Court
Entity Address, Address Line Two	Suite 350
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	443-6209
Document Quarterly Report	true
Document Transition Report	false

Condensed Balance Sheets

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 38,848	$ 20,505
Short-term marketable securities	70,508	69,456
Prepaid expenses and other current assets	2,805	706
Total current assets	112,161	90,667
Restricted cash	106	106
Property and equipment, net	4,767	5,152
Deferred offering costs		570
Right-of-use assets	5,279
Other non-current assets	18	46
Total assets	122,331	96,541
Current liabilities
Accounts payable	763	1,417
Accrued liabilities	3,992	3,583
Deferred rent		299
Lease liabilities	1,116
Other current liabilities	76	114
Total current liabilities	5,947	5,413
Deferred rent		1,572
Long-term lease liabilities	5,926
Other non-current liabilities	52	113
Total liabilities	11,925	7,098
Commitments and contingencies (Note 5)
Stockholder’s equity (deficit)
Preferred stock, $0.0001 par value, 10,000,000 shares authorized as of September 30, 2019 and no shares authorized as of December 31, 2018; no shares issued and outstanding as of September 30, 2019 and December 31, 2018
Common stock, $0.0001 par value, 300,000,000 shares authorized as of September 30, 2019 and 170,800,000 shares authorized as of December 31, 2018; 20,328,242 and 1,335,690 shares issued and outstanding as of September 30, 2019 and December 31, 2018	2
Additional paid-in capital	192,036	1,599
Accumulated other comprehensive income (loss)	78	(31)
Accumulated deficit	(81,710)	(50,516)
Total stockholders’ equity (deficit)	110,406	(48,948)
Total liabilities, redeemable convertible preferred stock and stockholders’ equity (deficit)	$ 122,331	96,541
Series A Redeemable Convertible Preferred Stock
Current liabilities
Temporary equity, value		40,735
Series B Redeemable Convertible Preferred Stock
Current liabilities
Temporary equity, value		$ 97,656

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	10,000,000	0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	170,800,000
Common stock, shares issued	20,328,242	1,335,690
Common stock, shares outstanding	20,328,242	1,335,690
Series A Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	59,433,105
Redeemable convertible preferred stock, shares issued	0	5,794,727
Redeemable convertible preferred stock, shares outstanding	0	5,794,727
Redeemable convertible preferred stock, liquidation value	$ 0	$ 47,178
Series B Redeemable Convertible Preferred Stock
Redeemable convertible preferred stock, par value	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock, shares authorized	0	75,500,000
Redeemable convertible preferred stock, shares issued	0	7,345,067
Redeemable convertible preferred stock, shares outstanding	0	7,345,067
Redeemable convertible preferred stock, liquidation value	$ 0	$ 97,520

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses
Research and development	$ 8,923	$ 12,538	$ 25,778	$ 24,172
General and administrative	2,700	1,088	7,174	3,112
Total operating expenses	11,623	13,626	32,952	27,284
Loss from operations	(11,623)	(13,626)	(32,952)	(27,284)
Interest income	654	573	1,752	1,423
Other income (expense), net		1	6	77
Net loss	(10,969)	(13,052)	(31,194)	(25,784)
Change in unrealized gains (losses) on marketable securities	41	16	109	(16)
Comprehensive loss	$ (10,928)	$ (13,036)	$ (31,085)	$ (25,800)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.54)	$ (13.06)	$ (3.15)	$ (27.94)
Weighted average number of shares outstanding, basic and diluted	20,158,223	999,369	9,895,574	922,664

Condensed Statements of Redeema

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred StockLicense Agreement	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2017		$ 26,084
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2017		4,379,916
Issuance of redeemable convertible preferred stock, value			$ 14,651	$ 93,844	$ 3,812
Issuance of redeemable convertible preferred stock, shares			1,414,811	7,081,452	263,615
Redeemable convertible preferred stock, ending balance at Sep. 30, 2018		$ 138,391
Redeemable convertible preferred stock, ending balance, shares at Sep. 30, 2018		13,139,794
Beginning balance at Dec. 31, 2017	$ (15,739)					$ 0	$ 432	$ (1)	$ (16,170)
Beginning balance, shares at Dec. 31, 2017						1,263,416
Issuance of common stock upon exercise of stock options	25					$ 0	25	0	0
Issuance of common stock upon exercise of stock options, shares						16,311
Early exercised common stock options	0					$ 0	0	0	0
Early exercised common stock options, shares						70,583
Repurchase of early exercised shares	$ 0					$ 0	0	0	0
Repurchase of early exercised shares, shares	(7,914)					(7,913)
Vesting of early exercised common stock options and restricted stock	$ 152					$ 0	152	0	0
Stock-based compensation	625					0	625	0	0
Other comprehensive income (loss)	(16)					0	0	(16)	0
Net loss	(25,784)					0	0	0	(25,784)
Ending balance at Sep. 30, 2018	(40,737)					$ 0	1,234	(17)	(41,954)
Ending balance, shares at Sep. 30, 2018						1,342,397
Redeemable convertible preferred stock, beginning balance at Jun. 30, 2018		$ 134,579
Redeemable convertible preferred stock, beginning balance, shares at Jun. 30, 2018		12,876,179
Issuance of redeemable convertible preferred stock, value					$ 3,812
Issuance of redeemable convertible preferred stock, shares					263,615
Redeemable convertible preferred stock, ending balance at Sep. 30, 2018		$ 138,391
Redeemable convertible preferred stock, ending balance, shares at Sep. 30, 2018		13,139,794
Beginning balance at Jun. 30, 2018	(28,049)					$ 0	886	(33)	(28,902)
Beginning balance, shares at Jun. 30, 2018						1,331,316
Issuance of common stock upon exercise of stock options	23					$ 0	23	0	0
Issuance of common stock upon exercise of stock options, shares						13,656
Early exercised common stock options	0					$ 0	0	0	0
Early exercised common stock options, shares						5,338
Repurchase of early exercised shares	0					$ 0	0	0	0
Repurchase of early exercised shares, shares						(7,913)
Vesting of early exercised common stock options and restricted stock	38					$ 0	38	0	0
Stock-based compensation	287					0	287	0	0
Other comprehensive income (loss)	16					0	0	16	0
Net loss	(13,052)					0	0	0	(13,052)
Ending balance at Sep. 30, 2018	(40,737)					$ 0	1,234	(17)	(41,954)
Ending balance, shares at Sep. 30, 2018						1,342,397
Redeemable convertible preferred stock, beginning balance at Dec. 31, 2018		$ 138,391	$ 40,735	$ 97,656
Redeemable convertible preferred stock, beginning balance, shares at Dec. 31, 2018		13,139,794	5,794,727	7,345,067
Conversion of redeemable convertible preferred stock into common stock		$ (138,391)
Conversion of redeemable convertible preferred stock into common stock, shares		(13,139,794)
Redeemable convertible preferred stock, ending balance, shares at Sep. 30, 2019		0	0	0
Beginning balance at Dec. 31, 2018	(48,948)					$ 0	1,599	(31)	(50,516)
Beginning balance, shares at Dec. 31, 2018						1,335,690
Conversion of redeemable convertible preferred stock into common stock	138,391					$ 1	138,390	0	0
Conversion of redeemable convertible preferred stock into common stock, shares						13,139,794
Issuance of common stock upon initial public offering, net of issuance cost	50,247					$ 1	50,246	0	0
Issuance of common stock upon initial public offering, net of issuance cost, shares						5,750,000
Issuance of common stock upon exercise of stock options	261					$ 0	261	0	0
Issuance of common stock upon exercise of stock options, shares						114,890
Early exercised common stock options	0					$ 0	0	0	0
Early exercised common stock options, shares						2,112
Repurchase of early exercised shares	$ (2)					$ 0	(2)	0	0
Repurchase of early exercised shares, shares	(14,244)					(14,244)
Vesting of early exercised common stock options and restricted stock	$ 93					$ 0	93	0	0
Stock-based compensation	1,449					0	1,449	0	0
Other comprehensive income (loss)	109					0	0	109	0
Net loss	(31,194)					0	0	0	(31,194)
Ending balance at Sep. 30, 2019	110,406					$ 2	192,036	78	(81,710)
Ending balance, shares at Sep. 30, 2019						20,328,242
Redeemable convertible preferred stock, ending balance, shares at Sep. 30, 2019		0	0	0
Beginning balance at Jun. 30, 2019	120,608					$ 2	191,310	37	(70,741)
Beginning balance, shares at Jun. 30, 2019						20,268,103
Issuance of common stock upon exercise of stock options	190					$ 0	190	0	0
Issuance of common stock upon exercise of stock options, shares						62,501
Repurchase of early exercised shares	(2)					$ 0	(2)	0	0
Repurchase of early exercised shares, shares						(2,362)
Vesting of early exercised common stock options and restricted stock	20					$ 0	20	0	0
Stock-based compensation	518					0	518	0	0
Other comprehensive income (loss)	41					0	0	41	0
Net loss	(10,969)					0	0	0	(10,969)
Ending balance at Sep. 30, 2019	$ 110,406					$ 2	$ 192,036	$ 78	$ (81,710)
Ending balance, shares at Sep. 30, 2019						20,328,242

Condensed Statements of Redee_2

Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) (Parenthetical) $ in Thousands	9 Months Ended
	Sep. 30, 2018USD ($)
Series A Redeemable Convertible Preferred Stock
Stock issuance cost	$ 5
Redeemable convertible preferred stock liability	3,137
Series B Redeemable Convertible Preferred Stock
Stock issuance cost	$ 176

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (31,194)	$ (25,784)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	915	623
Net amortization of premiums and discounts on marketable securities	(451)	(526)
Stock-based compensation	1,449	625
Change in fair value of redeemable convertible preferred stock liability		(70)
Landlord contributions for leasehold improvements		129
Loss (gain) on sale of property and equipment	14	(41)
Gain on sales of marketable securities	(7)	(11)
Changes in assets and liabilities
Prepaid expenses and other assets	(2,071)	(109)
Right-of-use assets	870
Accounts payable	(153)	352
Accrued and other liabilities	1,000	917
Deferred rent		(60)
Lease liabilities	(979)
Net cash used in operating activities	(30,607)	(20,143)
Cash flows from investing activities
Purchases of property and equipment, net	(1,137)	(1,400)
Purchases of marketable securities	(86,117)	(112,155)
Maturities of marketable securities	67,538	40,800
Sales of marketable securities	18,094	3,000
Net cash used in investing activities	(1,622)	(69,755)
Cash flows from financing activities
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		105,358
Proceeds from issuance of common stock in initial public offering, net of issuance costs	50,321
Proceeds from exercise of common stock options	251	97
Net cash provided by financing activities	50,572	105,455
Net increase in cash, cash equivalents and restricted cash	18,343	15,557
Cash, cash equivalents and restricted cash, at beginning of period	20,611	6,032
Cash, cash equivalents and restricted cash, at end of period	38,954	21,589
Reconciliation of cash, cash equivalents and restricted cash
Cash and cash equivalents	38,848	21,483
Restricted cash	106	106
Cash, cash equivalents and restricted cash, at end of period	38,954	21,589
Supplemental disclosure of cash flow information:
Cash paid for income taxes	1	1
Cash paid for interest	69	96
Supplemental non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	88	134
Purchases of property and equipment in accounts payable and accrued liabilities	11	180
Extinguishment of redeemable convertible preferred stock liability		3,137
Conversion of redeemable convertible preferred stock into common stock	$ 138,391
Series B Redeemable Convertible Preferred Stock
Adjustments to reconcile net loss to net cash used in operating activities
Issuance of Series B redeemable convertible preferred stock pursuant to license agreement		3,812
Supplemental non-cash investing and financing activities:
Issuance of Series B redeemable convertible preferred stock pursuant to license agreement		$ 3,812

Organization

Organization	9 Months Ended
	Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization Description of the Business IDEAYA Biosciences, Inc. (the “Company”) is an oncology-focused precision medicine company committed to the discovery and development of targeted therapeutics for patient populations selected using molecular diagnostics. The Company is headquartered in South San Francisco, California and was incorporated in the state of Delaware in June 2015. To date, the Company has been primarily engaged in business planning, research, development, recruiting and raising capital. Initial Public Offering On May 22, 2019, the Company’s registration statement on Form S-1 (File No. 333-231081) relating to its initial public offering (“IPO”) of common stock became effective. The IPO closed on May 28, 2019 at which time the Company issued 5,750,000 shares of its common stock at a price of $10.00 per share, which included shares issued upon the underwriters’ exercise of their overallotment option to purchase 750,000 additional shares. In addition, upon closing the IPO, all outstanding shares of the redeemable convertible preferred stock converted into 13,139,794 shares of common stock. The Company received an aggregate of $50.2 million in cash, net of underwriting discounts and commissions, and after deducting offering costs. Liquidity The Company has incurred significant losses and negative cash flows from operations in all periods since inception and had an accumulated deficit of $81.7 million as of September 30, 2019. The Company has historically financed its operations primarily through the sale of redeemable convertible preferred stock, common stock and convertible notes. To date, none of the Company’s product candidates have been approved for sale, and the Company has not generated any revenue since inception. Management expects operating losses to continue and increase for the foreseeable future, as the Company progresses into clinical development activities for its lead product candidates. The Company’s prospects are subject to risks, expenses and uncertainties frequently encountered by companies in the biotechnology industry as discussed above. While the Company has been able to raise multiple rounds of financing, there can be no assurance that in the event the Company requires additional financing, such financing will be available on terms which are favorable or at all. Failure to generate sufficient cash flows from operations, raise additional capital or reduce certain discretionary spending would have a material adverse effect on the Company’s ability to achieve its intended business objectives. As of September 30, 2019, the Company had cash, cash equivalents and short-term marketable securities of $109.4 million. Management believes that the Company’s current cash, cash equivalents and short-term marketable securities will be sufficient to fund its planned operations for at least 12 months from the date of the issuance of these financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The unaudited condensed financial statements and accompanying notes have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”). Certain footnotes or other financial information that are normally required by GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2018 has been derived from the audited financial statements at that date but does not include all of the information required by GAAP for complete financial statements. The accompanying balance sheet as of September 30, 2019, the statements of operations and comprehensive loss and of redeemable convertible preferred stock and stockholders’ equity (deficit) for the three and nine months ended September 30, 2019 and September 30, 2018, and the statements of cash flows for the nine months ended September 30, 2019 and September 30, 2018 are unaudited. In the opinion of management, the unaudited data reflects all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2019, the results of its operations and comprehensive loss for the three and nine months ended September 30, 2019 and September 30, 2018 and its cash flows for the nine months ended September 30, 2019 and September 30, 2018. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2019 and September 30, 2018 are also unaudited. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods or any future year or period. The accompanying interim unaudited condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto for the year ended December 31, 2018, which are included in the Company’s prospectus related to the Company’s IPO, filed with the SEC on May 24, 2019 (the “final prospectus”), pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “Securities Act”). Reverse Stock Split In May 2019, the Company’s board of directors approved a 1-for-10.2564 reverse stock split of the Company’s common stock and redeemable convertible preferred stock, which was effected on May 21, 2019. The par value and authorized shares of the common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. All issued and outstanding common stock, options to purchase common stock and per share amounts contained in these financial statements have been retroactively adjusted to give effect to the reverse stock split for all periods presented. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Such estimates include the valuation of the redeemable convertible preferred stock liability, deferred tax assets, useful lives of property and equipment, determination of the discount rate for operating leases and stock-based compensation. Actual results could differ from those estimates. Risks and Uncertainties The Company operates in a dynamic and highly competitive industry and is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, contract manufacturer and contract research organizations, compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical studies and clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance and reporting. The Company believes that changes in any of the following areas could have a material adverse effect on the Company’s future financial position, results of operations, or cash flows; ability to obtain future financing; advances and trends in new technologies and industry standards; results of clinical trials; regulatory approval and market acceptance of the Company’s products; development of sales channels; certain strategic relationships; litigation or claims against the Company base in intellectual property, patent, product, regulatory, or other factors; and the Company’s ability to attract and retain employees necessary to support its growth. Products developed by the Company require approvals from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained or maintained, that the products will receive the necessary approvals, or that any approved products will be commercially viable. If the Company was denied approval, approval was delayed or the Company was unable to maintain approval, it could have a materially adverse impact on the Company. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and other third parties. The Company has expended and will continue to expend substantial funds to complete the research, development and clinical testing of product candidates. The Company also will be required to expend additional funds to establish commercial-scale manufacturing arrangements and to provide for the marketing and distribution of products that receive regulatory approval. The Company may require additional funds to commercialize its products. The Company is unable to entirely fund these efforts with its current financial resources. If adequate funds are unavailable on a timely basis from operations or additional sources of financing, the Company may have to delay, reduce the scope of or eliminate one or more of its research or development programs which would materially and adversely affect its business, financial condition and operations. Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and short-term marketable securities. Substantially all the Company’s cash is held by one financial institution that management believes is of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company’s investment policy addresses credit ratings, diversification, and maturity dates. The Company invests its cash equivalents and short-term marketable securities in money market funds, U.S. government securities, commercial paper, corporate bonds, and asset-backed securities. The Company limits its credit risk associated with cash equivalents and short-term marketable securities by placing them with banks and institutions it believes are highly creditworthy and in highly rated investments and, by policy, limits the amount of credit exposure with any one commercial issuer. The Company has not experienced any credit losses on its deposits of cash, cash equivalents or short-term marketable securities. Summary of Significant Accounting Policies There have been no material changes in the accounting policies from those disclosed in the financial statements and the related notes included in the final prospectus. Deferred Initial Public Offering Costs Deferred offering costs, consisting of legal, accounting and other fees and costs relating to the Company’s IPO, are capitalized and recorded on the balance sheet. As of December 31, 2018, $0.6 million of deferred offering costs were recorded on the balance sheet. During the nine months ended September 30, 2019, an additional $2.7 million in deferred offering costs were incurred. In connection with the IPO, all deferred offering costs were charged against the proceeds from the IPO included in additional paid-in capital on the accompanying condensed balance sheet as of September 30, 2019. Leases The Company leases its facilities and meets the requirements to account for these leases as operating leases. For the three and nine months ended September 30, 2018, for facility leases that contain rent escalations or rent concession provisions, the Company records its lease expense during the lease term on a straight-line basis over the term of the lease. As of December 31, 2018, the Company recorded the difference between the rent paid and the straight-line rent as a deferred rent liability. As of December 31, 2018, leasehold improvements funded by landlord incentives or allowances were recorded as leasehold improvement assets and a corresponding deferred rent liability. The leasehold improvement asset is amortized over the lesser of the term of the lease or life of the asset. For the three and nine months ended September 30, 2018, the deferred rent liability is amortized on a straight-line basis as a reduction to rent expense over the term of the lease agreement. Upon adoption of ASC 842, Leases ROU assets and lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The ROU asset also includes any lease payments made to the lessor at or before the commencement date, minus lease incentives received, and initial direct costs incurred. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of ASC 842, the Company combines lease and nonlease components. Net Loss per Share Attributable to Common Stockholders Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock, stock options and common stock subject to repurchase related to unvested restricted stock awards and early exercise of stock options are considered to be potentially dilutive securities. Basic and diluted net loss attributable to common stockholders per share is presented in conformity with the two-class method required for participating securities as the redeemable convertible preferred stock is considered a participating security because it participates in dividends with common stock. The Company also considers the shares issued upon the early exercise of stock options subject to repurchase to be participating securities, because holders of such shares have non-forfeitable dividend rights in the event a dividend is paid on common stock. The holders of all series of redeemable convertible preferred stock and the holders of early exercised shares subject to repurchase do not have a contractual obligation to share in the Company’s losses. As such, the net loss was attributed entirely to common stockholders. Because the Company has reported a net loss for all periods presented, diluted net loss per common share is the same as basic net loss per common share for those periods. Recently Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) under its accounting standard codifications (“ASC”) or other standard setting bodies and adopted by the Company as of the specified effective date, unless otherwise discussed below. Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases Codification Improvements to Topic 842, Leases Leases (Topic 842): Targeted Improvements Leases (Topic 842): Codification Improvements. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Non-employee Share-Based Payment Accounting New Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.

Fair Value Measurements and Sho

Fair Value Measurements and Short-Term Marketable Securities	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements and Short-Term Marketable Securities	3. Fair Value Measurements and Short-Term Marketable Securities The Company applies fair value accounting for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, a three-tier fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows: Level 1—Observable inputs, such as quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Observable inputs other than Level 1 prices 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 which reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. In determining fair value, the Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible as well as considers counterparty credit risk in its assessment of fair value. As of September 30, 2019, financial assets measured and recognized at fair value are as follows (in thousands): September 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Assets U.S. government securities Level 1 $ 24,948 $ 19 $ — $ 24,967 Corporate bonds Level 2 41,108 59 — 41,167 Commercial paper Level 2 4,374 — — 4,374 Marketable securities 70,430 78 — 70,508 Money market funds (1) Level 1 38,680 — — 38,680 Total fair value of assets $ 109,110 $ 78 $ — $ 109,188 (1) Included in cash and cash equivalents on the balance sheet As of December 31, 2018, financial assets measured and recognized at fair value are as follows (in thousands): December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Assets U.S. government securities Level 1 $ 9,983 $ — $ (1 ) $ 9,982 Corporate bonds Level 2 18,820 — (19 ) 18,801 Commercial paper Level 2 29,699 — — 29,699 Asset-backed securities Level 2 10,984 — (10 ) 10,974 Marketable securities 69,486 — (30 ) 69,456 Money market funds (1) Level 1 20,419 — — 20,419 Total fair value of assets $ 89,905 $ — $ (30 ) $ 89,875 (1) Included in cash and cash equivalents on the balance sheet As of September 30, 2019 and December 31, 2018, marketable securities had a remaining maturity of twelve months or less. There were no financial liabilities measured and recognized at fair value as of September 30, 2019 and December 31, 2018.

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Balance Sheet Components	4. Balance Sheet Components Property and Equipment, Net Property and equipment, net consists of the following (in thousands): Useful Life September 30, December 31, (In Years) 2019 2018 Laboratory equipment 5 $ 3,830 $ 3,335 Computer equipment 3 117 117 Software 3 118 109 Leasehold improvements Shorter of useful life or lease term 2,581 2,570 Furniture and fixtures 5 308 304 Total property and equipment 6,954 6,435 Less: Accumulated depreciation and amortization (2,187 ) (1,283 ) Property and equipment, net $ 4,767 $ 5,152 Depreciation and amortization expense was $0.3 million and $0.2 million for the three months ended September 30, 2019 and September 30, 2018, respectively, and $0.9 million and $0.6 million for the nine months ended September 30, 2019 and September 30, 2018, respectively. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): September 30, December 31, 2019 2018 Accrued salaries and benefits $ 1,504 $ 1,279 Accrued research and development expenses 1,870 1,095 Legal and professional fees 486 999 Other 132 210 Accrued liabilities $ 3,992 $ 3,583

Leases, Lease Commitments and C

Leases, Lease Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Leases, Lease Commitments and Contingencies	5. Leases, Lease Commitments and Contingencies Operating Leases The Company leases its laboratory and office facilities in South San Francisco, California, and through September 30, 2019 in San Diego, California under non-cancelable operating leases with expiration dates in July 2024 and March 2020, respectively. On September 30, 2019, the Company and the landlord of the laboratory and office facilities in South San Francisco entered into a second amendment (“Second Amendment”) to lease additional office spaces at the same location. The Company will account for the Second Amendment as a separate contract and will recognize a related ROU asset and lease liability on the lease commencement date of the additional office spaces, which is currently expected in early 2020. On September 30, 2019, the Company and the landlord of the laboratory and office facilities in San Diego agreed to terminate the lease. Accordingly, the Company derecognized the remaining balances of the ROU asset and lease liability. As part of the lease agreement for the laboratory and office facilities in South San Francisco, the Company was provided a tenant improvement allowance of $1.6 million. In May 2018, the Company entered into an amendment to the lease agreement to expand the size of the laboratory and office facilities leased in South San Francisco, California by adding additional space to the lease commencing June 1, 2018. As part of the amendment, the landlord provided an additional tenant improvement allowance of $0.4 million. The Company is amortizing the tenant improvement allowances over the term of the lease agreement. Rent expense was $0.4 million and $1.0 million for the three and nine months ended September 30, 2018, respectively. As of December 31, 2018, $1.9 million of deferred rent representing future minimum rental payments for leases with scheduled rent escalations and tenant improvement allowances was included in current and long-term liabilities. Future minimum lease payments under the non-cancelable operating leases as of December 31, 2018 are as follows (in thousands): As of December 31, 2018: Operating Leases 2019 $ 1,711 2020 1,611 2021 1,604 2022 1,646 2023 1,690 Thereafter 1,441 Total lease payments $ 9,703 The maturities of operating lease liabilities as of September 30, 2019 are as follows (in thousands): As of September 30, 2019 Operating Leases Remaining fiscal 2019 $ 386 2020 1,564 2021 1,604 2022 1,646 2023 1,690 Thereafter 1,441 Total lease payments 8,331 Less: Interest (1,289 ) Present value of lease liabilities $ 7,042 Amounts recognized on the balance sheet Current lease liabilities $ 1,116 Long-term lease liabilities 5,926 Total lease liabilities $ 7,042 Operating lease expense was $0.4 million and $1.2 million for the three and nine months ended September 30, 2019, respectively. As of September 30, 2019, the ROU assets of $5.3 million are included in non-current assets on the balance sheet, and lease liabilities of $7.0 million are included in current liabilities and non-current liabilities on the balance sheet. As of September 30, 2019, the remaining term for the operating lease in South San Francisco, California is 4.8 years, and the discount rate is 7.0%. During the nine months ended September 30, 2019, cash paid for amounts included in operating lease liabilities of $1.3 million is included in cash flows from operating activities on the statement of cash flows. Contingencies From time to time, the Company may be involved in litigation related to claims that arise in the ordinary course of its business activities. The Company accrues for these matters when it is probable that future expenditures will be made and these expenditures can be reasonably estimated. As of September 30, 2019 and December 31, 2018, the Company does not believe that any such matters, individually or in the aggregate, will have a material adverse effect on the Company’s financial position, results of operations or cash flows. Indemnification The Company enters into standard indemnification arrangements in the ordinary course of business. Pursuant to these arrangements, the Company indemnifies, holds harmless and agrees to reimburse the indemnified parties for losses suffered or incurred by the indemnified party. The term of these indemnification agreements is generally perpetual any time after the execution of the agreement. The maximum potential amount of future payments the Company could be required to make under these arrangements is not determinable. The Company has never incurred costs to defend lawsuits or settle claims related to these indemnification agreements. As a result, the Company believes the fair value of these agreements is not material.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes The Company did not record a federal or state income tax provision or benefit for the three and nine months ended September 30, 2019 and September 30, 2018 as it has incurred net losses since inception. In addition, the net deferred tax assets generated from net operating losses are fully offset by a valuation allowance as the Company believes it is not more likely than not that the benefit will be realized.

Redeemable Convertible Preferre

Redeemable Convertible Preferred Stock	9 Months Ended
	Sep. 30, 2019
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	7. Redeemable Convertible Preferred Stock Issued and outstanding redeemable convertible preferred stock and its principal terms as of December 31, 2018 were as follows (in thousands, except share and per share amounts): Redeemable Convertible Preferred Stock Liquidation Carrying Original Issue Price Per Authorized Outstanding Value Value Share Series A redeemable convertible preferred stock 59,433,105 5,794,727 $ 47,178 $ 40,735 $ 8.1415 Series B redeemable convertible preferred stock 75,500,000 7,345,067 97,520 97,656 $ 13.2769 134,933,105 13,139,794 $ 144,698 $ 138,391 Following the closing of the IPO, all outstanding shares of the redeemable convertible preferred stock converted into 13,139,794 shares of common stock and the related carrying value was reclassified to common stock and additional paid-in capital. There were no shares of redeemable convertible preferred stock outstanding as of September 30, 2019.

Common Stock

Common Stock	9 Months Ended
	Sep. 30, 2019
Stockholders Equity Note [Abstract]
Common Stock	8. Common Stock As of September 30, 2019 and December 31, 2018, the Company’s certificate of incorporation authorized the Company to issue 300,000,000 and 170,800,000 shares of common stock, respectively, at a par value of $0.0001 per share. Each share of common stock is entitled to one vote. The holders of common stock are also entitled to receive dividends whenever funds are legally available and when declared by the Board of Directors, subject to prior rights of the preferred stockholders. As of September 30, 2019 and December 31, 2018, no dividends have been declared to date. The Company had reserved common stock, on an as-converted basis, for future issuance as follows: September 30, December 31, 2019 2018 Conversion of Series A redeemable convertible preferred stock — 5,794,727 Conversion of Series B redeemable convertible preferred stock — 7,345,067 Exercise of outstanding options under the 2015 and 2019 Plans 1,921,259 1,598,864 Issuance of common stock options under the 2015 Plan — 535,691 Issuance of common stock options under the 2019 Plan 1,089,038 — Issuance of common stock options under the Employee Stock Purchase Plan 195,000 — Total 3,205,297 15,274,349

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation 2019 Incentive Award Plan The Company’s board of directors adopted and the Company’s stockholders approved, effective on the day prior to the effectiveness of the registration statement on Form S-1, the 2019 Incentive Award Plan (the “2019 Plan”), under which the Company may grant cash and equity-based incentive awards to the Company’s employees, consultants and directors. Following the effectiveness of the 2019 Plan, the Company will not make any further grants under the 2015 Equity Incentive Plan (the “2015 Plan”). However, the 2015 Plan will continue to govern the terms and conditions of the outstanding awards granted under it. Shares of common stock subject to awards granted under the 2015 Plan that are forfeited or lapse unexercised and which following the effective date of the 2019 Plan are not issued under the 2015 Plan will be available for issuance under the 2019 Plan. Options granted under the 2019 Plan may be either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”). ISOs may be granted only to Company employees (including officers and directors who are also employees). NSOs may be granted to Company employees and consultants. The exercise price of an ISO and NSO shall not be less than 100% of the estimated fair value of the shares on the date of grant, respectively. The exercise price of an ISO granted to an employee who, at the time of grant, owns stock representing more than 10% (“10% stockholder”) of the voting power of all classes of stock of the Company shall be no less than 110% of the estimated fair value of the shares on the date of grant. To date, options have a term of 10 years (or five years if granted to a 10% stockholder) and generally vest over a 4-year period with 1-year cliff vesting. 2015 Equity Incentive Plan In 2015, the Company established its 2015 Plan which provides for the granting of stock options to employees and consultants of the Company. Options granted under the 2015 Plan may be either ISOs NSOs. Stock-Based Compensation Expense Total stock-based compensation expense recorded related to awards granted to employees and non-employees was as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development $ 227 $ 124 $ 679 $ 250 General and administrative 291 163 770 375 Total stock-based compensation expense $ 518 $ 287 $ 1,449 $ 625 Stock Options Activity under the Company’s 2015 and 2019 equity incentive plans is set forth below: Outstanding Options Shares available for Grant Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Balance, January 1, 2019 535,691 1,598,864 $ 4.128 9.22 Additional shares authorized 978,500 — Options granted (532,567 ) 532,567 $ 10.127 Options exercised — (117,002 ) $ 2.285 Options repurchased 14,244 — $ 0.835 Options canceled 93,170 (93,170 ) $ 4.805 Balance, September 30, 2019 1,089,038 1,921,259 $ 5.870 8.73 Exercisable as of September 30, 2019 491,434 $ 4.021 8.18 Vested and expected to vest as of September 30, 2019 1,921,259 $ 5.870 8.73 The weighted-average grant-date fair value of options granted during the nine months ended September 30, 2019 and September 30, 2018 was $7.197 and $3.120 per share, respectively. The aggregate intrinsic value of options exercised for the nine months ended September 30, 2019 and September 30, 2018 was $0.7 million and $0.1 million, respectively. Intrinsic values are calculated as the difference between the exercise price of the underlying options and the fair value of the common stock on the date of exercise. As of September 30, 2019 and December 31, 2018, total unrecognized stock-based compensation expense for stock options was $6.2 million and $4.2 million, respectively, which is expected to be recognized over a weighted-average period of 2.92 years and 3.26 years, respectively. The total fair value of options vested for the nine months ended September 30, 2019 and September 30, 2018 was $1.6 million and $0.2 million, respectively. Early Exercise of Stock Options The terms of the 2015 Plan permit the exercise of options granted under the 2015 Plan prior to vesting, subject to required approvals. The shares are subject to the Company’s lapsing repurchase right upon termination of employment at the original purchase price. The proceeds are initially recorded in other liabilities from the early exercise of stock options and are reclassified to additional paid-in capital as the Company’s repurchase right lapses. During the nine months ended September 30, 2019 and September 30, 2018, the Company repurchased 14,244 and 7,914 shares of common stock, respectively. As of September 30, 2019 and December 31, 2018, shares that were subject to repurchase were 116,306 and 237,853, respectively. The aggregate exercise prices of early exercised shares as of September 30, 2019 and December 31, 2018 was $0.1 million and $0.2 million, respectively, which were recorded in other current liabilities and other non-current liabilities. Stock-Based Compensation Associated with Awards to Employees During the nine months ended September 30, 2019 and September 30, 2018, the Company granted stock options to employees to purchase 532,567 and 1,281,150 shares of common stock, respectively. The fair values of options were calculated using the assumptions set forth below: Three Months Ended September 30, 2019 Three Months Ended September 30, 2018 Nine Months Ended September 30, 2019 Nine Months Ended September 30, 2018 Expected term 5.4 - 6.1 years 6.0 - 6.1 years 5.4 - 6.1 years 5.8 - 6.1 years Expected volatility 77.2% - 81.6% 81.6% - 81.9% 77.2% - 82.2% 80.3% - 82.3% Risk-free interest rate 1.4% - 1.9% 2.8% - 2.9% 1.4% - 2.5% 2.6% - 2.9% Dividend yield 0% 0% 0% 0% Expected term . The expected term represents the weighted-average period the stock options are expected to remain outstanding and is based on the options’ vesting terms, contractual terms and industry peers, as the Company does not have sufficient historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior. Expected Volatility . The Company uses an average historical stock price volatility of a peer group of publicly traded companies to be representative of its expected future stock price volatility, as the Company does not have sufficient trading history for its common stock. For purposes of identifying these peer companies, the Company considers the industry, stage of development, size and financial leverage of potential comparable companies. For each grant, the Company measures historical volatility over a period equivalent to the expected term. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. Risk-Free Interest Rate . The risk-free rate assumption is based on U.S. Treasury instruments whose term was consistent with the expected term of the Company’s stock options. Expected Dividend Rate . The Company has not paid and does not anticipate paying any dividends in the near future. Accordingly, the Company has estimated the dividend yield to be zero. The Company accounts for forfeitures as they occur. Fair Value of Common Stock Prior to the IPO, the fair value of the Company’s common stock was determined by the board of directors with assistance from management and, in part, on input from an independent third-party valuation firm. The board of directors determined the fair value of common stock by considering a number of objective and subjective factors, including valuations of comparable companies, sales of redeemable convertible preferred stock, operating and financial performance, the lack of liquidity of the Company’s common stock and the general and industry-specific economic outlook. Subsequent to the Company’s IPO, the fair value of the Company’s common stock is determined based on its closing market price on the date of grant. Stock-Based Compensation Associated with Awards to Non-employees During the nine months ended September 30, 2019 and September 30, 2018, the Company granted stock options to non-employees to purchase 0 and 3,900 shares of common stock, respectively. Stock-based compensation expense related to options granted to non-employees for all periods presented was not material. Restricted Stock Restricted stock activity was as follows: Number of Shares Underlying Outstanding Restricted Stock Awards Weighted Average Grant Date Fair Value Unvested, December 31, 2018 19,492 $ 0.63 Vested (4,593 ) $ 0.05 Unvested, September 30, 2019 14,899 $ 0.81 During the years ended December 31, 2015 and 2016, the Company issued 346,579 founder shares under restricted stock agreements. Under the terms of these agreements, the founder shares vest over the requisite service periods. Recipients of restricted stock awards have voting and dividend rights with respect to such shares upon grant without regard to vesting. Shares of unvested restricted stock are subject to the Company’s right of repurchase. As the restricted stock was purchased by employees at a price equal to its fair value at the time of issuance, there was no stock-based compensation expense related to these awards. The total fair value of restricted stock vested during the nine months ended September 30, 2019 and September 30, 2018 was less than $0.1 million in each period. As of September 30, 2019 and December 31, 2018, outstanding shares of unvested restricted stock that were subject to repurchase have an aggregate purchase price of less than $0.1 million, which is recorded in other current liabilities and other non-current liabilities on the balance sheets.

Net Loss Per Share Attributable

Net Loss Per Share Attributable to Common Stockholders	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Loss Per Share Attributable to Common Stockholders	10. Net Loss Per Share Attributable to Common Stockholders The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Numerator: Net loss attributable to common stockholders $ (10,969 ) $ (13,052 ) $ (31,194 ) $ (25,784 ) Denominator: Weighted-average shares outstanding 20,307,383 1,341,269 10,087,459 1,333,706 Less: weighted-average unvested restricted shares and shares subject to repurchase (149,160 ) (341,900 ) (191,885 ) (411,042 ) Weighted-average shares used in computing net loss per share attributable to common stock, basic and diluted 20,158,223 999,369 9,895,574 922,664 Net loss per share attributable to common stockholders, basic and diluted $ (0.54 ) $ (13.06 ) $ (3.15 ) $ (27.94 ) The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive: As of September 30, 2019 2018 Redeemable convertible preferred stock — 13,139,794 Options to purchase common stock 1,921,259 1,467,983 Unvested restricted stock awards 14,899 20,865 Unvested early exercised common stock options 116,306 298,745 Total 2,052,464 14,927,387

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2019
Related Party Transactions [Abstract]
Related Party Transactions	11. Related Party Transactions The Company incurred $0.4 million during the three months ended September 30, 2018, and $0.9 million during the nine months ended September 30, 2018 in relation to agreements for pharmaceutical research, development and manufacturing activities with entities affiliated with a former director of the Company, Edward Hu. As of December 31, 2018, $0.2 million is included in accounts payable in the Company’s balance sheet. Mr. Hu resigned from the Company’s board of directors in January 2019.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The unaudited condensed financial statements and accompanying notes have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”). Certain footnotes or other financial information that are normally required by GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2018 has been derived from the audited financial statements at that date but does not include all of the information required by GAAP for complete financial statements. The accompanying balance sheet as of September 30, 2019, the statements of operations and comprehensive loss and of redeemable convertible preferred stock and stockholders’ equity (deficit) for the three and nine months ended September 30, 2019 and September 30, 2018, and the statements of cash flows for the nine months ended September 30, 2019 and September 30, 2018 are unaudited. In the opinion of management, the unaudited data reflects all adjustments, which include only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of September 30, 2019, the results of its operations and comprehensive loss for the three and nine months ended September 30, 2019 and September 30, 2018 and its cash flows for the nine months ended September 30, 2019 and September 30, 2018. The financial data and other information disclosed in these notes related to the three and nine months ended September 30, 2019 and September 30, 2018 are also unaudited. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods or any future year or period. The accompanying interim unaudited condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto for the year ended December 31, 2018, which are included in the Company’s prospectus related to the Company’s IPO, filed with the SEC on May 24, 2019 (the “final prospectus”), pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “Securities Act”).
Reverse Stock Split	Reverse Stock Split In May 2019, the Company’s board of directors approved a 1-for-10.2564 reverse stock split of the Company’s common stock and redeemable convertible preferred stock, which was effected on May 21, 2019. The par value and authorized shares of the common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. All issued and outstanding common stock, options to purchase common stock and per share amounts contained in these financial statements have been retroactively adjusted to give effect to the reverse stock split for all periods presented.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of expenses during the reporting period. Such estimates include the valuation of the redeemable convertible preferred stock liability, deferred tax assets, useful lives of property and equipment, determination of the discount rate for operating leases and stock-based compensation. Actual results could differ from those estimates.
Risks and Uncertainties	Risks and Uncertainties The Company operates in a dynamic and highly competitive industry and is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, protection of proprietary technology, dependence on key personnel, contract manufacturer and contract research organizations, compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical studies and clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance and reporting. The Company believes that changes in any of the following areas could have a material adverse effect on the Company’s future financial position, results of operations, or cash flows; ability to obtain future financing; advances and trends in new technologies and industry standards; results of clinical trials; regulatory approval and market acceptance of the Company’s products; development of sales channels; certain strategic relationships; litigation or claims against the Company base in intellectual property, patent, product, regulatory, or other factors; and the Company’s ability to attract and retain employees necessary to support its growth. Products developed by the Company require approvals from the U.S. Food and Drug Administration (“FDA”) or other international regulatory agencies prior to commercial sales. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained or maintained, that the products will receive the necessary approvals, or that any approved products will be commercially viable. If the Company was denied approval, approval was delayed or the Company was unable to maintain approval, it could have a materially adverse impact on the Company. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, consultants and other third parties. The Company has expended and will continue to expend substantial funds to complete the research, development and clinical testing of product candidates. The Company also will be required to expend additional funds to establish commercial-scale manufacturing arrangements and to provide for the marketing and distribution of products that receive regulatory approval. The Company may require additional funds to commercialize its products. The Company is unable to entirely fund these efforts with its current financial resources. If adequate funds are unavailable on a timely basis from operations or additional sources of financing, the Company may have to delay, reduce the scope of or eliminate one or more of its research or development programs which would materially and adversely affect its business, financial condition and operations.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash, cash equivalents and short-term marketable securities. Substantially all the Company’s cash is held by one financial institution that management believes is of high credit quality. Such deposits may, at times, exceed federally insured limits. The Company’s investment policy addresses credit ratings, diversification, and maturity dates. The Company invests its cash equivalents and short-term marketable securities in money market funds, U.S. government securities, commercial paper, corporate bonds, and asset-backed securities. The Company limits its credit risk associated with cash equivalents and short-term marketable securities by placing them with banks and institutions it believes are highly creditworthy and in highly rated investments and, by policy, limits the amount of credit exposure with any one commercial issuer. The Company has not experienced any credit losses on its deposits of cash, cash equivalents or short-term marketable securities.
Deferred Initial Public Offering Costs	Deferred Initial Public Offering Costs Deferred offering costs, consisting of legal, accounting and other fees and costs relating to the Company’s IPO, are capitalized and recorded on the balance sheet. As of December 31, 2018, $0.6 million of deferred offering costs were recorded on the balance sheet. During the nine months ended September 30, 2019, an additional $2.7 million in deferred offering costs were incurred. In connection with the IPO, all deferred offering costs were charged against the proceeds from the IPO included in additional paid-in capital on the accompanying condensed balance sheet as of September 30, 2019.
Leases	Leases The Company leases its facilities and meets the requirements to account for these leases as operating leases. For the three and nine months ended September 30, 2018, for facility leases that contain rent escalations or rent concession provisions, the Company records its lease expense during the lease term on a straight-line basis over the term of the lease. As of December 31, 2018, the Company recorded the difference between the rent paid and the straight-line rent as a deferred rent liability. As of December 31, 2018, leasehold improvements funded by landlord incentives or allowances were recorded as leasehold improvement assets and a corresponding deferred rent liability. The leasehold improvement asset is amortized over the lesser of the term of the lease or life of the asset. For the three and nine months ended September 30, 2018, the deferred rent liability is amortized on a straight-line basis as a reduction to rent expense over the term of the lease agreement. Upon adoption of ASC 842, Leases ROU assets and lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. As most of the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The ROU asset also includes any lease payments made to the lessor at or before the commencement date, minus lease incentives received, and initial direct costs incurred. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. For lease agreements entered into or reassessed after the adoption of ASC 842, the Company combines lease and nonlease components.
Net Loss per Share Attributable to Common Stockholders	Net Loss per Share Attributable to Common Stockholders Basic net loss per common share is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted-average number of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock, stock options and common stock subject to repurchase related to unvested restricted stock awards and early exercise of stock options are considered to be potentially dilutive securities. Basic and diluted net loss attributable to common stockholders per share is presented in conformity with the two-class method required for participating securities as the redeemable convertible preferred stock is considered a participating security because it participates in dividends with common stock. The Company also considers the shares issued upon the early exercise of stock options subject to repurchase to be participating securities, because holders of such shares have non-forfeitable dividend rights in the event a dividend is paid on common stock. The holders of all series of redeemable convertible preferred stock and the holders of early exercised shares subject to repurchase do not have a contractual obligation to share in the Company’s losses. As such, the net loss was attributed entirely to common stockholders. Because the Company has reported a net loss for all periods presented, diluted net loss per common share is the same as basic net loss per common share for those periods.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) under its accounting standard codifications (“ASC”) or other standard setting bodies and adopted by the Company as of the specified effective date, unless otherwise discussed below. Recently Adopted Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) Leases Codification Improvements to Topic 842, Leases Leases (Topic 842): Targeted Improvements Leases (Topic 842): Codification Improvements. In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815): (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. In June 2018, the FASB issued ASU 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Non-employee Share-Based Payment Accounting New Accounting Pronouncements Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement In August 2018, the FASB issued ASU 2018-15, Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.
Fair Value Measurements and Short-Term Marketable Securities	The Company applies fair value accounting for all financial assets and liabilities and non-financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, a three-tier fair value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows: Level 1—Observable inputs, such as quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2—Observable inputs other than Level 1 prices 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 which reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. In determining fair value, the Company utilizes valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs to the extent possible as well as considers counterparty credit risk in its assessment of fair value.
Share Based Compensation Forfeiture	The Company accounts for forfeitures as they occur.

Fair Value Measurements and S_2

Fair Value Measurements and Short-Term Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Schedule of Financial Assets Measured and Recognized at Fair Value	As of September 30, 2019, financial assets measured and recognized at fair value are as follows (in thousands): September 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Assets U.S. government securities Level 1 $ 24,948 $ 19 $ — $ 24,967 Corporate bonds Level 2 41,108 59 — 41,167 Commercial paper Level 2 4,374 — — 4,374 Marketable securities 70,430 78 — 70,508 Money market funds (1) Level 1 38,680 — — 38,680 Total fair value of assets $ 109,110 $ 78 $ — $ 109,188 (1) Included in cash and cash equivalents on the balance sheet As of December 31, 2018, financial assets measured and recognized at fair value are as follows (in thousands): December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Assets U.S. government securities Level 1 $ 9,983 $ — $ (1 ) $ 9,982 Corporate bonds Level 2 18,820 — (19 ) 18,801 Commercial paper Level 2 29,699 — — 29,699 Asset-backed securities Level 2 10,984 — (10 ) 10,974 Marketable securities 69,486 — (30 ) 69,456 Money market funds (1) Level 1 20,419 — — 20,419 Total fair value of assets $ 89,905 $ — $ (30 ) $ 89,875 (1) Included in cash and cash equivalents on the balance sheet

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2019
Balance Sheet Related Disclosures [Abstract]
Summary of Property and Equipment, Net	Property and equipment, net consists of the following (in thousands): Useful Life September 30, December 31, (In Years) 2019 2018 Laboratory equipment 5 $ 3,830 $ 3,335 Computer equipment 3 117 117 Software 3 118 109 Leasehold improvements Shorter of useful life or lease term 2,581 2,570 Furniture and fixtures 5 308 304 Total property and equipment 6,954 6,435 Less: Accumulated depreciation and amortization (2,187 ) (1,283 ) Property and equipment, net $ 4,767 $ 5,152
Summary of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): September 30, December 31, 2019 2018 Accrued salaries and benefits $ 1,504 $ 1,279 Accrued research and development expenses 1,870 1,095 Legal and professional fees 486 999 Other 132 210 Accrued liabilities $ 3,992 $ 3,583

Leases, Lease Commitments and_2

Leases, Lease Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Schedule of Future Minimum Lease Payments Under Non-cancelable Operating Leases	Future minimum lease payments under the non-cancelable operating leases as of December 31, 2018 are as follows (in thousands): As of December 31, 2018: Operating Leases 2019 $ 1,711 2020 1,611 2021 1,604 2022 1,646 2023 1,690 Thereafter 1,441 Total lease payments $ 9,703
Schedule of Maturities of Operating Lease Liabilities	The maturities of operating lease liabilities as of September 30, 2019 are as follows (in thousands): As of September 30, 2019 Operating Leases Remaining fiscal 2019 $ 386 2020 1,564 2021 1,604 2022 1,646 2023 1,690 Thereafter 1,441 Total lease payments 8,331 Less: Interest (1,289 ) Present value of lease liabilities $ 7,042 Amounts recognized on the balance sheet Current lease liabilities $ 1,116 Long-term lease liabilities 5,926 Total lease liabilities $ 7,042

Redeemable Convertible Prefer_2

Redeemable Convertible Preferred Stock (Tables)	9 Months Ended
	Sep. 30, 2019
Temporary Equity Disclosure [Abstract]
Schedule of Issued and Outstanding Redeemable Convertible Preferred Stock and Principal Terms	Issued and outstanding redeemable convertible preferred stock and its principal terms as of December 31, 2018 were as follows (in thousands, except share and per share amounts): Redeemable Convertible Preferred Stock Liquidation Carrying Original Issue Price Per Authorized Outstanding Value Value Share Series A redeemable convertible preferred stock 59,433,105 5,794,727 $ 47,178 $ 40,735 $ 8.1415 Series B redeemable convertible preferred stock 75,500,000 7,345,067 97,520 97,656 $ 13.2769 134,933,105 13,139,794 $ 144,698 $ 138,391

Common Stock (Tables)

Common Stock (Tables)	9 Months Ended
	Sep. 30, 2019
Stockholders Equity Note [Abstract]
Schedule of Number of Common Stock Reserved for Future Issuance	The Company had reserved common stock, on an as-converted basis, for future issuance as follows: September 30, December 31, 2019 2018 Conversion of Series A redeemable convertible preferred stock — 5,794,727 Conversion of Series B redeemable convertible preferred stock — 7,345,067 Exercise of outstanding options under the 2015 and 2019 Plans 1,921,259 1,598,864 Issuance of common stock options under the 2015 Plan — 535,691 Issuance of common stock options under the 2019 Plan 1,089,038 — Issuance of common stock options under the Employee Stock Purchase Plan 195,000 — Total 3,205,297 15,274,349

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock-Based Compensation Expense	Total stock-based compensation expense recorded related to awards granted to employees and non-employees was as follows (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Research and development $ 227 $ 124 $ 679 $ 250 General and administrative 291 163 770 375 Total stock-based compensation expense $ 518 $ 287 $ 1,449 $ 625
Summary of Activity under Equity Incentive Plans	Activity under the Company’s 2015 and 2019 equity incentive plans is set forth below: Outstanding Options Shares available for Grant Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Balance, January 1, 2019 535,691 1,598,864 $ 4.128 9.22 Additional shares authorized 978,500 — Options granted (532,567 ) 532,567 $ 10.127 Options exercised — (117,002 ) $ 2.285 Options repurchased 14,244 — $ 0.835 Options canceled 93,170 (93,170 ) $ 4.805 Balance, September 30, 2019 1,089,038 1,921,259 $ 5.870 8.73 Exercisable as of September 30, 2019 491,434 $ 4.021 8.18 Vested and expected to vest as of September 30, 2019 1,921,259 $ 5.870 8.73
Assumptions Used to Calculate Fair Values of Options	The fair values of options were calculated using the assumptions set forth below: Three Months Ended September 30, 2019 Three Months Ended September 30, 2018 Nine Months Ended September 30, 2019 Nine Months Ended September 30, 2018 Expected term 5.4 - 6.1 years 6.0 - 6.1 years 5.4 - 6.1 years 5.8 - 6.1 years Expected volatility 77.2% - 81.6% 81.6% - 81.9% 77.2% - 82.2% 80.3% - 82.3% Risk-free interest rate 1.4% - 1.9% 2.8% - 2.9% 1.4% - 2.5% 2.6% - 2.9% Dividend yield 0% 0% 0% 0%
Summary of Restricted Stock Activity	Restricted stock activity was as follows: Number of Shares Underlying Outstanding Restricted Stock Awards Weighted Average Grant Date Fair Value Unvested, December 31, 2018 19,492 $ 0.63 Vested (4,593 ) $ 0.05 Unvested, September 30, 2019 14,899 $ 0.81

Net Loss Per Share Attributab_2

Net Loss Per Share Attributable to Common Stockholders (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	The following table sets forth the computation of basic and diluted net loss per share attributable to common stockholders (in thousands, except share and per share data): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Numerator: Net loss attributable to common stockholders $ (10,969 ) $ (13,052 ) $ (31,194 ) $ (25,784 ) Denominator: Weighted-average shares outstanding 20,307,383 1,341,269 10,087,459 1,333,706 Less: weighted-average unvested restricted shares and shares subject to repurchase (149,160 ) (341,900 ) (191,885 ) (411,042 ) Weighted-average shares used in computing net loss per share attributable to common stock, basic and diluted 20,158,223 999,369 9,895,574 922,664 Net loss per share attributable to common stockholders, basic and diluted $ (0.54 ) $ (13.06 ) $ (3.15 ) $ (27.94 )
Schedule of Outstanding Shares of Potentially Dilutive Securities Excluded From the Computation of Diluted Net Loss Per Share	The following outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the period presented because including them would have been antidilutive: As of September 30, 2019 2018 Redeemable convertible preferred stock — 13,139,794 Options to purchase common stock 1,921,259 1,467,983 Unvested restricted stock awards 14,899 20,865 Unvested early exercised common stock options 116,306 298,745 Total 2,052,464 14,927,387

Organization - Additional Infor

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	May 28, 2019	Sep. 30, 2019	Dec. 31, 2018
Subsidiary Sale Of Stock [Line Items]
Proceeds from issuance of common stock in initial public offering, net of issuance costs		$ 50,321
Accumulated deficit		81,710	$ 50,516
Cash, cash equivalents and short-term marketable securities		$ 109,400
IPO
Subsidiary Sale Of Stock [Line Items]
Common stock, shares issued	5,750,000
Common stock, price per share	$ 10
Conversion of redeemable convertible preferred stock into common stock, shares	13,139,794
Proceeds from issuance of common stock in initial public offering, net of issuance costs	$ 50,200
Over Allotment Option
Subsidiary Sale Of Stock [Line Items]
Common stock, shares issued	750,000

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	May 21, 2019	Sep. 30, 2019USD ($)	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)
Summary Of Significant Accounting Policies [Line Items]
Reverse stock split ratio	0.0975
Right-of-use assets		$ 5,279	$ 6,100
Operating lease liabilities		7,042	8,000
Deferred rent			$ 1,900	$ 1,900
IPO
Summary Of Significant Accounting Policies [Line Items]
Deferred offering costs		$ 2,700		$ 600

Fair Value Measurements and S_3

Fair Value Measurements and Short-Term Marketable Securities - Schedule of Financial Assets Measured and Recognized at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost	$ 70,430	$ 69,486
Marketable securities, gross unrealized gains	78
Marketable securities, gross unrealized losses		(30)
Marketable securities, estimated fair value	70,508	69,456
Amortized Cost	109,110	89,905
Estimated Fair Value	109,188	89,875
U.S. Government Securities | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost	24,948	9,983
Marketable securities, gross unrealized gains	19
Marketable securities, gross unrealized losses		(1)
Marketable securities, estimated fair value	24,967	9,982
Corporate Bonds | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost	41,108	18,820
Marketable securities, gross unrealized gains	59
Marketable securities, gross unrealized losses		(19)
Marketable securities, estimated fair value	41,167	18,801
Commercial Paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost	4,374
Marketable securities, estimated fair value	4,374
Commercial Paper | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost		29,699
Marketable securities, estimated fair value		29,699
Asset-Backed Securities | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities, amortized cost		10,984
Marketable securities, gross unrealized losses		(10)
Marketable securities, estimated fair value		10,974
Money Market Funds | Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market funds, amortized Cost	38,680	20,419
Money market funds, estimated Fair Value	$ 38,680	$ 20,419

Fair Value Measurements and S_4

Fair Value Measurements and Short-Term Marketable Securities - Additional Information (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Financial liabilities recognized at fair value	$ 0	$ 0
Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Marketable securities remaining maturity period	12 months	12 months

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 6,954	$ 6,435
Less: Accumulated depreciation and amortization	(2,187)	(1,283)
Property and equipment, net	4,767	5,152
Laboratory Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 3,830	3,335
Property and equipment net, useful life	5 years
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	$ 117	117
Property and equipment net, useful life	3 years
Software
Property Plant And Equipment [Line Items]
Total property and equipment	$ 118	109
Property and equipment net, useful life	3 years
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	$ 2,581	2,570
Property and equipment net, useful life	Shorter of useful life or lease term
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	$ 308	$ 304
Property and equipment net, useful life	5 years

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Balance Sheet Related Disclosures [Abstract]
Depreciation and amortization expense	$ 0.3	$ 0.2	$ 0.9	$ 0.6

Balance Sheet Components - Su_2

Balance Sheet Components - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Balance Sheet Related Disclosures [Abstract]
Accrued salaries and benefits	$ 1,504	$ 1,279
Accrued research and development expenses	1,870	1,095
Legal and professional fees	486	999
Other	132	210
Accrued liabilities	$ 3,992	$ 3,583

Leases, Lease Commitments and_3

Leases, Lease Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Jun. 01, 2018
Operating Leased Assets [Line Items]
Rent expense		$ 400		$ 1,000
Deferred rent					$ 1,900	$ 1,900
Operating lease expense	$ 400		$ 1,200
Right-of-use assets	5,279		5,279		6,100
Operating lease liabilities	7,042		7,042		$ 8,000
Cash paid included in operating lease liabilities included in cash flow from operating activities			1,300
Lease Agreement | Laboratory and Office Facilities
Operating Leased Assets [Line Items]
Tenant improvement allowance	$ 1,600		$ 1,600
Amendment of Lease Agreement | Laboratory and Office Facilities
Operating Leased Assets [Line Items]
Tenant improvement allowance							$ 400
South San Francisco, California
Operating Leased Assets [Line Items]
Operating lease, remaining lease term	4 years 9 months 18 days		4 years 9 months 18 days
Operating lease, discount rate	7.00%		7.00%
South San Francisco, California | Lease Agreement | Laboratory and Office Facilities
Operating Leased Assets [Line Items]
Lease expiration date			Jul. 31, 2024
San Diego, California | Lease Agreement | Laboratory and Office Facilities
Operating Leased Assets [Line Items]
Lease expiration date			Mar. 31, 2020

Leases, Lease Commitments and_4

Leases, Lease Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
2019	$ 1,711
2020	1,611
2021	1,604
2022	1,646
2023	1,690
Thereafter	1,441
Total lease payments	$ 9,703

Leases, Lease Commitments and_5

Leases, Lease Commitments and Contingencies - Schedule of Maturities of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019
Leases [Abstract]
Remaining fiscal 2019	$ 386
2020	1,564
2021	1,604
2022	1,646
2023	1,690
Thereafter	1,441
Total lease payments	8,331
Less: Interest	(1,289)
Operating lease liabilities	7,042	$ 8,000
Current lease liabilities	1,116
Long-term lease liabilities	5,926
Total lease liabilities	$ 7,042	$ 8,000

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Federal or state income tax provision or benefit	$ 0	$ 0	$ 0	$ 0

Redeemable Convertible Prefer_3

Redeemable Convertible Preferred Stock - Schedule of Issued and Outstanding Redeemable Convertible Preferred Stock and its Principal Terms (Details) - USD ($) $ / shares in Units, $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock Authorized	0	59,433,105
Redeemable Convertible Preferred Stock Outstanding	0	5,794,727
Liquidation Value	$ 0	$ 47,178
Carrying Value		$ 40,735
Original Issue Price Per Share		$ 8.1415
Series B Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock Authorized	0	75,500,000
Redeemable Convertible Preferred Stock Outstanding	0	7,345,067
Liquidation Value	$ 0	$ 97,520
Carrying Value		$ 97,656
Original Issue Price Per Share		$ 13.2769
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Stock Authorized		134,933,105
Redeemable Convertible Preferred Stock Outstanding	0	13,139,794	13,139,794	12,876,179	4,379,916
Liquidation Value		$ 144,698
Carrying Value		$ 138,391	$ 138,391	$ 134,579	$ 26,084

Redeemable Convertible Prefer_4

Redeemable Convertible Preferred Stock - Additional Information (Details) - shares	May 28, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Common Stock
Temporary Equity [Line Items]
Conversion of redeemable convertible preferred stock into common stock, shares	13,139,794	13,139,794
Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, shares outstanding		0	13,139,794	13,139,794	12,876,179	4,379,916

Common Stock - Additional Infor

Common Stock - Additional Information (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2019USD ($)Vote$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares
Stockholders Equity Note [Abstract]
Common stock, shares authorized | shares	300,000,000	170,800,000
Common stock, par value | $ / shares	$ 0.0001	$ 0.0001
Common stock, voting rights	Each share of common stock is entitled to one vote.
Number of common stock voting rights held per share | Vote	1
Common stock, dividends declared | $	$ 0	$ 0

Common Stock - Schedule of Numb

Common Stock - Schedule of Number of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Number of common stock reserved for future issuance	3,205,297	15,274,349
Conversion of Series A Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Number of common stock reserved for future issuance		5,794,727
Conversion of Series B Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Number of common stock reserved for future issuance		7,345,067
Exercise of Outstanding Options | 2015 and 2019 Plans
Class Of Stock [Line Items]
Number of common stock reserved for future issuance	1,921,259	1,598,864
Issuance of Common Stock Options | 2015 Plan
Class Of Stock [Line Items]
Number of common stock reserved for future issuance		535,691
Issuance of Common Stock Options | 2019 Plan
Class Of Stock [Line Items]
Number of common stock reserved for future issuance	1,089,038
Issuance of Common Stock Options | Employee Stock Purchase Plan
Class Of Stock [Line Items]
Number of common stock reserved for future issuance	195,000

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant-date fair value of options granted			$ 7.197	$ 3.120
Aggregate intrinsic value of options exercised			$ 700,000	$ 100,000
Total unrecognized stock-based compensation expense for stock options	$ 6,200,000		$ 6,200,000		$ 4,200,000
Total unrecognized stock-based compensation expense, weighted-average period of recognition			2 years 11 months 1 day		3 years 3 months 3 days
Total fair value of options vested			$ 1,600,000	$ 200,000
Common stock repurchased			14,244	7,914
Stock options granted to purchase shares			532,567	1,281,150
Estimated dividend yield	0.00%	0.00%	0.00%	0.00%
Founder shares issued under restricted stock agreements						346,579	346,579
Stock-based compensation expense	$ 518,000	$ 287,000	$ 1,449,000	$ 625,000
Issuance of Common Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares subject to repurchase	116,306		116,306		237,853
Other Current Liabilities and Other Non-Current Liabilities
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate exercise prices of early exercised shares	$ 100,000		$ 100,000		$ 200,000
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted to purchase shares			0	3,900
2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Term of options							10 years
Vesting period of options							4 years
2019 Equity Incentive Plan | Cliff Vesting
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period of options							1 year
2019 Equity Incentive Plan | 10% Stockholder
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Term of options							5 years
Incentive Stock Options and Nonqualified Stock Options | 2019 Equity Incentive Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price of stock option on estimated fair value of shares							100.00%
Incentive Stock Options | 2019 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price of stock option granted to employee							10.00%
Incentive Stock Options | 2019 Equity Incentive Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of exercise price of stock option on estimated fair value of shares							110.00%
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 0
Restricted Stock | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total fair value of stock vested			100,000	$ 100,000
Aggregate purchase price of unvested restricted stock subject to repurchase	$ 100,000		$ 100,000		$ 100,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 518	$ 287	$ 1,449	$ 625
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	227	124	679	250
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 291	$ 163	$ 770	$ 375

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Activity under Equity Incentive Plans (Details) - $ / shares	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding Options, Shares, Options granted	532,567	1,281,150
2015 and 2019 Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding Options, Shares available for Grant, Beginning balance	535,691
Outstanding Options, Shares available for Grant, Additional shares authorized	978,500
Outstanding Options, Shares available for Grant, Options granted	(532,567)
Outstanding Options, Shares available for Grant, Options repurchased	14,244
Outstanding Options, Shares available for Grant, Options canceled	93,170
Outstanding Options, Shares available for Grant, Ending balance	1,089,038		535,691
Outstanding Options, Shares, Beginning balance	1,598,864
Outstanding Options, Shares, Options granted	532,567
Outstanding Options, Shares, Options exercised	(117,002)
Outstanding Options, Shares, Options canceled	(93,170)
Outstanding Options, Shares, Ending balance	1,921,259		1,598,864
Outstanding Options, Shares, Exercisable	491,434
Outstanding Options, Shares, Vested and expected to vest	1,921,259
Outstanding Options, Weighted-Average Exercise Price, Beginning balance	$ 4.128
Outstanding Options, Weighted-Average Exercise Price, Options granted	10.127
Outstanding Options, Weighted-Average Exercise Price, Options exercised	2.285
Outstanding Options, Weighted-Average Exercise Price, Options repurchased	0.835
Outstanding Options, Weighted-Average Exercise Price, Options canceled	4.805
Outstanding Options, Weighted-Average Exercise Price, Ending balance	5.870		$ 4.128
Outstanding Options, Weighted-Average Exercise Price, Exercisable	4.021
Outstanding Options, Weighted-Average Exercise Price, Vested and expected to vest	$ 5.870
Outstanding Options, Weighted Average Remaining Contractual Term (Years)	8 years 8 months 23 days		9 years 2 months 19 days
Outstanding Options, Weighted Average Remaining Contractual Term (Years), Exercisable	8 years 2 months 4 days
Outstanding Options, Weighted Average Remaining Contractual Term (Years), Vested and expected to vest	8 years 8 months 23 days

Stock-Based Compensation - Assu

Stock-Based Compensation - Assumptions Used to Calculate Fair Values of Options (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	77.20%	81.60%	77.20%	80.30%
Expected volatility, maximum	81.60%	81.90%	82.20%	82.30%
Risk-free interest rate, minimum	1.40%	2.80%	1.40%	2.60%
Risk-free interest rate, maximum	1.90%	2.90%	2.50%	2.90%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	5 years 4 months 24 days	6 years	5 years 4 months 24 days	5 years 9 months 18 days
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

Stock-Based Compensation - Su_3

Stock-Based Compensation - Summary of Restricted Stock Activity (Details) - Restricted Stock	9 Months Ended
	Sep. 30, 2019$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares Underlying Outstanding Restricted Stock Awards, Unvested, December 31, 2018 | shares	19,492
Number of Shares Underlying Outstanding Restricted Stock Awards, Vested | shares	(4,593)
Number of Shares Underlying Outstanding Restricted Stock Awards, Unvested, September 30, 2019 | shares	14,899
Weighted Average Grant Date Fair Value, Unvested, December 31, 2018 | $ / shares	$ 0.63
Weighted Average Grant Date Fair Value, Vested | $ / shares	0.05
Weighted Average Grant Date Fair Value, Unvested, September 30, 2019 | $ / shares	$ 0.81

Net Loss Per Share Attributab_3

Net Loss Per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (10,969)	$ (13,052)	$ (31,194)	$ (25,784)
Denominator:
Weighted-average shares outstanding	20,307,383	1,341,269	10,087,459	1,333,706
Less: weighted-average unvested restricted shares and shares subject to repurchase	(149,160)	(341,900)	(191,885)	(411,042)
Weighted average number of shares outstanding, basic and diluted	20,158,223	999,369	9,895,574	922,664
Net loss per share attributable to common stockholders, basic and diluted	$ (0.54)	$ (13.06)	$ (3.15)	$ (27.94)

Net Loss Per Share Attributab_4

Net Loss Per Share Attributable to Common Stockholders - Schedule of Outstanding Shares of Potentially Dilutive Securities Excluded From the Computation of Diluted Net Loss Per Share (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	2,052,464	14,927,387
Issuance of Common Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	1,921,259	1,467,983
Unvested Restricted Stock Awards
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	14,899	20,865
Unvested Early Exercised Common Stock Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	116,306	298,745
Redeemable Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities		13,139,794

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Details) - Former Director - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018
Related Party Transaction [Line Items]
Expenses in relation to agreements with related parties	$ 0.4	$ 0.9
Accounts payable			$ 0.2