Unity Biotechnology (UBX) 10-Q 7 Aug 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2019	Aug. 01, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	UBX
Entity Registrant Name	Unity Biotechnology, Inc.
Entity Central Index Key	0001463361
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Current Reporting Status	Yes
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		43,203,111
Entity Shell Company	false
Entity File Number	001-38470
Entity Tax Identification Number	264726035
Entity Address, Address Line One	3280 Bayshore Blvd
Entity Address, Address Line Two	Suite 100
Entity Address, City or Town	Brisbane
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94005
City Area Code	650
Local Phone Number	416-1192

Condensed Balance Sheets (Unaud

Condensed Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 23,113	$ 15,399
Short-term marketable securities	104,889	155,736
Prepaid expenses and other current assets	2,775	1,830
Tenant improvement receivable	10,650
Total current assets	141,427	172,965
Property and equipment, net	5,658	6,238
Long-term marketable securities	4,040
Restricted cash	1,446	550
Other long-term assets	1,639	1,622
Total assets	154,210	181,375
Current liabilities:
Accounts payable	5,381	4,847
Accrued compensation	2,741	3,791
Accrued and other current liabilities	4,840	4,990
Settlement liability		2,059
Contingent consideration liability	2,270	895
Total current liabilities	15,232	16,582
Deferred rent, net of current portion	11,921	2,467
Contingent consideration liability, net of current portion		1,588
Other non-current liabilities	7	45
Total liabilities	27,160	20,682
Commitments and contingencies (Note 6)
Stockholders’ equity:
Common stock, $0.0001 par value; 300,000,000 shares authorized as of June 30, 2019 and December 31, 2018; 43,203,599 and 42,414,294 shares issued and outstanding as of June 30, 2019 and December 31, 2018, respectively	4	4
Additional paid-in capital	333,252	324,663
Related party promissory notes for purchase of common stock	(201)	(201)
Employee promissory notes for purchase of common stock	(400)	(400)
Accumulated other comprehensive loss	113	(95)
Accumulated deficit	(205,718)	(163,278)
Total stockholders’ equity	127,050	160,693
Total liabilities, convertible preferred stock, and stockholders’ equity	$ 154,210	$ 181,375

Condensed Balance Sheets (Paren

Condensed Balance Sheets (Parenthetical) (Unaudited) - $ / shares	Jun. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	10,000,000	10,000,000
Convertible preferred stock, shares issued	0	0
Convertible preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	43,203,599	42,414,294
Common stock, shares outstanding	43,203,599	42,414,294

Condensed Statements of Operati

Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating expenses:
Research and development	$ 18,468	$ 15,198	$ 34,973	$ 28,223
General and administrative	4,970	3,842	9,447	7,299
Change in fair value of contingent consideration	1,032	1,758	(213)	1,758
Total operating expenses	24,470	20,798	44,207	37,280
Loss from operations	(24,470)	(20,798)	(44,207)	(37,280)
Interest income (expense), net	900	826	1,906	1,178
Other expense, net	(103)	(30)	(139)	(33)
Net loss	(23,673)	(20,002)	(42,440)	(36,135)
Other comprehensive loss
Unrealized gain (loss) on marketable securities, net of tax	94	61	208	27
Comprehensive loss	$ (23,579)	$ (19,941)	$ (42,232)	$ (36,108)
Net loss per share, basic and diluted	$ (0.56)	$ (0.76)	$ (1)	$ (2.41)
Weighted-average number of shares used in computing net loss per share, basic and diluted	42,442,886	26,298,666	42,311,040	15,003,493

Statements of Stockholders Equi

Statements of Stockholders Equity (Unaudited) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series C Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Related Party Promissory Notes for Purchase of Commmon Stock	Employee Promissory Notes for Purchase of Commmon Stock	Accumulated Other Comprehensive (Gain) Loss	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ (83,113)			$ 1	$ 4,072	$ (202)		$ (104)	$ (86,880)
Beginning balance, (in shares) at Dec. 31, 2017		28,159,724
Beginning balance at Dec. 31, 2017		$ 173,956
Beginning balance, (in shares) at Dec. 31, 2017				4,830,389
Issuance of convertible preferred stock per share for cash, net of issuance costs (in shares)			3,590,573
Issuance of convertible preferred stock per share for cash, net of issuance costs			$ 54,950
Issuance of common stock upon exercise of stock options	31				31
Issuance of common stock upon exercise of stock options (in shares)				400,587
Vesting of early exercised stock options	38				38
Stock-based compensation	1,370				1,370
Change in unrealized gain (loss) on available-for-sale marketable securities, net of tax	(34)							(34)
Receipt of promissory note from related party for purchase of common stock	(390)					(390)
Receipt of promissory note from employee for purchase of common stock	(400)						$ (400)
Net loss	(16,133)								(16,133)
Ending balance at Mar. 31, 2018	(98,631)			$ 1	5,511	(592)	(400)	(138)	(103,013)
Ending balance, (in shares) at Mar. 31, 2018		31,750,297
Ending balance at Mar. 31, 2018		$ 228,906
Ending balance, (in shares) at Mar. 31, 2018				5,230,976
Beginning balance at Dec. 31, 2017	(83,113)			$ 1	4,072	(202)		(104)	(86,880)
Beginning balance, (in shares) at Dec. 31, 2017		28,159,724
Beginning balance at Dec. 31, 2017		$ 173,956
Beginning balance, (in shares) at Dec. 31, 2017				4,830,389
Change in unrealized gain (loss) on available-for-sale marketable securities, net of tax	27
Receipt of promissory note from related party for purchase of common stock	(390)
Receipt of promissory note from employee for purchase of common stock	(400)
Net loss	(36,135)
Ending balance at Jun. 30, 2018	195,070			$ 4	318,759	(201)	(400)	(77)	(123,015)
Ending balance, (in shares) at Jun. 30, 2018		0
Ending balance, (in shares) at Jun. 30, 2018				42,314,738
Beginning balance at Mar. 31, 2018	(98,631)			$ 1	5,511	(592)	(400)	(138)	(103,013)
Beginning balance, (in shares) at Mar. 31, 2018		31,750,297
Beginning balance at Mar. 31, 2018		$ 228,906
Beginning balance, (in shares) at Mar. 31, 2018				5,230,976
Issuance of convertible preferred stock per share for cash, net of issuance costs (in shares)			322,852
Issuance of convertible preferred stock per share for cash, net of issuance costs			$ 4,931
Conversion of preferred stock to common stock	233,839			$ 2	233,837
Conversion of preferred stock to common stock (in shares)		(32,073,149)
Conversion of preferred stock to common stock		$ (233,837)
Conversion of preferred stock to common stock (in shares)				32,073,149
Issuance of common stock upon exercise of stock options	5				5
Issuance of common stock upon exercise of stock options (in shares)				10,613
Vesting of early exercised stock options	213				213
Stock-based compensation	2,838				2,838
Common stock issued to third party for a license/ Issuance of common stock at initial public offering, net of offering costs	75,852			$ 1	75,851
Common stock issued to third party for a license/ Issuance of common stock at initial public offering, net of offering costs (in shares)				5,000,000
Change in unrealized gain (loss) on available-for-sale marketable securities, net of tax	61							61
Repayment of promissory note from related party from purchase of common stock	895				504	391
Net loss	(20,002)								(20,002)
Ending balance at Jun. 30, 2018	195,070			$ 4	318,759	(201)	(400)	(77)	(123,015)
Ending balance, (in shares) at Jun. 30, 2018		0
Ending balance, (in shares) at Jun. 30, 2018				42,314,738
Beginning balance at Dec. 31, 2018	$ 160,693			$ 4	324,663	(201)	(400)	(95)	(163,278)
Beginning balance, (in shares) at Dec. 31, 2018	0	0
Beginning balance, (in shares) at Dec. 31, 2018	42,414,294			42,414,294
Issuance of common stock upon exercise of stock options	$ 300				300
Issuance of common stock upon exercise of stock options (in shares)				340,731
Vesting of early exercised stock options	207				207
Stock-based compensation	1,997				1,997
Common stock issued to third party for a license/ Issuance of common stock at initial public offering, net of offering costs	2,059				2,059
Common stock issued to third party for a license/ Issuance of common stock at initial public offering, net of offering costs (in shares)				133,334
Change in unrealized gain (loss) on available-for-sale marketable securities, net of tax	114							114
Net loss	(18,767)								(18,767)
Ending balance at Mar. 31, 2019	146,603			$ 4	329,226	(201)	(400)	19	(182,045)
Ending balance, (in shares) at Mar. 31, 2019		0
Ending balance, (in shares) at Mar. 31, 2019				42,888,359
Beginning balance at Dec. 31, 2018	$ 160,693			$ 4	324,663	(201)	(400)	(95)	(163,278)
Beginning balance, (in shares) at Dec. 31, 2018	0	0
Beginning balance, (in shares) at Dec. 31, 2018	42,414,294			42,414,294
Change in unrealized gain (loss) on available-for-sale marketable securities, net of tax	$ 208
Net loss	(42,440)
Ending balance at Jun. 30, 2019	$ 127,050			$ 4	333,252	(201)	(400)	113	(205,718)
Ending balance, (in shares) at Jun. 30, 2019	0	0
Ending balance, (in shares) at Jun. 30, 2019	43,203,599			43,203,599
Beginning balance at Mar. 31, 2019	$ 146,603			$ 4	329,226	(201)	(400)	19	(182,045)
Beginning balance, (in shares) at Mar. 31, 2019		0
Beginning balance, (in shares) at Mar. 31, 2019				42,888,359
Issuance of common stock upon exercise of stock options	494				494
Issuance of common stock upon exercise of stock options (in shares)				147,832
Vesting of early exercised stock options	218				218
Stock-based compensation	1,944				1,944
Repurchased shares (in shares)				(3,793)
Issuance of common stock under employee stock purchase plan (“ESPP”)	405				405
Issuance of common stock under employee stock purchase plan (“ESPP”) (in Shares)				51,201
Common stock issued to third party for a license/ Issuance of common stock at initial public offering, net of offering costs	965				965
Common stock issued to third party for a license/ Issuance of common stock at initial public offering, net of offering costs (in shares)				120,000
Change in unrealized gain (loss) on available-for-sale marketable securities, net of tax	94							94
Net loss	(23,673)								(23,673)
Ending balance at Jun. 30, 2019	$ 127,050			$ 4	$ 333,252	$ (201)	$ (400)	$ 113	$ (205,718)
Ending balance, (in shares) at Jun. 30, 2019	0	0
Ending balance, (in shares) at Jun. 30, 2019	43,203,599			43,203,599

Statements of Stockholders Eq_2

Statements of Stockholders Equity (Unaudited) (Parenthetical) - Series C Convertible Preferred Stock - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2018	Mar. 31, 2018
Issuance price per share	$ 5.1972	$ 15.3317
Issuance cost	$ 19	$ 100

Condensed Statements of Cash Fl

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities
Net loss	$ (42,440)	$ (36,135)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,280	1,023
Amortization of premium and discounts on marketable securities	(766)	(203)
Stock-based compensation	3,941	4,208
Common stock issued to third party for a license	965
Accretion of tenant improvement allowance	(470)	(305)
Change in fair value of contingent consideration for license agreements	(213)	1,758
Changes in operating assets and liabilities:
Contribution receivable		1,382
Prepaid expenses and other current assets	(766)	(662)
Other long-term assets	(17)	(1,099)
Accounts payable	391	(633)
Accrued compensation	(1,050)	(240)
Accrued liabilities and other current liabilities	(1,031)	970
Deferred rent, net of current portion	279	4
Net cash used in operating activities	(39,897)	(29,932)
Investing activities
Purchase of marketable securities	(50,699)	(137,803)
Maturities of marketable securities	98,480	58,484
Purchase of property and equipment	(437)	(526)
Net cash provided by investing activities	47,344	(79,845)
Financing activities
Proceeds from issuance of convertible preferred stock, net of issuance costs		59,899
Proceeds from issuance of common stock under equity incentive plans, net of repurchases	1,199	33
Proceeds from initial public offering, net of issuance costs		79,050
Payments of deferred offering costs		(3,199)
Proceeds from repayment of recourse notes		895
Payments made on capital lease obligations	(36)	(34)
Net cash provided by financing activities	1,163	136,644
Net increase in cash, cash equivalents and restricted cash	8,610	26,867
Cash, cash equivalents and restricted cash at beginning of the period	15,949	7,848
Cash, cash equivalents and restricted cash at end of the period	24,559	34,715
Supplemental Disclosures of Non-Cash Investing and Financing Information
Lessor funded lease incentives included in tenant improvement receivable	10,650
Property and equipment included in accounts payable	$ 47	138
Receipt of promissory note from related party for purchase of common stock		390
Receipt of promissory note from employees for purchase of common stock		$ 400

Organization

Organization	6 Months Ended
	Jun. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization Description of Business Unity Biotechnology, Inc. (the “Company”) is a biotechnology company engaged in the research and development of therapeutics to extend human healthspan. The Company devotes substantially all of its time and efforts to performing research and development, raising capital and recruiting personnel. The Company is located in Brisbane, California, was incorporated in the State of Delaware in 2009 and operates in one segment. Need for Additional Capital The Company has incurred operating losses and has an accumulated deficit as a result of ongoing efforts to develop drug product candidates, including conducting preclinical and clinical trials and providing general and administrative support for these operations. The Company had an accumulated deficit of $205.7 million and $163.3 million as of June 30, 2019 and December 31, 2018, respectively. The Company had net losses of $42.4 million and $36.1 million for the six months ended June 30, 2019 and 2018, respectively, and net cash used in operating activities of $39.9 million and $29.9 million for the six months ended June 30, 2019 and 2018, respectively. To date, none of the Company’s drug candidates have been approved for sale. The Company has not generated any revenue from contracts with customers and does not expect positive cash flows from operations in the foreseeable future. The Company has historically financed its operations primarily through the issuance and sale of convertible preferred stock and convertible promissory notes. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of one year following the date that these financial statements are issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of, or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation These financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and the rules and regulations of United States Securities and Exchange Commission (“SEC”) for interim reporting. The condensed financial statements are unaudited and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, necessary for the fair presentation for interim reporting. The results of operations for any interim period are not necessarily indicative of results of operations for any future period. Certain information and footnote disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted. Accordingly, the unaudited condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 filed with the SEC. Reverse Stock Split On April 19, 2018, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.95 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock, which was effected on April 20, 2018. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information included in the accompanying financial statements have been adjusted to reflect the Reverse Split. Accordingly, all share and per share information presented in the condensed financial statements herein, and notes thereto, have been retroactively adjusted to reflect the Reverse Split. Initial Public Offering On May 7, 2018, the Company closed its initial public offering (“IPO”), of 5,000,000 shares of common stock, at an offering price to the public of $17.00 per share. The Company received net proceeds of approximately $75.9 million, after deducting underwriting discounts, commissions and offering related transaction costs of approximately $9.1 million. In connection with the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 32,073,149 shares of common stock. In addition, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. In connection with the completion of its IPO, on May 7, 2018, the Company’s certificate of incorporation was amended and restated to provide for 300,000,000 authorized shares of common stock with a par value of $0.0001 per share and 10,000,000 authorized shares of preferred stock with a par value of $0.0001 per share. Use of Estimates The condensed financial statements have been prepared in accordance with GAAP, which requires management to make estimates and assumptions that affect the amounts and disclosures reported in the condensed financial statements and accompanying notes. The Company bases its estimates on historical experience and market-specific or other relevant assumptions that it believes are reasonable under the circumstances. The amounts of assets and liabilities reported in the Company’s condensed balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining the fair value of assets and liabilities, common stock valuation, contingent consideration liability and stock-based compensation. Actual results could differ from such estimates or assumptions. Contingent Consideration Liability The Company has entered into, and may continue to enter into, license agreements to access and utilize certain technology. In each case, the Company evaluates if the license agreement results in the acquisition of an asset or a business. To date none of the Company’s license agreements have been considered an acquisition of a business. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments made before product approval, are immediately recognized as research and development expense when due, provided there is no alternative future use of the rights in other research and development projects. These license agreements also include contingent consideration in the form of additional issuances of the Company’s common stock based on the achievement of certain milestones. For asset acquisitions, the Company assesses on a continuous Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents primarily include money market funds that invest in U.S. Treasury obligations which are stated at fair value. The Company has issued a letter of credit under a lease agreement which has been collateralized. This cash is classified as noncurrent restricted cash on the balance sheet based on the term of the underlying lease. The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the balance sheets that sum to the total of the same amounts shown in the condensed statements of cash flows (in thousands). June 30, 2019 December 31, 2018 Cash and cash equivalents $ 23,113 $ 15,399 Restricted cash 1,446 550 Total cash, cash equivalents and restricted cash $ 24,559 $ 15,949 Variable Interest Entities The Company reviews agreements it enters into with third-party entities, pursuant to which the Company may have a variable interest in the entity, in order to determine if the entity is a variable interest entity (“VIE”). If the entity is a VIE, the Company assesses whether or not it is the primary beneficiary of that entity. In determining whether the Company is the primary beneficiary of an entity, the Company applies a qualitative approach that determines whether it has both (i) the power to direct the economically significant activities of the entity and (ii) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. If the Company determines it is the primary beneficiary of a VIE, it consolidates that VIE into the Company’s financial statements. The Company’s determination about whether it should consolidate such VIEs is made continuously as changes to existing relationships or future transactions may result in a consolidation or deconsolidation event. Concentrations of Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash and marketable securities. Substantially all of the Company’s cash and cash equivalents and restricted cash is deposited in accounts with financial institutions that management believes are of high credit quality. Such deposits have and will continue to exceed federally insured limits. The Company maintains its cash with accredited financial institutions, and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses on its cash deposits. The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents, restricted cash and marketable securities and issuers of marketable securities to the extent recorded on the balance sheets. As of June 30, 2019, the Company had no off-balance sheet concentrations of credit risk. The Company depends on third-party suppliers for key raw materials used in its manufacturing processes and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate raw materials. Leases The Company leases office space and laboratory facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. Incentives granted under the Company’s facilities lease, including allowances to fund leasehold improvements and rent holidays, and are recognized as reductions to rental expense on a straight-line basis over the term of the lease. Lessor funded leasehold improvement incentives not yet received are recorded in prepaid expense and other current assets on the balance sheet. The Company does not assume renewals in its determination of the lease term unless they are deemed to be reasonably assured at the inception of the lease and begins recognizing rent expense on the date that it obtains the legal right to use and control the leased space. Deferred rent consists of the difference between cash payments and the rent expense recognized. The Company entered into capital lease agreements for certain equipment with a lease term of three years. The current portion of capital lease obligations is included in accrued and other liabilities and the noncurrent capital lease obligations is included in other noncurrent liabilities in the balance sheet. Recently Adopted Accounting Pronouncements In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification. The Company adopted the amendments during the first quarter of fiscal year 2019, and as a result, disclosed in its statements of stockholders’ equity the quarterly activity of each caption of stockholders’ equity for the six months ended June 30, 2019 and 2018. The adoption of the amendments did not have a material impact on the Company’s condensed financial statements. In January 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-01, Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230: Classification of Certain Cash Receipts and Cash Payments) Recently Issued Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU No. 2018-15, Intangibles (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract 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 June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses, or Topic 326, Measurement of Credit Losses on Financial Instruments In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements The Company determines the fair value of financial and non-financial assets and liabilities based on the assumptions that market participants would use in pricing the asset or liability in an orderly transaction between market participants at the measurement date. The identification of market participant assumptions provides a basis for determining what inputs are to be used for pricing each asset or liability. A fair value hierarchy has been established which gives precedence to fair value measurements calculated using observable inputs over those using unobservable inputs. This hierarchy prioritized the inputs into three broad levels as follows: • Level 1: Quoted prices in active markets for identical instruments • Level 2: Other significant observable inputs (including quoted prices in active markets for similar instruments) • Level 3: Significant unobservable inputs (including assumptions in determining the fair value of certain investments) The carrying amounts of financial instruments such as cash and cash equivalents, restricted cash, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities approximate the related fair values due to the short maturities of these instruments. The fair value of the Company’s cost method investment is measured when it is deemed to be other-than-temporarily impaired. The Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands): June 30, 2019 Total Level 1 Level 2 Level 3 Assets: Cash equivalents: Money market funds $ 17,873 $ 17,873 $ — $ — Receivable from maturity of U.S. treasury security 4,250 4,250 — — Total cash equivalents 22,123 22,123 — — Short-term marketable securities: U.S. treasuries 7,494 — 7,494 — U.S. and foreign commercial paper 11,935 — 11,935 — U.S. and foreign corporate debt securities 19,646 — 19,646 — U.S. government debt securities 65,814 — 65,814 — Total short-term marketable securities 104,889 — 104,889 — Long-term marketable securities: U.S. government debt securities 4,040 — 4,040 — Total long-term marketable securities 4,040 — 4,040 — Total assets subject to fair value measurements on a recurring basis $ 131,052 $ 22,123 $ 108,929 $ — Liabilities: Contingent consideration liability $ 2,270 $ — $ — $ 2,270 Total liabilities subject to fair value measurements on a recurring basis $ 2,270 $ — $ — $ 2,270 December 31, 2018 Total Level 1 Level 2 Level 3 Assets: Cash equivalents: Money market funds $ 14,131 $ 14,131 $ — $ — Total cash equivalents 14,131 14,131 — — Short-term marketable securities: U.S. treasuries 34,121 — 34,121 — U.S. and foreign commercial paper 10,635 — 10,635 — U.S. and foreign corporate debt securities 26,533 — 26,533 — Asset-backed securities 2,748 — 2,748 — U.S. government debt securities 81,699 — 81,699 — Total short-term marketable securities 155,736 — 155,736 — Total assets subject to fair value measurements on a recurring basis $ 169,867 $ 14,131 $ 155,736 $ — Liabilities: Contingent consideration liability $ 2,483 $ — $ — $ 2,483 Total liabilities subject to fair value measurements on a recurring basis $ 2,483 $ — $ — $ 2,483 The Company estimates the fair value of its money market funds, U.S. and foreign commercial paper, U.S. and foreign corporate debt securities, asset-backed securities, U.S. treasuries and U.S. government debt securities by taking into consideration valuations obtained from third-party pricing services. The pricing services utilize industry standard valuation models, including both income and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, issuer credit spreads; benchmark securities; prepayment/default projections based on historical data; and other observable inputs. The fair value of the contingent consideration liability includes inputs not observable in the market and thus represents a Level 3 measurement. The Company has recorded a contingent consideration liability related to three agreements with a privately held clinical-stage biopharmaceutical company (see Note 5). As of June 30, 2019, these Commercial Agreements included contingent consideration of up to an aggregate of 533,336 additional shares of common stock to be issued in specified portions to the Licensor and an academic institution from which the Licensor had previously in-licensed the underlying technology based on achievement of certain specified preclinical and clinical development and sales milestone events. The probability of achieving the defined milestone events under the Commercial Agreements is estimated on a quarterly basis by the Company’s management using a probability-weighted valuation approach model which reflects the probability and timing of future issuances of shares. Total contingent consideration may change significantly as preclinical and clinical development related to the compounds covered by the Commercial Agreements progresses and additional data is obtained, impacting the Company’s assumptions regarding probabilities of and timing for successful achievement of the related milestone events. For example, significant increases in the estimated probability of achieving a milestone would result in a significantly higher fair value measurement while significant decreases in the estimated probability of achieving a milestone would result in a significantly lower fair value measurement. The potential outstanding contingent consideration value results in shares to be issued ranging from zero, if none of the milestones are achieved, to a maximum of $6.1 million (using the Company’s stock price as of June 30, 2019). As of June 30, 2019, and December 31, 2018, none of the commercial milestones had been achieved and no royalties were due from the sales of licensed products. As of December 31, 2018, the Company determined that the net settlement criteria of the definition of a derivative had been met for 133,334 shares of common stock to the third parties. The Company issued 106,667 of these shares to the Licensor in January 2019 and the remaining 26,667 shares to the academic institution in March 2019. The settlement liability of $2.0 million recorded at December 31, 2018 was reclassified into stockholders’ equity upon the issuance of these shares. The Company recorded a contingent consideration liability of $2.3 million at June 30, 2019 related to additional potential shares subject to the achievement of certain specified clinical development and sales milestone events under the agreements. The following table provides a reconciliation of assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (in thousands): Amount Balance at December 31, 2018 $ 2,483 Additions — Settlements — Change in fair value (213 ) Balance at June 30, 2019 $ 2,270 There were no transfers between the hierarchies during the six months ended June 30, 2019 and the year ended December 31, 2018.

Marketable Securities

Marketable Securities	6 Months Ended
	Jun. 30, 2019
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities Marketable securities, which are classified as available-for-sale, consisted of the following as of June 30, 2019 (in thousands): Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Money market funds $ 17,873 $ — $ — $ 17,873 Short-term marketable securities: U.S. and foreign commercial paper 11,921 14 — 11,935 U.S. and foreign corporate debt securities 19,621 25 — 19,646 U.S. government debt securities 65,743 72 (1 ) 65,814 U.S. treasuries 7,493 1 — 7,494 Total short-term marketable securities 104,778 112 (1 ) 104,889 Long-term marketable securities: U.S. government debt securities 4,038 3 (1 ) 4,040 Total long-term marketable securities 4,038 3 (1 ) 4,040 Total marketable securities $ 126,689 $ 115 $ (2 ) $ 126,802 Marketable securities, which are classified as available-for-sale, consisted of the following as of December 31, 2018 (in thousands): Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Money market funds $ 14,131 $ — $ — $ 14,131 Short-term marketable securities: U.S. and foreign commercial paper 10,638 — (3 ) 10,635 U.S. and foreign corporate debt securities 26,552 2 (21 ) 26,533 Asset-backed securities 2,750 — (2 ) 2,748 U.S. government debt securities 81,755 1 (57 ) 81,699 U.S. treasuries 34,136 1 (16 ) 34,121 Total short-term marketable securities 155,831 4 (99 ) 155,736 Total marketable securities $ 169,962 $ 4 $ (99 ) $ 169,867 At June 30, 2019, the remaining contractual maturities of available-for-sale securities were less than two years. There have been no significant realized gains or losses on available-for-sale securities for the periods presented. Available-for-sale debt securities that were in a continuous loss position but were not deemed to be other than temporarily impaired were immaterial at both June 30, 2019 and December 31, 2018.

License Agreements

License Agreements	6 Months Ended
	Jun. 30, 2019
Research And Development [Abstract]
License Agreements	5. License Agreements License and Compound Library and Option Agreement The Company is a party to three agreements with a privately held clinical-stage biopharmaceutical company (the “Licensor”): (a) a compound library and option agreement executed in February 2016 granting the Company the right to identify and take licenses to research, develop, and seek and obtain marketing approval for library compounds for the treatment of indications outside of oncology, (b) an initial license agreement executed in February 2016 granting the Company rights to an initial licensed compound, and (c) a second license agreement executed in January 2019 granting the Company rights to a second licensed compound (collectively, the “Commercial Agreements”). As of June 30, 2019, as part of these agreements, the Company has issued 640,002 shares of common stock to the Licensor and 160,000 shares of common stock to an academic institution from whom the Licensor had previously licensed the technology. The Commercial Agreements referenced above include cash payments of up to $70.3 million as well as the equity payments of up to an aggregate 533,336 additional shares of common stock, in each case to be issued In December 2018, the Company elected to advance a second compound into formal preclinical development which gave rise to an obligation under the compound library and option agreement to issue an additional 133,334 shares of common stock to the Licensor and the academic institution. These shares were issued to the Licensor in January 2019 and the academic institution in March 2019. In connection with the additional shares of common stock that the Company may be obligated to issue under the Commercial Agreements upon achievement of the specified milestones events, the Company recorded a contingent consideration liability of $2.3 million at June 30, 2019 and $2.5 million at December 31, 2018. The Company agreed to provide funding to the Licensor for research and development work performed at a cost of up to $2.0 million through February 2020. The research and development expense under the research services agreement was $0.3 million and $0.3 million for the six months ended June 30, 2019 and 2018, respectively. Under the consolidation guidance, the Company determined that the Licensor is a VIE. The Company does not have the power to direct the activities that most significantly affect the economic performance of this entity and as such the Company is not the primary beneficiary and consolidation is not required. As of June 30, 2019 and December 31, 2018, the Company has not provided financial, or other, support to the Licensor that was not contractually required. Other License Agreements with Research Institutions In May 2019, the Company entered into a license agreement with The Regents of the University of California on behalf its San Francisco campus (collectively, “UCSF”) which provides the Company the rights to certain patents and related know-how. In June 2019, as part of this license agreement, the Company issued 120,000 shares of its common stock to UCSF. In addition, the Company is obligated to pay an annual license maintenance fee and may be obligated to make milestone payments or issue up to an additional 34,000 shares of its common stock upon the occurrence of specified events and upon commercialization, to make royalty payments in the low single digit percentages (subject to a specified minimum annual royalty) based on sales of products commercialized pursuant to the agreement. None of these events had occurred and no milestone payments or royalty payments had been recognized as of June 30, 2019. The upfront issuance of 120,000 shares of the Company’s common stock, which was recorded as additional paid-in capital, was valued at $1.0 million for the three months ended June 30, 2019. The Company has entered into license agreements with various other research institutions which have provided the Company with rights to patents, and in certain cases, research “know-how” and proprietary research tools to research, develop and commercialize drug candidates. In addition to upfront consideration paid to these various research institutions in either cash or shares of the Company’s common stock, the Company may be obligated to pay milestone payments in cash or the issuance of the Company’s common stock upon achievement of certain specified clinical development and/or sales events. The contingent consideration liability considered to be a derivative associated with the potential issuance of common stock related to these license agreements was not significant at June 30, 2019 or December 31, 2018. To date, none of these events has occurred and no contingent consideration, milestone or royalty payments have been recognized.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Operating Leases In February 2019, the Company entered into a lease agreement for new office and laboratory space in South San Francisco, California. The term of the lease agreement commenced in May 2019. The lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional eight years at the then market rental rates as determined pursuant to the lease agreement. The total base rent payment escalates annually based on a fixed percentage beginning from the 13th month of the lease agreement. The Company will also be responsible for the operating expenses and tax expenses allocated to the building, and the operating expenses and tax expenses attributable to the common areas. Pursuant to the lease agreement, the landlord provided the Company with a tenant improvement allowance of up to $7.8 million and will finance up to $2.9 million for additional tenant improvements subject to repayment provisions as described in the lease agreement. The value of this tenant improvement allowance of $10.7 million was recorded as deferred rent and on the balance sheet at June 30, 2019. It will be reclassified to leasehold improvements within property and equipment when realized, and depreciated over the remaining lease term. The Company has a non-cancelable operating lease consisting of administrative and research and development office space for its current Brisbane, California headquarters that expires in October 2022. Future minimum lease payments under the Company’s non-cancellable operating leases at June 30, 2019 were as follows (in thousands): Amount 2019 (from July to December) 1,014 2020 5,373 2021 6,230 2022 5,859 2023 and later 33,658 Total future minimum lease payments $ 52,134 Rent expense for the six months ended June 30, 2019 and 2018 was $1.5 million and $0.9 million, respectively. Indemnifications The Company indemnifies each of its officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with the Company’s amended and restated certificate of incorporation and bylaws. The term of the indemnification period lasts as long as an officer or director may be subject to any proceeding arising out of acts or omissions of such officer or director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable the Company to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations for any period presented.

Related Party Transactions

Related Party Transactions	6 Months Ended
	Jun. 30, 2019
Related Party Transactions [Abstract]
Related-Party Transactions	7. Related-Party Transactions Recourse Notes In December 2015, April 2016, and July 2016, the Company issued three full-recourse promissory notes to two executive officers for an aggregate principal amount of $0.2 million with an interest rate of 2.5% per annum. All of the principal was used to early exercise options for 667,253 shares of the Company’s common stock, in aggregate. All of these related party full-recourse notes were repaid on April 4, 2018 in accordance with the terms of such notes. In October 2017, the Company issued two promissory notes to an executive officer for $1.6 million and $0.5 million, each with an interest rate of 1.85% per annum. The aggregate principal amount of $2.1 million was used to purchase 625,084 shares of restricted stock. The promissory notes were considered to be non-recourse in substance and accordingly, the shares sold subject to such promissory notes are considered to be an option for accounting purposes. In April 2018, the Company’s board of directors approved the forgiveness of all outstanding principal and accrued interest of the $1.6 million non-recourse promissory note. The non-recourse promissory note outstanding of $0.5 million was repaid on April 4, 2018 in accordance with the terms of the note. The forgiveness of the promissory note was accounted for as a modification of a share-based payment and $1.5 million of stock-based compensation from the modification was recorded in 2018. In January 2018, the Company issued full-recourse promissory notes to an executive and an executive officer of the Company for an aggregate principal amount of $0.4 million with an interest rate of 2.5% per annum. All of the principal was used to early exercise options for 114,406 shares of the Company’s common stock Financing Activities During the six months ended June 30, 2018, the Company issued convertible preferred stock for total proceeds of $3.0 million to shareholders who are considered to be related parties.

Equity Financing

Equity Financing	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Equity Financing	8. Equity Financing On June 3, 2019, the Company entered into a sales agreement (the “Sales Agreement”) with Cowen and Company, LLC (“Cowen”) to sell shares of the Company’s common stock, from time to time, with aggregate gross sales proceeds of up to $75,000,000, through an at-the-market equity offering program under which Cowen will act as its sales agent. The issuance and sale of shares of common stock by the Company pursuant to the Sales Agreement are deemed an “at-the-market” offering under the Securities Act of 1933, as amended. Cowen is entitled to compensation for its services equal to up to 3.0% of the gross proceeds of any shares of common stock sold through Cowen under the Sales Agreement. In addition, the Company has agreed to reimburse a portion of the expenses of Cowen in connection with the offering up to a maximum of $0.1 million. During the three months ended June 30, 2019, the Company received no proceeds from the sale of shares of common stock pursuant to the Sales Agreement. On June 3, 2019, the Company also filed a Registration Statement on Form S-3 (the “Shelf Registration Statement”), covering the offering of up to $250.0 million of common stock, preferred stock, debt securities, warrants and units. The Shelf Registration Statement included a prospectus covering the offering, issuance and sale of up to $75.0 million of the Company’s common stock from time to time through the “at-the-market” equity offering program. The SEC declared the Shelf Registration Statement effective on June 6, 2019.

Stock-Based Compensation

Stock-Based Compensation	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	9. Stock-Based Compensation Stock Option Activity A summary of the Company’s stock option activity under the 2013 Equity Incentive Plan and 2018 Incentive Award Plan for the six months ended June 30, 2019 is as follows: Number of Shares Weighted- Average Exercise Price Balances at December 31, 2018 5,500,531 $ 6.75 Granted 2,104,522 $ 9.03 Exercised (488,563 ) $ 1.63 Canceled (234,391 ) $ 8.49 Balances at June 30, 2019 6,882,099 $ 7.75 Stock Based Compensation Expense The following table sets forth the total stock-based compensation expense for all options granted to employees and nonemployees, including shares sold through the issuance of non-recourse promissory notes which are considered to be options for accounting purposes, as well as expense related to the Company’s 2018 Employee Stock Purchase Plan (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Research and development $ 932 $ 1,969 $ 1,927 $ 2,943 General and administrative 1,013 869 2,015 1,265 Total $ 1,944 $ 2,838 $ 3,941 $ 4,208

Net Loss per Common Share

Net Loss per Common Share	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Loss per Common Share	10. Net Loss per Common Share Basic net loss per share is calculated by dividing net loss by the weighted average number of shares outstanding for the period. Diluted net loss per share is calculated by dividing net loss by the weighted average number of shares of common stock and potential dilutive common stock equivalents outstanding during the period if the effect is dilutive. The calculation of diluted earnings (loss) per share also requires that, to the extent contingences are satisfied during the period and the presumed issuance of such additional shares as contingent consideration is dilutive to earnings (loss) per share for the period, adjustments to net income or net loss used in the calculation are required to remove the change in fair value of the contingent consideration liability for the period. Likewise, adjustments to the denominator are required to reflect the related dilutive shares. In all periods presented, the Company’s outstanding stock options, convertible preferred stock, early exercised common stock subject to future vesting, restricted stock accounted for as options common and preferred stock warrants and presumed issuance of additional shares as contingent consideration were excluded from the calculation of diluted net loss per share because their effects were antidilutive. A reconciliation of the numerators and denominators used in computing net loss from continuing operations per share is as follows (in thousands, except per share amounts): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Numerator: Net loss $ (23,673 ) $ (20,002 ) $ (42,440 ) $ (36,135 ) Denominator: Weighted-average number of shares outstanding—basic and diluted 42,442,886 26,298,666 42,311,040 15,003,493 Net loss per share—basic and diluted $ (0.56 ) $ (0.76 ) $ (1.00 ) $ (2.41 ) Since the Company was in a net loss position for all periods presented, basic net loss per common share is the same as diluted net loss per common share as the inclusion of all potential common shares outstanding would have been anti-dilutive. June 30, 2019 2018 Options to purchase common stock 7,051,590 5,545,004 Early exercised common stock subject to future vesting 478,288 938,545 Restricted stock accounted for as options — 359,229 Warrants to purchase common stock — 850,686 Shares subject to ESPP 52,047 — Total 7,581,925 7,693,464 Up to 640,218 shares may be contingently issued, if certain performance conditions are met under the Company’s in-licensing agreements .

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Need for Additional Capital	Need for Additional Capital The Company has incurred operating losses and has an accumulated deficit as a result of ongoing efforts to develop drug product candidates, including conducting preclinical and clinical trials and providing general and administrative support for these operations. The Company had an accumulated deficit of $205.7 million and $163.3 million as of June 30, 2019 and December 31, 2018, respectively. The Company had net losses of $42.4 million and $36.1 million for the six months ended June 30, 2019 and 2018, respectively, and net cash used in operating activities of $39.9 million and $29.9 million for the six months ended June 30, 2019 and 2018, respectively. To date, none of the Company’s drug candidates have been approved for sale. The Company has not generated any revenue from contracts with customers and does not expect positive cash flows from operations in the foreseeable future. The Company has historically financed its operations primarily through the issuance and sale of convertible preferred stock and convertible promissory notes. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of one year following the date that these financial statements are issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of, or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.
Basis of Presentation	Basis of Presentation These financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and the rules and regulations of United States Securities and Exchange Commission (“SEC”) for interim reporting. The condensed financial statements are unaudited and, in the opinion of management, reflect all adjustments, consisting only of normal recurring adjustments, necessary for the fair presentation for interim reporting. The results of operations for any interim period are not necessarily indicative of results of operations for any future period. Certain information and footnote disclosures normally included in annual financial statements prepared in accordance with GAAP have been condensed or omitted. Accordingly, the unaudited condensed financial statements should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 filed with the SEC.
Reverse Stock Split	Reverse Stock Split On April 19, 2018, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.95 reverse split (“Reverse Split”) of shares of the Company’s common and convertible preferred stock, which was effected on April 20, 2018. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the Reverse Split. All of the share and per share information included in the accompanying financial statements have been adjusted to reflect the Reverse Split. Accordingly, all share and per share information presented in the condensed financial statements herein, and notes thereto, have been retroactively adjusted to reflect the Reverse Split.
Initial Public Offering	Initial Public Offering On May 7, 2018, the Company closed its initial public offering (“IPO”), of 5,000,000 shares of common stock, at an offering price to the public of $17.00 per share. The Company received net proceeds of approximately $75.9 million, after deducting underwriting discounts, commissions and offering related transaction costs of approximately $9.1 million. In connection with the IPO, all of the Company’s outstanding shares of convertible preferred stock were automatically converted into 32,073,149 shares of common stock. In addition, all of our convertible preferred stock warrants were converted into warrants to purchase shares of common stock. In connection with the completion of its IPO, on May 7, 2018, the Company’s certificate of incorporation was amended and restated to provide for 300,000,000 authorized shares of common stock with a par value of $0.0001 per share and 10,000,000 authorized shares of preferred stock with a par value of $0.0001 per share.
Use of Estimates	Use of Estimates The condensed financial statements have been prepared in accordance with GAAP, which requires management to make estimates and assumptions that affect the amounts and disclosures reported in the condensed financial statements and accompanying notes. The Company bases its estimates on historical experience and market-specific or other relevant assumptions that it believes are reasonable under the circumstances. The amounts of assets and liabilities reported in the Company’s condensed balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining the fair value of assets and liabilities, common stock valuation, contingent consideration liability and stock-based compensation. Actual results could differ from such estimates or assumptions.
Contingent Consideration Liability	Contingent Consideration Liability The Company has entered into, and may continue to enter into, license agreements to access and utilize certain technology. In each case, the Company evaluates if the license agreement results in the acquisition of an asset or a business. To date none of the Company’s license agreements have been considered an acquisition of a business. For asset acquisitions, the upfront payments to acquire such licenses, as well as any future milestone payments made before product approval, are immediately recognized as research and development expense when due, provided there is no alternative future use of the rights in other research and development projects. These license agreements also include contingent consideration in the form of additional issuances of the Company’s common stock based on the achievement of certain milestones. For asset acquisitions, the Company assesses on a continuous
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. Cash equivalents primarily include money market funds that invest in U.S. Treasury obligations which are stated at fair value. The Company has issued a letter of credit under a lease agreement which has been collateralized. This cash is classified as noncurrent restricted cash on the balance sheet based on the term of the underlying lease. The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the balance sheets that sum to the total of the same amounts shown in the condensed statements of cash flows (in thousands). June 30, 2019 December 31, 2018 Cash and cash equivalents $ 23,113 $ 15,399 Restricted cash 1,446 550 Total cash, cash equivalents and restricted cash $ 24,559 $ 15,949
Variable Interest Entities	Variable Interest Entities The Company reviews agreements it enters into with third-party entities, pursuant to which the Company may have a variable interest in the entity, in order to determine if the entity is a variable interest entity (“VIE”). If the entity is a VIE, the Company assesses whether or not it is the primary beneficiary of that entity. In determining whether the Company is the primary beneficiary of an entity, the Company applies a qualitative approach that determines whether it has both (i) the power to direct the economically significant activities of the entity and (ii) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. If the Company determines it is the primary beneficiary of a VIE, it consolidates that VIE into the Company’s financial statements. The Company’s determination about whether it should consolidate such VIEs is made continuously as changes to existing relationships or future transactions may result in a consolidation or deconsolidation event.
Concentrations of Risk	Concentrations of Risk Financial instruments that potentially subject the Company to significant concentrations of credit risk consist primarily of cash, cash equivalents, restricted cash and marketable securities. Substantially all of the Company’s cash and cash equivalents and restricted cash is deposited in accounts with financial institutions that management believes are of high credit quality. Such deposits have and will continue to exceed federally insured limits. The Company maintains its cash with accredited financial institutions, and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses on its cash deposits. The Company’s investment policy limits investments to certain types of securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents, restricted cash and marketable securities and issuers of marketable securities to the extent recorded on the balance sheets. As of June 30, 2019, the Company had no off-balance sheet concentrations of credit risk. The Company depends on third-party suppliers for key raw materials used in its manufacturing processes and is subject to certain risks related to the loss of these third-party suppliers or their inability to supply the Company with adequate raw materials.
Leases	Leases The Company leases office space and laboratory facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. Incentives granted under the Company’s facilities lease, including allowances to fund leasehold improvements and rent holidays, and are recognized as reductions to rental expense on a straight-line basis over the term of the lease. Lessor funded leasehold improvement incentives not yet received are recorded in prepaid expense and other current assets on the balance sheet. The Company does not assume renewals in its determination of the lease term unless they are deemed to be reasonably assured at the inception of the lease and begins recognizing rent expense on the date that it obtains the legal right to use and control the leased space. Deferred rent consists of the difference between cash payments and the rent expense recognized. The Company entered into capital lease agreements for certain equipment with a lease term of three years. The current portion of capital lease obligations is included in accrued and other liabilities and the noncurrent capital lease obligations is included in other noncurrent liabilities in the balance sheet.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification. The Company adopted the amendments during the first quarter of fiscal year 2019, and as a result, disclosed in its statements of stockholders’ equity the quarterly activity of each caption of stockholders’ equity for the six months ended June 30, 2019 and 2018. The adoption of the amendments did not have a material impact on the Company’s condensed financial statements. In January 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2016-01, Financial Instruments Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230: Classification of Certain Cash Receipts and Cash Payments)
Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Issued Accounting Pronouncements Not Yet Adopted In August 2018, the FASB issued ASU No. 2018-15, Intangibles (Topic 350): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract 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 June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses, or Topic 326, Measurement of Credit Losses on Financial Instruments In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) Leases
Fair Value Measurements	Fair Value Measurements The Company determines the fair value of financial and non-financial assets and liabilities based on the assumptions that market participants would use in pricing the asset or liability in an orderly transaction between market participants at the measurement date. The identification of market participant assumptions provides a basis for determining what inputs are to be used for pricing each asset or liability. A fair value hierarchy has been established which gives precedence to fair value measurements calculated using observable inputs over those using unobservable inputs. This hierarchy prioritized the inputs into three broad levels as follows: • Level 1: Quoted prices in active markets for identical instruments • Level 2: Other significant observable inputs (including quoted prices in active markets for similar instruments) • Level 3: Significant unobservable inputs (including assumptions in determining the fair value of certain investments) The carrying amounts of financial instruments such as cash and cash equivalents, restricted cash, prepaid expenses and other current assets, accounts payable, accrued compensation, accrued and other current liabilities approximate the related fair values due to the short maturities of these instruments. The fair value of the Company’s cost method investment is measured when it is deemed to be other-than-temporarily impaired.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash within the balance sheets that sum to the total of the same amounts shown in the condensed statements of cash flows (in thousands). June 30, 2019 December 31, 2018 Cash and cash equivalents $ 23,113 $ 15,399 Restricted cash 1,446 550 Total cash, cash equivalents and restricted cash $ 24,559 $ 15,949

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Summary of Financial Assets and Liabilities Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Measurements	The Company’s financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements were as follows (in thousands): June 30, 2019 Total Level 1 Level 2 Level 3 Assets: Cash equivalents: Money market funds $ 17,873 $ 17,873 $ — $ — Receivable from maturity of U.S. treasury security 4,250 4,250 — — Total cash equivalents 22,123 22,123 — — Short-term marketable securities: U.S. treasuries 7,494 — 7,494 — U.S. and foreign commercial paper 11,935 — 11,935 — U.S. and foreign corporate debt securities 19,646 — 19,646 — U.S. government debt securities 65,814 — 65,814 — Total short-term marketable securities 104,889 — 104,889 — Long-term marketable securities: U.S. government debt securities 4,040 — 4,040 — Total long-term marketable securities 4,040 — 4,040 — Total assets subject to fair value measurements on a recurring basis $ 131,052 $ 22,123 $ 108,929 $ — Liabilities: Contingent consideration liability $ 2,270 $ — $ — $ 2,270 Total liabilities subject to fair value measurements on a recurring basis $ 2,270 $ — $ — $ 2,270 December 31, 2018 Total Level 1 Level 2 Level 3 Assets: Cash equivalents: Money market funds $ 14,131 $ 14,131 $ — $ — Total cash equivalents 14,131 14,131 — — Short-term marketable securities: U.S. treasuries 34,121 — 34,121 — U.S. and foreign commercial paper 10,635 — 10,635 — U.S. and foreign corporate debt securities 26,533 — 26,533 — Asset-backed securities 2,748 — 2,748 — U.S. government debt securities 81,699 — 81,699 — Total short-term marketable securities 155,736 — 155,736 — Total assets subject to fair value measurements on a recurring basis $ 169,867 $ 14,131 $ 155,736 $ — Liabilities: Contingent consideration liability $ 2,483 $ — $ — $ 2,483 Total liabilities subject to fair value measurements on a recurring basis $ 2,483 $ — $ — $ 2,483
Summary of Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs	As of December 31, 2018, the Company determined that the net settlement criteria of the definition of a derivative had been met for 133,334 shares of common stock to the third parties. The Company issued 106,667 of these shares to the Licensor in January 2019 and the remaining 26,667 shares to the academic institution in March 2019. The settlement liability of $2.0 million recorded at December 31, 2018 was reclassified into stockholders’ equity upon the issuance of these shares. The Company recorded a contingent consideration liability of $2.3 million at June 30, 2019 related to additional potential shares subject to the achievement of certain specified clinical development and sales milestone events under the agreements. The following table provides a reconciliation of assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (in thousands): Amount Balance at December 31, 2018 $ 2,483 Additions — Settlements — Change in fair value (213 ) Balance at June 30, 2019 $ 2,270

Marketable Securities (Tables)

Marketable Securities (Tables)	6 Months Ended
	Jun. 30, 2019
Investments Debt And Equity Securities [Abstract]
Summary of Marketable Securities Classified as Available-for-Sale	Marketable securities, which are classified as available-for-sale, consisted of the following as of June 30, 2019 (in thousands): Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Money market funds $ 17,873 $ — $ — $ 17,873 Short-term marketable securities: U.S. and foreign commercial paper 11,921 14 — 11,935 U.S. and foreign corporate debt securities 19,621 25 — 19,646 U.S. government debt securities 65,743 72 (1 ) 65,814 U.S. treasuries 7,493 1 — 7,494 Total short-term marketable securities 104,778 112 (1 ) 104,889 Long-term marketable securities: U.S. government debt securities 4,038 3 (1 ) 4,040 Total long-term marketable securities 4,038 3 (1 ) 4,040 Total marketable securities $ 126,689 $ 115 $ (2 ) $ 126,802 Marketable securities, which are classified as available-for-sale, consisted of the following as of December 31, 2018 (in thousands): Amortized Cost Basis Unrealized Gains Unrealized Losses Fair Value Money market funds $ 14,131 $ — $ — $ 14,131 Short-term marketable securities: U.S. and foreign commercial paper 10,638 — (3 ) 10,635 U.S. and foreign corporate debt securities 26,552 2 (21 ) 26,533 Asset-backed securities 2,750 — (2 ) 2,748 U.S. government debt securities 81,755 1 (57 ) 81,699 U.S. treasuries 34,136 1 (16 ) 34,121 Total short-term marketable securities 155,831 4 (99 ) 155,736 Total marketable securities $ 169,962 $ 4 $ (99 ) $ 169,867

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Summary of Future Minimum Lease Payments under Non-cancellable Operating Lease	Future minimum lease payments under the Company’s non-cancellable operating leases at June 30, 2019 were as follows (in thousands): Amount 2019 (from July to December) 1,014 2020 5,373 2021 6,230 2022 5,859 2023 and later 33,658 Total future minimum lease payments $ 52,134

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Activity	A summary of the Company’s stock option activity under the 2013 Equity Incentive Plan and 2018 Incentive Award Plan for the six months ended June 30, 2019 is as follows: Number of Shares Weighted- Average Exercise Price Balances at December 31, 2018 5,500,531 $ 6.75 Granted 2,104,522 $ 9.03 Exercised (488,563 ) $ 1.63 Canceled (234,391 ) $ 8.49 Balances at June 30, 2019 6,882,099 $ 7.75
Summary of Stock-based Compensation Expense	The following table sets forth the total stock-based compensation expense for all options granted to employees and nonemployees, including shares sold through the issuance of non-recourse promissory notes which are considered to be options for accounting purposes, as well as expense related to the Company’s 2018 Employee Stock Purchase Plan (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Research and development $ 932 $ 1,969 $ 1,927 $ 2,943 General and administrative 1,013 869 2,015 1,265 Total $ 1,944 $ 2,838 $ 3,941 $ 4,208

Net Loss per Common Share (Tabl

Net Loss per Common Share (Tables)	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Schedule of Reconciliation of Numerators and Denominators Used In Computing Net Loss From Continuing Operations Per Share	A reconciliation of the numerators and denominators used in computing net loss from continuing operations per share is as follows (in thousands, except per share amounts): Three Months Ended June 30, Six Months Ended June 30, 2019 2018 2019 2018 Numerator: Net loss $ (23,673 ) $ (20,002 ) $ (42,440 ) $ (36,135 ) Denominator: Weighted-average number of shares outstanding—basic and diluted 42,442,886 26,298,666 42,311,040 15,003,493 Net loss per share—basic and diluted $ (0.56 ) $ (0.76 ) $ (1.00 ) $ (2.41 )
Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Per Share	Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows: June 30, 2019 2018 Options to purchase common stock 7,051,590 5,545,004 Early exercised common stock subject to future vesting 478,288 938,545 Restricted stock accounted for as options — 359,229 Warrants to purchase common stock — 850,686 Shares subject to ESPP 52,047 — Total 7,581,925 7,693,464

Organization - Additional Infor

Organization - Additional Information (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Jun. 30, 2019USD ($)Segment	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of segments | Segment					1
Accumulated deficit	$ (205,718)				$ (205,718)		$ (163,278)
Net loss	$ (23,673)	$ (18,767)	$ (20,002)	$ (16,133)	(42,440)	$ (36,135)
Cash used in operations					$ (39,897)	$ (29,932)

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details)	May 07, 2018USD ($)$ / sharesshares	Apr. 20, 2018	Jun. 30, 2019USD ($)$ / sharesshares	Mar. 31, 2019shares	Jun. 30, 2018shares	Jun. 30, 2019USD ($)$ / sharesshares	Jun. 30, 2018USD ($)	Dec. 31, 2018$ / sharesshares
Summary Of Significant Accounting Policies [Line Items]
Reverse split of shares of common and convertible preferred stock, conversion ratio		0.33898
Reverse split of shares of common and convertible preferred stock, effective date		Apr. 20, 2018
Net proceeds from issuance initial public offering after Deducting Underwriting Discounts Commissions and offering related transaction costs | $							$ 79,050,000
Common stock, shares authorized			300,000,000			300,000,000		300,000,000
Common stock, par value | $ / shares			$ 0.0001			$ 0.0001		$ 0.0001
Off-balance sheet concentrations of credit risk description						As of June 30, 2019, the Company had no off-balance sheet concentrations of credit risk.
Off-balance sheet concentrations of credit risk | $			$ 0			$ 0
Capital lease term						3 years
Common Stock
Summary Of Significant Accounting Policies [Line Items]
Common stock issued to third parties			120,000	133,334	5,000,000
Common shares issued upon conversion of preferred shares					32,073,149
IPO
Summary Of Significant Accounting Policies [Line Items]
Common stock issued to third parties	5,000,000
Issuance price per share | $ / shares	$ 17
Net proceeds from issuance initial public offering after Deducting Underwriting Discounts Commissions and offering related transaction costs | $	$ 75,900,000
Approximate underwriting discounts commissions and offering related transaction costs | $	$ 9,100,000
Common stock, shares authorized	300,000,000
Common stock, par value | $ / shares	$ 0.0001
Preferred stock, shares authorized	10,000,000
Preferred stock, par value | $ / shares	$ 0.0001
IPO | Common Stock
Summary Of Significant Accounting Policies [Line Items]
Common shares issued upon conversion of preferred shares	32,073,149
Amended and Restated Certificate of Incorporation
Summary Of Significant Accounting Policies [Line Items]
Reverse split of shares of common and convertible preferred stock		1-for-2.95 reverse split

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Cash and cash equivalents	$ 23,113	$ 15,399
Restricted cash	1,446	550
Total cash, cash equivalents and restricted cash	$ 24,559	$ 15,949	$ 34,715	$ 7,848

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets and Liabilities Subject to Fair Value Measurements on Recurring Basis and Level of Inputs Used in Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Short-term marketable securities	$ 104,889	$ 155,736
Long-term marketable securities	4,040
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	22,123	14,131
Short-term marketable securities	104,889	155,736
Long-term marketable securities	4,040
Total assets subject to fair value measurements on a recurring basis	131,052	169,867
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	2,270	2,483
Fair Value, Measurements, Recurring | Contingent Consideration Liability
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	2,270	2,483
Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents	22,123	14,131
Total assets subject to fair value measurements on a recurring basis	22,123	14,131
Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	104,889	155,736
Long-term marketable securities	4,040
Total assets subject to fair value measurements on a recurring basis	108,929	155,736
Fair Value, Measurements, Recurring | Level 3
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	2,270	2,483
Fair Value, Measurements, Recurring | Level 3 | Contingent Consideration Liability
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	2,270	2,483
Money market funds | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	17,873	14,131
Money market funds | Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents	17,873	14,131
Receivable From Maturity Of U.S. Treasury Security | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	4,250
Receivable From Maturity Of U.S. Treasury Security | Fair Value, Measurements, Recurring | Level 1
Assets:
Cash equivalents	4,250
US Government Debt Securities | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	65,814	81,699
Long-term marketable securities	4,040
US Government Debt Securities | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	65,814	81,699
Long-term marketable securities	4,040
US Treasury Securities | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	7,494	34,121
US Treasury Securities | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	7,494	34,121
U.S. and Foreign Commercial Paper | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	11,935	10,635
U.S. and Foreign Commercial Paper | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	11,935	10,635
Corporate Debt Securities | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	19,646	26,533
Corporate Debt Securities | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities	$ 19,646	26,533
Asset-backed Securities | Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities		2,748
Asset-backed Securities | Fair Value, Measurements, Recurring | Level 2
Assets:
Short-term marketable securities		$ 2,748

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($)	1 Months Ended			6 Months Ended	12 Months Ended
	Mar. 31, 2019	Jan. 31, 2019	Dec. 31, 2018	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Settlement liability			$ 2,000,000		$ 2,000,000
Fair value assets hierarchy levels transfers amount				$ 0	$ 0
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liabilities fair value disclosure				2,300,000
Commercial Agreements | Contingent Consideration | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Maximum amount of potential contingent consideration value				$ 6,100,000
Commercial Agreements | Clinical Stage Biopharmaceutical Company | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Contingent consideration additional common stock issued				533,336
Compound Library And Option Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Common shares expected to be issued			133,334		133,334
Compound Library And Option Agreement | Licensor
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of common stock (in shares)		106,667
Compound Library And Option Agreement | Academic Institution
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of common stock (in shares)	26,667

Fair Value Measurements - Sum_2

Fair Value Measurements - Summary of Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Details) - Fair Value, Measurements, Recurring - Level 3 $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance at December 31, 2018	$ 2,483
Change in fair value	(213)
Balance at June 30, 2019	$ 2,270

Marketable Securities - Summary

Marketable Securities - Summary of Marketable Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 126,689	$ 169,962
Unrealized Gains	115	4
Unrealized Losses	(2)	(99)
Fair Value	126,802	169,867
Money market funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	17,873	14,131
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	17,873	14,131
Short Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	104,778	155,831
Unrealized Gains	112	4
Unrealized Losses	(1)	(99)
Fair Value	104,889	155,736
Short Term Marketable Securities | U.S. and Foreign Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	11,921	10,638
Unrealized Gains	14	0
Unrealized Losses	0	(3)
Fair Value	11,935	10,635
Short Term Marketable Securities | Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	19,621	26,552
Unrealized Gains	25	2
Unrealized Losses	0	(21)
Fair Value	19,646	26,533
Short Term Marketable Securities | US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	65,743	81,755
Unrealized Gains	72	1
Unrealized Losses	(1)	(57)
Fair Value	65,814	81,699
Short Term Marketable Securities | U S Treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	7,493	34,136
Unrealized Gains	1	1
Unrealized Losses	0	(16)
Fair Value	7,494	34,121
Short Term Marketable Securities | Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		2,750
Unrealized Gains		0
Unrealized Losses		(2)
Fair Value		$ 2,748
Long Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	4,038
Unrealized Gains	3
Unrealized Losses	(1)
Fair Value	4,040
Long Term Marketable Securities | US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	4,038
Unrealized Gains	3
Unrealized Losses	(1)
Fair Value	$ 4,040

Marketable Securities - Additio

Marketable Securities - Additional Information (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Investments Debt And Equity Securities [Abstract]
Long-term available-for sale securities maximum remaining contractual maturity	2 years
Realized gains or losses on available-for-sale securities	$ 0

License Agreements - Additional

License Agreements - Additional Information (Details) - USD ($)	Jun. 03, 2019	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued		43,203,599	42,414,294	43,203,599			43,203,599		42,414,294
Compound library and option agreement execution month and year							2016-02
Research and development expense				$ 18,468,000		$ 15,198,000	$ 34,973,000	$ 28,223,000
Upfront cosideration				$ 965,000	$ 2,059,000	$ 75,852,000
Common Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued				120,000	133,334	5,000,000
Upfront cosideration						$ 1,000
Additional Paid-In Capital
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront cosideration				$ 965,000	$ 2,059,000	$ 75,851,000
Licensed Products
Research And Development Arrangement Contract To Perform For Others [Line Items]
Royalties due from sales							$ 0
Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront cosideration	$ 250,000,000
Initial License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
License agreement execution month and year							2016-02
Second License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
License agreement execution month and year							2019-01
Commercial Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Funding provided for the research and development under the agreement in period							2020-02
Research and development expense							$ 300,000	$ 300,000
Commercial Agreements | Clinical Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued		640,002		640,002			640,002
Commercial Agreements | Clinical Stage Biopharmaceutical Company | Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent consideration additional common stock issued							533,336
Milestone payments							$ 70,300,000
Funding provided for the research and development under the agreement		$ 2,000,000		$ 2,000,000			$ 2,000,000
Commercial Agreements | Academic Institution
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued		160,000		160,000			160,000
Compound Library And Option Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common shares expected to be issued			133,334						133,334
Contingent consideration liability		$ 2,300,000	$ 2,500,000	$ 2,300,000			$ 2,300,000		$ 2,500,000
Other Licensing Agreements with Research Institutions
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent consideration, milestone or royalty payments		$ 0	$ 0	0			0		$ 0
Other Licensing Agreements with Research Institutions | UCSF
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common shares expected to be issued		34,000
Contingent consideration, milestone or royalty payments		$ 0		0			$ 0
Other Licensing Agreements with Research Institutions | UCSF | Common Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued		120,000
Other Licensing Agreements with Research Institutions | UCSF | Additional Paid-In Capital
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront cosideration				$ 1,000,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
South San Francisco, California
Lessee, operating lease, description	The term of the lease agreement commenced in May 2019
Lease, term	10 years
Operating lease, renewal term	8 years
Letter of credit, lease agreement	$ 900,000
South San Francisco, California | Deferred Rent and Tenant Improvement Receivable
Tenant improvement allowance	$ 10,700,000
Brisbane, California
Operating lease, expiration date	Oct. 31, 2022
Rent expense, operating lease	$ 1,500,000	$ 900,000
Maximum | South San Francisco, California
Tenant improvement allowance	7,800,000
Operating lease, Right-of-use Asset	$ 2,900,000

Commitments and Contingencies_2

Commitments and Contingencies - Summary of Future Minimum Lease Payments under Non-cancellable Operating Lease (Details) $ in Thousands	Jun. 30, 2019USD ($)
Commitments And Contingencies Disclosure [Abstract]
2019 (from July to December)	$ 1,014
2020	5,373
2021	6,230
2022	5,859
2023 and later	33,658
Total future minimum lease payments	$ 52,134

Related-Party Transactions - Ad

Related-Party Transactions - Additional Information (Details) $ in Thousands	Apr. 04, 2018USD ($)	Apr. 30, 2018USD ($)	Jan. 31, 2018USD ($)shares	Oct. 31, 2017USD ($)Promissoryshares	Jul. 31, 2016USD ($)shares	Apr. 30, 2016USD ($)shares	Dec. 31, 2015USD ($)shares	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2019PromissoryNoteExecutive
Related Party Transaction [Line Items]
Number of promissory notes | PromissoryNote										3
Number of executive officers | Executive										2
Proceeds from convertible preferred stock								$ 59,899
Executive Officer
Related Party Transaction [Line Items]
Due from related parties				$ 2,100
Restricted stock purchased by related party, shares | shares				625,084
Forgiveness of promissory note accounted as modification of share based payment									$ 1,500
Stockholders | Financing Activities
Related Party Transaction [Line Items]
Proceeds from convertible preferred stock								$ 3,000
Promissory Note | Executive Officer
Related Party Transaction [Line Items]
Number of promissory notes | Promissory				2
Due from related parties			$ 400		$ 200	$ 200	$ 200
Interest rate			2.50%		2.50%	2.50%	2.50%
Promissory Note | Common Stock | Executive Officer
Related Party Transaction [Line Items]
Early exercise options in aggregate | shares			114,406		667,253	667,253	667,253
Full-recourse Notes | Executive Officer
Related Party Transaction [Line Items]
Repayment of promissory notes	$ 200
Promissory Note One | Executive Officer
Related Party Transaction [Line Items]
Due from related parties				$ 1,600
Interest rate				1.85%
Promissory Note Two | Executive Officer
Related Party Transaction [Line Items]
Due from related parties				$ 500
Interest rate				1.85%
Non-Recourse Promissory Note | Executive Officer
Related Party Transaction [Line Items]
Outstanding principal and accrued interest, forgiveness		$ 1,600
Repayment of promissory notes	$ 500

Equity Financing - Additional I

Equity Financing - Additional Information (Details) - USD ($)	Jun. 03, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018
Subsidiary Sale Of Stock [Line Items]
Shares issued offering value		$ 965,000	$ 2,059,000	$ 75,852,000
Common Stock
Subsidiary Sale Of Stock [Line Items]
Shares issued offering value				$ 1,000
Maximum
Subsidiary Sale Of Stock [Line Items]
Shares issued offering value	$ 250,000,000
At-the-Market Equity Offering Program | Maximum | Common Stock
Subsidiary Sale Of Stock [Line Items]
Shares issued offering value	75,000,000
Sales Agreement | Cowen | At-the-Market Equity Offering Program
Subsidiary Sale Of Stock [Line Items]
Proceeds from sale of common stock		$ 0
Sales Agreement | Cowen | At-the-Market Equity Offering Program | Maximum
Subsidiary Sale Of Stock [Line Items]
Proceeds from sale of common stock	$ 75,000,000
Percentage of gross sales proceeds of common stock payable as compensation	3.00%
Reimburse of expenses	$ 100,000

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Details) - Two Thousand Thirteen Equity Incentive Plan and Two Thousand Eighteen Incentive Award Plan	6 Months Ended
	Jun. 30, 2019$ / sharesshares
Number of Shares
Balances at December 31, 2018 | shares	5,500,531
Granted | shares	2,104,522
Exercised | shares	(488,563)
Canceled | shares	(234,391)
Balances at June 30, 2019 | shares	6,882,099
Weighted-Average Exercise Price
Balances at December 31, 2018 | $ / shares	$ 6.75
Granted | $ / shares	9.03
Exercised | $ / shares	1.63
Canceled | $ / shares	8.49
Balances at June 30, 2019 | $ / shares	$ 7.75

Stock-Based Compensation - Su_2

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 1,944	$ 2,838	$ 3,941	$ 4,208
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	932	1,969	1,927	2,943
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 1,013	$ 869	$ 2,015	$ 1,265

Net Loss per Common Share - Sch

Net Loss per Common Share - Schedule of Reconciliation of Numerators and Denominators Used In Computing Net Loss From Continuing Operations Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (23,673)	$ (18,767)	$ (20,002)	$ (16,133)	$ (42,440)	$ (36,135)
Denominator:
Weighted-average number of shares used in computing net loss per share, basic and diluted	42,442,886		26,298,666		42,311,040	15,003,493
Net loss per share, basic and diluted	$ (0.56)		$ (0.76)		$ (1)	$ (2.41)

Net Loss per Common Share - Sum

Net Loss per Common Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Per Share (Details) - shares	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	7,581,925	7,693,464
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	7,051,590	5,545,004
Early Exercised Common Stock Subject to Future Vesting
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	478,288	938,545
Restricted Stock Accounted For as Options
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		359,229
Warrants to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share		850,686
Shares Subject To ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	52,047	0

Net Loss per Common Share - Add

Net Loss per Common Share - Additional Information (Details)	6 Months Ended
	Jun. 30, 2019shares
Maximum
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares contingently issued	640,218