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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2020	Jul. 27, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Amendment Flag	false
Document Period End Date	Jun. 30, 2020
Document Fiscal Year Focus	2020
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 Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Current Reporting Status	Yes
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		52,040,018
Entity Shell Company	false
Entity File Number	001-38470
Entity Tax Identification Number	26-4726035
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	285 East Grand Ave.
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	650
Local Phone Number	416-1192
Title of 12(b) Security	Common Stock, par value $0.0001
Security Exchange Name	NASDAQ

Condensed Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 39,592	$ 37,473
Short-term marketable securities	72,176	84,508
Strategic investment	7,768	5,507
Prepaid expenses and other current assets	4,001	1,999
Total current assets	123,537	129,487
Property and equipment, net	14,226	16,636
Operating lease right of use asset	24,891
Long-term marketable securities		3,025
Restricted cash	1,446	1,446
Other long-term assets	598	627
Total assets	164,698	151,221
Current liabilities:
Accounts payable	3,952	5,185
Accrued compensation	3,651	5,905
Accrued and other current liabilities	6,250	4,995
Contingent consideration liability	1,816	1,131
Total current liabilities	15,669	17,216
Operating lease liability, net of current portion	36,770
Deferred rent, net of current portion		13,298
Total liabilities	52,439	30,514
Commitments and contingencies (Note 6)
Stockholders’ equity:
Convertible preferred stock, $0.0001 par value; 10,000,000 shares authorized; no shares issued and outstanding	0	0
Common stock, $0.0001 par value; 300,000,000 shares authorized as of June 30, 2020 and December 31, 2019; 51,729,511 and 47,227,065 shares issued and outstanding as of June 30, 2020 and December 31, 2019, respectively	5	5
Additional paid-in capital	404,754	366,695
Related party promissory notes for purchase of common stock	(210)	(210)
Employee promissory notes for purchase of common stock	(362)	(418)
Accumulated other comprehensive income	232	90
Accumulated deficit	(292,160)	(245,455)
Total stockholders’ equity	112,259	120,707
Total liabilities and stockholders’ equity	$ 164,698	$ 151,221

Condensed Balance Sheets (Parenthetical) (Unaudited) - $ / shares	Jun. 30, 2020	Dec. 31, 2019
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	51,729,511	47,227,065
Common stock, shares outstanding	51,729,511	47,227,065

Condensed Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating expenses:
Research and development	$ 16,123	$ 18,468	$ 35,388	$ 34,973
General and administrative	6,320	4,970	12,273	9,447
Change in fair value of contingent consideration	906	1,032	685	(213)
Impairment of long-lived assets			2,159
Total operating expenses	23,349	24,470	50,505	44,207
Loss from operations	(23,349)	(24,470)	(50,505)	(44,207)
Interest income	340	900	867	1,906
Other income (expense), net	4,342	(103)	2,933	(139)
Net loss	(18,667)	(23,673)	(46,705)	(42,440)
Other comprehensive loss
Unrealized gain (loss) on marketable debt securities	(141)	94	142	208
Comprehensive loss	$ (18,808)	$ (23,579)	$ (46,563)	$ (42,232)
Net loss per share, basic and diluted	$ (0.38)	$ (0.56)	$ (0.96)	$ (1.00)
Weighted-average number of shares used in computing net loss per share, basic and diluted	49,659,153	42,442,886	48,606,768	42,311,040

Condensed Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	ATM Equity offering program	Common Stock	Common Stock ATM Equity offering program	Additional Paid-In Capital	Additional Paid-In Capital ATM Equity offering program	Related Party Promissory Notes for Purchase of Common Stock	Employee Promissory Notes for Purchase of Common Stock	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ 160,693		$ 4		$ 324,663		$ (201)	$ (400)	$ (95)	$ (163,278)
Beginning balance, (in shares) at Dec. 31, 2018			42,414,294
Issuance of common stock	2,059				2,059
Issuance of common stock (in shares)			133,334
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
Change in unrealized gain (loss) on available-for-sale marketable debt securities	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			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			42,414,294
Change in unrealized gain (loss) on available-for-sale marketable debt securities	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			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			42,888,359
Issuance of common stock	965				965
Issuance of common stock (in shares)			120,000
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			(3,793)
Issuance of common stock under employee stock purchase plan ("2018 ESPP")	405				405
Issuance of common stock under employee stock purchase plan ("ESPP") (in Shares)			51,201
Change in unrealized gain (loss) on available-for-sale marketable debt securities	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			43,203,599
Beginning balance at Dec. 31, 2019	$ 120,707		$ 5		366,695		(210)	(418)	90	(245,455)
Beginning balance, (in shares) at Dec. 31, 2019	47,227,065		47,227,065
Issuance of common stock		$ 8,763				$ 8,763
Issuance of common stock (in shares)				1,513,840
Issuance of common stock upon exercise of stock options	$ 249				249
Issuance of common stock upon exercise of stock options (in shares)			73,049
Vesting of early exercised stock options	40				40
Stock-based compensation	3,225				3,225
Common stock issued for services	100				100
Common stock issued for services (in shares)			43,550
Change in unrealized gain (loss) on available-for-sale marketable debt securities	283								283
Net loss	(28,038)									(28,038)
Ending balance at Mar. 31, 2020	105,329		$ 5		379,072		(210)	(418)	373	(273,493)
Ending balance, (in shares) at Mar. 31, 2020			48,857,504
Beginning balance at Dec. 31, 2019	$ 120,707		$ 5		366,695		(210)	(418)	90	(245,455)
Beginning balance, (in shares) at Dec. 31, 2019	47,227,065		47,227,065
Issuance of common stock upon exercise of stock options (in shares)	123,191
Change in unrealized gain (loss) on available-for-sale marketable debt securities	$ 142
Net loss	(46,705)
Ending balance at Jun. 30, 2020	$ 112,259		$ 5		404,754		(210)	(362)	232	(292,160)
Ending balance, (in shares) at Jun. 30, 2020	51,729,511		51,729,511
Beginning balance at Mar. 31, 2020	$ 105,329		$ 5		379,072		(210)	(418)	373	(273,493)
Beginning balance, (in shares) at Mar. 31, 2020			48,857,504
Issuance of common stock		$ 20,731				$ 20,731
Issuance of common stock (in shares)				2,594,030
Issuance of common stock upon exercise of stock options	355				355
Issuance of common stock upon exercise of stock options (in shares)			105,142
Issuance of common stock from restricted stock units, shares			103,020
Vesting of early exercised stock options	132				132
Stock-based compensation	4,079				4,079
Issuance of common stock under employee stock purchase plan ("2018 ESPP")	385				385
Issuance of common stock under employee stock purchase plan ("ESPP") (in Shares)			69,815
Repayment of promissory note from employee from purchase of common stock	56							56
Change in unrealized gain (loss) on available-for-sale marketable debt securities	(141)								(141)
Net loss	(18,667)									(18,667)
Ending balance at Jun. 30, 2020	$ 112,259		$ 5		$ 404,754		$ (210)	$ (362)	$ 232	$ (292,160)
Ending balance, (in shares) at Jun. 30, 2020	51,729,511		51,729,511

Condensed Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Operating activities
Net loss	$ (46,705)	$ (42,440)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,814	1,280
Net accretion and amortization of premium and discounts on marketable securities	(54)	(766)
Stock-based compensation	7,421	3,941
Common stock issued to third party for a license		965
Non-cash rent expense	(106)
Impairment of long-lived assets	2,159
Change in fair value of strategic investment	(3,060)
Accretion of tenant improvement allowance		(470)
Change in fair value of contingent consideration	685	(213)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(2,002)	(766)
Other long-term assets	29	(17)
Accounts payable	(1,262)	391
Accrued compensation	(2,271)	(1,050)
Accrued and other current liabilities	(1,005)	(1,031)
Deferred rent, net of current portion		279
Net cash used in operating activities	(44,357)	(39,897)
Investing activities
Purchase of marketable securities	(49,792)	(50,699)
Maturities of marketable securities	65,345	98,480
Realized gain on sale of strategic investments	799
Purchase of property and equipment	(377)	(437)
Net cash provided by investing activities	15,975	47,344
Financing activities
Proceeds from issuance of common stock under ATM Offering Program, net of issuance costs	29,494
Proceeds from issuance of common stock under equity incentive plans, net of repurchases	604	794
Proceeds from issuance of common stock under 2018 ESPP	385	405
Proceeds from repayment of recourse notes	56
Payments made on capital lease obligations	(38)	(36)
Net cash provided by financing activities	30,501	1,163
Net increase in cash, cash equivalents and restricted cash	2,119	8,610
Cash, cash equivalents and restricted cash at beginning of the period	38,919	15,949
Cash, cash equivalents and restricted cash at end of the period	41,038	24,559
Supplemental Disclosures of Non-Cash Investing and Financing Activities
Property and equipment included in accounts payable	30	47
Lessor funded lease incentives included in tenant improvement receivable		$ 10,650
Issuance of shares in settlement of share-based liability	$ 100

Organization

6 Months Ended

Jun. 30, 2020

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’s headquarters are located in South San Francisco, California. The Company 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 $292.2 million and $245.5 million as of June 30, 2020 and December 31, 2019, respectively. The Company had net losses of $46.7 million and $42.4 million for the six months ended June 30, 2020 and 2019, respectively, and net cash used in operating activities of $44.4 million and $39.9 million for the six months ended June 30, 2020 and 2019, respectively. To date, none of the Company’s drug candidates have been approved for sale, and therefore, 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 financed its operations primarily through private placements of preferred stock and promissory notes, public equity issuances and more recently from its ATM Offering Program (as defined below) and will continue to be dependent upon equity and/or debt financing until the Company is able to generate positive cash flows from its operations. See Note 10, “Subsequent Events”.

The Company had cash, cash equivalents and marketable securities of $111.8 million as of June 30, 2020. 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 condensed 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 Accounting Policies

6 Months Ended

Jun. 30, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

These condensed 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 the United States Securities and Exchange Commission (“SEC”) for interim reporting.

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, 2019 filed with the SEC.

Unaudited Condensed Financial Statements

The accompanying financial information for the three and six months ended June 30, 2020 and 2019 are unaudited. The unaudited condensed financial statements have been prepared on the same basis as the annual audited financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary to present fairly the Company’s financial position as of June 30, 2020 and its results of operations for the three and six months ended June 30, 2020 and 2019 and cash flows for the six months ended June 30, 2020 and 2019. The results for interim periods are not necessarily indicative of the results expected for the full fiscal year or any other period(s).

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, contingent consideration liability and stock-based compensation. Actual results could differ from such estimates or assumptions.

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 letters of credit under lease agreements which have 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, 2020		December 31, 2019
Cash and cash equivalents	$	39,592	$	37,473
Restricted cash		1,446		1,446
Total cash, cash equivalents and restricted cash	$	41,038	$	38,919

Marketable Securities

The Company generally invests its excess cash in investment grade, short to intermediate-term, fixed income securities. Such investments are considered available-for-sale debt securities, and reported at fair value with unrealized gains and losses included as a component of stockholders’ equity. Marketable securities with original maturities of greater than 90 days from the date of purchase but less than one year from the balance sheet date that are available to be converted into cash to fund current operations are classified as short-term, while marketable securities with maturities in one year or beyond one year from the balance sheet date are classified as long-term. The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity, which is included in interest income on the statements of operations and comprehensive loss. Realized gains and losses and declines in value judged to be other-than-temporary, if any, on marketable securities are included in interest income (expense), net. The cost of securities sold is determined using the specific identification method.

The Company periodically evaluates whether declines in fair values of its marketable securities below their book value are other-than-temporary. This evaluation consists of several qualitative and quantitative factors regarding the severity and duration of the unrealized loss as well as the Company’s ability and intent to hold the marketable

security until a forecasted recovery occurs. Additionally, the Company assesses whether it has plans to sell the security or it is more likely than not it will be required to sell any marketable securities before recovery of its amortized cost basis. Factors considered include quoted market prices, recent financial results and operating trends, implied values from any recent transactions or offers of investee securities, credit quality of debt instrument issuers, other publicly available information that may affect the value of the marketable security, duration and severity of the decline in value, and management’s strategy and intentions for holding the marketable security. To date, the Company has not recorded any impairment charges on its marketable securities related to other-than-temporary declines in market value.

Strategic Investments

The Company has previously made investments in strategic partners and may do so again in the future. The Company does not intend to have a controlling interest or significant influence when it makes these strategic investments. Investments in equity securities of strategic partners with readily determinable fair values are measured using quoted market prices, with changes recorded through other income (expense), net in the statement of operations and comprehensive loss. The Company currently holds a non-controlling equity investment in the common stock of Ascentage Pharma Group International (“Ascentage International”), an affiliate of a Hong-Kong based clinical-stage biopharmaceutical company called Ascentage Pharma Group Corp. Limited (“Ascentage Pharma”), which was acquired in connection with certain commercial agreements with Ascentage Pharma (see Note 5, “License Agreements”). In October 2019, Ascentage International completed an initial public offering of shares of its common stock on the Hong Kong Stock Exchange.

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 in marketable securities 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, 2020, the Company had no off-balance sheet concentrations of credit risk.

The Company is also exposed to market risk in its strategic investments. As of June 30, 2020, the Company held an investment in common stock which is publicly traded in Hong Kong.

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.

In March 2020 the World Health Organization declared the global novel coronavirus (“COVID-19”) outbreak a pandemic. To date, the Company’s operations have not been significantly impacted by the COVID-19 pandemic. However, the Company cannot at this time predict the specific extent, duration, or full impact that the COVID-19 pandemic will have on its financial condition and results of operations, including ongoing and planned clinical studies. The impact of the COVID-19 pandemic on the financial performance of the Company will depend on future developments, including the duration and spread of the COVID-19 pandemic and related governmental advisories and restrictions. These developments and the impact of COVID-19 on the financial markets and the overall economy are highly uncertain. If the financial markets and/or the overall economy are impacted for an extended period, the Company’s business, financial condition, results of operations and prospects may be adversely affected.

Research and Development Expenses and Accruals

Costs related to research, design and development of drug candidates are charged to research and development expense as incurred. Research and development costs include, but are not limited to, payroll and personnel expenses for personnel contributing to research and development activities, laboratory supplies, outside services, licenses acquired to be used in research and development, manufacturing of clinical material, pre-clinical testing and consultants and allocated overhead, including rent, equipment, depreciation and utilities. Research and development costs are expensed as incurred unless there is an alternative future use in other research and development projects. Payments made prior to the receipt of goods or services to be used in research and development are deferred and recognized as expense in the period in which the related goods are received or services are rendered. Such payments are evaluated for current or long-term classification based on when they will be realized.

As part of the process of preparing its condensed financial statements, the Company is required to estimate expenses resulting from its obligations under contracts with vendors and consultants and clinical site agreements in connection with conducting clinical trials. The financial terms of these contracts are subject to negotiations which vary from contract to contract and may result in payment flows that do not match the periods over which materials or services are provided under such contracts. The Company’s objective is to reflect the appropriate expenses in its condensed financial statements by matching those expenses with the period in which services and efforts are expended. The Company accounts for these expenses according to the progress of the production of clinical trial materials or based on progression of the clinical trial, as measured by patient progression and the timing of various aspects of the trial. The Company determines accrual estimates by taking into account discussion with applicable personnel and outside service providers as to the progress or state of consummation of goods and services, or the services completed. During the course of a clinical trial, the Company adjusts the rate of expense recognition if actual results differ from its estimates. The Company makes estimates of accrued expenses as of each balance sheet date in its condensed financial statements based on the facts and circumstances known at that time. The Company’s clinical trial accrual is dependent in part upon the timely and accurate reporting of contract research organizations, contract manufacturers and other third-party vendors. Although the Company does not expect its estimates to be materially different from amounts actually incurred, the Company’s understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and it may result in reporting changes in estimates in any particular period. Adjustments to prior period estimates have not been material for the six months ended June 30, 2020 and 2019.

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 whether the license agreement results in the acquisition of an asset or a business. To date, all of the Company’s license agreements have been considered acquisitions of assets and none have been considered acquisitions of a business. For license agreements that are considered to be acquisitions of assets, the upfront payments for such license, 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. Some of the Company’s license agreements also include contingent consideration in the form of an obligation to issue additional shares of the Company’s common stock based on the achievement of certain milestones. The Company assesses on a continuous basis whether (i) such contingent consideration meets the definition of a derivative, and (ii) whether it can be classified within stockholders’ equity. Until such time when equity classification criteria are met or the milestones expire, the contingent consideration is classified as a liability. The derivative related to this contingent consideration is measured at fair value as of each balance sheet date with the related change in fair value being reflected in operating expenses. Upon a reassessment event that results in the contingent consideration no longer meeting the definition of a derivative and/or meeting equity classification criteria, the final change in fair value of the instrument is recorded within operating expenses and the liability is reclassified into stockholders’ equity.

Leases

Prior to January 1, 2020, the Company accounted for its leases of office space and laboratory facilities under non-cancelable operating lease agreements and recognized 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, were 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 were recorded in prepaid

expenses and other current assets on the condensed balance sheets. The Company did not assume renewals in its determination of the lease term unless they were deemed to be reasonably assured at the inception of the lease and began recognizing rent expense on the date that it obtained the legal right to use and control the leased space. Deferred rent consisted of the difference between cash payments and the rent expense recognized. The Company recognized a liability for costs that would continue to be incurred under a lease contract for its remaining term without economic benefit at its fair value when the entity ceased using the right conveyed by the contract, which was when the space was completely vacated. The Company also entered into capital lease agreements for certain equipment with a lease term of three years. The current portion of capital lease obligations was included in accrued and other current liabilities and the noncurrent capital lease obligations was included in other noncurrent liabilities on the condensed balance sheets.

Subsequent to January 1, 2020, the Company determines whether the arrangement is or contains a lease at the inception of the arrangement and if so, whether such a lease is classified as a financing lease or an operating lease. Operating leases are included in operating lease right-of-use assets, (“ROU assets”), operating lease liabilities, net of current portion, and accrued and other current liabilities on the Company’s condensed balance sheets. The Company has elected not to recognize on the condensed balance sheets leases with terms of one year or less. Operating lease ROU assets represent the Company’s right to use an underlying asset for the lease term and are considered long-lived assets for purposes of identifying, recognizing and measuring impairment. Operating lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of lease payments over the expected lease term. As the Company’s leases do not provide an implicit rate, the Company uses its incremental borrowing rate, which is the rate incurred to borrow on a collateralized basis over a similar term an amount equal to the lease payments in a similar economic environment, in determining the present value of lease payments. The operating lease ROU asset also includes any lease payments made or incentives received and impairment charges if the Company determines the ROU asset is impaired and excludes lease incentives. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise such options to extend or terminate the lease. Lease expense for lease payments is recognized on a straight-line basis over the lease term.

The Company has elected to not separate lease and non-lease components for its leased assets and accounts for all lease and non-lease components of its agreements as a single lease component. The lease components resulting in a ROU asset have been recorded on the condensed balance sheets and are amortized as lease expense on a straight-line basis over the lease term.

The Company does not have any material financing leases.

Impairment of Long-Lived Assets

The Company evaluates long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be fully recoverable. If indicators of impairment exist and the undiscounted future cash flows that the assets are expected to generate are less than the carrying value of the assets, the Company reduces the carrying amount of the assets through an impairment charge, to their estimated fair value based on a discounted cash flow approach or, when available and appropriate, to comparable market values. During the first quarter of 2020, the Company evaluated indicators of impairment for the ROU asset and related leasehold improvements considering the current economic environment, its impact on subleasing activity and the exit of its headquarters previously located in Brisbane, California. The Company concluded the carrying value of these assets were not fully recoverable and recorded an impairment charge of $2.2 million (see Note 6, “Commitments and Contingencies”) during the six months ended June 30, 2020.

Determining estimated discounted cash flows for purposes of an impairment analysis requires the Company to make estimates and assumptions regarding the amount and timing of sublease income. There are often risks and uncertainties associated with the intent to sublease offices and laboratory space. Consequently, the eventual realized sublease revenues may vary from estimates as of the impairment testing date and adjustments may occur in future periods. Furthermore, the Company’s sublease assumptions could be further impacted by the COVID-19 outbreak.

Income Taxes

On March 18, 2020, the Families First Coronavirus Response Act (“FFCR Act”), and on March 27, 2020, the Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) were each enacted in response to the COVID-

19 pandemic. The FFCR Act and the CARES Act contain numerous tax-related provisions relating to refundable payroll tax credits, deferment of employer side social security payments, net operating loss carryback periods, alternative minimum tax credit refunds, modifications to the net interest deduction limitations and technical corrections to tax depreciation methods for qualified improvement property.

On June 29, 2020 California State Assembly Bill 85 (the “Trailer Bill”) was enacted which suspends the use of California net operating loss (“NOL”) deductions and certain tax credits, including research and development tax credits, for the 2020, 2021, and 2022 tax years.

The FFCR Act, CARES Act and Trailer Bill did not have a material impact on the Company’s condensed financial statements as of June 30, 2020; however, the Company continues to examine the impacts the FFCR Act, CARES Act and Trailer Bill may have on its business, results of operations, financial condition and liquidity.

Recently Adopted Accounting Pronouncements

In December 2019, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which removes certain exceptions to the general principles in Topic 740 related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The new guidance also simplifies aspects of the accounting for franchise taxes and enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. This ASU is effective for the Company for all interim and annual periods beginning January 1, 2022, with early adoption permitted. The Company early adopted ASU 2019-12 beginning January 1, 2020 on a prospective basis. The adoption of this standard did not have a material impact on its condensed financial statements and related disclosures. The only aspect of ASU 2019-12 that is currently applicable to the Company is the removal of the exception related to intraperiod tax allocation. The Company began applying the general methodology regarding the intraperiod allocation of tax expense in 2020. After the adoption of ASU 2019-12, in periods where the Company has a loss from continuing operations, the amount of taxes attributable to continuing operations will be determined without regard to the tax effect of other items, including changes in unrealized gains related to marketable securities.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 (ASU 2018-18), which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under the guidance for contracts with customers (Topic 606) when the collaborative arrangement participant is a customer in the context of a unit of account. The standard is effective for interim and annual periods beginning after December 15, 2020, with early adoption permitted, including adoption in any interim period for public business entities for periods in which financial statements have not been issued. The Company adopted this standard on January 1, 2020. The adoption of this ASU did not have a material impact on the Company’s condensed financial statements.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. This ASU eliminates, modifies and adds disclosure requirements for fair value measurements. The amendments in this ASU are effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2019, with early adoption permitted. The Company adopted this standard on January 1, 2020. The adoption of this ASU did not have a material impact on its condensed financial statements but has resulted in enhanced disclosures related to the recurring Level 3 fair value measurements.

In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. The amendments in this ASU expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. This new guidance is effective for the Company in fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted. The Company adopted this standard on January 1, 2020. The adoption of this ASU did not have a material impact on its condensed financial statements.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), and related amendments which supersedes the guidance in former ASC 840, Leases. The new standard, as amended by subsequent ASUs on the

Topic, requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. On November 15, 2019, the FASB issued ASU 2019-10 to delay the effective date of this standard, making it effective for the Company for annual reporting periods beginning after December 15, 2020, and interim periods within annual periods beginning after December 15, 2021, with early adoption permitted.

The Company adopted this standard on January 1, 2020 using the modified retrospective approach with a cumulative effect adjustment to accumulated deficit at the beginning of the period of adoption, if any. The Company elected the package of practical expedients permitted under the transition guidance within Topic 842, which allowed the Company to carry forward the historical lease classification, retain the initial direct costs for any leases that existed prior to the adoption of the standard and not reassess whether any contracts entered into prior to the adoption are leases. The Company also elected to account for lease and non-lease components in its lease agreements as a single lease component in determining lease assets and liabilities. In addition, the Company elected not to recognize the right-of-use assets and liabilities for leases with lease terms of one year or less. The Company did not elect the practical expedient allowing the use-of-hindsight, which would require the Company to reassess the lease term of its leases based on all facts and circumstances through the effective date and did not elect the practical expedient pertaining to land easements as this is not applicable to the current contract portfolio.

Upon adoption of Topic 842, the Company recorded $42.4 million of operating lease liabilities and $27.2 million of right-of-use assets after reclassification of deferred rent of $15.3 million, as of January 1, 2020. The adoption did not have a material impact on the Company’s condensed statements of operations and comprehensive loss or condensed statements of cash flows (see Note 6, “Commitments and Contingencies” for additional information).

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, which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. This new standard also requires customers to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This standard is effective for the Company for annual reporting periods beginning after December 15, 2020, and interim periods within annual periods beginning after December 15, 2021. This new standard can be applied either retrospectively or prospectively to all implementation costs incurred after the date of adoption. The Company is currently evaluating the impact of adoption on its condensed financial statements.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments, as clarified in subsequent amendments. ASU 2016-13 changes the impairment model for certain financial instruments. The new model is a forward-looking expected loss model and will apply to financial assets subject to credit losses and measured at amortized cost and certain off-balance sheet credit exposures. This includes loans, held-to-maturity debt securities, loan commitments, financial guarantees and net investments in leases, as well as trade receivables. For available-for-sale debt securities with unrealized losses, credit losses will be measured in a manner similar to today, except that the losses will be recognized as allowances rather than reductions in the amortized cost of the securities. In October 2019, the FASB voted to delay the effective date of this standard. Topic 326 will be effective for the Company for fiscal years beginning after December 15, 2022. Early adoption is permitted. The Company is currently assessing the effect that this ASU will have on its condensed financial position, results of operations, and disclosures.

Fair Value Measurements

6 Months Ended

Jun. 30, 2020

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 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, 2020
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents:
Money market funds	$	21,280	$	21,280	$	—	$	—
U.S. and foreign commercial paper		4,000		—		4,000		—
Total cash equivalents		25,280		21,280		4,000		—
Short-term marketable securities:
U.S. and foreign commercial paper		5,748		—		5,748		—
U.S. and foreign corporate debt securities		10,381		—		10,381		—
U.S. government debt securities		36,202		—		36,202		—
U.S. treasuries		19,845		—		19,845		—
Total short-term marketable securities		72,176		—		72,176		—
Strategic investment
Foreign equity securities		7,768		7,768		—		—
Total strategic investment		7,768		7,768		—		—
Total assets subject to fair value measurements on a recurring basis	$	105,224	$	29,048	$	76,176	$	—
Liabilities:
Contingent consideration liability	$	1,816	$	—	$	—	$	1,816
Total liabilities subject to fair value measurements on a recurring basis	$	1,816	$	—	$	—	$	1,816

	December 31, 2019
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents:
Money market funds	$	29,377	$	29,377	$	—	$	—
U.S. and foreign commercial paper		4,999		—		4,999		—
U.S government debt securities		2,550		—		2,550		—
Total cash equivalents		36,926		29,377		7,549		—
Short-term marketable securities:
U.S. treasuries		15,063		—		15,063		—
U.S. and foreign commercial paper		11,972		—		11,972		—
U.S. and foreign corporate debt securities		8,755		—		8,755		—
U.S. government debt securities		48,718		—		48,718		—
Total short-term marketable securities		84,508		—		84,508		—
Strategic investment
Foreign equity securities		5,507		5,507		—		—
Total strategic investment		5,507		5,507		—		—
Long-term marketable securities
U.S. treasuries		3,025		—		3,025		—
Total long-term marketable securities		3,025		—		3,025		—
Total assets subject to fair value measurements on a recurring basis	$	129,966	$	34,884	$	95,082	$	—
Liabilities:
Contingent consideration liability	$	1,131	$	—	$	—	$	1,131
Total liabilities subject to fair value measurements on a recurring basis	$	1,131	$	—	$	—	$	1,131

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, U.S. government debt securities and foreign equity 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 Company has recorded a contingent consideration liability related to three agreements (the “Commercial Agreements”) with Ascentage Pharma Group Corp. Limited, a clinical-stage biopharmaceutical company based in Hong Kong China (“Ascentage Pharma”) (see Note 5, “License Agreements”). As of June 30, 2020, 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 Ascentage Pharma and an academic institution from which Ascentage Pharma had previously in-licensed the underlying technology based on achievement of certain specified preclinical and clinical development and sales milestone events for two or more licensed compounds. The fair value of the contingent consideration liability includes inputs not observable in the market and thus represents a Level 3 measurement. 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 utilizes current stock price and reflects the probability and timing of future issuances of shares. The probability and timing of future issuances of shares is based on current scenarios and plans to research, develop, and seek to obtain marketing approval for two or more licensed compounds under the Commercial Agreements, with individual probabilities for each defined milestone event ranging from 90% to 100%, and with a cumulative probability of 90%. 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 clinical programs and 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 $5.6 million (using the Company’s stock price as of June 30, 2020). As of June 30, 2020, and December 31, 2019, none of the commercial milestones had been achieved and no royalties were due from the sales of licensed products.

The following table provides a reconciliation of the Company’s financial liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) at June 30, 2020 and 2019 (in thousands):

	Amount
Balance at December 31, 2019	$	1,131
Additions		—
Settlements		—
Change in fair value		685
Balance at June 30, 2020	$	1,816

	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 into and out of Level 3 of the fair value hierarchy during the six months ended June 30, 2020 and 2019.

The Company holds an equity investment in Ascentage International, an affiliate of Ascentage Pharma. The equity interest represents an insignificant level of ownership in the investee and has been recorded within strategic investment on the Company’s condensed balance sheets (see Note 5, “License Agreements”). In October 2019, Ascentage International completed an initial public offering of common stock on the Hong Kong Stock Exchange. Following the initial public offering, the Company’s underlying investment changed to be an equity security with a readily determinable fair value which is measured at fair value on a recurring basis based on quoted stock prices available on the Hong Kong Stock Exchange, which are considered observable inputs (Level 1). The fair value of the common stock was $7.8 million and $5.5 million as of June 30, 2020 and December 31, 2019, respectively, and is included in strategic investment on the Company’s condensed balance sheets. The change in fair value of this investment was $2.3 million and zero for the six months ended June 30, 2020 and 2019, respectively, and was recorded in other income (expense), net on the statements of operations and comprehensive loss.

See Note 4, “Marketable Securities,” for further information regarding the carrying value of the Company’s financial instruments.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Marketable Securities

6 Months Ended

Jun. 30, 2020

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, 2020 (in thousands):

	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	21,280	$	—	$	—		$	21,280
U.S. and foreign commercial paper		4,000		—		—			4,000
Total cash equivalents		25,280		—		—			25,280
Short-term marketable securities:
U.S. and foreign commercial paper		5,738		10		—			5,748
U.S. and foreign corporate debt securities		10,338		43		—			10,381
U.S. government debt securities		36,088		116		(2	)		36,202
U.S. treasuries		19,780		65		—			19,845
Total short-term marketable securities		71,944		234		(2	)		72,176
Total	$	97,224	$	234	$	(2	)	$	97,456

Marketable securities, which are classified as available-for-sale, consisted of the following as of December 31, 2019 (in thousands):

	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	29,377	$	—	$	—		$	29,377
U.S. and foreign commercial paper		4,999		—		—			4,999
U.S. government debt securities		2,550		—		—			2,550
Total cash equivalents		36,926		—		—			36,926
Short-term marketable securities:
U.S. and foreign commercial paper		11,965		7		—			11,972
U.S. and foreign corporate debt securities		8,748		8		(1	)		8,755
U.S. government debt securities		48,647		71		—			48,718
U.S. treasuries		15,057		6		—			15,063
Total short-term marketable securities		84,417		92		(1	)		84,508
Long-term marketable securities
U.S. treasuries		3,025		—		—			3,025
Total long-term marketable securities		3,025		—		—			3,025
Total	$	124,368	$	92	$	(1	)	$	124,459

At June 30, 2020, the remaining contractual maturities of available-for-sale securities were less than one year. 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, 2020 and December 31, 2019. The Company does not intend to and does not believe it is more likely than not that it will be required to sell these debt securities before their maturities.

See Note 3, “Fair Value Measurements,” for further information regarding the fair value of the Company’s financial instruments.

License Agreements

6 Months Ended

Jun. 30, 2020

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 Ascentage Pharma: (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 (this second license agreement, the “Bcl License Agreement” and collectively with the other two agreements, the “Commercial Agreements”).

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 of (a) 933,337 shares of common stock in the event there is only one licensed product, and (b) 1,333,338 shares of common stock in the event there are two or more licensed products, in each case to be issued based on the Company’s achievement of certain preclinical and clinical development and sales milestone events. The Company is required to make 80% of all equity payments to Ascentage Pharma and the remaining 20% to an academic institution from whom Ascentage Pharma had previously licensed the technology. The milestones include the advancement of additional compounds into Investigational New Drug application (“IND”) enabling studies, the filing of an IND, the commencement of clinical studies, Food and Drug Administration (“FDA”) and/or European Medicines Agency approval, and a net sales threshold. The Bcl License Agreement also includes tiered royalties in the low-single digits based on sales of licensed products. As of June 30, 2020, the Company had issued 640,002 shares of common stock to Ascentage Pharma and 160,000 shares of common stock to an academic institution from whom Ascentage Pharma had previously licensed the technology.

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 Ascentage Pharma and the academic institution. These shares were issued to Ascentage Pharma in January 2019 and the academic institution in March 2019.

In June 2020, the Company entered into a third amendment to the Bcl License Agreement. Under the terms of the original Bcl License Agreement, Ascentage Pharma granted the Company exclusive development and commercialization rights and non-exclusive manufacturing rights to an Ascentage Bcl inhibitor compound known as UBX1967 as well as the right to continue its preclinical development efforts with another Ascentage-controlled Bcl inhibitor compound, known as UBX1325, a small molecule inhibitor of the anti-apoptotic Bcl-2 family member, Bcl-xL, that served as a back-up to UBX1967. Under the terms of the third amendment to the Bcl License Agreement, the status of UBX1967 and UBX1325 were switched such that UBX1325 became the licensed compound and UBX1967 became the back-up compound under the Bcl License Agreement.

On July 1, 2020, the Company announced that it had completed IND-enabling studies for UBX1325. The Company plans to initiate a Phase 1 clinical study for UBX1325 in the second half of 2020. Upon filing an IND for the Phase 1 clinical study for UBX1325, the Company will become obligated to issue 133,333 shares of common stock to Ascentage Pharma and the academic institution. Upon commencement of a Phase 1 study, the Company will also be obligated to make a milestone payment of $1.0 million, which the Company may elect to pay in cash or shares of common stock and may also become obligated to issue an additional 133,333 shares of its common stock to these parties.

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 $1.8 million at June 30, 2020 and $1.1 million at December 31, 2019. The $1.8 million contingent consideration liability was recorded as a current liability based on the latest estimates for milestone achievements. To date, no royalties were due from the sales of licensed products.

In April 2016, in connection with the Commercial Agreements, the Company purchased an interest in an affiliate of Ascentage Pharma for an aggregate purchase price of $0.5 million. In May 2018, this interest was exchanged for an interest in a newly formed affiliate of Ascentage Pharma called Ascentage International as part of a reorganization

of those entities. The Company also invested an additional $0.5 million in Ascentage International in May 2018 which was recorded within other long-term assets on the Company’s balance sheet as of December 31, 2018.

In October 2019, Ascentage International completed an initial public offering of shares of its common stock on the Hong Kong Stock Exchange at HK$34.20 (approximately USD $4.36) per share. In connection with Ascentage International’s initial public offering, the Company’s interest converted into shares of common stock of Ascentage International. The Company determined that its investment in Ascentage International met the definition of an equity security with a readily determinable fair value which is measured at fair value on a recurring basis based on quoted stock price available on the Hong Kong Stock Exchange. The Company was subject to a lock-up agreement with Ascentage International that precluded the Company from selling shares prior to April 28, 2020.

The Company agreed to provide funding to Ascentage Pharma 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 not material for the six months ended June 30, 2020 and 2019.

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 of its San Francisco campus (collectively, “UCSF”) which provides the Company the rights to certain patents and related know-how to make, use, sell, offer for sale and import certain products and practice certain methods for use in the development of human therapeutics, which excludes the provision of services to third parties for consideration of any kind. The license to the Company is subject to UCSF’s reserved rights under the licensed intellectual property for educational and non-commercial research purposes and a requirement to substantially manufacture any licensed products in the United States. The Company is obligated to use diligent efforts to develop and obtain regulatory approval for at least one product commercialized pursuant to the agreement, and must meet certain regulatory and development milestones. 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 development events, up to aggregate milestone payments of $13.6 million for each product licensed under the agreement, 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, 2020. The upfront issuance of 120,000 shares of the Company’s common stock was valued at $1.0 million and recorded as additional paid-in capital upon issuance in June 2019.

The Company has also 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, 2020 or December 31, 2019. To date, none of these events has occurred and no contingent consideration, milestone or royalty payments have been recognized.

Commitments and Contingencies

6 Months Ended

Jun. 30, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

6. Commitments and Contingencies

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 from occupancy of approximately ten years ending on December 31, 2029 with an option to extend the term for an additional eight years at then-market rental rates. The total base rent payment escalates annually based on a fixed percentage beginning from the 13^th month of the lease agreement. The Company will also be responsible for the operating expenses and real estate taxes allocated to the building and common areas. Pursuant to the lease

agreement, the landlord provided the Company with a tenant improvement allowance of $10.7 million, which was included in deferred rent and leasehold improvements on the balance sheet at December 31, 2019. In connection with the execution of the lease agreement, the Company delivered a letter of credit of approximately $0.9 million to the landlord.

In May 2016, the Company executed a non-cancellable lease agreement for office and laboratory space in Brisbane, California which commenced in May 2016 and continues through October 2022. The lease agreement includes an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional four years by giving the landlord written notice of the election to exercise the option at least fifteen months prior to the original expiration of the lease term. The lease provides for monthly base rent amounts escalating over the term of the lease and the lessor provided the Company a $3.9 million tenant improvement allowance to complete the laboratory and office renovation which was recorded as deferred rent liability and leasehold improvements within property and equipment, net. In May 2017, the Company entered into an amendment to expand the leased space and received a three-month rent holiday for the expanded space.

The Company’s operating leases include various covenants, indemnities, defaults, termination rights, security deposits and other provisions customary for lease transactions of this nature.

The following table summarizes the components of lease expense, which are included in operating expenses in the Company’s condensed statements of operations and comprehensive loss (in thousands):

	Three Months Ended		Six Months Ended
	June 30, 2020		June 30, 2020
Operating lease cost	$	1,144	$	2,444
Variable lease cost		363		641
Impairment of operating lease right-of-use asset		—		1,157
Total lease cost	$	1,507	$	4,242

Variable lease payments include amounts relating to common area maintenance, real estate taxes and insurance and are recognized in the condensed statements of operations and comprehensive loss as incurred. Rent expense for the six months ended June 30, 2019 was $1.5 million.

The following table summarizes supplemental information related to leases (in thousands):

	Six Months Ended
	June 30, 2020
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$	2,557
Weighted-average remaining lease term (years)
Operating leases		8.8
Weighted-average discount rate (percentage)
Operating leases		5.8	%

The following table summarizes the maturities of lease liabilities as of June 30, 2020 (in thousands):

	Amount
2020 (remaining 6 months)	$	3,241
2021		6,653
2022		6,283
2023		4,810
2024		4,964
Thereafter		27,302
Total future minimum lease payments		53,253
Less: Amount representing interest		(12,171	)
Present value of future minimum lease payments		41,082
Less: Current portion of operating lease liability		(4,312	)
Noncurrent portion of operating lease liability	$	36,770

The cumulative effect on the Company’s condensed balance sheets at January 1, 2020 from the adoption of Topic 842 was as follows (in thousands):

	December 31, 2019		Topic 842 Adjustments			January 1, 2020
Operating lease right-of-use assets	$	—	$	27,174		$	27,174
Accrued and other current liabilities		4,995		(1,970	)		3,025
Operating lease liabilities, current portion		—		3,455			3,455
Deferred rent, net of current portion		13,298		(13,298	)		—
Operating lease liabilities, net of current portion		—		38,988			38,988

In February 2020, the Company completed its move into the new office and laboratory space in South San Francisco, exited its previous offices and laboratory space in Brisbane, California, and began to actively market this space for sublease. Concurrent with this move and in consideration of real estate market conditions, in particular due to the COVID-19 pandemic in March 2020, the Company identified indicators of impairment in the related asset group, which included the leased ROU asset and related leasehold improvements associated with the lease. The Company subsequently evaluated and compared the net book value of the asset group to the estimated undiscounted future cash flows over the remaining term of the lease and concluded that an impairment had occurred. The discounted estimated future cash flows include estimates of sublease rentals through the end of the lease term, which ends on October 31, 2022, utilizing a discount rate of 3.5% based on the Company’s estimated incremental borrowing rate. The estimated discounted cash flows were compared to the net book value of the ROU asset and leasehold improvements resulting in an impairment loss of $2.2 million. This impairment loss was allocated proportionally to the right-of-use asset of $1.2 million and leasehold improvements of $1.0 million and recorded in operating expense in the condensed statements of operations and comprehensive loss for the six months ended June 30, 2020. After recording the impairment, the remaining balance of the ROU asset and leasehold improvements was $1.8 million and $1.6 million, respectively. The Company continues to actively market this space for sublease. Additional impairments could be realized if the Company’s sublease assumptions are further impacted by real estate market conditions, including potentially as a result of the COVID-19 pandemic.

Indemnification

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.

Equity Financing

6 Months Ended

Jun. 30, 2020

Equity [Abstract]

Equity Financing

7. Equity Financing

In June 2019, the Company 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 an “at-the-market” offering under the Securities Act of 1933, as amended (the “ATM Offering Program”). The SEC declared the Shelf Registration Statement effective on June 6, 2019.

On June 3, 2019, the Company also entered into a sales agreement (the “June 2019 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 $75.0 million, through the ATM Offering Program under which Cowen acts as its sales agent. 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 June 2019 Sales Agreement. In addition, the Company has agreed to reimburse a portion of Cowen’s expenses in connection with the offering up to a maximum of $0.1 million. During the six months ended June 30, 2020, the Company issued and sold 4,107,870 shares of its common stock through its ATM Offering Program and received net proceeds of approximately $29.5 million, after deducting commissions and other offering expenses of $1.0 million.

Stock-Based Compensation

6 Months Ended

Jun. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

8. Stock-Based Compensation

Stock Options and Restricted Stock Units Activity

A summary of the Company’s stock option activity under the 2013 Equity Incentive Plan, 2018 Incentive Award Plan, and 2020 Employment Inducement Incentive Plan for the six months ended June 30, 2020 is as follows:

	Number of Shares			Weighted- Average Exercise Price
Balances at December 31, 2019		6,906,898		$	7.62
Granted		3,272,256		$	6.33
Exercised		(123,191	)	$	3.40
Canceled		(587,441	)	$	9.43
Balances at June 30, 2020		9,468,522		$	7.11

A summary of the Company’s restricted stock units (“RSUs”) and performance stock units (“PSUs”) activity for the six months ended June 30, 2020 is as follows:

	Number of Shares			Weighted- Average Grant Date Fair Value
Unvested at December 31, 2019		325,887		$	9.00
Granted		661,314		$	5.67
Vested		(103,020	)	$	9.00
Canceled		(43,539	)	$	9.00
Unvested at June 30, 2020		840,642		$	6.38

For stock options granted to employees with service-based vesting, the fair value of stock options granted to employees was estimated on the date of grant using the Black-Scholes option pricing model and utilizing assumptions that were determined as follows:

Expected Term—The expected term represents the period that the options granted are expected to be outstanding and is determined using the simplified method (based on the mid-point between the vesting date and the end of the contractual term) as the Company has concluded that its stock option exercise history does not provide a reasonable basis upon which to estimate expected term.

Expected Volatility—Due to limited historical data, the Company estimates stock price volatility based on a combined weighted-average of the Company’s historical average volatility and that of a selected peer group of comparable publicly traded companies within the biotechnology and pharmaceutical industry that were deemed to be representative of future stock price trends over the expected life of the award.

Risk-Free Interest Rate—The Company based the risk-free interest rate over the expected term of the options based on the constant maturity rate of U.S. Treasury securities with similar maturities as of the date of the grant.

Expected Dividends—The Company has never paid any dividends and does not plan to pay dividends in the foreseeable future. Therefore, the expected dividend yield is zero.

In March 2020, the board of directors granted the Company’s newly hired Chief Executive Officer stock-based awards covering an aggregate of 1.1 million shares of common stock, including options to purchase an aggregate of 800,000 shares of common stock, 120,000 RSUs, 150,000 PSUs and 30,000 shares of common stock. The stock-based awards were granted pursuant to the 2020 Employment Inducement Incentive Plan, which was approved by the board of directors in March 2020 to provide for grants to newly hired employees as a material inducement for them to commence employment with the Company.

The 30,000 shares of common stock were fully vested on the date of grant and thus, the related compensation expense of $0.2 million was recognized on the grant date. The stock options and RSUs will vest subject to continued service through the applicable vesting date.

The PSUs vest as to 50,000 PSUs upon the attainment of (a) a volume-weighted average per share closing trading price of the Company’s common stock of at least $36.875 over a trailing 30-day period or (b) a change in control transaction in which the price per share to the holders of the Company’s common stock is at least $36.875 and as to 100,000 PSUs (x) at such time as the Company’s market capitalization reaches at least $2.5 billion, as measured based on the volume weighted-average closing trading price over a trailing 30 day period or (y) a change in control transaction in which the consideration paid to the Company’s stockholders is equal to at least $2.5 billion, as determined by the board of directors.

For the PSU awards, the Company used the Monte-Carlo option pricing model to determine the fair value of awards at the date of grant. The Monte-Carlo option pricing model uses similar input assumptions as the Black-Scholes model; however, it further incorporates into the fair-value determination the possibility that the market condition may not be satisfied. Compensation costs related to awards with a market-based condition are recognized regardless of whether the market condition is ultimately satisfied. Compensation cost is not reversed if the achievement of the market condition does not occur. The total grant date fair value of the PSU awards was determined to be $0.7 million and will be recognized as compensation expense over the weighted-average derived service period of approximately 4.3 years.

During the six months ended June 30, 2020 the Company issued 13,550 shares in settlement of stock-based compensation awards accounted for as liability awards.

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, and costs associated with the Company’s 2018 Employee Stock Purchase Plan (“2018 ESPP”) included in the Company’s condensed statements of operations and comprehensive loss (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development	$	1,937	$	931	$	3,472	$	1,926
General and administrative		2,167		1,013		3,949		2,015
Total	$	4,104	$	1,944	$	7,421	$	3,941

Stock based compensation for the six months ended June 30, 2020 includes $0.1 million of expense related to awards accounted for as liability awards.

Net Loss per Common Share

6 Months Ended

Jun. 30, 2020

Earnings Per Share [Abstract]

Net Loss per Common Share

9. 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 contingencies are satisfied during the period and the presumed issuance of 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, RSUs (including PSUs), early exercised common stock subject to future vesting, restricted stock accounted for as options, shares subject to the 2018 ESPP 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 share and per share amounts):

	Three Months Ended June 30,						Six Months Ended June 30,
	2020			2019			2020			2019
Numerator:
Net loss	$	(18,667	)	$	(23,673	)	$	(46,705	)	$	(42,440	)
Denominator:
Weighted-average number of shares outstanding—basic and diluted		49,659,153			42,442,886			48,606,768			42,311,040
Net loss per share—basic and diluted	$	(0.38	)	$	(0.56	)	$	(0.96	)	$	(1.00	)

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. Potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive were as follows:

	June 30,
	2020		2019
Options to purchase common stock		9,468,522		7,051,590
Early exercised common stock subject to future vesting		80,285		478,288
RSUs		840,642		—
Shares subject to 2018 ESPP		68,188		52,047
Total		10,457,637		7,581,925

Up to 640,218 shares may be contingently issued, if certain performance conditions are met under the Company’s in-licensing agreements. See Note 5, “License Agreements,” to our condensed financial statements for additional information.

Subsequent Events

6 Months Ended

Jun. 30, 2020

Subsequent Events [Abstract]

Subsequent Events

10. Subsequent Events

On July 30, 2020, the Company notified Ascentage International of its intention to terminate its license agreement for its compound APG1252 due to the Company’s decision to prioritize the progression of other compounds from Ascentage International’s library of Bcl-2 inhibitors, such as UBX1325 and UBX1967. Pursuant to its terms, the APG1252 license agreement and all amendments thereto will terminate ninety days from the date of such notice.

On July 31, 2020, the Company entered into a sales agreement (the “July 2020 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 $50.0 million, 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 July 2020 Sales Agreement are deemed an “at-the-market” offering under the Securities Act of 1933, as amended (the “Securities Act”). 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 July 2020 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 July 2020, the Company issued and sold 882,106 shares of its common stock through its ATM Offering Program and received net proceeds of approximately $7.7 million, after deducting commissions of $0.2 million.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2020

Accounting Policies [Abstract]

Need for Additional Capital

Basis of Presentation

Unaudited Condensed Financial Statements

Use of Estimates

Cash, Cash Equivalents and Restricted Cash

	June 30, 2020		December 31, 2019
Cash and cash equivalents	$	39,592	$	37,473
Restricted cash		1,446		1,446
Total cash, cash equivalents and restricted cash	$	41,038	$	38,919

Marketable Securities

Strategic Investments

Concentrations of Risk

The Company is also exposed to market risk in its strategic investments. As of June 30, 2020, the Company held an investment in common stock which is publicly traded in Hong Kong.

Research and Development Expenses and Accruals

Contingent Consideration Liability

Leases

The Company does not have any material financing leases.

Impairment of Long-Lived Assets

Income Taxes

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements Not Yet Adopted

Fair Value Measurements

•

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)

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2020

Accounting Policies [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash

	June 30, 2020		December 31, 2019
Cash and cash equivalents	$	39,592	$	37,473
Restricted cash		1,446		1,446
Total cash, cash equivalents and restricted cash	$	41,038	$	38,919

Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2020

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, 2020
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents:
Money market funds	$	21,280	$	21,280	$	—	$	—
U.S. and foreign commercial paper		4,000		—		4,000		—
Total cash equivalents		25,280		21,280		4,000		—
Short-term marketable securities:
U.S. and foreign commercial paper		5,748		—		5,748		—
U.S. and foreign corporate debt securities		10,381		—		10,381		—
U.S. government debt securities		36,202		—		36,202		—
U.S. treasuries		19,845		—		19,845		—
Total short-term marketable securities		72,176		—		72,176		—
Strategic investment
Foreign equity securities		7,768		7,768		—		—
Total strategic investment		7,768		7,768		—		—
Total assets subject to fair value measurements on a recurring basis	$	105,224	$	29,048	$	76,176	$	—
Liabilities:
Contingent consideration liability	$	1,816	$	—	$	—	$	1,816
Total liabilities subject to fair value measurements on a recurring basis	$	1,816	$	—	$	—	$	1,816

	December 31, 2019
	Total		Level 1		Level 2		Level 3
Assets:
Cash equivalents:
Money market funds	$	29,377	$	29,377	$	—	$	—
U.S. and foreign commercial paper		4,999		—		4,999		—
U.S government debt securities		2,550		—		2,550		—
Total cash equivalents		36,926		29,377		7,549		—
Short-term marketable securities:
U.S. treasuries		15,063		—		15,063		—
U.S. and foreign commercial paper		11,972		—		11,972		—
U.S. and foreign corporate debt securities		8,755		—		8,755		—
U.S. government debt securities		48,718		—		48,718		—
Total short-term marketable securities		84,508		—		84,508		—
Strategic investment
Foreign equity securities		5,507		5,507		—		—
Total strategic investment		5,507		5,507		—		—
Long-term marketable securities
U.S. treasuries		3,025		—		3,025		—
Total long-term marketable securities		3,025		—		3,025		—
Total assets subject to fair value measurements on a recurring basis	$	129,966	$	34,884	$	95,082	$	—
Liabilities:
Contingent consideration liability	$	1,131	$	—	$	—	$	1,131
Total liabilities subject to fair value measurements on a recurring basis	$	1,131	$	—	$	—	$	1,131

Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Text Block]

	Amount
Balance at December 31, 2019	$	1,131
Additions		—
Settlements		—
Change in fair value		685
Balance at June 30, 2020	$	1,816

	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)

6 Months Ended

Jun. 30, 2020

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, 2020 (in thousands):

	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	21,280	$	—	$	—		$	21,280
U.S. and foreign commercial paper		4,000		—		—			4,000
Total cash equivalents		25,280		—		—			25,280
Short-term marketable securities:
U.S. and foreign commercial paper		5,738		10		—			5,748
U.S. and foreign corporate debt securities		10,338		43		—			10,381
U.S. government debt securities		36,088		116		(2	)		36,202
U.S. treasuries		19,780		65		—			19,845
Total short-term marketable securities		71,944		234		(2	)		72,176
Total	$	97,224	$	234	$	(2	)	$	97,456

Marketable securities, which are classified as available-for-sale, consisted of the following as of December 31, 2019 (in thousands):

	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value
Cash equivalents:
Money market funds	$	29,377	$	—	$	—		$	29,377
U.S. and foreign commercial paper		4,999		—		—			4,999
U.S. government debt securities		2,550		—		—			2,550
Total cash equivalents		36,926		—		—			36,926
Short-term marketable securities:
U.S. and foreign commercial paper		11,965		7		—			11,972
U.S. and foreign corporate debt securities		8,748		8		(1	)		8,755
U.S. government debt securities		48,647		71		—			48,718
U.S. treasuries		15,057		6		—			15,063
Total short-term marketable securities		84,417		92		(1	)		84,508
Long-term marketable securities
U.S. treasuries		3,025		—		—			3,025
Total long-term marketable securities		3,025		—		—			3,025
Total	$	124,368	$	92	$	(1	)	$	124,459

Commitments and Contingencies (Tables)

6 Months Ended

Jun. 30, 2020

Commitments And Contingencies Disclosure [Abstract]

Summary of Components of Lease Expense

The following table summarizes the components of lease expense, which are included in operating expenses in the Company’s condensed statements of operations and comprehensive loss (in thousands):

	Three Months Ended		Six Months Ended
	June 30, 2020		June 30, 2020
Operating lease cost	$	1,144	$	2,444
Variable lease cost		363		641
Impairment of operating lease right-of-use asset		—		1,157
Total lease cost	$	1,507	$	4,242

Summary of Supplemental Information Related to Leases

The following table summarizes supplemental information related to leases (in thousands):

	Six Months Ended
	June 30, 2020
Cash paid for amounts included in the measurement of lease liabilities
Operating cash flows from operating leases	$	2,557
Weighted-average remaining lease term (years)
Operating leases		8.8
Weighted-average discount rate (percentage)
Operating leases		5.8	%

Summary of Maturities of Lease Liabilities

The following table summarizes the maturities of lease liabilities as of June 30, 2020 (in thousands):

	Amount
2020 (remaining 6 months)	$	3,241
2021		6,653
2022		6,283
2023		4,810
2024		4,964
Thereafter		27,302
Total future minimum lease payments		53,253
Less: Amount representing interest		(12,171	)
Present value of future minimum lease payments		41,082
Less: Current portion of operating lease liability		(4,312	)
Noncurrent portion of operating lease liability	$	36,770

Adoption of New Accounting Pronouncements Cumulative Effect on Condensed Balance Sheets

The cumulative effect on the Company’s condensed balance sheets at January 1, 2020 from the adoption of Topic 842 was as follows (in thousands):

	December 31, 2019		Topic 842 Adjustments			January 1, 2020
Operating lease right-of-use assets	$	—	$	27,174		$	27,174
Accrued and other current liabilities		4,995		(1,970	)		3,025
Operating lease liabilities, current portion		—		3,455			3,455
Deferred rent, net of current portion		13,298		(13,298	)		—
Operating lease liabilities, net of current portion		—		38,988			38,988

Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Summary of Stock Option Activity

	Number of Shares			Weighted- Average Exercise Price
Balances at December 31, 2019		6,906,898		$	7.62
Granted		3,272,256		$	6.33
Exercised		(123,191	)	$	3.40
Canceled		(587,441	)	$	9.43
Balances at June 30, 2020		9,468,522		$	7.11

Summary of Restricted Stock Units and Performance Stock Units Activity

A summary of the Company’s restricted stock units (“RSUs”) and performance stock units (“PSUs”) activity for the six months ended June 30, 2020 is as follows:

	Number of Shares			Weighted- Average Grant Date Fair Value
Unvested at December 31, 2019		325,887		$	9.00
Granted		661,314		$	5.67
Vested		(103,020	)	$	9.00
Canceled		(43,539	)	$	9.00
Unvested at June 30, 2020		840,642		$	6.38

Summary of Stock-based Compensation Expense

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development	$	1,937	$	931	$	3,472	$	1,926
General and administrative		2,167		1,013		3,949		2,015
Total	$	4,104	$	1,944	$	7,421	$	3,941

Net Loss per Common Share (Tables)

6 Months Ended

Jun. 30, 2020

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 share and per share amounts):

	Three Months Ended June 30,						Six Months Ended June 30,
	2020			2019			2020			2019
Numerator:
Net loss	$	(18,667	)	$	(23,673	)	$	(46,705	)	$	(42,440	)
Denominator:
Weighted-average number of shares outstanding—basic and diluted		49,659,153			42,442,886			48,606,768			42,311,040
Net loss per share—basic and diluted	$	(0.38	)	$	(0.56	)	$	(0.96	)	$	(1.00	)

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,
	2020		2019
Options to purchase common stock		9,468,522		7,051,590
Early exercised common stock subject to future vesting		80,285		478,288
RSUs		840,642		—
Shares subject to 2018 ESPP		68,188		52,047
Total		10,457,637		7,581,925

Organization - Additional Information (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($) Segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of operating segment \| Segment					1
Accumulated deficit	$ (292,160)				$ (292,160)		$ (245,455)
Net loss	(18,667)	$ (28,038)	$ (23,673)	$ (18,767)	(46,705)	$ (42,440)
Cash used in operations					(44,357)	$ (39,897)
Cash, cash equivalents and marketable securities	$ 111,800				$ 111,800

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/exampleRef
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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Reference 10: http://www.xbrl.org/2003/role/disclosureRef
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Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 14: http://www.xbrl.org/2003/role/disclosureRef
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Reference 15: http://www.xbrl.org/2003/role/disclosureRef
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Reference 16: http://www.xbrl.org/2003/role/disclosureRef
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Reference 17: http://www.xbrl.org/2003/role/disclosureRef
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Reference 18: http://www.xbrl.org/2003/role/disclosureRef
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Reference 19: http://www.xbrl.org/2003/role/disclosureRef
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Reference 21: http://www.xbrl.org/2003/role/disclosureRef
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Reference 23: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Cash and cash equivalents	$ 39,592	$ 37,473
Restricted cash	1,446	1,446
Total cash, cash equivalents and restricted cash	$ 41,038	$ 38,919	$ 24,559	$ 15,949

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Jan. 01, 2020
Summary Of Significant Accounting Policies [Line Items]
Off-balance sheet concentrations of credit risk description	As of June 30, 2020, the Company had no off-balance sheet concentrations of credit risk.
Off-balance sheet concentrations of credit risk	$ 0
Capital lease agreements, term of contract	3 years
Impairment charge	$ 2,200,000
Operating lease, right of use assets	24,891,000
Operating lease liability	$ 41,082,000
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right of use assets		$ 27,200,000
Operating lease liability		42,400,000
Deferred rent		$ 15,300,000
Maximum
Summary Of Significant Accounting Policies [Line Items]
Lease term where right-of-use asset and lease liability not recognized	1 year

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, 2020	Dec. 31, 2019
Assets:
Short-term marketable securities	$ 72,176	$ 84,508
Strategic investment	7,768	5,507
Long-term marketable securities		3,025
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	25,280	36,926
Short-term marketable securities	72,176	84,508
Strategic investment	7,768	5,507
Long-term marketable securities		3,025
Total assets subject to fair value measurements on a recurring basis	105,224	129,966
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	1,816	1,131
Fair Value, Measurements, Recurring \| Level 1
Assets:
Cash equivalents	21,280	29,377
Strategic investment	7,768	5,507
Total assets subject to fair value measurements on a recurring basis	29,048	34,884
Fair Value, Measurements, Recurring \| Level 2
Assets:
Cash equivalents	4,000	7,549
Short-term marketable securities	72,176	84,508
Long-term marketable securities		3,025
Total assets subject to fair value measurements on a recurring basis	76,176	95,082
Fair Value, Measurements, Recurring \| Level 3
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	1,816	1,131
Fair Value, Measurements, Recurring \| Contingent Consideration Liability
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	1,816	1,131
Fair Value, Measurements, Recurring \| Contingent Consideration Liability \| Level 3
Liabilities:
Total liabilities subject to fair value measurements on a recurring basis	1,816	1,131
Money market funds \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	21,280	29,377
Money market funds \| Fair Value, Measurements, Recurring \| Level 1
Assets:
Cash equivalents	21,280	29,377
U.S. and Foreign Commercial Paper \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents	4,000	4,999
Short-term marketable securities	5,748	11,972
U.S. and Foreign Commercial Paper \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Cash equivalents	4,000	4,999
Short-term marketable securities	5,748	11,972
U.S. Treasuries \| Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	19,845	15,063
Long-term marketable securities		3,025
U.S. Treasuries \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Short-term marketable securities	19,845	15,063
Long-term marketable securities		3,025
US Government Debt Securities \| Fair Value, Measurements, Recurring
Assets:
Cash equivalents		2,550
Short-term marketable securities	36,202	48,718
US Government Debt Securities \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Cash equivalents		2,550
Short-term marketable securities	36,202	48,718
U.S. and Foreign Corporate Debt Securities \| Fair Value, Measurements, Recurring
Assets:
Short-term marketable securities	10,381	8,755
U.S. and Foreign Corporate Debt Securities \| Fair Value, Measurements, Recurring \| Level 2
Assets:
Short-term marketable securities	10,381	8,755
Foreign Equity Securities \| Fair Value, Measurements, Recurring
Assets:
Strategic investment	7,768	5,507
Foreign Equity Securities \| Fair Value, Measurements, Recurring \| Level 1
Assets:
Strategic investment	$ 7,768	$ 5,507

Fair Value Measurements - Additional Information (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cumulative probability of defined milestones	90.00%
Strategic investment	$ 7,768,000		$ 5,507,000
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Transfers into and out of fair value hierarchy	$ 0	$ 0
Minimum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone event range	90.00%
Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Milestone event range	100.00%
Commercial Agreements \| Clinical Stage Biopharmaceutical Company
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Strategic investment	$ 7,800,000		$ 5,500,000
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
Commercial Agreements \| Clinical Stage Biopharmaceutical Company \| Contingent Consideration \| Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Maximum amount of potential contingent consideration value	$ 5,600,000
License and Compound Library and Option Agreement
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Change in fair value of investment	$ 2,300,000	$ 0

Fair Value Measurements - Summary of Reconciliation of Financial Liabilities Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Details) - Fair Value, Measurements, Recurring - Level 3 - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Balance at December	$ 1,131	$ 2,483
Change in fair value	685	(213)
Balance at June	$ 1,816	$ 2,270

Marketable Securities - Summary of Marketable Securities Classified as Available-for-Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$ 97,224	$ 124,368
Unrealized Gains	234	92
Unrealized Losses	(2)	(1)
Fair Value	97,456	124,459
Cash Equivalents
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	25,280	36,926
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	25,280	36,926
Cash Equivalents \| Money market funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	21,280	29,377
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	21,280	29,377
Cash Equivalents \| U.S. and Foreign Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	4,000	4,999
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	4,000	4,999
Cash Equivalents \| U.S. and Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		2,550
Unrealized Gains		0
Unrealized Losses		0
Fair Value		2,550
Short Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	71,944	84,417
Unrealized Gains	234	92
Unrealized Losses	(2)	(1)
Fair Value	72,176	84,508
Short Term Marketable Securities \| U.S. and Foreign Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	5,738	11,965
Unrealized Gains	10	7
Unrealized Losses	0	0
Fair Value	5,748	11,972
Short Term Marketable Securities \| U.S. and Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	10,338	8,748
Unrealized Gains	43	8
Unrealized Losses	0	(1)
Fair Value	10,381	8,755
Short Term Marketable Securities \| US Government Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	36,088	48,647
Unrealized Gains	116	71
Unrealized Losses	(2)	0
Fair Value	36,202	48,718
Short Term Marketable Securities \| U S Treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	19,780	15,057
Unrealized Gains	65	6
Unrealized Losses	0	0
Fair Value	$ 19,845	15,063
Long Term Marketable Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		3,025
Unrealized Gains		0
Unrealized Losses		0
Fair Value		3,025
Long Term Marketable Securities \| U S Treasuries
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis		3,025
Unrealized Gains		0
Unrealized Losses		0
Fair Value		$ 3,025

Marketable Securities - Additional Information (Details)	6 Months Ended
	Jun. 30, 2020 USD ($)
Marketable Securities [Line Items]
Realized gains or losses on available-for-sale debt securities	$ 0
Maximum
Marketable Securities [Line Items]
Available-for sale securities, remaining contractual maturity	1 year

License Agreements - Additional Information (Details)		1 Months Ended		3 Months Ended		6 Months Ended
License Agreements - Additional Information (Details)	Jul. 01, 2020 USD ($) shares	Jun. 30, 2019 USD ($) shares	Apr. 30, 2016 USD ($)	Jun. 30, 2019 USD ($) shares	Mar. 31, 2019 USD ($) shares	Jun. 30, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Oct. 31, 2019 $ / shares	Oct. 31, 2019 $ / shares	May 31, 2018 USD ($)
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued \| shares						51,729,511	47,227,065
Compound library and option agreement execution month and year						2016-02
Upfront consideration				$ 965,000	$ 2,059,000
Common Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance of common stock \| shares				120,000	133,334
Additional Paid-In Capital
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront consideration				$ 965,000	$ 2,059,000
Licensed Products
Research And Development Arrangement Contract To Perform For Others [Line Items]
Royalties due from sales						$ 0
Commercial Agreements \| Clinical Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued \| shares						640,002
Equity payments percentage						80.00%
Contingent consideration liability						$ 1,800,000	$ 1,100,000
Funding provided for the research and development under the agreement in period						2020-02
Commercial Agreements \| Clinical Stage Biopharmaceutical Company \| Subsequent Event
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common shares expected to be issued \| shares	133,333
Milestone payments	$ 1,000,000.0
Additional common shares expected to be issued \| shares	133,333
Commercial Agreements \| Clinical Stage Biopharmaceutical Company \| Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Contingent consideration additional common stock issued for one licensed product \| shares						933,337
Contingent consideration additional common stock issued for two or more licensed product \| shares						1,333,338
Milestone payments						$ 70,300,000
Funding provided for the research and development under the agreement						$ 2,000,000.0
Commercial Agreements \| Clinical Stage Biopharmaceutical Company \| Initial License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
License agreement execution month and year						2016-02
Commercial Agreements \| Clinical Stage Biopharmaceutical Company \| Second License Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
License agreement execution month and year						2019-01
Commercial Agreements \| Academic Institution
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common stock, shares issued \| shares						160,000
Equity payments percentage						20.00%
Commercial Agreements \| Affiliate of Clinical-Stage Biopharmaceutical Company
Research And Development Arrangement Contract To Perform For Others [Line Items]
Aggregate purchase price of interest			$ 500,000
Additional amount invested										$ 500,000
License and Compound Library and Option Agreement \| IPO \| Common Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance price per share \| (per share)								$ 4.36	$ 34.20
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
Other Licensing Agreements with Research Institutions \| UCSF
Research And Development Arrangement Contract To Perform For Others [Line Items]
Common shares expected to be issued \| shares		34,000
Contingent consideration, milestone or royalty payments						$ 0
Milestone payments		$ 13,600,000
Other Licensing Agreements with Research Institutions \| UCSF \| Common Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Issuance of common stock \| shares		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 consideration		$ 1,000,000.0

Commitments and Contingencies - Additional Information (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Commitments And Contingencies [Line Items]
Rent expense		$ 1,500
Operating lease, right of use assets	$ 24,891
South San Francisco, California
Commitments And Contingencies [Line Items]
Lessee, operating lease, description	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 from occupancy of approximately ten years ending on December 31, 2029 with an option to extend the term for an additional eight years at then-market rental rates
Lease commencement date	2019-05
Lease expiration date	Dec. 31, 2029
Operating lease, initial lease term	10 years
Operating lease, option to extend description	option to extend the term for an additional eight years
Operating lease, existence of option to extend	true
Operating lease, renewal term	8 years
Letter of credit, delivered in connection of lease agreement	$ 900
Estimated discount rate	3.50%
Impairment loss	$ 2,200
Operating lease, right of use assets	1,800
Leasehold improvements	1,600
South San Francisco, California \| Right of Use Assets
Commitments And Contingencies [Line Items]
Impairment loss	1,200
South San Francisco, California \| Leasehold Improvements
Commitments And Contingencies [Line Items]
Impairment loss	$ 1,000
South San Francisco, California \| Deferred Rent and Leasehold Improvements
Commitments And Contingencies [Line Items]
Tenant improvement allowance			$ 10,700
Brisbane, California
Commitments And Contingencies [Line Items]
Operating lease, option to extend description	The lease agreement includes an escalation clause for increased rent and a renewal provision allowing the Company to extend this lease for an additional four years by giving the landlord written notice of the election to exercise the option at least fifteen months prior to the original expiration of the lease term.
Operating lease, renewal term	4 years
Tenant improvement allowance	$ 3,900
Lease expiration date	2022-10

Commitments and Contingencies -Summary of Components of Lease Expense (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020
Lease, Cost [Abstract]
Operating lease cost	$ 1,144	$ 2,444
Variable lease cost	363	641
Impairment of operating lease right-of-use asset		1,157
Total lease cost	$ 1,507	$ 4,242

Commitments and Contingencies - Summary of Supplemental Information Related to Leases (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Operating cash flows from operating leases	$ 2,557
Operating leases, Weighted-average remaining lease term (years)	8 years 9 months 18 days
Operating leases, Weighted-average discount rate (percentage)	5.80%

Commitments and Contingencies - Summary of Maturities of Lease Liabilities (Details) $ in Thousands	Jun. 30, 2020 USD ($)
Commitments And Contingencies Disclosure [Abstract]
2020 (remaining 6 months)	$ 3,241
2021	6,653
2022	6,283
2023	4,810
2024	4,964
Thereafter	27,302
Total future minimum lease payments	53,253
Less: Amount representing interest	(12,171)
Present value of future minimum lease payments	41,082
Less: Current portion of operating lease liability	(4,312)
Noncurrent portion of operating lease liability	$ 36,770

Commitments and Contingencies - Adoption of New Accounting Pronouncements Cumulative Effect on Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Item Effected [Line Items]
Operating lease right of use asset	$ 24,891
Accrued and other current liabilities	6,250		$ 4,995
Operating lease liabilities, current portion	4,312
Deferred rent, net of current portion			13,298
Operating lease liabilities, net of current portion	$ 36,770
Topic 842
Item Effected [Line Items]
Accrued and other current liabilities			4,995
Deferred rent, net of current portion			$ 13,298
Topic 842 \| Topic 842 Adjustments
Item Effected [Line Items]
Operating lease right of use asset		$ 27,174
Accrued and other current liabilities		(1,970)
Operating lease liabilities, current portion		3,455
Deferred rent, net of current portion		(13,298)
Operating lease liabilities, net of current portion		38,988
Topic 842 \| Net of Topic 842 Adjustments
Item Effected [Line Items]
Operating lease right of use asset		27,174
Accrued and other current liabilities		3,025
Operating lease liabilities, current portion		3,455
Operating lease liabilities, net of current portion		$ 38,988

Equity Financing - Additional Information (Details) - USD ($)		1 Months Ended	3 Months Ended				6 Months Ended
	Jun. 03, 2019	Jun. 30, 2019	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020
Subsidiary Sale Of Stock [Line Items]
Equity financing, authorized amount		$ 250,000,000.0
Proceeds from issuance of common stock under ATM Offering Program, net of issuance costs							$ 29,494,000
Common Stock
Subsidiary Sale Of Stock [Line Items]
Issuance of common stock (in shares)					120,000	133,334
At-the-Market Equity Offering Program \| June 2019 Sales Agreement \| Cowen
Subsidiary Sale Of Stock [Line Items]
Proceeds from issuance of common stock under ATM Offering Program, net of issuance costs							$ 29,500,000
Issuance of common stock (in shares)							4,107,870
Payments for other offering expenses							$ 1,000,000.0
At-the-Market Equity Offering Program \| June 2019 Sales Agreement \| Cowen \| Maximum
Subsidiary Sale Of Stock [Line Items]
Proceeds from issuance of common stock under ATM Offering Program, net of issuance costs	$ 75,000,000.0
Percentage of gross sales proceeds of common stock payable as compensation	3.00%
Reimbursement of expenses	$ 100,000
At-the-Market Equity Offering Program \| Common Stock
Subsidiary Sale Of Stock [Line Items]
Equity financing, authorized amount		$ 75,000,000.0
Issuance of common stock (in shares)			2,594,030	1,513,840

Stock-Based Compensation - Summary of Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2020 $ / shares shares
Number of Shares
Balances at December 31, 2019 \| shares	6,906,898
Granted \| shares	3,272,256
Exercised \| shares	(123,191)
Canceled \| shares	(587,441)
Balances at June 30, 2020 \| shares	9,468,522
Weighted-Average Exercise Price
Balances at December 31, 2019 \| $ / shares	$ 7.62
Granted \| $ / shares	6.33
Exercised \| $ / shares	3.40
Canceled \| $ / shares	9.43
Balances at June 30, 2020 \| $ / shares	$ 7.11

Stock-Based Compensation - Summary of Restricted Stock Units and Performance Stock Units Activity (Details) - Restricted Stock Units (RSUs) and Performance Stock Units (PSUs)	6 Months Ended
	Jun. 30, 2020 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares, Unvested at December 31, 2019 \| shares	325,887
Shares, Granted \| shares	661,314
Shares, Vested \| shares	(103,020)
Shares, Canceled \| shares	(43,539)
Shares, Unvested at June 30, 2020 \| shares	840,642
Unvested at December 31, 2019 \| $ / shares	$ 9.00
Granted \| $ / shares	5.67
Vested \| $ / shares	9.00
Canceled \| $ / shares	9.00
Unvested at June 30, 2020 \| $ / shares	$ 6.38

Stock-Based Compensation - Additional Information (Details) - USD ($)	1 Months Ended	6 Months Ended
	Mar. 31, 2020	Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation cost recognized	$ 200,000
Settlement of stock based compensation for liability awards		13,550
Stock based compensation expense for liability awards		$ 100,000
2020 Employment Inducement Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance	1,100,000
2020 Employment Inducement Incentive Plan \| Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance	800,000
2020 Employment Inducement Incentive Plan \| RSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance	120,000
2020 Employment Inducement Incentive Plan \| PSUs
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance	150,000
Grant date fair value	$ 700,000
Weighted-average derived service period for recognition of compensation expense	4 years 3 months 18 days
2020 Employment Inducement Incentive Plan \| PSUs \| Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance	50,000
Volume-weighted average per Share closing trading price	$ 36.875
Trailing period	30 days
2020 Employment Inducement Incentive Plan \| PSUs \| Tranche Two
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance	100,000
Trailing period	30 days
Price per share to holders of company’s common stock	$ 36.875
Market capitalization trigger	$ 2,500,000,000
2020 Employment Inducement Incentive Plan \| Common Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized for issuance	30,000
Number of shares vested	30,000
Vesting rights		The stock options and RSUs will vest subject to continued service through the applicable vesting date.

Stock-Based Compensation - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 4,104	$ 1,944	$ 7,421	$ 3,941
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	1,937	931	3,472	1,926
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 2,167	$ 1,013	$ 3,949	$ 2,015

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, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net loss	$ (18,667)	$ (28,038)	$ (23,673)	$ (18,767)	$ (46,705)	$ (42,440)
Denominator:
Weighted-average number of shares outstanding—basic and diluted	49,659,153		42,442,886		48,606,768	42,311,040
Net loss per share, basic and diluted	$ (0.38)		$ (0.56)		$ (0.96)	$ (1.00)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References

+ Details

Net Loss per Common Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Per Share (Details) - shares	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	10,457,637	7,581,925
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	9,468,522	7,051,590
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	80,285	478,288
RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	840,642
Shares Subject to 2018 ESPP
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	68,188	52,047

Net Loss per Common Share - Additional Information (Details)	6 Months Ended
	Jun. 30, 2020 shares
Maximum
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Shares contingently issued	640,218

Subsequent Events - Additional Information (Details) - USD ($) $ in Thousands			1 Months Ended	6 Months Ended
	Jul. 31, 2020	Jul. 30, 2020	Jul. 31, 2020	Jun. 30, 2020
Subsequent Event [Line Items]
Proceeds from issuance of common stock under ATM Offering Program, net of issuance costs				$ 29,494
Subsequent Event \| July 2020 Sales Agreement \| At-the-Market Equity Offering Program
Subsequent Event [Line Items]
Proceeds from issuance of common stock under ATM Offering Program, net of issuance costs			$ 7,700
Issuance of common stock (in shares)			882,106
Commissions			$ 200
Subsequent Event \| Ascentage International \| License Agreement
Subsequent Event [Line Items]
Notice period to terminate license agreement		90 days
Subsequent Event \| Cowen \| July 2020 Sales Agreement \| At-the-Market Equity Offering Program \| Maximum
Subsequent Event [Line Items]
Proceeds from issuance of common stock under ATM Offering Program, net of issuance costs	$ 50,000
Percentage of gross sales proceeds of common stock payable as compensation	3.00%
Reimbursement of expenses	$ 100