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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2019	Mar. 20, 2020	Oct. 31, 2019
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	RAPT
Entity Registrant Name	RAPT Therapeutics, Inc.
Entity Central Index Key	0001673772
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	001-38997
Entity Tax Identification Number	47-3313701
Entity Address, Address Line One	561 Eccles Avenue
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	489-9000
Entity Common Stock, Shares Outstanding		24,334,288
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock, par value $0.0001 per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Annual Report	true
Document Transition Report	false
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Public Float			$ 187.4
Documents Incorporated by Reference	DOCUMENTS INCORPORATED BY REFERENCE Portions of the Registrant’s Definitive Proxy Statement for the 2020 Annual Meeting of Stockholders to be filed with the U.S. Securities and Exchange Commission pursuant to Regulation 14A within 120 days after the end of the fiscal year covered by this Annual Report on Form 10-K are incorporated by reference in Part III, Items 10-14 of this Annual Report on Form 10-K.

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 77,383	$ 63,798
Prepaid expenses and other current assets	3,123	1,264
Total current assets	80,506	65,062
Property and equipment, net	3,707	4,159
Other assets	389	389
Total assets	84,602	69,610
Current liabilities:
Accounts payable	1,143	1,771
Accrued expenses	3,642	2,488
Deferred revenue, current	4,000
Other current liabilities	471	384
Total current liabilities	9,256	4,643
Deferred rent, net of current portion	2,225	969
Commitments
Convertible preferred stock, $0.0001 par value: no shares and 104,018,468 shares authorized at December 31, 2019 and 2018, respectively; no shares and 16,415,281 shares issued and outstanding at December 31, 2019 and 2018, respectively		161,111
Stockholders' equity (deficit):
Preferred stock, $0.0001 par value: 50,000,000 shares authorized; no shares issued and outstanding at December 31, 2019 and 2018
Common stock, $0.0001 par value; 500,000,000 shares authorized; 21,833,037 and 878,413 shares issued and outstanding at December 31, 2019 and 2018, respectively	2	1
Additional paid-in capital	235,049	22,441
Related party promissory note for the purchase of common stock		(598)
Accumulated other comprehensive income (loss)	20	(4)
Accumulated deficit	(161,950)	(118,953)
Total stockholders' equity (deficit)	73,121	(97,113)
Total liabilities, convertible preferred stock and stockholders' equity (deficit)	$ 84,602	$ 69,610

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	0	104,018,468
Convertible preferred stock, shares issued	0	16,415,281
Convertible preferred stock, shares outstanding	0	16,415,281
Preferred stock, par value	$ 0.0001	$ 0.0001
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	21,833,037	878,413
Common stock, shares outstanding	21,833,037	878,413

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 34,910	$ 31,767
General and administrative	8,719	5,180
Total operating expenses	43,629	36,947
Loss from operations	43,629	36,947
Other income
Other income, net	1,292	800
Net loss before taxes	42,337	36,147
Provision for income taxes	660
Net loss	42,997	36,147
Other comprehensive (income) loss	(24)	4
Total comprehensive loss	$ 42,973	$ 36,151
Net loss per share, basic and diluted	$ 9.89	$ 58.09
Weighted average number of shares used in computing net loss per share, basic and diluted	4,346,400	622,289

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Series C Convertible Preferred Stock	Series C-2 Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Related Party Promissory Notes for the Purchase of Common Stock	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2017		$ 108,643
Beginning balance, shares at Dec. 31, 2017		75,563,784
Beginning balance at Dec. 31, 2017	$ (62,405)				$ 1	$ 21,005	$ (605)	$ (82,806)
Beginning balance, shares at Dec. 31, 2017					880,191
Issuance of convertible preferred stock, net of issuance cost			$ 29,914	$ 22,554
Issuance of convertible preferred stock, net of issuance cost, shares			13,054,684	9,873,412
Issuance of common stock upon exercise of stock options, net of repurchase	266					266
Issuance of common stock upon exercise of stock options, net of repurchase, shares					6,555
Repurchase of common stock from related party	17						17
Repurchase of common stock from related party, shares					(8,333)
Interest on promissory notes from related parties for purchase of common stock	(10)						(10)
Stock-based compensation	1,170					1,170
Foreign currency translation adjustment	(4)								$ (4)
Net loss	(36,147)							(36,147)
Ending balance at Dec. 31, 2018	$ 161,111	$ 161,111	$ 59,770	$ 22,554
Ending balance, shares at Dec. 31, 2018	16,415,281	98,491,880	4,351,554	1,645,562
Ending balance at Dec. 31, 2018	$ (97,113)				$ 1	22,441	(598)	(118,953)	(4)
Ending balance, shares at Dec. 31, 2018	878,413				878,413
Issuance of common stock upon initial public offering, net of issuance cost	$ 34,721					34,721
Issuance of common stock upon initial public offering, net of issuance cost, shares					3,427,360
Issuance of convertible preferred stock, net of issuance cost				$ 14,379
Issuance of convertible preferred stock, net of issuance cost, shares				6,311,445
Conversion of Series A, B, C, C-2 convertible preferred stock to common stock	175,490				$ 1	175,489
Temporary equity, Conversion of Series A, B, C, C-2 convertible preferred stock to common stock		(104,803,325)
Temporary equity, Conversion of Series A, B, C, C-2 convertible preferred stock to common stock, shares		$ (175,490)
Conversion of Series A, B, C, C-2 convertible preferred stock to common stock, shares					17,467,184
Issuance of common stock upon exercise of stock options, net of repurchase	347					347
Issuance of common stock upon exercise of stock options, net of repurchase, shares					143,415
Repurchase of common stock from related party	174						174
Repurchase of common stock from related party, shares					(83,335)
Paydown of promissory notes from related parties for purchase of common stock	73						73
Forgiveness of promissory notes from related parties	353						353
Interest on promissory notes from related parties for purchase of common stock	(2)						$ (2)
Stock-based compensation	2,051					2,051
Foreign currency translation adjustment	24								24
Net loss	$ (42,997)							(42,997)
Ending balance, shares at Dec. 31, 2019	0
Ending balance at Dec. 31, 2019	$ 73,121				$ 2	$ 235,049		$ (161,950)	$ 20
Ending balance, shares at Dec. 31, 2019	21,833,037				21,833,037

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net loss	$ (42,997)	$ (36,147)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,330	1,237
Stock-based compensation expense	2,051	1,170
Loss (gain) on disposal of capital equipment	(35)	17
Gain on foreign exchange	24	0
Other noncash income (loss), net	20	(14)
Changes in operating assets and liabilities:
Prepaid expenses and other long-term assets	(1,859)	(691)
Deferred revenue	4,000	0
Accounts payable and accrued liabilities	736	1,437
Deferred rent	1,256	38
Net cash used in operating activities	(35,474)	(32,953)
Investing activities
Proceeds from sale of equipment	45	0
Purchase of property and equipment	(888)	(3,500)
Net cash used in investing activities	(843)	(3,500)
Financing activities
Proceeds from the sale of convertible preferred stock, net of issuance costs	14,379	52,468
Proceeds from initial public offering	34,721	0
Proceeds from issuance of common stock, net of repurchases	802	266
Net cash provided by financing activities	49,902	52,734
Net increase in cash and cash equivalents	13,585	16,281
Cash and cash equivalents at beginning of year	63,798	47,517
Cash and cash equivalents at end of year	77,383	63,798
Supplemental disclosures of cash flow information:
Cash paid for income taxes	660	0
Supplemental disclosures of non-cash investing and financing information
Property and equipment purchases included in accounts payable	0	753
Convertible preferred stock converted to common stock	175,490	$ 0
Forgiveness of promissory notes from related party for purchase of common stock	$ 397

Organization

12 Months Ended

Dec. 31, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Organization

1. Organization

Description of the Business

RAPT Therapeutics, Inc. (“RAPT” or the “Company”) is a clinical-stage, immunology-based biopharmaceutical company focused on discovering, developing and commercializing oral small molecule therapies for patients with significant unmet needs in oncology and inflammatory diseases. Utilizing its proprietary drug discovery and development engine, the Company develops highly selective small molecules that are designed to modulate the critical immune responses underlying these diseases. In May 2019, the Company changed its name from FLX Bio, Inc. to RAPT Therapeutics, Inc.

The Company is located in South San Francisco, California.

Equity Financings

In November 2019, the Company completed its initial public offering (“IPO”), pursuant to which the Company issued an aggregate of 3,427,360 shares of its common stock at an offering price of $12.00 per share for net proceeds of $34.7 million after deducting underwriting discounts and other offering related costs. Immediately prior to the closing of the IPO, all outstanding shares of the Company’s convertible preferred stock converted into 17,467,184 shares of the Company’s common stock.

In connection with the completion of its IPO, the Company’s certificate of incorporation was amended and restated to provide for 500,000,000 authorized shares of common stock with a par value of $0.0001 per share and 50,000,000 authorized shares of preferred stock with a par value of $0.0001 per share.

In February 2020, the Company completed an underwritten follow-on public offering (“Follow‑on Offering”) of 2,500,000 shares of its common stock issued at an offering price of $30.00 per share. The shares issued in the Follow-on Offering generated approximately $69.7 million in net proceeds after deducting underwriting discounts and other offering related costs.

Liquidity and Management Plans

The accompanying consolidated financial statements have been prepared assuming that the Company will continue as a going concern. Since inception, the Company has incurred net losses and negative cash flows from operations. During the year ended December 31, 2019, the Company incurred a net loss of $43.0 million and used $35.5 million of cash in operations. At December 31, 2019, the Company had cash and cash equivalents of $77.4 million and working capital of $71.3 million.

Management plans to continue to incur substantial costs in order to conduct research and development activities and additional capital will be needed to undertake these activities. The Company intends to raise such capital through the issuance of additional equity, borrowings and strategic alliances with other companies. However, if such arrangements are not available at adequate levels or on acceptable terms, the Company would be required to significantly reduce operating expenses and delay or reduce the scope of or eliminate some of its development programs. Management believes that the Company’s current cash and cash equivalents, including the net proceeds of approximately $69.7 million from the Follow-on Offering in February 2020, will provide sufficient funds to enable the Company to meet its obligations for at least twelve months from the filing date of this report.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and include the consolidated accounts of the Company and its wholly-owned subsidiary, RAPT Therapeutics Australia Pty Ltd., which was established in 2018. All intercompany balances and transactions have been eliminated in consolidation.

Reverse Stock Split

On July 19, 2019, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock on a one-for-six basis (the “Reverse Stock Split”). In connection with the Reverse Stock Split, the conversion ratio for the Company’s outstanding convertible preferred stock was proportionately adjusted such that the common stock issuable upon conversion of such preferred stock was decreased in proportion to the Reverse Stock Split. The par value of the common stock was not adjusted as a result of the Reverse Stock Split. All references to common stock, options to purchase common stock, early exercised options, share data, per share data, convertible preferred stock (to the extent presented on an as-converted to common stock basis) and related information contained in these consolidated financial statements have been retrospectively adjusted to reflect the effect of the Reverse Stock Split for all periods presented.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the 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 balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining the fair value of assets and liabilities, common stock valuation and stock-based compensation. Actual results could differ from such estimates or assumptions.

Concentration of Credit Risk and Other Risks and Uncertainties

The Company is subject to a number of risks similar to other biopharmaceutical companies in the early stage, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s products and protection of proprietary technology. If the Company does not successfully obtain regulatory approval, commercialize or partner any of its product candidates, it will be unable to generate revenue from product sales or achieve profitability.

Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents. Substantially all the Company’s cash and cash equivalents are held by financial institutions. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents in highly rated money market funds.

Segments

The Company operates as a single operating segment. The Company’s chief operating decision maker, its President and Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purposes of allocating resources, making operating decisions and evaluating financial performance.

Fair Value of Financial Instruments

The carrying amount of the Company’s financial instruments, including certain prepaid and accrued expenses, approximates fair value due to their short-term maturities.

Cash Equivalents

The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents.

Property and Equipment

Property and equipment consist of computer equipment, laboratory equipment, leasehold improvements and furniture and fixtures, and is recorded at cost, less accumulated depreciation and amortization. Depreciation and amortization are calculated using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the improvements.

Depreciation and amortization begin at the time the asset is placed in service. Maintenance and repairs are charged to expense as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation and amortization are removed from the balance sheet and the resulting gain or loss is reflected in the results of operations.

Impairment of Long-Lived Assets

The Company evaluates its long-lived assets for impairment annually or more frequently whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparing the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. For the years ended December 31, 2019 and 2018, the Company did not record any impairment losses on long-lived assets.

Leases

The Company leases office space and laboratory facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. Funding of leasehold improvements by the Company’s landlord is accounted for as a tenant improvement allowance and recorded as current and non-current deferred rent liabilities and amortized on a straight-line basis as a reduction of rent expense over the term of the lease.

Convertible Preferred Stock

The Company recorded all shares of convertible preferred stock at their respective fair values on the dates of issuance, less issuance costs. In the event of a change of control of the Company, proceeds received from the sale of such shares would have been distributed in accordance with the liquidation preferences set forth in the Company’s Amended and Restated Certificate of Incorporation unless the holders of the convertible preferred stock had converted their shares of convertible preferred stock into shares of common stock. Convertible preferred stock was previously classified outside of stockholders’ equity (deficit) on the balance sheet as events triggering redemption were not solely within the Company’s control. Immediately prior to the consummation of the Company’s IPO in November 2019, the convertible preferred stock converted into 17,467,184 shares of common stock.

Revenue

License and collaborative agreements consist of license, milestone and royalty payments generated through agreements with strategic partners for the development and commercialization of certain product candidates. The terms of an agreement may include a non-refundable upfront fee, payments based upon achievement of milestones and royalties on net product sales. If a portion of the nonrefundable upfront fee or other payments received is allocated to continuing performance obligations under the terms of an agreement, such portion is recorded as deferred revenue and recognized as revenue when (or as) the underlying performance obligation is satisfied.

The Company recognizes revenue when it transfers promised goods or services to customers or counterparties in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those goods or services. In determining the appropriate amount of revenue to be recognized, the Company performs the following steps: (i) identification of the promised goods or services in the agreement; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the agreement; (iii) measurement of the transaction price, including any constraint on variable consideration; (iv) allocation of the transaction price to performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation.

Licenses: If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in an agreement, the Company will recognize revenue from the nonrefundable, upfront fee allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. If a license is bundled with other performance obligations, the Company utilizes judgment to assess the nature of the combined performance obligations to determine whether the combined performance obligations are satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

Milestone payments: If an agreement includes event-based or milestone payments, the Company evaluates whether the events or milestones are considered likely to be achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is unlikely that a significant revenue reversal would occur, the value of the associated event-based or milestone payments is included in the transaction price. Event-based or milestone payments that are not within the control of the Company are not included in the transaction price until they become likely to be achieved.

Royalties: If an agreement includes sales-based royalties and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied).

As of December 31, 2019, the Company recorded deferred revenue of $4.0 million on the consolidated balance sheet related to the license and collaborative agreement with Hanmi Pharmaceutical LTD (“Hanmi”).

Research and Development Costs

Research and development costs are expensed as incurred. Research and development costs consist primarily of salaries and benefits of research and development personnel, costs related to research activities, preclinical studies, clinical trials, drug manufacturing and allocated overhead and facility-related expenses. The Company accounts for non-refundable advance payments for goods or services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made.

Clinical trial costs are a component of research and development expenses. The Company expenses costs for its clinical trial activities performed by third parties, including clinical research organizations (“CROs”) and other service providers, as they are incurred, based upon estimates of the work completed over the life of the individual study in accordance with associated agreements. The Company uses information it receives from internal personnel and outside service providers to estimate the clinical trial costs incurred.

Stock-Based Compensation

The Company measures employee and director stock-based compensation expense for all stock-based awards based on their grant date fair value using the Black-Scholes option-pricing model. For stock-based awards with service conditions only, stock-based compensation expense is recognized over the requisite service period using the straight-line method. For awards with performance conditions, the Company evaluates the probability of achieving performance conditions at each reporting date. The Company begins to recognize stock-based compensation expense using an accelerated attribution method when it is deemed probable that the performance condition will be met. Forfeitures are recognized as they occur.

Stock-based compensation expense for nonemployee stock-based awards is measured at fair value using the Black-Scholes option-pricing model. The Company recognizes stock-based compensation expense for the estimated fair value of the vested portion of nonemployee awards in its consolidated statements of operations and comprehensive loss. Stock-based compensation expense related to stock-based awards to nonemployees is subject to re-measurement over the service period, which approximates the vesting period.

Stock-based compensation expense related to restricted stock awards is determined using the estimated fair value of the Company’s common stock on the date of grant for the period prior to the IPO. The fair value of restricted stock awards granted after the IPO is determined based on the stock price on the date of grant. The estimated fair value is amortized as compensation expense over the service period of the award.

Foreign Currency Transactions

The functional currency of RAPT Therapeutics Australia Pty Ltd., the Company’s wholly-owned subsidiary, is the Australian dollar. Accordingly, all monetary assets and liabilities of the subsidiary are translated into U.S. dollars at the current period-end exchange rates and non-monetary assets are translated using historical exchange rates. Income and expense elements are remeasured to U.S. dollars using the average exchange rates in effect during the period. Remeasurement gains and losses are recorded as other income (expense).

The Company is subject to foreign currency risk with respect to its clinical contracts denominated in currencies other than the U.S. dollar. Payments on contracts denominated in foreign currencies are made at the spot rate on the day of payment. Changes in the exchange rate between billing dates and payment dates are recorded to other (income), net on the consolidated statements of operations.

Income Taxes

Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period such tax rate changes are enacted.

The Company recognizes the effect of income tax positions only if those positions are more likely than not to be sustained. Recognized income tax positions are measured at the largest amount that is greater than 50% likely to be realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Valuation allowances are established when necessary to reduce deferred tax assets to amounts more likely than not to be realized. Interest and penalties related to unrecognized tax benefits are recognized as a component of income tax expense.

Comprehensive Loss

Comprehensive loss includes net loss and certain changes in stockholders’ deficit that are excluded from net loss, primarily unrealized losses from foreign currency translation adjustments.

Net Loss Per Share

Basic net loss per share is computed by dividing the net loss by the weighted average number of common shares outstanding during the period, without consideration of potential dilutive securities. Diluted net loss per common share is computed by dividing the net loss by the sum of the weighted average number of common shares outstanding during the period plus the number of potential dilutive securities outstanding during the period calculated in accordance with the treasury stock method. Diluted net loss per share is the same as basic net loss per share since the effect of potentially dilutive securities is anti-dilutive.

Recent Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) or other standard setting bodies and adopted by the Company as of the specified effective date. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), the Company meets the definition of an emerging growth company and has elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act.

Adopted Accounting Pronouncements

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230: Classification of Certain Cash Receipts and Cash Payments), which addresses specific cash flow issues with the objective of reducing the diversity in practice for the treatment of these issues. The areas identified include: debt prepayment or debt extinguishment costs; settlement of zero-coupon debt instruments; contingent consideration payments made after a business combination; proceeds from the settlement of insurance claims; proceeds from the settlement of corporate-owned life insurance policies; distributions received from equity method investees; beneficial interests in securitization transactions; and application of the predominance principle with respect to separately identifiable cash flows. The Company adopted ASU 2016-15 in the fourth quarter of 2019 and there was no impact to the financial position or results of operations related to this adoption.

In February 2018, the FASB issued ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. The amendments in this standard allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. The Company adopted ASU 2018-02 in the fourth quarter of 2019 and there was no impact to the financial position or results of operations related to this adoption.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (ASC 808): Clarifying the Interaction between ASC 808 and ASC 606, which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements. ASU 2018-18 adds unit-of-account guidance in ASC 808 to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606, and requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under ASC 606 is precluded if the collaborative arrangement participant is not a customer. The Company adopted ASU 2018-18 in the fourth quarter of 2019 and there was no impact to the financial position or results of operations related to this adoption.

In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The amendments also improve consistent application of and simplify other areas of Topic 740 by clarifying and amending existing guidance. The Company adopted ASU 2019-12 in the fourth quarter of 2019 and there was no impact to the financial position or results of operations related to this adoption.

Recently Issued Accounting Pronouncement Not Yet Adopted

In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU 2016-02 requires lessees to record most leases on their balance sheet while recognizing expense in a manner similar to existing accounting. ASU 2016‑02 states that a lessee would recognize a lease liability for the obligation to make lease payments and a right-to-use asset for the right to use the underlying asset for the lease term. The new accounting guidance is effective for the Company for fiscal periods beginning after December 15, 2020 and early adoption is permitted. The Company is currently assessing the timing of adoption and the impact that the adoption will have on its consolidated financial statements and related disclosures.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. ASU 2016-13 amended guidance on reporting credit losses for assets held at amortized cost basis and available-for-sale debt securities. For available-for-sale debt securities, credit losses will be presented as an allowance rather than as a write-down. This standard is effective for the Company’s fiscal year beginning after December 15, 2020. Early adoption is permitted for all entities. The Company is currently assessing the timing of adoption and the impact that the adoption of ASU 2016-13 will have on its consolidated financial statements and related disclosures.

In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Non-Employee Share-Based Payment Accounting as part of the FASB simplification initiative. ASU 2018-07 expands the scope of Topic 718, allowing the Company to apply the requirements of Topic 718 to certain non-employee awards to acquire goods and services from non-employees. This ASU will be effective for the Company for fiscal years beginning after December 15, 2019, and early adoption is permitted. The Company is currently assessing the impact that the adoption of ASU 2018-07 will have on its consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurements (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement as part of the FASB’s disclosure framework project. ASU 2018-13 modifies the disclosure requirements on fair value measurements in Topic 820, Fair Value Measurement by removing the requirement to disclose amounts of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels and the valuation process for Level 3 fair value measurements. This ASU also modifies existing disclosure requirements by clarifying that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date, and it adds required disclosures for the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period, and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. This ASU will be effective for the Company for fiscal years and interim periods within those fiscal years, beginning after December 15, 2019, and early adoption is permitted. The Company is currently assessing the impact of this ASU on its consolidated financial statements.

Fair Value Measurements

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

Fair value accounting is applied for all financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis (at least annually). Financial instruments include cash and cash equivalents, accounts payable and accrued liabilities that approximate fair value due to their relatively short maturities.

Assets and liabilities recorded at fair value on a recurring basis in the balance sheet are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows:

Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date;

Level 2—Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and

Level 3—Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data.

Financial assets subject to fair value measurements on a recurring basis comprise money market funds that are measured using Level 1 inputs. The money market funds subject to fair value measurements at December 31, 2019 and 2018 were $77.4 million and $63.7 million, respectively, and are included in cash and cash equivalents.

- 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

Property and Equipment

12 Months Ended

Dec. 31, 2019

Property Plant And Equipment [Abstract]

Property and Equipment

4. Property and Equipment

Property and equipment consists of the following (in thousands):

	December 31,
	2019			2018
Laboratory equipment	$	5,752		$	5,466
Leasehold improvements		3,294			2,989
Computer equipment		326			308
Furniture and fixtures		394			365
Total property and equipment		9,766			9,128
Less accumulated depreciation and amortization		(6,059	)		(4,969	)
Property and equipment, net	$	3,707		$	4,159

Depreciation and amortization expenses were $1.3 million and $1.2 million for the years ended December 31, 2019 and 2018, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2019

Payables And Accruals [Abstract]

Accrued Expenses

5. Accrued Expenses

Accrued expenses consist of the following (in thousands):

	December 31,
	2019		2018
Accrued research and clinical development expenses	$	1,353	$	519
Accrued compensation		1,779		1,433
Accrued professional and consulting services		192		182
Accrued property and equipment		—		202
Accrued lab supplies		29		80
Other		289		72
Total accrued expenses	$	3,642	$	2,488

Commitments

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Commitments

6. Commitments

The Company enters into contracts in the normal course of business with CROs for preclinical studies and clinical trials. These agreements provide for notice of termination by either party and are, therefore, cancelable contracts.

In May 2015, the Company entered into an operating lease for 30,376 square feet of laboratory and office facilities in South San Francisco, California, which expires in May 2022 and provides for tenant improvement allowances of $0.8 million. In April 2018, the Company amended the lease agreement to include an additional 6,378 square feet of laboratory and office space increasing the total leased premises to 36,754 square feet. The lease amendment extended the lease term to November 2026, and the amendment contains scheduled rent increases over the lease term and an option for the Company to extend the lease for an additional five-year term. The lease amendment contains a tenant improvement allowance of $1.4 million that the Company used in 2018 toward $2.4 million in total leasehold improvements, which is being amortized over the remaining lease term.

In February 2019, the Company entered into an agreement to sublease its facility lease of 6,378 square feet of laboratory and office space.

As of December 31, 2019, future minimum non-cancelable lease payments, net of sublease rental income, are as follows (in thousands):

Year ending December 31:
2020	$	1,602
2021		1,969
2022		2,037
2023		2,109
Thereafter		6,579
Total minimum lease payments	$	14,296

The terms of the lease agreement provide for rental payments on a monthly basis and on a graduated scale. The Company recognizes rent expense on a straight-line basis over the lease period and has accrued for rent expense incurred but not paid. Rent expense includes certain monthly charges that do not represent non-cancelable obligations, as defined. These costs are determined based on actual charges incurred. In addition, tenant improvement allowances recorded are amortized as a reduction to rent expense on a straight-line basis over the lease term. Rent expense was $2.0 million and $1.8 million in the years ended December 31, 2019 and 2018, respectively.

From time to time, the Company may be subject to various legal proceedings and claims arising in the ordinary course of business. The Company assesses contingencies to determine the degree of probability and range of possible loss for potential accrual in its financial statements. An estimated loss contingency is accrued in the financial statements if it is probable that a liability has been incurred and the amount of the loss can be reasonably estimated. The Company is not subject to any current pending legal matters or claims and no contingency loss had been accrued.

Collaboration Agreements

12 Months Ended

Dec. 31, 2019

Collaboration Agreements [Abstract]

Collaboration Agreements

7. Collaboration Agreements

Collaboration and License Agreement with Hanmi

In December 2019, the Company entered into a Collaboration and License Agreement (“Hanmi Agreement”) with Hanmi, pursuant to which the Company granted Hanmi an exclusive license to develop, manufacture and commercialize FLX475 and related compounds and products with respect to human cancers in the Republic of Korea, the Republic of China (Taiwan) and the People’s Republic of China, including the special administrative regions of Macau and Hong Kong (the “Hanmi Territory”) and certain sublicense rights.

In consideration of such rights, under the Hanmi Agreement, the Company is entitled to $10.0 million in an upfront payment of $4.0 million and an expected near-term milestone payment of $6.0 million. Additionally, the Company will be eligible to receive contingent payments of up to $108.0 million upon the achievement of specified milestones, consisting of up to $48.0 million upon the achievement of development milestones and up to $60.0 million upon the achievement of sales milestones, as well as double-digit royalties on future net sales of FLX475 in specified territories.

The Company identified the following performance obligations as defined by ASC 606 at the inception of the Hanmi Agreement, including (1) the exclusive development, manufacturing and commercialization license in the Hanmi Territory; (2) the transfer of know-how, technology, research data and information, and any improvements in technology; (3) the obligation to participate in the joint steering committee and appoint an alliance manager; (4) the responsibility to complete Phase 2 clinical trials; and (5) the supply of FLX475 for use in Hanmi’s Phase 2 clinical trials and Hanmi will reimburse the Company for manufacturing costs.

The Company determined that the identified performance obligations, except for the supply of FLX475, are not distinct and should be combined into one distinct performance obligation. The Company considered factors such as the novelty of the drug candidate and that the promised goods and services are highly interdependent and are expected to significantly modify one another.

The Company determined that the transaction price as of December 31, 2019 was $10.0 million, consisting of the upfront fee of $4.0 million and an expected near-term milestone payment of $6.0 million, which were determined to not be constrained. Other future development milestones were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company’s control and were considered to be fully constrained. The Company expects that the revenue from sales milestone and royalty payments will be recognized when the sales occur or the milestone is achieved. The Company will re-evaluate the transaction price at each reporting period.

The Company recognizes revenue for the performance obligation by applying the cost-based input method over the estimated service period. The Company determined that this method most faithfully depicts the transfer of its performance obligations to Hanmi as it reflects the progress made towards providing Hanmi with the necessary know-how to continue developing FLX475 in the Hanmi Territory.

For the year ended December 31, 2019, no revenue was recognized pursuant to the Hanmi Agreement. As of December 31, 2019, deferred revenue related to the Hanmi Agreement was $4.0 million and is expected to be recognized over the Phase 2 clinical trial period.

Clinical Trial Collaboration and Supply Agreement with Merck

In November 2018, the Company entered into a clinical trial collaboration and supply agreement with Merck (known as MSD outside the United States and Canada), through an affiliate, under which the Company will conduct a clinical trial evaluating FLX475 in combination with Keytruda^® (pembrolizumab), Merck’s anti-PD-1 therapy, in patients with advanced cancers. The Company is the sponsor of the clinical trial, and Merck will supply Keytruda for use in the clinical trial.

Related-Party Promissory Notes

12 Months Ended

Dec. 31, 2019

Related-Party Promissory Notes

8. Related Party Promissory Notes

In August 2015 and June 2016, the Company entered into limited recourse promissory notes with the Company’s chief executive officer and chief operating officer for the purchase of restricted common stock. The principal amount of the loan with the Company’s chief executive officer was $0.3 million (the “CEO Note”). The principal amount of the loan with the Company’s chief operating officer was $0.3 million (the “COO Note”). The loans were secured by the shares of common stock of the Company held by the individuals. The loans accrued interest at a rate of 1.82% and 1.41% per annum, respectively, and were due upon the earlier of voluntary termination of services to the Company, filing by the Company of its first registration statement with the SEC under the Securities Act of 1933 or sale of substantially all of the Company’s assets. As of December 31, 2018, the total outstanding balances under these notes, including accrued interest, were approximately $0.6 million. In June 2019, the Company forgave $0.4 million, which was the entire amount of principal and accrued interest due on the CEO Note.

In March 2018, the board of directors reduced the number of performance-based options of its former chief operating officer by 8,333 shares resulting in a $17,000 reduction to the principal of the COO Note. In March 2019, the chief operating officer resigned from the Company and, under the terms of a separation agreement, there were 63,019 vested shares and 28,645 unvested shares subject to repurchase. In March 2019, the Company reduced the principal on the COO Note by $0.1 million relating to the unvested shares, which shares were cancelled and returned to the option pool. In July 2019, the Company repurchased 29,686 vested shares from the chief operating officer in exchange for canceling $0.1 million of principal and interest on the COO Note. The Company received cash proceeds of $0.1 million as payment for the remaining principal and interest on the COO Notes relating to the remaining 33,333 vested shares.

Convertible Preferred Stock

12 Months Ended

Dec. 31, 2019

Temporary Equity Disclosure [Abstract]

Convertible Preferred Stock

9. Convertible Preferred Stock

In June 2018, the Company completed a subsequent closing of Series C convertible preferred stock financing at $2.2925 per share for $29.9 million in gross proceeds. Additionally, in December 2018, the Company completed a $22.6 million Series C-2 convertible preferred stock financing at $2.2925 per share, and between January 2019 and June 2019, the Company closed additional sales of Series C-2 convertible preferred stock at $2.2925 per share for $14.4 million in gross proceeds.

As of December 31, 2018, convertible preferred stock consisted of the following (in thousands, except share amounts):

			Shares		Net		Aggregate
	Shares		Issued and		Carrying		Liquidation
	Authorized		Outstanding		Value		Preference
Series A		37,509,105		6,251,502	$	28,861	$	37,509
Series B		25,000,000		4,166,663		49,926		50,000
Series C		26,109,363		4,351,554		59,770		59,856
Series C-2		15,400,000		1,645,562		22,554		22,635
Total convertible preferred stock		104,018,468		16,415,281	$	161,111	$	170,000

Immediately prior to the closing of the Company’s IPO in November 2019, all outstanding shares of the Company’s convertible preferred stock converted into 17,467,184 shares of the Company’s common stock. As such, no convertible preferred stock shares were outstanding as of December 31, 2019.

The rights, privileges and preferences of the convertible preferred stock were as follows:

Conversion

Shares of Series A, Series B, Series C and Series C-2 convertible preferred stock were initially convertible, at the option of the holder at any time, into shares of common stock as determined by dividing the applicable original issue price for such series by the applicable conversion price for such series, subject to adjustment in the event of any stock splits, stock dividends, combinations, subdivisions or similar recapitalization affecting such shares, and subject also to adjustment for certain dilutive issuances. Conversion of all outstanding convertible stock was automatic upon (i) the closing of a firm commitment underwritten public offering resulting in at least $30,000,000 in gross proceeds to the Company, prior to underwriting commissions and expenses, provided that the public offering price was at least $13.7550 per share, as adjusted for any stock dividends, combinations, splits, recapitalizations and the like or (ii) the election of the holders of 55% or more of the then outstanding shares of preferred stock.

Dividends

The holders of shares of Series A, Series B, Series C and Series C-2 convertible preferred stock were entitled to receive dividends, when, as and if declared by the board of directors, at the rate per annum of $0.08, $0.16, $0.18, $0.18 per share, respectively, subject to adjustment in the event of any stock splits, stock dividends, combinations, subdivisions or similar recapitalization affecting such shares.

Accrued dividends were payable when, as and if declared by the board of directors, and were not cumulative. After payment of the above dividend, any additional dividends would be distributed among all holders of common and preferred stock in proportion to the number of shares of common stock into which the representative shares were convertible.

Voting

Each holder of shares of Series A, Series B, Series C and Series C-2 convertible preferred stock was entitled to one vote for each share of common stock into which such shares of preferred stock were convertible, had voting rights and powers equal to the voting rights and powers of the common stock and would vote together with the common stock on all matters as to which holders of common stock had the right to vote, in each case, except as provided by law or by other provisions of the Company’s Restated Certificate of Incorporation.

Election of board of directors

As long as at least 6,000,000 shares of preferred stock were outstanding, as adjusted for any stock dividends, combinations, splits, recapitalizations and the like, the holders of shares of Series A, Series B, Series C and Series C-2 convertible preferred stock, voting as a separate class, were entitled to elect two members of the board of directors. The holders of shares of common stock, voting as a separate class, were entitled to elect two members of the board of directors. The holders of the shares of preferred stock and common stock, voting together as a single class, and on an as-converted basis, were entitled to elect all remaining members of the board of directors.

Protective provisions

As long as at least 6,000,000 shares of preferred stock were outstanding, as adjusted for any stock dividends, combinations, splits, recapitalizations and the like, the Company would first obtain the approval by vote or written consent of the holders of at least 65% of the then outstanding shares of preferred stock, voting together as a single class and not as a separate series, and on an as-converted basis with respect to: (i) consummation of liquidation event or effect any other merger or consolidation, (ii) amend, alter or repeal any provision of the Company’s certificate of incorporation of bylaws, (iii) increase or decrease the total number of authorized shares of common stock or preferred stock or designated shares of any series of preferred stock, (iv) authorize, issue or obligate the Company to issue any equity security having preference over any series of preferred stock, (v) redeem, purchase or otherwise acquire any share or shares of preferred stock or common stock, (vi) change the authorized number of directors of the Company, (vii) increase the number of shares of common stock reserved under any employee equity incentive plan, (viii) permit any subsidiary to sell or issue equity securities, (ix) pay or declare any dividend on any shares of capital stock and (x) authorize, issue or obligate the Company to issue any debt security if the aggregate indebtedness exceeds $5,000,000.

Liquidation preferences

In the event of any voluntary or involuntary liquidation, dissolution or winding-up of the Company or other “Liquidation Event” (as defined in the Company’s Restated Certificate of Incorporation), the holders of shares of Series A, Series B, Series C and Series C-2 convertible preferred stock would be entitled to be paid an amount equal to the original issue price per share, subject to adjustment in the event of any stock splits, stock dividends, combinations, subdivisions or similar recapitalization affecting such shares together with any dividends declared but unpaid, prior to the payment of any distributions to the holders of common stock. If, upon the occurrence of such event, the assets and funds distributed among the holders of the Series A, Series B, Series C and Series C‑2 convertible preferred stock were insufficient to permit the payment to such holders of the full aforesaid preferential amounts, then the entire assets and funds of the Company legally available for distribution were to be distributed ratably among the holders of the Series A, Series B, Series C and Series C-2 convertible preferred stock.

All holders of Series A, Series B, Series C and Series C-2 convertible preferred stock would be deemed to have converted if, as a result of an actual conversion, such holder would have received, in the aggregate, a greater amount than the amount that would be distributed to such holder if such holder did not convert such shares of Series A, Series B, Series C and Series C-2 convertible preferred stock into common stock.

Classification

The Company classified the convertible preferred stock outside of permanent equity on the balance sheet as these shares can be redeemed upon the occurrence of certain change in control events that are outside of the Company’s control, including liquidation, sale or transfer of the Company. The Company did not adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because it was uncertain whether or when an event would occur that would obligate the Company to pay the liquidation preferences to holders of shares of redeemable convertible preferred stock, and at the balance sheet dates these circumstances were not probable.

Common Stock

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Common Stock

10. Common Stock

The holders of the Company’s common stock have one vote for each share of common stock held by them. Holders of shares of the Company’s common stock are entitled to dividends when, as and if declared by the board of directors. No dividends had been declared as of December 31, 2019 or December 31, 2018. As of December 31, 2019 and 2018, the Company had 21,833,037 and 878,413 shares of common stock outstanding, respectively.

As of December 31, 2019, the Company had reserved the following shares of common stock, on an as-converted basis, for future issuance as follows:

	December 31,
	2019
Options issued and outstanding		406,939
Options available for future grants		1,874,759
ESPP		240,336
Total		2,522,034

Stock Option Plan

12 Months Ended

Dec. 31, 2019

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Option Plan

11. Stock Option Plan

In 2015, the Company adopted the FLX Bio, Inc. 2015 Stock Plan (the “2015 Plan”).

In connection with the consummation of the IPO in November 2019, the Company’s board of directors adopted the Company’s 2019 Equity Incentive Plan (the “2019 Plan” and collectively with the 2015 Plan, the “Option Plans”). Upon the effectiveness of the 2019 Plan, the Company’s 2015 Plan terminated and no further grants may be made thereunder. However, the 2015 Plan will continue to govern the terms and conditions of the outstanding awards previously granted thereunder. As of December 31, 2019, the Company had 1,874,759 shares of common stock available for grant. In addition, the number of shares reserved for issuance under the Company’s 2019 Plan will automatically increase on January 1 of each year beginning January 1, 2020 by a number equal to (i) 4% of the shares of common stock outstanding on the last business day of the prior fiscal year or (ii) the number of shares determined by the Company’s board of directors.

The Company’s Option Plans provided for the granting of incentive and non-statutory stock options and restricted shares of common stock options to eligible employees, officers, directors, advisors and consultants. Terms of the stock option agreements, including vesting requirements, are determined by the board of directors, subject to the provisions of the Options Plans. Options granted generally vest over four years and expire no later than ten years from the date of grant. As a private company, the estimated fair value of the Company’s underlying common stock was determined by the board of directors. Following the Company’s IPO, the Company’s board of directors intend to determine the fair value of the Company’s common stock based on the closing price of its common stock on the date of grant. The exercise price of the incentive stock options must be equal to or greater than the estimated fair value of the underlying common stock on the date of grant.

Activity under the Company’s stock option plans is set forth below:

	Shares Available			Number of Shares Outstanding			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Balances at December 31, 2018		693,879			768,239		$	4.62
Stock options authorized		1,786,166			—
Stock options granted		(823,858	)		823,858			12.96
Stock options exercised		—			(145,160	)		2.63
Unvested common shares repurchased		85,103			—			2.03
Stock options forfeited		133,469			(133,469	)		7.39
Balances at December 31, 2019		1,874,759			1,313,468		$	9.77		8.93	$	23,438
Vested and expected to vest at December 31, 2019					1,313,468		$	9.77		8.93	$	23,438
Exercisable at December 31, 2019					317,694		$	5.26		7.93	$	7,102

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the Company’s common stock, as determined by the board of directors, as of December 31, 2019.

The options granted in the years ended December 31, 2019 and 2018 had a weighted average per share grant-date fair value of $7.12 and $4.32, respectively, and a total grant date fair value of $7.2 million and $2.1 million.

The aggregate intrinsic value of stock options exercised in the years ended December 31, 2019 and 2018 was $3.6 million and $0.1 million, respectively.

The aggregate fair value of options that vested in the years ended December 31, 2019 and 2018 was $1.6 million and $0.9 million, respectively.

The Company had 25,000 shares of performance-based stock options outstanding as of December 31, 2018. The grant date fair value of the award was $0.2 million. As of December 31, 2018, the Company has not recognized any of the related stock-based compensation expense, as vesting of the awards was not determined to be probable. These performance-based stock options were cancelled in March 2019 upon the chief operating officer’s resignation from the Company.

Employee stock option valuation

The assumptions used to value employee and director stock option awards granted under the Option Plans during the years ended December 31, 2019 and 2018, using the Black-Scholes option pricing model, were as follows:

	Year Ended December 31,
	2019	2018
Fair value of common stock	$6.30 - $27.53	$1.03 - $1.05
Expected term (in years)	5.01 - 6.08	5.67 - 6.08
Volatility	83.00% - 84.99%	80.69% - 81.48%
Risk-free interest rate	1.58% - 2.23%	2.62% - 2.88%
Dividend yield	—	—

Prior to the Company’s IPO, the grant date fair value of the shares of common stock underlying stock options was determined by the Company’s board of directors with the assistance of management and an independent third-party valuation specialist. The grant date fair value of common stock was determined using valuation methodologies by considering a number of objective and subjective factors including important developments in the Company’s operations, valuations performed by independent third parties, sales of convertible preferred stock, actual operating results and financial performance, the conditions in the biotechnology industry and the economy in general, the stock price performance and volatility of comparable public companies and the lack of liquidity of the Company’s common stock, among other factors.

In determining the fair value of the options granted, the Company uses the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment to determine.

Expected term

The expected term represents the period that the Company’s 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). The Company has very limited historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior for its stock option grants.

Expected volatility

Since the Company was privately held and did not have any trading history for its common stock, the expected volatility was estimated based on the average volatility for comparable publicly traded biopharmaceutical companies over a period, where available, equal to the expected term of the stock option grants. The comparable companies were chosen based on their similar size, life cycle stage or area of specialty.

Risk-free interest rate

The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of the options.

Expected dividend

The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero.

Stock options granted to nonemployees

Stock-based compensation related to stock options granted to non-employees is recognized as the services are rendered. The assumptions used to value non-employee stock option awards granted under the 2015 Plan during the years ended December 31, 2019 and 2018, using the Black-Scholes option pricing model, were as follows:

	Year Ended December 31,
	2019	2018
Expected term (in years)	5.34 - 9.74	6.34 - 10.00
Volatility	81.43% - 84.85%	78.29% - 85.47%
Risk-free interest rate	1.40% - 2.67%	1.32% - 3.19%
Dividend yield	—	—

During the years ended December 31, 2019 and 2018, the Company granted 39,165 and 4,166 options to nonemployee consultants and recognized related expense of $0.5 million and $0.1 million, respectively.

Early exercise of stock options

The terms of the 2015 Plan permit option holders to exercise stock options before they are vested, subject to certain limitations. Such unvested shares are subject to repurchase by the Company at the original exercise price in the event the option holder’s service to the Company is terminated either voluntarily or involuntarily. As a result of early exercises under the 2015 Plan, approximately 0.1 million and 0.2 million shares were subject to repurchase as of December 31, 2019 and 2018, respectively. The Company treats cash received from the exercise of unvested options as a refundable deposit and classifies such amounts as a liability in its balance sheet. As of December 31, 2019 and 2018, the Company included cash received for the early exercise of unvested options of $0.1 million and $0.2 million, respectively, in other current liabilities. Amounts included in liabilities are transferred into common stock and additional paid-in capital as the shares vest, which is generally over a period of 48 months and may include a one-year cliff.

Stock-based compensation expense

Total stock-based compensation recognized for both employees and non-employees was as follows (in thousands):

	Year Ended December 31,
	2019		2018
Research and development	$	1,076	$	542
General and administrative		975		628
Total stock-based compensation expense	$	2,051	$	1,170

As of December 31, 2019, unrecognized stock-based compensation cost related to outstanding unvested stock options that are expected to vest was $7.9 million. This unrecognized stock-based compensation cost is expected to be recognized over 2.5 years.

Income Taxes

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Income Taxes

12. Income Taxes

The following table presents domestic and foreign components of income (loss) before income taxes for the periods presented (in thousands):

	December 31,
	2019			2018
United States	$	(41,301	)	$	(36,034	)
Foreign		(1,036	)		(113	)
	$	(42,337	)	$	(36,147	)

The components of the provision for income taxes are as follows (in thousands):

	December 31,
	2019		2018
Current
Foreign	$	660	$	—
Total provision for income taxes	$	660	$	—

A reconciliation of the statutory U.S. federal rate and effective rate is as follows:

	December 31,
	2019			2018
Federal tax		21.00	%		21.00	%
Stock-based compensation		(0.63	)		(0.60	)
Research and development tax credit		2.97			3.68
Withholding taxes, net of federal benefit		(1.23	)		—
Change in valuation allowance		(23.18	)		(24.13	)
Other		(0.49	)		0.05
Income tax expense		(1.56	%)		—	%

The Company has incurred net operating losses for all periods since inception. The Company has not reflected any benefit of such net operating loss carryforwards in the accompanying financial statements. The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding the realization of such assets.

The components of the Company’s deferred tax assets and liabilities are as follows (in thousands):

	December 31,
	2019			2018
Deferred tax assets:
Net operating loss carryforwards	$	29,014		$	20,810
Federal and state research and development tax credits		4,636			3,378
Depreciation and amortization		—			105
Accrued liabilities and reserves		837			448
Stock-based compensation		215			50
Gross deferred tax assets		34,702			24,791
Valuation allowance		(34,605	)		(24,791	)
Deferred tax liabilities:
Depreciation and amortization		(97	)		—
Gross deferred tax liabilities		(97	)		—
Net deferred taxes	$	—		$	—

Realization of deferred tax assets is dependent upon future taxable income, if any. The Company has established a valuation allowance to offset deferred tax assets as of December 31, 2019 and 2018, due to the uncertainty of realizing future tax benefits from its net operating loss carryforwards and other deferred tax assets. The valuation allowance increased by approximately $9.8 million and $8.7 million during the years ended December 31, 2019 and 2018, respectively. The increase in the valuation allowance is mainly related to the increase in net operating loss carryforwards incurred during the respective taxable years.

As of December 31, 2019, the Company had federal net operating loss carryforwards of approximately $134.2 million. The federal net operating loss carryforwards generated during and after fiscal 2018 totaling $73.2 million are carried forward indefinitely, while all others along with the federal tax credit carryforwards, expire in years beginning in 2035. As of December 31, 2019, the Company had approximately $12.1 million of state net operating loss carryforwards, which begin to expire in 2035 and are available to offset future taxable income. As of December 31, 2019, the Company had research and development tax credit carryforwards of approximately $3.7 million and approximately $3.2 million available to reduce future federal and state income taxes, respectively. Moreover, as of December 31, 2019, the Company recorded federal and state reserves of $0.9 million and approximately $0.8, respectively, as uncertain tax positions. If not utilized, the federal credit carryforwards will begin expiring in 2035. The state credits carry forward indefinitely.

Federal and state laws impose substantial restrictions on the utilization of net operating loss and tax credit carryforwards in the event of an ownership change for tax purposes, as defined in Section 382 of the Internal Revenue Code. As a result of such ownership changes, the Company’s ability to realize the potential future benefit of tax losses and tax credits that existed at the time of the ownership change may be significantly reduced. The Company’s deferred tax asset and related valuation allowance would be reduced as a result. The Company has not yet performed a Section 382 study to determine the amount of reduction, if any. The annual limitation may result in the expiration of net operating losses and credits before utilization. Under the new enacted law, the carryforward period of net operating losses generated from 2018 forward is indefinite; however, the carryforward period for net operating losses generated prior to 2018 remains the same. Therefore, the annual limitation may still result in the expiration of certain net operating losses and tax credit carryforwards before their utilization.

Tax benefits from uncertain tax positions are recognized when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on the technical merits. The amount recognized is measured as the largest amount of tax benefit that is greater than 50 percent likely of being realized upon effective settlement.

A reconciliation of the beginning and ending amounts of unrecognized tax benefits for the years ended December 31, 2019 and 2018 resulting primarily from research and development tax credits claimed on the Company’s annual tax returns were as follows (in thousands):

	December 31,
	2019		2018
Balance at beginning of year	$	1,281	$	789
Additions on tax positions related to prior years		—		19
Additions on tax positions related to current year		466		473
Balance at end of year	$	1,747	$	1,281

The Company does not expect that its uncertain tax positions will materially change in the next twelve months. The reversal of uncertain tax benefits would not impact the Company’s effective tax rate as the Company continues to maintain a full valuation allowance against its deferred tax assets. In accordance with ASC 740, the Company would classify interest and penalties related to uncertain tax positions in income tax expense, if applicable. There was no interest expense or penalties related to unrecognized tax benefits through December 31, 2019.

The Company files income tax returns with varying statutes of limitations in the United States, various states and foreign jurisdictions. The Company is not currently under examination by income tax authorities in federal, state or other jurisdictions. All tax returns remain open for examination by federal and state authorities. The tax years from inception in 2015 forward remain open to examination due to the carryover of unused net operating losses and tax credits.

Net Loss Per Share

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Net Loss Per Share

13. Net Loss Per Share

The following table sets forth the computation of the basic and diluted net loss per share of the years ended December 31, 2019 and 2018 (in thousands, except share and per share data):

	Year Ended December 31
	2019			2018
Numerator:
Net loss	$	42,997		$	36,147
Denominator:
Weighted average common shares outstanding		4,404,486			866,348
Less: weighted-average unvested restricted common stock subject to repurchase		(21,294	)		(137,691	)
Less: weighted-average unvested early exercised common shares subject to repurchase		(36,792	)		(106,368	)
Weighted-average shares used to compute net loss per common share, basic and diluted		4,346,400			622,289
Net loss per share, basic and diluted	$	9.89		$	58.09

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

	December 31,
	2019		2018
Convertible preferred stock		—		16,415,281
Common stock options issued and outstanding		1,313,468		693,879
Total		1,313,468		17,109,160

Subsequent Events

12 Months Ended

Dec. 31, 2019

Subsequent Events [Abstract]

Subsequent Events

14. Subsequent Events

In February 2020, the Company completed an underwritten Follow‑on Offering of 2,500,000 shares of its common stock issued at an offering price of $30.00 per share. The shares issued in the Follow-on Offering generated approximately $69.7 million in net proceeds after deducting underwriting discounts and other offering related costs.

Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) and include the consolidated accounts of the Company and its wholly-owned subsidiary, RAPT Therapeutics Australia Pty Ltd., which was established in 2018. All intercompany balances and transactions have been eliminated in consolidation.
Reverse Stock Split	Reverse Stock Split On July 19, 2019, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock on a one-for-six basis (the “Reverse Stock Split”). In connection with the Reverse Stock Split, the conversion ratio for the Company’s outstanding convertible preferred stock was proportionately adjusted such that the common stock issuable upon conversion of such preferred stock was decreased in proportion to the Reverse Stock Split. The par value of the common stock was not adjusted as a result of the Reverse Stock Split. All references to common stock, options to purchase common stock, early exercised options, share data, per share data, convertible preferred stock (to the extent presented on an as-converted to common stock basis) and related information contained in these consolidated financial statements have been retrospectively adjusted to reflect the effect of the Reverse Stock Split for all periods presented.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the 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 balance sheets and the amount of expenses and income reported for each of the periods presented are affected by estimates and assumptions, which are used for, but are not limited to, determining the fair value of assets and liabilities, common stock valuation and stock-based compensation. Actual results could differ from such estimates or assumptions.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties The Company is subject to a number of risks similar to other biopharmaceutical companies in the early stage, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s products and protection of proprietary technology. If the Company does not successfully obtain regulatory approval, commercialize or partner any of its product candidates, it will be unable to generate revenue from product sales or achieve profitability. Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents. Substantially all the Company’s cash and cash equivalents are held by financial institutions. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents in highly rated money market funds.
Segments	Segments The Company operates as a single operating segment. The Company’s chief operating decision maker, its President and Chief Executive Officer, manages the Company’s operations on a consolidated basis for the purposes of allocating resources, making operating decisions and evaluating financial performance.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying amount of the Company’s financial instruments, including certain prepaid and accrued expenses, approximates fair value due to their short-term maturities.
Cash Equivalents	Cash Equivalents The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents.
Property and Equipment	Property and Equipment Property and equipment consist of computer equipment, laboratory equipment, leasehold improvements and furniture and fixtures, and is recorded at cost, less accumulated depreciation and amortization. Depreciation and amortization are calculated using the straight-line method over the estimated useful lives of the respective assets, generally three to five years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the improvements. Depreciation and amortization begin at the time the asset is placed in service. Maintenance and repairs are charged to expense as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation and amortization are removed from the balance sheet and the resulting gain or loss is reflected in the results of operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company evaluates its long-lived assets for impairment annually or more frequently whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparing the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. For the years ended December 31, 2019 and 2018, the Company did not record any impairment losses on long-lived assets.
Leases	Leases The Company leases office space and laboratory facilities under non-cancelable operating lease agreements and recognizes related rent expense on a straight-line basis over the term of the lease. Funding of leasehold improvements by the Company’s landlord is accounted for as a tenant improvement allowance and recorded as current and non-current deferred rent liabilities and amortized on a straight-line basis as a reduction of rent expense over the term of the lease.
Convertible Preferred Stock	Convertible Preferred Stock The Company recorded all shares of convertible preferred stock at their respective fair values on the dates of issuance, less issuance costs. In the event of a change of control of the Company, proceeds received from the sale of such shares would have been distributed in accordance with the liquidation preferences set forth in the Company’s Amended and Restated Certificate of Incorporation unless the holders of the convertible preferred stock had converted their shares of convertible preferred stock into shares of common stock. Convertible preferred stock was previously classified outside of stockholders’ equity (deficit) on the balance sheet as events triggering redemption were not solely within the Company’s control. Immediately prior to the consummation of the Company’s IPO in November 2019, the convertible preferred stock converted into 17,467,184 shares of common stock.
Revenue	Revenue License and collaborative agreements consist of license, milestone and royalty payments generated through agreements with strategic partners for the development and commercialization of certain product candidates. The terms of an agreement may include a non-refundable upfront fee, payments based upon achievement of milestones and royalties on net product sales. If a portion of the nonrefundable upfront fee or other payments received is allocated to continuing performance obligations under the terms of an agreement, such portion is recorded as deferred revenue and recognized as revenue when (or as) the underlying performance obligation is satisfied. The Company recognizes revenue when it transfers promised goods or services to customers or counterparties in an amount that reflects the consideration to which the Company expects to be entitled in exchange for those goods or services. In determining the appropriate amount of revenue to be recognized, the Company performs the following steps: (i) identification of the promised goods or services in the agreement; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the agreement; (iii) measurement of the transaction price, including any constraint on variable consideration; (iv) allocation of the transaction price to performance obligations based on estimated selling prices; and (v) recognition of revenue when (or as) the Company satisfies each performance obligation. Licenses: If a license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in an agreement, the Company will recognize revenue from the nonrefundable, upfront fee allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. If a license is bundled with other performance obligations, the Company utilizes judgment to assess the nature of the combined performance obligations to determine whether the combined performance obligations are satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition. Milestone payments: If an agreement includes event-based or milestone payments, the Company evaluates whether the events or milestones are considered likely to be achieved and estimates the amount to be included in the transaction price using the most likely amount method. If it is unlikely that a significant revenue reversal would occur, the value of the associated event-based or milestone payments is included in the transaction price. Event-based or milestone payments that are not within the control of the Company are not included in the transaction price until they become likely to be achieved. Royalties: If an agreement includes sales-based royalties and the license is deemed to be the predominant item to which the royalties relate, the Company will recognize revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). As of December 31, 2019, the Company recorded deferred revenue of $4.0 million on the consolidated balance sheet related to the license and collaborative agreement with Hanmi Pharmaceutical LTD (“Hanmi”).
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Research and development costs consist primarily of salaries and benefits of research and development personnel, costs related to research activities, preclinical studies, clinical trials, drug manufacturing and allocated overhead and facility-related expenses. The Company accounts for non-refundable advance payments for goods or services that will be used in future research and development activities as expenses when the goods have been received or when the service has been performed rather than when the payment is made. Clinical trial costs are a component of research and development expenses. The Company expenses costs for its clinical trial activities performed by third parties, including clinical research organizations (“CROs”) and other service providers, as they are incurred, based upon estimates of the work completed over the life of the individual study in accordance with associated agreements. The Company uses information it receives from internal personnel and outside service providers to estimate the clinical trial costs incurred.
Share-Based Compensation	Stock-Based Compensation The Company measures employee and director stock-based compensation expense for all stock-based awards based on their grant date fair value using the Black-Scholes option-pricing model. For stock-based awards with service conditions only, stock-based compensation expense is recognized over the requisite service period using the straight-line method. For awards with performance conditions, the Company evaluates the probability of achieving performance conditions at each reporting date. The Company begins to recognize stock-based compensation expense using an accelerated attribution method when it is deemed probable that the performance condition will be met. Forfeitures are recognized as they occur. Stock-based compensation expense for nonemployee stock-based awards is measured at fair value using the Black-Scholes option-pricing model. The Company recognizes stock-based compensation expense for the estimated fair value of the vested portion of nonemployee awards in its consolidated statements of operations and comprehensive loss. Stock-based compensation expense related to stock-based awards to nonemployees is subject to re-measurement over the service period, which approximates the vesting period. Stock-based compensation expense related to restricted stock awards is determined using the estimated fair value of the Company’s common stock on the date of grant for the period prior to the IPO. The fair value of restricted stock awards granted after the IPO is determined based on the stock price on the date of grant. The estimated fair value is amortized as compensation expense over the service period of the award.
Foreign Currency Transactions	Foreign Currency Transactions The functional currency of RAPT Therapeutics Australia Pty Ltd., the Company’s wholly-owned subsidiary, is the Australian dollar. Accordingly, all monetary assets and liabilities of the subsidiary are translated into U.S. dollars at the current period-end exchange rates and non-monetary assets are translated using historical exchange rates. Income and expense elements are remeasured to U.S. dollars using the average exchange rates in effect during the period. Remeasurement gains and losses are recorded as other income (expense). The Company is subject to foreign currency risk with respect to its clinical contracts denominated in currencies other than the U.S. dollar. Payments on contracts denominated in foreign currencies are made at the spot rate on the day of payment. Changes in the exchange rate between billing dates and payment dates are recorded to other (income), net on the consolidated statements of operations.
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period such tax rate changes are enacted. The Company recognizes the effect of income tax positions only if those positions are more likely than not to be sustained. Recognized income tax positions are measured at the largest amount that is greater than 50% likely to be realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. Valuation allowances are established when necessary to reduce deferred tax assets to amounts more likely than not to be realized. Interest and penalties related to unrecognized tax benefits are recognized as a component of income tax expense.
Comprehensive Loss	Comprehensive Loss Comprehensive loss includes net loss and certain changes in stockholders’ deficit that are excluded from net loss, primarily unrealized losses from foreign currency translation adjustments.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing the net loss by the weighted average number of common shares outstanding during the period, without consideration of potential dilutive securities. Diluted net loss per common share is computed by dividing the net loss by the sum of the weighted average number of common shares outstanding during the period plus the number of potential dilutive securities outstanding during the period calculated in accordance with the treasury stock method. Diluted net loss per share is the same as basic net loss per share since the effect of potentially dilutive securities is anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (“FASB”) or other standard setting bodies and adopted by the Company as of the specified effective date. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), the Company meets the definition of an emerging growth company and has elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act.
Adopted Accounting Pronouncements	Adopted Accounting Pronouncements In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230: Classification of Certain Cash Receipts and Cash Payments), which addresses specific cash flow issues with the objective of reducing the diversity in practice for the treatment of these issues. The areas identified include: debt prepayment or debt extinguishment costs; settlement of zero-coupon debt instruments; contingent consideration payments made after a business combination; proceeds from the settlement of insurance claims; proceeds from the settlement of corporate-owned life insurance policies; distributions received from equity method investees; beneficial interests in securitization transactions; and application of the predominance principle with respect to separately identifiable cash flows. The Company adopted ASU 2016-15 in the fourth quarter of 2019 and there was no impact to the financial position or results of operations related to this adoption. In February 2018, the FASB issued ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. The amendments in this standard allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. The Company adopted ASU 2018-02 in the fourth quarter of 2019 and there was no impact to the financial position or results of operations related to this adoption. In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (ASC 808): Clarifying the Interaction between ASC 808 and ASC 606, which clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in ASC 606 should be applied, including recognition, measurement, presentation and disclosure requirements. ASU 2018-18 adds unit-of-account guidance in ASC 808 to align with the guidance in ASC 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of ASC 606, and requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under ASC 606 is precluded if the collaborative arrangement participant is not a customer. The Company adopted ASU 2018-18 in the fourth quarter of 2019 and there was no impact to the financial position or results of operations related to this adoption. In December 2019, the FASB issued ASU No. 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The amendments also improve consistent application of and simplify other areas of Topic 740 by clarifying and amending existing guidance. The Company adopted ASU 2019-12 in the fourth quarter of 2019 and there was no impact to the financial position or results of operations related to this adoption.
Recently Issued Accounting Pronouncements Not Yet Adopted	Recently Issued Accounting Pronouncement Not Yet Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU 2016-02 requires lessees to record most leases on their balance sheet while recognizing expense in a manner similar to existing accounting. ASU 2016‑02 states that a lessee would recognize a lease liability for the obligation to make lease payments and a right-to-use asset for the right to use the underlying asset for the lease term. The new accounting guidance is effective for the Company for fiscal periods beginning after December 15, 2020 and early adoption is permitted. The Company is currently assessing the timing of adoption and the impact that the adoption will have on its consolidated financial statements and related disclosures. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. ASU 2016-13 amended guidance on reporting credit losses for assets held at amortized cost basis and available-for-sale debt securities. For available-for-sale debt securities, credit losses will be presented as an allowance rather than as a write-down. This standard is effective for the Company’s fiscal year beginning after December 15, 2020. Early adoption is permitted for all entities. The Company is currently assessing the timing of adoption and the impact that the adoption of ASU 2016-13 will have on its consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Non-Employee Share-Based Payment Accounting as part of the FASB simplification initiative. ASU 2018-07 expands the scope of Topic 718, allowing the Company to apply the requirements of Topic 718 to certain non-employee awards to acquire goods and services from non-employees. This ASU will be effective for the Company for fiscal years beginning after December 15, 2019, and early adoption is permitted. The Company is currently assessing the impact that the adoption of ASU 2018-07 will have on its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurements (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement as part of the FASB’s disclosure framework project. ASU 2018-13 modifies the disclosure requirements on fair value measurements in Topic 820, Fair Value Measurement by removing the requirement to disclose amounts of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels and the valuation process for Level 3 fair value measurements. This ASU also modifies existing disclosure requirements by clarifying that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date, and it adds required disclosures for the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period, and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. This ASU will be effective for the Company for fiscal years and interim periods within those fiscal years, beginning after December 15, 2019, and early adoption is permitted. The Company is currently assessing the impact of this ASU on its consolidated financial statements.

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2019

Property Plant And Equipment [Abstract]

Schedule of Property and Equipment

Property and equipment consists of the following (in thousands):

	December 31,
	2019			2018
Laboratory equipment	$	5,752		$	5,466
Leasehold improvements		3,294			2,989
Computer equipment		326			308
Furniture and fixtures		394			365
Total property and equipment		9,766			9,128
Less accumulated depreciation and amortization		(6,059	)		(4,969	)
Property and equipment, net	$	3,707		$	4,159

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2019

Payables And Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):

	December 31,
	2019		2018
Accrued research and clinical development expenses	$	1,353	$	519
Accrued compensation		1,779		1,433
Accrued professional and consulting services		192		182
Accrued property and equipment		—		202
Accrued lab supplies		29		80
Other		289		72
Total accrued expenses	$	3,642	$	2,488

Commitments (Tables)

12 Months Ended

Dec. 31, 2019

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Non-Cancelable Lease Payments, Net of Sublease Rental Income

As of December 31, 2019, future minimum non-cancelable lease payments, net of sublease rental income, are as follows (in thousands):

Year ending December 31:
2020	$	1,602
2021		1,969
2022		2,037
2023		2,109
Thereafter		6,579
Total minimum lease payments	$	14,296

Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2019

Temporary Equity Disclosure [Abstract]

Schedule of Convertible Preferred Stock Outstanding

As of December 31, 2018, convertible preferred stock consisted of the following (in thousands, except share amounts):

			Shares		Net		Aggregate
	Shares		Issued and		Carrying		Liquidation
	Authorized		Outstanding		Value		Preference
Series A		37,509,105		6,251,502	$	28,861	$	37,509
Series B		25,000,000		4,166,663		49,926		50,000
Series C		26,109,363		4,351,554		59,770		59,856
Series C-2		15,400,000		1,645,562		22,554		22,635
Total convertible preferred stock		104,018,468		16,415,281	$	161,111	$	170,000

Common Stock (Tables)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Summary of Reserved Shares of Common Stock on Converted Basis for Future Issuance

As of December 31, 2019, the Company had reserved the following shares of common stock, on an as-converted basis, for future issuance as follows:

	December 31,
	2019
Options issued and outstanding		406,939
Options available for future grants		1,874,759
ESPP		240,336
Total		2,522,034

Stock Option Plan (Tables)

12 Months Ended

Dec. 31, 2019

Summary of Option Plans

Activity under the Company’s stock option plans is set forth below:

	Shares Available			Number of Shares Outstanding			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Balances at December 31, 2018		693,879			768,239		$	4.62
Stock options authorized		1,786,166			—
Stock options granted		(823,858	)		823,858			12.96
Stock options exercised		—			(145,160	)		2.63
Unvested common shares repurchased		85,103			—			2.03
Stock options forfeited		133,469			(133,469	)		7.39
Balances at December 31, 2019		1,874,759			1,313,468		$	9.77		8.93	$	23,438
Vested and expected to vest at December 31, 2019					1,313,468		$	9.77		8.93	$	23,438
Exercisable at December 31, 2019					317,694		$	5.26		7.93	$	7,102

Summary of Assumptions Used to Value Under Stock Option Plans

	Year Ended December 31,
	2019	2018
Fair value of common stock	$6.30 - $27.53	$1.03 - $1.05
Expected term (in years)	5.01 - 6.08	5.67 - 6.08
Volatility	83.00% - 84.99%	80.69% - 81.48%
Risk-free interest rate	1.58% - 2.23%	2.62% - 2.88%
Dividend yield	—	—

Summary of Stock-based Compensation Expense

Total stock-based compensation recognized for both employees and non-employees was as follows (in thousands):

	Year Ended December 31,
	2019		2018
Research and development	$	1,076	$	542
General and administrative		975		628
Total stock-based compensation expense	$	2,051	$	1,170

Non-employees

Summary of Assumptions Used to Value Under Stock Option Plans

The assumptions used to value non-employee stock option awards granted under the 2015 Plan during the years ended December 31, 2019 and 2018, using the Black-Scholes option pricing model, were as follows:

	Year Ended December 31,
	2019	2018
Expected term (in years)	5.34 - 9.74	6.34 - 10.00
Volatility	81.43% - 84.85%	78.29% - 85.47%
Risk-free interest rate	1.40% - 2.67%	1.32% - 3.19%
Dividend yield	—	—

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Schedule of Domestic and Foreign Components of Income (Loss) before Income Taxes

The following table presents domestic and foreign components of income (loss) before income taxes for the periods presented (in thousands):

	December 31,
	2019			2018
United States	$	(41,301	)	$	(36,034	)
Foreign		(1,036	)		(113	)
	$	(42,337	)	$	(36,147	)

Schedule of Components of Provision for Income Taxes

The components of the provision for income taxes are as follows (in thousands):

	December 31,
	2019		2018
Current
Foreign	$	660	$	—
Total provision for income taxes	$	660	$	—

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the statutory U.S. federal rate and effective rate is as follows:

	December 31,
	2019			2018
Federal tax		21.00	%		21.00	%
Stock-based compensation		(0.63	)		(0.60	)
Research and development tax credit		2.97			3.68
Withholding taxes, net of federal benefit		(1.23	)		—
Change in valuation allowance		(23.18	)		(24.13	)
Other		(0.49	)		0.05
Income tax expense		(1.56	%)		—	%

Schedule of Components of Deferred Tax Assets and Liabilities

The components of the Company’s deferred tax assets and liabilities are as follows (in thousands):

	December 31,
	2019			2018
Deferred tax assets:
Net operating loss carryforwards	$	29,014		$	20,810
Federal and state research and development tax credits		4,636			3,378
Depreciation and amortization		—			105
Accrued liabilities and reserves		837			448
Stock-based compensation		215			50
Gross deferred tax assets		34,702			24,791
Valuation allowance		(34,605	)		(24,791	)
Deferred tax liabilities:
Depreciation and amortization		(97	)		—
Gross deferred tax liabilities		(97	)		—
Net deferred taxes	$	—		$	—

Schedule of Reconciliation of Unrecognized Tax Benefits

	December 31,
	2019		2018
Balance at beginning of year	$	1,281	$	789
Additions on tax positions related to prior years		—		19
Additions on tax positions related to current year		466		473
Balance at end of year	$	1,747	$	1,281

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2019

Earnings Per Share [Abstract]

Computation of Basic and Diluted Net Loss per Share

The following table sets forth the computation of the basic and diluted net loss per share of the years ended December 31, 2019 and 2018 (in thousands, except share and per share data):

	Year Ended December 31
	2019			2018
Numerator:
Net loss	$	42,997		$	36,147
Denominator:
Weighted average common shares outstanding		4,404,486			866,348
Less: weighted-average unvested restricted common stock subject to repurchase		(21,294	)		(137,691	)
Less: weighted-average unvested early exercised common shares subject to repurchase		(36,792	)		(106,368	)
Weighted-average shares used to compute net loss per common share, basic and diluted		4,346,400			622,289
Net loss per share, basic and diluted	$	9.89		$	58.09

Potentially Dilutive Securities not Included in Diluted per Share Calculations

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

	December 31,
	2019		2018
Convertible preferred stock		—		16,415,281
Common stock options issued and outstanding		1,313,468		693,879
Total		1,313,468		17,109,160

Organization - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		12 Months Ended
	Feb. 29, 2020	Nov. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Net proceeds received after deducting underwriting discounts and other offering related costs			$ 34,721	$ 0
Authorized shares of common stock			500,000,000	500,000,000
Common stock, par value			$ 0.0001	$ 0.0001
Authorized shares of preferred stock			50,000,000	50,000,000
Preferred stock, par value			$ 0.0001	$ 0.0001
Net loss			$ 42,997	$ 36,147
Cash used in operations			35,474	32,953
Cash and cash equivalents			77,383	$ 63,798
Working capital			$ 71,300
Common Stock
Class Of Stock [Line Items]
Shares issued during period			3,427,360
Conversion of convertible preferred stock, common stock shares issued		17,467,184	17,467,184
Initial Public Offering
Class Of Stock [Line Items]
Shares issued during period		3,427,360
Shares issued, price per share		$ 12.00
Net proceeds received after deducting underwriting discounts and other offering related costs		$ 34,700
Authorized shares of common stock		500,000,000
Common stock, par value		$ 0.0001
Authorized shares of preferred stock		50,000,000
Preferred stock, par value		$ 0.0001
Follow-on Offering \| Subsequent Event
Class Of Stock [Line Items]
Shares issued during period	2,500,000
Shares issued, price per share	$ 30.00
Net proceeds received after deducting underwriting discounts and other offering related costs	$ 69,700

Summary of Significant Accounting Policies - Additional Information (Details)		1 Months Ended	12 Months Ended
	Jul. 19, 2019	Nov. 30, 2019 shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Impairment losses on long-lived assets			$ 0	$ 0
Deferred revenue			4,000,000
Collaboration and License Agreement \| Hanmi Pharmaceutical Co., LTD
Significant Accounting Policies [Line Items]
Deferred revenue			$ 4,000,000
Minimum
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life			3 years
Maximum
Significant Accounting Policies [Line Items]
Property, plant and equipment, useful life			5 years
Common Stock
Significant Accounting Policies [Line Items]
Reverse stock split, description			On July 19, 2019, the Company filed an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of shares of the Company’s common stock on a one-for-six basis (the “Reverse Stock Split”). In connection with the Reverse Stock Split, the conversion ratio for the Company’s outstanding convertible preferred stock was proportionately adjusted such that the common stock issuable upon conversion of such preferred stock was decreased in proportion to the Reverse Stock Split. The par value of the common stock was not adjusted as a result of the Reverse Stock Split
Reverse stock split, conversion ratio	0.17
Conversion of Series A, B, C, C-2 convertible preferred stock to common stock, shares \| shares		17,467,184	17,467,184

Fair Value Measurements - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Recurring \| Level 1 \| Cash and Cash Equivalents
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Money market fund, fair value	$ 77.4	$ 63.7

Property and Equipment - Schedule of Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Line Items]
Total property and equipment	$ 9,766	$ 9,128
Less accumulated depreciation and amortization	(6,059)	(4,969)
Property and equipment, net	3,707	4,159
Laboratory Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	5,752	5,466
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	3,294	2,989
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	326	308
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	$ 394	$ 365

Property and Equipment - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Depreciation and amortization expenses	$ 1,330	$ 1,237

Accrued Expenses - Schedule of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued research and clinical development expenses	$ 1,353	$ 519
Accrued compensation	1,779	1,433
Accrued professional and consulting services	192	182
Accrued property and equipment		202
Accrued lab supplies	29	80
Other	289	72
Total accrued expenses	$ 3,642	$ 2,488

Commitments - Additional Information (Details)	1 Months Ended		12 Months Ended
Commitments - Additional Information (Details)	Apr. 30, 2018 ft²	May 31, 2015 USD ($) ft²	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 31, 2019 ft²
Lessee Lease Description [Line Items]
Lease expiration period		2022-05
Tenant improvements allowances		$ 800,000		$ 1,400,000
Additional laboratory and office facilities under lease \| ft²	6,378				6,378
Increased of leased premises \| ft²	36,754
Lessee operating lease extended lease term	2026-11
Lease existence of option to extend		true
Lessee operating lease option to extend		5 years
Leasehold improvements on amortized over the remaining lease term				2,400,000
Operating lease rent expense			$ 2,000,000	$ 1,800,000
Contingency loss, accrued			$ 0
South San Francisco and California
Lessee Lease Description [Line Items]
Laboratory and office facilities under lease \| ft²		30,376

Commitments - Schedule of Future Minimum Non-Cancelable Lease Payments, Net of Sublease Rental Income (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Operating Lease Liabilities Payments Due [Abstract]
2020	$ 1,602
2021	1,969
2022	2,037
2023	2,109
Thereafter	6,579
Total minimum lease payments	$ 14,296

Collaboration Agreements - Additional Information (Details)	12 Months Ended
	Dec. 31, 2019 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue	$ 4,000,000
Collaboration and License Agreement \| Hanmi Pharmaceutical Co., LTD
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront and expected near-term milestone payment	10,000,000
Upfront fee receivable	4,000,000
Milestone payment	6,000,000
Revenue recognized	0
Deferred revenue	4,000,000
Collaboration and License Agreement \| Hanmi Pharmaceutical Co., LTD \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payment upon achievement	48,000,000
Sales milestone payment upon achievement	60,000,000
Contingent payments upon Achievement of specified milestones	$ 108,000,000

Related-Party Promissory Notes - Additional Information (Details) - Promissory Notes - USD ($)	1 Months Ended
	Jul. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2016	Aug. 31, 2015	Dec. 31, 2018
Related Party Transaction [Line Items]
Total outstanding balances including accrued interest							$ 600,000
Chief Executive Officer
Related Party Transaction [Line Items]
Principal amount of loan						$ 300,000
Interest rate percentage						1.82%
Forgiveness of principal amount and accrued interest due on note		$ 400,000
Chief Operating Officer
Related Party Transaction [Line Items]
Principal amount of loan					$ 300,000
Interest rate percentage					1.41%
Reduction of promissory note			$ (100,000)	$ (17,000)
Vested shares subject to repurchase			63,019
Unvested shares subject to repurchase			28,645
Repurchase of vested shares	29,686
Principal and interest on promissory note cancelled for repurchase of vested shares	$ 100,000
Proceeds from remaining number of vested shares	$ 100,000
Remaining number of vested shares issued	33,333
Chief Operating Officer \| Performance-based Options
Related Party Transaction [Line Items]
Reduction of performance-based options				8,333

Convertible Preferred Stock - Additional Information (Details) - USD ($)	1 Months Ended			6 Months Ended	12 Months Ended
	Nov. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Proceeds from the sale of convertible preferred stock, net of issuance costs					$ 14,379,000	$ 52,468,000
Convertible preferred stock, shares outstanding		16,415,281			0	16,415,281
Preferred stock, shares outstanding		0			0	0
Aggregate indebtedness		$ 5,000,000				$ 5,000,000
Minimum
Temporary Equity [Line Items]
Preferred stock, shares outstanding		6,000,000				6,000,000
Preferred stock voting rights percent						65.00%
Underwritten Public Offering \| Minimum
Temporary Equity [Line Items]
Shares issued, price per share		$ 13.7550				$ 13.7550
Proceeds from the sale of convertible preferred stock, net of issuance costs						$ 30,000,000
Common Stock
Temporary Equity [Line Items]
Conversion of convertible preferred stock, common stock shares issued	17,467,184				17,467,184
Series C Convertible Preferred Stock
Temporary Equity [Line Items]
Shares issued, price per share			$ 2.2925
Proceeds from the sale of convertible preferred stock, net of issuance costs			$ 29,900,000
Convertible preferred stock, shares outstanding		4,351,554				4,351,554
Dividends per share, declared						$ 0.18
Preferred stock, voting rights						one vote for each share of common stock into which such shares of preferred stock were convertible
Series C-2 Convertible Preferred Stock
Temporary Equity [Line Items]
Shares issued, price per share		$ 2.2925		$ 2.2925		$ 2.2925
Proceeds from the sale of convertible preferred stock, net of issuance costs		$ 22,600,000		$ 14,400,000
Convertible preferred stock, shares outstanding		1,645,562				1,645,562
Dividends per share, declared						$ 0.18
Preferred stock, voting rights						one vote for each share of common stock into which such shares of preferred stock were convertible
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding		6,251,502				6,251,502
Dividends per share, declared						$ 0.08
Preferred stock, voting rights						one vote for each share of common stock into which such shares of preferred stock were convertible
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Convertible preferred stock, shares outstanding		4,166,663				4,166,663
Dividends per share, declared						$ 0.16
Preferred stock, voting rights						one vote for each share of common stock into which such shares of preferred stock were convertible

Convertible Preferred Stock - Schedule of Convertible Preferred Stock Outstanding (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Temporary Equity [Line Items]
Shares Authorized	0	104,018,468
Shares Issued	0	16,415,281
Shares Outstanding	0	16,415,281
Net Carrying Value		$ 161,111
Aggregate Liquidation Preference		$ 170,000
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		37,509,105
Shares Issued		6,251,502
Shares Outstanding		6,251,502
Net Carrying Value		$ 28,861
Aggregate Liquidation Preference		$ 37,509
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		25,000,000
Shares Issued		4,166,663
Shares Outstanding		4,166,663
Net Carrying Value		$ 49,926
Aggregate Liquidation Preference		$ 50,000
Series C Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		26,109,363
Shares Issued		4,351,554
Shares Outstanding		4,351,554
Net Carrying Value		$ 59,770
Aggregate Liquidation Preference		$ 59,856
Series C-2 Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized		15,400,000
Shares Issued		1,645,562
Shares Outstanding		1,645,562
Net Carrying Value		$ 22,554
Aggregate Liquidation Preference		$ 22,635

Common Stock - Additional Information (Details)	12 Months Ended
Common Stock - Additional Information (Details)	Dec. 31, 2019 USD ($) vote $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Nov. 30, 2019 $ / shares shares
Class Of Stock [Line Items]
Authorized shares of common stock	500,000,000	500,000,000
Common stock, par value \| $ / shares	$ 0.0001	$ 0.0001
Authorized shares of preferred stock	50,000,000	50,000,000
Preferred stock, par value \| $ / shares	$ 0.0001	$ 0.0001
Number of votes for each share of common stock	The holders of the Company’s common stock have one vote for each share of common stock held by them
Number of votes per share \| vote	1
Dividends declared \| $	$ 0	$ 0
Common stock, shares outstanding	21,833,037	878,413
Initial Public Offering
Class Of Stock [Line Items]
Authorized shares of common stock			500,000,000
Common stock, par value \| $ / shares			$ 0.0001
Authorized shares of preferred stock			50,000,000
Preferred stock, par value \| $ / shares			$ 0.0001

Common Stock - Summary of Reserved Shares of Common Stock on Converted Basis for Future Issuance (Detail)	Dec. 31, 2019 shares
Class Of Stock [Line Items]
Total shares reserved	2,522,034
ESPP
Class Of Stock [Line Items]
Total shares reserved	240,336
Options Issued and Outstanding
Class Of Stock [Line Items]
Total shares reserved	406,939
Options Available for Future Grants
Class Of Stock [Line Items]
Total shares reserved	1,874,759

Stock Option Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average grant-date fair value per share	$ 7.12	$ 4.32
Stock options grant date fair value	$ 7,200,000	$ 2,100,000
Number of stock options, exercises intrinsic value	3,600,000	100,000
Stock option vested aggregate fair value	1,600,000	900,000
Stock-based compensation expense	2,051,000	1,170,000
Expected dividend yield	0
Stock options related expense	2,051,000	1,170,000
Unrecognized stock-based compensation cost related to outstanding unvested stock options that are expected to vest	$ 7,900,000
Unrecognized stock-based compensation cost, expected period to be recognized	2 years 6 months
Performance-based Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vested aggregate fair value		$ 200,000
Stock options, outstanding		25,000
Stock-based compensation expense		$ 0
2019 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for grant	1,874,759
Percentage of increase in shares reserved for issuance on common stock outstanding	4.00%
Option Plans \| Share-based Payment Arrangement, Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for grant	1,874,759	693,879
Stock options, outstanding	1,313,468	768,239
Stock options granted	823,858
Option Plans \| Employees, Officers, Directors, Advisors, and Consultants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	4 years
Option Plans \| Maximum \| Employees, Officers, Directors, Advisors, and Consultants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted, expiration period	10 years
2015 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Award vesting period	48 months
Shares not vested and subject to repurchase	100,000	200,000
Cash received from early exercise of unvested options	$ 100,000	$ 200,000
2015 Stock Plan \| Non-employees \| Share-based Payment Arrangement, Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock options granted	39,165	4,166
Stock options related expense	$ 500,000	$ 100,000

Stock Option Plan - Summary of Option Plans (Details) - Option Plans - Share-based Payment Arrangement, Option $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Available, Beginning Balance	693,879
Shares Available, Stock options authorized	1,786,166
Shares Available, Stock options granted	(823,858)
Shares Available, Unvested common shares repurchased	85,103
Shares Available, Stock options forfeited	133,469
Shares Available, Ending Balance	1,874,759
Number of Shares Outstanding, Beginning Balance	768,239
Number of Shares Outstanding, Stock options granted	823,858
Number of Shares Outstanding, Stock options exercised	(145,160)
Number of Shares Outstanding, Stock options forfeited	(133,469)
Number of Shares Outstanding, Ending Balance	1,313,468
Number of Shares Outstanding, Vested and expected to vest	1,313,468
Number of Shares Outstanding, Exercisable	317,694
Weighted Average Exercise Price Per Share, Beginning Balance \| $ / shares	$ 4.62
Weighted Average Exercise Price Per Share, Stock options granted \| $ / shares	12.96
Weighted Average Exercise Price Per Share, Stock options exercised \| $ / shares	2.63
Weighted Average Exercise Price Per Share, Unvested common shares repurchased \| $ / shares	2.03
Weighted Average Exercise Price Per Share, Stock options forfeited \| $ / shares	7.39
Weighted Average Exercise Price Per Share, Ending Balance \| $ / shares	9.77
Weighted Average Exercise Price Per Share, Vested and expected to vest \| $ / shares	9.77
Weighted Average Exercise Price Per Share, Exercisable \| $ / shares	$ 5.26
Weighted Average Remaining Contractual Term (Years), Balance	8 years 11 months 4 days
Weighted Average Remaining Contractual Term (Years), Vested and expected to vest	8 years 11 months 4 days
Weighted Average Remaining Contractual Term (Years), Exercisable	7 years 11 months 4 days
Aggregate Intrinsic Value \| $	$ 23,438
Aggregate Intrinsic Value, Vested and expected to vest \| $	23,438
Aggregate Intrinsic Value, Exercisable \| $	$ 7,102

Stock Option Plan - Summary of Assumptions Used to Value Under Stock Option Plans (Details) - Share-based Payment Arrangement, Option - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Employee and Director \| Option Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Volatility, minimum	83.00%	80.69%
Volatility, maximum	84.99%	81.48%
Risk-free interest rate, minimum	1.58%	2.62%
Risk-free interest rate, maximum	2.23%	2.88%
Non-employees \| 2015 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Volatility, minimum	81.43%	78.29%
Volatility, maximum	84.85%	85.47%
Risk-free interest rate, minimum	1.40%	1.32%
Risk-free interest rate, maximum	2.67%	3.19%
Minimum \| Employee and Director \| Option Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 6.30	$ 1.03
Expected term (in years)	5 years 3 days	5 years 8 months 1 day
Minimum \| Non-employees \| 2015 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 4 months 2 days	6 years 4 months 2 days
Maximum \| Employee and Director \| Option Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 27.53	$ 1.05
Expected term (in years)	6 years 29 days	6 years 29 days
Maximum \| Non-employees \| 2015 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	9 years 8 months 26 days	10 years

Stock Option Plan - Summary of Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 2,051	$ 1,170
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	1,076	542
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 975	$ 628

Income Taxes - Schedule of Domestic and Foreign Components of Income (Loss) before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
United States	$ (41,301)	$ (36,034)
Foreign	(1,036)	(113)
Income (loss) before income taxes	$ (42,337)	$ (36,147)

Income Taxes - Schedule of Components of Provision for Income Taxes (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)
Income Tax Disclosure [Abstract]
Foreign	$ 660
Total provision for income taxes	$ 660

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal tax	21.00%	21.00%
Stock-based compensation	(0.63%)	(0.60%)
Research and development tax credit	2.97%	3.68%
Withholding taxes, net of federal benefit	(1.23%)
Change in valuation allowance	(23.18%)	(24.13%)
Other	(0.49%)	0.05%
Income tax expense	(1.56%)

Income Taxes - Schedule of Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 29,014	$ 20,810
Federal and state research and development tax credits	4,636	3,378
Depreciation and amortization		105
Accrued liabilities and reserves	837	448
Stock-based compensation	215	50
Gross deferred tax assets	34,702	24,791
Valuation allowance	(34,605)	(24,791)
Deferred tax liabilities:
Depreciation and amortization	(97)
Gross deferred tax liabilities	(97)
Net deferred taxes	$ 0	$ 0

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Line Items]
Increase in valuation allowance	$ 9,800,000	$ 8,700,000
Interest expense or penalties related to unrecognized tax benefits		$ 0
Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount	3,700,000
Federal
Income Tax Disclosure [Line Items]
Operating loss carryforwards	134,200,000
Operating loss carryforwards indefinitely	$ 73,200,000
Operating loss carryforwards expiration beginning year	2035
Tax credit carryforwards amount	$ 3,200,000
Uncertain tax positions reserves	900,000
Uncertain tax positions	800,000
State
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 12,100,000
Operating loss carryforwards expiration beginning year	2035
Tax credit carryforwards amount	$ 3,200,000
Uncertain tax positions reserves	900,000
Uncertain tax positions	$ 800,000

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 1,281	$ 789
Additions on tax positions related to prior years		19
Additions on tax positions related to current year	466	473
Balance at end of year	$ 1,747	$ 1,281

Net Loss Per Share - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ 42,997	$ 36,147
Denominator:
Weighted average common shares outstanding	4,404,486	866,348
Weighted-average shares used to compute net loss per common share, basic and diluted	4,346,400	622,289
Net loss per share, basic and diluted	$ 9.89	$ 58.09
Early Exercised Stock
Denominator:
Less: weighted-average unvested common stock subject to repurchase	(36,792)	(106,368)
Restricted Stock
Denominator:
Less: weighted-average unvested common stock subject to repurchase	(21,294)	(137,691)

- 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 Share - Potentially Dilutive Securities not Included in Diluted per Share Calculations (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	1,313,468	17,109,160
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities		16,415,281
Common Stock Options Issued and Outstanding
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Potentially dilutive securities	1,313,468	693,879

Subsequent Events - Additional Information (Details) - Follow-on Offering - Subsequent Event $ / shares in Units, $ in Millions	1 Months Ended
	Feb. 29, 2020 USD ($) $ / shares shares
Subsequent Event [Line Items]
Issuance of common stock upon initial public offering, net of issuance cost, shares \| shares	2,500,000
Shares issued, price per share \| $ / shares	$ 30.00
Net proceeds received after deducting underwriting discounts and other offering related costs \| $	$ 69.7