Cover	9 Months Ended
Cover	Sep. 30, 2019
Cover page.
Document Type	S-1
Amendment Flag	false
Entity Registrant Name	RAPT Therapeutics, Inc.
Entity Central Index Key	0001673772
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 48,310	$ 63,798	$ 47,517
Prepaid expenses and other current assets	2,914	1,264	725
Total current assets	51,224	65,062	48,242
Property and equipment, net	4,002	4,159	1,913
Other assets	3,143	389	236
Total assets	58,369	69,610	50,391
Current liabilities:
Accounts payable	1,590	1,771	1,106
Accrued expenses	3,410	2,488	1,492
Other current liabilities	457	384	650
Total current liabilities	5,457	4,643	3,248
Deferred rent, net of current portion	2,219	969	905
Commitments
Convertible preferred stock	175,490	161,111	108,643
Stockholders' Deficit
Preferred stock
Common stock	1	1	1
Additional paid-in capital	23,923	22,441	21,005
Related party promissory note for the purchase of common stock		(598)	(605)
Accumulated other comprehensive income/(loss)	13	(4)
Accumulated deficit	(148,734)	(118,953)	(82,806)
Total stockholders' (deficit) equity	(124,797)	(97,113)	(62,405)
Total liabilities, convertible preferred stock and stockholders' (deficit)	$ 58,369	69,610	$ 50,391
Pro Forma [Member]
Current liabilities:
Commitments
Stockholders' Deficit
Common stock		2
Additional paid-in capital		183,551
Related party promissory note for the purchase of common stock		(598)
Accumulated other comprehensive income/(loss)		(4)
Accumulated deficit		(118,953)
Total stockholders' (deficit) equity		$ 63,998

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Convertible preferred stock, par value	$ 0.0001	$ 0.0001
Convertible preferred stock, shares authorized	104,018,468	104,018,468
Convertible preferred stock, shares issued	98,491,880	75,563,784
Convertible preferred stock, shares outstanding	98,491,880	75,563,784
Liquidation preference	$ 170,000	$ 117,437
Common stock, par value	$ 0.0001	$ 0.0001
Common stock, shares authorized	119,200,000	119,200,000
Common stock, shares issued	878,413	880,191
Common stock, shares outstanding	878,413	880,191
Pro Forma [Member]
Convertible preferred stock, shares issued	0
Convertible preferred stock, shares outstanding	0
Common stock, shares issued	17,293,694
Common stock, shares outstanding	17,293,694

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 8,582	$ 9,181	$ 24,720	$ 23,387	$ 31,767	$ 25,618
General and administrative	1,733	1,364	6,094	3,889	5,180	3,713
Total operating expenses	10,315	10,545	30,814	27,276	36,947	29,331
Loss from operations	10,315	10,545	30,814	27,276	36,947	29,331
Other income
Other income, net	344	261	1,033	559	800	216
Net loss	(9,971)	(10,284)	(29,781)	(26,717)	(36,147)	(29,115)
Other comprehensive income/(loss)	(15)	(4)	17	(4)	(4)
Total comprehensive loss	$ (9,956)	$ (10,288)	$ (29,764)	$ (26,721)	$ (36,151)	$ (29,115)
Net loss per share, basic and diluted	$ (12.41)	$ (15.90)	$ (40.15)	$ (45.11)	$ (58.09)	$ (67.45)
Weighted average number of shares used in computing net loss per share, basic and diluted	803,229	646,800	741,711	592,237	622,289	431,679
Pro Forma [Member]
Other income
Net loss per share, basic and diluted					$ (2.50)
Weighted average number of shares used in computing net loss per share, basic and diluted					14,461,086

CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS 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, 2016		$ 78,787
Beginning balance, shares at Dec. 31, 2016		62,509,105
Beginning balance at Dec. 31, 2016	$ (34,243)				$ 1	$ 20,043	$ (596)	$ (53,691)
Issuance of convertible preferred stock, net of issuance costs			$ 29,856
Issuance of convertible preferred stock, net of issuance costs, shares			13,054,679
Beginning balance, shares at Dec. 31, 2016					868,056
Issuance of common stock upon exercise of stock options, net of repurchase	$ 246					246
Issuance of common stock upon exercise of stock options, net of repurchase, shares	50,034				12,135
Interest on promissory notes from related parties for purchase of common stock	$ (9)						(9)
Stock-based compensation	716					716
Net loss	(29,115)							(29,115)
Ending balance at Dec. 31, 2017	$ (62,405)				$ 1	21,005	(605)	(82,806)
Ending balance, shares at Dec. 31, 2017	75,563,784	75,563,784	13,054,679
Ending balance at Dec. 31, 2017	$ 108,643	$ 108,643	$ 29,856
Ending balance, shares at Dec. 31, 2017	880,191				880,191
Issuance cost related to convertible preferred stock			(4)
Issuance of common stock upon exercise of stock options, net of repurchase	$ 90					90
Issuance of common stock upon exercise of stock options, net of repurchase, shares					5,159
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	(3)						(3)
Stock-based compensation	294					294
Net loss	(8,232)							(8,232)
Ending balance at Mar. 31, 2018	(70,239)				$ 1	21,389	(591)	(91,038)
Ending balance, shares at Mar. 31, 2018		75,563,784
Ending balance at Mar. 31, 2018		$ 108,639
Ending balance, shares at Mar. 31, 2018					877,017
Beginning balance at Dec. 31, 2017	$ 108,643	$ 108,643	$ 29,856
Beginning balance, shares at Dec. 31, 2017	75,563,784	75,563,784	13,054,679
Beginning balance at Dec. 31, 2017	$ (62,405)				$ 1	21,005	(605)	(82,806)
Beginning balance, shares at Dec. 31, 2017	880,191				880,191
Forgiveness of promissory notes from related parties for purchase of common stock	$ 0
Net loss	(26,717)
Ending balance at Sep. 30, 2018	(88,048)				$ 1	22,077	(596)	(109,526)	$ (4)
Ending balance, shares at Sep. 30, 2018		88,618,468
Ending balance at Sep. 30, 2018		$ 138,557
Ending balance, shares at Sep. 30, 2018					868,892
Beginning balance at Dec. 31, 2017	$ 108,643	$ 108,643	$ 29,856
Beginning balance, shares at Dec. 31, 2017	75,563,784	75,563,784	13,054,679
Beginning balance at Dec. 31, 2017	$ (62,405)				$ 1	21,005	(605)	(82,806)
Issuance of convertible preferred stock, net of issuance costs			$ 29,914	$ 22,554
Issuance of convertible preferred stock, net of issuance costs, shares			13,054,684	9,873,412
Beginning balance, shares at Dec. 31, 2017	880,191				880,191
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	17,390				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	$ (97,113)				$ 1	22,441	(598)	(118,953)	(4)
Ending balance, shares at Dec. 31, 2018	98,491,880	98,491,880	26,109,363	9,873,412
Ending balance at Dec. 31, 2018	$ 161,111	$ 161,111	$ 59,770	$ 22,554
Ending balance, shares at Dec. 31, 2018	878,413				878,413
Beginning balance at Mar. 31, 2018		$ 108,639
Beginning balance, shares at Mar. 31, 2018		75,563,784
Beginning balance at Mar. 31, 2018	$ (70,239)				$ 1	21,389	(591)	(91,038)
Issuance of convertible preferred stock, net of issuance costs			$ 29,918
Issuance of convertible preferred stock, net of issuance costs, shares			13,054,684
Beginning balance, shares at Mar. 31, 2018					877,017
Issuance of common stock upon exercise of stock options, net of repurchase	47					47
Issuance of common stock upon exercise of stock options, net of repurchase, shares					(5,849)
Interest on promissory notes from related parties for purchase of common stock	(2)						(2)
Stock-based compensation	286					286
Net loss	(8,204)							(8,204)
Ending balance at Jun. 30, 2018	(78,112)				$ 1	21,722	(593)	(99,242)
Ending balance, shares at Jun. 30, 2018		88,618,468
Ending balance at Jun. 30, 2018		$ 138,557
Ending balance, shares at Jun. 30, 2018					871,169
Issuance of common stock upon exercise of stock options, net of repurchase	60					60
Issuance of common stock upon exercise of stock options, net of repurchase, shares					(2,277)
Interest on promissory notes from related parties for purchase of common stock	(3)						(3)
Stock-based compensation	295					295
Foreign currency translation adjustment	(4)								(4)
Net loss	(10,284)							(10,284)
Ending balance at Sep. 30, 2018	(88,048)				$ 1	22,077	(596)	(109,526)	(4)
Ending balance, shares at Sep. 30, 2018		88,618,468
Ending balance at Sep. 30, 2018		$ 138,557
Ending balance, shares at Sep. 30, 2018					868,892
Beginning balance at Dec. 31, 2018	$ 161,111	$ 161,111	$ 59,770	$ 22,554
Beginning balance, shares at Dec. 31, 2018	98,491,880	98,491,880	26,109,363	9,873,412
Beginning balance at Dec. 31, 2018	$ (97,113)				$ 1	22,441	(598)	(118,953)	(4)
Issuance of convertible preferred stock, net of issuance costs				$ 6,947
Issuance of convertible preferred stock, net of issuance costs, shares				3,039,908
Beginning balance, shares at Dec. 31, 2018	878,413				878,413
Issuance of common stock upon exercise of stock options, net of repurchase	$ 66					66
Issuance of common stock upon exercise of stock options, net of repurchase, shares					3,685
Repurchase of common stock from related party	109						109
Repurchase of common stock from related party, shares					(53,649)
Interest on promissory notes from related parties for purchase of common stock	(2)						(2)
Stock-based compensation	377					377
Net loss	(9,188)							(9,188)
Ending balance at Mar. 31, 2019	(105,751)				$ 1	22,884	(491)	(128,141)	(4)
Ending balance, shares at Mar. 31, 2019		101,531,788
Ending balance at Mar. 31, 2019		$ 168,058
Ending balance, shares at Mar. 31, 2019					828,449
Beginning balance at Dec. 31, 2018	$ 161,111	$ 161,111	$ 59,770	$ 22,554
Beginning balance, shares at Dec. 31, 2018	98,491,880	98,491,880	26,109,363	9,873,412
Beginning balance at Dec. 31, 2018	$ (97,113)				$ 1	22,441	(598)	(118,953)	(4)
Beginning balance, shares at Dec. 31, 2018	878,413				878,413
Issuance of common stock upon exercise of stock options, net of repurchase, shares	141,707
Forgiveness of promissory notes from related parties for purchase of common stock	$ 382
Net loss	(29,781)
Ending balance at Sep. 30, 2019	$ (124,797)				$ 1	23,923		(148,734)	13
Ending balance, shares at Sep. 30, 2019	104,803,325	104,803,325	26,109,363	16,184,857
Ending balance at Sep. 30, 2019	$ 175,490	$ 175,490	$ 59,770	$ 36,933
Ending balance, shares at Sep. 30, 2019	935,040				935,040
Beginning balance at Mar. 31, 2019		$ 168,058
Beginning balance, shares at Mar. 31, 2019		101,531,788
Beginning balance at Mar. 31, 2019	$ (105,751)				$ 1	22,884	(491)	(128,141)	(4)
Issuance of convertible preferred stock, net of issuance costs				$ 7,451
Issuance of convertible preferred stock, net of issuance costs, shares				3,271,537
Beginning balance, shares at Mar. 31, 2019					828,449
Issuance of common stock upon exercise of stock options, net of repurchase	34					34
Issuance of common stock upon exercise of stock options, net of repurchase, shares					13,996
Repurchase of common stock from related party	65						65
Repurchase of common stock from related party, shares					(29,686)
Paydown of promissory notes from related parties for purchase of common stock	73						73
Forgiveness of promissory notes from related parties for purchase of common stock	353						$ 353
Stock-based compensation	347					347
Foreign currency translation adjustment	2								2
Net loss	(10,622)							(10,622)
Ending balance at Jun. 30, 2019	(115,499)				$ 1	23,265		(138,763)	(2)
Ending balance, shares at Jun. 30, 2019		104,803,325
Ending balance at Jun. 30, 2019		$ 175,509
Ending balance, shares at Jun. 30, 2019					812,759
Issuance cost related to convertible preferred stock				$ (19)
Issuance of common stock upon exercise of stock options, net of repurchase	227					227
Issuance of common stock upon exercise of stock options, net of repurchase, shares					122,281
Stock-based compensation	431					431
Foreign currency translation adjustment	15								15
Net loss	(9,971)							(9,971)
Ending balance at Sep. 30, 2019	$ (124,797)				$ 1	$ 23,923		$ (148,734)	$ 13
Ending balance, shares at Sep. 30, 2019	104,803,325	104,803,325	26,109,363	16,184,857
Ending balance at Sep. 30, 2019	$ 175,490	$ 175,490	$ 59,770	$ 36,933
Ending balance, shares at Sep. 30, 2019	935,040				935,040

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (29,781)	$ (26,717)	$ (36,147)	$ (29,115)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	1,011	943	1,237	1,384
Stock-based compensation expense	1,155	874	1,170	716
Loss on disposal of capital equipment	9	54	17	15
Gain on foreign currency translation	(17)	(4)
Other noncash income (loss), net			(14)	(9)
Noncash interest income (loss), net	(20)	(8)
Changes in operating assets and liabilities:
Prepaid expenses and other long-term assets	(4,404)	(400)	(691)	(34)
Accounts payable and accrued liabilities	2,187	1,516	1,475	(80)
Net cash used in operating activities	(29,786)	(23,742)	(32,953)	(27,123)
Investing activities
Purchase of property and equipment	(863)	(1,846)	(3,500)	(1,124)
Net cash used in investing activities	(863)	(1,846)	(3,500)	(1,124)
Financing activities
Proceeds from the sale of convertible preferred stock, net of issuance costs	14,379	29,914	52,468	29,856
Proceeds from issuance of common stock, net of repurchases	782	198	266	246
Net cash provided by financing activities	15,161	30,112	52,734	30,102
Net increase in cash and cash equivalents	(15,488)	(4,524)	16,281	1,855
Cash and cash equivalents at beginning of period	63,798	47,517	47,517	45,662
Cash and cash equivalents at end of period	48,310	52,041	63,798	47,517
Supplemental disclosures of non-cash investing and financing information
Deferred offering costs included in accounts payable and accrued expenses	454	0
Property and equipment included in accounts payable	0	409	$ 753	$ 0
Forgiveness of promissory notes from related party for purchase of common stock	$ 382	$ 0

Organization and Liquidity Risks	9 Months Ended	12 Months Ended
Organization and Liquidity Risks	Sep. 30, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Liquidity Risks	1. Organization and Liquidity Risks *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. The Company changed its name from FLX Bio, Inc. to RAPT Therapeutics, Inc. on May 21, 2019. The Company is located in South San Francisco, California. *Initial Public Offering* On November 4, 2019, the Company completed its initial public offering (“IPO”). The Company’s Registration Statement on Form S-1 (File Nos. 333-232572) relating to the IPO was declared effective by the Securities and Exchange Commission (“SEC”) on October 30, 2019. The shares began trading on the Nasdaq Global Market on October 31, 2019. The Company issued 3,000,000 shares of its common stock at an offering price at $12.00 per share. Immediately prior to the closing of the Company’s IPO on November 4, 2019, all outstanding shares of the Company’s convertible preferred stock converted into 17,467,184 shares of the Company’s common stock. Shortly following the close of the offering, the underwriters exercised their option to purchase an additional 427,360 shares at the IPO price per share. In aggregate, the shares issued in the offering generated approximately $33.8 million in net proceeds after deducting underwriting discounts and other offering related costs. In connection with the completion of its IPO, on November 4, 2019, 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. *Liquidity and Management Plans* The accompanying condensed consolidated financial statements have been prepared assuming that the Company will continue as a going concern. However, since inception, the Company has incurred net losses and negative cash flows from operations and such losses are expected to continue for the foreseeable future. As of September 30, 2019, the Company had an accumulated deficit of $148.7 million, cash and cash equivalents of $48.3 million and working capital of $45.8 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 $33.8 million from the closing of its IPO in November 2019 as described above, will provide sufficient funds to enable the Company to meet its obligations for at least twelve months from the filing date of this report.	1. Organization and Liquidity Risks *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. (“FLX”) to RAPT Therapeutics, Inc. The Company is located in South San Francisco, California. *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, 2018, the Company incurred a net loss of $36.1 million and used $33.0 million of cash in operations. At December 31, 2018, the Company had cash and cash equivalents of $63.8 million and an accumulated deficit of $119.0 million. Management expects losses to continue for the next several years and does not expect positive cash flows in the foreseeable future. The Company has historically financed its operations through the sale of convertible preferred stock. The Company has evaluated and concluded there are no conditions or events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern for a period of at least one year following the date that these consolidated financial statements were issued. Management expects operating losses to continue for the foreseeable future. As a result, the Company will need to raise additional capital. If sufficient funds on acceptable terms are not available when needed, the Company could be required to significantly reduce its operating expenses and delay, reduce the scope of or eliminate one or more of its development programs. Failure to manage discretionary spending or raise additional financing, as needed, may adversely impact the Company’s ability to achieve its intended business objectives.

Summary of Significant Accounting Policies	9 Months Ended	12 Months Ended
Summary of Significant Accounting Policies	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies *Basis of Presentation* The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) for interim financial information and pursuant to Article 10 of Regulation S-X of the Securities Act of 1933, as amended (Securities Act). Accordingly, they do not include all of the information and notes required by U.S. GAAP for complete financial statements. These unaudited condensed consolidated financial statements include only normal and recurring adjustments that the Company believes are necessary to fairly state the Company’s financial position and the results of its operations and cash flows. Interim-period results are not necessarily indicative of results of operations or cash flows for a full year or any subsequent interim period. The condensed balance sheet at December 31, 2018 has been derived from the audited financial statements at that date but does not include all disclosures required by U.S. GAAP for complete financial statements. Because all of the disclosures required by U.S. GAAP for complete financial statements are not included herein, these unaudited condensed consolidated financial statements and the notes accompanying them should be read in conjunction with the Company’s audited consolidated financial statements included in the Registration Statement on Form S-1 and related Prospectus dated October 30, 2019 filed with the SEC pursuant to Rule 424(b)(4) of the Securities Act of 1933, as amended (“Prospectus”). *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. *Deferred Offering Costs* Deferred offering costs consisting of direct legal, accounting, printing and other fees and costs related to the IPO are capitalized. The deferred offering costs were reclassified to additional paid-in capital upon the effectiveness of the IPO in November 2019. As of September 30, 2019, $2.8 million of deferred offering costs were capitalized and included in other assets. *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 potential dilutive effects 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 us as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on our condensed consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), we meet the definition of an emerging growth company, and have elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. *Recently Issued Accounting Pronouncement Not Yet Adopted* In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU 2016-02 requires lessees to put 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, 2019 and early adoption is permitted. The Company is currently assessing 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 31, 2020. Early adoption is permitted for all entities. The Company is currently assessing the impact that the adoption of ASU 2016-13 will have on its condensed consolidated financial statements and related disclosures. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230), which provides greater clarity to preparers on the treatment of certain items within an entity’s statement of cash flows. ASU 2016-15 is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The guidance is effective for the Company on January 1, 2019, and the Company is currently evaluating the impact that the adoption of ASU 2016-15 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 interim periods within fiscal years beginning after December 15, 2020, with early adoption 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. The Company is currently assessing the impact of this ASU on its consolidated financial statements. 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. ASU 2018-18 will be effective for the Company for fiscal years beginning after December 15, 2020, and interim periods within fiscal years beginning after December 15, 2021. The Company is currently assessing the impact of this ASU on its condensed consolidated financial statements.	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. *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. *Unaudited Pro Forma Financial Information* The unaudited pro forma consolidated stockholders’ equity as of December 31, 2018, assumes the conversion of all outstanding shares of convertible preferred stock into 98,491,880 shares of common stock immediately prior to the completion of the Company’s planned initial public offering (“IPO”). The shares of common stock issuable and the proceeds expected to be received in the IPO are excluded from such pro forma financial information. Pro forma basic and diluted net loss per share has been computed to give effect to the conversion of all outstanding convertible preferred stock into shares of common stock. The unaudited pro forma net loss per share for the year ended December 31, 2018, was computed using the weighted-average number of shares of common stock outstanding, including the pro forma effect of the conversion of all outstanding shares of convertible preferred stock into shares of common stock, as if such conversion had occurred at the beginning of the period, or their issuance dates, if later. *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. *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 and Cash Equivalents* Cash equivalents are financial instruments that potentially subject the Company to concentrations of credit risk. The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. The Company invests its cash and cash equivalents in money market funds. The Company limits its credit risk associated with cash and cash equivalents by placing its cash with banks and institutions it believes are highly credit worthy and in highly-rated investments. *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. As of December 31, 2017 and 2018, the Company has not recorded 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 records 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 will be 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 have converted their shares of convertible preferred stock into shares of common stock. Convertible preferred stock is classified outside of stockholders’ deficit on the balance sheet as events triggering redemption are not solely within the Company’s control. The Company has not adjusted the carrying values of its convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur. As of December 31, 2018, it was not probable that such a redemption would occur. *Research and Development Costs* Research and development costs are charged to expense 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 option grants 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. 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., our 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 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 potential dilutive effects of potential dilutive shares outstanding during the period. Potential dilutive securities include stock options, warrants and convertible preferred stock. The dilutive effect of stock options and warrants is computed using the treasury stock method and the dilutive effect of convertible preferred stock is calculated using the “if-converted method”. For all periods presented, diluted net loss per share is the same as basic net loss per share since the effect of including potential common shares 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 us as of the specified effective date. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), we meet the definition of an emerging growth company, and have elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act. *Recently Adopted Accounting Pronouncements* In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition. This standard is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The standard also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. The standard is effective for annual periods beginning after December 15, 2018 using one of two retrospective application methods. The Company has elected to adopt this standard as of January 1, 2018. The adoption of ASU No. 2014-09 did not have any impact on the Company’s consolidated financial statements and related disclosures. In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718), Scope of Modification Accounting. This pronouncement provides guidance about which changes to the terms or conditions of a share-based payment award may require an entity to apply modification accounting under Topic 718. This guidance is effective for the Company for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. The Company adopted this standard on January 1, 2018. The adoption of ASU No. 2017-09 did not have a significant impact on the Company’s consolidated financial statements and related disclosures. *Recently Issued Accounting Pronouncements* In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU 2016-02 requires lessees to put 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, 2019 and early adoption is permitted. The Company is currently assessing the impact that the adoption will have on its consolidated financial statements and related disclosures. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230), which provides greater clarity to preparers on the treatment of certain items within an entity’s statement of cash flows. The new guidance is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The guidance is effective for the Company on January 1, 2019, and the Company is currently evaluating the impact that the adoption of ASU 2016-15 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. The new standard 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 interim periods within fiscal years beginning after December 15, 2020, with early adoption permitted. The Company is currently assessing the timing of adoption and 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. This ASU 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. The Company is currently assessing the impact of this ASU on its consolidated financial statements.

Fair Value Measurements	9 Months Ended	12 Months Ended
Fair Value Measurements	Sep. 30, 2019	Dec. 31, 2018
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 September 30, 2019 and December 31, 2018 were $48.3 million and $63.8 million, respectively, and are included in cash and cash equivalents.	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, 2017 and 2018 were $47.5 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

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Property and Equipment

4. Property and Equipment

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


	December 31, 2018			September 30, 2019
Laboratory equipment	$	5,466		$	5,832
Leasehold improvements		2,989			3,294
Computer equipment		308			316
Furniture and fixtures		365			394

Total property and equipment		9,128			9,836
Less accumulated depreciation and amortization		(4,969	)		(5,834	)

Property and equipment, net	$	4,159		$	4,002

Depreciation and amortization expenses were $0.3 million and $0.3 million for the three months ended September 30, 2019 and 2018, respectively, and $1.0 million and $0.9 million for the nine months ended September 30, 2019 and 2018, respectively.

4. Property and Equipment

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


	December 31,
	2017			2018
Laboratory equipment	$	4,603		$	5,466
Leasehold improvements		598			2,989
Computer equipment		244			308
Furniture and fixtures		237			365

Total property and equipment		5,682			9,128
Less accumulated depreciation and amortization		(3,769	)		(4,969	)

Property and equipment, net	$	1,913		$	4,159

Accrued Expenses

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Payables and Accruals [Abstract]

Accrued Expenses

5. Accrued Expenses

Accrued expenses consist of the following (in thousands):


	December 31, 2018		September 30, 2019
Accrued clinical expenses	$	519	$	973
Accrued compensation		1,433		1,604
Accrued professional and consulting services		182		662
Accrued property and equipment		202		—
Accrued lab supplies		80		78
Other		72		93

Total accrued expenses	$	2,488	$	3,410

5. Accrued Expenses

Accrued expenses consist of the following (in thousands):


	December 31,
	2017		2018
Accrued clinical expenses	$	39	$	519
Accrued compensation		1,019		1,433
Accrued professional and consulting services		317		182
Accrued property and equipment		—		202
Accrued lab supplies		70		80
Other		47		72

Total accrued expenses	$	1,492	$	2,488

Commitments

12 Months Ended

Dec. 31, 2018

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 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 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 with a related party. See Note 14 for further discussion.

As of December 31, 2018, future minimum non-cancelable lease payments, net of sublease rental income, are as follows (in thousands):


Year ending December 31:
2019	$	1,432
2020		1,639
2021		1,969
2022		2,038
Thereafter		8,687

Total minimum lease payments	$	15,765

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 $1.6 million and $1.8 million in the years ended December 31, 2017 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.

Clinical Trial Collaboration and Supply Agreement	12 Months Ended
Clinical Trial Collaboration and Supply Agreement	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Clinical Trial Collaboration and Supply Agreement	7. Clinical Trial Collaboration and Supply Agreement 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	9 Months Ended	12 Months Ended
Related-Party Promissory Notes	Sep. 30, 2019	Dec. 31, 2018
Related Party Transactions [Abstract]
Related-Party Promissory Notes	6. 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.	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 loans was $0.3 million and $0.3 million, respectively. The loans are secured by the shares of common stock of the Company held by the individuals. The loans accrue interest at a rate of 1.82% and 1.41% per annum, respectively, and are due upon the earlier of voluntary termination of services to the Company, filing by the Company of its first registration statement with the Securities and Exchange Commission under the Securities Act of 1933 or sale of substantially all of the Company’s assets. As of December 31, 2017 and 2018, the total outstanding balance under these notes, including accrued interest, was approximately $0.6 million and $0.6 million, respectively. The notes are recorded within stockholders’ deficit.

Convertible Preferred Stock and Stockholders' Deficit

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Convertible Preferred Stock and Stockholders' Deficit

7. Convertible Preferred Stock and Stockholders’ Deficit

Convertible preferred stock

In June 2018, the Company closed a subsequent sale of Series C convertible preferred stock at $2.2925 per share for $29.9 million in gross proceeds. In December 2018, the Company closed the sale of Series C-2 convertible preferred stock at $2.2925 per share for $22.6 million in gross proceeds, 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 Authorized		Shares Issued and Outstanding		Net Carrying Value		Aggregate Liquidation Preference
Series A		37,509,105		37,509,105	$	28,861	$	37,509
Series B		25,000,000		25,000,000		49,926		50,000
Series C		26,109,363		26,109,363		59,770		59,856
Series C-2		15,400,000		9,873,412		22,554		22,635

Total convertible preferred stock		104,018,468		98,491,880	$	161,111	$	170,000

As of September 30, 2019, convertible preferred stock outstanding consisted of the following (in thousands, except share amounts):


	Shares Authorized		Shares Issued and Outstanding		Net Carrying Value		Aggregate Liquidation Preference
Series A		37,509,105		37,509,105	$	28,861	$	37,509
Series B		25,000,000		25,000,000		49,926		50,000
Series C		26,109,363		26,109,363		59,770		59,856
Series C-2		29,271,007		16,184,857		36,933		37,104

Total convertible preferred stock		117,889,475		104,803,325	$	175,490	$	184,469

9. Convertible Preferred Stock and Stockholders’ Deficit

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 March 2019, the Company completed subsequent closings of Series C-2 convertible preferred stock financing at $2.2925 per share for $7.0 million.

As of December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share amounts):


	Shares Authorized		Shares Issued and Outstanding		Net Carrying Value		Aggregate Liquidation Preference
Series A		37,509,105		37,509,105	$	28,861	$	37,509
Series B		25,000,000		25,000,000		49,926		50,000
Series C		26,240,224		13,054,679		29,856		29,928

Total convertible preferred stock		88,749,329		75,563,784	$	108,643	$	117,437

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


	Shares Authorized		Shares Issued and Outstanding		Net Carrying Value		Aggregate Liquidation Preference
Series A		37,509,105		37,509,105	$	28,861	$	37,509
Series B		25,000,000		25,000,000		49,926		50,000
Series C		26,109,363		26,109,363		59,770		59,856
Series C-2		15,400,000		9,873,412		22,554		22,635

Total convertible preferred stock		104,018,468		98,491,880	$	161,111	$	170,000

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

Conversion

Each share of Series A, Series B, Series C and Series C-2 convertible preferred stock are 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 is 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 is 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 shall be 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 are payable when, as and if declared by the Board of Directors, and are not cumulative. After payment of the above dividend, any additional dividends shall 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 are convertible.

Voting

Each holder of shares of Series A, Series B, Series C and Series C-2 convertible preferred stock is entitled to one vote for each share of common stock into which such shares of preferred stock are convertible, has voting rights and powers equal to the voting rights and powers of the common stock and shall vote together with the common stock on all matters as to which holders of common stock have 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 are 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, are entitled to elect two members of the Board of Directors. The holders of shares of common stock, voting as a separate class, are 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, are 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 are outstanding, as adjusted for any stock dividends, combinations, splits, recapitalizations and the like, the Company shall 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 shall 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 are 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 are 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 shall be deemed to have converted if, as a result of an actual conversion, such holder would receive, 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 has 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 has not adjusted the carrying values of the convertible preferred stock to the liquidation preferences of such shares because it is 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. Subsequent adjustments to the carrying values of the liquidation preferences will be made only when it becomes probable that such a liquidation event will occur.

Common Stock

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Equity [Abstract]

Common Stock

8. 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 September 30, 2019 or December 31, 2018. As of September 30, 2019 and December 31, 2018, 935,040 shares and 878,413 shares of common stock were outstanding, respectively.

As of September 30, 2019, the Company had reserved the following shares of common stock, on an as-converted basis, for future issuance as follows:


Series A convertible preferred stock outstanding			6,251,502
Series B convertible preferred stock outstanding			4,166,663
Series C convertible preferred stock outstanding			4,351,554
Series C-2 convertible preferred stock outstanding			2,697,465
Options issued and outstanding			943,610
Options available for future grants			461,904

Total			18,872,698

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, subject to the prior rights of the holders of convertible preferred stock. As of December 31, 2017 and 2018, no dividends had been declared.

As of December 31, 2018, the Company had reserved the following shares of common stock, on an as-converted basis, for future issuance as follows:


Series A convertible preferred stock outstanding			6,251,502
Series B convertible preferred stock outstanding			4,166,663
Series C convertible preferred stock outstanding			4,351,554
Series C-2 convertible preferred stock outstanding			2,566,666
Options issued and outstanding			768,239
Options available for future grants			693,879

Total			18,798,503

Stock Option Plan

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Share-based Payment Arrangement [Abstract]

Stock Option Plan

9. Stock Option Plan

The Company’s 2019 Equity Incentive Plan (the “2019 Plan”) which was adopted by the Company’s Board of Directors on June 27, 2019 and approved by the Company’s stockholders on July 18, 2019, became effective upon the consummation of the IPO. Upon the effectiveness of the 2019 Plan, the Company’s 2015 Stock Plan (the “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. A summary of the Company’s stock option activity under the 2015 Plan for the nine months ended September 30, 2019 is as follows:


	Shares Available			Number of Shares Outstanding			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Balances at December 31, 2018		693,879			768,239		$	4.62		8.84	$	1,291
Stock options authorized		—			—
Stock options granted		(416,919	)		416,919			8.63
Stock options exercised		—			(141,707	)		4.37
Unvested common shares repurchased		85,103			—			1.02
Stock options forfeited		99,841			(99,841	)		5.07

Balances at September 30, 2019		461,904			943,610		$	6.82		8.79	$	6,416

Employee stock option valuation

The assumptions used to value employee and director stock option awards granted under the 2015 Plan during the three and nine months ended September 30, 2019 and 2018, using the Black-Scholes option pricing model, were as follows:


	For the Three Months Ended September 30,		For the Nine Months Ended September 30,
	2018	2019	2018	2019
Fair value of common stock	$6.18	$13.62	$6.18	$13.62
Expected term (in years)	5.96 - 6.08	5.01 - 6.07	5.67 - 6.08	5.01 - 6.08
Volatility	81.37% - 81.48%	83.29% - 84.81%	83.69% - 81.48%	83.00% - 84.81%
Risk-free interest rate	2.88%	1.58% - 1.62%	2.62% - 2.88%	1.58% - 2.23%
Dividend yield	—	—	—	—

Stock options granted to nonemployees

Stock-based compensation related to stock options granted to non-employees is recognized as services are rendered. The assumptions used to value non-employee stock option awards granted under the 2015 Plan during the three and nine months ended September 30, 2019 and 2018, using the Black-Scholes option pricing model, were as follows:


	For the Three Months Ended September 30,		For the Nine Months Ended September 30,
	2018	2019	2018	2019
Expected term (in years)	6.60 - 9.70	5.60 - 9.99	6.60 - 9.70	5.60 - 9.99
Volatility	80.12% - 81.73%	82.30% - 83.79%	80.02% - 82.32%	81.43% - 83.79%
Risk-free interest rate	2.75% - 3.02%	1.44% - 2.02%	2.39% - 3.02%	1.44% - 2.67%
Dividend yield	—	—	—	—

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 September 30, 2019 and December 31, 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 September 30, 2019 and December 31, 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):


	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
	2018		2019		2018		2019
Research and development	$	131	$	212	$	404	$	611
General and administrative		164		219		470		544

Total stock-based compensation expense	$	295	$	431	$	874	$	1,155

11. Stock Option Plan

In 2015, the Company adopted the FLX Bio, Inc. 2015 Stock Plan (the 2015 Plan) for eligible employees, officers, directors, advisors, and consultants, which provides for the grant of incentive and non-statutory stock options and restricted shares of common stock. Terms of the stock option agreements, including vesting requirements, are determined by the Board of Directors, subject to the provisions of the 2015 Plan. Options granted generally vest over four years and expire no later than ten years from the date of grant. The estimated fair value of the underlying common stock is determined by the Board of Directors. 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.

The following summarizes option activity under the 2015 Plan:


	Shares Available			Number of Shares Outstanding			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Balances at December 31, 2016		539,900			142,037		$	1.56		9.47	$	426
Stock options authorized		540,554			—
Stock options granted		(306,804	)		306,804			1.99
Stock options exercised		—			(50,034	)		1.91
Unvested common shares repurchased		37,893			—			1.06
Stock options forfeited		50,147			(50,147	)		1.84

Balances at December 31, 2017		861,690			348,660		$	1.85		9.02	$	526
Stock options authorized		250,000			—
Stock options granted		(497,417	)		497,417			6.18
Stock options exercised		—			(17,390	)		1.93
Unvested common shares repurchased		19,158			—			1.39
Stock options forfeited		60,448			(60,448	)		2.74

Balances at December 31, 2018		693,879			768,239		$	4.62		8.84	$	1,291

Vested and expected to vest at December 31, 2018					768,223					8.84	$	1,291
Exercisable at December 31, 2018					137,159					7.99	$	580

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, 2017 and 2018.

During the year ended December 31, 2017, the Company granted 0.3 million stock options to purchase shares of common stock with a weighted-average grant date fair value of $3.60 per share and a weighted-average exercise price of $1.99 per share. The weighted average grant date fair value of the common stock was $4.38 per share. The grant date fair value of those awards was $1.2 million. During the year ended December 31, 2018, the Company granted 0.5 million stock options to purchase shares of common stock with a weighted-average grant date fair value of $4.32 per share and a weighted-average exercise price of $6.18 per share. The grant date fair value of those awards was $2.1 million. The intrinsic value of options exercised for the years ended December 31, 2017 and 2018 was $0.2 million and $0.1 million, respectively. The fair value of the 0.3 million and 0.3 million stock options vested during 2017 and 2018 was $0.6 million and $0.9 million, respectively.

The Company had 33,333 shares and 25,000 shares of performance-based stock options outstanding as of December 31, 2017 and 2018, respectively. The grant date fair value of the award was $0.2 million. As of December 31, 2017 and 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.

Employee stock option valuation

The assumptions used to value employee and director stock option awards granted under the 2015 Plan during the years ended December 31, 2017 and 2018, using the Black-Scholes option pricing model, were as follows:


	Year Ended December 31,
	2017	2018
Fair value of common stock	$3.84 - $6.18	$6.18 - $6.30
Expected term (in years)	5.96 - 6.07	5.67 - 6.08
Volatility	81.50% - 83.06%	80.69% - 81.48%
Risk-free interest rate	1.75% - 2.22%	2.62% - 2.88%
Dividend yield	—	—

The fair value of the shares of common stock underlying stock options has historically been determined by the Company’s Board of Directors. Because there has been no public market for the Company’s common stock, the Board of Directors has determined fair value of the common stock at the time of grant of the option 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 is privately held and does 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, 2017 and 2018, using the Black-Scholes option pricing model, were as follows:


	Year Ended December 31,
	2017	2018
Expected term (in years)	7.34 - 10.00	6.34 - 10.00
Volatility	78.92% - 86.03%	78.29% - 85.47%
Risk-free interest rate	1.32% - 2.57%	1.32% - 3.19%
Dividend yield	—	—

During the years ended December 31, 2017 and 2018, the Company granted 33,332 and 4,166 options to nonemployee consultants and recognized related expense of $0.1 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 0.4 million and 0.2 million shares were subject to repurchase as of December 31, 2017 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, 2017 and 2018, the Company included cash received for the early exercise of unvested options of $0.5 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,
	2017		2018
Research and development		394		542
General and administrative		322		628

Total stock-based compensation expense	$	716	$	1,170

As of December 31, 2018, unrecognized stock-based compensation cost related to outstanding unvested stock options that are expected to vest was $1.3 million. This unrecognized stock-based compensation cost is expected to be recognized over 1.72 years.

Income Taxes

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

The Company did not record a provision for income taxes for the nine months ended September 30, 2019 and 2018 because all of its taxable income is expected to be fully offset by net operating losses generated in prior years. In addition, the Company’s deferred tax assets continue to be subject to a full valuation allowance.

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,
	2017			2018
United States	$	(29,114	)	$	(36,033	)
Foreign		—			(113	)

	$	(29,114	)	$	(36,146	)

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


	December 31⁽¹⁾
	2017			2018
Federal tax		34.0	%		21.0	%
State, net of federal benefit		—			—
Stock based compensation		(1.0	)		(0.6	)
R&D credit		2.0			3.68
Change in valuation allowance		(8.0	)		(24.13	)
Other		—			0.05
Re-measurement of deferred tax assets		(27.0	)		—

Income tax expense		0.0	%		0.0	%

Note:

(1)	For the year ended December 31, 2017, the statutory tax rate was 34%. For the year ended December 31, 2018, as a result of Tax Reform, the statutory tax rate was decreased to 21%.

The Company has not recorded income tax expense or benefit through December 31, 2018 because of the Company’s history of operating losses. 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 Tax Cuts and Job Act (the “Act”) was enacted on December 22, 2017. The Act reduces the top U.S. federal corporate tax rate from 34% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred, changes the rules related to uses and limitations of net operating loss carryforwards created in tax years beginning after December 31, 2017, allows for immediate expensing of fixed asset additions beginning after September 27, 2017 and creates new taxes on certain foreign-sourced earnings. In 2017, the Company was not subject to a one-time transition tax as no foreign accumulated earnings and profits existed.

The Tax Act created a new requirement that global intangible low-taxed income (“GILTI”) earned by the Company’s foreign wholly-owned subsidiary must be included in gross U.S. taxable income. While the Tax Act provides for a modified territorial tax system, beginning in 2018, GILTI provisions will be applied providing an incremental tax on low taxed foreign income. The GILTI provisions require the Company to include in its U.S. income tax return foreign subsidiary earnings in excess of an allowable return on the foreign subsidiary’s tangible assets. During 2018, the Company made an accounting policy election to treat taxes related to GILTI as a current period expense when incurred.

We applied the guidance in Staff Accounting Bulletin No. 118 to reasonably estimate the effects of the 2017 Act and recorded provisional amounts in our financial statements as of December 31, 2017. In 2017, as a result of the signing of the Act, the Company recorded a $7.9 million reduction in our deferred tax assets due to the decrease in the Federal rate along with a corresponding reduction of our valuation allowance. In 2018, we completed our determination of the accounting implications of the 2017 Act and recorded no adjustments to the provisional amounts.

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


	December 31
	2017			2018
Net operating loss carryforwards	$	13,635		$	20,810
Federal and state tax credits		2,035			3,378
Depreciation and amortization		60			105
Accrued liabilities and reserves		313			448
Stock-based compensation		25			50

Gross deferred tax assets	$	16,068		$	24,791
Valuation allowance		(16,068	)		(24,791	)

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, 2017 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 $2.4 million and $8.7 million during the years ended December 31, 2017 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, 2017, and 2018, the Company had federal net operating loss carryforwards of approximately $60.9 million and $95.0 million, respectively. The federal NOL carryforwards generated during and after fiscal 2018 totaling $34.1 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, 2017 and 2018, the Company had approximately $12.1 million and $12.1 million of state net operating loss carryforwards, respectively, which begin to expire in 2035 and are available to offset future taxable income. As of December 31, 2017, and 2018, the Company had research and development tax credit carryforwards of approximately $1.7 million and $2.7 million, and approximately $1.4 million and $2.3 million, available to reduce future federal and state income taxes, respectively. Moreover, as of December 31, 2017 and 2018, the Company recorded federal and state reserves of $0.4 million and $0.7 million and approximately $0.3 and $0.6 million, respectively, as uncertain tax positions as of December 31, 2018. 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.

A reconciliation of the beginning and ending amounts of unrecognized tax benefits for the years ended December 31, 2017 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
	2017		2018
Balance at beginning of year	$	360	$	789
Additions on tax positions related to prior years		23		19
Additions on tax positions related to current year		406		473

Balance at end of year	$	789	$	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, 2018.

The Company files income tax returns in the United States, the State of California and the State of Colorado. 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.

Net Loss Per Share

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

11. Net Loss Per Share

Net loss per share

The following table sets forth the computation of the basic and diluted net loss per share for the nine months ended September 30, 2018 and 2019 (in thousands, except share and per share data):


	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2019			2018			2019
Numerator:
Net loss	$	(10,284	)	$	(9,971	)	$	(26,717	)	$	(29,781	)

Denominator:
Weighted average common shares outstanding		867,565			838,961			864,855			822,644
Less: weighted-average unvested restricted common stock subject to repurchase		(124,514	)		(19,097	)		(150,868	)		(45,451	)
Less: weighted-average unvested common shares subject to repurchase		(96,251	)		(16,635	)		(121,750	)		(35,482	)

Weighted-average shares used to compute net loss per share, basic and diluted		646,800			803,229			592,237			741,711

Net loss per share, basic and diluted	$	(15.90	)	$	(12.41	)	$	(45.11	)	$	(40.15	)

Outstanding potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive


	As of September 30,
	2018		2019
Convertible preferred stock		88,618,468		104,803,325
Common stock options issued and outstanding		768,406		943,610

Total		89,386,874		105,746,935

Immediately prior to the closing of the Company’s IPO on November 4, 2019, all outstanding shares of the Company’s convertible preferred stock converted into 17,467,184 shares of the Company’s common stock. For the nine months ended September 30, 2019, with the assumed conversion of the convertible preferred stock, the basic and diluted unaudited pro forma net loss was $1.69 per share.

13. Net Loss Per Share

Historical 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, 2017 and 2018 (in thousands, except share and per share data):


	Year Ended December 31,
	2017			2018
Numerator:
Net loss	$	(29,115	)	$	(36,147	)

Denominator:
Weighted average common shares outstanding		856,451			866,348
Less: weighted-average unvested restricted common stock subject to repurchase		(250,269	)		(137,691	)
Less: weighted-average unvested early exercised common shares subject to repurchase		(174,504	)		(106,368	)

Weighted-average shares used to compute net loss per common share, basic and diluted		431,679			622,289

Net loss per share, basic and diluted	$	(67.45	)	$	(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,
	2017		2018
Convertible preferred stock		12,593,942		16,415,281
Common stock options issued and outstanding		861,690		693,879

Total		13,455,632		17,109,160

Unaudited pro forma net loss per share

The following table presents the computation of pro forma basic and diluted net loss per share (in thousands, except share and per share data):


	For the Year Ended December 31, 2018
Numerator:
Net loss	$	(36,147	)

Denominator:
Weighted-average shares used to compute net loss per common share, basic and diluted		622,289
Pro forma adjustments to reflect:
Assumed conversion of convertible preferred stock		13,838,797

Weighted-average shares used to compute net loss per share, basic and diluted		14,461,086

Pro forma net loss per share, basic and diluted	$	(2.50	)

Subsequent Events	9 Months Ended	12 Months Ended
Subsequent Events	Sep. 30, 2019	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events On December 1, 2019, the Company entered into a Collaboration and License Agreement (“Agreement”) with Hanmi Pharmaceutical Co., LTD. for FLX475. Under the terms of the Agreement, the Company will receive $10.0 million in an upfront and expected near-term milestone payment. Additionally, the Company will be eligible to receive additional 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.	14. Subsequent Events In February 2019, the Company entered into an agreement to sublease its facility lease of 6,378 square feet of laboratory and office space with a related party. The sublease has an initial term of eighteen months, expiring August 2020, with an option to extend by an additional six months. In March 2019, the Company completed a subsequent closing of Series C-2 convertible preferred stock financing at $2.2925 per share with gross proceeds of $7.0 million. Management has reviewed and evaluated subsequent events through May 24, 2019, the date the audited financial statements were available to be issued. No subsequent events have been identified for disclosure, other than the subsequent events noted above.

Summary of Significant Accounting Policies (Policies)	9 Months Ended	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	*Basis of Presentation* The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”) for interim financial information and pursuant to Article 10 of Regulation S-X of the Securities Act of 1933, as amended (Securities Act). Accordingly, they do not include all of the information and notes required by U.S. GAAP for complete financial statements. These unaudited condensed consolidated financial statements include only normal and recurring adjustments that the Company believes are necessary to fairly state the Company’s financial position and the results of its operations and cash flows. Interim-period results are not necessarily indicative of results of operations or cash flows for a full year or any subsequent interim period. The condensed balance sheet at December 31, 2018 has been derived from the audited financial statements at that date but does not include all disclosures required by U.S. GAAP for complete financial statements. Because all of the disclosures required by U.S. GAAP for complete financial statements are not included herein, these unaudited condensed consolidated financial statements and the notes accompanying them should be read in conjunction with the Company’s audited consolidated financial statements included in the Registration Statement on Form S-1 and related Prospectus dated October 30, 2019 filed with the SEC pursuant to Rule 424(b)(4) of the Securities Act of 1933, as amended (“Prospectus”).	*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.	*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.
Unaudited Pro Forma Financial Information		*Unaudited Pro Forma Financial Information* The unaudited pro forma consolidated stockholders’ equity as of December 31, 2018, assumes the conversion of all outstanding shares of convertible preferred stock into 98,491,880 shares of common stock immediately prior to the completion of the Company’s planned initial public offering (“IPO”). The shares of common stock issuable and the proceeds expected to be received in the IPO are excluded from such pro forma financial information. Pro forma basic and diluted net loss per share has been computed to give effect to the conversion of all outstanding convertible preferred stock into shares of common stock. The unaudited pro forma net loss per share for the year ended December 31, 2018, was computed using the weighted-average number of shares of common stock outstanding, including the pro forma effect of the conversion of all outstanding shares of convertible preferred stock into shares of common stock, as if such conversion had occurred at the beginning of the period, or their issuance dates, if later.
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.
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 and Cash Equivalents		*Cash and Cash Equivalents* Cash equivalents are financial instruments that potentially subject the Company to concentrations of credit risk. The Company considers all highly liquid investments with original maturities of 90 days or less from the date of purchase to be cash equivalents. The Company invests its cash and cash equivalents in money market funds. The Company limits its credit risk associated with cash and cash equivalents by placing its cash with banks and institutions it believes are highly credit worthy and in highly-rated investments.
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. As of December 31, 2017 and 2018, the Company has not recorded 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 records 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 will be 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 have converted their shares of convertible preferred stock into shares of common stock. Convertible preferred stock is classified outside of stockholders’ deficit on the balance sheet as events triggering redemption are not solely within the Company’s control. The Company has not adjusted the carrying values of its convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur. As of December 31, 2018, it was not probable that such a redemption would occur.
Research and Development Costs		*Research and Development Costs* Research and development costs are charged to expense 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		*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 option grants 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. 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., our 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 potential dilutive effects 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.	*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 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 potential dilutive effects of potential dilutive shares outstanding during the period. Potential dilutive securities include stock options, warrants and convertible preferred stock. The dilutive effect of stock options and warrants is computed using the treasury stock method and the dilutive effect of convertible preferred stock is calculated using the “if-converted method”. For all periods presented, diluted net loss per share is the same as basic net loss per share since the effect of including potential common shares 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 us as of the specified effective date. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), we meet the definition of an emerging growth company, and have elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act.
Recently Adopted Accounting Pronouncements		*Recently Adopted Accounting Pronouncements* In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in ASC 605, Revenue Recognition. This standard is based on the principle that revenue is recognized to depict the transfer of goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. The standard also requires additional disclosure about the nature, amount, timing and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. The standard is effective for annual periods beginning after December 15, 2018 using one of two retrospective application methods. The Company has elected to adopt this standard as of January 1, 2018. The adoption of ASU No. 2014-09 did not have any impact on the Company’s consolidated financial statements and related disclosures. In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718), Scope of Modification Accounting. This pronouncement provides guidance about which changes to the terms or conditions of a share-based payment award may require an entity to apply modification accounting under Topic 718. This guidance is effective for the Company for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period, with early adoption permitted. The Company adopted this standard on January 1, 2018. The adoption of ASU No. 2017-09 did not have a significant impact on the Company’s consolidated financial statements and related disclosures.
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 us as of the specified effective date. Unless otherwise discussed, the impact of recently issued standards that are not yet effective will not have a material impact on our condensed consolidated financial statements upon adoption. Under the Jumpstart Our Business Startups Act of 2012, as amended (the “JOBS Act”), we meet the definition of an emerging growth company, and have elected the extended transition period for complying with new or revised accounting standards pursuant to Section 107(b) of the JOBS Act.	*Recently Issued Accounting Pronouncements* In February 2016, the FASB issued ASU No. 2016-02, Leases. ASU 2016-02 requires lessees to put 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, 2019 and early adoption is permitted. The Company is currently assessing the impact that the adoption will have on its consolidated financial statements and related disclosures. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230), which provides greater clarity to preparers on the treatment of certain items within an entity’s statement of cash flows. The new guidance is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The guidance is effective for the Company on January 1, 2019, and the Company is currently evaluating the impact that the adoption of ASU 2016-15 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. The new standard 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 interim periods within fiscal years beginning after December 15, 2020, with early adoption permitted. The Company is currently assessing the timing of adoption and 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. This ASU 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. The Company is currently assessing the impact of this ASU on its consolidated financial statements.
Deferred Offering Costs	*Deferred Offering Costs* Deferred offering costs consisting of direct legal, accounting, printing and other fees and costs related to the IPO are capitalized. The deferred offering costs were reclassified to additional paid-in capital upon the effectiveness of the IPO in November 2019. As of September 30, 2019, $2.8 million of deferred offering costs were capitalized and included in other assets.
Recently Issued Accounting Pronouncement 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 put 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, 2019 and early adoption is permitted. The Company is currently assessing 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 31, 2020. Early adoption is permitted for all entities. The Company is currently assessing the impact that the adoption of ASU 2016-13 will have on its condensed consolidated financial statements and related disclosures. In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230), which provides greater clarity to preparers on the treatment of certain items within an entity’s statement of cash flows. ASU 2016-15 is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. The guidance is effective for the Company on January 1, 2019, and the Company is currently evaluating the impact that the adoption of ASU 2016-15 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 interim periods within fiscal years beginning after December 15, 2020, with early adoption 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. The Company is currently assessing the impact of this ASU on its consolidated financial statements. 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. ASU 2018-18 will be effective for the Company for fiscal years beginning after December 15, 2020, and interim periods within fiscal years beginning after December 15, 2021. The Company is currently assessing the impact of this ASU on its condensed consolidated financial statements.

Property and Equipment (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Schedule of Property and Equipment

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


	December 31, 2018			September 30, 2019
Laboratory equipment	$	5,466		$	5,832
Leasehold improvements		2,989			3,294
Computer equipment		308			316
Furniture and fixtures		365			394

Total property and equipment		9,128			9,836
Less accumulated depreciation and amortization		(4,969	)		(5,834	)

Property and equipment, net	$	4,159		$	4,002

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


	December 31,
	2017			2018
Laboratory equipment	$	4,603		$	5,466
Leasehold improvements		598			2,989
Computer equipment		244			308
Furniture and fixtures		237			365

Total property and equipment		5,682			9,128
Less accumulated depreciation and amortization		(3,769	)		(4,969	)

Property and equipment, net	$	1,913		$	4,159

Accrued Expenses (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Payables and Accruals [Abstract]

Schedule of Accrued Expenses

Accrued expenses consist of the following (in thousands):


	December 31, 2018		September 30, 2019
Accrued clinical expenses	$	519	$	973
Accrued compensation		1,433		1,604
Accrued professional and consulting services		182		662
Accrued property and equipment		202		—
Accrued lab supplies		80		78
Other		72		93

Total accrued expenses	$	2,488	$	3,410

Accrued expenses consist of the following (in thousands):


	December 31,
	2017		2018
Accrued clinical expenses	$	39	$	519
Accrued compensation		1,019		1,433
Accrued professional and consulting services		317		182
Accrued property and equipment		—		202
Accrued lab supplies		70		80
Other		47		72

Total accrued expenses	$	1,492	$	2,488

Commitments (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

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

As of December 31, 2018, future minimum non-cancelable lease payments, net of sublease rental income, are as follows (in thousands):


Year ending December 31:
2019	$	1,432
2020		1,639
2021		1,969
2022		2,038
Thereafter		8,687

Total minimum lease payments	$	15,765

Convertible Preferred Stock and Stockholders' Deficit (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

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 Authorized		Shares Issued and Outstanding		Net Carrying Value		Aggregate Liquidation Preference
Series A		37,509,105		37,509,105	$	28,861	$	37,509
Series B		25,000,000		25,000,000		49,926		50,000
Series C		26,109,363		26,109,363		59,770		59,856
Series C-2		15,400,000		9,873,412		22,554		22,635

Total convertible preferred stock		104,018,468		98,491,880	$	161,111	$	170,000

As of September 30, 2019, convertible preferred stock outstanding consisted of the following (in thousands, except share amounts):


	Shares Authorized		Shares Issued and Outstanding		Net Carrying Value		Aggregate Liquidation Preference
Series A		37,509,105		37,509,105	$	28,861	$	37,509
Series B		25,000,000		25,000,000		49,926		50,000
Series C		26,109,363		26,109,363		59,770		59,856
Series C-2		29,271,007		16,184,857		36,933		37,104

Total convertible preferred stock		117,889,475		104,803,325	$	175,490	$	184,469

As of December 31, 2017, convertible preferred stock consisted of the following (in thousands, except share amounts):


	Shares Authorized		Shares Issued and Outstanding		Net Carrying Value		Aggregate Liquidation Preference
Series A		37,509,105		37,509,105	$	28,861	$	37,509
Series B		25,000,000		25,000,000		49,926		50,000
Series C		26,240,224		13,054,679		29,856		29,928

Total convertible preferred stock		88,749,329		75,563,784	$	108,643	$	117,437

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


	Shares Authorized		Shares Issued and Outstanding		Net Carrying Value		Aggregate Liquidation Preference
Series A		37,509,105		37,509,105	$	28,861	$	37,509
Series B		25,000,000		25,000,000		49,926		50,000
Series C		26,109,363		26,109,363		59,770		59,856
Series C-2		15,400,000		9,873,412		22,554		22,635

Total convertible preferred stock		104,018,468		98,491,880	$	161,111	$	170,000

Common Stock (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Equity [Abstract]

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

As of September 30, 2019, the Company had reserved the following shares of common stock, on an as-converted basis, for future issuance as follows:


Series A convertible preferred stock outstanding			6,251,502
Series B convertible preferred stock outstanding			4,166,663
Series C convertible preferred stock outstanding			4,351,554
Series C-2 convertible preferred stock outstanding			2,697,465
Options issued and outstanding			943,610
Options available for future grants			461,904

Total			18,872,698

As of December 31, 2018, the Company had reserved the following shares of common stock, on an as-converted basis, for future issuance as follows:


Series A convertible preferred stock outstanding			6,251,502
Series B convertible preferred stock outstanding			4,166,663
Series C convertible preferred stock outstanding			4,351,554
Series C-2 convertible preferred stock outstanding			2,566,666
Options issued and outstanding			768,239
Options available for future grants			693,879

Total			18,798,503

Stock Option Plan (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

Summary of Stock Option Activity

A summary of the Company’s stock option activity under the 2015 Plan for the nine months ended September 30, 2019 is as follows:


	Shares Available			Number of Shares Outstanding			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Balances at December 31, 2018		693,879			768,239		$	4.62		8.84	$	1,291
Stock options authorized		—			—
Stock options granted		(416,919	)		416,919			8.63
Stock options exercised		—			(141,707	)		4.37
Unvested common shares repurchased		85,103			—			1.02
Stock options forfeited		99,841			(99,841	)		5.07

Balances at September 30, 2019		461,904			943,610		$	6.82		8.79	$	6,416

The following summarizes option activity under the 2015 Plan:


	Shares Available			Number of Shares Outstanding			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Balances at December 31, 2016		539,900			142,037		$	1.56		9.47	$	426
Stock options authorized		540,554			—
Stock options granted		(306,804	)		306,804			1.99
Stock options exercised		—			(50,034	)		1.91
Unvested common shares repurchased		37,893			—			1.06
Stock options forfeited		50,147			(50,147	)		1.84

Balances at December 31, 2017		861,690			348,660		$	1.85		9.02	$	526
Stock options authorized		250,000			—
Stock options granted		(497,417	)		497,417			6.18
Stock options exercised		—			(17,390	)		1.93
Unvested common shares repurchased		19,158			—			1.39
Stock options forfeited		60,448			(60,448	)		2.74

Balances at December 31, 2018		693,879			768,239		$	4.62		8.84	$	1,291

Vested and expected to vest at December 31, 2018					768,223					8.84	$	1,291
Exercisable at December 31, 2018					137,159					7.99	$	580

Summary of Stock-based Compensation Expense

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


	For the Three Months Ended September 30,				For the Nine Months Ended September 30,
	2018		2019		2018		2019
Research and development	$	131	$	212	$	404	$	611
General and administrative		164		219		470		544

Total stock-based compensation expense	$	295	$	431	$	874	$	1,155

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


	Year Ended December 31,
	2017		2018
Research and development		394		542
General and administrative		322		628

Total stock-based compensation expense	$	716	$	1,170

Employees, Officers, Directors, Advisors, and Consultants

Summary of Assumptions Used to Value Stock Option Awards under 2015 Plan


	For the Three Months Ended September 30,		For the Nine Months Ended September 30,
	2018	2019	2018	2019
Fair value of common stock	$6.18	$13.62	$6.18	$13.62
Expected term (in years)	5.96 - 6.08	5.01 - 6.07	5.67 - 6.08	5.01 - 6.08
Volatility	81.37% - 81.48%	83.29% - 84.81%	83.69% - 81.48%	83.00% - 84.81%
Risk-free interest rate	2.88%	1.58% - 1.62%	2.62% - 2.88%	1.58% - 2.23%
Dividend yield	—	—	—	—


	Year Ended December 31,
	2017	2018
Fair value of common stock	$3.84 - $6.18	$6.18 - $6.30
Expected term (in years)	5.96 - 6.07	5.67 - 6.08
Volatility	81.50% - 83.06%	80.69% - 81.48%
Risk-free interest rate	1.75% - 2.22%	2.62% - 2.88%
Dividend yield	—	—

Non-employees

Summary of Assumptions Used to Value Stock Option Awards under 2015 Plan

The assumptions used to value non-employee stock option awards granted under the 2015 Plan during the three and nine months ended September 30, 2019 and 2018, using the Black-Scholes option pricing model, were as follows:


	For the Three Months Ended September 30,		For the Nine Months Ended September 30,
	2018	2019	2018	2019
Expected term (in years)	6.60 - 9.70	5.60 - 9.99	6.60 - 9.70	5.60 - 9.99
Volatility	80.12% - 81.73%	82.30% - 83.79%	80.02% - 82.32%	81.43% - 83.79%
Risk-free interest rate	2.75% - 3.02%	1.44% - 2.02%	2.39% - 3.02%	1.44% - 2.67%
Dividend yield	—	—	—	—

The assumptions used to value non-employee stock option awards granted under the 2015 Plan during the years ended December 31, 2017 and 2018, using the Black-Scholes option pricing model, were as follows:


	Year Ended December 31,
	2017	2018
Expected term (in years)	7.34 - 10.00	6.34 - 10.00
Volatility	78.92% - 86.03%	78.29% - 85.47%
Risk-free interest rate	1.32% - 2.57%	1.32% - 3.19%
Dividend yield	—	—

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Domestic and Foreign Pretax (Loss) Income from Continuing Operations

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


	December 31,
	2017			2018
United States	$	(29,114	)	$	(36,033	)
Foreign		—			(113	)

	$	(29,114	)	$	(36,146	)

Schedule of the statutory U.S. federal rate and Effective rate

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


	December 31⁽¹⁾
	2017			2018
Federal tax		34.0	%		21.0	%
State, net of federal benefit		—			—
Stock based compensation		(1.0	)		(0.6	)
R&D credit		2.0			3.68
Change in valuation allowance		(8.0	)		(24.13	)
Other		—			0.05
Re-measurement of deferred tax assets		(27.0	)		—

Income tax expense		0.0	%		0.0	%

Note:

(1)	For the year ended December 31, 2017, the statutory tax rate was 34%. For the year ended December 31, 2018, as a result of Tax Reform, the statutory tax rate was decreased to 21%.

Significant Components of Company's Net Deferred Tax Assets/(Liabilities)

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


	December 31
	2017			2018
Net operating loss carryforwards	$	13,635		$	20,810
Federal and state tax credits		2,035			3,378
Depreciation and amortization		60			105
Accrued liabilities and reserves		313			448
Stock-based compensation		25			50

Gross deferred tax assets	$	16,068		$	24,791
Valuation allowance		(16,068	)		(24,791	)

Net deferred taxes	$	—		$	—

Reconciliation of Unrecognized Tax Benefits Including Interest and Penalties


	December 31
	2017		2018
Balance at beginning of year	$	360	$	789
Additions on tax positions related to prior years		23		19
Additions on tax positions related to current year		406		473

Balance at end of year	$	789	$	1,281

Net Loss Per Share (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2019

Dec. 31, 2018

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 for the nine months ended September 30, 2018 and 2019 (in thousands, except share and per share data):


	Three Months Ended September 30,						Nine Months Ended September 30,
	2018			2019			2018			2019
Numerator:
Net loss	$	(10,284	)	$	(9,971	)	$	(26,717	)	$	(29,781	)

Denominator:
Weighted average common shares outstanding		867,565			838,961			864,855			822,644
Less: weighted-average unvested restricted common stock subject to repurchase		(124,514	)		(19,097	)		(150,868	)		(45,451	)
Less: weighted-average unvested common shares subject to repurchase		(96,251	)		(16,635	)		(121,750	)		(35,482	)

Weighted-average shares used to compute net loss per share, basic and diluted		646,800			803,229			592,237			741,711

Net loss per share, basic and diluted	$	(15.90	)	$	(12.41	)	$	(45.11	)	$	(40.15	)

The following table sets forth the computation of the basic and diluted net loss per share of the years ended December 31, 2017 and 2018 (in thousands, except share and per share data):


	Year Ended December 31,
	2017			2018
Numerator:
Net loss	$	(29,115	)	$	(36,147	)

Denominator:
Weighted average common shares outstanding		856,451			866,348
Less: weighted-average unvested restricted common stock subject to repurchase		(250,269	)		(137,691	)
Less: weighted-average unvested early exercised common shares subject to repurchase		(174,504	)		(106,368	)

Weighted-average shares used to compute net loss per common share, basic and diluted		431,679			622,289

Net loss per share, basic and diluted	$	(67.45	)	$	(58.09	)

Outstanding Potentially Dilutive Securities not Included in Diluted per Share Calculations

Outstanding potentially dilutive securities that were not included in the diluted per share calculations because they would be anti-dilutive


	As of September 30,
	2018		2019
Convertible preferred stock		88,618,468		104,803,325
Common stock options issued and outstanding		768,406		943,610

Total		89,386,874		105,746,935

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


	December 31,
	2017		2018
Convertible preferred stock		12,593,942		16,415,281
Common stock options issued and outstanding		861,690		693,879

Total		13,455,632		17,109,160

Schedule of Pro forma basic and diluted net loss per share

The following table presents the computation of pro forma basic and diluted net loss per share (in thousands, except share and per share data):


	For the Year Ended December 31, 2018
Numerator:
Net loss	$	(36,147	)

Denominator:
Weighted-average shares used to compute net loss per common share, basic and diluted		622,289
Pro forma adjustments to reflect:
Assumed conversion of convertible preferred stock		13,838,797

Weighted-average shares used to compute net loss per share, basic and diluted		14,461,086

Pro forma net loss per share, basic and diluted	$	(2.50	)

Organization and Liquidity Risks - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended						9 Months Ended		12 Months Ended
	Nov. 04, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Net loss		$ 9,971	$ 10,622	$ 9,188	$ 10,284	$ 8,204	$ 8,232	$ 29,781	$ 26,717	$ 36,147	$ 29,115
Cash used in operations								29,786	$ 23,742	32,953	27,123
Cash and cash equivalents		48,310						48,310		63,798	47,517
Accumulated deficit		148,734						148,734		$ 118,953	$ 82,806
Authorized shares of common stock										119,200,000	119,200,000
Common stock, par value										$ 0.0001	$ 0.0001
Working capital		$ 45,800						$ 45,800
Subsequent Event \| Common Stock
Class Of Stock [Line Items]
Conversion of convertible preferred stock, common stock shares issued	17,467,184
Subsequent Event \| Initial Public Offering
Class Of Stock [Line Items]
Shares issued during period	3,000,000
Shares issued, price per share	$ 12.00
Net proceeds received after deducting underwriting discounts and other offering related costs	$ 33,800
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
Subsequent Event \| Underwriters Exercised Option to Purchase at IPO
Class Of Stock [Line Items]
Shares issued during period	427,360

Summary of Significant Accounting Policies - Additional Information (Details) (Detail)		9 Months Ended	12 Months Ended
	Jul. 19, 2019	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Conversion of convertible preferred stock into common stock \| shares			98,491,880
Impairment losses on long-lived assets			$ 0	$ 0
Other Assets
Significant Accounting Policies [Line Items]
Deferred offering costs		$ 2,800,000
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

Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 48.3	$ 63.8	$ 47.5

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment	$ 9,836	$ 9,128	$ 5,682
Less accumulated depreciation and amortization	(5,834)	(4,969)	(3,769)
Property and equipment, net	4,002	4,159	1,913
Laboratory Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	5,832	5,466	4,603
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment	3,294	2,989	598
Computer Equipment
Property Plant And Equipment [Line Items]
Total property and equipment	316	308	244
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	$ 394	$ 365	$ 237

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Depreciation and amortization expenses	$ 300	$ 300	$ 1,011	$ 943	$ 1,237	$ 1,384

Accrued Expenses - Schedule of Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Payables and Accruals [Abstract]
Accrued clinical expenses	$ 973	$ 519	$ 39
Accrued compensation	1,604	1,433	1,019
Accrued professional and consulting services	662	182	317
Accrued property and equipment		202
Accrued lab supplies	78	80	70
Other	93	72	47
Total accrued expenses	$ 3,410	$ 2,488	$ 1,492

Commitments - Additional Information (Detail) $ in Millions	1 Months Ended		12 Months Ended
Commitments - Additional Information (Detail) $ in Millions	Apr. 30, 2018 ft²	May 31, 2015 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Feb. 28, 2019 ft²
Lessee, Lease, Description [Line Items]
Lease expiration period		2022-05
Tenant improvements allowances \| $		$ 0.8	$ 1.4
Additional laboratory and office facilities under lease	6,378
Increased of leased premises	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 \| $			2.4
Sublease its facility lease					6,378
Operating lease rent expense \| $			$ 1.8	$ 1.6
South San Francisco and California
Lessee, Lease, Description [Line Items]
Laboratory and office facilities under lease		30,376

Commitments - Schedule of Future Minimum Non-Cancelable Lease Payments, Net of Sublease Rental Income (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2019	$ 1,432
2020	1,639
2021	1,969
2022	2,038
Thereafter	8,687
Total minimum lease payments	$ 15,765

Related-Party Promissory Notes - Additional Information (Detail) - 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	Dec. 31, 2017
Related Party Transaction [Line Items]
Total outstanding balances including accrued interest							$ 600,000	$ 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
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
Interest rate percentage					1.41%
Chief Operating Officer \| Performance-based Options
Related Party Transaction [Line Items]
Reduction of performance-based options				8,333

Convertible Preferred Stock and Stockholders' Deficit - Additional Information (Detail) - USD ($)		1 Months Ended			3 Months Ended	6 Months Ended	9 Months Ended		12 Months Ended
	Nov. 04, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2019	Jun. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Proceeds from the sale of convertible preferred stock, net of issuance costs							$ 14,379,000	$ 29,914,000	$ 52,468,000	$ 29,856,000
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 \| Subsequent Event
Temporary Equity [Line Items]
Conversion of convertible preferred stock, common stock shares issued	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
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 are convertible
Series C-2 Convertible Preferred Stock
Temporary Equity [Line Items]
Shares issued, price per share		$ 2.2925	$ 2.2925		$ 2.2925	$ 2.2925			$ 2.2925
Proceeds from the sale of convertible preferred stock, net of issuance costs		$ 7,000,000	$ 22,600,000			$ 14,400,000			$ 22,600,000
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 are convertible
Series C-2 Convertible Preferred Stock \| Subsequent Event
Temporary Equity [Line Items]
Shares issued, price per share		$ 2.2925			$ 2.2925
Proceeds from the sale of convertible preferred stock, net of issuance costs					$ 7,000,000
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
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 are convertible
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
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 are convertible

Convertible Preferred Stock and Stockholders' Deficit - Schedule of Convertible Preferred Stock Outstanding (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Shares Authorized	117,889,475	104,018,468	104,018,468
Shares Issued	104,803,325	98,491,880	75,563,784
Shares Outstanding	104,803,325	98,491,880	75,563,784
Net Carrying Value	$ 175,490	$ 161,111	$ 108,643
Aggregate Liquidation Preference	$ 184,469	$ 170,000	$ 117,437
Series A Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	37,509,105	37,509,105	37,509,105
Shares Issued	37,509,105	37,509,105	37,509,105
Shares Outstanding	37,509,105	37,509,105	37,509,105
Net Carrying Value	$ 28,861	$ 28,861	$ 28,861
Aggregate Liquidation Preference	$ 37,509	$ 37,509	$ 37,509
Series B Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	25,000,000	25,000,000	25,000,000
Shares Issued	25,000,000	25,000,000	25,000,000
Shares Outstanding	25,000,000	25,000,000	25,000,000
Net Carrying Value	$ 49,926	$ 49,926	$ 49,926
Aggregate Liquidation Preference	$ 50,000	$ 50,000	$ 50,000
Series C Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	26,109,363	26,109,363	26,240,224
Shares Issued	26,109,363	26,109,363	13,054,679
Shares Outstanding	26,109,363	26,109,363	13,054,679
Net Carrying Value	$ 59,770	$ 59,770	$ 29,856
Aggregate Liquidation Preference	$ 59,856	$ 59,856	$ 29,928
Series C-2 Convertible Preferred Stock
Temporary Equity [Line Items]
Shares Authorized	29,271,007	15,400,000
Shares Issued	16,184,857	9,873,412
Shares Outstanding	16,184,857	9,873,412
Net Carrying Value	$ 36,933	$ 22,554
Aggregate Liquidation Preference	$ 37,104	$ 22,635
Adjustment
Temporary Equity [Line Items]
Shares Authorized			88,749,329

Common Stock - Additional Information (Detail)	9 Months Ended	12 Months Ended
Common Stock - Additional Information (Detail)	Sep. 30, 2019 USD ($) Vote shares	Dec. 31, 2018 USD ($) Vote shares	Dec. 31, 2017 USD ($) shares
Equity [Abstract]
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	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	1
Dividends declared \| $	$ 0	$ 0	$ 0
Common stock shares outstanding \| shares	935,040	878,413	880,191

Common Stock - Summary of Reserved Shares of Common Stock on Converted Basis for Future Issuance (Detail) - shares	Sep. 30, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Total shares reserved	18,872,698	18,798,503
Series A Convertible Preferred Stock
Class Of Stock [Line Items]
Total shares reserved	6,251,502	6,251,502
Series B Convertible Preferred Stock
Class Of Stock [Line Items]
Total shares reserved	4,166,663	4,166,663
Series C Convertible Preferred Stock
Class Of Stock [Line Items]
Total shares reserved	4,351,554	4,351,554
Series C-2 Convertible Preferred Stock
Class Of Stock [Line Items]
Total shares reserved	2,697,465	2,566,666
Options Issued and Outstanding
Class Of Stock [Line Items]
Total shares reserved	943,610	768,239
Options Available for Future Grants
Class Of Stock [Line Items]
Total shares reserved	461,904	693,879

Stock Option Plan - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock options shares granted			416,919		497,417	306,804
Weighted-average grant date fair value per share					$ 4.32	$ 3.60
Weighted-average exercise price per share			$ 8.63		$ 6.18	1.99
Weighted average grant date fair value of common stock						$ 4.38
Stock options grant date fair value					$ 2,100,000	$ 1,200,000
Number of stock options, exercises intrinsic value					$ 100,000	$ 200,000
Number of stock options shares vested					300,000	300,000
Stock option vested value					$ 900,000	$ 600,000
Stock options, outstanding	943,610		943,610		768,239	348,660	142,037
Stock-based compensation expense			$ 1,155,000	$ 874,000	$ 1,170,000	$ 716,000
Expected dividend yield					0
Stock options related expense	$ 431,000	$ 295,000	$ 1,155,000	$ 874,000	1,170,000	$ 716,000
Unrecognized stock-based compensation cost related to outstanding unvested stock options that are expected to vest					$ 1,300,000
Unrecognized stock-based compensation cost, expected period to be recognized					1 year 8 months 19 days
2015 Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			48 months			48 months
Shares not vested and subject to repurchase	100,000		100,000		200,000	400,000
Cash received from early exercise of unvested options	$ 100,000		$ 100,000		$ 200,000	$ 500,000
2015 Stock Plan \| Employees, Officers, Directors, Advisors, and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period					4 years
Performance-based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option vested value					$ 0.2	$ 0.2
Stock options, outstanding					25,000	33,333
Stock-based compensation expense					$ 0	$ 0
Share-based Payment Arrangement, Option \| 2015 Stock Plan \| Non-employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock options shares granted					4,166	33,332
Stock options related expense					$ 100,000	$ 100,000
Maximum \| 2015 Stock Plan \| Employees, Officers, Directors, Advisors, and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, expiration period					10 years

Stock Option Plan - Summary of Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement [Abstract]
Shares Available, Beginning Balance	693,879	861,690	539,900
Shares Available, Stock options authorized		250,000	540,554
Shares Available, Stock options granted	(416,919)	(497,417)	(306,804)
Shares Available, Stock options exercised		0	0
Shares Available, Unvested common shares repurchased	85,103	19,158	37,893
Shares Available, Stock options forfeited	99,841	60,448	50,147
Shares Available, Ending Balance	461,904	693,879	861,690	539,900
Number of Shares Outstanding, Beginning Balance	768,239	348,660	142,037
Number of Shares Outstanding, Stock options authorized	0
Number of Shares Outstanding, Stock options granted	416,919	497,417	306,804
Number of Shares Outstanding, Stock options exercised	(141,707)	(17,390)	(50,034)
Number of Shares Outstanding, Unvested common shares repurchased	0
Number of Shares Outstanding, Stock options forfeited	(99,841)	(60,448)	(50,147)
Number of Shares Outstanding, Ending Balance	943,610	768,239	348,660	142,037
Weighted Average Exercise Price Per Share, Beginning Balance	$ 4.62	$ 1.85
Number of Shares Outstanding, Vested and expected to vest						768,223
Weighted Average Exercise Price Per Share, Stock options authorized	0	0	$ 0
Number of Shares Outstanding, Exercisable						137,159
Weighted Average Exercise Price Per Share, Stock options granted	8.63	6.18	1.99
Weighted Average Exercise Price Per Share, Ending Balance	4.62	4.62	1.85		$ 6.82	$ 4.62	$ 1.85
Weighted Average Exercise Price Per Share, Stock options exercised	$ 4.37	$ 1.93	$ 1.91
Weighted Average Remaining Contractual Term (Years), Balance	8 years 9 months 14 days	8 years 10 months 2 days	9 years 7 days	9 years 5 months 19 days
Weighted Average Exercise Price Per Share, Unvested common shares repurchased	$ 1.02	$ 1.39	$ 1.06
Weighted Average Remaining Contractual Term (Years), Vested and expected to vest		8 years 10 months 2 days
Weighted Average Exercise Price Per Share, Stock options forfeited	5.07	$ 2.74	1.84
Weighted Average Remaining Contractual Term (Years), Exercisable		7 years 11 months 26 days
Weighted Average Exercise Price Per Share, Ending Balance	$ 6.82	$ 4.62	$ 1.85
Aggregate Intrinsic Value				$ 426	$ 6,416	$ 1,291	$ 526
Weighted Average Exercise Price Per Share, Vested and expected to vest						$ 0
Aggregate Intrinsic Value, Vested and expected to vest						$ 1,291
Weighted Average Exercise Price Per Share, Exercisable						$ 0
Aggregate Intrinsic Value, Exercisable						$ 580

Stock Option Plan - Summary of Assumptions Used to Value Stock Option Awards under 2015 Plan (Detail) - Share-based Payment Arrangement, Option - 2015 Stock Plan - $ / shares	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Employees, Officers, Directors, Advisors, and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of common stock	$ 13.62	$ 6.18	$ 13.62	$ 6.18
Risk-free interest rate		2.88%
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 7 months 6 days	6 years 7 months 6 days	5 years 7 months 6 days	6 years 7 months 6 days	6 years 4 months 2 days	7 years 4 months 2 days
Volatility	82.30%	80.12%	81.43%	80.02%	78.29%	78.92%
Risk-free interest rate	1.44%	2.75%	1.44%	2.39%	1.32%	1.32%
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Minimum \| Employees, Officers, Directors, Advisors, and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of common stock					$ 6.18	$ 3.84
Expected term (in years)	5 years 3 days	5 years 11 months 15 days	5 years 3 days	5 years 8 months 1 day	5 years 8 months 1 day	5 years 11 months 15 days
Volatility	83.29%	81.37%	83.00%	81.48%	80.69%	81.50%
Risk-free interest rate	1.58%		1.58%	2.62%	2.62%	1.75%
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	9 years 11 months 26 days	9 years 8 months 12 days	9 years 11 months 26 days	9 years 8 months 12 days	10 years	10 years
Volatility	83.79%	81.73%	83.79%	82.32%	85.47%	86.03%
Risk-free interest rate	2.02%	3.02%	2.67%	3.02%	3.19%	2.57%
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Maximum \| Employees, Officers, Directors, Advisors, and Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of common stock					$ 6.30	$ 6.18
Expected term (in years)	6 years 25 days	6 years 29 days	6 years 29 days	6 years 29 days	6 years 29 days	6 years 25 days
Volatility	84.81%	81.48%	84.81%	83.69%	81.48%	83.06%
Risk-free interest rate	1.62%		2.23%	2.88%	2.88%	2.22%
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%

Stock Option Plan - Summary of Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 431	$ 295	$ 1,155	$ 874	$ 1,170	$ 716
Research and Development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	212	131	611	404	542	394
General and Administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 219	$ 164	$ 544	$ 470	$ 628	$ 322

Income Taxes - Schedule of Domestic and Foreign Pretax (Loss) Income from Continuing Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ (36,033)	$ (29,114)
Foreign	(113)
Income (loss) before income taxes	$ (36,146)	$ (29,114)

Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail)		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal tax	34.00%	21.00%	34.00%
State, net of federal benefit		0.00%	0.00%
Stock based compensation		(0.60%)	(1.00%)
R&D credit		3.68%	2.00%
Change in valuation allowance		(24.13%)	(8.00%)
Other		0.05%
Re-measurement of deferred tax assets			(27.00%)
Income tax expense		0.00%	0.00%

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Federal statutory income tax rate	34.00%	21.00%	34.00%
Reduction in deferred tax assets due to decrease in Federal tax rate			$ 7.9
Increase in valuation allowance		$ 8.7	2.4
Research and Development
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount		2.7	1.7
Federal
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount		1.4	1.4
Operating loss carryforwards		95.0	60.9
Operating loss carryforwards indefinitely		$ 34.1
Operating loss carryforwards expiration beginning year		2035
Uncertain tax positions reserves		$ 0.4	0.4
Uncertain tax positions		0.3
State
Income Tax Disclosure [Line Items]
Tax credit carryforwards amount		2.3	2.3
Operating loss carryforwards		$ 12.1	12.1
Operating loss carryforwards expiration beginning year		2035
Uncertain tax positions reserves		$ 0.7	$ 0.7
Uncertain tax positions		$ 0.6

Income Taxes - Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 20,810	$ 13,635
Federal and state tax credits	3,378	2,035
Depreciation and amortization	105	60
Accrued liabilities and reserves	448	313
Stock-based compensation	50	25
Gross deferred tax assets	24,791	16,068
Valuation allowance	(24,791)	(16,068)
Net deferred taxes	$ 0	$ 0

Income Taxes - Reconciliation of Unrecognized Tax Benefits Including Interest and Penalties (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Beginning Balance	$ 789	$ 360
Additions on tax positions related to prior years	19	23
Additions on tax positions related to current year	473	406
Ending Balance	$ 1,281	$ 789

Net Loss Per Share - Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (9,971)	$ (10,622)	$ (9,188)	$ (10,284)	$ (8,204)	$ (8,232)	$ (29,781)	$ (26,717)	$ (36,147)	$ (29,115)
Denominator:
Weighted average common shares outstanding	838,961			867,565			822,644	864,855	866,348	856,451
Weighted-average shares used to compute net loss per common share, basic and diluted	803,229			646,800			741,711	592,237	622,289	431,679
Net loss per share, basic and diluted	$ (12.41)			$ (15.90)			$ (40.15)	$ (45.11)	$ (58.09)	$ (67.45)
Restricted Stock
Denominator:
Less: weighted-average unvested common shares subject to repurchase	(19,097)			(124,514)			(45,451)	(150,868)	(137,691)	(250,269)
Early Exercised Stock Options
Denominator:
Less: weighted-average unvested common shares subject to repurchase	(16,635)			(96,251)			(35,482)	(121,750)	(106,368)	(174,504)

- 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 (Detail) - shares	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities	105,746,935	89,386,874	17,109,160	13,455,632
Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities	104,803,325	88,618,468	16,415,281	12,593,942
Common Stock Options Issued and Outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities	943,610	768,406	693,879	861,690

Net Loss Per Share - Schedule of ProForma basic and diluted net loss per share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (9,971)	$ (10,622)	$ (9,188)	$ (10,284)	$ (8,204)	$ (8,232)	$ (29,781)	$ (26,717)	$ (36,147)	$ (29,115)
Denominator:
Weighted-average shares used to compute net loss per common share, basic and diluted	803,229			646,800			741,711	592,237	622,289	431,679
Pro forma adjustments to reflect
Assumed conversion of convertible preferred stock									14,461,086
Pro forma net loss per share, basic and diluted									$ (2.50)
Convertible Preferred Stock
Pro forma adjustments to reflect
Assumed conversion of convertible preferred stock									13,838,797

- Definition

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

+ References + Details

Subsequent Events - Additional Informations (Detail) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended	6 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) $ / shares	Feb. 28, 2019 ft²	Dec. 31, 2018 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Dec. 01, 2019 USD ($)
Subsequent Event [Line Items]
Lessee, Operating Lease, Option to Extend		The sublease has an initial term of eighteen months, expiring August 2020, with an option to extend by an additional six months.
Sublease its facility lease \| ft²		6,378
Sublease, expiration period		2020-08
Sublease, initial term		18 months
Sublease, option to extend period		6 months
Proceeds from the sale of convertible preferred stock, net of issuance costs						$ 14,379	$ 29,914	$ 52,468	$ 29,856
Subsequent Event \| Collaboration and License Agreement \| Hanmi Pharmaceutical Company Limited
Subsequent Event [Line Items]
Upfront and expected near-term milestone payment										$ 10,000
Subsequent Event \| Collaboration and License Agreement \| Hanmi Pharmaceutical Company Limited \| Maximum
Subsequent Event [Line Items]
Additional contingent payments upon Achievement of specified milestones										108,000
Sales milestone payment upon achievement										60,000
Development milestone payment upon achievement										$ 48,000
Series C-2 Convertible Preferred Stock
Subsequent Event [Line Items]
Shares issued, price per share \| $ / shares	$ 2.2925		$ 2.2925	$ 2.2925	$ 2.2925			$ 2.2925
Proceeds from the sale of convertible preferred stock, net of issuance costs	$ 7,000		$ 22,600		$ 14,400			$ 22,600
Series C-2 Convertible Preferred Stock \| Subsequent Event
Subsequent Event [Line Items]
Shares issued, price per share \| $ / shares	$ 2.2925			$ 2.2925
Proceeds from the sale of convertible preferred stock, net of issuance costs				$ 7,000

Net Loss Per Share - Additional Information (Detail) - $ / shares		9 Months Ended
	Nov. 04, 2019	Sep. 30, 2019
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Basic and diluted unaudited pro forma net loss per share		$ 1.69
Subsequent Event \| Common Stock
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Conversion of convertible preferred stock, common stock shares issued	17,467,184