Tricida (TCDAQ) 10-Q 14 Nov 19 2019 Q3 Quarterly report Financial data

Cover Page

Cover Page - $ / shares	9 Months Ended
	Sep. 30, 2019	Nov. 07, 2019	Dec. 31, 2018	Jul. 02, 2018
Cover page.
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2019
Document Transition Report	false
Entity File Number	001-38558
Entity Registrant Name	TRICIDA, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-3372526
Entity Address, Address Line One	7000 Shoreline Court
Entity Address, Address Line Two	Suite 201
Entity Address, City or Town	South San Francisco
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94080
City Area Code	415
Local Phone Number	429-7800
Title of 12(b) Security	Common stock, par value $0.001 per share
Trading Symbol	TCDA
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		49,613,330
Common stock, par value (in USD per share)	$ 0.001		$ 0.001	$ 0.001
Amendment Flag	false
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001595585
Current Fiscal Year End Date	--12-31

CONDENSED BALANCE SHEETS

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 30,368	$ 37,172
Short-term investments	291,864	203,906
Prepaid expenses and other current assets	4,856	3,269
Total current assets	327,088	244,347
Long-term investments	41,342	2,287
Property and equipment, net	2,053	1,215
Operating lease right-of-use assets	9,694
Total assets	380,177	247,849
Current liabilities:
Accounts payable	4,471	8,460
Current operating lease liabilities	1,064
Accrued expenses and other current liabilities	15,601	6,344
Total current liabilities	21,136	14,804
Long-term liabilities
Term Loan	38,016	38,071
Non-current operating lease liabilities	8,914
Other long-term liabilities	488	449
Total liabilities	68,554	53,324
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.001 par value; 40,000,000 shares authorized, no shares issued or outstanding as of September 30, 2019 and December 31, 2018	0	0
Common stock, $0.001 par value; 400,000,000 shares authorized as of September 30, 2019 and December 31, 2018; 49,531,916 and 42,148,247 shares issued and outstanding as of September 30, 2019 and December 31, 2018, respectively	50	42
Additional paid-in capital	621,923	386,830
Accumulated other comprehensive income (loss)	486	(153)
Accumulated deficit	(310,836)	(192,194)
Total stockholders’ equity	311,623	194,525
Total liabilities and stockholders’ equity	$ 380,177	$ 247,849

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in USD per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	40,000,000	40,000,000
Preferred stock, issued (in shares)	0	0
Preferred stock, outstanding (in shares)	0	0
Common stock, par value (in USD per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	400,000,000	400,000,000
Common stock, shares issued	49,531,916	42,148,247
Common stock, shares outstanding	49,531,916	42,148,247

CONDENSED STATEMENTS OF OPERATI

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses:
Research and development	$ 31,976	$ 25,230	$ 92,375	$ 62,897
General and administrative	13,120	4,178	28,333	11,888
Total operating expenses	45,096	29,408	120,708	74,785
Loss from operations	(45,096)	(29,408)	(120,708)	(74,785)
Other income (expense), net	2,387	1,247	6,256	1,987
Interest expense	(1,410)	(937)	(4,190)	(2,166)
Net loss	(44,119)	(29,098)	(118,642)	(74,964)
Other comprehensive income (loss):
Net unrealized gain (loss) on available-for-sale investments	(143)	(14)	639	(27)
Total comprehensive loss	$ (44,262)	$ (29,112)	$ (118,003)	$ (74,991)
Net loss per share, basic and diluted (in USD per share)	$ (0.89)	$ (0.71)	$ (2.53)	$ (4.86)
Weighted-average number of shares outstanding, basic and diluted (in shares)	49,418,025	41,261,703	46,813,876	15,415,194

CONDENSED STATEMENTS OF CONVERT

CONDENSED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017	104,129,702
Beginning balance at Dec. 31, 2017	$ 147,070
Beginning balance (in shares) at Dec. 31, 2017		2,272,609
Beginning balance at Dec. 31, 2017	(88,041)	$ 2	$ 1,356	$ (13)	$ (89,386)
Total Stockholders’ Equity (Deficit)
Issuance of common stock under equity incentive plans (in shares)		29,519
Issuance of common stock under equity incentive plans	24		24
Stock-based compensation	353		353
Net unrealized gain (loss) on available-for-sale investments	(54)			(54)
Net loss	$ (20,504)				(20,504)
Ending balance (in shares) at Mar. 31, 2018	104,129,702
Ending balance at Mar. 31, 2018	$ 147,070
Ending balance (in shares) at Mar. 31, 2018		2,302,128
Ending balance at Mar. 31, 2018	$ (108,222)	$ 2	1,733	(67)	(109,890)
Beginning balance (in shares) at Dec. 31, 2017	104,129,702
Beginning balance at Dec. 31, 2017	$ 147,070
Beginning balance (in shares) at Dec. 31, 2017		2,272,609
Beginning balance at Dec. 31, 2017	(88,041)	$ 2	1,356	(13)	(89,386)
Total Stockholders’ Equity (Deficit)
Net unrealized gain (loss) on available-for-sale investments	(27)
Net loss	(74,964)
Ending balance (in shares) at Sep. 30, 2018		42,100,323
Ending balance at Sep. 30, 2018	$ 219,071	$ 42	383,419	(40)	(164,350)
Beginning balance (in shares) at Mar. 31, 2018	104,129,702
Beginning balance at Mar. 31, 2018	$ 147,070
Beginning balance (in shares) at Mar. 31, 2018		2,302,128
Beginning balance at Mar. 31, 2018	(108,222)	$ 2	1,733	(67)	(109,890)
Total Stockholders’ Equity (Deficit)
Issuance of warrant in connection with Term Loan	$ 194		194
Series A warrant exercise (in shares)	95,936
Series A warrant exercise	$ 458
Issuance of common stock under equity incentive plans (in shares)		151,478
Issuance of common stock under equity incentive plans	108		108
Stock-based compensation	970		970
Net unrealized gain (loss) on available-for-sale investments	41			41
Net loss	$ (25,362)				(25,362)
Ending balance (in shares) at Jun. 30, 2018	104,225,638
Ending balance at Jun. 30, 2018	$ 147,528
Ending balance (in shares) at Jun. 30, 2018		2,453,606
Ending balance at Jun. 30, 2018	(132,271)	$ 2	3,005	(26)	(135,252)
Total Stockholders’ Equity (Deficit)
Issuance of common stock under equity incentive plans (in shares)		4,396
Issuance of common stock under equity incentive plans	34		34
Stock-based compensation	1,706		1,706
Issuance of common stock in connection with initial public offering, net of underwriter discounts and issuance costs (in shares)		13,455,000
Issuance of common stock in connection with public offering, net of underwriter discounts and issuance costs	231,186	$ 14	231,172
Net unrealized gain (loss) on available-for-sale investments	(14)			(14)
Net loss	(29,098)				(29,098)
Number of stock issued on conversion of preferred shares (in shares)		26,187,321
Preferred shares converted into common stock	(147,528)	$ (26)	(147,502)
Ending balance (in shares) at Sep. 30, 2018		42,100,323
Ending balance at Sep. 30, 2018	$ 219,071	$ 42	383,419	(40)	(164,350)
Total Stockholders’ Equity (Deficit)
Number of common stock issued upon conversion (in shares)		(104,225,638)
Beginning balance (in shares) at Dec. 31, 2018	0
Beginning balance at Dec. 31, 2018	$ 0
Beginning balance (in shares) at Dec. 31, 2018	42,148,247	42,148,247
Beginning balance at Dec. 31, 2018	$ 194,525	$ 42	386,830	(153)	(192,194)
Total Stockholders’ Equity (Deficit)
Issuance of warrant in connection with Term Loan	284		284
Issuance of common stock under equity incentive plans (in shares)		527,859
Issuance of common stock under equity incentive plans	737	$ 1	736
Stock-based compensation	2,658		2,658
Net unrealized gain (loss) on available-for-sale investments	302			302
Net loss	$ (37,897)				(37,897)
Ending balance (in shares) at Mar. 31, 2019	0
Ending balance at Mar. 31, 2019	$ 0
Ending balance (in shares) at Mar. 31, 2019		42,676,106
Ending balance at Mar. 31, 2019	$ 160,609	$ 43	390,508	149	(230,091)
Beginning balance (in shares) at Dec. 31, 2018	0
Beginning balance at Dec. 31, 2018	$ 0
Beginning balance (in shares) at Dec. 31, 2018	42,148,247	42,148,247
Beginning balance at Dec. 31, 2018	$ 194,525	$ 42	386,830	(153)	(192,194)
Total Stockholders’ Equity (Deficit)
Net unrealized gain (loss) on available-for-sale investments	639
Net loss	$ (118,642)
Ending balance (in shares) at Sep. 30, 2019	49,531,916	49,531,916
Ending balance at Sep. 30, 2019	$ 311,623	$ 50	621,923	486	(310,836)
Beginning balance (in shares) at Mar. 31, 2019	0
Beginning balance at Mar. 31, 2019	$ 0
Beginning balance (in shares) at Mar. 31, 2019		42,676,106
Beginning balance at Mar. 31, 2019	160,609	$ 43	390,508	149	(230,091)
Total Stockholders’ Equity (Deficit)
Issuance of common stock under equity incentive plans (in shares)		160,339
Issuance of common stock under equity incentive plans	959		959
Stock-based compensation	4,413		4,413
Issuance of common stock in connection with initial public offering, net of underwriter discounts and issuance costs (in shares)		6,440,000
Issuance of common stock in connection with public offering, net of underwriter discounts and issuance costs	217,009	$ 6	217,003
Net unrealized gain (loss) on available-for-sale investments	480			480
Net loss	$ (36,626)				(36,626)
Ending balance (in shares) at Jun. 30, 2019	0
Ending balance at Jun. 30, 2019	$ 0
Ending balance (in shares) at Jun. 30, 2019		49,276,445
Ending balance at Jun. 30, 2019	346,844	$ 49	612,883	629	(266,717)
Total Stockholders’ Equity (Deficit)
Issuance of common stock under equity incentive plans (in shares)		255,471
Issuance of common stock under equity incentive plans	359	$ 1	358
Stock-based compensation	8,682		8,682
Net unrealized gain (loss) on available-for-sale investments	(143)			(143)
Net loss	$ (44,119)				(44,119)
Ending balance (in shares) at Sep. 30, 2019	49,531,916	49,531,916
Ending balance at Sep. 30, 2019	$ 311,623	$ 50	$ 621,923	$ 486	$ (310,836)

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating activities:
Net loss	$ (118,642)	$ (74,964)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	524	455
Amortization of operating lease right-of-use assets	646
Accretion (amortization) of premiums and discounts on investments	(3,143)	(392)
Amortization of Term Loan discount and issuance costs	1,564	902
Stock-based compensation	15,753	3,029
Changes in fair value of compound derivative liability and warrants	259	(238)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,562)	(184)
Accounts payable	(4,235)	2,251
Accrued expenses and other liabilities	9,544	7,166
Operating lease liabilities	(584)
Net cash used in operating activities	(99,876)	(61,975)
Investing activities:
Purchases of investments	(375,385)	(103,012)
Maturities of investments	252,155	55,804
Purchases of property and equipment	(1,117)	(712)
Net cash used in investing activities	(124,347)	(47,920)
Financing activities:
Proceeds from equity offerings, net	217,930	237,750
Payments of equity offering costs	(921)	(6,564)
Proceeds from exercise of common stock under equity incentive plans	1,944	251
Proceeds from issuance of convertible preferred stock, net	0	85
Proceeds from leasehold improvement loan	0	276
Repayment of leasehold improvement loan	(85)	(81)
Proceeds (payments) under Term Loan, net	(1,449)	23,622
Net cash provided by financing activities	217,419	255,339
Net increase (decrease) in cash and cash equivalents	(6,804)	145,444
Cash and cash equivalents at beginning of period	37,172	9,774
Cash and cash equivalents at end of period	30,368	155,218
Supplemental disclosures
Cash paid for interest	2,504	1,073
Supplemental disclosures of non-cash investing and financing activities
Right-of-use assets obtained in exchange for lease obligations	8,084
Warrants and compound derivative liability related to Term Loan	284	810
Purchase of property and equipment in accounts payable and accrued expenses	$ 271	$ 116

ORGANIZATION AND BASIS OF PRESE

ORGANIZATION AND BASIS OF PRESENTATION	9 Months Ended
	Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGANIZATION AND BASIS OF PRESENTATION	ORGANIZATION AND BASIS OF PRESENTATION Organization —Tricida, Inc. (the Company) was incorporated in the state of Delaware on May 22, 2013 and was granted its certification of qualification in the state of California on August 5, 2013, or inception. The Company is focused on the development and commercialization of its drug candidate, veverimer (also known as TRC101), a non-absorbed, orally-administered polymer designed to treat metabolic acidosis in patients with chronic kidney disease. The Company has sustained operating losses and expects such annual losses to continue over the next several years. The Company’s ultimate success depends on the outcome of its research and development and commercialization activities for veverimer, for which it expects to incur additional losses in the future. Through September 30, 2019 , the Company has relied primarily on the proceeds from equity offerings and debt financing to finance its operations. The Company recognizes that it may need to raise additional capital to fully implement its business plan, and if market conditions are favorable or if the Company identifies specific strategic opportunities or needs, intends to do so through the issuance of equity, borrowings, or strategic alliances with partner companies. However, if such financing is not available at adequate levels or on reasonable terms, the Company will need to reevaluate its operating plans. Basis of Presentation —The accompanying unaudited condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. The condensed balance sheet as of September 30, 2019 , the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2019 and 2018 , the condensed statements of convertible preferred stock and stockholders' equity (deficit) for the three and nine months ended September 30, 2019 and 2018 and the condensed statements of cash flows for the nine months ended September 30, 2019 and 2018 are unaudited, but include all adjustments, consisting only of normal recurring adjustments, which the Company consider necessary for a fair presentation of the financial position, operating results and cash flows for the periods presented. The condensed balance sheet at December 31, 2018 has been derived from audited financial statements. Although the Company believes that the disclosures in these financial statements are adequate to make the information presented not misleading, certain information and footnote information normally included in financial statements prepared in accordance with U.S. GAAP have been condensed or omitted pursuant to the rules and regulations of the Securities and Exchange Commission. Results for any interim period are not necessarily indicative of results for any future interim period or for the entire year. The accompanying financial statements should be read in conjunction with the financial statements and notes thereto included in the Company's Annual Report on Form 10-K for the year ended December 31, 2018

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES With the exception of the change for the accounting of leases as a result of the adoption of Accounting Standards Update (ASU) No. 2016-02, Leases (Topic 842), or Topic 842, there have been no new or material changes to the significant accounting policies discussed in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, that are of significance, or potential significance, to the Company. Leases —The Company determines if an arrangement contains a lease at inception. For arrangements where the Company is the lessee, operating leases are included in operating lease right-of-use (ROU) assets; current operating lease liabilities; and non-current operating lease liabilities on its condensed balance sheets. The Company currently does not have any finance leases. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at the commencement date. ROU assets also include any initial direct costs incurred and any lease payments made on or before the lease commencement date, less lease incentives received. The Company uses its incremental borrowing rate based on the information available at the commencement date in determining the lease liabilities as the Company’s leases generally do not provide an implicit rate. The incremental borrowing rate is reevaluated upon a lease modification. The operating lease ROU asset also includes any initial direct costs and prepaid lease payments made less any lease incentives. The Company considered information available at the adoption date of Topic 842 to determine the incremental borrowing rate for leases in existence as of this date. Lease terms may include options to extend or terminate the lease when the Company is reasonably certain that the option will be exercised. Lease expense is recognized on a straight-line basis over the lease term. The Company elected to apply each of the practical expedients described in Accounting Standards Codification (ASC) Topic 842-10-65-1(f) which allow companies not to reassess: (i) whether any expired or existing agreements contain leases, (ii) the classification of any expired or existing leases, and (iii) the capitalization of initial direct costs for any existing leases. The Company also elected to apply the short-term lease measurement and recognition exemption in which ROU assets and lease liabilities are not recognized for short-term operating leases. A short-term is a lease that, at the commencement date, has a lease term of 12 months or less and does not include an option to purchase the underlying asset that the lessee is reasonably certain to exercise. Recent Accounting Pronouncements Adopted Standards In February 2016, the Financial Accounting Standards Board (FASB) issued Topic 842, which amended prior accounting standards for leases. The Company adopted Topic 842 on January 1, 2019, using the alternative modified transition method, which applies the standard as of the effective date and therefore, the Company has not applied the standard to the comparative periods presented on the Company's condensed financial statements . The Company elected the following practical expedients when assessing the transition impact available to lessees: (i) not to reassess whether any expired or existing contracts as of January 1, 2019, are or contain leases; (ii) not to reassess the lease classification for any expired or existing leases as of January 1, 2019; and (iii) not to reassess initial direct costs for any existing leases as of January 1, 2019. As a lessee, the primary impact of the adoption of Topic 842 was the recognition of operating lease ROU assets of $2.3 million and operating lease liabilities of $2.5 million on its condensed balance sheet as of January 1, 2019. See Note 4 " Leases " for additional details. Standards Not Yet Effective In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326) : Measurement of Credit Losses on Financial Instruments (ASU 2016-13). ASU 2016-13 implements an impairment model, known as the current expected credit loss model, based on expected losses rather than incurred losses. Under the new guidance, an entity will recognize, as an allowance, its estimate of expected credit losses. The ASU is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted. The Company does not expect the adoption of ASU 2016-13 will have a material impact on its financial statements. In September 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement , which amends ASC Topic 820, Fair Value Measurement . The FASB issued final guidance that eliminates, adds and modifies certain disclosure requirements for fair value measurements as part of its disclosure framework project. Under the ASU, entities will no longer be required to disclose the amount of transfers between Level 1 and Level 2 of the fair value hierarchy. Public companies will be required to disclose 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. ASU No. 2018-13 is effective for public business entities for annual reporting periods, and interim periods within those annual periods, beginning after December 15, 2019. Early adoption will be permitted in any interim or annual period. The Company plans to adopt this guidance on January 1, 2020. The new guidance only affects disclosures in the notes to the financial statements and will not affect the Company's financial statements.

FAIR VALUE MEASUREMENTS AND FAI

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS	FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS The fair value of the Company’s financial assets and liabilities are determined in accordance with the fair value hierarchy established in the FASB's ASC Topic 820, Fair Value Measurements and Disclosures . ASC Topic 820 defines fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The fair value hierarchy of ASC Topic 820 requires an entity to maximize the use of observable inputs when measuring fair value and classifies those inputs into three levels: Level 1 —Observable inputs, such as quoted prices in active markets; Level 2 —Inputs, other than the quoted prices in active markets, which are observable either directly or indirectly such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the instrument’s anticipated life; and Level 3 —Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions. The Company's policy is to recognize transfers in and out of Level 1, 2 and 3 as of the end of the reporting period. There were no transfers of assets or liabilities between the fair value measurement levels during the nine months ended September 30, 2019 . The Company's financial instruments consist primarily of cash, cash equivalents, short-term and long-term investments, accounts payable and the Term Loan with Hercules. Cash, cash equivalents and investments are reported at their respective fair values on the Company's condensed balance sheets. Where quoted prices are available in an active market, securities are classified as Level 1. The Company classifies money market funds as Level 1. When quoted market prices are not available for the specific security, then the Company estimates the fair value by using quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant inputs are observable in the market or can be corroborated by observable market data for substantially the full term of the assets. Where applicable, these models project future cash flows and discount the future amounts to a present value using market-based observable inputs obtained from various third-party data providers, including but not limited to benchmark yields, reported trades and broker/dealer quotes. Where applicable the market approach utilizes prices and information from market transactions for similar or identical assets. The Company classifies U.S. government agency securities, commercial paper, corporate debt securities and asset-backed securities as Level 2. The Company's short-term and long-term investments are classified as available-for-sale. The following tables set forth the fair value of the Company's cash and financial assets remeasured on a recurring basis based on the three-tier fair value hierarchy by significant investment category as of September 30, 2019 and December 31, 2018 . September 30, 2019 Reported as: (in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and Cash Equivalents Short-Term Investments Long-Term Investments Cash $ 1,644 $ — $ — $ 1,644 $ 1,644 $ — $ — Level 1: Money market fund 28,724 — — 28,724 28,724 — — Level 2: U.S. government agency securities 16,492 3 (2 ) 16,493 — 2,000 14,493 Commercial paper 139,242 164 (2 ) 139,404 — 139,404 — Corporate debt securities 161,438 309 (6 ) 161,741 — 134,892 26,849 Asset-backed securities 15,548 20 — 15,568 — 15,568 — Subtotal 332,720 496 (10 ) 333,206 — 291,864 41,342 Total assets measured at fair value $ 363,088 $ 496 $ (10 ) $ 363,574 $ 30,368 $ 291,864 $ 41,342 December 31, 2018 Reported as: (in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and Cash Equivalents Short-Term Investments Long-Term Investments Cash $ 3,021 $ — $ — $ 3,021 $ 3,021 $ — $ — Level 1: Money market fund 33,154 — — 33,154 33,154 — — Level 2: Commercial paper 68,467 — (63 ) 68,404 997 67,407 — Corporate debt securities 89,038 4 (63 ) 88,979 — 86,692 2,287 Asset-backed securities 49,838 3 (34 ) 49,807 — 49,807 — Subtotal 207,343 7 (160 ) 207,190 997 203,906 2,287 Total assets measured at fair value $ 243,518 $ 7 $ (160 ) $ 243,365 $ 37,172 $ 203,906 $ 2,287 Interest income related to the Company's cash, cash equivalents and available-for-sale investments included in other income (expense), net was approximately $2.5 million and $1.3 million for the three months ended September 30, 2019 and 2018 , respectively, and $6.7 million and $1.9 million for the nine months ended September 30, 2019 and 2018 , respectively. There were no gross realized gains and gross realized losses for the three and nine months ended September 30, 2019 and 2018 . The following table summarizes the Company's available-for-sale investments that were in a continuous unrealized loss position but were not deemed to be other-than-temporarily impaired, as of September 30, 2019 and December 31, 2018 . September 30, 2019 December 31, 2018 (in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses U.S. government agency securities $ 9,997 $ (2 ) $ — $ — Commercial paper 3,924 (2 ) 67,407 (63 ) Corporate debt securities 15,733 (6 ) 85,699 (63 ) Asset-backed securities — — 36,730 (34 ) Total $ 29,654 $ (10 ) $ 189,836 $ (160 ) The Company held a total of 10 and 48 positions which were in an unrealized loss position as of September 30, 2019 and December 31, 2018 , respectively. All available-for-sale investments in an unrealized loss position were in a continuous loss position for less than 12 months. As of September 30, 2019 , unrealized losses on available-for-sale investments were not attributable to credit risk. The Company determined that there were no other-than-temporary impairments as of September 30, 2019 because the Company does not intend to sell these securities nor does the Company believe that it will be required to sell these securities before the recovery of their amortized cost basis. The following table summarizes the maturities of the Company’s cash equivalents (excluding money market funds) and available-for-sale investments, as of September 30, 2019 . (in thousands) Amortized Cost Fair Value Mature in less than one year $ 291,396 $ 291,864 Mature in one to five years 41,324 41,342 Total $ 332,720 $ 333,206 The following table presents a reconciliation of financial liabilities measured at fair value on a recurring basis using Level 3 unobservable inputs for the nine months ended September 30, 2019 and 2018 . Nine Months Ended September 30, 2019 2018 (in thousands) Compound Derivative Liability Compound Derivative Liability Warrant Liability Fair value at beginning of period $ 161 $ — $ 106 Additions — 654 156 Change in fair value 259 (543 ) 305 Reclassification to equity — — (194 ) Issuance of convertible preferred stock on exercise of warrant — — (373 ) Fair value at end of period $ 420 $ 111 $ — The following table presents information about significant unobservable inputs related to the Company's Level 3 financial liabilities as of September 30, 2019 . September 30, 2019 (in thousands) Fair Value Valuation Technique Significant Unobservable Input Input Compound derivative liability $ 420 Discounted cash flow Discount rate 11.4 % Probability of the occurrence of certain events 10.0 % Term Loan The estimated fair value of the Term Loan was $39.7 million as of September 30, 2019 which approximates the carrying value and is classified as Level 3. The Company utilized a market yield analysis and income approach to estimate a value for the Term Loan. The key valuation assumptions used consist of the discount rate of 11.4% and the probability of the occurrence of certain events of 10.0% .

LEASES

LEASES	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
LEASES	LEASES As described in Note 2 . " Summary of Significant Accounting Policies ", the Company adopted Topic 842 as of January 1, 2019. Comparative prior period amounts have not been adjusted and are reported in accordance with historic accounting under Topic 840. In July 2014, the Company entered into a five -year noncancelable operating lease for its offices and laboratory space in South San Francisco, California that was scheduled to expire in June 2019, with an option for the Company to extend the lease for an additional three years . In August 2017, the Company entered into an amendment which extended the existing operating lease to June 2021 and added 13,258 square feet of additional lease space resulting in a total of 26,987 square feet being leased in the aggregate under the amended lease. In November 2017, the Company entered into a second amendment which reduced the common areas resulting in a total of 26,897 square feet being leased in aggregate under the second amendment. On August 14, 2019 , the Company entered into a third amendment to the existing operating lease which will extend the leased space by an additional 19,177 square feet, or Second Expansion Premises, which will result in a total of 46,074 square feet being leased in aggregate. The operating lease for the Second Expansion Premises will commence on the date they are delivered to the Company, which is expected to be September 1, 2020 (the Second Expansion Premises Commencement Date). In conjunction with the third amendment, the Company also agreed to lease 5,569 square feet of temporary office space effective August 15, 2019 to the Second Expansion Premises Commencement Date. The third amendment will extend the lease by 84 months from the Second Expansion Premises Commencement Date, with an option to extend the lease for an additional 36 months subject to certain conditions. The Company determined that the Second Expansion Premises shall be accounted for as a new lease at the Second Expansion Premises Commencement Date. Further, the Company determined that the amendment to the existing operating lease and temporary office space shall be accounted as a lease modification as of September 30, 2019. The Company recognized an operating lease ROU asset of $8.1 million and operating lease liability of $8.1 million on its condensed balance sheet upon the execution of the third amendment on August 14, 2019, and will measure and record an additional ROU asset and operating lease liability for the Second Expansion Premises upon the Second Expansion Premises Commencement Date. Operating lease expense was $0.4 million and $0.9 million for the three and nine months ended September 30, 2019 , respectively. Operating cash flows for the nine months ended September 30, 2019 included $0.8 million in cash payments for operating leases. Expense related to short-term leases was not significant for the three and nine months ended September 30, 2019 . The following table presents the maturity analysis of the Company's operating lease liabilities showing the aggregate lease payments as of September 30, 2019 . (in thousands) September 30, 2019 2019 (remaining three months) $ 273 2020 1,107 2021 1,377 2022 1,662 2023 and thereafter 8,434 Total lease payments (1) 12,853 Less: imputed interest (2,875 ) Total operating lease liabilities $ 9,978 (1) As noted above, The operating lease for the Second Expansion Premises will commence in fiscal year 2020 and therefore the lease related to the Second Expansion Premises is not recognized on the condensed consolidated balance sheet as of September 30, 2019. As of September 30, 2019, future minimum lease payments related to the Second Expansion Premises are expected to be $8.2 million over the lease term of 7.0 years . Operating lease liabilities are based on the net present value of the remaining lease payments over the remaining lease term. In determining the present value of lease payments, the Company uses its incremental borrowing rate. The weighted average discount rate used to determine the operating lease liabilities as of September 30, 2019 was 6.0% . The Company's weighted average remaining lease term was 7.9 years as of September 30, 2019 . ASC Topic 840 Disclosures The Company elected the alternative modified transition method and is required to present previously disclosed information under the prior accounting standard for leases. The following table presents the future minimum lease commitments under the Company’s operating leases as of December 31, 2018, as previously disclosed. (in thousands) December 31, 2018 2019 $ 1,076 2020 1,108 2021 562 2022 and thereafter — Total future minimum lease payments $ 2,746

OTHER BALANCE SHEET COMPONENTS

OTHER BALANCE SHEET COMPONENTS	9 Months Ended
	Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
OTHER BALANCE SHEET COMPONENTS	OTHER BALANCE SHEET COMPONENTS Property and Equipment, Net The following table presents the components of property and equipment, net as of September 30, 2019 and December 31, 2018 . (in thousands) September 30, December 31, Furniture and fixtures $ 210 $ 193 Computer and lab equipment 3,125 1,888 Leasehold improvements 1,163 1,055 4,498 3,136 Less: accumulated depreciation and amortization (2,445 ) (1,921 ) Total property and equipment, net $ 2,053 $ 1,215 Depreciation and amortization expense was approximately $0.2 million and $0.2 million for the three months ended September 30, 2019 and 2018 , respectively, and $0.5 million for the nine months ended September 30, 2019 and 2018 . Accrued Expenses and Other Current Liabilities The following table presents the components of accrued expenses and other current liabilities as of September 30, 2019 and December 31, 2018 . (in thousands) September 30, December 31, Accrued clinical and nonclinical study costs $ 1,679 $ 2,168 Accrued contract manufacturing 8,081 1,676 Accrued compensation 2,890 1,565 Accrued professional fees and other 2,951 935 Total accrued expenses and other current liabilities $ 15,601 $ 6,344

BORROWINGS

BORROWINGS	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
BORROWINGS	BORROWINGS Term Loan On February 28, 2018 , the Company entered into the Term Loan with Hercules. The Term Loan provided for a loan in an aggregate principal amount of up to $100.0 million to be funded in five tranches subject to certain performance‑based milestones. The first tranche, in the amount of $25.0 million , was funded on the closing date of the Term Loan. On October 15, 2018, the Company entered into the second amendment to the Term Loan with Hercules, which amended certain terms of the Term Loan. After giving effect to the second amendment, the Term Loan continued to provide for a loan in an aggregate principal amount of up to $100.0 million to be funded in five tranches subject to certain performance‑based milestones. The second tranche was reduced from $25.0 million to $15.0 million and was funded on December 28, 2018 . The Company accounted for the second amendment as a modification to the existing Term Loan. On March 27, 2019 , the Company modified the Term Loan with Hercules by entering into the third amendment to the Term Loan. After giving effect to the third amendment, the amount available under the Term Loan is increased from up to $100.0 million to up to $200.0 million to be funded in tranches, subject to certain performance-based milestones, and the maturity of the Term Loan is extended. Under the terms of the Term Loan, as amended by the third amendment, the $40.0 million principal outstanding remains outstanding, and additional tranches of $20.0 million and $15.0 million will be available for draw down prior to December 15, 2019 and December 15, 2020, respectively. An additional tranche of $75.0 million will be available for draw down between January 1, 2020 and December 15, 2020, on the condition that the Company obtains final approval from the FDA for the NDA for veverimer. A final tranche of $50.0 million will be available for draw down on or prior to December 15, 2021, upon request by the Company and the approval of Hercules' investment committee. The Company accounted for the third amendment as a modification to the existing Term Loan. The Term Loan bears interest at a floating per annum interest rate equal to the greater of either (i) 8.35% or (ii) the lesser of (x) 8.35% plus the prime rate as reported in The Wall Street Journal minus 6.00% and (y) 9.85% . The maturity date is extended to April 1, 2023 and may be extended to April 1, 2024 if the tranche of $75.0 million described above is drawn. The Company will initially be making interest-only payments until April 1, 2021. If the Company achieves certain performance milestones and financial covenants, the interest-only period could be extended for up to an additional 24 months. Upon expiration of the interest-only period, the Company will repay the Term Loan in equal monthly installments comprised of principal and interest, based on a 30 -month amortization schedule, through maturity. The Company will pay an additional amount of (a) $2.6 million due on March 1, 2022 and (b) the product of 7.55% and the aggregate loans funded under the Term Loan due at maturity or on any earlier date on which the loans become due. If the Company prepays the Term Loan, the Company will be required to pay a prepayment charge equal to (i) 2.00% of the amount being prepaid at any time during the first 12 months following the effective date of the third amendment (ii) 1.50% of the amount being prepaid after 12 months but prior to 24 months following the effective date of the third amendment (iii) 1.00% of the amount being prepaid after 24 months but prior to 36 months following the effective date of the third amendment and (iv) zero if prepaid any time after 36 months following the effective date of the third amendment but prior to the maturity. The Term Loan is secured by substantially all of the Company's assets, except its intellectual property, which is the subject of a negative pledge; however, the collateral does consist of rights to payments and proceeds from the sale, licensing or disposition of all or any part of, or rights in, its intellectual property. Under the Term Loan, the Company is subject to certain covenants, including but not limited to requirements to deliver financial reports at designated times of the year and maintain a minimum level of cash. These covenants also limit or restrict the Company's ability to incur additional indebtedness or liens, acquire, own or make any investments, pay cash dividends, repurchase stock or enter into certain corporate transactions, including mergers and changes of control. Warrants In conjunction with the Term Loan entered into on February 28, 2018 , the Company issued warrants to Hercules to purchase 53,458 shares of its common stock with an exercise price of $9.35 per share. The estimated fair value of the warrants at the date of issuance was approximately $0.2 million . The fair value of the common stock warrant liability was determined using the probability‑weighted expected return method. It was recorded at its fair value at inception and was remeasured at each financial reporting period with any changes in fair value being recognized as a component of other income (expense), net in the accompanying condensed statements of operations and comprehensive loss. On April 10, 2018, the Company entered into amendments with Hercules that resulted in the reclassification of the warrant liability to stockholders' equity (deficit) as the amended terms of the warrants qualified for them to be accounted for as equity instruments and, as such, were no longer subject to remeasurement. The fair value of the common stock warrants of approximately $0.2 million was reclassified to stockholders' equity (deficit) upon execution of the amendment. In connection with the funding of the second tranche on December 28, 2018 , the Company issued to Hercules a warrant to purchase 53,458 shares of its common stock with an exercise price of $9.35 per share. The common stock warrant was recorded in stockholders' equity (deficit) at its fair value of approximately $0.9 million on December 28, 2018 . In conjunction with the third amendment, the Company issued warrants to Hercules to purchase 16,721 shares of its common stock with an exercise price of $23.92 per share. The common stock warrants were recorded in stockholders' equity (deficit) at its fair value of approximately $0.3 million on March 27, 2019. The fair value of the common stock warrants were determined using an option-pricing model with the following assumptions: time to liquidity of 7.0 years, volatility of 75.0% , risk-free rate of 2.3% and stock price based on the March 27, 2019 closing price of the Company's common stock reported by The Nasdaq Global Select Market. In connection with each subsequent draw down under the tranches described above, the Company is obligated to issue additional warrants to purchase a number of shares of the Company's common stock determined by dividing (x) an amount equal to 1.0% of the principal amount of the applicable tranche by (y) $23.92 subject to adjustments following certain corporate events. Embedded Derivatives and Other Debt Issuance Costs The Company determined that certain loan features were embedded derivatives requiring bifurcation and separate accounting. Those embedded derivatives were bundled together as a single, compound embedded derivative and then bifurcated and accounted for separately from the host contract. The Company initially recorded a compound derivative liability of $0.7 million , which is required to be marked to market in future periods. As of September 30, 2019 , the Company calculated the fair values of the compound derivative by computing the difference between the fair value of the Term Loan with the compound derivative using the “with and without” method under the income approach, and the fair value of the Term Loan without the compound derivative. The Company calculated the fair values using a probability‑weighted discounted cash flow analysis. The key valuation assumptions used consist of the discount rate of 11.4% and the probability of the occurrence of certain events of 10.0% . The compound derivative liability is being remeasured at each financial reporting period with any changes in fair value being recognized as a component of other income (expense), net in the condensed statements of operations and comprehensive loss. The fair value of the compound derivative liability was approximately $0.4 million as of September 30, 2019 and was classified as other long-term liabilities on the condensed balance sheet. The facility fee, fair value of warrants at issuance, fair value of embedded derivatives which were bifurcated, and other debt issuance costs have been treated as debt discounts on the Company’s condensed balance sheets and together with the additional payment are being amortized to interest expense throughout the life of the Term Loan using the effective interest rate method. As of September 30, 2019 and December 31, 2018 , there were unamortized issuance costs and debt discounts of $3.6 million and $2.7 million , respectively, which were recorded as a direct deduction from the Term Loan on the condensed balance sheets. The following table presents future payments of principal and interest on the Term Loan as of September 30, 2019 . (in thousands) September 30, 2019 2019 (remaining three months) $ 844 2020 3,396 2021 14,173 2022 20,403 2023 16,282 55,098 Less: amount representing interest (15,098 ) Present value of Term Loan 40,000 Less: current portion — Long-term portion of Term Loan $ 40,000 Convertible Preferred Stock Warrant Liability The Company entered into a Note and Warrant Purchase Agreement with Sibling Co—Investment LLC, or Sibling, in 2013, the principal and interest of which was subsequently converted into the Company’s Series A convertible preferred stock in the same year. In accordance with the agreement a warrant to purchase 95,936 shares of Series A convertible preferred stock was established in conjunction with the Series A financing round. The warrant had a contractual life of 7 years and an exercise price of $0.886 . The fair value of the warrant liability was determined using a Black-Scholes option pricing model and was recorded at its fair value at inception and remeasured at each financial reporting period with any changes in fair value being recognized as a component of other income (expense), net in the accompanying statement of operations and comprehensive loss. On June 16, 2018, Sibling provided the Notice of Exercise and purchased 95,936 shares of Series A convertible preferred stock. The fair value adjustment recognized upon exercise was determined using the intrinsic value which was calculated as the initial public offering, or IPO, price of $19.00 less the warrant exercise price, with the change in fair value being recognized as a component of other income (expense), net in the condensed statements of operations and comprehensive loss. The resultant fair value was reclassified to Series A convertible preferred stock on June 16, 2018.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Facilities The Company has an operating lease for its offices and laboratory space in South San Francisco. See Note 4 . " Leases " for additional information about this lease. In addition, associated with the operating lease, the Company has a tenant improvement loan with remaining payments totaling approximately $0.2 million as of September 30, 2019 . Other Commitments On May 8, 2018, the Company and Patheon Austria GmbH & Co KG, or Patheon, entered into a master development/validation services and clinical/launch supply agreement, or MDS, pursuant to which Patheon will manufacture and supply the Company drug substances. Statements of work under the MDS commit the Company to certain purchase obligations of approximately $134.2 million , of which $22.8 million was paid through September 30, 2019 and $111.4 million will be paid over the next 24 months, with approximately one-half of this amount occurring in each of the two successive 12 -month periods. On October 4, 2019, the Company and Patheon entered into the Manufacturing and Commercial Supply Agreement under which Patheon agreed to manufacture and supply veverimer to support the Company's commercialization efforts. See Note 10 " Subsequent Events " for additional information. The Company also enters into other contracts in the normal course of business with contract research organizations, contract development and manufacturing organizations and other service providers and vendors. These contracts generally provide for termination on short notice and are cancelable contracts and accordingly, are not included in the contractual obligations and disclosures summarized above. Contingencies While there are no legal proceedings the Company is aware of, the Company may become party to various claims and complaints arising in the ordinary course of business. The Company does not believe that any ultimate liability resulting from any of these claims will have a material adverse effect on its results of operations, financial position, or liquidity. However, the Company cannot give any assurance regarding the ultimate outcome of these claims, and their resolution could be material to operating results for any particular period, depending upon the level of income for the period. Guarantees and Indemnifications The Company indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director and officer liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, it has not recognized any liabilities relating to these obligations for any period presented.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS' EQUITY On July 2, 2018, the Company’s amended and restated certificate of incorporation became effective, authorizing the Company to issue a total of 440,000,000 shares of all classes of capital stock, consisting of 400,000,000 shares of common stock, par value $0.001 per share, and 40,000,000 shares of preferred stock, par value $0.001 per share. As of September 30, 2019 and December 31, 2018 , the Company had 49,531,916 and 42,148,247 shares of common stock outstanding, respectively. As of September 30, 2019 , and December 31, 2018 , the Company had no shares of preferred stock outstanding. Common Stock On July 2, 2018 , the Company completed its IPO and issued 13,455,000 shares of common stock at an offering price of $19.00 per share for net proceeds of approximately $237.7 million , after deducting underwriting discounts and commissions of $17.9 million . Upon the closing of the IPO, all of the 104,225,638 shares of convertible preferred stock outstanding were automatically converted on a 1:3.98 basis into 26,187,321 shares of common stock. On April 8, 2019 , the Company consummated an underwritten public offering and issued 6,440,000 shares of common stock, which included the exercise in full by the underwriters of their option to purchase 840,000 additional shares of common stock at an offering price of $36.00 per share for net proceeds of approximately $217.9 million , after deducting underwriting discounts and commissions of $13.9 million . Common stock reserved for future issuance as of September 30, 2019 and December 31, 2018 , consisted of the following. September 30, December 31, Stock options and RSUs issued and outstanding 6,685,989 4,599,307 Stock options, RSUs and ESPP shares authorized for future issuance 3,611,844 4,534,784 Total 10,297,833 9,134,091

NET LOSS PER SHARE

NET LOSS PER SHARE	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
NET LOSS PER SHARE	NET LOSS PER SHARE The following table sets forth the computation of the basic and diluted net loss per share attributable to common stockholders for the three and nine months ended September 30, 2019 and 2018 . Three Months Ended Nine Months Ended (In thousands, except share and per share amounts) 2019 2018 2019 2018 Numerator: Net loss $ (44,119 ) $ (29,098 ) $ (118,642 ) $ (74,964 ) Denominator: Weighted average common shares outstanding 49,425,382 41,295,394 46,824,045 15,436,198 Less: weighted average shares subject to repurchase (7,357 ) (33,691 ) (10,169 ) (21,004 ) Weighted average number of shares used in basic and diluted net loss per share 49,418,025 41,261,703 46,813,876 15,415,194 Net loss per share, basic and diluted $ (0.89 ) $ (0.71 ) $ (2.53 ) $ (4.86 ) Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share for all periods as the inclusion of all potential common shares outstanding would have been anti-dilutive. The following weighted-average outstanding common stock equivalents were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive. September 30, 2019 2018 Warrants to purchase common stock 123,637 53,458 Common stock subject to repurchase 6,300 28,793 Options and RSUs issued and outstanding 6,685,989 4,580,888 Total potential common shares excluded from the computation of diluted net loss per share 6,815,926 4,663,139

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	9 Months Ended
	Sep. 30, 2019
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS On October 4, 2019, the Company entered into a Manufacturing and Commercial Supply Agreement, or the Supply Agreement, with Patheon. Under the Supply Agreement, Patheon has agreed to manufacture and supply veverimer to support the Company’s commercialization efforts. Patheon has also agreed to manufacture and supply veverimer to support the Company’s drug development and clinical trial activities. The Company’s obligation to purchase veverimer is subject to minimum and maximum annual commitments, with the minimum commitments subject to reduction in certain circumstances. The Company has agreed to reimburse Patheon for certain capital expenditures Patheon incurs to make improvements to its manufacturing facility as required to meet the Company’s supply requirements. The Supply Agreement has an initial term ending on June 30, 2030, and will automatically extend for two three-year renewal periods, unless a notice of non-renewal is delivered at least three years before the expiration of the initial term or the first renewal period, as applicable. The Company is currently evaluating the accounting impact and disclosure requirements associated with the Supply Agreement.

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation —The accompanying unaudited condensed financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP) for interim financial information and with the instructions to Form 10-Q and Article 10 of Regulation S-X. The condensed balance sheet as of September 30, 2019 , the condensed statements of operations and comprehensive loss for the three and nine months ended September 30, 2019 and 2018 , the condensed statements of convertible preferred stock and stockholders' equity (deficit) for the three and nine months ended September 30, 2019 and 2018 and the condensed statements of cash flows for the nine months ended September 30, 2019 and 2018 are unaudited, but include all adjustments, consisting only of normal recurring adjustments, which the Company consider necessary for a fair presentation of the financial position, operating results and cash flows for the periods presented. The condensed balance sheet at December 31, 2018 has been derived from audited financial statements.
Leases	Leases —The Company determines if an arrangement contains a lease at inception. For arrangements where the Company is the lessee, operating leases are included in operating lease right-of-use (ROU) assets; current operating lease liabilities; and non-current operating lease liabilities on its condensed balance sheets. The Company currently does not have any finance leases. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at the commencement date. ROU assets also include any initial direct costs incurred and any lease payments made on or before the lease commencement date, less lease incentives received. The Company uses its incremental borrowing rate based on the information available at the commencement date in determining the lease liabilities as the Company’s leases generally do not provide an implicit rate. The incremental borrowing rate is reevaluated upon a lease modification. The operating lease ROU asset also includes any initial direct costs and prepaid lease payments made less any lease incentives. The Company considered information available at the adoption date of Topic 842 to determine the incremental borrowing rate for leases in existence as of this date. Lease terms may include options to extend or terminate the lease when the Company is reasonably certain that the option will be exercised. Lease expense is recognized on a straight-line basis over the lease term. The Company elected to apply each of the practical expedients described in Accounting Standards Codification (ASC) Topic 842-10-65-1(f) which allow companies not to reassess: (i) whether any expired or existing agreements contain leases, (ii) the classification of any expired or existing leases, and (iii) the capitalization of initial direct costs for any existing leases. The Company also elected to apply the short-term lease measurement and recognition exemption in which ROU assets and lease liabilities are not recognized for short-term operating leases. A short-term is a lease that, at the commencement date, has a lease term of 12 months or less and does not include an option to purchase the underlying asset that the lessee is reasonably certain to exercise.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Adopted Standards In February 2016, the Financial Accounting Standards Board (FASB) issued Topic 842, which amended prior accounting standards for leases. The Company adopted Topic 842 on January 1, 2019, using the alternative modified transition method, which applies the standard as of the effective date and therefore, the Company has not applied the standard to the comparative periods presented on the Company's condensed financial statements . The Company elected the following practical expedients when assessing the transition impact available to lessees: (i) not to reassess whether any expired or existing contracts as of January 1, 2019, are or contain leases; (ii) not to reassess the lease classification for any expired or existing leases as of January 1, 2019; and (iii) not to reassess initial direct costs for any existing leases as of January 1, 2019. As a lessee, the primary impact of the adoption of Topic 842 was the recognition of operating lease ROU assets of $2.3 million and operating lease liabilities of $2.5 million on its condensed balance sheet as of January 1, 2019. See Note 4 " Leases " for additional details. Standards Not Yet Effective In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326) : Measurement of Credit Losses on Financial Instruments (ASU 2016-13). ASU 2016-13 implements an impairment model, known as the current expected credit loss model, based on expected losses rather than incurred losses. Under the new guidance, an entity will recognize, as an allowance, its estimate of expected credit losses. The ASU is effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted. The Company does not expect the adoption of ASU 2016-13 will have a material impact on its financial statements. In September 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement , which amends ASC Topic 820, Fair Value Measurement . The FASB issued final guidance that eliminates, adds and modifies certain disclosure requirements for fair value measurements as part of its disclosure framework project. Under the ASU, entities will no longer be required to disclose the amount of transfers between Level 1 and Level 2 of the fair value hierarchy. Public companies will be required to disclose 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. ASU No. 2018-13 is effective for public business entities for annual reporting periods, and interim periods within those annual periods, beginning after December 15, 2019. Early adoption will be permitted in any interim or annual period. The Company plans to adopt this guidance on January 1, 2020. The new guidance only affects disclosures in the notes to the financial statements and will not affect the Company's financial statements.

FAIR VALUE MEASUREMENTS AND F_2

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIALINSTRUMENTS (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Cash and financial assets remeasured on a recurring basis by hierarchy	The following tables set forth the fair value of the Company's cash and financial assets remeasured on a recurring basis based on the three-tier fair value hierarchy by significant investment category as of September 30, 2019 and December 31, 2018 . September 30, 2019 Reported as: (in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and Cash Equivalents Short-Term Investments Long-Term Investments Cash $ 1,644 $ — $ — $ 1,644 $ 1,644 $ — $ — Level 1: Money market fund 28,724 — — 28,724 28,724 — — Level 2: U.S. government agency securities 16,492 3 (2 ) 16,493 — 2,000 14,493 Commercial paper 139,242 164 (2 ) 139,404 — 139,404 — Corporate debt securities 161,438 309 (6 ) 161,741 — 134,892 26,849 Asset-backed securities 15,548 20 — 15,568 — 15,568 — Subtotal 332,720 496 (10 ) 333,206 — 291,864 41,342 Total assets measured at fair value $ 363,088 $ 496 $ (10 ) $ 363,574 $ 30,368 $ 291,864 $ 41,342 December 31, 2018 Reported as: (in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Cash and Cash Equivalents Short-Term Investments Long-Term Investments Cash $ 3,021 $ — $ — $ 3,021 $ 3,021 $ — $ — Level 1: Money market fund 33,154 — — 33,154 33,154 — — Level 2: Commercial paper 68,467 — (63 ) 68,404 997 67,407 — Corporate debt securities 89,038 4 (63 ) 88,979 — 86,692 2,287 Asset-backed securities 49,838 3 (34 ) 49,807 — 49,807 — Subtotal 207,343 7 (160 ) 207,190 997 203,906 2,287 Total assets measured at fair value $ 243,518 $ 7 $ (160 ) $ 243,365 $ 37,172 $ 203,906 $ 2,287
Available-for-sale investment in a continuous unrealized loss position but not deemed OTTI	The following table summarizes the Company's available-for-sale investments that were in a continuous unrealized loss position but were not deemed to be other-than-temporarily impaired, as of September 30, 2019 and December 31, 2018 . September 30, 2019 December 31, 2018 (in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses U.S. government agency securities $ 9,997 $ (2 ) $ — $ — Commercial paper 3,924 (2 ) 67,407 (63 ) Corporate debt securities 15,733 (6 ) 85,699 (63 ) Asset-backed securities — — 36,730 (34 ) Total $ 29,654 $ (10 ) $ 189,836 $ (160 )
Maturities of cash equivalents and available-for-sale investments	The following table summarizes the maturities of the Company’s cash equivalents (excluding money market funds) and available-for-sale investments, as of September 30, 2019 . (in thousands) Amortized Cost Fair Value Mature in less than one year $ 291,396 $ 291,864 Mature in one to five years 41,324 41,342 Total $ 332,720 $ 333,206
Reconciliation of liabilities using Level 3 unobservable inputs	The following table presents a reconciliation of financial liabilities measured at fair value on a recurring basis using Level 3 unobservable inputs for the nine months ended September 30, 2019 and 2018 . Nine Months Ended September 30, 2019 2018 (in thousands) Compound Derivative Liability Compound Derivative Liability Warrant Liability Fair value at beginning of period $ 161 $ — $ 106 Additions — 654 156 Change in fair value 259 (543 ) 305 Reclassification to equity — — (194 ) Issuance of convertible preferred stock on exercise of warrant — — (373 ) Fair value at end of period $ 420 $ 111 $ —
Significant fair value unobservable inputs	The following table presents information about significant unobservable inputs related to the Company's Level 3 financial liabilities as of September 30, 2019 . September 30, 2019 (in thousands) Fair Value Valuation Technique Significant Unobservable Input Input Compound derivative liability $ 420 Discounted cash flow Discount rate 11.4 % Probability of the occurrence of certain events 10.0 %

LEASES (Tables)

LEASES (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Operating lease maturity schedule	The following table presents the maturity analysis of the Company's operating lease liabilities showing the aggregate lease payments as of September 30, 2019 . (in thousands) September 30, 2019 2019 (remaining three months) $ 273 2020 1,107 2021 1,377 2022 1,662 2023 and thereafter 8,434 Total lease payments (1) 12,853 Less: imputed interest (2,875 ) Total operating lease liabilities $ 9,978 (1) As noted above, The operating lease for the Second Expansion Premises will commence in fiscal year 2020 and therefore the lease related to the Second Expansion Premises is not recognized on the condensed consolidated balance sheet as of September 30, 2019. As of September 30, 2019, future minimum lease payments related to the Second Expansion Premises are expected to be $8.2 million over the lease term of 7.0 years .
Operating lease maturity schedule under prior guidance	The following table presents the future minimum lease commitments under the Company’s operating leases as of December 31, 2018, as previously disclosed. (in thousands) December 31, 2018 2019 $ 1,076 2020 1,108 2021 562 2022 and thereafter — Total future minimum lease payments $ 2,746

OTHER BALANCE SHEET COMPONENTS

OTHER BALANCE SHEET COMPONENTS (Tables)	9 Months Ended
	Sep. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Property and Equipment, Net	The following table presents the components of property and equipment, net as of September 30, 2019 and December 31, 2018 . (in thousands) September 30, December 31, Furniture and fixtures $ 210 $ 193 Computer and lab equipment 3,125 1,888 Leasehold improvements 1,163 1,055 4,498 3,136 Less: accumulated depreciation and amortization (2,445 ) (1,921 ) Total property and equipment, net $ 2,053 $ 1,215
Schedule of Accrued Expenses and Other Current Liabilities	The following table presents the components of accrued expenses and other current liabilities as of September 30, 2019 and December 31, 2018 . (in thousands) September 30, December 31, Accrued clinical and nonclinical study costs $ 1,679 $ 2,168 Accrued contract manufacturing 8,081 1,676 Accrued compensation 2,890 1,565 Accrued professional fees and other 2,951 935 Total accrued expenses and other current liabilities $ 15,601 $ 6,344

BORROWINGS (Tables)

BORROWINGS (Tables)	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Schedule of Future Payments of Principal and Interest	The following table presents future payments of principal and interest on the Term Loan as of September 30, 2019 . (in thousands) September 30, 2019 2019 (remaining three months) $ 844 2020 3,396 2021 14,173 2022 20,403 2023 16,282 55,098 Less: amount representing interest (15,098 ) Present value of Term Loan 40,000 Less: current portion — Long-term portion of Term Loan $ 40,000

STOCKHOLDERS' EQUITY (Tables)

STOCKHOLDERS' EQUITY (Tables)	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Schedule of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance as of September 30, 2019 and December 31, 2018 , consisted of the following. September 30, December 31, Stock options and RSUs issued and outstanding 6,685,989 4,599,307 Stock options, RSUs and ESPP shares authorized for future issuance 3,611,844 4,534,784 Total 10,297,833 9,134,091

NET LOSS PER SHARE (Tables)

NET LOSS PER SHARE (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Basic and diluted net loss per share	The following table sets forth the computation of the basic and diluted net loss per share attributable to common stockholders for the three and nine months ended September 30, 2019 and 2018 . Three Months Ended Nine Months Ended (In thousands, except share and per share amounts) 2019 2018 2019 2018 Numerator: Net loss $ (44,119 ) $ (29,098 ) $ (118,642 ) $ (74,964 ) Denominator: Weighted average common shares outstanding 49,425,382 41,295,394 46,824,045 15,436,198 Less: weighted average shares subject to repurchase (7,357 ) (33,691 ) (10,169 ) (21,004 ) Weighted average number of shares used in basic and diluted net loss per share 49,418,025 41,261,703 46,813,876 15,415,194 Net loss per share, basic and diluted $ (0.89 ) $ (0.71 ) $ (2.53 ) $ (4.86 )
Antidilutive securities excluded from computation	The following weighted-average outstanding common stock equivalents were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented because including them would have been antidilutive. September 30, 2019 2018 Warrants to purchase common stock 123,637 53,458 Common stock subject to repurchase 6,300 28,793 Options and RSUs issued and outstanding 6,685,989 4,580,888 Total potential common shares excluded from the computation of diluted net loss per share 6,815,926 4,663,139

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recent accounting pronouncements (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Aug. 14, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Operating lease ROU asset	$ 9,694	$ 8,100	$ 2,300
Total operating lease liabilities	$ 9,978	$ 8,100	$ 2,500

FAIR VALUE MEASUREMENTS AND F_3

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2019USD ($)Position	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)Position	Sep. 30, 2018USD ($)	Dec. 31, 2018Position
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment interest income	$ 2,500,000	$ 1,300,000	$ 6,700,000	$ 1,900,000
Gross realized gains and gross realized losses on Available-for-sale securities	$ 0	$ 0	$ 0	$ 0
Debt instruments held in continuous unrealized loss position | Position	10		10		48
Discount rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, measurement input	0.114		0.114
Term Loan | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument, fair value	$ 39,700,000		$ 39,700,000
Minimum | Discount rate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, measurement input	0.100		0.100
Maximum | Discount rate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, measurement input	0.114		0.114

FAIR VALUE MEASUREMENTS AND F_4

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Marketable securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash	$ 30,368	$ 37,172
Level 2:
Short-term investments	291,864	203,906
Long-term investments	41,342	2,287
Fair Value, Measurements, Recurring
Total assets measured at fair value
Amortized Cost	363,088	243,518
Gross Unrealized Gains	496	7
Gross Unrealized Losses	(10)	(160)
Total assets measured at fair value	363,574	243,365
Cash | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	1,644	3,021
Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	30,368	37,172
Cash and Cash Equivalents | Cash | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	1,644	3,021
Level 1:
Money market fund	1,644	3,021
Short-term Investments | Fair Value, Measurements, Recurring
Level 2:
Short-term investments	291,864	203,906
Investments | Fair Value, Measurements, Recurring
Level 2:
Long-term investments	41,342	2,287
Level 1 | Money market fund | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	28,724	33,154
Level 1:
Money market fund	28,724	33,154
Level 1 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	28,724	33,154
Level 2
Level 2:
Gross Unrealized Losses	(10)	(160)
Level 2 | U.S. government agency securities
Level 2:
Gross Unrealized Losses	(2)	0
Level 2 | Commercial paper
Level 2:
Gross Unrealized Losses	(2)	(63)
Level 2 | Corporate debt securities
Level 2:
Gross Unrealized Losses	(6)	(63)
Level 2 | Asset-backed securities
Level 2:
Gross Unrealized Losses	0	(34)
Level 2 | Fair Value, Measurements, Recurring
Level 2:
Amortized Cost	332,720	207,343
Gross Unrealized Gains	496	7
Gross Unrealized Losses	(10)	(160)
Estimated Fair Value	333,206	207,190
Level 2 | Fair Value, Measurements, Recurring | U.S. government agency securities
Level 2:
Amortized Cost	16,492
Gross Unrealized Gains	3
Gross Unrealized Losses	(2)
Estimated Fair Value	16,493
Level 2 | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Amortized Cost	139,242	68,467
Gross Unrealized Gains	164	0
Gross Unrealized Losses	(2)	(63)
Estimated Fair Value	139,404	68,404
Level 2 | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Amortized Cost	161,438	89,038
Gross Unrealized Gains	309	4
Gross Unrealized Losses	(6)	(63)
Estimated Fair Value	161,741	88,979
Level 2 | Fair Value, Measurements, Recurring | Asset-backed securities
Level 2:
Amortized Cost	15,548	49,838
Gross Unrealized Gains	20	3
Gross Unrealized Losses	0	(34)
Estimated Fair Value	15,568	49,807
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Level 2:
Estimated Fair Value	0	997
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | U.S. government agency securities
Level 2:
Estimated Fair Value	0
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Estimated Fair Value	0	997
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Estimated Fair Value	0	0
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Asset-backed securities
Level 2:
Estimated Fair Value	0	0
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring
Level 2:
Short-term investments	291,864	203,906
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring | U.S. government agency securities
Level 2:
Short-term investments	2,000
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Short-term investments	139,404	67,407
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Short-term investments	134,892	86,692
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring | Asset-backed securities
Level 2:
Short-term investments	15,568	49,807
Level 2 | Investments | Fair Value, Measurements, Recurring
Level 2:
Long-term investments	41,342	2,287
Level 2 | Investments | Fair Value, Measurements, Recurring | U.S. government agency securities
Level 2:
Long-term investments	14,493
Level 2 | Investments | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Long-term investments	0	0
Level 2 | Investments | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Long-term investments	26,849	2,287
Level 2 | Investments | Fair Value, Measurements, Recurring | Asset-backed securities
Level 2:
Long-term investments	$ 0	$ 0

FAIR VALUE MEASUREMENTS AND F_5

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Available-for-sale investments in continuous unrealized loss position not deemed to be OTTI (Details) - Level 2 - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 29,654	$ 189,836
Unrealized Losses	(10)	(160)
U.S. government agency securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	9,997	0
Unrealized Losses	(2)	0
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,924	67,407
Unrealized Losses	(2)	(63)
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	15,733	85,699
Unrealized Losses	(6)	(63)
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	36,730
Unrealized Losses	$ 0	$ (34)

FAIR VALUE MEASUREMENTS AND F_6

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Maturities of cash equivalents and available-for-sale investments (Details) $ in Thousands	Sep. 30, 2019USD ($)
Amortized Cost
Mature in less than one year	$ 291,396
Mature in one to five years	41,324
Total	332,720
Fair Value
Mature in less than one year	291,864
Mature in one to five years	41,342
Total	$ 333,206

FAIR VALUE MEASUREMENTS AND F_7

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Reconciliation of liabilities measured using Level 3 (Details) - Level 3 - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Compound Derivative
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	$ 161	$ 0
Addition	0	654
Change in fair value	259	(543)
Reclassification to equity	0	0
Issuance of convertible preferred stock on exercise of warrant	0	0
Fair value at end of period	$ 420	111
Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period		106
Addition		156
Change in fair value		305
Reclassification to equity		(194)
Issuance of convertible preferred stock on exercise of warrant		(373)
Fair value at end of period		$ 0

FAIR VALUE MEASUREMENTS AND F_8

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Significant unobservable inputs related to level 3 assets and liabilities (Details) $ in Thousands	Sep. 30, 2019USD ($)
Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.114
Measurement input, probability of occurrence of certain events
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.100
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Compound derivative liability	$ 420
Level 3 | Measurement input, probability of occurrence of certain events
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.100
Minimum | Level 3 | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.100
Maximum | Level 3 | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.114

LEASES - Information related to

LEASES - Information related to operating lease cost (Details) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019USD ($)	Sep. 30, 2019USD ($)	Sep. 01, 2020ft²	Aug. 15, 2019ft²	Aug. 14, 2019USD ($)ft²	Jan. 01, 2019USD ($)	Nov. 30, 2017ft²	Aug. 31, 2017ft²	Jul. 31, 2014
Lessee, Lease, Description [Line Items]
Term of operating lease									5 years
Operating lease renewal term									3 years
Extension of square footage leased					19,177
Term of third lease amendment lease					84 months
Term of operating lease not yet commenced	36 months	36 months
Temporary office space leased				5,569
Operating lease ROU asset | $	$ 9,694	$ 9,694			$ 8,100	$ 2,300
Total operating lease liabilities | $	9,978	9,978			$ 8,100	$ 2,500
Operating lease expense | $	$ 400	900
Cash paid within operating cash flows for the three months ended March 31, 2019 | $		$ 800
Property Subject to Operating Lease
Lessee, Lease, Description [Line Items]
Lease rentable area (in square feet)								26,987
Amended Lease | Property Subject to Operating Lease
Lessee, Lease, Description [Line Items]
Lease rentable area (in square feet)								13,258
Second Amendment To Lease | Property Subject to Operating Lease
Lessee, Lease, Description [Line Items]
Lease rentable area (in square feet)							26,897
Forecast
Lessee, Lease, Description [Line Items]
Lease rentable area (in square feet)			46,074

LEASES - Supplemental balance s

LEASES - Supplemental balance sheet information regarding operating leases (Details)	Sep. 30, 2019
Leases [Abstract]
Weighted average remaining lease term	7 years 10 months 24 days
Weighted average discount rate	6.00%

LEASES - Operating lease schedu

LEASES - Operating lease schedule of maturity (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Aug. 14, 2019	Jan. 01, 2019	Dec. 31, 2018
Leases [Abstract]
2019 (remaining three months)	$ 273
2020	1,107
2021	1,377
2022	1,662
2023 and thereafter	8,434			$ 0
Total lease payments	12,853
Less: imputed interest	(2,875)
Total operating lease liabilities	9,978	$ 8,100	$ 2,500
Future minimum payments of operating lease	$ 8,200
Term of operating lease not yet commenced, second expansion premises	7 years

LEASES - Operating lease sche_2

LEASES - Operating lease schedule of maturity under previous guidance (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Leases [Abstract]
2019		$ 1,076
2020		1,108
2021		562
2022 and thereafter	$ 8,434	0
Total future minimum lease payments		$ 2,746

OTHER BALANCE SHEET COMPONENT_2

OTHER BALANCE SHEET COMPONENTS - Property and equipment, net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 4,498		$ 4,498		$ 3,136
Less: accumulated depreciation and amortization	(2,445)		(2,445)		(1,921)
Total property and equipment, net	2,053		2,053		1,215
Depreciation and amortization expense	200	$ 200	524	$ 455
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	210		210		193
Computer and lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	3,125		3,125		1,888
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,163		$ 1,163		$ 1,055

OTHER BALANCE SHEET COMPONENT_3

OTHER BALANCE SHEET COMPONENTS - Accrued expense and other current liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued clinical and nonclinical study costs	$ 1,679	$ 2,168
Accrued contract manufacturing	8,081	1,676
Accrued compensation	2,890	1,565
Accrued professional fees and other	2,951	935
Total accrued expenses and other current liabilities	$ 15,601	$ 6,344

BORROWINGS - Additional informa

BORROWINGS - Additional information (Details)	Mar. 27, 2019USD ($)$ / sharesshares	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 28, 2018USD ($)$ / sharesshares	Oct. 15, 2018USD ($)tranche	Jul. 02, 2018$ / shares	Jun. 16, 2018shares	Apr. 10, 2018USD ($)	Feb. 28, 2018USD ($)tranche$ / sharesshares	Dec. 31, 2013$ / shares
Debt Instrument [Line Items]
Long-term portion of Term Loan		$ 38,016,000	$ 38,071,000
Number of shares called by warrant (in shares) | shares							95,936
Exercise price of warrant (in USD per share) | $ / shares										$ 0.886
Percentage of principal		1.00%
Contractual life of warrant										7 years
Term Loan
Debt Instrument [Line Items]
Debt aggregate principal amount					$ 100,000,000				$ 100,000,000
Debt, number of tranches | tranche					5				5
Long-term portion of Term Loan	$ 40,000,000
Debt payment terms, multiplier	7.55%
Number of shares called by warrant (in shares) | shares									53,458
Exercise price of warrant (in USD per share) | $ / shares									$ 9.35
Estimated fair value of warrants									$ 200,000
Increase (decrease) in equity due to warrants				$ 900,000				$ 200,000
Fair value of embedded derivative liability		$ 400,000						$ 700,000
Unamortized issuance costs and debt discounts		$ 3,600,000	$ 2,700,000
Term Loan | Minimum
Debt Instrument [Line Items]
Debt prepayment fee, percentage	2.00%
Term Loan | Tranche one
Debt Instrument [Line Items]
Debt aggregate principal amount									25,000,000
Term Loan | Tranche two
Debt Instrument [Line Items]
Debt aggregate principal amount					$ 15,000,000				$ 25,000,000
Number of shares called by warrant (in shares) | shares				53,458
Exercise price of warrant (in USD per share) | $ / shares				$ 9.35
New Term Loan
Debt Instrument [Line Items]
Maximum borrowing capacity of term loan	$ 200,000,000
Term of principal and interest payments	30 months
One time payment of principal	$ 2,600,000
Debt prepayment fee, percentage year two	1.50%
Debt prepayment fee, percentage year three	1.00%
Debt prepayment fee, percentage after year three	0.00%
New Term Loan | Maximum
Debt Instrument [Line Items]
Extended period for payment of interest only	24 months
New Term Loan | Tranche one
Debt Instrument [Line Items]
Debt aggregate principal amount	$ 20,000,000
New Term Loan | Tranche two
Debt Instrument [Line Items]
Debt aggregate principal amount	15,000,000
New Term Loan | Tranche three
Debt Instrument [Line Items]
Debt aggregate principal amount	75,000,000
New Term Loan | Tranche four
Debt Instrument [Line Items]
Debt aggregate principal amount	$ 50,000,000
Third Amendment to Term Loan
Debt Instrument [Line Items]
Number of shares called by warrant (in shares) | shares	16,721
Exercise price of warrant (in USD per share) | $ / shares	$ 23.92
Increase (decrease) in equity due to warrants	$ 300,000
Floating variable rate 1 | Term Loan
Debt Instrument [Line Items]
Debt interest rate			8.35%
Floating variable rate 2 | Term Loan | Prime rate
Debt Instrument [Line Items]
Debt, basis spread on variable rate			8.35%
Floating variable rate 2 | New Term Loan | Prime rate
Debt Instrument [Line Items]
Debt, basis spread on adjusted variable rate	6.00%
Floating variable rate 3 | Term Loan
Debt Instrument [Line Items]
Debt interest rate			9.85%
Time To Liquidity | Third Amendment to Term Loan
Debt Instrument [Line Items]
Measurement input, time to liquidity	7 years
Volatility | Third Amendment to Term Loan
Debt Instrument [Line Items]
Warrants measurement input	0.750
Risk-free interest rate | Third Amendment to Term Loan
Debt Instrument [Line Items]
Warrants measurement input	0.023
Discount Rate
Debt Instrument [Line Items]
Derivative liability, measurement input		0.114
Measurement input, probability of occurrence of certain events
Debt Instrument [Line Items]
Derivative liability, measurement input		0.100
IPO
Debt Instrument [Line Items]
Share price (in USD per share) | $ / shares						$ 19

BORROWINGS - Debt maturity sche

BORROWINGS - Debt maturity schedule (Details) $ in Thousands	Sep. 30, 2019USD ($)
Debt Disclosure [Abstract]
2019 (remaining three months)	$ 844
2020	3,396
2021	14,173
2022	20,403
2023	16,282
Long-term debt including interest payable	55,098
Less: amount representing interest	(15,098)
Present value of Term Loan	40,000
Less: current portion	0
Long-term portion of Term Loan	$ 40,000

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Millions	May 08, 2018	Sep. 30, 2019
Debt Instrument [Line Items]
Purchase obligation	$ 134.2	$ 111.4
Payment of long-term purchase commitment		22.8
Purchase obligation term	24 months
Period one half remaining commitment due	12 months
Tenant Improvement Loan
Debt Instrument [Line Items]
Tenant improvement loan, remaining payments		$ 0.2

STOCKHOLDERS' EQUITY - Addition

STOCKHOLDERS' EQUITY - Additional information (Details) $ / shares in Units, $ in Millions	Apr. 08, 2019USD ($)$ / sharesshares	Jul. 02, 2018USD ($)$ / sharesshares	Jun. 14, 2018	Sep. 30, 2019$ / sharesshares	Dec. 31, 2018$ / sharesshares
Class of Stock [Line Items]
Total number of shares authorized for issue (in shares)		440,000,000
Common stock, shares authorized (in shares)		400,000,000		400,000,000	400,000,000
Common stock, par value (in USD per share) | $ / shares		$ 0.001		$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)		40,000,000		40,000,000	40,000,000
Preferred stock, par value (in USD per share) | $ / shares		$ 0.001		$ 1	$ 1
Common stock, shares outstanding (in shares)				49,531,916	42,148,247
Common stock, shares, Issued				49,531,916	42,148,247
Preferred stock, outstanding (in shares)				0	0
Number of common stock issued upon conversion (in shares)		26,187,321
Reserve stock split ratio			0.2513
IPO
Class of Stock [Line Items]
Number of common stock issued (in shares)		13,455,000
Share price (in USD per share) | $ / shares		$ 19
Proceeds from initial public offering, net of issuance costs | $		$ 237.7
Underwriting discounts and commissions | $		$ 17.9
Over-Allotment Option
Class of Stock [Line Items]
Number of common stock issued (in shares)	840,000
Underwritten Public Offering
Class of Stock [Line Items]
Number of common stock issued (in shares)	6,440,000
Share price (in USD per share) | $ / shares	$ 36
Proceeds from initial public offering, net of issuance costs | $	$ 217.9
Underwriting discounts and commissions | $	$ 13.9
Convertible Preferred Stock
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares)		104,225,638

STOCKHOLDERS' EQUITY - Common s

STOCKHOLDERS' EQUITY - Common stock (Details) - shares	Sep. 30, 2019	Dec. 31, 2018
Equity [Abstract]
Stock options and RSUs issued and outstanding (in shares)	6,685,989	4,599,307
Stock options, RSUs and ESPP shares authorized for future issuance (in shares)	3,611,844	4,534,784
Total (in shares)	10,297,833	9,134,091

NET LOSS PER SHARE - Basic and

NET LOSS PER SHARE - Basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 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
Numerator:
Net loss	$ (44,119)	$ (36,626)	$ (37,897)	$ (29,098)	$ (25,362)	$ (20,504)	$ (118,642)	$ (74,964)
Denominator:
Weighted average common shares outstanding (in shares)	49,425,382			41,295,394			46,824,045	15,436,198
Less: weighted average shares subject to repurchase (in shares)	(7,357)			(33,691)			(10,169)	(21,004)
Weighted-average number of shares used in basic and diluted net loss per share (in shares)	49,418,025			41,261,703			46,813,876	15,415,194
Net loss per share, basic and diluted (in USD per share)	$ (0.89)			$ (0.71)			$ (2.53)	$ (4.86)

NET LOSS PER SHARE - Antidiluti

NET LOSS PER SHARE - Antidilutive securities (Details) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	6,815,926	4,663,139
Warrants to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	123,637	53,458
Common stock subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	6,300	28,793
Options and RSUs issued and outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	6,685,989	4,580,888

SUBSEQUENT EVENTS - Lease agree

SUBSEQUENT EVENTS - Lease agreement (Details) - RenewalPeriod	Oct. 04, 2019	Jul. 31, 2014
Subsequent Event [Line Items]
Operating lease renewal term		3 years
Subsequent Event
Subsequent Event [Line Items]
Number of renewal periods	2
Notice period before renewal	3 years
Renewal period	3 years