Tricida (TCDAQ) 10-Q 10 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 06, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Registrant Name	TRICIDA, INC.
Entity Central Index Key	0001595585
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Small Business	false
Entity Common Stock, Shares Outstanding		49,140,257

CONDENSED BALANCE SHEETS

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 21,928	$ 37,172
Short-term investments	197,040	203,906
Prepaid expenses and other current assets	3,018	3,269
Total current assets	221,986	244,347
Long-term investments	0	2,287
Property and equipment, net	1,209	1,215
Operating lease right-of-use assets	2,052
Deferred offering costs	717	0
Total assets	225,964	247,849
Current liabilities:
Accounts payable	5,045	8,460
Current operating lease liabilities	1,038
Accrued expenses and other current liabilities	20,663	6,344
Total current liabilities	26,746	14,804
Long-term liabilities
Term Loan	36,940	38,071
Non-current operating lease liabilities	1,223
Other long-term liabilities	446	449
Total liabilities	65,355	53,324
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.001 par value; 40,000,000 shares authorized, no shares issued or outstanding as of March 31, 2019 and December 31, 2018	0	0
Common stock, $0.001 par value; 400,000,000 shares authorized as of March 31, 2019 and December 31, 2018; 42,676,106 and 42,148,247 shares issued and outstanding as of March 31, 2019 and December 31, 2018, respectively	43	42
Additional paid-in capital	390,508	386,830
Accumulated other comprehensive income (loss)	149	(153)
Accumulated deficit	(230,091)	(192,194)
Total stockholders’ equity	160,609	194,525
Total liabilities and stockholders’ equity	$ 225,964	$ 247,849

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - $ / shares	Mar. 31, 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 (in shares)	42,676,106
Common stock, shares outstanding (in shares)		42,148,247

CONDENSED STATEMENTS OF OPERATI

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 31,423	$ 16,633
General and administrative	6,352	3,465
Total operating expenses	37,775	20,098
Loss from operations	(37,775)	(20,098)
Other income (expense), net	1,267	(87)
Interest expense	(1,389)	(319)
Net loss	(37,897)	(20,504)
Other comprehensive income (loss):
Net unrealized gain (loss) on available-for-sale securities	302	(54)
Total comprehensive loss	$ (37,595)	$ (20,558)
Net loss per share, basic and diluted (in USD per share)	$ (0.90)	$ (9)
Weighted-average number of shares outstanding, basic and diluted (in shares)	42,268,062	2,278,266

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 warrants in connection with Term Loan	24
Issuance of common stock upon exercise of stock options (in shares)		29,519
Issuance of common stock upon exercise of stock options			24
Stock-based compensation	353		353
Unrealized gain 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, 2018	0
Beginning balance at Dec. 31, 2018	$ 0
Beginning balance (in shares) at Dec. 31, 2018		42,148,247
Beginning balance at Dec. 31, 2018	194,525	$ 42	386,830	(153)	(192,194)
Total Stockholders’ Equity (Deficit)
Issuance of warrants in connection with Term Loan	284		284
Issuance of common stock upon exercise of stock options (in shares)		527,859
Issuance of common stock upon exercise of stock options	737	$ 1	736
Stock-based compensation	2,658		2,658
Unrealized gain 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	42,676,106
Ending balance at Mar. 31, 2019	$ 160,609	$ 43	$ 390,508	$ 149	$ (230,091)

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating activities:
Net loss	$ (37,897)	$ (20,504)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	172	138
Operating Lease, Right-Of-Use Asset, Amortization	204
Amortization of Term Loan discount and issuance costs	488	128
Accretion (amortization) of premiums and discounts on investments	(756)	(123)
Stock-based compensation	2,658	353
Changes in fair value of compound derivative liabilities and warrants	174	136
Changes in operating assets and liabilities:
Prepaid expenses and other assets	243	1,110
Accounts payable	(3,390)	4,699
Accrued expenses and other liabilities	14,479	(1,033)
Operating lease liabilities	(217)
Net cash used in operating activities	(23,842)	(15,096)
Investing activities:
Purchase of investments	(44,769)	(16,802)
Maturities of investments	54,981	11,865
Purchase of property and equipment	(193)	(494)
Net cash provided by (used in) investing activities	10,019	(5,431)
Financing activities:
Payments of equity offering costs	(321)	(726)
Proceeds from exercise of common stock under equity award plans	251	77
Proceeds from leasehold improvement loan	0	250
Repayment of leasehold improvement loan	(32)	0
Proceeds (repayment) of Term Loan, net	(1,319)	23,634
Net cash provided by (used in) financing activities	(1,421)	23,235
Net increase (decrease) in cash and cash equivalents	(15,244)	2,708
Cash and cash equivalents at beginning of period	37,172	9,774
Cash and cash equivalents at end of period	21,928	12,482
Supplemental disclosures
Cash paid for interest	782	0
Supplemental disclosures of non-cash financing activities
Deferred offering costs incurred but not paid	396	1,452
Warrants and compound derivative related to Term Loan	284	810
Purchase of property and equipment in accounts payable and accrued expenses	$ 0	$ 51

ORGANIZATION AND BASIS OF PRESE

ORGANIZATION AND BASIS OF PRESENTATION	3 Months Ended
	Mar. 31, 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 (TRC101), a non-absorbed, orally-administered polymer designed to treat metabolic acidosis by binding and removing acid from the gastrointestinal, or GI, tract. In May 2018, the Company completed its pivotal Phase 3 clinical trial, TRCA-301, that met both its primary and secondary endpoints in a highly statistically significant manner. In March 2019, the Company completed its extension trial, TRCA-301E. Based on the initial topline data analyses, the TRCA-301E trial met all of its primary and secondary endpoints. The Company plans to submit a New Drug Application, or NDA, in the second half of 2019, pursuant to the U.S. Food and Drug Administration’s, or FDA’s, Accelerated Approval Program. As part of the Accelerated Approval Program, the Company has committed to conduct a confirmatory postmarketing trial, VALOR-CKD (also known as TRCA-303), to evaluate the efficacy and safety of veverimer in delaying CKD progression in subjects with metabolic acidosis. The VALOR-CKD trial has been initiated, and the Company has committed to completely enrolling, or nearly completely enrolling, subjects in the trial prior to the Company's NDA submission. 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. The Company recognizes the need to raise additional capital to fully implement its business plan. Through March 31, 2019 , the Company has relied primarily on the proceeds from equity offerings and debt financing to finance its operations. The Company intends to raise additional capital 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 March 31, 2019 , the condensed statements of operations and comprehensive loss for the three months ended March 31, 2019 and 2018 , the condensed statements of convertible preferred stock and stockholders' equity (deficit) for the three months ended March 31, 2019 and 2018 and the condensed statements of cash flows for the three months ended March 31, 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	3 Months Ended
	Mar. 31, 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 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 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 for the Company 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 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 OF FINANCIAL INSTRUM

FAIR VALUE OF FINANCIAL INSTRUMENTS	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
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 FASB, 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 three months ended March 31, 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 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 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 summarize the Company's cash, cash equivalents and available-for-sale investments' amortized cost, gross unrealized gains, gross unrealized losses and estimated fair value by significant investment category reported as cash and cash equivalents, short-term investments or long-term investments as of March 31, 2019 and December 31, 2018 . March 31, 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 $ 636 $ — $ — $ 636 $ 636 $ — $ — Level 1: Money market fund 21,292 — — 21,292 21,292 — — Level 2: Commercial paper 76,665 40 (1 ) 76,704 — 76,704 — Corporate debt securities 68,114 59 (4 ) 68,169 — 68,169 — Asset-backed securities 52,112 55 — 52,167 — 52,167 — Subtotal 196,891 154 (5 ) 197,040 — 197,040 — Total assets measured at fair value $ 218,819 $ 154 $ (5 ) $ 218,968 $ 21,928 $ 197,040 $ — 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 $1.6 million and $0.3 million for the three months ended March 31, 2019 and 2018 , respectively. There were no gross realized gains and gross realized losses for the three months ended March 31, 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 March 31, 2019 and December 31, 2018 . March 31, 2019 December 31, 2018 (in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 7,935 $ (1 ) $ 67,407 $ (63 ) Corporate debt securities 27,552 (4 ) 85,699 (63 ) Asset-backed securities — — 36,730 (34 ) Total $ 35,487 $ (5 ) $ 189,836 $ (160 ) The Company held a total of 10 and 48 positions which were in an unrealized loss position as of March 31, 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 March 31, 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 March 31, 2019 because the Company does not intend to sell these securities nor does the Company believes that the Company will be required to sell these securities before the recovery of their amortized cost basis. The following table presents a reconciliation of financial liabilities measured at fair value on a recurring basis using Level 3 unobservable inputs for the three months ended March 31, 2019 and 2018 . Three Months Ended March 31, 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 174 35 101 Fair value at end of period $ 335 $ 689 $ 363 The following table presents information about significant unobservable inputs related to the Company's significant Level 3 financial liabilities as of March 31, 2019 . March 31, 2019 (in thousands) Fair Value Valuation Technique Significant Unobservable Input Range of Inputs Compound derivative liability $ 335 Discounted cash flow Discount rate 11.0 % - 11.2 % 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 March 31, 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 range of values for the Term Loan. The discount rate ranged between 11.0% and 11.2% .

LEASES

LEASES	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
LEASES	LEASES The Company has an operating lease for its office and laboratory space in South San Francisco, California. Operating lease expense was $0.3 million for the three months ended March 31, 2019 . Cash paid within operating cash flows for operating leases was $0.3 million for the three months ended March 31, 2019 . Short-term lease expense for the months ended March 31, 2019 was not material. The following table presents supplemental balance sheet information related to operating leases as of March 31, 2019 . (in thousands, except lease term and discount rate) March 31, 2019 Operating lease right-of-use assets $ 2,052 Operating lease liabilities: Current operating lease liabilities 1,038 Non-current operating lease liabilities 1,223 Total operating lease liabilities $ 2,261 Weighted average remaining lease term 2.25 years Weighted average discount rate 8.0 % The following table presents the maturities of the Company's operating lease liabilities as of March 31, 2019 . (in thousands) March 31, 2019 2019 (remaining nine months) $ 811 2020 1,108 2021 562 Total lease payments 2,481 Less: imputed interest (220 ) Total operating lease liabilities $ 2,261 ASC Topic 840 Disclosures The Company elected the alternative modified transition method. 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	3 Months Ended
	Mar. 31, 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 March 31, 2019 and December 31, 2018 . (in thousands) March 31, December 31, Furniture and fixtures $ 193 $ 193 Computer and lab equipment 2,051 1,888 Leasehold improvements 1,058 1,055 3,302 3,136 Less: accumulated depreciation and amortization (2,093 ) (1,921 ) Total property and equipment, net $ 1,209 $ 1,215 Depreciation and amortization expense was approximately $0.2 million and $0.1 million for the three months ended March 31, 2019 and 2018 , respectively. Accrued Expenses and Other Current Liabilities The following table presents the components of accrued expenses and other current liabilities as of March 31, 2019 and December 31, 2018 . (in thousands) March 31, December 31, Accrued clinical and nonclinical study costs $ 6,112 $ 2,168 Accrued contract manufacturing 10,694 1,676 Accrued compensation 2,077 1,565 Accrued professional fees and other 1,780 935 Total accrued expenses and other current liabilities $ 20,663 $ 6,344

BORROWINGS

BORROWINGS	3 Months Ended
	Mar. 31, 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 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) a volume weighted average price. 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 March 31, 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 11.2% 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 $0.3 million as of March 31, 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 March 31, 2019 and December 31, 2018 there were unamortized issuance costs and debt discounts of $4.1 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 March 31, 2019 . (in thousands) March 31, 2019 2019 (remaining nine months) $ 2,566 2020 3,396 2021 14,173 2022 20,403 2023 16,282 56,820 Less: amount representing interest (16,820 ) 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.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	3 Months Ended
	Mar. 31, 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 March 31, 2019 and December 31, 2018 , the Company had 42,676,106 and 42,148,247 shares of common stock outstanding, respectively. As of March 31, 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 . See Note 10 " Subsequent Events " for additional information about the underwritten public offering. Common stock reserved for future issuance as of March 31, 2019 and December 31, 2018 , consisted of the following. March 31, December 31, Stock options and RSUs issued and outstanding 5,798,844 4,599,307 Stock options, RSUs and ESPP shares authorized for future issuance 4,914,799 4,534,784 Total 10,713,643 9,134,091

NET LOSS PER SHARE

NET LOSS PER SHARE	3 Months Ended
	Mar. 31, 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 months ended March 31, 2019 and 2018 . Three Months Ended March 31, (In thousands, except share and per share amounts) 2019 2018 Numerator: Net loss $ (37,897 ) $ (20,504 ) Denominator: Weighted average common shares outstanding 42,283,526 2,290,598 Less: weighted average shares subject to repurchase (15,464 ) (12,332 ) Weighted average number of shares used in basic and diluted net loss per share 42,268,062 2,278,266 Net loss per share, basic and diluted $ (0.90 ) $ (9.00 ) 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. March 31, 2019 2018 Series A convertible preferred stock — 2,839,886 Series B convertible preferred stock — 8,172,579 Series C convertible preferred stock — 8,996,586 Series D convertible preferred stock — 6,154,166 Warrants to purchase preferred or common stock 123,637 77,562 Common stock subject to repurchase 12,621 16,959 Options and RSUs issued and outstanding 5,798,844 4,383,674 Total potential common shares excluded from the computation of diluted net loss per share 5,935,102 30,641,412

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Facilities In July 2014, the Company entered into a five -year noncancelable operating lease that expires 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. See Note 4 " Leases " for details of related commitments. In addition, associated with the operating lease, the Company has a tenant improvement loan with remaining payments totaling approximately $0.2 million as of March 31, 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 $54.3 million , of which $12.6 million was paid through March 31, 2019 and $41.7 million will be paid over the next 36 months, with the majority of this amount occurring in the first successive 12 -month period. The Company expects to incur additional purchase obligations relating to future statements of work under such MDS. The Company also enters into other contracts in the normal course of business with CROs, 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 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.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	3 Months Ended
	Mar. 31, 2019
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS 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 . The table below shows, on a pro forma basis, the impact of the Company’s underwritten public offering on certain condensed balance sheet items as if all of the transactions occurred on March 31, 2019 . Pro Forma (unaudited) (in thousands) March 31, 2019 March 31, 2019 Cash, cash equivalents and investments $ 218,968 $ 436,898 Deferred offering costs 717 — Common stock 43 49 Additional paid-in capital 390,508 607,715 Total stockholders' equity 160,609 377,822

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	3 Months Ended
	Mar. 31, 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 March 31, 2019 , the condensed statements of operations and comprehensive loss for the three months ended March 31, 2019 and 2018 , the condensed statements of convertible preferred stock and stockholders' equity (deficit) for the three months ended March 31, 2019 and 2018 and the condensed statements of cash flows for the three months ended March 31, 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 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 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 for the Company 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 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 OF FINANCIAL INSTR_2

FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Financial assets and liabilities measured on a recurring basis	The following tables summarize the Company's cash, cash equivalents and available-for-sale investments' amortized cost, gross unrealized gains, gross unrealized losses and estimated fair value by significant investment category reported as cash and cash equivalents, short-term investments or long-term investments as of March 31, 2019 and December 31, 2018 . March 31, 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 $ 636 $ — $ — $ 636 $ 636 $ — $ — Level 1: Money market fund 21,292 — — 21,292 21,292 — — Level 2: Commercial paper 76,665 40 (1 ) 76,704 — 76,704 — Corporate debt securities 68,114 59 (4 ) 68,169 — 68,169 — Asset-backed securities 52,112 55 — 52,167 — 52,167 — Subtotal 196,891 154 (5 ) 197,040 — 197,040 — Total assets measured at fair value $ 218,819 $ 154 $ (5 ) $ 218,968 $ 21,928 $ 197,040 $ — 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
Amortized cost, unrealized holding gains or losses, and fair value of marketable securities	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 March 31, 2019 and December 31, 2018 . March 31, 2019 December 31, 2018 (in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 7,935 $ (1 ) $ 67,407 $ (63 ) Corporate debt securities 27,552 (4 ) 85,699 (63 ) Asset-backed securities — — 36,730 (34 ) Total $ 35,487 $ (5 ) $ 189,836 $ (160 )
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 three months ended March 31, 2019 and 2018 . Three Months Ended March 31, 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 174 35 101 Fair value at end of period $ 335 $ 689 $ 363
Significant fair value unobservable inputs	The following table presents information about significant unobservable inputs related to the Company's significant Level 3 financial liabilities as of March 31, 2019 . March 31, 2019 (in thousands) Fair Value Valuation Technique Significant Unobservable Input Range of Inputs Compound derivative liability $ 335 Discounted cash flow Discount rate 11.0 % - 11.2 % Probability of the occurrence of certain events 10.0 %

LEASES (Tables)

LEASES (Tables)	3 Months Ended
	Mar. 31, 2019
Leases [Abstract]
Supplemental balance sheet information related to leases	The following table presents supplemental balance sheet information related to operating leases as of March 31, 2019 . (in thousands, except lease term and discount rate) March 31, 2019 Operating lease right-of-use assets $ 2,052 Operating lease liabilities: Current operating lease liabilities 1,038 Non-current operating lease liabilities 1,223 Total operating lease liabilities $ 2,261 Weighted average remaining lease term 2.25 years Weighted average discount rate 8.0 %
Operating lease maturity schedule	The following table presents the maturities of the Company's operating lease liabilities as of March 31, 2019 . (in thousands) March 31, 2019 2019 (remaining nine months) $ 811 2020 1,108 2021 562 Total lease payments 2,481 Less: imputed interest (220 ) Total operating lease liabilities $ 2,261
Operating lease maturity schedule under prior guidance	The Company elected the alternative modified transition method. 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

BALANCE SHEET COMPONENTS (Table

BALANCE SHEET COMPONENTS (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2019 and December 31, 2018 . (in thousands) March 31, December 31, Furniture and fixtures $ 193 $ 193 Computer and lab equipment 2,051 1,888 Leasehold improvements 1,058 1,055 3,302 3,136 Less: accumulated depreciation and amortization (2,093 ) (1,921 ) Total property and equipment, net $ 1,209 $ 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 March 31, 2019 and December 31, 2018 . (in thousands) March 31, December 31, Accrued clinical and nonclinical study costs $ 6,112 $ 2,168 Accrued contract manufacturing 10,694 1,676 Accrued compensation 2,077 1,565 Accrued professional fees and other 1,780 935 Total accrued expenses and other current liabilities $ 20,663 $ 6,344

TERM LOAN (Tables)

TERM LOAN (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2019 . (in thousands) March 31, 2019 2019 (remaining nine months) $ 2,566 2020 3,396 2021 14,173 2022 20,403 2023 16,282 56,820 Less: amount representing interest (16,820 ) Present value of Term Loan 40,000 Less: current portion — Long-term portion of Term Loan $ 40,000

STOCKHOLDERS' EQUITY (Tables)

STOCKHOLDERS' EQUITY (Tables)	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Schedule of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance as of March 31, 2019 and December 31, 2018 , consisted of the following. March 31, December 31, Stock options and RSUs issued and outstanding 5,798,844 4,599,307 Stock options, RSUs and ESPP shares authorized for future issuance 4,914,799 4,534,784 Total 10,713,643 9,134,091

NET LOSS PER SHARE (Tables)

NET LOSS PER SHARE (Tables)	3 Months Ended
	Mar. 31, 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 months ended March 31, 2019 and 2018 . Three Months Ended March 31, (In thousands, except share and per share amounts) 2019 2018 Numerator: Net loss $ (37,897 ) $ (20,504 ) Denominator: Weighted average common shares outstanding 42,283,526 2,290,598 Less: weighted average shares subject to repurchase (15,464 ) (12,332 ) Weighted average number of shares used in basic and diluted net loss per share 42,268,062 2,278,266 Net loss per share, basic and diluted $ (0.90 ) $ (9.00 )
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. March 31, 2019 2018 Series A convertible preferred stock — 2,839,886 Series B convertible preferred stock — 8,172,579 Series C convertible preferred stock — 8,996,586 Series D convertible preferred stock — 6,154,166 Warrants to purchase preferred or common stock 123,637 77,562 Common stock subject to repurchase 12,621 16,959 Options and RSUs issued and outstanding 5,798,844 4,383,674 Total potential common shares excluded from the computation of diluted net loss per share 5,935,102 30,641,412

SUBSEQUENT EVENTS SUBSEQUENT EV

SUBSEQUENT EVENTS SUBSEQUENT EVENTS (Tables)	3 Months Ended
	Mar. 31, 2019
Subsequent Events [Abstract]
Stockholders' Equity, Pro Forma Information	The table below shows, on a pro forma basis, the impact of the Company’s underwritten public offering on certain condensed balance sheet items as if all of the transactions occurred on March 31, 2019 . Pro Forma (unaudited) (in thousands) March 31, 2019 March 31, 2019 Cash, cash equivalents and investments $ 218,968 $ 436,898 Deferred offering costs 717 — Common stock 43 49 Additional paid-in capital 390,508 607,715 Total stockholders' equity 160,609 377,822

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Accounting Policies [Abstract]
Operating lease right-of-use assets	$ 2,052	$ 2,300
Total operating lease liabilities	$ 2,261	$ 2,500

FAIR VALUE OF FINANCIAL INSTR_3

FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)Position	Dec. 31, 2017Position
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment interest income	$ 1.6	$ 0.3
Debt instruments held in continuous unrealized loss position | Position		10	48
Discount Rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, measurement input	0.112
Term Loan | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument, fair value	$ 39.7
Minimum | Discount Rate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, measurement input	0.110
Maximum | Discount Rate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liability, measurement input	0.112

FAIR VALUE OF FINANCIAL INSTR_4

FAIR VALUE OF FINANCIAL INSTRUMENTS - Financial assets and liabilities by hierarchy (Details) $ in Thousands	Mar. 31, 2019USD ($)
Level 3
Financial liabilities:
Compound derivative liability	$ 335

FAIR VALUE OF FINANCIAL INSTR_5

FAIR VALUE OF FINANCIAL INSTRUMENTS - Reconciliation of liabilities measured using Level 3 (Details) - Level 3 - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Compound Derivative
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening Balance	$ 161	$ 0
Addition	0	654
Reclassification to equity	174	35
Ending balance	$ 335	689
Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Opening Balance		106
Addition		156
Reclassification to equity		101
Ending balance		$ 363

FAIR VALUE OF FINANCIAL INSTR_6

FAIR VALUE OF FINANCIAL INSTRUMENTS - Marketable securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash	$ 21,928	$ 37,172
Level 2:
Short-term investments	197,040	203,906
Long-term investments	0	2,287
Fair Value, Measurements, Recurring
Total assets measured at fair value
Amortized Cost	218,819	243,518
Gross Unrealized Gains	154	7
Gross Unrealized Losses	(5)	(160)
Estimated Fair Value	218,968	243,365
Cash | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	636	3,021
Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	21,928	37,172
Cash and Cash Equivalents | Cash | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	636	3,021
Level 1:
Money market fund	636	3,021
Short-term Investments | Fair Value, Measurements, Recurring
Level 2:
Short-term investments	197,040	203,906
Investments | Fair Value, Measurements, Recurring
Level 2:
Long-term investments	0	2,287
Level 1 | Money market fund | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	21,292	33,154
Level 1:
Money market fund	21,292	33,154
Level 1 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Debt Securities, Available-for-sale [Line Items]
Cash	21,292	33,154
Level 2
Level 2:
Unrealized Holding Losses	(5)	(160)
Level 2 | Commercial paper
Level 2:
Unrealized Holding Losses	(1)	(63)
Level 2 | Corporate debt securities
Level 2:
Unrealized Holding Losses	(4)	(63)
Level 2 | Asset-backed securities
Level 2:
Unrealized Holding Losses	0	(34)
Level 2 | Fair Value, Measurements, Recurring
Level 2:
Amortized Cost	196,891	207,343
Unrealized Holding Gains	154	7
Unrealized Holding Losses	(5)	(160)
Short-term investments	197,040	207,190
Level 2 | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Amortized Cost	76,665	68,467
Unrealized Holding Gains	40	0
Unrealized Holding Losses	(1)	(63)
Short-term investments	76,704	68,404
Level 2 | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Amortized Cost	68,114	89,038
Unrealized Holding Gains	59	4
Unrealized Holding Losses	(4)	(63)
Short-term investments	68,169	88,979
Level 2 | Fair Value, Measurements, Recurring | Asset-backed securities
Level 2:
Amortized Cost	52,112	49,838
Unrealized Holding Gains	55	3
Unrealized Holding Losses	0	(34)
Short-term investments	52,167	49,807
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring
Level 2:
Short-term investments	0	997
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Short-term investments	0	997
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Short-term investments	0	0
Level 2 | Cash and Cash Equivalents | Fair Value, Measurements, Recurring | Asset-backed securities
Level 2:
Short-term investments	0	0
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring
Level 2:
Short-term investments	197,040	203,906
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring | Commercial paper
Level 2:
Short-term investments	76,704	67,407
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring | Corporate debt securities
Level 2:
Short-term investments	68,169	86,692
Level 2 | Short-term Investments | Fair Value, Measurements, Recurring | Asset-backed securities
Level 2:
Short-term investments	52,167	49,807
Level 2 | Investments | Fair Value, Measurements, Recurring
Level 2:
Long-term investments	0	2,287
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	0	2,287
Level 2 | Investments | Fair Value, Measurements, Recurring | Asset-backed securities
Level 2:
Long-term investments	$ 0	$ 0

FAIR VALUE OF FINANCIAL INSTR_7

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	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 35,487	$ 189,836
Unrealized Losses	5	160
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,935	67,407
Unrealized Losses	1	63
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	27,552	85,699
Unrealized Losses	4	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 OF FINANCIAL INSTR_8

FAIR VALUE OF FINANCIAL INSTRUMENTS Significant unobservable inputs related to level 3 assets and liabilities (Details) $ in Thousands	Mar. 31, 2019USD ($)
Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.112
Measurement Input, Probability Of Occurrence Of Certain Events
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.10
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Compound derivative liability	$ 335
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	0.110
Maximum | Level 3 | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.112

LEASES Information related to o

LEASES Information related to operating lease cost (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Leases [Abstract]
Operating lease expense	$ 300
Cash paid within operating cash flows for the three months ended March 31, 2019	$ 265

LEASES Supplemental balance she

LEASES Supplemental balance sheet information regarding operating leases (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Assets
Operating lease right-of-use assets	$ 2,052	$ 2,300
Operating lease liabilities:
Current operating lease liabilities	1,038
Non-current operating lease liabilities	1,223
Total operating lease liabilities	$ 2,261	$ 2,500
Weighted average remaining lease term	2 years 3 months
Weighted average discount rate	8.00%

LEASES Operating lease schedule

LEASES Operating lease schedule of maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
Leases [Abstract]
2019 (remaining nine months)	$ 811
2020	1,108
2021	562
Total lease payments	2,481
Less: imputed interest	(220)
Total operating lease liabilities	$ 2,261	$ 2,500

LEASES Operating lease schedu_2

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

OTHER BALANCE SHEET COMPONENTS

OTHER BALANCE SHEET COMPONENTS - Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,302		$ 3,136
Less: accumulated depreciation and amortization	(2,093)		(1,921)
Total property and equipment, net	1,209		1,215
Depreciation and amortization expense	172	$ 138
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	193		193
Computer and lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,051		1,888
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,058		$ 1,055

OTHER BALANCE SHEET COMPONENT_2

OTHER BALANCE SHEET COMPONENTS - Accrued Expense and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued clinical and nonclinical study costs	$ 6,112	$ 2,168
Accrued contract manufacturing	10,694	1,676
Accrued compensation	2,077	1,565
Accrued professional fees and other	1,780	935
Total accrued expenses and other current liabilities	$ 20,663	$ 6,344

BORROWINGS - Additional Informa

BORROWINGS - Additional Information (Details)	Mar. 27, 2019USD ($)$ / sharesshares	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 28, 2018USD ($)$ / sharesshares	Oct. 15, 2018USD ($)tranche	Jul. 02, 2018$ / shares	Apr. 10, 2018USD ($)	Feb. 28, 2018USD ($)tranche$ / sharesshares	Dec. 31, 2013$ / sharesshares
Debt Instrument [Line Items]
Long-term portion of Term Loan		$ 36,940,000	$ 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
Class of Warrant or Right, Percentage of Principal Amount of Applicable Tranche		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
Warrants and Rights Outstanding								$ 156,000
Increase (decrease) in equity due to warrants				$ 900,000			$ 200,000
Fair value of embedded derivative liability		$ 300,000					$ 700,000
Unamortized issuance costs and debt discounts		$ 4,100,000	$ 2,700,000
Term Loan | Minimum
Debt Instrument [Line Items]
Debt prepayment fee, percentage	2.00%
Term Loan | Term Loan, Tranche One
Debt Instrument [Line Items]
Debt aggregate principal amount								25,000,000
Term Loan | 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 | Term Loan, Tranche One
Debt Instrument [Line Items]
Debt aggregate principal amount	$ 20,000,000
New Term Loan | Term Loan, Tranche Two
Debt Instrument [Line Items]
Debt aggregate principal amount	15,000,000
New Term Loan | Term Loan, Tranche Three
Debt Instrument [Line Items]
Debt aggregate principal amount	75,000,000
New Term Loan | 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	0.75
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		0.112
Measurement Input, Probability Of Occurrence Of Certain Events
Debt Instrument [Line Items]
Derivative liability, measurement input		0.10
IPO
Debt Instrument [Line Items]
Share price (in USD per share) | $ / shares						$ 19

BORROWINGS - Debt Maturity Sche

BORROWINGS - Debt Maturity Schedule (Details) $ in Thousands	Mar. 31, 2019USD ($)
Debt Disclosure [Abstract]
2019 (remaining nine months)	$ 2,566
2020	3,396
2021	14,173
2022	20,403
2023	16,282
Long-term debt including interest payable	56,820
Less: amount representing interest	(16,820)
Present value of Term Loan	40,000
Less: current portion	0
Long-term portion of Term Loan	$ 40,000

STOCKHOLDERS' EQUITY Additional

STOCKHOLDERS' EQUITY Additional Information (Details) $ / shares in Units, $ in Millions	Apr. 08, 2019USD ($)$ / sharesshares	Jul. 02, 2018USD ($)$ / sharesshares	Jun. 14, 2018	Mar. 31, 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
Preferred stock, par value (in USD per share) | $ / shares		$ 0.001		$ 1	$ 1
Common stock, shares issued (in shares)				42,676,106
Common stock, shares outstanding (in shares)					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
Subsequent Event | 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
Subsequent Event | Over-Allotment Option
Class of Stock [Line Items]
Number of common stock issued (in shares)	840,000
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	Mar. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Stock options and RSUs issued and outstanding	5,798,844	4,599,307
Stock options, RSUs and ESPP shares authorized for future issuance	4,914,799	4,534,784
Total	10,713,643	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
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (37,897)	$ (20,504)
Denominator:
Weighted average common shares outstanding (in shares)	42,283,526	2,290,598
Less: weighted average shares subject to repurchase (in shares)	(15,464)	(12,332)
Weighted-average number of shares used in basic and diluted net loss per share (in shares)	42,268,062	2,278,266
Net loss per share, basic and diluted (in USD per share)	$ (0.90)	$ (9)

NET LOSS PER SHARE - Antidiluti

NET LOSS PER SHARE - Antidilutive securities (Details) - shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	5,935,102	30,641,412
Warrants to purchase preferred or common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	123,637	77,562
Common stock subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	12,621	16,959
Options and RSUs issued and outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	5,798,844	4,383,674
Series A Preferred Stock | Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	0	2,839,886
Series B Preferred Stock | Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	0	8,172,579
Series C Preferred Stock | Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	0	8,996,586
Series D Preferred Stock | Convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	0	6,154,166

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Details) $ in Millions	May 08, 2018USD ($)	Mar. 31, 2019USD ($)	Aug. 31, 2017ft²	Jul. 31, 2014
Debt Instrument [Line Items]
Operating lease, term				5 years
Operating lease renewal term				3 years
Purchase obligation	$ 54.3	$ 41.7
Payment of long-term purchase commitment		12.6
Purchase obligation term	36 months
Tenant Improvement Loan
Debt Instrument [Line Items]
Tenant improvement loan, remaining payments		$ 0.2
Subject to Operating Lease
Debt Instrument [Line Items]
Lease rentable area (in square feet) | ft²			13,258
Amended Lease | Subject to Operating Lease
Debt Instrument [Line Items]
Lease rentable area (in square feet) | ft²			26,987

SUBSEQUENT EVENTS Underwritten

SUBSEQUENT EVENTS Underwritten public offering (Details) - Subsequent Event $ / shares in Units, $ in Millions	Apr. 08, 2019USD ($)$ / sharesshares
Underwritten Public Offering
Subsequent Event [Line Items]
Number of common stock issued (in shares) | 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
Over-Allotment Option
Subsequent Event [Line Items]
Number of common stock issued (in shares) | shares	840,000

SUBSEQUENT EVENTS Underwritte_2

SUBSEQUENT EVENTS Underwritten public offering pro forma information (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Cash, Cash Equivalents and Investments	$ 218,968
Cash and cash equivalents	21,928	$ 37,172
Deferred offering costs	717	0
Common stock	43	42
Additional paid-in capital	390,508	386,830
Total stockholders' equity	160,609	$ 194,525	$ (108,222)	$ (88,041)
Pro Forma
Subsequent Event [Line Items]
Cash and cash equivalents	436,898
Deferred offering costs	0
Common stock	49
Additional paid-in capital	607,715
Total stockholders' equity	$ 377,822