Tricida (TCDAQ) 10-K 29 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 22, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
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		42,386,192
Entity Shell Company	false
Entity Voluntary Filers	No
Entity Well Known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Public Float			$ 0

BALANCE SHEETS

BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 37,172	$ 9,774
Short-term investments	203,906	57,740
Prepaid expenses and other current assets	3,269	1,910
Total current assets	244,347	69,424
Long-term investments	2,287	0
Property and equipment, net	1,215	1,150
Total assets	247,849	70,574
Current liabilities:
Accounts payable	8,460	3,861
Accrued expenses and other current liabilities	6,344	7,361
Total current liabilities	14,804	11,222
Long-term liabilities
Term Loan	38,071	0
Other long-term liabilities	449	323
Total liabilities	53,324	11,545
Commitments and Contingencies
Convertible preferred stock	0	147,070
Stockholders’ equity (deficit):
Preferred stock	0	0
Common stock	42	2
Additional paid-in capital	386,830	1,356
Accumulated other comprehensive loss	(153)	(13)
Accumulated deficit	(192,194)	(89,386)
Total stockholders’ equity (deficit)	194,525	(88,041)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	247,849	70,574
Series A convertible preferred stock, $0.001 par value; zero shares authorized, issued and outstanding as of September 30, 2018; 11,398,694 shares authorized and 11,302,758 shares issued and outstanding as of December 31, 2017, aggregate liquidation preference of $10,014 as of December 31, 2017
Long-term liabilities
Convertible preferred stock	0	9,800
Series B convertible preferred stock, $0.001 par value; zero shares authorized, issued and outstanding as of September 30, 2018; 32,526,878 shares authorized, issued and outstanding as of December 31, 2017, aggregate liquidation preference of $30,250 as of December 31, 2017
Long-term liabilities
Convertible preferred stock	0	29,618
Series C convertible preferred stock, $0.001 par value; zero shares authorized, issued and outstanding as of September 30, 2018; 35,806,451 shares authorized, issued and outstanding as of December 31, 2017, aggregate liquidation preference of $55,500 as of December 31, 2017
Long-term liabilities
Convertible preferred stock	0	50,347
Series D convertible preferred stock, $0.001 par value; zero shares authorized, issued and outstanding as of September 30, 2018; 24,500,000 shares authorized as of December 31, 2017 and 24,493,615 shares issued and outstanding as of December 31, 2017, aggregate liquidation preference of $57,560 as of December 31, 2017
Long-term liabilities
Convertible preferred stock	$ 0	$ 57,305

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Convertible preferred stock, par value (in USD per share)		$ 0.001
Convertible preferred stock, shares authorized (in shares)		104,232,023
Convertible preferred stock, shares issued (in shares)	0	104,129,702
Convertible preferred stock, shares outstanding (in shares)		104,129,702
Aggregate Liquidation Preference		$ 153,324
Preferred stock, par value (in USD per share)	$ 0.001	$ 0
Preferred stock, shares authorized (in shares)	40,000,000	0
Preferred stock, issued (in shares)	0	0
Preferred stock outstanding	0	0
Common stock, par value (in USD per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	400,000,000	134,000,000
Common stock, shares issued (in shares)	42,148,247	2,272,609
Common stock, shares outstanding (in shares)	42,148,247	2,272,609
Series A Preferred Stock
Convertible preferred stock, par value (in USD per share)		$ 0.001
Convertible preferred stock, shares authorized (in shares)	0	11,398,694
Convertible preferred stock, shares issued (in shares)	0	11,302,758
Convertible preferred stock, shares outstanding (in shares)	0	11,302,758
Aggregate Liquidation Preference	$ 0	$ 10,014
Series B Preferred Stock
Convertible preferred stock, par value (in USD per share)	$ 0.001	$ 0.001
Convertible preferred stock, shares authorized (in shares)	0	32,526,878
Convertible preferred stock, shares issued (in shares)	0	32,526,878
Convertible preferred stock, shares outstanding (in shares)	0	32,526,878
Aggregate Liquidation Preference	$ 0	$ 30,250
Series C Preferred Stock
Convertible preferred stock, shares authorized (in shares)	0	35,806,451
Convertible preferred stock, shares issued (in shares)	0	35,806,451
Convertible preferred stock, shares outstanding (in shares)	0	35,806,451
Aggregate Liquidation Preference	$ 0	$ 55,500
Series D Preferred Stock
Convertible preferred stock, shares authorized (in shares)	0	24,500,000
Convertible preferred stock, shares issued (in shares)	0	24,493,615
Convertible preferred stock, shares outstanding (in shares)	0	24,493,615
Aggregate Liquidation Preference	$ 0	$ 57,560

STATEMENTS OF OPERATIONS AND CO

STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 85,594	$ 35,906	$ 21,820
General and administrative	18,001	11,216	5,363
Total operating expenses	103,595	47,122	27,183
Loss from operations	(103,595)	(47,122)	(27,183)
Change in fair value—preferred stock tranche obligation	0	5,649	(1,571)
Other income (expense), net	3,924	183	103
Interest expense	(3,137)	0	0
Net loss	(102,808)	(41,290)	(28,651)
Other comprehensive loss:
Net unrealized gain (loss) on available-for-sale securities, net of tax	(140)	(13)	0
Comprehensive loss	$ (102,948)	$ (41,303)	$ (28,651)
Net loss per share, basic and diluted	$ (4.64)	$ (19.32)	$ (15.69)
Weighted-average number of shares outstanding, basic and diluted (in shares)	22,146,192	2,137,690	1,826,040

STATEMENTS OF CONVERTIBLE PREFE

STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series A Preferred Stock	Series B Preferred Stock	Series C Preferred Stock	Series D Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2015	29,734,656
Beginning balance at Dec. 31, 2015	$ 25,023
Beginning balance (in shares) at Dec. 31, 2015						2,244,806
Beginning balance at Dec. 31, 2015	(19,257)					$ 2	$ 186	$ 0	$ (19,445)
Total Stockholders’ Equity (Deficit)
Issuance of convertible preferred stock –subsequent closing, for cash, net of issuance cost and settlement of the preferred stock obligation (in shares)			14,094,980	19,532,259
Issuance of convertible preferred stock –subsequent closing, for cash, net of issuance cost and settlement of the preferred stock obligation			$ 14,395	$ 27,465
Issuance of common stock upon exercise of stock options						11,724
Issuance of common stock upon exercise of stock options	8						8
Stock-based compensation	272						272
Unrealized loss on investments	0
Net loss	$ (28,651)								(28,651)
Ending balance (in shares) at Dec. 31, 2016	63,361,895
Ending balance at Dec. 31, 2016	$ 66,883
Ending balance (in shares) at Dec. 31, 2016						2,256,530
Ending balance at Dec. 31, 2016	$ (47,628)					$ 2	466	0	(48,096)
Total Stockholders’ Equity (Deficit)
Issuance of convertible preferred stock –subsequent closing, for cash, net of issuance cost and settlement of the preferred stock obligation (in shares)				16,274,192	24,493,615
Issuance of convertible preferred stock –subsequent closing, for cash, net of issuance cost and settlement of the preferred stock obligation				$ 22,882	$ 57,305
Issuance of common stock upon exercise of stock options	7,286					16,079
Issuance of common stock upon exercise of stock options	$ 14						14
Stock-based compensation	876						876
Unrealized loss on investments	(13)							(13)
Net loss	$ (41,290)								(41,290)
Ending balance (in shares) at Dec. 31, 2017	104,129,702	11,302,758	32,526,878	35,806,451	24,493,615
Ending balance at Dec. 31, 2017	$ 147,070	$ 9,800	$ 29,618	$ 50,347	$ 57,305
Ending balance (in shares) at Dec. 31, 2017	2,272,609					2,272,609
Ending balance at Dec. 31, 2017	$ (88,041)					$ 2	1,356	(13)	(89,386)
Total Stockholders’ Equity (Deficit)
Issuance of Series A warrant for cash (in shares)	95,936
Issuance of Series A convertible preferred stock upon exercise of warrant	$ 458
Conversion of convertible preferred shares to common stock (in shares)		(104,225,638)
Conversion of convertible preferred shares to common stock		$ (147,528)
Common stock issued in reclassification of convertible preferred stock						26,187,321
Stock Issued During Period, Value, Conversion of Convertible Securities, Net of Adjustments	147,528					$ 26	147,502
Issuance of common stock in connection with initial public offering, net of underwriter discounts and issuance costs (in shares0						13,455,000
Issuance of common stock in connection with initial public offering, net of underwriter discounts and issuance costs	231,186					$ 13	231,173
Reclassification of common stock warrant liability to equity	194						194
Issuance of warrant in connection with Term Loan	$ 884						884
Issuance of common stock upon exercise of stock options	219,974
Issuance of common stock upon exercise of stock options, warrants and employee stock purchase plan, shares						233,317
Issuance of common stock upon exercise of stock options, warrants and employee stock purchase plan	$ 570					$ 1	569
Stock-based compensation	5,152						5,152
Unrealized loss on investments	(140)							(140)
Net loss	$ (102,808)								(102,808)
Ending balance (in shares) at Dec. 31, 2018	0	0	0	0	0
Ending balance at Dec. 31, 2018	$ 0	$ 0	$ 0	$ 0	$ 0
Ending balance (in shares) at Dec. 31, 2018	42,148,247					42,148,247
Ending balance at Dec. 31, 2018	$ 194,525					$ 42	$ 386,830	$ (153)	$ (192,194)

STATEMENTS OF CONVERTIBLE PRE_2

STATEMENTS OF CONVERTIBLE PREFERRED SHARES AND STOCKHOLDERS' EQUITY (DEFICIT) - (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Series B Preferred Stock
Settlement of preferred stock obligation	$ 0	$ 1,324
Series C Preferred Stock
Issued at	$ 1.55
Issuance costs	$ 65
Settlement of preferred stock obligation	$ 2,278	$ 2,471
Series D Preferred Stock
Issued at	$ 2.35
Issuance costs	$ 255
Preferred Stock | Series B Preferred Stock
Issued at		$ 0.93
Issuance costs		$ 37
Settlement of preferred stock obligation		$ 1,324
Preferred Stock | Series C Preferred Stock
Issued at		$ 1.55
Issuance costs		$ 339
Settlement of preferred stock obligation		$ 2,471

STATEMENTS OF CASH FLOWS

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net loss	$ (102,808)	$ (41,290)	$ (28,651)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	618	335	424
Amortization of term loan discount and issuance costs	1,316	0	0
Net amortization of premiums and discounts on investments	(1,094)	(42)	14
Stock-based compensation	5,152	876	272
Changes in fair value of warrants and compound derivative liabilities	(188)	0	0
Changes in fair value of preferred stock tranche obligation	0	(5,649)	1,571
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,338)	(1,157)	(69)
Accounts payable	4,573	1,757	1,636
Accrued expenses and other liabilities	(1,087)	4,769	1,655
Net cash used in operating activities	(94,856)	(40,401)	(23,115)
Investing activities:
Purchase of investments	(233,928)	(76,846)	(39,064)
Maturities of investments	86,429	39,903	18,533
Proceeds from sale of investments	0	0	499
Purchase of property and equipment	(855)	(1,004)	(249)
Net cash used in investing activities	(148,354)	(37,947)	(20,281)
Financing activities:
Proceeds from initial public offering, net	237,750	0	0
Payments of offering costs	(6,564)	0	0
Proceeds from exercise of common stock under equity plan awards	632	14	8
Proceeds from issuance of convertible preferred stock, net	85	82,465	43,007
Proceeds from leasehold improvement loan	276	0	0
Repayment of leasehold improvement loan	(113)	(39)	(42)
Proceeds from issuance of term loan, net	38,542	0	0
Net cash provided by financing activities	270,608	82,440	42,973
Net increase (decrease) in cash and cash equivalents	27,398	4,092	(423)
Cash and cash equivalents at beginning of year	9,774	5,682	6,105
Cash and cash equivalents at end of year	37,172	9,774	5,682
Supplemental disclosures
Cash paid for interest	1,612	0	0
Supplemental disclosures of non-cash investing and financing activities
Compound derivative and warrant related to the term loan	1,693	0	0
Purchase of property and equipment in accounts payable and accrued expenses	27	0	0
Series B Preferred Stock
Supplemental disclosures of non-cash investing and financing activities
Fair value of preferred stock obligation upon closing	0	0	1,324
Series C Preferred Stock
Supplemental disclosures of non-cash investing and financing activities
Fair value of preferred stock obligation upon closing	$ 0	$ 2,278	$ 2,471

ORGANIZATION AND BASIS OF PRESE

ORGANIZATION AND BASIS OF PRESENTATION	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGAINZATION AND BASIS OF PRESENTATION	ORGANIZATION AND BASIS OF PRESENTATION Organization —Tricida, Inc., or 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, TRC101 (veverimer), 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 TRC101 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 our 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 which it expects to incur additional losses in the future. Management recognizes the need to raise additional capital to fully implement its business plan. Through December 31, 2018, 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. The accompanying financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America, or GAAP.

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Use of Estimates —The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates. Cash and Cash Equivalents —All highly liquid investments with maturities at the date of purchase of three months or less are classified as cash equivalents. There are no restrictions on cash and cash equivalents. Investments —All investments with maturities of greater than three months at the date of purchase and maturities of less than one year at the reporting date are classified as short-term investments, while investments with maturities of a year or more at the reporting date are classified as long-term investments. Management has classified the Company’s investments as available-for-sale in the accompanying financial statements. Available-for-sale investments are carried at fair value, with unrealized gains and losses reported as a component of accumulated other comprehensive loss. Realized gains and losses on the sale of all such securities are reported in other income (expense), net and are computed using the specific identification method. Available-for-sale investments are subject to a periodic impairment review. The Company may recognize an impairment charge when a decline in the fair value of investments below the cost basis is determined to be other-than-temporary. In determining whether a decline in market value is other-than-temporary, various factors are considered, including the cause, duration of time and severity of the impairment, any adverse changes in the investees’ financial condition, and the Company’s intent and ability to hold the security for a period of time sufficient to allow for an anticipated recovery in market value. When the Company determines that the decline in the fair value of an investment is below the accounting basis and the decline is other-than-temporary, the carrying value of the security is reduced and a loss is recorded for the amount of such decline. There were no investments deemed to be impaired as of December 31, 2018. The Company’s investments are in commercial paper, corporate debt securities and asset-backed securities. Pursuant to the Company’s investment policy, all purchased securities have a minimum short-term rating of A1 (Moody’s) or P1 (Standard & Poor’s) or equivalent. If there is no short-term rating, a purchased security is required to have a long-term rating no lower than A3/A- or equivalent. Concentration of Credit Risk and Other Risks and Uncertainties —Financial instruments that potentially subject the Company to a concentration of credit risk, consist primarily of cash, cash equivalents and short-term and long-term investments. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. Management believes that these financial institutions are financially sound, and, accordingly, minimal credit risk exists with respect to those financial institutions. The Company is exposed to credit risk in the event of default by the financial institutions holding its cash, cash equivalents and short-term and long-term investments and by the issuers of the securities to the extent recorded in the balance sheet. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredient and drug product related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs . Property and Equipment —Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is determined using the straight-line method over the estimated useful lives of the respective assets, which is three years. Leasehold improvements are amortized using the straight-line method over the shorter of the lease term or their estimated useful economic lives. Impairment of Long-Lived Assets —Long-lived assets consist of property and equipment. The Company assesses potential impairment losses on long-lived assets used in operations when events and circumstances indicate that assets might be impaired. If such events or changes in circumstances arise, the Company compares the carrying amount of the long-lived assets to the estimated future undiscounted cash flows expected to be generated by the long-lived assets. If the estimated aggregate undiscounted cash flows are less than the carrying amount of the long-lived assets, an impairment charge, calculated as the amount by which the carrying amount of the assets exceeds the fair value of the assets, is recorded. The fair value of the long-lived assets is determined based on the estimated discounted cash flows expected to be generated from the long-lived assets. The Company has no t recognized any impairment losses in the years ended December 31, 2018 , 2017 and 2016 . Deferred Offering Costs —The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the equity financing. Clinical and Manufacturing Accruals —The Company records accruals for estimated costs of research, nonclinical and clinical studies and manufacturing development, which are a significant component of research and development expenses. A substantial portion of the Company’s ongoing research and development activities is conducted by third-party service providers, including clinical research organizations, or CROs, and contract manufacturing organizations, or CMOs. The Company’s contracts with CROs generally include pass-through fees such as regulatory expenses, investigator fees, travel costs and other miscellaneous costs, including shipping and printing fees. The financial terms of these contracts are subject to negotiations, which vary from contract to contract and may result in payment flows that do not match the periods over which materials or services are provided to the Company under such contracts. The Company accrues the costs incurred under agreements with these third parties based on estimates of actual work completed in accordance with the respective agreements. The Company determines the estimated costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fees to be paid for such services. The Company makes significant judgments and estimates in determining the accrual balance in each reporting period. As actual costs become known, the Company adjusts its accruals. Although the Company does not expect its estimates to be materially different from amounts actually incurred, the Company’s understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and could result in the Company reporting amounts that are too high or too low in any particular period. The Company’s accrual is dependent, in part, upon the receipt of timely and accurate reporting from information provided as part of its clinical and nonclinical studies and other third-party vendors. Through December 31, 2018 , there have been no material differences from the Company’s accrued estimated expenses to the actual clinical trial and manufacturing expenses. However, variations in the assumptions used to estimate accruals, including, but not limited to the number of patients enrolled, the rate of patient enrollment, the actual services performed, and the amount of manufactured drug substance and/or drug product, and related costs may vary from the Company’s estimates, resulting in adjustments to research and development expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect its financial position and results of operations. Lease —The Company leases its office and laboratory facility and the lease is classified as an operating lease. Rent expense is recognized on a straight-line basis over the term of the lease and, accordingly, the Company records the difference between cash rent payments and rent expense recognized as a deferred rent liability. Incentives granted under the Company’s facility lease, including allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the term of the lease. Term Loan —The Company accounts for the Loan and Security Agreement, or Term Loan, with Hercules Capital, Inc., or Hercules, as a liability measured at net proceeds less debt discount and is accreted to the face value of the Term Loan over its expected term using the effective interest method. The Company considers whether there are any embedded features in its debt instruments that require bifurcation and separate accounting as derivative financial instruments pursuant to Accounting Standards Codification, or ASC, Topic 815, Derivatives and Hedging . Convertible Preferred Stock —The Company recorded all shares of convertible preferred stock at their respective fair values, net of issuance costs, on the dates of issuance. In the event of a change of control of the Company, proceeds would have been distributed in accordance with the liquidation preferences set forth in its Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock had converted their convertible preferred shares into common shares. Therefore, convertible preferred stock was classified outside of stockholders’ equity (deficit) on the accompanying balance sheets as events triggering the liquidation preferences were not solely within the Company’s control. The Company elected not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur. Upon the closing of the IPO on July 2, 2018 , the 104,225,638 shares of convertible preferred stock outstanding were automatically converted into 26,187,321 shares of common stock. Warrants —The Company issued freestanding warrants to purchase shares of its Series A convertible preferred stock. Freestanding warrants for shares of the Company’s convertible preferred stock were classified outside of permanent equity, were recorded at fair value and were subject to remeasurement at each balance sheet date until the earlier of the exercise of the warrants or the completion of a liquidation event, including the completion of an initial public offering. Immediately prior to exercise, the warrant liability was remeasured to fair value and, upon exercise, the warrant liability was reclassified to additional paid-in capital, with any change in fair value recognized as a component of other income (expense), net. See Note 3 " Fair Value Measurements and Fair Value of Financial Instruments " to these financial statements for additional details. The Company also issued freestanding warrants to purchase shares of common stock in connection with its Term Loan. The warrants are recorded at fair value using the Black-Scholes option pricing model. See Note 5 " Borrowings " to these financial statements for additional details. Preferred Stock Tranche Obligation —The Company entered into convertible preferred stock financings where, in addition to the initial closing, investors agreed to buy, and the Company agreed to sell, additional shares of that convertible preferred stock at a set price in the event that certain agreed milestones are achieved (a tranched financing). The Company evaluated this tranche obligation and determined that it met the definition of a freestanding instrument, and accordingly, determined the fair value of this obligation and recorded it on the balance sheet with the residual of the proceeds raised being allocated to convertible preferred stock. The preferred stock tranche obligation was revalued each reporting period with changes in the fair value of the obligation recorded as a component of other income (expense), net in the statements of operations and comprehensive loss. The preferred stock tranche obligation was revalued at settlement and the resultant fair value was then reclassified to convertible preferred stock at that time. Research and Development Expense —Research and development expense is charged to the statements of operations and comprehensive loss in the period in which they are incurred. Research and development expense consists primarily of salaries and related costs, including stock-based compensation expense, for personnel and consultants in our research and development functions; fees paid to clinical consultants, clinical trial sites and vendors, including CROs, costs related to pre-commercialization manufacturing activities including payments to CMOs and other vendors and consultants, costs related to regulatory activities, expenses related to lab supplies and services and depreciation and other allocated facility-related and overhead expenses. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed. Stock-Based Compensation —Stock-based compensation expense represents the grant-date fair value of awards recognized on a straight-line basis over the employee's requisite service period (generally the vesting period). The Black-Scholes option-pricing model is used to calculate stock-based compensation expense for stock option awards and shares purchased under the Employee Stock Purchase Plan, or ESPP. For restricted stock units, or RSUs, the grant-date fair value is the closing price of the Company's common stock on the date of grant as reported on The Nasdaq Global Select Market. Because stock compensation expense is an estimate of awards ultimately expected to vest, it is reduced by an estimate for future forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates. The Company records the expense attributed to nonemployee services paid with stock option awards based on the estimated fair value of the awards determined using the Black-Scholes option pricing model. The measurement of stock-based compensation for nonemployees was previously subject to remeasurement as the options vested. As of July 1, 2018, the Company adopted ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting , which no longer subjects nonemployee awards to remeasurement. The expense is recognized over the period during which services are received. Income Taxes —The Company accounts for income taxes using the liability method, whereby deferred tax asset and liability account balances are determined based on differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company provides a valuation allowance when it is more likely than not that some portion or all of its deferred tax assets will not be realized. The Company accounts for income tax contingencies using a benefit recognition model. If it considers that a tax position is more likely than not to be sustained upon audit, based solely on the technical merits of the position, it recognizes the benefit. The Company measures the benefit by determining the amount that is greater than 50% likely of being realized upon settlement, presuming that the tax position is examined by the appropriate taxing authority that has full knowledge of all relevant information. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Comprehensive Loss —Comprehensive loss includes net loss and certain changes in stockholders’ equity (deficit) that are excluded from net loss, primarily unrealized losses on the Company’s available-for-sale securities. Net Loss per Share —Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented. Segment Reporting —The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. All of the Company's assets are maintained in the United States. Reverse Stock Split —On June 14, 2018 the Company’s board of directors approved a 1-for-3.98 reverse split of shares of the Company's common stock. The par values and the authorized shares of the common stock and convertible preferred stock were not adjusted as a result of the reverse stock split, nor were the outstanding shares of convertible preferred stock. All issued and outstanding common stock and related per share amounts contained in the accompanying financial statements have been retroactively adjusted to reflect this reverse stock split for all periods presented. The reverse stock split was effected on June 15, 2018. Recent Accounting Pronouncements Adopted Standards In May 2017, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2017-09, Compensation - Stock Compensation (Topic 718) : Scope of Modification Accounting . This ASU specifies the modification accounting applicable to any entity which changes the terms or conditions of a share-based payment award. The Company elected to prospectively adopt this ASU beginning January 1, 2018. The adoption of this ASU did not have a material impact on the Company's financial statements. In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815), (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception . The guidance in ASU No. 2017-11 allows for the exclusion of a down round feature, when evaluating whether or not an instrument or embedded feature requires derivative classification. The Company early adopted this guidance beginning January 1, 2018. The adoption of this standard had no material impact on the Company’s financial statements. In June 2018, the FASB issued ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (Topic 718) , which amends ASC Topic 718, “Compensation—Stock Compensation”. The ASU simplifies the accounting for share-based payments granted to nonemployees for goods and services. Under the ASU, most of the guidance on such payments to nonemployees would be aligned with the requirements for share-based payments granted to employees. The Company early adopted this guidance effective July 1, 2018. Upon adoption of this standard, share-based awards issued to nonemployees are measured at the grant date and are not subject to remeasurement. The Company has elected to continue to use the contractual term as the estimated expected term. The adoption of ASU No. 2018-07 did not have a material impact on the Company's financial statements and is expected to reduce the volatility in stock-based compensation expense for nonemployees recognized from period to period. Standards Not Yet Effective In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) , which for operating leases requires the lessee to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, on its balance sheet. The guidance requires a lessee to recognize single lease costs, calculated so that the cost of the lease is allocated over the lease term, on a generally straight-line basis. The new standard also requires expanded disclosures regarding leasing arrangements. The Company leases certain facilities for some of its operations, which represents a majority of its operating leases it has entered into as a lessee. The Company expects the implementation of ASC Topic 842 to have an impact on its financial statements and related disclosures as it had aggregate future minimum lease payments of approximately $2.7 million as of December 31, 2018 . The new standard becomes effective for the Company beginning January 1, 2019. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements , which provides an alternative modified retrospective transition method. Under this method, the cumulative-effect adjustment to the opening balance of retained earnings is recognized on the date of adoption and it allows entities not to apply the new leases standard, including its disclosure requirements, in the comparative periods presented in their financial statements in the year of adoption. The Company intends to elect this transition method at the adoption date of January 1, 2019, and as a result, will record a right-of-use asset and a corresponding lease liability on the balance sheet for all leases with terms longer than 12 months. The Company expects to elect the practical expedients upon transition that will retain the lease classification and initial direct costs for any leases that existed prior to the adoption of the new guidance. The Company is continuing to evaluate the effect that this guidance will have on its financial statements and related disclosures. 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 used to develop significant unobservable inputs for 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	12 Months Ended
	Dec. 31, 2018
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 ASC Topic 820, Fair Value Measurements and Disclosures issued by the FASB. 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. Our financial instruments consist primarily of cash and cash equivalents, short-term and long-term investments, accounts payable and the Term Loan with Hercules. Cash and cash equivalents and investments are reported at their respective fair values on our 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 and cash equivalents and available-for-sale investments' amortized cost, gross unrealized gains, gross unrealized losses and fair values by significant investment category reported as cash and cash equivalents, short-term investments or long-term investments as of December 31, 2018 and 2017 . December 31, 2018 Reported as: (in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses 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 December 31, 2017 Reported as: (in thousands) Amortized cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Short-term Investments Long-term investments Cash $ 86 $ — $ — $ 86 $ 86 $ — $ — Level 1: Money market fund 6,758 — — 6,758 6,758 — — Level 2: Commercial paper 25,780 — (7 ) 25,773 — 25,773 — Corporate debt securities 20,545 3 (5 ) 20,543 2,930 17,613 — Asset-backed securities 14,358 — (4 ) 14,354 — 14,354 — Subtotal 60,683 3 (16 ) 60,670 2,930 57,740 — Total assets measured at fair value $ 67,527 $ 3 $ (16 ) $ 67,514 $ 9,774 $ 57,740 $ — Interest income related to our cash and cash equivalents and available-for-sale investments included in other income (expense), net was approximately $3.5 million , $0.4 million and $0.2 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. The following table summarizes the maturities of the Company’s cash equivalents (excluding money market funds) and available-for-sale investments, at December 31, 2018 . (in thousands) Amortized Cost Fair Value Mature in less than one year $ 205,060 $ 204,903 Mature in one to five years 2,283 2,287 Total $ 207,343 $ 207,190 As of December 31, 2018 , unrealized losses on available-for-sale investments are not attributable to credit risk and are considered to be temporary. The following table summarizes our available-for-sale debt securities that were in a continuous unrealized loss position for less than 12 months, but were not deemed to be other-than-temporarily impaired, as of December 31, 2018 and 2017 . December 31, 2018 December 31, 2017 (in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 67,407 $ (63 ) $ 19,798 $ (7 ) Corporate debt securities 85,699 (63 ) 13,394 (5 ) Asset-backed securities 36,730 (34 ) 14,354 (4 ) Total $ 189,836 $ (160 ) $ 47,546 $ (16 ) We held a total of 48 and 21 positions, which were in an unrealized loss position as of December 31, 2018 and 2017 , respectively. Based on our review of these securities, we believe we had no other-than-temporary impairments as of December 31, 2018 and 2017 , because we do not intend to sell these securities nor do we believe that we will be required to sell these securities before the recovery of their amortized cost basis. Gross realized gains and gross realized losses were not material for the years presented. 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 years ended December 31, 2018 and 2017 . The following table presents a reconciliation of financial assets and liabilities measured at fair value on a recurring basis using Level 3 unobservable inputs for the years ended December 31, 2018 and 2017 . 2018 2017 (in thousands) Compound Derivative Liability Warrant Liability Convertible Preferred Stock Tranche Obligation Warrant Liability Fair value at beginning of year $ — $ 106 $ 3,371 $ 38 Addition 654 156 2,278 — Change in fair value (493 ) 305 (5,649 ) 68 Reclassification to equity — (194 ) — — Issuance of convertible preferred stock on exercise of warrant — (373 ) — — Fair value at end of year $ 161 $ — $ — $ 106 The following table presents information about significant unobservable inputs related to the Company's significant Level 3 financial assets and liabilities as of December 31, 2018 . December 31, 2018 (in thousands) Fair Value Valuation Technique Significant Unobservable Input Range of Inputs Compound derivative liability $ 161 Discounted cash flow Discount rate 13.0 % - 14.0 % Probability of the occurrence of certain events 10 % Term Loan The estimated fair value of the Term Loan was $37.8 million as of December 31, 2018 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 13% and 14% .

OTHER BALANCE SHEET COMPONENTS

OTHER BALANCE SHEET COMPONENTS	12 Months Ended
	Dec. 31, 2018
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 December 31, 2018 and 2017 . (in thousands) December 31, December 31, Furniture and fixtures $ 193 $ 193 Computer and lab equipment 1,888 1,382 Leasehold improvements 1,055 878 3,136 2,453 Less: accumulated depreciation and amortization (1,921 ) (1,303 ) Total property and equipment, net $ 1,215 $ 1,150 Depreciation and amortization expense was approximately $0.6 million , $0.3 million and $0.4 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. Accrued Expenses and Other Current Liabilities The following table presents the components of accrued expenses and other current liabilities as of December 31, 2018 and 2017 . (in thousands) December 31, December 31, Accrued clinical and nonclinical study costs $ 2,168 $ 2,235 Accrued contract manufacturing 1,676 4,157 Accrued compensation 1,565 — Accrued professional fees and other 935 969 Total accrued expenses and other current liabilities $ 6,344 $ 7,361

BORROWINGS

BORROWINGS	12 Months Ended
	Dec. 31, 2018
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 and Hercules entered into the second amendment to the Term Loan, which amended certain terms of the Term Loan. After giving effect to the second amendment, the Term Loan continues 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 third tranche was increased from $15.0 million to $35.0 million and will be available on or before December 31, 2019, on condition that the Company submits a New Drug Application, or NDA, to the U. S. Food and Drug Administration, or FDA, which the FDA accepts for review. The fourth tranche of $10.0 million was not modified and will be available on or before December 15, 2020, provided that the Company obtains product approval from the FDA for the NDA for TRC101 on or before December 15, 2020. The fifth tranche was reduced from $25.0 million to $15.0 million and will be available on or before December 31, 2020, upon request by the Company and the approval of Hercules' investment committee. The Company accounted for the second 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 5.00% and (y) 9.85% . The Term Loan repayment schedule provides for interest only payments for the first 16 months, followed by consecutive equal monthly payments of principal and interest commencing on April 1, 2020 and continuing through the maturity date of March 1, 2022. The Term Loan also provides for a $650,000 facility fee that was paid at closing and an additional payment equal to 6.55% multiplied by the greater of (i) the aggregate term loans funded and (ii)(a) the aggregate term loans funded plus (b) one half of (x) $60.0 million minus (y) the aggregate term loans funded, which is due when the Term Loan becomes due or upon prepayment of the facility. If the Company elects to prepay the Term Loan, there is also a prepayment fee of between 1% and 1.5% of the principal amount being prepaid depending on the timing and circumstances of prepayment. The Term Loan is secured by substantially all of the Company’s assets, except the Company’s intellectual property, which is the subject of a negative pledge. On March 27, 2019, the Company modified the Term Loan with Hercules. See Note 11 " Subsequent Events " for additional information about the third amendment to the Term Loan. Warrants In conjunction with the Term Loan entered into on February 28, 2018, the Company issued a warrant 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 warrant 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 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 warrants as of April 10, 2018 was determined using an option pricing model with the following assumptions: time to liquidity of 0.25 to 1.70 years, volatility of 72% , risk-free rate of 2.4% and equity value of $306.0 million to $420.0 million . 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 at 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 . The fair value of the common stock warrant was determined using an option-pricing model with the following assumptions: time to liquidity of 2.5 years, volatility of 75% , risk-free rate of 2.51% and equity value based on the December 28, 2018 closing price of the Company's common stock reported by The Nasdaq Global Select Market. In connection with any subsequent funding of tranches three through five, the Company is obligated to issue 53,458 shares of common stock for the third tranche, 21,383 shares of common stock for the fourth tranche and 32,075 shares of common stock for the fifth tranche. 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 $654,000 , which is required to be marked to market in future periods. As of December 31, 2018 , 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 13.5% and the probability of the occurrence of certain events of 10% . 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 statements of operations and comprehensive loss. The fair value of the compound derivative liability was approximately $0.2 million and was classified as other long-term liabilities on the 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 balance sheet 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 December 31, 2018 , there were unamortized issuance costs and debt discounts of $2.7 million, which were recorded as a direct deduction from the Term Loan on the balance sheet. The following table presents future payments of principal and interest on the Term Loan as of December 31, 2018 . (in thousands) December 31, 2018 2019 $ 3,486 2020 17,317 2021 21,896 2022 8,820 51,519 Less: amount representing interest (11,519 ) 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 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 accompanying statements of operations and comprehensive loss. The resultant fair value was reclassified to Series A convertible preferred stock at this time. As of December 31, 2017 , the fair value of the warrant was approximately $0.1 million and was classified as a long-term liability on the balance sheet.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2018
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 through June 30, 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. The Company recognizes rent expense on a straight-line basis over the noncancelable term of its operating lease. Rent expense was $1.0 million , $0.7 million , and $0.5 million during the years ended December 31, 2018, 2017, and 2016, respectively. Future minimum lease payments under the lease as of December 31, 2018 , not including any related common area maintenance charges or real estate taxes, were as follows. (in thousands) December 31, 2018 2019 $ 1,076 2020 1,108 2021 562 2022 and thereafter — Total future minimum lease payments $ 2,746 In addition, associated with the operating lease, the Company has a tenant improvement loan with remaining payments totaling approximately $0.3 million over the remaining life of the lease. 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 $49.8 million , of which $12.6 million was paid through December 31, 2018 and $37.2 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 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. Management 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, management 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.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
	Dec. 31, 2018
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 December 31, 2018 and 2017 , the Company had 42,148,247 and 2,272,609 shares of common stock outstanding, respectively. As of December 31, 2018 and 2017 , the Company had zero shares of preferred stock outstanding. Common Stock On July 2, 2018 , the Company completed its initial public offering, or 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. Common stock reserved for future issuance, on an as if converted basis, consisted of the following. December 31, December 31, Convertible preferred stock, issued and outstanding — 26,163,217 Stock options and RSUs issued and outstanding 4,599,307 3,588,663 Stock options, RSUs and ESPP shares authorized for future issuance 4,534,784 884,947 Total 9,134,091 30,636,827 Convertible Preferred Stock As of December 31, 2017 , convertible preferred stock consisted of the following. December 31, 2017 (in thousands, except share amounts) Authorized Shares Shares Issued and Outstanding Net Proceeds Aggregate Liquidation Preference Shares designated as: Series A convertible preferred stock 11,398,694 11,302,758 $ 9,800 $ 10,014 Series B convertible preferred stock 32,526,878 32,526,878 29,618 30,250 Series C convertible preferred stock 35,806,451 35,806,451 50,347 55,500 Series D convertible preferred stock 24,500,000 24,493,615 57,305 57,560 Total 104,232,023 104,129,702 $ 147,070 $ 153,324

STOCK-BASED COMPENSATION

STOCK-BASED COMPENSATION	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
STOCK-BASED COMPENSATION	STOCK-BASED COMPENSATION Equity Incentive Plans During 2013, the Company adopted an equity compensation plan, the 2013 Equity Incentive Plan, or 2013 Plan, for eligible employees, officers, directors, advisors, and consultants. The 2013 Plan provided for the grant of incentive and non-statutory stock options. In the year ending December 31, 2016 , in conjunction with the closing of the Company’s Series C financing, the number of shares of common stock reserved for issuance under the 2013 Plan was increased to 2,912,058 . In the year ending December 31, 2017 , in conjunction with the closing of the Company’s Series D financing, the number of shares of common stock reserved for issuance under the 2013 Plan was increased to 4,532,661 . In the year ending December 31, 2018 , the number of shares of common stock reserved for issuance under the 2013 Plan was increased to 4,935,929 . In June 2018, the Company's board of directors and stockholders approved the 2018 Equity Incentive Plan, or 2018 Plan. Any shares of common stock covered by awards granted under the 2013 Plan that terminate after June 22, 2018 by expiration, forfeiture, or cancellation were added to the 2018 Plan reserve and shares available for future issuance under the 2013 Plan were canceled. The initial number of shares of common stock available for issuance under the 2018 Plan was 4,000,000 . Unless our board of directors provides otherwise, beginning on January 1, 2019 and continuing until the expiration of the 2018 Plan, the total number of shares of common stock available for issuance under the 2018 Plan will automatically increase annually on January 1 by the lesser of (i) 3,200,000 shares of Common Stock, (ii) 4% of the total number of issued and outstanding shares of common stock as of December 31 of the immediately preceding year and (iii) an amount determined by the board of directors. Under the 2018 Plan, any shares that are forfeited or expired are added back to the shares available for issuance. As of December 31, 2018 , 3,752,893 shares of common stock were available for future issuance of options, restricted stock and other stock-based awards under the 2018 Plan. The terms of the stock option agreements, including vesting requirements, are determined by the board of directors, subject to the provisions of the plans. Options granted by the Company vest over a period of one to four years and are exercisable after they have been granted for up to 10 years from the date of grant. Per the company’s equity incentive plan, the term of the option expires, upon the earliest of 1) termination of continuous service for cause 2) three months after the termination of continuous service for reasons other than cause, death or disability 3) twelve months after the termination of continuous service due to disability 4) eighteen months after the employee’s death if the employee died during the period of continuous service 5) expiration date in the grant notice or 6) the day before the tenth anniversary of the date of grant. The exercise price of the incentive stock options must equal at least the fair market value of the stock on the date of grant. The 2013 Plan and the 2018 Plan allow for early exercise where the option holders may exercise their options prior to vesting. Common stock that is issued upon the early exercise of options is subject to repurchase by the Company at the original exercise price at the option of the Company. As of December 31, 2018 and December 31, 2017 , there were 20,193 and 366 shares of common stock that were subject to repurchase with an aggregate purchase price of approximately $88,940 and $350 at an average repurchase price of $4.40 and $0.96 per share, respectively. The following table summarizes stock option activity under the plans for the years ended December 31, 2018 and 2017 . Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (Thousands) Balance at December 31, 2016 2,259,011 $ 1.13 9.2 Granted 1,354,235 2.21 Exercised (7,286 ) 0.76 Forfeited or canceled (17,297 ) 0.87 Balance at December 31, 2017 3,588,663 1.54 8.5 Granted 1,627,556 15.21 Exercised (219,974 ) 1.37 Forfeited or canceled (418,730 ) 6.13 Balance at December 31, 2018 4,577,515 $ 5.99 8.0 $ 84,290 Vested and expected to vest at December 31, 2018 4,304,521 $ 5.62 7.9 $ 80,514 All outstanding options can be early exercised. Beginning in the fiscal year ending December 31, 2018 , the Company began issuing restricted stock units, or RSUs, to directors under the 2018 Plan. Awards granted to directors vest on the earlier of the one -year anniversary of the award's date of grant, or the date of the Company’s next annual meeting of stockholders that occurs following the date of grant. The following table summarizes RSU activity under the 2018 Plan for the year ended December 31, 2018 . Shares Weighted- Fair Value Unvested balance at December 31, 2017 — — Granted 21,792 19.00 Vested — Forfeited — Unvested balance at December 31, 2018 21,792 $ 19.00 There were no RSUs that vested during the year ended December 31, 2018 . Employee Stock Purchase Plan In June 2018, the Company's board of directors and stockholders approved the Tricida Inc. ESPP. The ESPP allows eligible employees to have up to 15 percent of their eligible compensation withheld and used to purchase common stock, subject to a maximum of $25,000 worth of stock purchased in a calendar year or no more than 2,500 shares in an offering period, whichever is less. An offering period consists of a six -month purchase period, with a look back feature to our stock price at the commencement of the offering period. Eligible employees can purchase the Company’s common stock at the end of the offering period at 85% of the lower of the closing price of our common stock on The Nasdaq Global Select Market on the first and last day of the offering periods. The initial number of shares of common stock available for issuance under the ESPP, was 800,000 . Unless the Company's board of directors provides otherwise, beginning on January 1, 2019, the maximum number of shares which shall be made available for sale under the ESPP will automatically increase on the first trading day in January of each calendar year during the term of the ESPP by an amount equal to the lesser of (i) one percent ( 1% ) of the total number of shares issued and outstanding on December 31 of the immediately preceding calendar year, (ii) 800,000 shares or (iii) an amount determined by the board of directors. In the year ended December 31, 2018 , the Company issued 18,109 shares under the ESPP, representing approximately $0.4 million in employee contributions. As of December 31, 2018 , there were 781,891 shares reserved for future issuance under the ESPP. Stock Option Valuation Assumptions As stock-based compensation recognized is based on options ultimately expected to vest, the expense has been reduced for estimated forfeitures. The Company uses the Black-Scholes option pricing model to determine the estimated fair value of stock options at the date of the grant. The Black-Scholes model includes inputs that require the input of subjective assumptions, including expected volatility, expected dividend yield, expected term, risk-free rate of return, and the estimated fair value of the underlying common stock on the date of grant. Expected Term: The expected term of the options represents the average period the stock options are expected to remain outstanding. As the Company does not have sufficient historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior, the expected term of options granted is derived from the average midpoint between the weighted average vesting term and the contractual term, also known as the simplified method. Expected Volatility: Since the Company does not have sufficient trading history for its common stock, the expected volatility is based on the historical volatilities of the common stock of comparable publicly traded companies. The Company selected companies with comparable characteristics, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of Tricida’s stock-based awards. Risk-Free Interest Rate: The risk-free interest rate is based on the yield of U.S. Treasury notes as of the grant date with terms commensurate with the expected term of the option. Expected Dividends: The expected dividends assumption is based on the Company’s expectation of not paying dividends in the foreseeable future. The fair value of the stock options granted to employees was calculated with the following assumptions. Years Ended December 31, 2018 2017 2016 Risk-free interest rate 2.7 % 1.7 % 1.7% - 1.8% Expected volatility 64.6 % 72.7% - 78.5% 76.3% - 78.4% Expected term (in years) 5.8 6.2 - 6.3 6.2 - 6.3 Expected dividends — % — % — % The fair value of the stock options granted to nonemployees was calculated with the following assumptions. Years Ended December 31, 2018 2017 2016 Risk-free interest rate 2.7% 1.7% - 2.2% 2.1% - 2.2% Expected volatility 68.4% 70.8% - 73.1% 80.1% - 80.4% Expected term (in years) 10.0 6.5 - 8.9 8.6 - 8.7 Expected dividends —% — % — % The following table summarizes the weighted-average fair value per share of stock options granted and the total intrinsic value of stock options exercised (in thousands) for the years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, (in thousands, except per share amounts) 2018 2017 2016 Stock options granted - weighted-average grant date fair value per share $ 10.39 $ 1.67 $ 1.11 Stock options exercised - intrinsic value 1,935 30 17 Stock-Based Compensation The following table presents stock-based compensation expense as reported in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, (in thousands) 2018 2017 2016 Research and development $ 2,643 $ 379 $ 185 General and administrative 2,509 497 87 Total $ 5,152 $ 876 $ 272 The following table presents stock-based compensation expense by award type as reported in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, (in thousands) 2018 2017 2016 Stock options $ 4,806 $ 876 $ 272 RSUs 202 — — ESPP 144 — — Total $ 5,152 $ 876 $ 272 As of December 31, 2018 and December 31, 2017 , there was approximately $15.0 million and $2.7 million , respectively, of unrecognized stock-based compensation associated with stock options, which the Company expects to recognize over a weighted-average period of 2.4 and 2.8 years, respectively. As of December 31, 2018 , there was approximately $0.2 million of unrecognized stock-based compensation associated with RSUs, which the Company expects to recognize over a weighted-average period of 0.5 years.

NET LOSS PER SHARE

NET LOSS PER SHARE	12 Months Ended
	Dec. 31, 2018
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 years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, (In thousands, except share and per share amounts) 2018 2017 2016 Numerator: Net loss $ (102,808 ) $ (41,290 ) $ (28,651 ) Denominator: Weighted average common shares outstanding 22,158,354 2,267,732 2,255,897 Less: weighted average shares subject to repurchase (12,162 ) (130,042 ) (429,857 ) Weighted average number of shares used in basic and diluted net loss per share 22,146,192 2,137,690 1,826,040 Net loss per share, basic and diluted $ (4.64 ) $ (19.32 ) $ (15.69 ) 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. December 31, 2018 2017 2016 Series A convertible preferred stock — 2,839,886 2,839,886 Series B convertible preferred stock — 8,172,579 8,172,579 Series C convertible preferred stock — 8,996,586 4,907,597 Series D convertible preferred stock — 6,154,166 — Warrants to purchase preferred or common stock 106,916 24,104 24,104 Common stock subject to repurchase 20,193 366 8,793 Options and RSUs issued and outstanding 4,599,307 3,588,663 2,259,011 Total 4,726,416 29,776,350 18,211,970

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Tax	INCOME TAXES The Company did not record a provision or benefit for income taxes during the years ended December 31, 2018 , 2017 and 2016 . The significant components of the Company’s net deferred tax assets as of December 31, 2018 and 2017 are shown below. December 31, (in thousands) 2018 2017 Deferred tax assets Net operating loss carryforwards $ 45,215 $ 24,939 Research and development credits 4,090 2,807 Capitalized assets 23 79 Accruals and reserves 390 149 Other—net 806 90 Gross deferred tax assets 50,524 28,064 Valuation allowance (50,524 ) (28,064 ) Net deferred tax assets $ — $ — A valuation allowance is established when it is more likely than not that a deferred tax asset will not be realized. As of December 31, 2018 and 2017 , the Company's valuation allowance was $50.5 million and $28.1 million , respectively. The valuation allowance increased by $22.4 million for the year ended December 31, 2018 . The change in the 2018 valuation allowance was primarily due to the addition of the 2018 net operating loss carryforwards. The following is a reconciliation between the U.S. federal income statutory tax rate and the Company’s effective tax rate for the years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, 2018 2017 2016 U.S. federal statutory rate 21 % 34.0 % 34.0 % State income taxes, net of federal benefit — 5.8 5.8 Stock-based compensation (0.2 ) (0.7 ) (0.3 ) Permanent adjustments (0.1 ) 4.2 (3.4 ) Change to valuation allowance (21.9 ) (19.6 ) (39.3 ) Tax Cuts and Jobs Act Impact — (25.4 ) — Research and development credits 1.3 1.6 3.2 Other (0.1 ) — — Effective tax rate — % — % — % On December 22, 2017, the Tax Cut and Jobs Act, or Tax Act, was signed into law. Among other changes under the Tax Act was a permanent reduction in the federal corporate income tax rate from 35% to 21% effective January 1, 2018. As a result of the reduction in the corporate income tax rate, the Company revalued its net deferred tax assets at December 31, 2017 , as the changes in tax law are accounted for in the period of enactment, resulting in a reduction in the value of our net deferred tax assets of approximately $10.5 million, offset by a $10.5 million change in valuation allowance as of that date. As of December 31, 2018 , the Company had approximately $185.5 million of federal net operating losses available for future use. Federal net operating losses incurred prior to January 1, 2018 of approximately $89.1 million expire beginning in 2033 while federal net operating losses incurred after December 31, 2017 of approximately $96.4 million will have an indefinite carryforward period, subject to annual limitations. Federal research credits of approximately $3.6 million that are available for future use expire beginning in 2033 . At December 31, 2018 , the Company also had approximately $89.6 million of state net operating losses available for future use that expire beginning in 2033 and state research credits of approximately $2.1 million that have no expiration date. Utilization of the net operating loss carryforwards and the research and development credits carryforwards may be subject to a substantial annual limitation due to ownership change limitations that may have occurred or that could occur in the future, as required by Sections 382 and 383 of the Internal Revenue Code of 1986, or the Code, as amended, as well as similar state and foreign provisions. The Company has not completed a study to assess whether an ownership change has occurred or whether there have been multiple ownership changes since the Company’s formation due to the complexity and cost associated with such a study and the fact that there may be additional such ownership changes in the future. If the Company has experienced an ownership change at any time since its formation, utilization of the net operating loss or research credit carryforwards would be subject to an annual limitation under Section 382 of the Code. Such limitation is determined by first multiplying the value of the Company’s stock at the time of the ownership change by the applicable long-term and tax-exempt rate, and then could be subject to additional adjustments, as required. Any limitation may result in expiration of a portion of the net operating loss or research credit carryforwards before utilization. Further, until a study is completed, and any limitation known, no amounts are being considered as an uncertain tax position or disclosed as an unrecognized tax benefit. Due to the existence of the valuation allowance, future changes in the Company’s unrecognized tax benefits will not impact its effective tax rate. Any carryforwards that will expire prior to utilization as a result of such limitations will be removed from deferred tax assets with a corresponding reduction of the valuation allowance. ASC Topic 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of any uncertain tax positions that have been taken or expected to be taken on a tax return. As of December 31, 2018 and 2017 , the Company had unrecognized tax benefits of $1.3 million and $0.8 million , respectively. The amount of unrecognized tax benefits is not expected to significantly change over the next twelve months. No amounts, outside of valuation allowance, would impact the effective tax rate on continuing operations. The beginning and ending gross unrecognized tax benefits amounts are as follows. Years Ended December 31, (in thousands) 2018 2017 2016 Gross unrecognized tax benefits at beginning of year $ 819 $ 574 $ 390 Additions for tax positions related to prior year — 2 19 Decrease related to prior year tax provisions — (92 ) (101 ) Additions for tax positions related to current year 478 335 266 Gross unrecognized tax benefits at end of year $ 1,297 $ 819 $ 574 It is the Company’s policy to include penalties and interest expense related to income taxes as a component of income tax expense as necessary. Management determined that no accrual for interest and penalties was required as of December 31, 2018 . The Company’s tax jurisdictions are the United States and California. The Company’s tax years from 2013 to 2018 will remain open for examination by the federal and state authorities for three and four years respectively, from the date of utilization of any net operating loss or tax credits. The Company is not currently subject to income tax examinations by any authority.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS 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 under the Term Loan with Hercules 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 TRC101. 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 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 we prepay the Term Loan we 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. 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. 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. The Company is currently evaluating the accounting impact of the loan modification.

SELECTED QUARTERLY FINANCIAL DA

SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)	12 Months Ended
	Dec. 31, 2018
Condensed Financial Information Disclosure [Abstract]
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)	SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) The following presents certain unaudited quarterly financial information for the years ended December 31, 2018 and 2017 . This information has been prepared on the same basis as the audited financial statements and includes all adjustments (consisting only of normal recurring adjustments) necessary to present fairly the unaudited quarterly results of operations set forth herein. Net loss per share for all periods presented has been retroactively adjusted to reflect the 1-for-3.98 reverse stock split effected on June 15, 2018. 2018 (in thousands, except per share data) Q1 Q2 Q3 Q4 Operating expenses $ 20,098 $ 25,279 $ 29,408 $ 28,810 Net loss (20,504 ) (25,362 ) (29,098 ) (27,844 ) Net loss per share, basic and diluted $ (9.00 ) $ (10.89 ) $ (0.71 ) $ (0.66 ) 2017 (in thousands, except per share data) Q1 Q2 Q3 Q4 Operating expenses $ 8,669 $ 6,922 $ 10,874 $ 20,657 Net loss (2,209 ) (7,726 ) (10,805 ) (20,550 ) Net loss per share, basic and diluted $ (1.08 ) $ (3.60 ) $ (4.81 ) $ (9.05 )

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	The accompanying financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America, or GAAP.
Use of Estimates	Use of Estimates —The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents —All highly liquid investments with maturities at the date of purchase of three months or less are classified as cash equivalents. There are no restrictions on cash and cash equivalents.
Investments	Investments —All investments with maturities of greater than three months at the date of purchase and maturities of less than one year at the reporting date are classified as short-term investments, while investments with maturities of a year or more at the reporting date are classified as long-term investments. Management has classified the Company’s investments as available-for-sale in the accompanying financial statements. Available-for-sale investments are carried at fair value, with unrealized gains and losses reported as a component of accumulated other comprehensive loss. Realized gains and losses on the sale of all such securities are reported in other income (expense), net and are computed using the specific identification method. Available-for-sale investments are subject to a periodic impairment review. The Company may recognize an impairment charge when a decline in the fair value of investments below the cost basis is determined to be other-than-temporary. In determining whether a decline in market value is other-than-temporary, various factors are considered, including the cause, duration of time and severity of the impairment, any adverse changes in the investees’ financial condition, and the Company’s intent and ability to hold the security for a period of time sufficient to allow for an anticipated recovery in market value. When the Company determines that the decline in the fair value of an investment is below the accounting basis and the decline is other-than-temporary, the carrying value of the security is reduced and a loss is recorded for the amount of such decline. There were no investments deemed to be impaired as of December 31, 2018. The Company’s investments are in commercial paper, corporate debt securities and asset-backed securities. Pursuant to the Company’s investment policy, all purchased securities have a minimum short-term rating of A1 (Moody’s) or P1 (Standard & Poor’s) or equivalent. If there is no short-term rating, a purchased security is required to have a long-term rating no lower than A3/A- or equivalent.
Concentration of Credit Risk and Other Risks and Uncertainties	Concentration of Credit Risk and Other Risks and Uncertainties —Financial instruments that potentially subject the Company to a concentration of credit risk, consist primarily of cash, cash equivalents and short-term and long-term investments. The Company maintains deposits in federally insured financial institutions in excess of federally insured limits. Management believes that these financial institutions are financially sound, and, accordingly, minimal credit risk exists with respect to those financial institutions. The Company is exposed to credit risk in the event of default by the financial institutions holding its cash, cash equivalents and short-term and long-term investments and by the issuers of the securities to the extent recorded in the balance sheet. The Company is dependent on third-party manufacturers to supply products for research and development activities in its programs. In particular, the Company relies and expects to continue to rely on a small number of manufacturers to supply it with its requirements for the active pharmaceutical ingredient and drug product related to these programs. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and formulated drugs .
Property and Equipment	Property and Equipment —Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is determined using the straight-line method over the estimated useful lives of the respective assets, which is three years. Leasehold improvements are amortized using the straight-line method over the shorter of the lease term or their estimated useful economic lives.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets —Long-lived assets consist of property and equipment. The Company assesses potential impairment losses on long-lived assets used in operations when events and circumstances indicate that assets might be impaired. If such events or changes in circumstances arise, the Company compares the carrying amount of the long-lived assets to the estimated future undiscounted cash flows expected to be generated by the long-lived assets. If the estimated aggregate undiscounted cash flows are less than the carrying amount of the long-lived assets, an impairment charge, calculated as the amount by which the carrying amount of the assets exceeds the fair value of the assets, is recorded. The fair value of the long-lived assets is determined based on the estimated discounted cash flows expected to be generated from the long-lived assets.
Deferred Offering Costs	Deferred Offering Costs —The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with in-process equity financings as deferred offering costs until such financings are consummated. After consummation of the equity financing, these costs are recorded in stockholders’ equity (deficit) as a reduction of additional paid-in capital generated as a result of the equity financing.
Clinical and Manufacturing Accruals	Clinical and Manufacturing Accruals —The Company records accruals for estimated costs of research, nonclinical and clinical studies and manufacturing development, which are a significant component of research and development expenses. A substantial portion of the Company’s ongoing research and development activities is conducted by third-party service providers, including clinical research organizations, or CROs, and contract manufacturing organizations, or CMOs. The Company’s contracts with CROs generally include pass-through fees such as regulatory expenses, investigator fees, travel costs and other miscellaneous costs, including shipping and printing fees. The financial terms of these contracts are subject to negotiations, which vary from contract to contract and may result in payment flows that do not match the periods over which materials or services are provided to the Company under such contracts. The Company accrues the costs incurred under agreements with these third parties based on estimates of actual work completed in accordance with the respective agreements. The Company determines the estimated costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fees to be paid for such services. The Company makes significant judgments and estimates in determining the accrual balance in each reporting period. As actual costs become known, the Company adjusts its accruals. Although the Company does not expect its estimates to be materially different from amounts actually incurred, the Company’s understanding of the status and timing of services performed relative to the actual status and timing of services performed may vary and could result in the Company reporting amounts that are too high or too low in any particular period. The Company’s accrual is dependent, in part, upon the receipt of timely and accurate reporting from information provided as part of its clinical and nonclinical studies and other third-party vendors. Through December 31, 2018 , there have been no material differences from the Company’s accrued estimated expenses to the actual clinical trial and manufacturing expenses. However, variations in the assumptions used to estimate accruals, including, but not limited to the number of patients enrolled, the rate of patient enrollment, the actual services performed, and the amount of manufactured drug substance and/or drug product, and related costs may vary from the Company’s estimates, resulting in adjustments to research and development expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect its financial position and results of operations.
Lease	Lease —The Company leases its office and laboratory facility and the lease is classified as an operating lease. Rent expense is recognized on a straight-line basis over the term of the lease and, accordingly, the Company records the difference between cash rent payments and rent expense recognized as a deferred rent liability. Incentives granted under the Company’s facility lease, including allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the term of the lease.
Term Loan	Term Loan —The Company accounts for the Loan and Security Agreement, or Term Loan, with Hercules Capital, Inc., or Hercules, as a liability measured at net proceeds less debt discount and is accreted to the face value of the Term Loan over its expected term using the effective interest method. The Company considers whether there are any embedded features in its debt instruments that require bifurcation and separate accounting as derivative financial instruments pursuant to Accounting Standards Codification, or ASC, Topic 815, Derivatives and Hedging .
Convertible Preferred Stock	Convertible Preferred Stock —The Company recorded all shares of convertible preferred stock at their respective fair values, net of issuance costs, on the dates of issuance. In the event of a change of control of the Company, proceeds would have been distributed in accordance with the liquidation preferences set forth in its Amended and Restated Certificate of Incorporation unless the holders of convertible preferred stock had converted their convertible preferred shares into common shares. Therefore, convertible preferred stock was classified outside of stockholders’ equity (deficit) on the accompanying balance sheets as events triggering the liquidation preferences were not solely within the Company’s control. The Company elected not to adjust the carrying values of the convertible preferred stock to the liquidation preferences of such shares because of the uncertainty of whether or when such an event would occur.
Warrants	Warrants —The Company issued freestanding warrants to purchase shares of its Series A convertible preferred stock. Freestanding warrants for shares of the Company’s convertible preferred stock were classified outside of permanent equity, were recorded at fair value and were subject to remeasurement at each balance sheet date until the earlier of the exercise of the warrants or the completion of a liquidation event, including the completion of an initial public offering. Immediately prior to exercise, the warrant liability was remeasured to fair value and, upon exercise, the warrant liability was reclassified to additional paid-in capital, with any change in fair value recognized as a component of other income (expense), net. See Note 3 " Fair Value Measurements and Fair Value of Financial Instruments " to these financial statements for additional details. The Company also issued freestanding warrants to purchase shares of common stock in connection with its Term Loan. The warrants are recorded at fair value using the Black-Scholes option pricing model. See Note 5 " Borrowings " to these financial statements for additional details.
Preferred Stock Tranche Obligation	Preferred Stock Tranche Obligation —The Company entered into convertible preferred stock financings where, in addition to the initial closing, investors agreed to buy, and the Company agreed to sell, additional shares of that convertible preferred stock at a set price in the event that certain agreed milestones are achieved (a tranched financing). The Company evaluated this tranche obligation and determined that it met the definition of a freestanding instrument, and accordingly, determined the fair value of this obligation and recorded it on the balance sheet with the residual of the proceeds raised being allocated to convertible preferred stock. The preferred stock tranche obligation was revalued each reporting period with changes in the fair value of the obligation recorded as a component of other income (expense), net in the statements of operations and comprehensive loss. The preferred stock tranche obligation was revalued at settlement and the resultant fair value was then reclassified to convertible preferred stock at that time.
Research and Development Expense	Research and Development Expense —Research and development expense is charged to the statements of operations and comprehensive loss in the period in which they are incurred. Research and development expense consists primarily of salaries and related costs, including stock-based compensation expense, for personnel and consultants in our research and development functions; fees paid to clinical consultants, clinical trial sites and vendors, including CROs, costs related to pre-commercialization manufacturing activities including payments to CMOs and other vendors and consultants, costs related to regulatory activities, expenses related to lab supplies and services and depreciation and other allocated facility-related and overhead expenses. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are deferred and capitalized. The capitalized amounts are expensed as the related goods are delivered or the services are performed.
Stock-Based Compensation	Stock-Based Compensation —Stock-based compensation expense represents the grant-date fair value of awards recognized on a straight-line basis over the employee's requisite service period (generally the vesting period). The Black-Scholes option-pricing model is used to calculate stock-based compensation expense for stock option awards and shares purchased under the Employee Stock Purchase Plan, or ESPP. For restricted stock units, or RSUs, the grant-date fair value is the closing price of the Company's common stock on the date of grant as reported on The Nasdaq Global Select Market. Because stock compensation expense is an estimate of awards ultimately expected to vest, it is reduced by an estimate for future forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from estimates. The Company records the expense attributed to nonemployee services paid with stock option awards based on the estimated fair value of the awards determined using the Black-Scholes option pricing model. The measurement of stock-based compensation for nonemployees was previously subject to remeasurement as the options vested. As of July 1, 2018, the Company adopted ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting , which no longer subjects nonemployee awards to remeasurement. The expense is recognized over the period during which services are received.
Income Taxes	Income Taxes —The Company accounts for income taxes using the liability method, whereby deferred tax asset and liability account balances are determined based on differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company provides a valuation allowance when it is more likely than not that some portion or all of its deferred tax assets will not be realized. The Company accounts for income tax contingencies using a benefit recognition model. If it considers that a tax position is more likely than not to be sustained upon audit, based solely on the technical merits of the position, it recognizes the benefit. The Company measures the benefit by determining the amount that is greater than 50% likely of being realized upon settlement, presuming that the tax position is examined by the appropriate taxing authority that has full knowledge of all relevant information. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit.
Comprehensive Loss	Comprehensive Loss —Comprehensive loss includes net loss and certain changes in stockholders’ equity (deficit) that are excluded from net loss, primarily unrealized losses on the Company’s available-for-sale securities.
Net Loss per Share	Net Loss per Share —Basic net loss per share is calculated by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given the net loss for each period presented.
Segment Reporting	Segment Reporting —The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. All of the Company's assets are maintained in the United States.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Adopted Standards In May 2017, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2017-09, Compensation - Stock Compensation (Topic 718) : Scope of Modification Accounting . This ASU specifies the modification accounting applicable to any entity which changes the terms or conditions of a share-based payment award. The Company elected to prospectively adopt this ASU beginning January 1, 2018. The adoption of this ASU did not have a material impact on the Company's financial statements. In July 2017, the FASB issued ASU No. 2017-11, Earnings Per Share (Topic 260); Distinguishing Liabilities from Equity (Topic 480); Derivatives and Hedging (Topic 815), (Part I) Accounting for Certain Financial Instruments with Down Round Features, (Part II) Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception . The guidance in ASU No. 2017-11 allows for the exclusion of a down round feature, when evaluating whether or not an instrument or embedded feature requires derivative classification. The Company early adopted this guidance beginning January 1, 2018. The adoption of this standard had no material impact on the Company’s financial statements. In June 2018, the FASB issued ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (Topic 718) , which amends ASC Topic 718, “Compensation—Stock Compensation”. The ASU simplifies the accounting for share-based payments granted to nonemployees for goods and services. Under the ASU, most of the guidance on such payments to nonemployees would be aligned with the requirements for share-based payments granted to employees. The Company early adopted this guidance effective July 1, 2018. Upon adoption of this standard, share-based awards issued to nonemployees are measured at the grant date and are not subject to remeasurement. The Company has elected to continue to use the contractual term as the estimated expected term. The adoption of ASU No. 2018-07 did not have a material impact on the Company's financial statements and is expected to reduce the volatility in stock-based compensation expense for nonemployees recognized from period to period. Standards Not Yet Effective In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) , which for operating leases requires the lessee to recognize a right-of-use asset and a lease liability, initially measured at the present value of the lease payments, on its balance sheet. The guidance requires a lessee to recognize single lease costs, calculated so that the cost of the lease is allocated over the lease term, on a generally straight-line basis. The new standard also requires expanded disclosures regarding leasing arrangements. The Company leases certain facilities for some of its operations, which represents a majority of its operating leases it has entered into as a lessee. The Company expects the implementation of ASC Topic 842 to have an impact on its financial statements and related disclosures as it had aggregate future minimum lease payments of approximately $2.7 million as of December 31, 2018 . The new standard becomes effective for the Company beginning January 1, 2019. In July 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842): Targeted Improvements , which provides an alternative modified retrospective transition method. Under this method, the cumulative-effect adjustment to the opening balance of retained earnings is recognized on the date of adoption and it allows entities not to apply the new leases standard, including its disclosure requirements, in the comparative periods presented in their financial statements in the year of adoption. The Company intends to elect this transition method at the adoption date of January 1, 2019, and as a result, will record a right-of-use asset and a corresponding lease liability on the balance sheet for all leases with terms longer than 12 months. The Company expects to elect the practical expedients upon transition that will retain the lease classification and initial direct costs for any leases that existed prior to the adoption of the new guidance. The Company is continuing to evaluate the effect that this guidance will have on its financial statements and related disclosures. 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 used to develop significant unobservable inputs for 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 FINANCIAL INSTRUMENTS (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities Measured on a Recurring Basis	The following tables summarize the Company's cash and cash equivalents and available-for-sale investments' amortized cost, gross unrealized gains, gross unrealized losses and fair values by significant investment category reported as cash and cash equivalents, short-term investments or long-term investments as of December 31, 2018 and 2017 . December 31, 2018 Reported as: (in thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses 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 December 31, 2017 Reported as: (in thousands) Amortized cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Short-term Investments Long-term investments Cash $ 86 $ — $ — $ 86 $ 86 $ — $ — Level 1: Money market fund 6,758 — — 6,758 6,758 — — Level 2: Commercial paper 25,780 — (7 ) 25,773 — 25,773 — Corporate debt securities 20,545 3 (5 ) 20,543 2,930 17,613 — Asset-backed securities 14,358 — (4 ) 14,354 — 14,354 — Subtotal 60,683 3 (16 ) 60,670 2,930 57,740 — Total assets measured at fair value $ 67,527 $ 3 $ (16 ) $ 67,514 $ 9,774 $ 57,740 $ —
Contractual Maturities of Cash Equivalents (Excluding Money Market Funds)	The following table summarizes the maturities of the Company’s cash equivalents (excluding money market funds) and available-for-sale investments, at December 31, 2018 . (in thousands) Amortized Cost Fair Value Mature in less than one year $ 205,060 $ 204,903 Mature in one to five years 2,283 2,287 Total $ 207,343 $ 207,190
Amortized Cost, Unrealized Holding Gains or Losses, and Fair Value of Marketable Securities	The following table summarizes our available-for-sale debt securities that were in a continuous unrealized loss position for less than 12 months, but were not deemed to be other-than-temporarily impaired, as of December 31, 2018 and 2017 . December 31, 2018 December 31, 2017 (in thousands) Fair Value Unrealized Losses Fair Value Unrealized Losses Commercial paper $ 67,407 $ (63 ) $ 19,798 $ (7 ) Corporate debt securities 85,699 (63 ) 13,394 (5 ) Asset-backed securities 36,730 (34 ) 14,354 (4 ) Total $ 189,836 $ (160 ) $ 47,546 $ (16 )
Reconciliation of Liabilities Using Level 3 Unobservable Inputs	The following table presents a reconciliation of financial assets and liabilities measured at fair value on a recurring basis using Level 3 unobservable inputs for the years ended December 31, 2018 and 2017 . 2018 2017 (in thousands) Compound Derivative Liability Warrant Liability Convertible Preferred Stock Tranche Obligation Warrant Liability Fair value at beginning of year $ — $ 106 $ 3,371 $ 38 Addition 654 156 2,278 — Change in fair value (493 ) 305 (5,649 ) 68 Reclassification to equity — (194 ) — — Issuance of convertible preferred stock on exercise of warrant — (373 ) — — Fair value at end of year $ 161 $ — $ — $ 106
Significant Unobservable Inputs Related to Level 3 Assets and Liabilities	The following table presents information about significant unobservable inputs related to the Company's significant Level 3 financial assets and liabilities as of December 31, 2018 . December 31, 2018 (in thousands) Fair Value Valuation Technique Significant Unobservable Input Range of Inputs Compound derivative liability $ 161 Discounted cash flow Discount rate 13.0 % - 14.0 % Probability of the occurrence of certain events 10 %

OTHER BALANCE SHEET COMPONENTS

OTHER BALANCE SHEET COMPONENTS (Tables)	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Property and Equipment, Net	Property and Equipment, Net The following table presents the components of property and equipment, net as of December 31, 2018 and 2017 . (in thousands) December 31, December 31, Furniture and fixtures $ 193 $ 193 Computer and lab equipment 1,888 1,382 Leasehold improvements 1,055 878 3,136 2,453 Less: accumulated depreciation and amortization (1,921 ) (1,303 ) Total property and equipment, net $ 1,215 $ 1,150
Schedule of Accrued Expenses and Other Current Liabilities	Accrued Expenses and Other Current Liabilities The following table presents the components of accrued expenses and other current liabilities as of December 31, 2018 and 2017 . (in thousands) December 31, December 31, Accrued clinical and nonclinical study costs $ 2,168 $ 2,235 Accrued contract manufacturing 1,676 4,157 Accrued compensation 1,565 — Accrued professional fees and other 935 969 Total accrued expenses and other current liabilities $ 6,344 $ 7,361

BORROWINGS (Tables)

BORROWINGS (Tables)	12 Months Ended
	Dec. 31, 2018
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 December 31, 2018 . (in thousands) December 31, 2018 2019 $ 3,486 2020 17,317 2021 21,896 2022 8,820 51,519 Less: amount representing interest (11,519 ) Present value of Term Loan 40,000 Less: current portion — Long-term portion of Term Loan $ 40,000

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases	Future minimum lease payments under the lease as of December 31, 2018 , not including any related common area maintenance charges or real estate taxes, were as follows. (in thousands) December 31, 2018 2019 $ 1,076 2020 1,108 2021 562 2022 and thereafter — Total future minimum lease payments $ 2,746

STOCKHOLDERS' EQUITY (Tables)

STOCKHOLDERS' EQUITY (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Common Stock Reserved for Future Issuance	Common stock reserved for future issuance, on an as if converted basis, consisted of the following. December 31, December 31, Convertible preferred stock, issued and outstanding — 26,163,217 Stock options and RSUs issued and outstanding 4,599,307 3,588,663 Stock options, RSUs and ESPP shares authorized for future issuance 4,534,784 884,947 Total 9,134,091 30,636,827
Schedule of Convertible Preferred Stock	As of December 31, 2017 , convertible preferred stock consisted of the following. December 31, 2017 (in thousands, except share amounts) Authorized Shares Shares Issued and Outstanding Net Proceeds Aggregate Liquidation Preference Shares designated as: Series A convertible preferred stock 11,398,694 11,302,758 $ 9,800 $ 10,014 Series B convertible preferred stock 32,526,878 32,526,878 29,618 30,250 Series C convertible preferred stock 35,806,451 35,806,451 50,347 55,500 Series D convertible preferred stock 24,500,000 24,493,615 57,305 57,560 Total 104,232,023 104,129,702 $ 147,070 $ 153,324

STOCK-BASED COMPENSATION (Table

STOCK-BASED COMPENSATION (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock Option Activity	The following table summarizes stock option activity under the plans for the years ended December 31, 2018 and 2017 . Shares Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value (Thousands) Balance at December 31, 2016 2,259,011 $ 1.13 9.2 Granted 1,354,235 2.21 Exercised (7,286 ) 0.76 Forfeited or canceled (17,297 ) 0.87 Balance at December 31, 2017 3,588,663 1.54 8.5 Granted 1,627,556 15.21 Exercised (219,974 ) 1.37 Forfeited or canceled (418,730 ) 6.13 Balance at December 31, 2018 4,577,515 $ 5.99 8.0 $ 84,290 Vested and expected to vest at December 31, 2018 4,304,521 $ 5.62 7.9 $ 80,514
Schedule of Restricted Stock Activity	The following table summarizes RSU activity under the 2018 Plan for the year ended December 31, 2018 . Shares Weighted- Fair Value Unvested balance at December 31, 2017 — — Granted 21,792 19.00 Vested — Forfeited — Unvested balance at December 31, 2018 21,792 $ 19.00
Schedule of Stock Options, Valuation Assumptions	The fair value of the stock options granted to employees was calculated with the following assumptions. Years Ended December 31, 2018 2017 2016 Risk-free interest rate 2.7 % 1.7 % 1.7% - 1.8% Expected volatility 64.6 % 72.7% - 78.5% 76.3% - 78.4% Expected term (in years) 5.8 6.2 - 6.3 6.2 - 6.3 Expected dividends — % — % — % The fair value of the stock options granted to nonemployees was calculated with the following assumptions. Years Ended December 31, 2018 2017 2016 Risk-free interest rate 2.7% 1.7% - 2.2% 2.1% - 2.2% Expected volatility 68.4% 70.8% - 73.1% 80.1% - 80.4% Expected term (in years) 10.0 6.5 - 8.9 8.6 - 8.7 Expected dividends —% — % — %
Schedule of Weighted Average Fair Value and Intrinsic Value	The following table summarizes the weighted-average fair value per share of stock options granted and the total intrinsic value of stock options exercised (in thousands) for the years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, (in thousands, except per share amounts) 2018 2017 2016 Stock options granted - weighted-average grant date fair value per share $ 10.39 $ 1.67 $ 1.11 Stock options exercised - intrinsic value 1,935 30 17
Schedule of Stock-based Compensation Expense	The following table presents stock-based compensation expense as reported in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, (in thousands) 2018 2017 2016 Research and development $ 2,643 $ 379 $ 185 General and administrative 2,509 497 87 Total $ 5,152 $ 876 $ 272 The following table presents stock-based compensation expense by award type as reported in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, (in thousands) 2018 2017 2016 Stock options $ 4,806 $ 876 $ 272 RSUs 202 — — ESPP 144 — — Total $ 5,152 $ 876 $ 272

NET LOSS PER SHARE (Tables)

NET LOSS PER SHARE (Tables)	12 Months Ended
	Dec. 31, 2018
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 years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, (In thousands, except share and per share amounts) 2018 2017 2016 Numerator: Net loss $ (102,808 ) $ (41,290 ) $ (28,651 ) Denominator: Weighted average common shares outstanding 22,158,354 2,267,732 2,255,897 Less: weighted average shares subject to repurchase (12,162 ) (130,042 ) (429,857 ) Weighted average number of shares used in basic and diluted net loss per share 22,146,192 2,137,690 1,826,040 Net loss per share, basic and diluted $ (4.64 ) $ (19.32 ) $ (15.69 )
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. December 31, 2018 2017 2016 Series A convertible preferred stock — 2,839,886 2,839,886 Series B convertible preferred stock — 8,172,579 8,172,579 Series C convertible preferred stock — 8,996,586 4,907,597 Series D convertible preferred stock — 6,154,166 — Warrants to purchase preferred or common stock 106,916 24,104 24,104 Common stock subject to repurchase 20,193 366 8,793 Options and RSUs issued and outstanding 4,599,307 3,588,663 2,259,011 Total 4,726,416 29,776,350 18,211,970

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Deferred Tax Assets	The significant components of the Company’s net deferred tax assets as of December 31, 2018 and 2017 are shown below. December 31, (in thousands) 2018 2017 Deferred tax assets Net operating loss carryforwards $ 45,215 $ 24,939 Research and development credits 4,090 2,807 Capitalized assets 23 79 Accruals and reserves 390 149 Other—net 806 90 Gross deferred tax assets 50,524 28,064 Valuation allowance (50,524 ) (28,064 ) Net deferred tax assets $ — $ —
Schedule of Effective Income Tax Rate Reconciliation	The following is a reconciliation between the U.S. federal income statutory tax rate and the Company’s effective tax rate for the years ended December 31, 2018 , 2017 and 2016 . Years Ended December 31, 2018 2017 2016 U.S. federal statutory rate 21 % 34.0 % 34.0 % State income taxes, net of federal benefit — 5.8 5.8 Stock-based compensation (0.2 ) (0.7 ) (0.3 ) Permanent adjustments (0.1 ) 4.2 (3.4 ) Change to valuation allowance (21.9 ) (19.6 ) (39.3 ) Tax Cuts and Jobs Act Impact — (25.4 ) — Research and development credits 1.3 1.6 3.2 Other (0.1 ) — — Effective tax rate — % — % — %
Schedule of Unrecognized Tax Benefits	The beginning and ending gross unrecognized tax benefits amounts are as follows. Years Ended December 31, (in thousands) 2018 2017 2016 Gross unrecognized tax benefits at beginning of year $ 819 $ 574 $ 390 Additions for tax positions related to prior year — 2 19 Decrease related to prior year tax provisions — (92 ) (101 ) Additions for tax positions related to current year 478 335 266 Gross unrecognized tax benefits at end of year $ 1,297 $ 819 $ 574

SELECTED QUARTERLY FINANCIAL _2

SELECTED QUARTERLY FINANCIAL DATE (UNAUDITED) (Tables)	12 Months Ended
	Dec. 31, 2018
Condensed Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information	Net loss per share for all periods presented has been retroactively adjusted to reflect the 1-for-3.98 reverse stock split effected on June 15, 2018. 2018 (in thousands, except per share data) Q1 Q2 Q3 Q4 Operating expenses $ 20,098 $ 25,279 $ 29,408 $ 28,810 Net loss (20,504 ) (25,362 ) (29,098 ) (27,844 ) Net loss per share, basic and diluted $ (9.00 ) $ (10.89 ) $ (0.71 ) $ (0.66 ) 2017 (in thousands, except per share data) Q1 Q2 Q3 Q4 Operating expenses $ 8,669 $ 6,922 $ 10,874 $ 20,657 Net loss (2,209 ) (7,726 ) (10,805 ) (20,550 ) Net loss per share, basic and diluted $ (1.08 ) $ (3.60 ) $ (4.81 ) $ (9.05 )

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)	Jun. 14, 2018	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)	Jul. 02, 2018shares
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares			104,129,702
Number of common stock issued upon conversion (in shares) | shares					26,187,321
Property and equipment, useful life		3 years
Impairment of long lived assets | $		$ 0	$ 0	$ 0
Operating leases, aggregate future minimum lease payments | $		2,746,000
To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. | $		$ 0	$ 0	$ 0
Reserve stock split ratio	0.2513	0.2513
Convertible Preferred Stock
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares					104,225,638

FAIR VALUE MEASUREMENTS AND F_3

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018USD ($)Position	Dec. 31, 2017USD ($)Position	Dec. 31, 2016USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest Income	$ 3,500,000	$ 400,000	$ 200,000
Securities positions in an unrealized loss position | Position	48	21
Securities with other-than-temporary-impairments	$ 0	$ 0
Term Loan | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument, fair value	$ 37,800,000
Minimum | Term Loan | Discount Rate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt measurement input	0.13
Maximum | Term Loan | Discount Rate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt measurement input	0.14

FAIR VALUE MEASUREMENTS AND F_4

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Financial assets and liabilities by hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 37,172	$ 9,774
Level 2:
Amortized Cost	207,343
Fair Value	207,190
Total assets measured at fair value
Short-term investments	203,906	57,740
Long-term investments	2,287	0
Level 2
Level 2:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		16
Level 2 | Commercial paper
Level 2:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		7
Level 2 | Corporate debt securities
Level 2:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		5
Level 2 | Asset-backed securities
Level 2:
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax		4
Fair Value, Measurements, Recurring
Total assets measured at fair value
Amortized Cost	243,518	67,527
Gross Unrealized Gains	7	3
Gross Unrealized Losses	(160)	(16)
Fair Value	243,365	67,514
Fair Value, Measurements, Recurring | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	37,172	9,774
Fair Value, Measurements, Recurring | Short-term Investments
Total assets measured at fair value
Short-term investments	203,906	57,740
Fair Value, Measurements, Recurring | Long-term investments
Total assets measured at fair value
Long-term investments	2,287	0
Fair Value, Measurements, Recurring | Level 1 | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	33,154	6,758
Fair Value, Measurements, Recurring | Level 2
Level 2:
Amortized Cost	207,343	60,683
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7	3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	160	16
Fair Value	207,190	60,670
Fair Value, Measurements, Recurring | Level 2 | Cash and Cash Equivalents
Level 2:
Fair Value	997	2,930
Fair Value, Measurements, Recurring | Level 2 | Short-term Investments
Total assets measured at fair value
Short-term investments	203,906	57,740
Fair Value, Measurements, Recurring | Level 2 | Long-term investments
Total assets measured at fair value
Long-term investments	2,287	0
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Level 2:
Amortized Cost	68,467	25,780
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	63	7
Fair Value	68,404	25,773
Fair Value, Measurements, Recurring | Level 2 | Commercial paper | Cash and Cash Equivalents
Level 2:
Fair Value	997	0
Fair Value, Measurements, Recurring | Level 2 | Commercial paper | Short-term Investments
Total assets measured at fair value
Short-term investments	67,407	25,773
Fair Value, Measurements, Recurring | Level 2 | Commercial paper | Long-term investments
Total assets measured at fair value
Long-term investments	0	0
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Level 2:
Amortized Cost	89,038	20,545
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4	3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	63	5
Fair Value	88,979	20,543
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities | Cash and Cash Equivalents
Level 2:
Fair Value	0	2,930
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities | Short-term Investments
Total assets measured at fair value
Short-term investments	86,692	17,613
Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities | Long-term investments
Total assets measured at fair value
Long-term investments	2,287	0
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Level 2:
Amortized Cost	49,838	14,358
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	34	4
Fair Value	49,807	14,354
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities | Cash and Cash Equivalents
Level 2:
Fair Value	0	0
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities | Short-term Investments
Total assets measured at fair value
Short-term investments	49,807	14,354
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities | Long-term investments
Total assets measured at fair value
Long-term investments	0	0
Cash | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	3,021	86
Cash | Fair Value, Measurements, Recurring | Cash and Cash Equivalents
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	3,021	86
Cash and Cash Equivalents, Fair Value Disclosure	3,021
Money market fund | Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	33,154	6,758
Cash and Cash Equivalents, Fair Value Disclosure	$ 33,154	$ 6,758

FAIR VALUE MEASUREMENTS AND F_5

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Maturities of Debt Securities, Available-For-Sale (Details) $ in Thousands	Dec. 31, 2018USD ($)
Amortized Cost
Mature in less than one year	$ 205,060
Mature in one to five years	2,283
Amortized Cost	207,343
Fair Value
Mature in less than one year	204,903
Mature in one to five years	2,287
Total	$ 207,190

FAIR VALUE MEASUREMENTS AND F_6

FAIR VALUE MEASUREMENTS AND FAIR VALUE OF FINANCIAL INSTRUMENTS - Unrealized losses on Debt Securities, Available-For-Sale (Details) - Level 2 - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 189,836	$ 47,546
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	160
Unrealized Losses		(16)
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	67,407	19,798
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	63
Unrealized Losses		(7)
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	85,699	13,394
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	63
Unrealized Losses		(5)
Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	36,730	14,354
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 34
Unrealized Losses		$ (4)

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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Compound Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of year	$ 0
Addition	654
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)	(493)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0
Issuance of convertible preferred stock on exercise of warrant	0
Fair value at end of year	161	$ 0
Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of year	106	38
Addition	156	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)	305	68
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	(194)	0
Issuance of convertible preferred stock on exercise of warrant	(373)	0
Fair value at end of year	0	106
Convertible Preferred Stock Tranche Obligation
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of year	$ 0	3,371
Addition		2,278
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		(5,649)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3		0
Issuance of convertible preferred stock on exercise of warrant		0
Fair value at end of year		$ 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	Dec. 31, 2018USD ($)
Probability of the occurrence of certain events
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.10
Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.135
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Compound derivative liability	$ 161
Level 3 | Probability of the occurrence of certain events
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.10
Level 3 | Minimum | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.130
Level 3 | Maximum | Discount Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative liability, measurement input	0.140

OTHER BALANCE SHEET COMPONENT_2

OTHER BALANCE SHEET COMPONENTS - Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,136	$ 2,453
Less: accumulated depreciation and amortization	(1,921)	(1,303)
Total property and equipment, net	1,215	1,150
Depreciation and amortization expense	618	335	$ 424
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	1,888	1,382
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,055	$ 878

OTHER BALANCE SHEET COMPONENT_3

OTHER BALANCE SHEET COMPONENTS - Accrued Expense and Other Current Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued clinical and nonclinical study costs	$ 2,168	$ 2,235
Accrued contract manufacturing	1,676	4,157
Accrued compensation	1,565	0
Accrued professional fees and other	935	969
Total accrued expenses and other current liabilities	$ 6,344	$ 7,361

BORROWINGS - Additional Informa

BORROWINGS - Additional Information (Details)	Dec. 28, 2018USD ($)$ / sharesshares	Apr. 10, 2018USD ($)	Dec. 31, 2018USD ($)$ / sharesshares	Oct. 15, 2018USD ($)tranche	Jul. 02, 2018$ / shares	Jun. 16, 2018shares	Feb. 28, 2018USD ($)tranche$ / sharesshares	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Number of shares called by warrant (in shares) | shares						95,936
Warrants term			7 years
Exercise price of warrant (in USD per share) | $ / shares			$ 0.886
Warrant liability								$ 100,000
Compound Derivative Liability			$ 161,000
Term Loan
Debt Instrument [Line Items]
Debt aggregate principal amount				$ 100,000,000			$ 100,000,000
Debt, number of tranches | tranche				5			5
Interest only payment term			16 months
Debt facility fee							$ 650,000
Debt payment terms, multiplier			6.55%
Debt payment terms, percentage of aggregate term loans funded							50.00%
Debt payment terms, spread amount over aggregate term loans funded			$ 60,000,000
Number of shares called by warrant (in shares) | shares							53,458
Exercise price of warrant (in USD per share) | $ / shares							$ 9.35
Warrant liability							$ 156,000
Increase (decrease) in equity due to warrants		$ 200,000
Fair value of embedded derivative liability		$ 654,000
Unamortized issuance costs and debt discounts			$ 2,700,000
Term Loan | Minimum
Debt Instrument [Line Items]
Debt prepayment fee, percentage			1.00%
Term Loan | Maximum
Debt Instrument [Line Items]
Debt prepayment fee, percentage			1.50%
Term Loan | Tranche one, funded on closing date
Debt Instrument [Line Items]
Debt aggregate principal amount							25,000,000
Term Loan | Tranche two, funded on December 28, 2018
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
Increase (decrease) in equity due to warrants	$ 900,000
Term Loan | Tranche three, funded on or before December 31, 2019
Debt Instrument [Line Items]
Debt aggregate principal amount				35,000,000			15,000,000
Number of shares called by warrant (in shares) | shares			53,458
Term Loan | Tranche four, funded on or before December 15, 2020
Debt Instrument [Line Items]
Debt aggregate principal amount							10,000,000
Number of shares called by warrant (in shares) | shares			21,383
Term Loan | Tranche five, funded on or before December 31, 2020
Debt Instrument [Line Items]
Debt aggregate principal amount				$ 15,000,000			$ 25,000,000
Number of shares called by warrant (in shares) | shares			32,075
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%
Debt, basis spread on adjusted variable rate			5.00%
Floating variable rate 3 | Term Loan
Debt Instrument [Line Items]
Debt interest rate			9.85%
Liquidity | Minimum
Debt Instrument [Line Items]
Warrants measurement input term		3 months 1 day
Liquidity | Maximum
Debt Instrument [Line Items]
Warrants measurement input term		1 year 8 months 12 days
Liquidity | Tranche two, funded on December 28, 2018
Debt Instrument [Line Items]
Warrants measurement input term	2 years 6 months
Volatility
Debt Instrument [Line Items]
Warrants measurement input		0.72
Volatility | Tranche two, funded on December 28, 2018
Debt Instrument [Line Items]
Warrants measurement input	0.75
Risk-free interest rate
Debt Instrument [Line Items]
Warrants measurement input		0.024
Risk-free interest rate | Tranche two, funded on December 28, 2018
Debt Instrument [Line Items]
Warrants measurement input	0.0251
Equity value | Minimum
Debt Instrument [Line Items]
Warrants measurement input		306,000,000
Equity value | Maximum
Debt Instrument [Line Items]
Warrants measurement input		420,000,000
Discount Rate
Debt Instrument [Line Items]
Derivative liability, measurement input			0.135
Probability of the 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	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
2019	$ 3,486
2020	17,317
2021	21,896
2022	8,820
Long-term debt including interest payable	51,519
Less: amount representing interest	(11,519)
Present value of Term Loan	40,000
Less: current portion	0
Long-term portion of notes payable	$ 40,000

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Additional Information (Details) $ in Millions	May 08, 2018USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)ft²	Dec. 31, 2016USD ($)	Aug. 31, 2017ft²	Jul. 31, 2014
Debt Instrument [Line Items]
Term of noncancelable operating lease						5 years
Term of lease extension option						3 years
Annual rent expense		$ 1	$ 0.7	$ 0.5
Purchase obligation	$ 49.8	37.2
Payment of long-term purchase commitment		12.6
Purchase obligation term	36 months
Tenant Improvement Loan
Debt Instrument [Line Items]
Term loan		$ 0.3
Subject to Operating Lease
Debt Instrument [Line Items]
Lease rentable area (in square feet) | ft²			26,987		13,258

COMMITMENTS AND CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES - Summary of Contractual Obligation (Details) $ in Thousands	Dec. 31, 2018USD ($)
Operating lease obligations and commitments
2019	$ 1,076
2020	1,108
2021	562
2022 and Thereafter	0
Total future minimum lease payments	$ 2,746

STOCKHOLDERS' EQUITY - Addition

STOCKHOLDERS' EQUITY - Additional Information (Details) $ / shares in Units, $ in Millions	Jul. 02, 2018USD ($)$ / sharesshares	Jun. 14, 2018	Dec. 31, 2018$ / sharesshares	Dec. 31, 2017$ / sharesshares
Class of Stock [Line Items]
Number of all classes of stock authorized	440,000,000
Common stock, shares authorized (in shares)			400,000,000	134,000,000
Common stock, par value (in USD per share) | $ / shares	$ 0.001		$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)			40,000,000	0
Preferred stock, par value (in USD per share) | $ / shares	$ 0.001		$ 0.001	$ 0
Common stock, shares outstanding (in shares)			42,148,247	2,272,609
Convertible preferred stock, shares outstanding (in shares)				104,129,702
Number of common stock issued upon conversion (in shares)	26,187,321
Reserve stock split ratio		0.2513	0.2513
Preferred stock outstanding			0	0
Convertible Preferred Stock
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares)	104,225,638
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
Initial public offering, underwriting discounts, commissions and issuance costs | $	$ 17.9

STOCKHOLDERS' EQUITY - Common S

STOCKHOLDERS' EQUITY - Common Stock (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Convertible preferred stock, issued and outstanding (in shares)	0	26,163,217
Stock options and RSUs issued and outstanding (in shares)	4,599,307	3,588,663
Stock options, RSUs and ESPP shares authorized for future issuance (in shares)	4,534,784	884,947
Total (in shares)	9,134,091	30,636,827

STOCKHOLDERS' EQUITY - Converti

STOCKHOLDERS' EQUITY - Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Temporary Equity [Line Items]
Authorized Shares		104,232,023
Shares Issued and Outstanding	0	104,129,702	63,361,895	29,734,656
Convertible preferred stock, shares outstanding (in shares)		104,129,702
Net Proceeds		$ 147,070
Aggregate Liquidation Preference		$ 153,324
Series A Preferred Stock
Temporary Equity [Line Items]
Authorized Shares	0	11,398,694
Shares Issued and Outstanding	0	11,302,758
Convertible preferred stock, shares outstanding (in shares)	0	11,302,758
Net Proceeds		$ 9,800
Aggregate Liquidation Preference	$ 0	$ 10,014
Series B Preferred Stock
Temporary Equity [Line Items]
Authorized Shares	0	32,526,878
Shares Issued and Outstanding	0	32,526,878
Convertible preferred stock, shares outstanding (in shares)	0	32,526,878
Net Proceeds		$ 29,618
Aggregate Liquidation Preference	$ 0	$ 30,250
Series C Preferred Stock
Temporary Equity [Line Items]
Authorized Shares	0	35,806,451
Shares Issued and Outstanding	0	35,806,451
Convertible preferred stock, shares outstanding (in shares)	0	35,806,451
Net Proceeds		$ 50,347
Aggregate Liquidation Preference	$ 0	$ 55,500
Series D Preferred Stock
Temporary Equity [Line Items]
Authorized Shares	0	24,500,000
Shares Issued and Outstanding	0	24,493,615
Convertible preferred stock, shares outstanding (in shares)	0	24,493,615
Net Proceeds		$ 57,305
Aggregate Liquidation Preference	$ 0	$ 57,560

STOCK-BASED COMPENSATION - Addi

STOCK-BASED COMPENSATION - Additional Information (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, RSUs and ESPP shares authorized for future issuance (in shares)		4,534,784	884,947
Award expiration period		10 years
Expiration term of options after the termination of continuous service for reasons other than cause, death, or disability		3 months
Expiration term of options after the termination of continuous service for disability		12 months
Expiration term of options after the termination of continuous service for employee's death		18 months
Shares of common stock subject to repurchase		20,193	366
Aggregate purchase price of shares available for repurchase		$ 88,940	$ 350
Average price per share of shares available for repurchase		$ 4.40	$ 0.96
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		1 year
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		4 years
2013 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, RSUs and ESPP shares authorized for future issuance (in shares)		4,935,929	4,532,661		2,912,058
2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, RSUs and ESPP shares authorized for future issuance (in shares)					4,000,000
Number of shares available for future issuance		3,752,893
Subsequent Event | Scenario, Forecast | 2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual increase in number of shares authorized				3,200,000
Annual percentage increase in number of shares authorized				4.00%
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation		$ 200,000
Unrecognized stock-based compensation, period for recognition		6 months
Employee Stock | Tricida, Inc. ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, RSUs and ESPP shares authorized for future issuance (in shares)	800,000
Number of shares available for future issuance		781,891
Maximum percentage of employee pay to be withheld for employee stock purchase plan	15.00%
Maximum number of shares purchased by employee	2,500
Employee purchase plan offering period	6 months
Employee stock purchase price, percentage of market closing price	85.00%
Shares issued under employee stock purchase plan		18,109
ESPP cash contributions		$ 400,000
Employee Stock | Subsequent Event | Scenario, Forecast | Tricida, Inc. ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual increase in number of shares authorized				800,000
Annual percentage increase in number of shares authorized				1.00%
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized stock-based compensation		$ 15,000,000	$ 2,700,000
Unrecognized stock-based compensation, period for recognition		2 years 5 months	2 years 9 months
Director | Restricted Stock Units (RSUs) | 2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		1 year

STOCK-BASED COMPENSATION - Sche

STOCK-BASED COMPENSATION - Schedule of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Options
Beginning balance (in shares)	3,588,663	2,259,011
Granted (in shares)	1,627,556	1,354,235
Exercised (in shares)	(219,974)	(7,286)
Forfeited (in shares)	(418,730)	(17,297)
Ending balance (in shares)	4,577,515	3,588,663	2,259,011
Vested and expected to vest at end of period, (in shares)	4,304,521
Weighted- Average Exercise Price
Beginning balance (in USD per share)	$ 1.54	$ 1.13
Granted (in USD per share)	15.21	2.21
Exercised (in USD per share)	1.37	0.76
Forfeited (in USD per share)	6.13	0.87
Ending balance (in USD per share)	5.99	$ 1.54	$ 1.13
Vested and expected to vest at end of period (in USD per share)	$ 5.62
Weighted- Average Remaining Contractual Term (Years)	8 years	8 years 6 months	9 years 2 months
Weighted-average remaining contractual term, vested and expected to vest	7 years 11 months
Aggregate intrinsic value	$ 84,290
Aggregate intrinsic value, vested and expected to vest	$ 80,514

STOCK-BASED COMPENSATION - Sc_2

STOCK-BASED COMPENSATION - Schedule of Restricted Stock Activity (Details)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Weighted- Average Grant Date Fair Value
Unvested balance beginning of period | $ / shares	$ 0
Granted | $ / shares	19
Vested | $ / shares
Forfeited | $ / shares
Unvested balance end of period | $ / shares	$ 19
Restricted Stock Units (RSUs)
Shares
Unvested balance beginning of period | shares	0
Granted | shares	21,792
Vested | shares	0
Forfeited | shares	0
Unvested balance end of period | shares	21,792

STOCK-BASED COMPENSATION - Stoc

STOCK-BASED COMPENSATION - Stock Options Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Stock Option
Stock options granted to employees
Risk-free interest rate	2.70%	1.70%
Expected volatility	64.60%
Expected term (in years)	5 years 9 months
Expected dividends	0.00%	0.00%	0.00%
Non-Employee Stock Option
Stock options granted to nonemployees
Risk-free interest rate	2.70%
Expected volatility	68.40%
Expected term (in years)	10 years
Expected dividends	0.00%	0.00%	0.00%
Minimum | Employee Stock Option
Stock options granted to employees
Risk-free interest rate		1.70%	1.70%
Expected volatility		72.70%	76.30%
Expected term (in years)		6 years 2 months	6 years 2 months
Minimum | Non-Employee Stock Option
Stock options granted to nonemployees
Risk-free interest rate		1.70%	2.10%
Expected volatility		70.80%	80.10%
Expected term (in years)		6 years 6 months	8 years 7 months
Maximum | Employee Stock Option
Stock options granted to employees
Risk-free interest rate		1.70%	1.80%
Expected volatility		78.50%	78.40%
Expected term (in years)		6 years 3 months	6 years 3 months
Maximum | Non-Employee Stock Option
Stock options granted to nonemployees
Risk-free interest rate		2.20%	2.20%
Expected volatility		73.10%	80.40%
Expected term (in years)		8 years 11 months	8 years 8 months

STOCK-BASED COMPENSATION - Weig

STOCK-BASED COMPENSATION - Weighted average fair value per share and intrinsic value (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock options granted - weighted-average grant date fair value per share	$ 10.39	$ 1.67	$ 1.11
Stock options exercised - intrinsic value	$ 1,935	$ 30	$ 17

STOCK-BASED COMPENSATION - Sc_3

STOCK-BASED COMPENSATION - Schedule of Stock-based Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 5,152	$ 876	$ 272
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	4,806	876	272
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	202	0	0
Employee Stock Purchase Plan (ESPP)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	144	0	0
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	2,643	379	185
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,509	$ 497	$ 87

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								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (27,844)	$ (29,098)	$ (25,362)	$ (20,504)	$ (20,550)	$ (10,805)	$ (7,726)	$ (2,209)	$ (102,808)	$ (41,290)	$ (28,651)
Denominator:
Weighted average common shares outstanding (in shares)									22,158,354	2,267,732	2,255,897
Less: weighted average shares subject to repurchase (in shares)									(12,162)	(130,042)	(429,857)
Weighted-average number of shares used in basic and diluted net loss per share (in shares)									22,146,192	2,137,690	1,826,040
Net loss per share, basic and diluted	$ (0.66)	$ (0.71)	$ (10.89)	$ (9)	$ (9.05)	$ (4.81)	$ (3.60)	$ (1.08)	$ (4.64)	$ (19.32)	$ (15.69)

NET LOSS PER SHARE - Antidiluti

NET LOSS PER SHARE - Antidilutive securities (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	4,726,416	29,776,350	18,211,970
Warrants to purchase preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	106,916	24,104	24,104
Common stock subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	20,193	366	8,793
Options issued and outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	4,599,307	3,588,663	2,259,011
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	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	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	4,907,597
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	0

INCOME TAXES - Additional Infor

INCOME TAXES - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Valuation allowance	$ (50,524)	$ (28,064)
Increase in valuation allowance	22,400	10,500
Reduction in the value of net deferred assets		10,500
Unrecognized tax benefits	1,297	819	$ 574	$ 390
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	185,500
Operating losses subject to expiration	89,100
Operating loss carryforwards not subject to expiration	96,400
Research tax credit carryforward		3,600
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 89,600
Research tax credit carryforward	$ 2,100

INCOME TAXES - Schedule of Defe

INCOME TAXES - Schedule of Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 45,215	$ 24,939
Research and development credits	4,090	2,807
Capitalized assets	23	79
Accruals and reserves	390	149
Other—net	806	90
Gross deferred tax assets	50,524	28,064
Valuation allowance	(50,524)	(28,064)
Net deferred tax assets	$ 0	$ 0

INCOME TAXES - Schedule of Inco

INCOME TAXES - Schedule of Income Tax Reconciliation (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S. federal statutory rate	21.00%	34.00%	34.00%
State income taxes, net of federal benefit	0.00%	5.80%	5.80%
Stock-based compensation	(0.20%)	(0.70%)	(0.30%)
Permanent adjustments	(0.10%)	4.20%	(3.40%)
Change to valuation allowance	(21.90%)	(19.60%)	(39.30%)
Tax Cuts and Jobs Act Impact	0.00%	(25.40%)	0.00%
Research and development credits	1.30%	1.60%	3.20%
Other	(0.10%)	0.00%	0.00%
Effective tax rate	0.00%	0.00%	0.00%

INCOME TAXES - Schedule of Unre

INCOME TAXES - Schedule of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefits at beginning of year	$ 819	$ 574	$ 390
Additions for tax positions related to prior year	0	2	19
Decrease related to prior year tax provisions	0	(92)	(101)
Additions for tax positions related to current year	478	335	266
Gross unrecognized tax benefits at end of year	$ 1,297	$ 819	$ 574

SUBSEQUENT EVENTS - New Term Lo

SUBSEQUENT EVENTS - New Term Loan (Details) - USD ($)	Mar. 27, 2019	Dec. 31, 2018	Dec. 28, 2018	Oct. 15, 2018	Feb. 28, 2018	Dec. 31, 2017
Subsequent Event [Line Items]
Remaining portion of previous term loan		$ 38,071,000				$ 0
Exercise price of warrant (in USD per share)		$ 0.886
Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount				$ 100,000,000	$ 100,000,000
Remaining portion of previous term loan		$ 40,000,000
Exercise price of warrant (in USD per share)					$ 9.35
Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Borrowing capacity of new term loan	$ 200,000,000
Term of interest and principal payment	30 months
Additional payment of principal	$ 2,600,000
End of term fee	7.55%
Prepayment fee, percentage for first 12 months	2.00%
Prepayment fee, percentage after month 12 before month 24	1.50%
Prepayment fee, percentage after 24 months before 36 months	1.00%
Prepayment fee, percentage after 36 months until maturity	0.00%
Warrants issued to purchase common stock (in shares)	16,721
Exercise price of warrant (in USD per share)	$ 23.92
Percentage of principal amount of applicable tranche	1.00%
Tranche one, funded on closing date | Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount					$ 25,000,000
Tranche one, funded on closing date | Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount	$ 20,000,000
Tranche two, funded on December 28, 2018 | Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount				15,000,000	25,000,000
Exercise price of warrant (in USD per share)			$ 9.35
Tranche two, funded on December 28, 2018 | Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount	15,000,000
Tranche three, funded on or before December 31, 2019 | Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount				$ 35,000,000	15,000,000
Tranche three, funded on or before December 31, 2019 | Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount	75,000,000
Tranche four, funded on or before December 15, 2020 | Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount					$ 10,000,000
Tranche four, funded on or before December 15, 2020 | Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Debt aggregate principal amount	$ 50,000,000
Floating variable rate 1 | Term Loan
Subsequent Event [Line Items]
Debt interest rate		8.35%
Floating variable rate 1 | Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Debt interest rate	8.35%
Floating variable rate 3 | Term Loan
Subsequent Event [Line Items]
Debt interest rate		9.85%
Floating variable rate 3 | Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Debt interest rate	9.85%
Prime rate | Floating variable rate 2 | Term Loan
Subsequent Event [Line Items]
Debt, basis spread on variable rate		8.35%
Debt, basis spread on adjusted variable rate		5.00%
Prime rate | Floating variable rate 2 | Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Debt, basis spread on variable rate	8.35%
Debt, basis spread on adjusted variable rate	6.00%
Maximum | Subsequent Event | New Term Loan
Subsequent Event [Line Items]
Expanded interest only payments contingent on meeting certain milestones	24 months

SELECTED QUARTERLY FINANCIAL _3

SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) $ / shares in Units, $ in Thousands	Jun. 14, 2018	Dec. 31, 2018USD ($)$ / shares	Sep. 30, 2018USD ($)$ / shares	Jun. 30, 2018USD ($)$ / shares	Mar. 31, 2018USD ($)$ / shares	Dec. 31, 2017USD ($)$ / shares	Sep. 30, 2017USD ($)$ / shares	Jun. 30, 2017USD ($)$ / shares	Mar. 31, 2017USD ($)$ / shares	Dec. 31, 2018USD ($)$ / shares	Dec. 31, 2017USD ($)$ / shares	Dec. 31, 2016USD ($)$ / shares
Condensed Financial Information Disclosure [Abstract]
Reserve stock split ratio	0.2513									0.2513
Operating expenses		$ 28,810	$ 29,408	$ 25,279	$ 20,098	$ 20,657	$ 10,874	$ 6,922	$ 8,669	$ 103,595	$ 47,122	$ 27,183
Net loss		$ (27,844)	$ (29,098)	$ (25,362)	$ (20,504)	$ (20,550)	$ (10,805)	$ (7,726)	$ (2,209)	$ (102,808)	$ (41,290)	$ (28,651)
Net loss per share, basic and diluted | $ / shares		$ (0.66)	$ (0.71)	$ (10.89)	$ (9)	$ (9.05)	$ (4.81)	$ (3.60)	$ (1.08)	$ (4.64)	$ (19.32)	$ (15.69)