Axcella Health (AXLA) 10-Q 20 Jun 19 2019 Q1 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	3 Months Ended
	Mar. 31, 2019	Jun. 18, 2019
Entity [Abstract]
Entity Registrant Name	Axcella Health Inc.
Entity Central Index Key	0001633070
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	No
Entity Filer Category	Non-accelerated Filer
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Entity Common Stock, Shares Outstanding		23,008,283
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Small Business	false
Entity Shell Company	false
Amendment Flag	false

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 66,769	$ 79,466
Prepaid expenses and other current assets	1,679	835
Total current assets	68,448	80,301
Property and equipment, net	920	1,076
Security deposits and other assets	216	216
Deferred offering costs	1,334	251
Total assets	70,918	81,844
Current liabilities:
Accounts payable	2,470	1,612
Accrued expenses	3,735	5,299
Total current liabilities	6,205	6,911
Long term debt, net of discount	24,624	24,521
Other liabilities	1,948	1,898
Preferred stock warrant liability	476	425
Total liabilities	33,253	33,755
Commitments and contingencies (Note 8)
Redeemable convertible preferred stock (Note 7)	197,888	197,842
Stockholders' equity (deficit):
Common stock, $0.001 par value; 47,000,000 and 47,000,000 shares authorized, 5,195,250 and 5,193,915 shares issued and 4,776,269 and 4,774,934 shares outstanding at March 31, 2019 and December 31, 2018, respectively	6	6
Additional paid-in-capital	8,393	7,290
Treasury stock, 418,981 shares at cost	0	0
Accumulated deficit	(168,622)	(157,049)
Total stockholders' equity (deficit)	(160,223)	(149,753)
Total liabilities, redeemable convertible preferred stock and stockholders' equity (deficit)	$ 70,918	$ 81,844

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Common stock, par value (USD per share)	$ 0.001	$ 0.001
Common stock authorized (in shares)	47,000,000	47,000,000
Common stock issued (in shares)	5,195,250	5,193,915
Common stock outstanding (in shares)	4,776,269	4,774,934
Treasury stock at cost (in shares)	418,981	418,981

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 7,563	$ 5,455
General and administrative	3,468	2,136
Total operating expenses	11,031	7,591
Loss from operations	(11,031)	(7,591)
Other income (expense):
Change in fair value of preferred stock warrant liability	(51)	37
Interest income (expense), net	(491)	(546)
Total other income (expense), net	(542)	(509)
Net loss	$ (11,573)	$ (8,100)
Net loss per share, basic and diluted (USD per share)	$ (2.43)	$ (1.92)
Weighted average common shares outstanding, basic and diluted (in shares)	4,775,828	4,229,118

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (11,573)	$ (8,100)
Adjustment to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	203	355
Stock-based compensation expense	1,137	368
Change in fair value of preferred stock warrant liability	51	(37)
Non-cash interest expense	153	75
Changes in current assets and liabilities:
Prepaid expenses and other current assets	(844)	92
Accounts payable	858	291
Accrued expenses	(2,182)	(1,169)
Net cash used in operating activities	(12,197)	(8,125)
Cash flows from investing activities:
Purchases of property and equipment	(47)	(288)
Net cash used in investing activities	(47)	(288)
Cash flows from financing activities:
Payment of deferred offering costs	(465)	0
Proceeds from issuance of long term debt	0	1,000
Payment of debt issuance costs	0	(844)
Proceeds from exercise of common stock options	12	0
Net cash (used in) provided by financing activities	(453)	156
Net decrease in cash and cash equivalents	(12,697)	(8,257)
Cash and cash equivalents, beginning of period	79,466	46,817
Cash and cash equivalents, end of period	66,769	38,560
Supplemental cash flow information:
Cash paid for interest	715	535
Deferred offering costs included in accounts payable and accrued expenses	$ 618	$ 0

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Treasury stock	Accumulated deficit
Redeemable convertible preferred stock, beginning (in shares) at Dec. 31, 2017	21,549,244
Redeemable convertible preferred stock, beginning at Dec. 31, 2017	$ 138,828
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of preferred stock to redemption value	$ 38
Redeemable convertible preferred stock, ending (in shares) at Mar. 31, 2018	21,549,244
Redeemable convertible preferred stock, ending at Mar. 31, 2018	$ 138,866
Beginning balance (in shares) at Dec. 31, 2017		4,648,078		418,981
Beginning balance at Dec. 31, 2017	(116,354)	$ 5	$ 4,621	$ 0	$ (120,980)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of preferred stock to redemption value	(38)		(38)
Stock-based compensation	368		368
Net loss	(8,100)				(8,100)
Ending balance (in shares) at Mar. 31, 2018		4,648,078		418,981
Ending balance at Mar. 31, 2018	$ (124,124)	$ 5	4,951	$ 0	(129,080)
Redeemable convertible preferred stock, beginning (in shares) at Dec. 31, 2018	26,831,246
Redeemable convertible preferred stock, beginning at Dec. 31, 2018	$ 197,842
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of preferred stock to redemption value	$ 46
Redeemable convertible preferred stock, ending (in shares) at Mar. 31, 2019	26,831,246
Redeemable convertible preferred stock, ending at Mar. 31, 2019	$ 197,888
Beginning balance (in shares) at Dec. 31, 2018		5,193,915		418,981
Beginning balance at Dec. 31, 2018	(149,753)	$ 6	7,290	$ 0	(157,049)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of preferred stock to redemption value	$ (46)		(46)
Exercise of common stock options (in shares)	1,336	1,335
Exercise of common stock options	$ 12		12
Stock-based compensation	1,137		1,137
Net loss	(11,573)				(11,573)
Ending balance (in shares) at Mar. 31, 2019		5,195,250		418,981
Ending balance at Mar. 31, 2019	$ (160,223)	$ 6	$ 8,393	$ 0	$ (168,622)

NATURE OF BUSINESS

NATURE OF BUSINESS	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
NATURE OF BUSINESS	NATURE OF BUSINESS Axcella Health Inc. and subsidiaries ("Axcella," the "Company" or "we") is a biotechnology company that was incorporated in Delaware on August 27, 2008 and has a principal place of business in Cambridge, Massachusetts. The Company is a biotechnology company pioneering the research and development of novel multifactorial interventions to support health and address dysregulated metabolism across a broad spectrum of consumers and patients who have limited options. The Company is subject to risks common to early-stage companies in the biotechnology industry, including, but not limited to, successful development of technology, obtaining additional funding, protection of proprietary technology, compliance with government regulations, risks of failure of preclinical studies, clinical studies and Clinical Trials, the need to obtain marketing approval for its product candidates, if required, and successfully market consumer products, fluctuations in operating results, economic pressure impacting therapeutic pricing, dependence on key personnel, risks associated with changes in technologies, development by competitors of technological innovations and the ability to scale manufacturing to large scale production. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and any necessary regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. On April 29, 2019, the Company filed an amended and restated certificate of incorporation to effect a one -for- 1.842 reverse stock split of the Company’s common stock. All share and per share data shown in the accompanying condensed consolidated financial statements and related notes have been retroactively revised to reflect the reverse stock split. On May 13, 2019, the Company completed an initial public offering (the “IPO”) of 3,571,428 shares of its common stock for aggregate gross proceeds of $71.4 million and its shares started trading on The Nasdaq Global Market under the ticker symbol “AXLA.” The Company received approximately $64.5 million in net proceeds after deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company. Upon closing of the IPO, all of the Company's outstanding shares of redeemable convertible preferred stock automatically converted into 14,641,997 shares of common stock. The accompanying condensed consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. The Company has historically funded its operations with proceeds from sales of preferred stock and borrowings under a loan and security agreement. As of March 31, 2019 , the Company had an accumulated deficit of $168.6 million . The Company expects to continue to generate operating losses in the foreseeable future. The Company expects that its cash and cash equivalents at March 31, 2019 , along with the net proceeds it received from completion of the IPO, will be sufficient to fund its operations for at least the next twelve months from the date of the issuance of the interim condensed consolidated financial statements.

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation and Principles of Consolidation The Company's condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America ("US GAAP"). The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. Unaudited Interim Financial Information The condensed consolidated financial statements of the Company included herein have been prepared, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP") have been condensed or omitted from this report, as is permitted by such rules and regulations. Accordingly, these condensed consolidated financial statements should be read in conjunction with the financial statements and notes thereto included in the Company’s Prospectus that forms a part of the Company’s Registration Statement on Form S-1 (File No. 333-230822), which was filed with the SEC pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended ("Securities Act"), on May 9, 2019 (the “Prospectus”). The results for any interim period are not necessarily indicative of results for any future period. Use of Estimates The preparation of the condensed consolidated financial statements in accordance with US GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expenses during the reporting period. The Company bases its estimates on historical experience, known trends and other market-specific or relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions. Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with the proposed IPO as deferred offering costs. The deferred offering costs will be offset against the IPO proceeds upon the consummation of the IPO, which closed on May 13, 2019. As of March 31, 2019 and December 31, 2018 , the Company capitalized $1.3 million and $0.3 million , respectively, of deferred issuance costs related to the IPO. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses. Summary of Significant Accounting Policies The Company’s significant accounting policies are described in Note 2 “Summary of Significant Accounting Policies,” in the Prospectus. There have been no material changes to the significant accounting policies during the period ended March 31, 2019 . Accounting Pronouncements Issued and Not Adopted In February 2016, the Financial Accounting Standards Board issued Accounting Standards Update, or ASU, No. 2016-02, Leases , which alters the accounting model and financial statement presentation and disclosure of leases. The new standard increases the transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and disclosing key information about leasing arrangements. ASU 2016-02 is effective for the Company's fiscal years beginning January 1, 2020. The Company is assessing the impact the adoption of ASU 2016-02 will have on its condensed consolidated financial statements and will recognize a lease obligation and right of use asset for its existing operating leases upon adoption. See additional information regarding the Company's lease obligations in Note 8.

PROPERTY AND EQUIPMENT

PROPERTY AND EQUIPMENT	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
PROPERTY AND EQUIPMENT	PROPERTY AND EQUIPMENT Property and equipment consist of the following (in thousands): March 31, December 31, Laboratory equipment $ 3,536 $ 3,489 Leasehold improvements 564 564 Office and computer equipment 294 294 Furniture and fixtures 122 122 Property and equipment 4,516 4,469 Less: accumulated depreciation and amortization (3,596 ) (3,393 ) Property and equipment, net $ 920 $ 1,076 Depreciation and amortization expense for the three months ended March 31, 2019 and 2018 was $0.2 million and $0.4 million , respectively.

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS The following table sets forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis (in thousands): Fair value measurements at March 31, 2019 using: Level 1 Level 2 Level 3 Total Assets: Cash Equivalents $ 66,519 $ — $ — $ 66,519 Total $ 66,519 $ — $ — $ 66,519 Liabilities: Success Fee Liability $ — $ — $ 1,220 $ 1,220 Preferred Stock Warrant Liability — — 476 476 Total $ — $ — $ 1,696 $ 1,696 Fair value measurements at December 31, 2018 using: Level 1 Level 2 Level 3 Total Assets: Cash Equivalents $ 79,216 $ — $ — $ 79,216 Total $ 79,216 $ — $ — $ 79,216 Liabilities: Success Fee Liability $ — $ — $ 1,220 $ 1,220 Preferred Stock Warrant Liability — — 425 425 Total $ — $ — $ 1,645 $ 1,645 Cash equivalents are comprised of funds held in an exchange traded money market fund and the fair value of the cash equivalents is determined based upon quoted market price for that fund. The fair value of the preferred stock warrant was determined using the Black-Scholes option-pricing model with the assumptions as disclosed in Note 7. These assumptions include significant judgments including the fair value of the underlying preferred stock. An increase or decrease in the estimated fair value will result in increases or decreases in the fair value of the warrant liability and such changes could be material. The fair value of the success fee liability was determined using a probability weighted present value of cash flows. The Company has projected that 100% of the liability will be paid and that the time value of discounting those cash flows does not have a material impact on the fair value measurement due to the expected term. The liability could decrease if the Company changes its assessment of the probability of paying the success fee. As a result of the IPO, the preferred stock warrants were converted to warrants to purchase common stock and the fair value of the warrant liability was reclassified to stockholders’ equity and the success fee liability was settled. As such, in the periods subsequent to the IPO, the carrying value of these instruments will no longer be adjusted. A roll forward of the fair value of the success fee liability and preferred stock warrant liability categorized with Level 3 inputs for the period ended March 31, 2019 is as follows (in thousands): Success fee Preferred stock warrant liability Balance — January 1, 2019 $ 1,220 $ 425 Increase in warrant fair value included in other expense — 51 Balance — March 31, 2019 $ 1,220 $ 476 There were no transfers among Level 1, Level 2, or Level 3 categories in the periods presented. The carrying value of cash, cash equivalents, accounts payable and accrued expenses that are reported on the consolidated balance sheets approximate their fair value due to the short-term nature of these assets and liabilities. The carrying value of the long-term debt approximates fair value as evidenced by the recent amendment.

ACCRUED EXPENSES AND OTHER CURR

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES	3 Months Ended
	Mar. 31, 2019
Payables and Accruals [Abstract]
ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES	ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES Accrued expenses and other liabilities consisted of the following (in thousands): ​ March 31, December 31, Accrued employee compensation and benefits $ 527 $ 1,957 Accrued external research and development expenses 2,007 1,679 Accrued professional fees 376 678 Other 825 985 Total accrued expenses and other current liabilities $ 3,735 $ 5,299

DEBT FINANCING

DEBT FINANCING	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
DEBT FINANCING	DEBT FINANCING Long term debt consisted of the following (in thousands): ​ March 31, December 31, Principal amount of long term debt $ 26,000 $ 26,000 Debt discount (569 ) (612 ) Deferred financing fees (807 ) (867 ) Long term debt, net of discount $ 24,624 $ 24,521 In January 2018, the Company entered into a new secured debt facility (the "2018 Facility") with the existing lender that replaced the February 2017 amended facility. The 2018 Facility provided funding of $21.0 million . The Company paid a transaction fee of $0.9 million to the lender in connection with the 2018 Facility, and that fee was recognized as debt discount. The 2018 Facility required interest only payments through January 2019 with the ability to extend the interest only payment period through January 2020 if certain conditions were met. Monthly principal payments of $0.6 million were to commence in February 2019 for 36 months . The 2018 Facility has an interest rate equal to the LIBOR plus 8.50% per annum ( 10.99% as of March 31, 2019 ) payable monthly and a $1.1 million success fee which is payable upon the occurrence of certain events, including the IPO. The success fee is comprised of $0.7 million associated with the February 2017 amended facility and an additional $0.4 million arising from the January 2018 amendment. The fair value of the additional success fee was recorded as an obligation to the lenders and created an additional debt discount. The Company granted the lender a first security interest in all assets of the Company, excluding intellectual property and granted a negative pledge on such intellectual property. In October 2018, the Company amended the 2018 Facility (the "Amended 2018 Facility") to extend the interest only period through July 2020 or January 2021 and the Maturity Date to July 2022 or January 2023 if certain conditions are met. The Amended 2018 Facility provides additional funding in the amounts of $5.0 million ("Term B Loan") and $4.0 million ("Term C Loan") if certain conditions are met. The Term B Loan of $5.0 million was drawn in December 2018. The success fee, increased to $1.2 million , which increased the debt discount by $0.1 million . Deferred financing costs of $0.1 million were incurred related to the amendment. The interest rate was not changed through the amendment. Upon completion of the IPO in May 2019, the interest only period was extended through January 2021 and the Maturity Date was extended to January 2023. Monthly principal payments of $1.1 million are to commence February 2021 for 24 months . The $1.2 million success fee was also paid. For the three months ended March 31, 2019 and 2018 , interest expense arising from the amortization of the debt discount and deferred financing fees was $0.2 million and $0.1 million , respectively. Terminal Interest Fee The Company's August 2015 debt facility, as amended, and the 2018 Facility, included a terminal interest fee obligation, which is due with the final principal payment of the loan and has been modified from time to time as the facilities were amended. The Company is accruing the terminal fee obligation over the term of the facility. At December 31, 2017 , the terminal fee obligation was $1.1 million . The October 2018 amendment increased the terminal interest fee to $1.4 million . There was no change in the terminal interest fee during the 3 months ended March 31, 2019 . The carrying value of the terminal interest fee was $0.7 million and $0.7 million at March 31, 2019 and December 31, 2018 , respectively. The scheduled principal maturity of the long-term debt, reflecting the 2018 Amended Facility and the Term B Loan draw offers, as of March 31, 2019 is as follows (in thousands): Year Ending December 31, 2020 $ 5,417 2021 13,000 2022 7,583 $ 26,000

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS' EQUITY At March 31, 2019 , the redeemable convertible preferred stock (the “Preferred Stock”) consisted of the following (in thousands, except for share data): March 31, 2019 Preferred Stock Authorized Preferred Carrying Value Liquidation Preference Common stock issuable upon conversion Series A Preferred Stock 5,874,334 5,761,539 $ 11,235 $ 11,235 3,127,870 Series B Preferred Stock 4,737,041 4,737,041 12,250 12,250 2,571,679 Series B-1 Preferred Stock 1,084,441 1,084,441 3,000 3,000 588,730 Series C Preferred Stock 6,969,044 6,969,044 70,091 70,248 3,783,401 Series D Preferred Stock 2,997,179 2,997,179 42,442 42,500 1,702,785 Series E Preferred Stock 6,266,786 5,282,002 58,870 59,000 2,867,532 27,928,825 26,831,246 $ 197,888 $ 198,233 14,641,997 Upon closing of the IPO in May 2019, all of the Preferred Stock converted into 14,641,997 shares of common stock. Preferred Stock Warrants — In connection with the issuance of debt in 2012, the Company issued warrants to purchase 112,795 shares of Series A Preferred Stock with an exercise price of $1.95 per share and an expiration date of April 13, 2020. The fair value of the warrants has been estimated using a Black-Scholes option-pricing model with the following assumptions: March 31, 2019 2018 Expected volatility 73.41 % 65 % Risk-free interest rate 2.22 % 2.48 % Weighted-average remaining contractual term (in years) 1.03 2.03 Expected dividend yield 0 % 0 % Fair value of underlying Series A Preferred Stock $6.06 $4.98 Upon closing of the IPO on May 13, 2019, the outstanding warrants to purchase Series A Preferred Stock became outstanding warrants to purchase an aggregate of 61,235 shares of common stock at a weighted average exercise price of $3.59 per share. The Company typically grants stock options to employees and non-employees at exercise prices deemed by the Company’s board of directors (the “Board”) to be equal to the fair value of the common stock at the time of grant. The fair value of the common stock has been determined by the Board at each measurement date based on a variety of different factors, including the results obtained from independent third-party appraisals, the Company's financial position and historical financial performance, the status of development of the Company's services, the current climate in the marketplace, the illiquid nature of the common stock, the effect of the rights and preferences of the preferred stockholders, and the prospects of a liquidity event, among others. The Company utilized the Black-Scholes option-pricing model to estimate the fair value of stock options awarded to employees. The Black-Scholes option-pricing model requires several key assumptions. The key assumptions used to apply this pricing model during the three months ended March 31, 2019 and 2018 , were as follows: 2019 2018 Risk-free interest rate 2.30% - 2.50 2.05% - 2.68% Expected term (in years) 6.25 1.83 - 6.25 Expected dividend yield 0 % 0 % Expected volatility of underlying common stock 73.4 % 65 % The risk-free interest rate was based on rates associated with U.S. Treasury issues approximating the expected life of the stock options. The expected term of options granted to employees was determined using the simplified method, which represents the midpoint of the contractual term of the option and the weighted-average vesting period of the option. The Company uses the simplified method because it does not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate the expected term. The expected dividend-yield assumption was based on the Company's expectation of no future dividend payments. The expected volatility of the underlying stock was based on the average historical volatility of comparable publicly traded companies based on weekly price returns as reported by a pricing service, as the Company does not have a trading history for its stock. The weighted-average grant date fair value of the options granted during the three months ended March 31, 2019 , was $5.02 per share. As of March 31, 2019 , there was $12.8 million of unrecognized compensation expense related to unvested stock options that is expected to be recognized over a weighted-average period of 3.1 years. Stock-based compensation related to stock options and unvested stock awards are classified as follows (in thousands): Three Months Ended March 31, 2019 2018 Research and development $ 482 $ 211 General and administrative 655 157 $ 1,137 $ 368 The following table summarizes the option activity under the 2010 Stock Incentive Plan: Options Weighted Average Exercise Price Weighted Average Remaining Life (in Years) Intrinsic Value (in thousands) Outstanding January 1, 2019 4,039,487 $ 5.67 8.6 $ 5,331 Granted 542,996 13.83 Exercised (1,336 ) 6.16 Canceled (8,028 ) 6.63 Outstanding March 31, 2019 4,573,119 $ 6.55 7.6 Options vested or expected to vest as of March 31, 2019 4,402,433 $ 6.55 7.6 $ 32,050 Options exercisable as of March 31, 2019 1,330,837 $ 4.41 5.8 $ 12,536 The intrinsic value of options exercised during the three months ended March 31, 2019 was nominal.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Leases The Company entered into a facility lease agreement for laboratory and office space with an expiration date of April 1, 2021. The lease agreement and most recent amendment contained escalating rent payments. Rent expense is recorded on a straight-line basis. The Company had deferred rent of $0.1 million as of both March 31, 2019 and 2018 . The Company is obligated to make minimum lease payments under the facility lease as follows (in thousands): Years Ending December 31, 2019 $ 896 2020 1,226 2021 415 Total $ 2,537 Rent expense for the three months ended March 31, 2019 and 2018 was $0.3 million and $0.3 million , respectively. We enter into contracts in the normal course of business with contract research organizations ("CROs"), contract manufacturing organizations ("CMOs") and other third parties for preclinical research studies, Non-IND, IRB-Approved Clinical Studies, Clinical Trials and testing and manufacturing services. These contracts do not contain minimum purchase commitments and are cancelable by upon prior written notice. Payments due upon cancellation consist only of payments for services provided or expenses incurred, including noncancelable obligations of service providers, up to the date of cancellation.

RETIREMENT PLAN

RETIREMENT PLAN	3 Months Ended
	Mar. 31, 2019
Retirement Benefits [Abstract]
RETIREMENT PLAN	RETIREMENT PLAN The Company has a 401(k) retirement and savings plan (the "Plan") covering all qualified employees. The Plan allows each participant to contribute a portion of his or her base wages up to an amount not to exceed an annual statutory maximum. The Company is permitted to make discretionary matching contributions to the Plan. The Company has not made any discretionary contributions.

NET LOSS PER SHARE

NET LOSS PER SHARE	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
NET LOSS PER SHARE	NET LOSS PER SHARE Net Loss Per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended March 31, 2019 2018 Numerator: Net loss $ (11,573 ) $ (8,100 ) Accretion of redeemable convertible preferred stock (46 ) (38 ) Net loss attributable to common stockholders $ (11,619 ) $ (8,138 ) Denominator: Weighted average common shares outstanding, basic and diluted 4,775,828 4,229,118 Net loss per share, basic and diluted $ (2.43 ) $ (1.92 ) The Company's potential dilutive securities, which include redeemable convertible preferred stock, warrants to purchase redeemable convertible preferred stock, vested and unvested stock purchased with promissory notes and common stock options, have been excluded from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended March 31, 2019 2018 Redeemable convertible preferred stock (as converted to common stock) 14,641,997 11,735,178 Warrants to purchase redeemable convertible preferred stock (as converted to common stock) 61,235 61,235 Options to purchase common stock 4,573,119 2,673,453 19,276,351 14,469,866 Upon closing of the IPO, all of the Preferred Stock converted into 14,641,997 shares of common stock. 3,571,428 shares of common stock were issued as part of the IPO.

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	3 Months Ended
	Mar. 31, 2019
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	SUBSEQUENT EVENTS The Company has evaluated subsequent events for financial statement purposes occurring through June 20, 2019, the date that these condensed consolidated financial statements were issued. 2019 Stock Option and Incentive Plan — The 2019 Stock Option and Incentive Plan (the "2019 Plan") was approved by our board of directors on April 29, 2019 and became effective immediately prior to May 9, 2019, which was the date the registration statement for the Company's proposed IPO was declared effective. The 2019 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock units, restricted stock awards, unrestricted stock awards, cash-based awards and dividend equivalent rights to the Company's officers, employees, directors and consultants. The number of shares initially reserved for issuance under the 2019 Plan is 905,000 , which shall be cumulatively increased on January 1, 2020 and each January 1 thereafter by 4% of the number of shares of the Company's common stock outstanding on the immediately preceding December 31, or such lesser number of shares determined by the Company's board of directors or compensation committee of the board of directors. 2019 Employee Stock Purchase Plan — The 2019 Employee Stock Purchase Plan (the "2019 ESPP") was approved by our board of directors on April 29, 2019 and became effective immediately prior to May 9, 2019, which was the date the registration statement for the Company's proposed IPO was declared effective. A total of 237,181 shares of common stock were initially reserved for issuance under this plan, which shall be cumulatively increased on January 1, 2020 and each January 1 thereafter by 1% of the number of shares of the Company's common stock outstanding on the immediately preceding December 31, or such lesser number of shares determined by the Company's board of directors or compensation committee of the board of directors.

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation and Principles of Consolidation The Company's condensed consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America ("US GAAP").
Principles of Consolidation	The accompanying condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of the condensed consolidated financial statements in accordance with US GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expenses during the reporting period. The Company bases its estimates on historical experience, known trends and other market-specific or relevant factors that it believes to be reasonable under the circumstances. On an ongoing basis, management evaluates its estimates as there are changes in circumstances, facts and experience. Actual results may differ from those estimates or assumptions.
Deferred Offering Costs	Deferred Offering Costs The Company capitalizes certain legal, professional accounting and other third-party fees that are directly associated with the proposed IPO as deferred offering costs. The deferred offering costs will be offset against the IPO proceeds upon the consummation of the IPO, which closed on May 13, 2019.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.
Accounting Pronouncements Issued and Not Adopted	Accounting Pronouncements Issued and Not Adopted In February 2016, the Financial Accounting Standards Board issued Accounting Standards Update, or ASU, No. 2016-02, Leases , which alters the accounting model and financial statement presentation and disclosure of leases. The new standard increases the transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and disclosing key information about leasing arrangements. ASU 2016-02 is effective for the Company's fiscal years beginning January 1, 2020. The Company is assessing the impact the adoption of ASU 2016-02 will have on its condensed consolidated financial statements and will recognize a lease obligation and right of use asset for its existing operating leases upon adoption. See additional information regarding the Company's lease obligations in Note 8.

PROPERTY AND EQUIPMENT (Tables)

PROPERTY AND EQUIPMENT (Tables)	3 Months Ended
	Mar. 31, 2019
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment	Property and equipment consist of the following (in thousands): March 31, December 31, Laboratory equipment $ 3,536 $ 3,489 Leasehold improvements 564 564 Office and computer equipment 294 294 Furniture and fixtures 122 122 Property and equipment 4,516 4,469 Less: accumulated depreciation and amortization (3,596 ) (3,393 ) Property and equipment, net $ 920 $ 1,076

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	3 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Fair Value Hierarchy of Assets and Liabilities	The following table sets forth by level, within the fair value hierarchy, the assets and liabilities carried at fair value on a recurring basis (in thousands): Fair value measurements at March 31, 2019 using: Level 1 Level 2 Level 3 Total Assets: Cash Equivalents $ 66,519 $ — $ — $ 66,519 Total $ 66,519 $ — $ — $ 66,519 Liabilities: Success Fee Liability $ — $ — $ 1,220 $ 1,220 Preferred Stock Warrant Liability — — 476 476 Total $ — $ — $ 1,696 $ 1,696 Fair value measurements at December 31, 2018 using: Level 1 Level 2 Level 3 Total Assets: Cash Equivalents $ 79,216 $ — $ — $ 79,216 Total $ 79,216 $ — $ — $ 79,216 Liabilities: Success Fee Liability $ — $ — $ 1,220 $ 1,220 Preferred Stock Warrant Liability — — 425 425 Total $ — $ — $ 1,645 $ 1,645
Rollforward of the Fair Value of the Success Fee Liability and Preferred Stock Warrant Liability	A roll forward of the fair value of the success fee liability and preferred stock warrant liability categorized with Level 3 inputs for the period ended March 31, 2019 is as follows (in thousands): Success fee Preferred stock warrant liability Balance — January 1, 2019 $ 1,220 $ 425 Increase in warrant fair value included in other expense — 51 Balance — March 31, 2019 $ 1,220 $ 476

ACCRUED EXPENSES AND OTHER CU_2

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES Payables and Accruals (Tables)	3 Months Ended
	Mar. 31, 2019
Payables and Accruals [Abstract]
Schedule of Accrued Expenses and Other Liabilities	Accrued expenses and other liabilities consisted of the following (in thousands): ​ March 31, December 31, Accrued employee compensation and benefits $ 527 $ 1,957 Accrued external research and development expenses 2,007 1,679 Accrued professional fees 376 678 Other 825 985 Total accrued expenses and other current liabilities $ 3,735 $ 5,299

DEBT FINANCING (Tables)

DEBT FINANCING (Tables)	3 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Summary of Long-term Debt	Long term debt consisted of the following (in thousands): ​ March 31, December 31, Principal amount of long term debt $ 26,000 $ 26,000 Debt discount (569 ) (612 ) Deferred financing fees (807 ) (867 ) Long term debt, net of discount $ 24,624 $ 24,521
Schedule of Maturities of Long-term Debt	The scheduled principal maturity of the long-term debt, reflecting the 2018 Amended Facility and the Term B Loan draw offers, as of March 31, 2019 is as follows (in thousands): Year Ending December 31, 2020 $ 5,417 2021 13,000 2022 7,583 $ 26,000

STOCKHOLDERS' EQUITY (Tables)

STOCKHOLDERS' EQUITY (Tables)	3 Months Ended
	Mar. 31, 2019
Equity [Abstract]
Summary of Preferred Stock and Common Stock Reserved for Future Issuance	At March 31, 2019 , the redeemable convertible preferred stock (the “Preferred Stock”) consisted of the following (in thousands, except for share data): March 31, 2019 Preferred Stock Authorized Preferred Carrying Value Liquidation Preference Common stock issuable upon conversion Series A Preferred Stock 5,874,334 5,761,539 $ 11,235 $ 11,235 3,127,870 Series B Preferred Stock 4,737,041 4,737,041 12,250 12,250 2,571,679 Series B-1 Preferred Stock 1,084,441 1,084,441 3,000 3,000 588,730 Series C Preferred Stock 6,969,044 6,969,044 70,091 70,248 3,783,401 Series D Preferred Stock 2,997,179 2,997,179 42,442 42,500 1,702,785 Series E Preferred Stock 6,266,786 5,282,002 58,870 59,000 2,867,532 27,928,825 26,831,246 $ 197,888 $ 198,233 14,641,997
Summary of Fair Value Measurement Inputs	The fair value of the warrants has been estimated using a Black-Scholes option-pricing model with the following assumptions: March 31, 2019 2018 Expected volatility 73.41 % 65 % Risk-free interest rate 2.22 % 2.48 % Weighted-average remaining contractual term (in years) 1.03 2.03 Expected dividend yield 0 % 0 % Fair value of underlying Series A Preferred Stock $6.06 $4.98
Summary of Fair Value Assumptions for Stock Options	The key assumptions used to apply this pricing model during the three months ended March 31, 2019 and 2018 , were as follows: 2019 2018 Risk-free interest rate 2.30% - 2.50 2.05% - 2.68% Expected term (in years) 6.25 1.83 - 6.25 Expected dividend yield 0 % 0 % Expected volatility of underlying common stock 73.4 % 65 %
Summary of Stock Based Compensation Expense	Stock-based compensation related to stock options and unvested stock awards are classified as follows (in thousands): Three Months Ended March 31, 2019 2018 Research and development $ 482 $ 211 General and administrative 655 157 $ 1,137 $ 368
Summary of Options Under the 2010 Plan	The following table summarizes the option activity under the 2010 Stock Incentive Plan: Options Weighted Average Exercise Price Weighted Average Remaining Life (in Years) Intrinsic Value (in thousands) Outstanding January 1, 2019 4,039,487 $ 5.67 8.6 $ 5,331 Granted 542,996 13.83 Exercised (1,336 ) 6.16 Canceled (8,028 ) 6.63 Outstanding March 31, 2019 4,573,119 $ 6.55 7.6 Options vested or expected to vest as of March 31, 2019 4,402,433 $ 6.55 7.6 $ 32,050 Options exercisable as of March 31, 2019 1,330,837 $ 4.41 5.8 $ 12,536

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Tables)	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments	The Company is obligated to make minimum lease payments under the facility lease as follows (in thousands): Years Ending December 31, 2019 $ 896 2020 1,226 2021 415 Total $ 2,537

NET LOSS PER SHARE (Tables)

NET LOSS PER SHARE (Tables)	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Schedule of Earnings Per Share, Basic and Diluted	Basic and diluted net loss per share attributable to common stockholders was calculated as follows (in thousands, except share and per share amounts): Three Months Ended March 31, 2019 2018 Numerator: Net loss $ (11,573 ) $ (8,100 ) Accretion of redeemable convertible preferred stock (46 ) (38 ) Net loss attributable to common stockholders $ (11,619 ) $ (8,138 ) Denominator: Weighted average common shares outstanding, basic and diluted 4,775,828 4,229,118 Net loss per share, basic and diluted $ (2.43 ) $ (1.92 )
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect: Three Months Ended March 31, 2019 2018 Redeemable convertible preferred stock (as converted to common stock) 14,641,997 11,735,178 Warrants to purchase redeemable convertible preferred stock (as converted to common stock) 61,235 61,235 Options to purchase common stock 4,573,119 2,673,453 19,276,351 14,469,866

NATURE OF BUSINESS (Details)

NATURE OF BUSINESS (Details) $ in Thousands	May 13, 2019USD ($)shares	Apr. 29, 2019	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Class of Stock [Line Items]
Accumulated deficit			$ 168,622	$ 157,049
Subsequent Event
Class of Stock [Line Items]
Reverse stock split conversion ratio		0.5429
IPO | Subsequent Event
Class of Stock [Line Items]
Shares sold | shares	3,571,428
Total consideration received	$ 71,400
Net proceeds on offering	$ 64,500
Shares converted | shares	14,641,997

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Deferred offering costs	$ 1,334	$ 251

PROPERTY AND EQUIPMENT - Summar

PROPERTY AND EQUIPMENT - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment	$ 4,516	$ 4,469
Less: accumulated depreciation and amortization	(3,596)	(3,393)
Property and equipment, net	920	1,076
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment	3,536	3,489
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment	564	564
Office and computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment	294	294
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment	$ 122	$ 122

PROPERTY AND EQUIPMENT - Narrat

PROPERTY AND EQUIPMENT - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Property, Plant and Equipment [Abstract]
Depreciation and amortization	$ 203	$ 355

FAIR VALUE MEASUREMENTS - Fair

FAIR VALUE MEASUREMENTS - Fair Value Hierarchy (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Liabilities:
Success fee liability expected to be paid (percent)	100.00%
Fair Value, Recurring
Assets:
Cash Equivalents	$ 66,519	$ 79,216
Total	66,519	79,216
Liabilities:
Success Fee Liability	1,220	1,220
Preferred Stock Warrant Liability	476	425
Total	1,696	1,645
Fair Value, Recurring | Level 1
Assets:
Cash Equivalents	66,519	79,216
Total	66,519	79,216
Liabilities:
Success Fee Liability	0	0
Preferred Stock Warrant Liability	0	0
Total	0	0
Fair Value, Recurring | Level 2
Assets:
Cash Equivalents	0	0
Total	0	0
Liabilities:
Success Fee Liability	0	0
Preferred Stock Warrant Liability	0	0
Total	0	0
Fair Value, Recurring | Level 3
Assets:
Cash Equivalents	0	0
Total	0	0
Liabilities:
Success Fee Liability	1,220	1,220
Preferred Stock Warrant Liability	476	425
Total	$ 1,696	$ 1,645

FAIR VALUE MEASUREMENTS - Rollf

FAIR VALUE MEASUREMENTS - Rollforward of Fair Value of Success Fee Liability and Preferred Stock Warrant Liability (Details) - Fair Value, Recurring $ in Thousands	3 Months Ended
	Mar. 31, 2019USD ($)
Success fee
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning	$ 1,220
Balance, ending	1,220
Preferred stock warrant liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning	425
Increase in warrant fair value included in other expense	51
Balance, ending	$ 476

ACCRUED EXPENSES AND OTHER CU_3

ACCRUED EXPENSES AND OTHER CURRENT LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Payables and Accruals [Abstract]
Accrued employee compensation and benefits	$ 527	$ 1,957
Accrued external research and development expenses	2,007	1,679
Accrued professional fees	376	678
Other	825	985
Total accrued expenses and other current liabilities	$ 3,735	$ 5,299

DEBT FINANCING - Summary of Lon

DEBT FINANCING - Summary of Long-term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
Principal amount of long term debt	$ 26,000	$ 26,000
Debt discount	(569)	(612)
Deferred financing fees	(807)	(867)
Long term debt, net of discount	$ 24,624	$ 24,521

DEBT FINANCING - Narrative (Det

DEBT FINANCING - Narrative (Details) - USD ($)	May 17, 2019	Feb. 28, 2021	Dec. 31, 2018	Oct. 31, 2018	Jan. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Transaction fee						$ 0	$ 844,000
Debt discount			$ 612,000			569,000
Deferred financing fees			867,000			807,000
Interest expense						$ 200,000	$ 100,000
Secured Debt
Line of Credit Facility [Line Items]
Success fee					$ 1,100,000
2018 Facility | Secured Debt
Line of Credit Facility [Line Items]
Funding amount					21,000,000
Transaction fee					900,000
Monthly principal payments					$ 600,000
Term					36 months
Interest rate at period end						10.99%
Success fee					$ 400,000
2018 Facility | LIBOR | Secured Debt
Line of Credit Facility [Line Items]
Basis spread on interest rate					8.50%
February 2017 Amended Facility | Secured Debt
Line of Credit Facility [Line Items]
Success fee					$ 700,000
Term Loan B | Line of Credit
Line of Credit Facility [Line Items]
Amounts drawn			5,000,000
Amended 2018 Facility | Secured Debt
Line of Credit Facility [Line Items]
Terminal fee								$ 1,100,000
Amended 2018 Facility | Line of Credit
Line of Credit Facility [Line Items]
Success fee				$ 1,200,000
Debt discount				100,000
Deferred financing fees				100,000
Terminal fee				1,400,000
Terminal fee, change in period						$ 0
Terminal fee, at carrying value			$ 700,000			$ 700,000
Pro Forma | Term Loan B | Line of Credit
Line of Credit Facility [Line Items]
Funding amount				5,000,000
Pro Forma | Term Loan C | Line of Credit
Line of Credit Facility [Line Items]
Funding amount				$ 4,000,000
Forecast | Amended 2018 Facility | Line of Credit
Line of Credit Facility [Line Items]
Monthly principal payments		$ 1,100,000
Term		24 months
Subsequent Event | Amended 2018 Facility | Line of Credit
Line of Credit Facility [Line Items]
Success fee	$ 1,200,000

DEBT FINANCING - Maturities of

DEBT FINANCING - Maturities of Long-term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
2020	$ 5,417
2023	13,000
2022	7,583
Principal amount of long-term debt	$ 26,000	$ 26,000

STOCKHOLDERS' EQUITY - Summary

STOCKHOLDERS' EQUITY - Summary of Preferred Stock (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Preferred Stock Authorized (in shares)	27,928,825
Preferred Stock Issued (in shares)	26,831,246
Preferred Stock Outstanding (in shares)	26,831,246	26,831,246	21,549,244	21,549,244
Carrying Value	$ 197,888	$ 197,842	$ 138,866	$ 138,828
Liquidation Preference	$ 198,233
Common stock issuable upon conversion (in shares)	14,641,997
Series A Preferred Stock
Class of Stock [Line Items]
Preferred Stock Authorized (in shares)	5,874,334
Preferred Stock Issued (in shares)	5,761,539
Preferred Stock Outstanding (in shares)	5,761,539
Carrying Value	$ 11,235
Liquidation Preference	$ 11,235
Common stock issuable upon conversion (in shares)	3,127,870
Series B Preferred Stock
Class of Stock [Line Items]
Preferred Stock Authorized (in shares)	4,737,041
Preferred Stock Issued (in shares)	4,737,041
Preferred Stock Outstanding (in shares)	4,737,041
Carrying Value	$ 12,250
Liquidation Preference	$ 12,250
Common stock issuable upon conversion (in shares)	2,571,679
Series B-1 Preferred Stock
Class of Stock [Line Items]
Preferred Stock Authorized (in shares)	1,084,441
Preferred Stock Issued (in shares)	1,084,441
Preferred Stock Outstanding (in shares)	1,084,441
Carrying Value	$ 3,000
Liquidation Preference	$ 3,000
Common stock issuable upon conversion (in shares)	588,730
Series C Preferred Stock
Class of Stock [Line Items]
Preferred Stock Authorized (in shares)	6,969,044
Preferred Stock Issued (in shares)	6,969,044
Preferred Stock Outstanding (in shares)	6,969,044
Carrying Value	$ 70,091
Liquidation Preference	$ 70,248
Common stock issuable upon conversion (in shares)	3,783,401
Series D Preferred Stock
Class of Stock [Line Items]
Preferred Stock Authorized (in shares)	2,997,179
Preferred Stock Issued (in shares)	2,997,179
Preferred Stock Outstanding (in shares)	2,997,179
Carrying Value	$ 42,442
Liquidation Preference	$ 42,500
Common stock issuable upon conversion (in shares)	1,702,785
Series E Preferred Stock
Class of Stock [Line Items]
Preferred Stock Authorized (in shares)	6,266,786
Preferred Stock Issued (in shares)	5,282,002
Preferred Stock Outstanding (in shares)	5,282,002
Carrying Value	$ 58,870
Liquidation Preference	$ 59,000
Common stock issuable upon conversion (in shares)	2,867,532

STOCKHOLDERS' EQUITY - Preferre

STOCKHOLDERS' EQUITY - Preferred Stock Narrative (Details)	May 13, 2019shares
Subsequent Event | IPO
Subsidiary, Sale of Stock [Line Items]
Shares converted	14,641,997

STOCKHOLDERS' EQUITY - Prefer_2

STOCKHOLDERS' EQUITY - Preferred Stock Warrants (Details) - $ / shares	12 Months Ended
	Dec. 31, 2012	May 13, 2019
Series A Preferred Stock
Class of Warrant or Right [Line Items]
Warrants issued in the period (in shares)	112,795
Warrant exercise price per share (USD per share)	$ 1.95
Subsequent Event | Common stock
Class of Warrant or Right [Line Items]
Warrant exercise price per share (USD per share)		$ 3.59
Warrants outstanding (in shares)		61,235

STOCKHOLDERS' EQUITY - Fair Val

STOCKHOLDERS' EQUITY - Fair Value Inputs of Warrants (Details)	Mar. 31, 2019	Mar. 31, 2018
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, term	1 year 11 days	2 years 11 days
Expected volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.7341	0.6500
Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0.0222	0.0248
Expected dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	0	0
Fair value of underlying Series A Preferred Stock
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants and rights outstanding, measurement input	6.06	4.98

STOCKHOLDERS' EQUITY - Stock Op

STOCKHOLDERS' EQUITY - Stock Option Plan (Details) - Stock options - 2010 Plan $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019USD ($)$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grand date fair value of options granted in the period (USD per share) | $ / shares	$ 5.02
Unrecognized compensation expense related to unvested stock options | $	$ 12.8
Unrecognized compensation expense, recognition period	3 years 1 month 7 days

STOCKHOLDERS' EQUITY - Stock _2

STOCKHOLDERS' EQUITY - Stock Option Fair Value Measurement Inputs (Details) - 2010 Plan - Stock options	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	2.30%	2.05%
Risk-free interest rate, maximum	2.50%	2.68%
Expected term (in years)	6 years 3 months
Expected dividend yield	0.00%	0.00%
Expected volatility of underlying common stock	73.40%	65.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		1 year 9 months 29 days
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 years 3 months

STOCKHOLDERS' EQUITY - Summar_2

STOCKHOLDERS' EQUITY - Summary of Stock Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 1,137	$ 368
Research and development
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	482	211
General and administrative
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation expense	$ 655	$ 157

STOCKHOLDERS' EQUITY - Summar_3

STOCKHOLDERS' EQUITY - Summary of Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	4,039,487
Granted (in shares)	542,996
Exercised (in shares)	(1,336)
Canceled (in shares)	(8,028)
Outstanding, ending balance (in shares)	4,573,119	4,039,487
Options vested or expected to vest (in shares)	4,402,433
Options exercisable (in shares)	1,330,837
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price, outstanding, beginning balance (USD per share)	$ 5.67
Weighted-average exercise price, granted (USD per share)	13.83
Weighted-average exercise price, exercised (USD per share)	6.16
Weighted-average exercise price, canceled (USD per share)	6.63
Weighted average exercise price, outstanding, ending balance (USD per share)	6.55	$ 5.67
Weighted-average exercise price, options vested or expected to vest (USD per share)	6.55
Weighted-average exercise price, options exercisable (USD per share)	$ 4.41
Weighted Average Remaining Life, Outstanding (in Years)	7 years 7 months 6 days	8 years 7 months 6 days
Weighted Average Remaining Life, Options vested or expected to vest (in Years)	7 years 7 months 6 days
Weighted Average Remaining Life, Options exercisable (in Years)	5 years 9 months 18 days
Intrinsic Value, Outstanding		$ 5,331
Intrinsic Value, Options vested or expected to vest	$ 32,050
Intrinsic Value, Options exercisable	$ 12,536

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Deferred rent	$ 0.1	$ 0.1
Rent expense	$ 0.3	$ 0.3

COMMITMENTS AND CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES - Future Minimum Lease Payments (Details) $ in Thousands	Mar. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 896
2020	1,226
2021	415
Total	$ 2,537

NET LOSS PER SHARE - Calculatio

NET LOSS PER SHARE - Calculation of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (11,573)	$ (8,100)
Accretion of redeemable convertible preferred stock	(46)	(38)
Net loss attributable to common stockholders	$ (11,619)	$ (8,138)
Denominator:
Weighted average common shares outstanding, basic and diluted (in shares)	4,775,828	4,229,118
Net loss per share, basic and diluted (USD per share)	$ (2.43)	$ (1.92)

NET LOSS PER SHARE - Summary of

NET LOSS PER SHARE - Summary of Anti-dilutive Securities (Details) - shares	May 13, 2019	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share (in shares)		19,276,351	14,469,866
Redeemable convertible preferred stock (as converted to common stock)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share (in shares)		14,641,997	11,735,178
Warrants to purchase redeemable convertible preferred stock (as converted to common stock)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share (in shares)		61,235	61,235
Options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of diluted net loss per share (in shares)		4,573,119	2,673,453
IPO | Subsequent Event
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares converted	14,641,997
Shares sold	3,571,428

SUBSEQUENT EVENTS - 2019 Stock

SUBSEQUENT EVENTS - 2019 Stock Option, Incentive Plan and Employee Stock Purchase Plan (Details) - Subsequent Event	Apr. 29, 2019shares
Stock Option And Incentive Plan 2019
Subsequent Event [Line Items]
Number of shares initially reserved for future issuance (in shares)	905,000
Annual percent increase in authorized shares available for issuance under share based payment arrangements	4.00%
Employee Stock Purchase Plan 2019
Subsequent Event [Line Items]
Number of shares initially reserved for future issuance (in shares)	237,181
Annual percent increase in authorized shares available for issuance under share based payment arrangements	1.00%