CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating activities
Net loss	$ (27,916)	$ (18,978)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	124	82
Stock-based compensation	999	262
Non-cash interest expense	354	3,580
Change in fair value warrant liability		2,709
Change in fair value derivative liability		676
Changes in operating assets and liabilities:
Other receivable	(350)	(411)
Prepaid expenses and other assets	340	130
Accounts payable	(114)	558
Accrued expenses	(556)	1,182
Deferred rent	14	(5)
Net cash used in operating activities	(27,105)	(10,215)
Investing activities
Purchases of property and equipment	(824)	(31)
Net cash used in investing activities	(824)	(31)
Financing activities
Proceeds from issuance of redeemable convertible preferred stock, net		17,712
Proceeds from issuance of common stock in public offering, net	46,277
Long-term borrowings, net	3,089	3,995
Payments of initial public offering costs		(595)
Proceeds from exercise of stock options	221	53
Proceeds from exercise of warrants		16
Repayments of long-term debt	(938)
Net cash provided by financing activities	48,649	21,181
Net increase in cash and cash equivalents	20,720	10,935
Cash, cash equivalents and restricted cash at the beginning of the period	61,051	13,406
Cash, cash equivalents and restricted cash at the end of the period	81,771	24,341
Supplemental disclosure for cash flow
Cash paid for interest	357	161
Supplemental disclosure of non-cash investing and financing activities
Accrued interest on term loan refinanced to principal	308
Conversion of convertible promissory notes into redeemable convertible preferred stock		19,778
Issuance of warrants in conjunction with debt	296	2,822
Debt refinanced with new term loan	6,563
Initial recognition of operating lease right-of-use assets and operating lease liabilities	1,991
Accretion of redeemable convertible preferred stock		95
Warrant liability converted to redeemable convertible preferred stock upon the exercise of warrants		1,196
Deferred initial public offering costs included in accounts payable and accrued expenses		$ 1,623
Purchases of property and equipment included in accounts payable	$ 6