Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (43,632)	$ (39,188)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	345	336
Stock-based compensation expense	6,966	1,984
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,563)	356
Accounts receivable	117	344
Accounts payable	(139)	(263)
Accrued expenses and other liabilities	1,279	3,966
Net cash used in operating activities	(36,627)	(32,465)
Cash flows from investing activities:
Purchases of property and equipment	(43)	(391)
Net cash used in investing activities	(43)	(391)
Cash flows from financing activities:
Payment of deferred issuance costs associated with Series B convertible preferred stock financing		(232)
Proceeds from initial public offering, net of underwriters discounts		109,517
Proceeds from stock options exercised	282	2
Payment of deferred offering costs	(181)	(2,971)
Net cash provided by financing activities	101	106,316
Net (decrease) increase in cash, cash equivalents and restricted cash	(36,569)	73,460
Cash, cash equivalents and restricted cash, at beginning of period	151,128	92,609
Cash, cash equivalents and restricted cash, at end of period	114,559	166,069
Supplemental disclosure of non-cash investing and financing activities:
Deferred offering costs not yet paid	$ 17	$ 41