Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net loss	$ (27,227)	$ (28,842)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	211	233
Stock-based compensation expense	4,834	4,325
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,889	(530)
Accounts receivable	(46)	127
Accounts payable	(60)	(1,065)
Accrued expenses and other liabilities	(946)	(178)
Net cash used in operating activities	(21,345)	(25,930)
Cash flows from investing activities:
Purchases of property and equipment	(35)	(43)
Net cash used in investing activities	(35)	(43)
Cash flows from financing activities:
Proceeds from stock options exercised	473	251
Proceeds from public offering, net of underwriters' discounts	33,672
Proceeds from at the market offering, net of sales commission	4,668
Payment of offering costs	(404)	(24)
Net cash provided by financing activities	38,409	227
Net increase (decrease) in cash, cash equivalents and restricted cash	17,029	(25,746)
Cash, cash equivalents and restricted cash, at beginning of period	99,194	151,128
Cash, cash equivalents and restricted cash, at end of period	116,223	125,382
Supplemental disclosure of non-cash investing and financing activities:
Offering costs not yet paid	12	$ 104
Property and equipment in accounts payable	$ 38