Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities:
Net loss	$ (29,401)	$ (23,450)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion on marketable securities	(625)	(57)
Depreciation and amortization expense	55	31
Stock-based compensation expense	1,607	1,583
Changes in operating assets and liabilities:
Restricted cash	(161)	(84)
Prepaid expenses and other current assets	(1,786)	(1,573)
Accounts payable	5,197	3,119
Accrued liabilities	(1,263)	2,819
Funding advance		500
Other liabilities	(8)	25
Net cash used in operating activities	(26,385)	(17,087)
Investing activities:
Purchases of property and equipment	(48)	(304)
Maturities of marketable securities	67,500
Purchases of marketable securities	(77,104)	(74,543)
Net cash used in investing activities	(9,652)	(74,847)
Financing activities:
Proceeds from public offering, net of issuance costs	49,748	89,938
Proceeds from at-the-market offering, net of issuance costs	627
Proceeds from exercise of stock options	(9)
Net cash provided by financing activities	50,366	89,938
Net increase (decrease) in cash and cash equivalents	14,329	(1,996)
Cash and cash equivalents at beginning of period	7,042	53,349
Cash and cash equivalents at end of period	21,371	51,353
Supplemental disclosure of non-cash investing and financing activities:
Purchases of property and equipment included in accounts payable	11	12
Conversion of redeemable convertible preferred stock into common stock		$ 81,620
Issuance costs associated with at-the-market offering included in accounts payable	$ 45