Condensed Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (11,984)	$ (11,424)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	276	295
Stock-based compensation	1,496	2,713
Accretion of discounts on short-term investments	(217)	(57)
Amortization of debt discount	44	0
Deferred rent	0	27
Changes in operating assets and liabilities:
Accounts receivable, net	94	(26)
Prepaid and other assets	584	(23)
Accounts payable	123	1,503
Accrued expenses	462	(1,275)
Accrued compensation	(1,392)	(1,531)
Operating leases	10	0
Net cash used in operating activities	(10,504)	(9,798)
Cash flows from investing activities:
Purchases of short-term investments	(32,763)	(21,898)
Maturities of short-term investments	26,000	36,500
Purchases of property and equipment	(36)	(74)
Net cash (used in) provided by investing activities	(6,799)	14,528
Cash flows from financing activities:
Proceeds from exercise of stock options	0	32
Payments of debt issuance costs	(52)	0
Net cash (used in) provided by financing activities	(52)	32
Net change in cash, cash equivalents and restricted cash	(17,355)	4,762
Cash, cash equivalents and restricted cash at beginning of period	34,329	19,614
Cash, cash equivalents and restricted cash at end of period	16,974	24,376
Cash and cash equivalents at end of period	16,276	23,217
Restricted cash at end of period	698	1,159
Supplemental cash flow disclosures
Cash paid for interest	347	0
Supplemental disclosure of non-cash investing activities:
Purchase of property and equipment in accounts payable and accrued expenses	$ 0	$ 59