CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash Flows from Operating Activities:
Net loss	$ (15,409)	$ (22,150)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	1,504	1,905
Warrant liability revaluation gain	(188)
Future tranche right liability revaluation gain	(5,362)
Issuance of common stock for services rendered	98	59
Accretion of discounts on short-term investments	(36)	(353)
Unrealized gain on available-for-sale securities		2
Depreciation and amortization expense	41	67
Gain on disposal of property and equipment		(11)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	819	(423)
Accounts payable, accrued expenses, and other liabilities	(575)	(2,426)
Other	8
Net cash used in operating activities	(19,100)	(23,330)
Cash Flows from Investing Activities:
Purchases of available-for-sale securities	(5,535)	(35,486)
Proceeds from maturity of available-for-sale securities	5,149	23,350
Proceeds from the sale of property and equipment		11
Purchases of property and equipment	(7)	(11)
Net cash used in investing activities	(393)	(12,136)
Cash Flows from Financing Activities:
Proceeds from common stock financings, net	7,230	3,857
Proceeds from employee stock purchases	54	68
Other		(6)
Net cash provided by financing activities	7,284	3,919
Net decrease in cash and cash equivalents	(12,209)	(31,547)
Cash and cash equivalent, beginning of period	40,019	71,431
Cash and cash equivalents, end of period	27,810	39,884
Supplemental disclosure of cash flow information:
Increase to right-of-use asset upon adoption of ASC 842		261
Increase to lease liability upon adoption of ASC 842		$ 261
Supplemental disclosure of non-cash financing and investing activities:
Capitalized offering costs in accounts payable and accrued expenses	$ 25