Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (18,606)	$ (19,779)	$ (28,304)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	18	23	23
Amortization	145
Noncash stock based compensation	1,762	3,630	3,651
Noncash interest		282	667
Change in fair value of warrants		(29)	(143)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(233)	155	2,006
Accounts payable and accrued expenses	1,377	(1,177)	(2,460)
Lease liability	(152)
Net cash used in operating activities	(15,689)	(16,895)	(24,560)
Cash flows from investing activities:
Acquisition of property and equipment	(98)	(318)	(1,313)
Net cash used in investing activities	(98)	(318)	(1,313)
Cash flows from financing activities:
Principal payments of long-term debt		(10,888)	(5,612)
Proceeds from exercise of stock options	164		152
Net cash provided by (used in) financing activities	42,415	(10,888)	13,075
Net increase (decrease) in cash and cash equivalents	26,628	(28,101)	(12,798)
Cash and cash equivalents, beginning of year	7,851	35,952	48,750
Cash and cash equivalents, end of year	34,479	7,851	35,952
Supplemental cash flow information
Interest paid during the year		$ 1,370	1,295
Non-cash transactions
Property and equipment purchases included in accounts payable	49		242
At-the-market sales
Cash flows from financing activities:
Proceeds from sales or issuance of common stock, net of offering costs	$ 42,251		$ 18,535