Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (19,779)	$ (28,304)	$ (28,741)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	23	23	19
Noncash stock based compensation	3,630	3,651	3,425
Noncash interest	282	667	946
Change in fair value of warrants	(29)	(143)	(234)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	155	2,006	922
Accounts payable and accrued expenses	(1,177)	(2,460)	362
Net cash used in operating activities	(16,895)	(24,560)	(23,301)
Cash flows from investing activities:
Acquisition of property and equipment	(318)	(1,313)	(31)
Net cash used in investing activities	(318)	(1,313)	(31)
Cash flows from financing activities:
Principal payments of long-term debt	(10,888)	(5,612)	(985)
Return of principal payments of long-term debt			985
Proceeds from issuance of common stock, in public offering, net of offering costs		18,535	37,528
Cash paid for debt financing costs			(175)
Proceeds from exercise of stock options		152	334
Net cash (used in) provided by financing activities	(10,888)	13,075	37,687
Net (decrease) increase in cash and cash equivalents	(28,101)	(12,798)	14,355
Cash and cash equivalents, beginning of year	35,952	48,750	34,395
Cash and cash equivalents, end of year	7,851	35,952	48,750
Supplemental cash flow information
Interest paid during the year	$ 1,370	1,295	$ 1,500
Non-cash transactions
Property and equipment purchases included in accounts payable		$ 242