Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (12,177)	$ (14,961)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	12	11
Stock based compensation expense	2,106	1,760
Noncash interest	262	360
Change in fair value of warrants	(29)	(102)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	562	2,243
Accounts payable and accrued expenses	(644)	(1,230)
Net cash used in operating activities	(9,908)	(11,919)
Cash flows from investing activities:
Acquisition of property and equipment	(316)	(450)
Net cash used in investing activities	(316)	(450)
Cash flows from financing activities:
Principal payments of loan payable	(3,263)	(2,496)
Proceeds from exercise of stock options		53
Net cash used in financing activities	(3,263)	(2,443)
Net decrease in cash and cash equivalents	(13,487)	(14,812)
Cash and cash equivalents, beginning of period	35,952	48,750
Cash and cash equivalents, end of period	22,465	33,938
Supplemental cash flow information
Interest paid	$ 449	$ 713