Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (15,969)	$ (22,064)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	18	16
Stock based compensation expense	2,827	2,727
Noncash interest	367	520
Change in fair value of warrants	(29)	(82)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	31	1,606
Accounts payable and accrued expenses	(1,004)	(1,473)
Net cash used in operating activities	(13,759)	(18,750)
Cash flows from investing activities:
Acquisition of property and equipment	(318)	(771)
Net cash used in investing activities	(318)	(771)
Cash flows from financing activities:
Principal payments of loan payable	(4,949)	(4,035)
Proceeds from the issuance of common stock, net		18,536
Proceeds from exercise of stock options		76
Net cash (used in) provided by financing activities	(4,949)	14,577
Net decrease in cash and cash equivalents	(19,026)	(4,944)
Cash and cash equivalents, beginning of period	35,952	48,750
Cash and cash equivalents, end of period	16,926	43,806
Supplemental cash flow information
Interest paid	$ 625	$ 1,023