CONDENSED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating activities
Net loss	$ (7,574)	$ (6,524)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	54	96
Non-cash interest expense	26	2
Stock-based compensation expense	586	664
Change in fair value of warrant liability	(115)	(1,304)
Loss on extinguishment of debt	221
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(162)	(155)
Restricted cash	(80)	0
Accounts payable	342	(415)
Accrued expenses and other liabilities	(1,645)	(598)
Deferred revenue	(203)	(244)
Net cash used in operating activities	(8,550)	(8,478)
Investing activities
Purchases of property and equipment	0	(32)
Net cash used in investing activities	0	(32)
Financing activities
Proceeds from issuance of common stock and common stock warrants, net of offering costs	0	(46)
Payments on notes payable	(14,124)	0
Proceeds from exercise of common stock options and warrants	14	46
Net cash used in financing activities	(14,110)	0
Net decrease in cash and cash equivalents	(22,660)	(8,510)
Cash and cash equivalents at beginning of period	36,079	54,059
Cash and cash equivalents at end of period	13,419	45,549
Supplemental cash flow information
Cash paid for interest	$ 663	$ 175