Condensed Statements of Cash Flows (Unaudited) - USD ($) shares in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Operating activities
Net loss	$ (23,123)	$ (26,017)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	292	311
Non-cash interest expense	67	28
Stock-based compensation expense	1,895	1,945
Change in fair value of warrant liability	(3,186)	(50)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(311)	(258)
Accounts payable	(179)	1,692
Accrued expenses and other liabilities	(568)	984
Deferred revenue	(35)	(804)
Net cash used in operating activities	(25,148)	(22,169)
Investing activities
Purchases of property and equipment	(286)	(137)
Net cash used in investing activities	(286)	(137)
Financing activities
Proceeds from issuance of common stock and common stock warrants, net of offering costs	12,675	51,505
Payments on notes payable	0	(1,249)
Proceeds from notes payable	5,000	0
Proceeds from exercise of common stock options and warrants	62	51
Net cash provided by financing activities	17,737	50,307
Net (decrease) increase in cash and cash equivalents	(7,697)	28,001
Cash and cash equivalents at beginning of period	54,059	7,942
Cash and cash equivalents at end of period	46,362	35,943
Supplemental non-cash financing activities
Conversion of preferred stock into common stock	0	56,678
Conversion of preferred stock warrants into common stock warrants	$ 0	$ 247
Issuance of warrants to purchase common stock	328	0
Supplemental cash flow information
Cash paid for interest	$ 644	$ 177