Condensed Statements of Cash Flows (Unaudited) - USD ($) shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Operating activities
Net loss	$ (13,430)	$ (15,343)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	192	209
Non-cash interest expense	26	18
Stock-based compensation expense	1,266	1,137
Change in fair value of warrant liability	(3,123)	(50)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(596)	(913)
Accounts payable	(578)	1,706
Accrued expenses and other liabilities	(1,209)	468
Deferred revenue	(55)	(715)
Net cash used in operating activities	(17,507)	(13,483)
Investing activities
Purchases of property and equipment	(286)	(132)
Net cash used in investing activities	(286)	(132)
Financing activities
Proceeds from issuance of common stock and common stock warrants, net of offering costs	12,172	51,505
Payments on notes payable	0	(833)
Proceeds from notes payable	5,000	0
Proceeds from exercise of common stock options and warrants	56	32
Net cash provided by financing activities	17,228	50,704
Net (decrease) increase in cash and cash equivalents	(565)	37,089
Cash and cash equivalents at beginning of period	54,059	7,942
Cash and cash equivalents at end of period	53,494	45,031
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 for common stock	328	0
Supplemental cash flow information
Cash paid for interest	$ 374	$ 123