Condensed Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	$ (6,524)	$ (7,222)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	96	101
Non-cash interest expense	2	8
Stock-based compensation expense	664	636
Change in fair value of warrant liability	(1,304)	(50)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(155)	(64)
Accounts payable	(415)	1,885
Accrued expenses and other liabilities	(598)	(215)
Deferred revenue	(244)	(113)
Net cash used in operating activities	(8,478)	(5,034)
Investing activities
Purchases of property and equipment	(32)	(50)
Net cash used in investing activities	(32)	(50)
Financing activities
Proceeds from issuance of common stock and common stock warrants, net of offering costs	(46)	51,507
Payments on notes payable		(416)
Proceeds from exercise of common stock options and warrants	46	15
Net cash provided by financing activities		51,106
Net (decrease) increase in cash and cash equivalents	(8,510)	46,022
Cash and cash equivalents at beginning of period	54,059	7,942
Cash and cash equivalents at end of period	45,549	53,964
Supplemental non-cash financing activities
Conversion of preferred stock into common stock		56,678
Conversion of preferred stock warrants into common stock warrants		247
Supplemental cash flow information
Cash paid for interest	$ 175	$ 64