Condensed Balance Sheets (Unaudited) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 45,549	$ 54,059
Restricted cash	94
Prepaid expenses and other current assets	495	342
Total current assets	46,138	54,401
Property and equipment, net	422	486
Restricted cash		94
Other assets	18	19
Total assets	46,578	55,000
Current liabilities:
Accounts payable	2,043	2,458
Accrued expenses	1,327	1,987
Notes payable, current portion	1,010	251
Deferred revenue	262	506
Total current liabilities	4,642	5,202
Other liabilities	65	4
Notes payable, net of current portion	8,990	9,749
Warrant liability	1,915	3,219
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.001 par value per share; 5,000,000 shares authorized at March 31, 2015 and December 31, 2014 and no shares issued and outstanding at March 31, 2015 and December 31, 2014
Common stock, $0.001 par value per share; 200,000,000 shares authorized at March 31, 2015 and December 31, 2014 and 18,015,423 and 17,933,260 shares issued and outstanding at March 31, 2015 and December 31, 2014, respectively	18	18
Additional paid-in capital	129,222	128,558
Accumulated deficit	(98,274)	(91,750)
Total stockholders' equity	30,966	36,826
Total liabilities and stockholders' equity	$ 46,578	$ 55,000