Balance Sheets (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 23,631	$ 73,306
Receivables	449	58
Prepaid expenses and other current assets	3,195	6,912
Total current assets	27,275	80,276
Property and equipment, net	1,336	1,994
Deposits	128	133
Other non-current assets	739	1,001
Total assets	29,478	83,404
Current liabilities:
Accounts payable	891	1,509
Accrued expenses	11,573	16,516
Deferred revenue-current portion	800	800
Deferred rent-current portion	98	39
Total current liabilities	13,362	18,864
Deferred revenue	2,133	2,733
Deferred rent	349	400
Warrant liabilities	9,983	12,962
Other long-term liabilities	20
Total liabilities	25,847	34,959
Commitments and contingencies (note 7)
Stockholders' equity:
Preferred stock, $0.001 par value; 30,000,000 shares authorized and no shares issued and outstanding
Common stock, $0.001 par value; 250,000,000 shares authorized; 83,681,579 and 83,236,840 shares issued and outstanding at September 30, 2013 and December 31, 2012, respectively	84	83
Additional paid-in capital-common stock	331,817	325,177
Additional paid-in capital-warrants issued	3,657	6,909
Deficit accumulated during the development stage	(331,927)	(283,724)
Total stockholders' equity	3,631	48,445
Total liabilities and stockholders' equity	$ 29,478	$ 83,404