Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 55,655	$ 73,306
Receivables	72	58
Prepaid expenses and other current assets	4,834	6,912
Total current assets	60,561	80,276
Property and equipment, net	1,909	1,994
Deposits	133	133
Other non-current assets	743	1,001
Total assets	63,346	83,404
Current liabilities:
Accounts payable	983	1,509
Accrued expenses	18,521	16,516
Deferred revenue-current portion	800	800
Deferred rent-current portion	58	39
Total current liabilities	20,362	18,864
Deferred revenue	2,533	2,733
Deferred rent	384	400
Warrant liabilities	2,174	12,962
Total liabilities	25,453	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; 83,680,849 and 83,236,840 shares issued and outstanding at March 31, 2013 and December 31, 2012, respectively	84	83
Additional paid-in capital-common stock	327,495	325,177
Additional paid-in capital-warrants issued	6,837	6,909
Deficit accumulated during the development stage	(296,523)	(283,724)
Total stockholders' equity	37,893	48,445
Total liabilities and stockholders' equity	$ 63,346	$ 83,404