Balance Sheets (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets:
Cash and cash equivalents	$ 38,932	$ 73,306
Receivables	31	58
Prepaid expenses and other current assets	4,067	6,912
Total current assets	43,030	80,276
Property and equipment, net	1,713	1,994
Deposits	128	133
Other non-current assets	732	1,001
Total assets	45,603	83,404
Current liabilities:
Accounts payable	1,165	1,509
Accrued expenses	18,969	16,516
Deferred revenue - current portion	800	800
Deferred rent - current portion	63	39
Total current liabilities	20,997	18,864
Deferred revenue	2,333	2,733
Deferred rent	365	400
Warrant liabilities	2,576	12,962
Total liabilities	26,271	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; 83,540,682 and 83,236,840 shares issued and outstanding at June 30, 2013 and December 31, 2012, respectively	84	83
Additional paid-in capital - common stock	330,735	325,177
Additional paid-in capital - warrants issued	3,728	6,909
Deficit accumulated during the development stage	(315,215)	(283,724)
Total stockholders' equity	19,332	48,445
Total liabilities and stockholders' equity	$ 45,603	$ 83,404