Statements of Cash Flows(USD ($)) In Thousands, unless otherwise specified	12 Months Ended			96 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012
Cash flows from operating activities:
Net loss	$ (24,534)	$ (23,187)	$ (24,357)	$ (121,136)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation expense	103	78	47	358
Loss on asset disposal	5			29
Increase in fair value of warrants	1,287			1,287
Loss on debt extinguishment	799			799
Cash paid for interest on debt extinguishment	(350)			(350)
Acquired in-process research and development				5,500
Stock-based compensation	2,623	1,216	543	4,971
Noncash interest expense	243	291	3,336	5,758
Changes in operating assets and liabilities:
Prepaid expenses and other assets	317	1,071	300	964
Accounts payable	(1,167)	769	(266)	800
Accrued expenses	60	(1,155)	1,993	2,057
Net cash used in operating activities	(20,614)	(20,917)	(18,404)	(98,963)
Cash flows from investing activities:
Purchase of in-process research and development				(5,500)
Payments under equipment funding agreement		(3,353)	(3,410)	(6,763)
Purchases of property and equipment	(477)	(193)	(75)	(968)
Net cash used in investing activities	(477)	(3,546)	(3,485)	(13,231)
Cash flows from financing activities:
Proceeds from issuance of debt	8,500	10,000	5,000	26,000
Proceeds from convertible notes			10,063	14,467
Payment of debt issuance costs	(103)	(76)	(174)	(428)
Repayment of debt	(14,139)	(1,725)	(988)	(18,788)
Proceeds from sale of preferred stock, net	26,287			69,863
Proceeds from sale of common stock, net	57	405	42,979	43,650
Net cash provided by financing activities	20,602	8,604	56,880	134,764
Net increase (decrease) in cash and cash equivalents	(489)	(15,859)	34,991	22,570
Cash and cash equivalents, beginning of period	23,059	38,918	3,927
Cash and cash equivalents, end of period	22,570	23,059	38,918	22,570
Noncash investing and financing activities:
Conversion of note principal and accrued interest to redeemable convertible preferred stock				4,547
Conversion of note principal and accrued interest to common stock			10,337	10,337
Conversion of preferred stock plus accrued dividends to common stock			58,072	58,072
Reclassification of warrant liability			1,113	1,113
Fair value of warrants issued in connection with loan facilities	213	272		485
Fair value of warrants issued in connection with equity financing and loan modification	14,949			14,949
Financing arrangement with third party vendors		401	386	991
Accretion of redeemable convertible preferred stock			(2,536)	9,948
Deemed dividend-beneficial conversion feature	13,250			13,250
Cash paid for interest	$ 1,267	$ 1,108	$ 381	$ 3,277