Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
CASH FLOWS USED IN OPERATING ACTIVITIES:
Net loss	$ (8,305)	$ (12,872)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	13	14
Amortization of premium (accretion of discount) on short-term investments, net		41
Stock-based compensation expense	334	425
Research and development expense settled in stock	3,570	2,578
Change in fair value of other current assets, warrant liability and other liability	(11,256)	465
Non-cash interest	113	92
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	942	(1,483)
Other long-term assets		18
Accounts payable	887	690
Accrued expenses and other current liabilities	6,423	(2,473)
Net cash used in operating activities	(7,279)	(12,505)
CASH FLOWS PROVIDED BY INVESTING ACTIVITIES:
Proceeds from sale of equipment		56
Purchases of marketable securities		(1,730)
Sales and maturities of marketable securities	4,000	7,250
Net cash provided by investing activities	4,000	5,576
CASH FLOWS USED IN FINANCING ACTIVITIES
Proceeds from exercise of stock options		256
Payments on note payable	(1,969)	(469)
Net cash used in financing activities	(1,969)	(213)
NET DECREASE IN CASH AND CASH EQUIVALENTS	(5,248)	(7,142)
CASH AND CASH EQUIVALENTS AT BEGINNING OF YEAR	18,653	25,128
CASH AND CASH EQUIVALENTS AT END OF PERIOD	13,405	17,986
SUPPLEMENTAL DISCLOSURES:
Cash paid for interest	508	311
NON-CASH FINANCING ACTIVITIES:
Accretion of dividends on preferred stock	$ (3,582)	$ 3,354