Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2012

Stock-Based Compensation Expense Related to Common Stock Options, Restricted Common Stock and Common Stock Warrants

Following is the stock-based compensation expense related to common stock options, restricted common stock and common stock warrants:

	Year Ended December 31,
	2012		2011
Research and development	$	922	$	1,558
General and administrative		1,870		1,687
Total stock-based compensation expense	$	2,792	$	3,245

Weighted Average Assumptions Used to Determine Fair Value of Options Granted

The fair value of the options granted is determined using the Black-Scholes option-pricing model. The following weighted average assumptions were used:

	2012			2011			Cumulative Period  from Inception (July 10, 2000)  to December 31, 2012
Risk-free interest rate		0.90	%		1.90	%		1.84	%
Expected life of the options		5.81 years			5.13 years			5.19 years
Expected volatility of the underlying stock		116	%		131	%		119	%
Expected dividend rate		0	%		0	%		0	%

Summary of Stock Option Activity

The following table summarizes the stock option activity in the stock based compensation plans:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2010		1,965,748		$	6.42
Granted		1,676,712			7.02
Forfeited/Cancelled		(334,543	)		8.93
Exercised		(216,443	)		1.21
Outstanding, December 31, 2011		3,091,474		$	6.83
Granted		800,000			2.13
Forfeited/Cancelled		(299,683	)		8.93
Exercised		(51,830	)		2.31
Outstanding, December 31, 2012		3,539,961		$	5.66		6.79	$	279
Exercisable, December 31, 2012		2,067,910		$	5.96		5.41	$	268

Unvested Restricted Shares

	Number of Shares			Weighted Average Price on  Grant
Unvested restricted shares outstanding, December 31, 2010		26,042		$	1.08
Restricted shares issued		20,834			8.04
Restricted shares vested		(46,876	)		4.17
Unvested restricted shares outstanding, December 31, 2011		—		$	—