Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2014

Stock-Based Compensation Expense on All Employee and Non-Employee Awards

The Company recognized stock-based compensation expense on all employee and non-employee awards as follows:

	For the three months ended March 31,
(in thousands)	2014		2013
Research and development	$	376	$	537
General and administrative		847		1,004
Stock-based employee compensation expense	$	1,223	$	1,541

Fair Value of Stock Options Assumptions Using Black-Scholes Option Valuation Model

For the three months ended March 30, 2014 and 2013, the fair value of stock options was estimated on the date of grant using a Black-Scholes option valuation model with the following assumptions:

	For the three months ended March 31,
	2014	2013
Risk-free interest rate	1.76 - 2.11%	1.0 - 1.15%
Expected life in years	6	6
Expected volatility	94.46 - 94.55%	83.40 - 83.69%
Expected dividend yield	0	0

Stock Option Activity Under Stock Option Plan

Stock option activity under the Company’s stock option plan for the three months ended March 31, 2014 is as follows:

(in thousands, except share and per share data)	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding, December 31, 2013		6,747,302		$	3.81
Granted		80,500			4.36
Exercised		(479,705	)		2.02
Cancelled		(188,269	)		5.10
Outstanding, March 31, 2014		6,159,828		$	3.92		7.45	$	6,459
Vested and unvested expected to vest at March 31, 2014		6,105,689		$	4.28		5.37	$	5,845
Options exercisable, March 31, 2014		2,843,928		$	4.28		5.37	$	1,810
Options exercisable, December 31, 2013		3,471,935		$	4.01		5.03	$	2,654
Options available for future grant		338,631

Summary of Unvested Restricted Stock

A summary of the status of unvested restricted stock for the three months ended March 31, 2014 is as follows:

	Number of Shares			Weighted-Average Grant Date Fair Value
Non-vested, December 31, 2013		352,865		$	4.38
Granted		—			—
Vested		(48,433	)		4.51
Cancelled		(8,026	)		4.41
Non-vested, March 31, 2014		296,406		$	4.36