Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 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 June 30,					For the six months ended June 30,
(in thousands)	2014		2013			2014		2013
Research and development	$	410	$	(145	)	$	786	$	392
General and administrative		802		277			1,649		1,281
Stock-based employee compensation expense	$	1,212	$	132		$	2,435	$	1,673

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

For the three months ended June 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 June 30,
	2014			2013
Risk-free interest rate		1.98	%		1.65	%
Expected life in years		6			6
Expected volatility		94.07	%		95.45	%
Expected dividend yield		0			0

Stock Option Activity Under Stock Option Plan

Stock option activity under the Company’s stock option plan for the six months ended June 30, 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		104,300			3.54
Exercised		(583,038	)		2.26
Cancelled		(227,402	)		5.05
Outstanding, June 30, 2014		6,041,162		$	3.91		7.24	$	4,456
Vested and unvested expected to vest at June 30, 2014		5,988,240		$	4.11		5.61	$	4,417
Options exercisable, June 30, 2014		3,176,594		$	4.11		5.61	$	1,846
Options exercisable, December 31, 2013		3,471,935		$	4.01		5.03	$	2,654
Options available for future grant		5,317,831

Summary of Unvested Restricted Stock

A summary of the status of unvested restricted stock for the six months ended June 30, 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		(8,936	)		4.41
Non-vested, June 30, 2014		295,496		$	4.36