Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2015

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)	2015		2014
Research and development	$	295	$	376
General and administrative		798		847
Stock-based employee compensation expense	$	1,093	$	1,223

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

For the three months ended March 31, 2015 and 2014, 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,
	2015	2014
Risk-free interest rate	1.63 - 1.64%	1.76 - 2.11%
Expected life in years	6	6
Expected volatility	85.48 - 86.81%	94.46 - 94.55%
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, 2015 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, 2014		6,505,663		$	4.07
Granted		50,000			9.10
Exercised		(592,200	)		4.29
Cancelled		(39,166	)		4.26
Outstanding, March 31, 2015		5,924,297		$	4.09		6.36	$	39,564
Vested and unvested expected to vest at March 31, 2015		5,878,611		$	4.08		5.33	$	39,259
Options exercisable, March 31, 2015		3,185,297		$	4.08		5.33	$	21,297
Options exercisable, December 31, 2014		3,781,162		$	4.10		5.80	$	4,130
Options available for future grant		4,572,935

Summary of Unvested Restricted Stock

A summary of the status of unvested restricted stock for the three months ended March 31, 2015 is as follows:

	Number of Shares			Weighted-Average Grant Date Fair Value
Non-vested, December 31, 2014		144,508		$	4.70
Granted		—			—
Vested		(48,435	)		4.51
Cancelled		—			—
Non-vested, March 31, 2015		96,073		$	4.79