Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 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 June 30,				For the six months ended June 30,
(in thousands)	2015		2014		2015		2014
Research and development	$	316	$	410	$	610	$	786
General and administrative		3,169		802		3,967		1,649
Stock-based employee compensation expense	$	3,485	$	1,212	$	4,577	$	2,435

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

For the three months ended June 30, 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 June 30,
	2015			2014
Risk-free interest rate		1.69	%		1.98	%
Expected life in years		6			6
Expected volatility		85.57	%		94.07	%
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 June 30, 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		51,800			9.11
Exercised		(2,860,299	)		3.97
Cancelled		(153,833	)		4.13
Outstanding, June 30, 2015		3,543,331		$	4.23		3.89	$	27,534
Vested and unvested expected to vest at June 30, 2015		3,491,349		$	4.12		5.76	$	27,130
Options exercisable, June 30, 2015		1,912,531		$	4.12		5.76	$	15,074
Options exercisable, December 31, 2014		3,781,162		$	4.10		5.80	$	4,130
Options available for future grant		3,635,802

Summary of Unvested Restricted Stock

A summary of the status of unvested restricted stock for the three months ended June 30, 2015 is as follows:

	Number of Shares			Weighted-Average Grant Date Fair Value
Non-vested, December 31, 2014		144,508		$	4.70
Granted		1,050,000			9.35
Vested		(70,991	)		4.62
Cancelled		—			—
Non-vested, June 30, 2015		1,123,517		$	9.05