Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2014

Stock-Based Compensation

Summary of stock option activity

        The following table summarizes stock option activity during the year ended December 31, 2014:

	Shares			Weighted average exercise price		Weighted average remaining contractual life (years)		Aggregate intrinsic value
Outstanding at January 1		6,814,417		$	6.90
Options granted		3,276,427			4.90
Options exercised		(206,389	)		4.14
Options cancelled		(1,055,112	)		7.35
Outstanding at December 31		8,829,343		$	6.17		6.35	$	70,082
Exercisable at December 31		4,915,662		$	6.67		4.17	$	70,082

Summary of unvested restricted share activity

        The following table summarizes unvested restricted share activity during the year ended December 31, 2014:

	Shares			Weighted average grant date fair value
Outstanding at January 1		45,000		$	4.63
Vested		(39,508	)		4.56
Granted		764,022			3.67
Forfeited		(25,000	)		4.34
Outstanding at December 31		744,514		$	3.65

Schedule of weighted average assumptions used to estimate fair value of each employee stock option award

	Years ended December 31,
	2014			2013			2012
Risk-free interest rate		1.88	%		1.20	%		1.11	%
Expected life in years		6.25 years			6.25 years			6.25 years
Volatility		104	%		102	%		101	%
Expected dividend yield		—			—			—

Schedule of stock-based compensation expense

        Stock-based compensation expense during the years ended December 31, 2014, 2013 and 2012 was as follows (in thousands):

	Years ended December 31,
	2014		2013		2012
Stock-based compensation expense by type of award:
Employee stock options	$	7,076	$	5,757	$	3,082
Restricted stock		355		273		240
Total stock-based compensation expense	$	7,431	$	6,030	$	3,322
Effect of stock-based compensation expense by line item:
Research and development	$	4,412	$	3,220	$	2,485
General and administrative		3,019		2,810		837
Total stock-based compensation expense included in net loss	$	7,431	$	6,030	$	3,322

Schedule of unrecognized stock-based compensation expense

        Unrecognized stock-based compensation expense as of December 31, 2014 was as follows (dollars in thousands):

	Unrecognized stock compensation expense as of December 31, 2014		Weighted average remaining period (in years)
Employee stock options	$	13,468		2.77
Restricted stock		2,545		3.39
Total	$	16,013		2.87