Share-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2016

Equity [Abstract]

Summary of Share-based Compensation Expense

Total share-based compensation expense related to all share-based awards for the three months ended March 31, 2016 and 2015 was comprised of the following (in thousands):

	Three Months Ended   March 31,
	2016		2015
Research and development:
Stock options	$	1,261	$	558
Restricted stock	30		478
Warrants	11		11
Research and development share-based compensation expense	1,302		1,047
General and administrative:
Stock options	1,710		589
Restricted stock	619		1,579
Warrants	74		187
General and administrative share-based compensation expense	2,403		2,355
Total share-based compensation expense included in expenses	$	3,705	$	3,402

Stock Options, Valuation Assumptions

The Company estimated the fair value of each stock option grant on the grant date using the Black-Scholes model with the following weighted-average assumptions:

	Three Months Ended   March 31,
	2016		2015
Expected volatility	148	%	174	%
Expected life (years)	5.41 years		6.82 years
Risk-free interest rate	1.4	%	1.8	%
Dividend yield	—		—

Summary of Stock Option Activity

The Company’s 2013 Equity Plan stock option activity for the three months ended March 31, 2016 is comprised of the following:

	Outstanding Stock Options
	Shares Underlying Stock Options		Weighted-Average Exercise Price per Share
Outstanding at December 31, 2015	2,318,685		$	22.01
Granted	79,800		$	17.59
Exercised	(10,000	)	$	8.52
Forfeited	(27,666	)	$	25.71
Outstanding at March 31, 2016	2,360,819		$	21.87

Summary of Restricted Stock Activity

The Company’s restricted stock award activity for the three months ended March 31, 2016 is comprised of the following:

	Number of Shares		Weighted-Average Grant Date Fair Market Value
Unvested at December 31, 2015	1,072,899		$	13.00
Vested	(30,220	)	$	5.53
Unvested at March 31, 2016	1,042,679		$	13.22