Share-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2016

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Share-based Compensation Expense

Total share-based compensation expense related to all share-based awards for the three and six months ended June 30, 2016 and 2015 was comprised of the following (in thousands):

	Three Months Ended   June 30,				Six Months Ended   June 30,
	2016		2015		2016		2015
Research and development:
Stock options	$	1,215	$	413	$	2,476	$	972
Restricted stock	—		735		30		1,213
Warrants	4		14		15		25
Research and development share-based compensation expense	1,219		1,162		2,521		2,210
General and administrative:
Stock options	1,559		918		3,269		1,507
Restricted stock	510		1,596		1,129		3,174
Warrants	53		204		127		391
General and administrative share-based compensation expense	2,122		2,718		4,525		5,072
Total share-based compensation expense included in expenses	$	3,341	$	3,880	$	7,046	$	7,282

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:

	Six Months Ended   June 30,
	2016		2015
Expected volatility	143	%	173	%
Expected life (years)	5.27 years		6.70 years
Risk-free interest rate	1.3	%	1.7	%
Dividend yield	—		—

Summary of Stock Option Activity

The Company’s 2013 Equity Incentive Plan stock option activity for the six months ended June 30, 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	207,200		$	17.25
Exercised	(10,000	)	$	8.52
Forfeited	(46,900	)	$	25.75
Outstanding at June 30, 2016	2,468,985		$	21.59

Summary of Restricted Stock Activity

The Company’s restricted stock award activity for the six months ended June 30, 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 June 30, 2016	1,042,679		$	13.22