Equity-Based Compensation

3 Months Ended

Mar. 31, 2017

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

Equity-Based Compensation

6. Equity-Based Compensation

Equity-based compensation expense is classified in the consolidated statements of operations as follows:

	Three Months Ended March 31,
	2017		2016
	(In thousands)
Research and development	$	1,503	$	454
General and administrative		1,127		358
Total	$	2,630	$	812

Restricted Stock

The following table summarizes the Company’s restricted stock activity, including converted Founder Stock, for the three months ended March 31, 2017:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of January 1, 2017		1,361,855		$	0.81
Vested		(186,917	)		0.73
Unvested restricted stock as of March 31, 2017		1,174,938		$	0.82

As of March 31, 2017, there was $2.7 million of unrecognized equity-based compensation expense related to restricted stock that is expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 1.8 years.

Stock Options

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $10.38 per option and $4.98 per option for those options granted during the three months ended March 31, 2017 and 2016, respectively. Key assumptions used to apply this pricing model were as follows:

	Three Months Ended March 31, 2017			Three Months Ended March 31, 2016
Risk-free interest rate		1.9	%		1.3	%
Expected life of options		6.0 years			6.0 years
Expected volatility of underlying stock		92.6	%		87.7	%
Expected dividend yield		0.0	%		0.0	%

The following is a summary of stock option activity for the three months ended March 31, 2017:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at January 1, 2017		3,040,214		$	8.35
Granted		1,343,428			13.71
Exercised		(7,136	)		6.56
Forfeited		(3,284	)		16.03
Outstanding at March 31, 2017		4,373,222		$	9.99		9.1	$	17,929
Exercisable at March 31, 2017		780,588		$	7.12		8.8	$	5,440

As of March 31, 2017, there was $25.0 million of unrecognized compensation cost related to stock options that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 3.2 years.