Equity-Based Compensation

3 Months Ended

Mar. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

6.	Equity-Based Compensation

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

	Three Months Ended March 31,
	2018		2017
	(In thousands)
Research and development	$	2,393	$	1,503
General and administrative		1,714		1,127
Total	$	4,107	$	2,630

Restricted Stock

The following table summarizes the Company’s restricted stock activity for the three months ended March 31, 2018:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of January 1, 2018		479,822		$	0.90
Granted		71,875			23.46
Vested		(149,846	)		0.78
Cancelled		(2,022	)		1.34
Unvested restricted stock as of March 31, 2018		399,829		$	5.02

As of March 31, 2018, there was $3.3 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 1.7 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 $18.28 per option and $10.38 per option for those options granted during the three months ended March 31, 2018 and 2017, respectively. Key assumptions used to apply this pricing model were as follows:

	Three Months Ended March 31, 2018			Three Months Ended March 31, 2017
Risk-free interest rate		2.5	%		1.9	%
Expected life of options	6.0 years			6.0 years
Expected volatility of underlying stock		89.4	%		92.6	%
Expected dividend yield		0.0	%		0.0	%

The following is a summary of stock option activity for the three months ended March 31, 2018:

	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, 2018		4,705,448		$	12.09
Granted		792,414			24.44
Exercised		(709,321	)		9.47
Forfeited		(17,224	)		14.62
Outstanding at March 31, 2018		4,771,317		$	14.52		8.68	$	35,070
Exercisable at March 31, 2018		1,343,841		$	9.64		7.95	$	15,420

As of March 31, 2018, there was $39.2 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 3.1 years.