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

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
	(In thousands)
Research and development	$	2,096	$	2,633	$	3,879	$	5,026
General and administrative		2,308		2,268		5,117		3,982
Total	$	4,404	$	4,901	$	8,996	$	9,008

Restricted stock is measured at fair value based on the quoted price of the Company’s common stock.

The following table summarizes the Company’s restricted stock activity for the six months ended June 30, 2019:

	Number of Shares			Weighted Average Grant Date Fair Value per Share
Unvested restricted stock as of December 31, 2018		109,073		$	15.53
Granted		-			-
Vested		(36,462	)		1.34
Cancelled		-			-
Unvested restricted stock as of June 30, 2019		72,611		$	22.66

As of June 30, 2019, there was $0.8 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 1.4 years.

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $9.22 and $9.07 per option for those options granted during the three and six months ended June 30, 2019, respectively, and $16.64 and $17.94 per option for those options granted during the three and six months ended June 30, 2018, respectively. Key assumptions used to apply this pricing model were as follows:

	Three Months Ended June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
Risk-free interest rate	2.1%	2.8%	2.4%	2.6%
Expected life of options	5.5-6.0 years	5.5-6.25 years	5.5-6.0 years	5.5-6.25 years
Expected volatility of underlying stock	67.9%	87.7%	68.6%	89.0%
Expected dividend yield	0.0%	0.0%	0.0%	0.0%

The following is a summary of stock option activity for the six months ended June 30, 2019:

	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(In years)		(In thousands)
Outstanding at December 31, 2018		5,037,663		$	15.63
Granted		793,813			14.54
Exercised		(233,872	)		8.65
Forfeited		(282,108	)		18.79
Outstanding at June 30, 2019		5,315,496		$	15.61		8.14	$	14,978
Exercisable at June 30, 2019		2,165,526		$	13.43		7.01	$	10,193

As of June 30, 2019, there was $34.1 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 2.6 years.

Of the unvested restricted stock outstanding and stock options outstanding as of June 30, 2019, 71,875 are performance-based restricted stock units and 213,750 are performance-based stock options. The performance-based restricted stock units and performance-based stock options vest upon obtaining certain scientific, financial and regulatory milestones through 2020. At June 30, 2019, 71,875 performance-based restricted stock units and 188,750 performance-based options are not included in computing the diluted (loss) earnings per share because the performance criteria had not been met as of the end of the reporting period.