In April 2016, the Company adopted the Amended and Restated 2015 Stock Option and Incentive Plan (the “2015 Plan”). The 2015 Plan provides for the grant of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. Recipients of incentive stock options and non-qualified stock options are eligible to purchase shares of the Company’s common stock at an exercise price equal to the fair value of such stock on the grant date. Stock options granted under the 2015 Plan generally vest 25% on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years, unless they contain specific performance-based vesting provisions. The maximum term of stock options granted under the 2015 Plan is ten years.

As of December 31, 2019, there were 2,655,673 shares available for future issuance. The number of shares reserved for issuance under the 2015 Plan shall be cumulatively increased by four percent of the number of shares of stock issued and outstanding on the immediately preceding December 31 or such lesser number of shares of stock as determined by the board of directors.

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

	Year Ended December 31,
	2019		2018		2017
	(In thousands)
Research and development	$	6,986	$	8,994	$	7,280
General and administrative		8,105		8,052		8,042
Total	$	15,091	$	17,046	$	15,322

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 year ended December 31, 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		(37,198	)		1.34
Cancelled		-			-
Unvested restricted stock as of December 31, 2019		71,875		$	22.88

The weighted average grant date fair value of restricted stock, was $22.98 for restricted stock granted during 2018. There was no restricted stock granted in 2017 or 2019. As of December 31, 2019, there was $0.2 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 1.0 year. All of the unvested restricted stock outstanding as of December 31, 2019 are performance-based restricted stock units that vest upon obtaining certain scientific, financial and regulatory milestones through 2020. These 71,875 restricted stock units 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.

The weighted average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $9.21 per option for options granted during the year ended December 31, 2019, $15.05 per option for options granted during the year ended December 31, 2018 and $12.43 per option for options granted during the year ended December 31, 2017. The total intrinsic value (the amount by which the fair market value exceeded the exercise price) of stock options exercised during the year ended December 31, 2019, 2018 and 2017 was $2.3 million, $18.0 million, and $1.6 million, respectively. Key assumptions used to apply this pricing model were as follows:

	Year Ended December 31,
	2019			2018			2017
Risk-free interest rate		2.1	%		2.7	%		2.0	%
Expected life of options	6.0 years			6.0 years			6.0 years
Expected volatility of underlying stock		68.1	%		87.1	%		93.9	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Risk-free Interest Rate. The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant with maturities approximately equal to the option’s expected term.

Expected Dividend Yield. The expected dividend yield assumption is based on the fact that the Company has never paid cash dividends and has no present intention to pay cash dividends.

Expected Volatility. The expected volatility was derived from a blend of average historical stock volatilities of several unrelated public companies within the Company’s industry and the Company’s historical volatility, both over a period equivalent to the expected term of the stock option grants.

Expected Term. The expected term represents the period that stock options awards are expected to be outstanding. For option grants that are considered to be “plain vanilla,” the Company determines the expected term using the simplified method. The simplified method deems the term to be the average of the time-to-vesting and the contractual life of the options. The Company uses the simplified method because it does not have sufficient historical option exercise data to provide a reasonable basis upon which to estimate the expected term.

The Company uses the market closing price of its common stock as reported on the Nasdaq Global Select Market to determine the fair value of the shares of common stock underlying stock options. The following is a summary of stock option activity for the year ended December 31, 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		1,220,613			14.90
Exercised		(364,404	)		8.47
Forfeited		(527,901	)		18.50
Outstanding at December 31, 2019		5,365,971		$	15.67		7.86	$	9,082
Exercisable at December 31, 2019		2,578,717		$	14.11		6.92	$	8,017

As of December 31, 2019, there was $28.2 million of unrecognized compensation cost related to stock options that have not yet vested. These costs are expected to be recognized over a weighted average remaining vesting period of 2.5 years.

Of the unvested stock options outstanding as of December 31, 2019, 213,750 are performance-based stock options that vest upon obtaining certain scientific, financial and regulatory milestones through 2020. At December 31, 2019, 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.