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Stock-Based Compensation

12 Months Ended

Dec. 31, 2020

12. Stock-Based Compensation

Total compensation cost recognized for all stock-based compensation awards in the consolidated statements of operations was as follows (in thousands):




	Year Ended
	December 31,
	2020		2019
Research and development	$	11,580	$	13,538
General and administrative		11,576		13,705
Total stock-based compensation expense	$	23,156	$	27,243

Restricted Stock and Restricted Stock Unit Awards

The following table summarizes restricted stock and restricted stock unit awards activity for the instruments discussed above as of December 31, 2019 and 2020 is as follows:


		Weighted
		Average
		Grant Date
		Fair Value
	Shares	Per Share
Unvested restricted stock and restricted stock unit awards as of December 31, 2019	581,408	$	24.03
Issued	364,549	$	29.57
Vested	(304,638)	$	23.37
Forfeited	(133,869)	$	24.72
Unvested restricted stock and restricted stock unit awards as of December 31, 2020	507,450	$	27.35

The expense related to restricted stock and restricted stock unit awards granted to employees and non-employees was $2.8 million and $1.6 million, respectively, for the year ended December 31, 2020. The expense related to restricted stock and restricted stock unit awards granted to employees and non-employees was $4.7 million and $1.6 million, respectively, for the year ended December 31, 2019. The expense related to restricted stock and restricted stock unit awards granted to employees and non-employees was $0 million and $2.4 million, respectively, for the year ended December 31, 2018.

As of December 31, 2020, total unrecognized compensation expense related to unvested restricted stock and restricted stock unit awards was $11.9 million, which the Company expects to recognize over a remaining weighted-average period of 2.7 years.

Stock Options

The following is a summary of stock option activity for the year ended December 31, 2020:


		Weighted Average		Remaining	Aggregate Intrinsic
	Shares	Exercise Price		Contractual Life (years)	Value (in thousands)
Outstanding at December 31, 2019	4,358,291	$	25.40	7.4	$	26,060
Granted	1,721,748	$	30.44
Exercised	(964,412)	$	20.22
Cancelled	(1,203,370)	$	30.72
Outstanding at December 31, 2020	3,912,257	$	27.26	7.9	$	167,640
Exercisable at December 31, 2020	1,463,668	$	24.05	6.8	$	67,413

The total intrinsic value of options exercised for the years ended December 31, 2020, 2019 and 2018 was $15.6 million, $14.6 million, and $15.9 million, respectively.

Using the Black-Scholes option pricing model, the weighted average fair value of options containing service-based vesting granted to employees and directors during the years ended December 31, 2020, 2019, and 2018 was $16.60, $15.67, and $24.91, respectively. The expense related to options containing service-based vesting granted to employees and directors was $16.1 million, $18.1 million, and $19.9 million for the years ended December 31, 2020, 2019, and 2018, respectively.

The fair value of each service-based vesting option issued to employees and directors was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:


	Year Ended
	December 31,

	2020		2019		2018
Expected volatility	60.0	%	73.8	%	77.5	%
Expected option term (in years)	6.25		6.25		6.25
Risk free interest rate	1.5	%	2.0	%	2.9	%
Expected dividend yield	—		—		—

As of December 31, 2020, total unrecognized compensation expense related to stock options was $40.4 million, which the Company expects to recognize over a remaining weighted-average period of 2.7 years.