(8) Stock-Based Compensation

A summary of stock option activity for the three months ended March 31, 2021 is as follows:


		Weighted		Weighted
		Average		Average
		Exercise		Remaining
		Price		Contractual
	Shares	Per Share		Term (In Years)
Options outstanding at December 31, 2020	3,042,229	$	28.93	8.2
Granted	4,500	$	19.66
Exercised	—	$	—
Canceled	(405)	$	126.27
Options outstanding at March 31, 2021	3,046,324	$	28.90	8.0
Options vested and expected to vest at March 31, 2021	2,953,396	$	29.53	8.0
Options exercisable at March 31, 2021	982,848	$	71.05	6.1
Shares available for grant under the 2008 Plan	894,882

The weighted average grant-date fair value of stock options granted during the three months ended March 31, 2021 was $15.20.

The aggregate intrinsic value of stock options vested and expected to vest at March 31, 2021 was $31.6 million. The aggregate intrinsic value of stock options exercisable at March 31, 2021 was $8.3 million. As of March 31, 2021, total compensation cost related to non-vested employee and non-employee director stock options not yet recognized was approximately $10.6 million, net of estimated forfeitures, which is expected to be recognized as expense over a weighted average period of 2.8 years.

Stock-based compensation expense for the three months ended March 31, 2021 and 2020 was recorded as follows:


	Three months ended March 31,
	2021		2020
	(In thousands)
Research and development	$	661	$	310
General and administrative		614		377
Total stock-based compensation expense	$	1,275	$	687

The fair values of employee and director stock options granted during the three months ended March 31, 2021 and 2020 were valued using the Black-Scholes option pricing model with the following assumptions:


	Three months ended March 31,
	2021		2020
Expected stock price volatility	97 – 98	%	91	%
Expected option term	6.0	Years	6.0	Years
Risk-free interest rate	0.8 - 1.2	%	0.6	%
Expected dividend yield	None		None