The Company recognized stock-based compensation expense on all employee and non-employee awards as follows:


	For the Three Months Ended June 30,				For the Six Months Ended June 30,
(in thousands)	2021		2020		2021		2020
Research and development		847		535		1,580		1,065
General and administrative		4,443		1,126		5,906		2,536
Stock-based compensation expense	$	5,290	$	1,661	$	7,486	$	3,601

The Company granted an aggregate of 41,500 and 4,395,438 stock options during the three and six months ended June 30, 2021 with a weighted-average grant date fair value of $2.27 and $2.44 per share, respectively. The Company granted an aggregate of 153,000 and 1,049,000 stock options during the three and six months ended June 30, 2020 with a weighted-average grant date fair value of $1.99 and $2.55 per share, respectively.

On March 4, 2021, the Company extended the contractual life of 216,700 fully vested stock options held by a former director of the Company. On April 5, 2021, the Company extended the contractual life of 751,371 stock options and accelerated the vesting of 226,889 shares of restricted stock held by a former officer of the Company. On April 29, 2021, the Company extended the contractual life of 10,417 vested and 167,023 unvested stock options and accelerated the vesting of 4,137 shares of restricted stock held by a former director. On May 17, 2021, the Company extended the contractual life of 347,267 vested stock options held by a former officer of the Company. These extensions resulted in additional stock compensation expense of approximately $1.9 million and $2.0 million during the three and six months ended June 30, 2021, respectively.

For the three months ended June 30, 2021 and 2020, the fair value of stock options was estimated on the date of grant using a Black-Scholes option valuation model with the following assumptions:


	For the Three Months Ended June 30,						For the Six Months Ended June 30,
	2021			2020			2021			2020
Risk-free interest rate		1.15	%	0.43 - 0.60%			0.09-1.15%			0.43 - 1.68%
Expected life in years	6.25			6-6.25			5.50 - 6.25			6 - 6.25
Expected volatility		74.08	%	73.10 - 73.90%			72.92 -74.80%			71.11 - 73.74%
Expected dividend yield		—	%		—	%		—	%		—	%

At June 30, 2021, there were 463,333 stock options that had been issued outside the 2012 Equity Incentive Plan, or the 2012 Plan and the 2020 Equity Incentive Plan, or the 2020 Plan. These options are excluded from the schedule below.

Stock option activity under the Company’s stock option plans for the three months ended June 30, 2021 is as follows:


(in thousands, except share and per share data)	Number of Shares			Weighted- Average Exercise Price		Weighted- Average Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding, December 31, 2020		6,840,719		$	3.81
Granted		4,395,438			3.79
Exercised		(363,109	)		2.86
Cancelled		(686,219	)		3.86
Outstanding, June 30, 2021		10,186,829		$	3.83		8.27	$	899
Options exercisable, June 30, 2021		4,970,314		$	4.13		7.23	$	771
Options exercisable, December 31, 2020		3,596,315		$	4.17		6.90	$	598
Options available for future grant		1,427,152

At June 30, 2021, total unrecognized compensation costs related to unvested stock options outstanding amounted to $11.3 million. The cost is expected to be recognized over a weighted-average period of 1.82 years.

A summary of the status of unvested restricted stock for the three months ended June 30, 2021 is as follows:


	Number of Shares			Weighted-Average Grant Date Fair Value
Unvested, December 31, 2020		786,280		$	3.08
Granted		726,224			3.75
Vested		(325,238	)		3.58
Cancelled		(122,091	)		3.61
Unvested, June 30, 2021		1,065,175		$	3.32

At June 30, 2021, total unrecognized compensation costs related to unvested restricted stock outstanding amounted to $2.2 million. The cost is expected to be recognized over a weighted-average period of 1.68 years.