The Company maintains one stock incentive plan, the 2013 Stock Incentive Plan, as well as the 2013 Employee Stock Purchase Plan.

In addition, during the year ended December 31, 2021, the Company granted options to purchase an aggregate of 248,366 shares of the Company's common stock and 106,955 restricted stock units (RSUs) to four new employees as equity inducement awards outside of the Company's 2013 Stock Incentive Plan and material to the employees’ acceptance of employment with the Company. These equity awards were approved in accordance with Nasdaq Listing Rule 5635(c)(4), and these equity awards remained outstanding as of March 31, 2022. No additional equity inducement awards were granted during the three months ended March 31, 2022. These options have a weighted average exercise price of $5.12 per share, and the RSUs have a weighted average grant date fair value of $5.08 per unit. These inducement awards are included in stock-based compensation expense and the following tables.

Total stock-based compensation expense related to stock options, restricted stock units, shares issued under the employee stock purchase plan, and shares granted to non-employee directors in lieu of board fees was $5.3 million and $7.0 million for the three months ended March 31, 2022 and 2021, respectively.

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


	Three Months Ended March 31,
	2022		2021
	(In thousands)
Research and development	$	1,792	$	2,230
General and administrative		3,497		4,785
Total	$	5,289	$	7,015

The weighted-average grant date fair value of options, estimated as of the grant date using the Black-Scholes option pricing model, was $0.91 and $6.75 per option for those options granted during the three months ended March 31, 2022 and 2021, respectively.

Key assumptions used to apply this pricing model were as follows:


	Three Months Ended March 31,
	2022			2021
Risk-free interest rate		2.1	%		0.4	%
Expected life of options	5.96 years			6.0 years
Expected volatility of underlying stock		72.8	%		70.5	%
Expected dividend yield		0.0	%		0.0	%

The following is a summary of stock option activity for the three months ended March 31, 2022:


	Number of Options			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
	(In thousands)					(In years)		(In thousands)
Outstanding at December 31, 2021		12,946		$	11.61
Granted		4,284			1.41
Exercised		-			-
Forfeited		(1,583	)		10.82
Outstanding at March 31, 2022		15,647		$	8.89		7.61	$	-
Exercisable at March 31, 2022		5,923		$	14.14		5.02	$	-

As of March 31, 2022, there was $29.0 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 2.74 years.

During the three months ended March 31, 2022, 87,500 RSUs were granted to an executive. The awards were service-based. Assuming all service conditions are achieved, 25% of the RSUs would vest annually for four years.


	Number of Service Based RSU Shares (in thousands)			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2021		2,222		$	8.70
Granted		88			1.41
Vested		(277	)		13.58
Forfeited		(345	)		7.98
Outstanding at March 31, 2022		1,688		$	7.67

Compensation expense totaling $1.3 million and $1.1 million was recognized for the service-based RSUs for the three months ended March 31, 2022 and 2021, respectively.

As of March 31, 2022, there was $10.3 million of unrecognized compensation cost related to service-based RSUs that are expected to vest. These costs are expected to be recognized over a weighted average remaining vesting period of 1.92 years.