During the second quarter of 2021, the Company approved the 2021 Inducement Plan to reserve 850,000 shares of its common stock to be used exclusively for grants of awards to individuals that were not previously employees or directors of the Company as a material inducement to such individuals’ entry into employment with the Company within the meaning of Rule 5635(c)(4) of the Nasdaq Listing Rules. As of March 31, 2022, there were 670,000 shares remaining available to be issued under the 2021 Inducement Plan.

The following table summarizes stock option activity under the Company’s stock-based compensation plan during the three months ended March 31, 2022 (in thousands, except option and share data):

	Shares Available for Grant			Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (in Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2021		1,327,645			5,262,185		$	10.24		7.25	$	530
Additional shares authorized		1,728,441			—
Granted		(1,353,300	)		1,353,300		$	5.17
Restricted stock units granted		(298,150	)		—
Exercised		—			(15,995	)	$	8.98
Forfeited or cancelled		109,039			(109,039	)	$	12.06
Restricted stock units forfeited		33,225			—
Outstanding as of March 31, 2022		1,546,900			6,490,451		$	9.16		7.75	$	7,636
Vested and exercisable as of March 31, 2022					3,115,184		$	11.07		6.22	$	1,934

During the three months ended March 31, 2022 and 2021, the weighted-average grant date fair value of options granted were $3.23 and $7.76 per share, respectively.

The Company records stock-based compensation of stock options granted by estimating the fair value of stock-based awards using the Black-Scholes option pricing model and amortizes the fair value of the stock-based awards granted over the applicable vesting period of the awards on a straight-line basis. The fair value of stock options was estimated using the following weighted-average assumptions:

In the first quarter of 2020, the Company revised its non-employee director compensation policy to approve the granting of restricted stock units (RSUs) in accordance with the Restated 2013 Equity Incentive Plan (the Restated 2013 Plan). Each eligible director who has served for at least six months during the prior calendar year and continues to serve as a non-employee member of the Board of Directors (the “Board”) is granted RSUs. Each eligible director who has served on the Board for less than six months during the prior calendar year and who continues to serve as a non-employee member of the Board, is granted RSUs which are pro-rated for the period served during the prior calendar year.

The RSUs granted to non-employee directors will vest on the one-year anniversary of the grant date, subject to the eligible director’s continuous services through the vesting date, and will vest in full upon a change in control, as defined under the Restated 2013 Plan. The RSUs granted to employees will vest annually on the one-year, two-year, and three-year anniversaries of the grant date. The fair value of all RSUs is measured at the grant date based on the closing market price of the Company’s common stock and is recognized as stock-based compensation expense on a straight-line basis over the vesting period.

The performance stock units (PSUs) are also available for grant pursuant to the Restated 2013 Plan. Each PSU, which is a stock award, is earned through the achievement of performance-based metrics over a defined performance period. The length of the defined performance period, the performance-based metric to be achieved during the defined performance period, and the measure of whether and to what degree such performance-based metric has been achieved will be conclusively determined by the compensation committee of the Company’s board of directors, in its sole discretion. The estimated fair value of equity awards that contain performance conditions is expensed over the term of the award once the Company has determined that it is probable that performance conditions will be satisfied. There was no PSUs granted during the three months ended March 31, 2022. As of March 31, 2022, no PSUs have vested as the performance-based metrics of the PSUs have not yet been achieved.

During the three months ended March 31, 2022 and 2021, the Company granted 298,150 and 203,500 RSUs, respectively, with a weighted-average grant date fair value of $5.22 and $9.99 per share, respectively.

In relation to the RSUs granted, the Company recognized $0.4 million and $0.1 million in stock-based compensation expense for the three months ended March 31, 2022 and 2021, respectively, which were included in selling, general and administrative expenses. As of March 31, 2022, the total unrecognized compensation expense related to unvested RSUs and PSUs was $5.1 million, which the Company expects to recognize over an estimated weighted-average period of 2.7 years.

The following table summarizes RSU and PSU activity and weighted average grant date fair value for the three months ended March 31, 2022:

	Shares			Weighted- Average Grant Date Fair Value
Unvested shares as of December 31, 2021		623,000		$	8.63
Granted		298,150		$	5.22
Vested		(85,106	)	$	10.09
Forfeited		(33,225	)	$	8.33
Unvested shares as of March 31, 2022		802,819		$	7.22

Total stock-based compensation expense recognized was as follows (in thousands):

As of March 31, 2022, the total unrecognized compensation expense related to unvested options was $15.2 million, which the Company expects to recognize over an estimated weighted average period of 2.9 years.