The Company currently grants equity-based awards under its 2020 Incentive Award Plan (“2020 Plan”) and its 2022 Inducement Plan (“Inducement Award Plan”). The Company previously granted equity-based awards under its 2005 Equity Incentive Plan (“2005 Plan”) and 2016 Equity Incentive Plan (“2016 Plan” and together with the 2020 Plan, the Inducement Award Plan and the 2005 Plan, the “Plans”).

A summary of the restricted stock unit activity under the Plans for the three months ended March 31, 2024 was as follows:


	Shares			Weighted- Average Grant Date Fair Value
Restricted stock units outstanding as of December 31, 2023		105,048		$	3.20
Granted		984,929		$	5.49
Vested		(27,869	)	$	3.29
Forfeited		(1,079	)	$	4.61
Restricted stock units outstanding as of March 31, 2024		1,061,029		$	5.33

A summary of the stock option activity under the Plans for the three months ended March 31, 2024 was as follows:


	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2023		5,709,980		$	5.68		8.0	$	6,334
Granted		2,346,602			5.24
Exercised		(918	)		3.90
Cancelled		(1,890	)		5.63
Outstanding at March 31, 2024		8,053,774		$	5.55		8.4	$	12,871

The fair value of each stock option granted to employees, directors and non-employees was estimated on the date of grant using the Black-Scholes option-pricing model, or a Monte Carlo simulation in the case of certain options granted to executive officers, with the following weighted-average assumptions for the three months ended March 31, 2024 and 2023:


	Three Months Ended March 31,
	2024			2023
Risk-free interest rate		4.1	%		3.7	%
Expected dividend yield		—	%		—	%
Expected term (in years)		6.0			6.0
Expected volatility		77.1	%		83.3	%

The weighted-average fair value of options granted to employees, directors and non-employees during the three months ended March 31, 2024 and 2023 was $3.64 and $2.00, respectively.

Stock-based compensation expense included in the Company’s consolidated statements of operations and comprehensive loss was as follows (in thousands):


	Three Months Ended March 31,
	2024		2023
Research and development	$	703	$	284
General and administrative		1,459		1,326
Total	$	2,162	$	1,610

For the period ended March 31, 2024, approximately $281,000 of the stock-based compensation expense was recorded as a liability. For the period ended March 31, 2023, the Company did not record any stock-based compensation expense as a liability.

Unrecognized share-based compensation related to stock-based options amounted to $12.5 million at March 31, 2024 and is expected to be recognized over a weighted average period of approximately 3.21 years. Unrecognized share-based compensation related to restricted stock units amounted to $3.9 million at March 31, 2024 and is expected to be recognized over a weighted average period of approximately 2.6 years.

In March 2024, the Company granted options to purchase 550,000 shares of common stock at an exercise price of $6.07 per share to its Chief Executive Officer (“2024 Performance Option”) and granted 385,000 RSUs to its Executive Chair (“2024 PSUs”), each with performance-based vesting conditions under the 2020 Plan. Vesting of the 2024 Performance Option and the 2024 PSUs are based on the achievement of various clinical and regulatory milestones during the specified periods.

For the performance-based stock awards, the Company measures stock compensation expense based on the fair value of the award on the grant date and then once the Company determines the performance criteria is probable of achievement, recognizes such amount over the vesting term of the award, which is approximately 3.8 years. The Company determined the fair value of the 2024 Performance Option using the Black-Scholes option-pricing model using the following inputs: risk-free interest rate of 4.26%, volatility of 76.82%, expected dividend yield 0%, and expected term of 6.0 years.

The Company measured the 2024 PSUs based on the fair value of the Company’s stock on the date of grant.

As of March 31, 2024, the Company has determined that 183,333 shares of common stock subject to the 2024 Performance Option and 128,333 shares of common stock underlying the 2024 PSUs are probable to vest and has recognized approximately $12 thousand of expense, which is included in stock-based compensation expense. As of March 31, 2024, there is $1.5 million of unrecognized stock compensation expense to be recognized in connection with all of the performance-based awards that are deemed to be probable as of March 31, 2024.

As of March 31, 2024, there were 761,474 shares available for grant under the 2020 Plan and 200,800 shares available for grant under the Inducement Award Plan.

In April 2020, the Board of Directors adopted the Company’s 2020 Employee Stock Purchase Plan (“2020 ESPP”). As of December 31, 2023, the Company reserved 438,834 shares of common stock for issuance under the 2020 ESPP and on January 1, 2024, the shares available for issuance under the 2020 ESPP were automatically increased by 286,072. As of March 31, 2024, there are a total of 724,906 shares available for issuance under the 2020 ESPP and no shares have been issued under the 2020 ESPP.