On January 1, 2023, an additional 417,875 shares were added to the 2021 ESPP, representing 1% of total common shares outstanding at December 31, 2022, pursuant to the terms of the plan. The expense incurred under this plan for the six months ended June 30, 2023 and 2022 was immaterial to the financial statements. The amounts have been included in the total stock-based compensation line items in the accompanying financial statements and disclosures.

On January 1, 2023 an additional 2,089,379 shares were added to the 2021 Plan, representing 5% of total common shares outstanding at December 31, 2022, pursuant to the terms of the plan.

As of June 30, 2023, 1,966,616 shares remained available for future grant under the 2021 Plan. 7,374,118 stock-based award units were outstanding under the 2021 Plan and 2018 Plan as of June 30, 2023.

The Company’s 2021 Plan provides for the Company to sell or issue common stock or restricted common stock or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, nonemployees and members of the board of directors of the Company. The 2021 Plan is administered by the board of directors or at the discretion of the board of directors by the compensation committee of the board. The exercise prices, vesting periods, and other restrictions are determined at the discretion of the compensation committee of the board of directors, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the contractual term of stock option may not be greater than 10 years. Stock options granted to date typically vest over four years.

The assumptions used to determine the fair values of stock options granted to employees and directors are presented as follows:


	Six months ended June 30,
	2023	2022
Fair value of common stock	$1.73 - $2.54	$6.54 - $16.56
Dividend yield	—%	—%
Volatility	90.36% - 91.64%	82.49% - 86.69%
Risk-free interest rate	3.46% - 3.98%	1.46% - 3.08%
Expected term (years)	5.50 - 6.25	5.50 - 6.25

The Company’s stock option activity regarding employees, directors, and nonemployees is summarized as follows (in thousands excepts share and per share amounts):


	Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Life (in years)		Aggregate intrinsic value
Options outstanding—December 31, 2022		6,264,898		$	6.92		8.43	$	42
Granted		1,184,500			1.86
Exercised		(197,078	)		0.94
Cancelled		(244,786	)		8.15
Forfeited		(1,278,807	)		6.97
Options outstanding—June 30, 2023		5,728,727		$	6.02		8.23	$	2,009
Options exercisable—June 30, 2023		2,871,893		$	6.11		7.81

Additional information with regard to stock option activity involving employees and directors is as follows (in thousands except per share amounts):


	Six months ended June 30,
	2023		2022
Weighted-average grant-date fair value per option of total options granted	$	1.37	$	6.40
Aggregate intrinsic value of stock options exercised		329		89

As of June 30, 2023, total unrecognized compensation cost related to the unvested awards to employees, directors, and nonemployees is $2.8 million, which is expected to be recognized over a weighted-average period of 1.8 years.

The fair values of restricted stock units (“RSUs”) are based on the fair market value of the Company’s common stock on the date of grant. Each RSU represents a contingent right to receive one share of the Company’s common stock upon vesting. In general, RSUs vest (i) annually in four equal installments on the grant anniversary or (ii) incrementally over two years. The following table summarizes the Company’s RSU activity for the three months ended June 30, 2023:


	Number of Restricted Stock Units			Weighted- Average Grant Date Fair Value
Outstanding at December 31, 2022		1,144,994		$	6.76
Granted		1,419,000			1.73
Vested		(650,321	)		3.93
Forfeited		(1,193,516	)		3.89
Outstanding at June 30, 2023		720,157		$	4.16

As of June 30, 2023, total unrecognized compensation cost related to the unvested awards to employees is $0.4 million, which is expected to be recognized over a weighted-average period of 1.2 years.

The assumptions used to determine the fair values of stock appreciation right ("SAR") awards granted to employees and directors under the 2021 Plan are presented as follows:


		Six months ended June 30,
		2023
Fair value of common stock		$1.73
Dividend yield		—%
Volatility		90.83%
Risk-free interest rate		3.46%
Expected term (years)		4.0

All SARs granted to date vest incrementally over two years. The Company’s SAR grant activity regarding employees is summarized as follows (in thousands excepts share and per share amounts):


	Number of SARs		Weighted- Average Exercise Price per SAR		Weighted- Average Remaining Contractual Life (in years)		Aggregate intrinsic value
Outstanding—January 1, 2023		—	$	—		—	$	—
Granted		1,000,000		1.73
Exercised		—		—
Forfeited		—		—
Outstanding—June 30, 2023		1,000,000	$	1.73		9.60	$	1,310
Exercisable—June 30, 2023		258,332	$	1.73		9.60

As of June 30, 2023, total unrecognized compensation cost related to unvested awards to employees is $0.2 million, which is expected to be recognized over a weighted-average period of 1.1 years.

The Company recorded stock-based compensation expense regarding its employees, directors, and nonemployees as follows (in thousands):


	Three months ended June 30,					Six months ended June 30,
	2023			2022		2023		2022
Research and development expense	$	(169	)	$	1,703	$	1,816	$	2,979
General and administrative expense		1,343			1,143		3,017		2,064
Total	$	1,174		$	2,846	$	4,833	$	5,043