In August 2015, the Company’s board of directors approved the 2015 Equity Incentive Plan (“2015 Plan”). In connection with the Company’s IPO and the effectiveness of the 2018 Award Incentive Plan (“2018 Plan”), discussed below, the 2015 Plan terminated. The 92,815 shares of common stock that were then unissued and available for future issuance under the 2015 Plan became available under the 2018 Plan.

In September 2018, the Company’s board of directors approved the 2018 Plan. Under the 2018 Plan, a total of 2,690,000 shares of common stock were initially reserved for issuance under the 2018 Plan, plus the number of shares remaining available for future awards under the 2015 Plan, as of the effective date of the 2018 Plan. The number of shares of common stock reserved for issuance under the 2018 Plan automatically increases on January 1 of each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by 4% of the total number of shares of the Company’s outstanding stock on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s board of directors. The 2018 Plan provides, among other things, for the grant of options, stock appreciation rights, restricted stock awards, restricted stock unit awards and performance bonus awards.

The maximum number of shares that may be issued upon the exercise of stock options under the 2018 Plan is 45,000,000.

The Company’s board of directors has the authority to determine to whom options will be granted, the number of shares, the term, and the exercise price. If an individual owns stock representing 10% or more of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the board of directors. Options granted have a term of up to 10 years and generally vest over a 4-year period with a straight-line vesting.

In April 2021, the Company’s board of directors adopted the 2021 Employment Inducement Incentive Award Plan (the “2021 Plan”), pursuant to Nasdaq Listing Rule 5635(c)(4). The principal purpose of the 2021 Plan is to promote the success and enhance the value of the Company by inducing new employees to commence employment with us, and by aligning the individual interests of new employees with the interests of our stockholders. Awards granted under the 2021 Plan are intended to constitute “employment inducement awards” under Nasdaq Listing Rule 5635(c)(4), and, therefore, the 2021 Plan is intended to be exempt from the Nasdaq Listing Rules regarding shareholder approval of stock option and stock purchase plans. A total of 790,400 shares of our common stock were initially reserved for issuance under the 2021 Plan. The 2021 Plan provides for the grant of non-qualified stock options, restricted stock units, restricted stock awards, stock appreciation rights, and other stock-based and cash-based awards. The 2021 Plan does not provide for the grant of incentive stock options. Awards under the 2021 Plan may be granted to eligible employees who are either new employees or who are commencing employment with us or one of our subsidiaries following a bona fide period of non-employment with us, and for whom such awards are granted as a material inducement to commencing employment with us or one of our subsidiaries. Awards under the 2021 Plan may not be granted to our consultants or non-employee directors.

The 2021 Plan is administered by our board of directors and, to the extent our board of directors delegates its authority to it, our compensation committee. In the event of a change in control in which the successor corporation refuses to assume or substitute any outstanding award under the 2021 Plan, the vesting of such award will accelerate in full. Our board of directors may terminate, amend, or modify the 2021 Plan at any time, provided that no termination or amendment may materially impair any rights under any outstanding award under the 2021 Plan without the consent of the holder.

On April 21, 2022, the Company’s board of directors increased the number of shares available under the 2021 Plan by 700,000 shares. On February 2, 2023, the Company’s board of directors increased the number of shares available under the 2021 Plan by 1,300,000 shares.

In September 2018, the Company’s Board of Directors approved the 2018 Employee Stock Purchase Plan (“2018 ESPP”). The 2018 ESPP also became effective in September 2018. A total of 282,334 shares were initially reserved for issuance under the 2018 ESPP. Additionally, the number of shares of common stock reserved for issuance under the 2018 ESPP will increase automatically each year, beginning on January 1, 2019 and continuing through and including January 1, 2028, by the lesser of (1) 1% of the shares of common stock outstanding on December 31 of the preceding calendar year or (2) such lesser number of shares determined by the Company’s Board of Directors. The maximum number of shares that may be issued under the 2018 ESPP is 5,000,000. The offering periods are scheduled to start on the first trading day on or after June 1 or December 1 of each year. Contributions under the 2018 ESPP are limited to a maximum of 15% of an employee’s eligible compensation.

The estimated fair value of stock purchase rights granted under the 2018 ESPP were calculated using the Black-Scholes option-pricing model using the following assumptions:


	Year ended December 31,
	2023			2022			2021
Expected dividend yield		—	%		—	%		—	%
Expected term	0.50 years			0.50 years			0.57 years
Risk-free interest rate		5.1	%		1.3	%		0.1	%
Expected volatility		90.1	%		94.9	%		88.0	%

The fair value of each stock option granted to an employee or a director was estimated as of the date of grant using the Black-Scholes model with the following weighted-average assumptions:


	Year Ended December 31,
	2023			2022			2021
Expected dividend yield		—	%		—	%		—	%
Expected term	5.77 years			5.98 years			6.01 years
Risk-free interest rate		4.0	%		2.1	%		1.1	%
Expected volatility		98.9	%		77.9	%		79.0	%

Management’s calculations are based on a grant date valuation approach. Using the Black-Scholes model, the weighted-average grant-date fair value of employee stock options granted was $1.73, $3.20, and $8.03 per share during the years ended December 31, 2023, 2022, and 2021, respectively.

A summary of the 2018 Plan and 2021 Plan activity is as follows:


				Options Outstanding
	Number of Shares Available for Issuance			Number of Shares			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2022		4,814,394			6,951,620		$	7.92		8.08	$	1,089
Authorized		4,775,796			—
Granted		(4,227,353	)		712,566		$	2.20
Exercised					(6,000	)	$	0.76
Cancelled		824,088			(384,725	)	$	5.35
Balance at December 31, 2023		6,186,925			7,273,461		$	7.50		7.26	$	264
Vested and exercisable – December 31, 2023					4,647,325		$	8.53		6.67	$	200
Vested and expected to vest – December 31, 2023					6,997,158		$	7.60		7.22	$	254

For the years ended December 31, 2023, 2022, and 2021, the total intrinsic value of stock option awards exercised was de minimis, $0.5 million, and $5.8 million, respectively, determined at the date of option exercise, and the total cash received upon exercise of stock options was de minimis for 2023, $0.2 million in 2022, and $3.4 million in 2021. The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying stock option awards and the estimated fair value of the common stock on the date of exercise.

As of December 31, 2023, $8.5 million of total cost related to non-vested employee and consultant options is expected to be recognized over a weighted-average period of 1.67 years. The total fair value of shares vested during the year ended December 31, 2023 was $7.8 million.

Stock-based compensation expense and awards granted to non-employees was $0.8 million, $0.6 million, and $0.7 million for the years ended December 31, 2023, 2022, and 2021, respectively.

The Company has granted restricted stock unit awards under the 2018 Equity Plan. The Company's restricted stock unit awards have a term of up to 10 years and generally vest over a 6 month, 1 or 2-year period. The following table summarizes the Company's restricted stock unit activity during the year ended December 31, 2023:


	Number of Shares			Weighted Average Grant Date Fair Value
Outstanding, unvested as December 31, 2022		561,526		$	5.38
Issued		3,514,787		$	3.29
Vested		(871,385	)	$	4.64
Canceled/Forfeited		(115,958	)	$	3.29
Outstanding, unvested as December 31, 2023		3,088,970		$	3.29

Total stock-based compensation for all awards granted to employees, consultants and the Company's 2018 ESPP, before taxes, is as follows (in thousands):


	Year Ended December 31,
	2023		2022		2021
Research and development expenses	$	6,266	$	6,730	$	6,626
General and administrative expenses		5,352		5,827		3,926
Total	$	11,618	$	12,557	$	10,552