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 the Company or one of our subsidiaries following a bona fide period of non-employment with the Company, and for whom such awards are granted as a material inducement to commencing employment with the Company or one of its subsidiaries. Awards under the 2021 Plan may not be granted to the Company's 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. The Company's 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.

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, 2023		6,186,925			7,273,461		$	7.50		7.26	$	264
Authorized		3,903,416			—
Granted		(8,947,004	)		7,015,522		$	1.74
Exercised		—			(30,797	)	$	1.99
Cancelled		2,403,755			(1,512,706	)	$	3.84
Balance at June 30, 2024		3,547,092			12,745,480		$	4.78		8.16	$	14
Vested and exercisable at June 30, 2024					5,838,765		$	7.55		6.76	$	14
Vested and expected to vest at June 30, 2024					12,139,168		$	4.90		8.10	$	14

For the six months ended June 30, 2024, the total intrinsic value of stock option awards exercised was de minimis, determined at the date of option exercise, and the total cash received upon exercise of stock options was not significant for the period. 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 June 30, 2024, $10.2 million of total unrecognized compensation cost related to non-vested employee and consultant options is expected to be recognized over a weighted-average period of 1.40 years. The total fair value of shares vested during the six months ended June 30, 2024 was $4.8 million.

Stock-based compensation expense and awards granted to non-employees were $0.4 million and $0.3 million, respectively, for the six months ended June 30, 2024 and 2023.

The Company has granted restricted stock unit awards under the 2018 Equity Plan. The 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 six months ended June 30, 2024:


	Number of Shares			Weighted- Average Grant Date Fair Value
Outstanding, unvested at December 31, 2023		3,088,970		$	3.29
Issued		814,462		$	2.15
Vested		(826,642	)	$	3.28
Canceled/Forfeited		(461,245	)	$	3.01
Outstanding, unvested at June 30, 2024		2,615,545		$	2.99

In March 2024, the Company granted 1,117,020 performance-based restricted stock unit awards ("PSUs") to certain executives under the 2018 Equity Plan. Vesting of the PSUs is dependent upon achievement of certain performance-based metrics through December 31, 2025. Assuming achievement of each performance-based metric, the executive must also generally remain in the Company's service at the date of achievement of the performance-based metric. PSUs are converted into shares of the Company's common stock once vested. The number of shares earned at the end of the performance period will vary, based on actual performance. Upon grant of the PSUs, the Company recognizes stock-based compensation expense related to these awards based on assumptions as to what percentage of each target will be achieved. The Company evaluates these target assumptions on a quarterly basis and adjusts stock-based compensation expense related to these awards, as appropriate.

As of June 30, 2024, 167,555 shares were vested, and therefore the Company recorded $0.1 million stock-based compensation expense related to the PSUs for the three and six months ended June 30, 2024.

The following table summarizes the performance-based restricted stock activity under the 2018 Equity Plan during the six months ended June 30, 2024:


	Number of Shares			Weighted- Average Grant Date Fair Value
Unvested performance-based restricted stock at December 31, 2023		—		$	—
Granted		1,117,020		$	0.75
Vested		(167,555	)	$	0.75
Canceled/Forfeited		(111,698	)	$	0.75
Unvested performance-based restricted stock at June 30, 2024		837,767		$	0.75

Total stock-based compensation for all awards granted to employees, directors and non-employees and purchase rights under the Company's 2018 and 2021 Equity Plans and the 2018 Employee Stock Purchase Plan (“ESPP”), before taxes, is as follows (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Research and development expenses	$	1,609	$	1,631	$	3,269	$	3,244
General and administrative expenses		1,858		1,309		3,444		2,587
Total	$	3,467	$	2,940	$	6,713	$	5,831