In connection with the closing of the Company’s initial public offering, the Company's Board of Directors and stockholders approved the 2018 Omnibus Incentive Plan (the “2018 Plan”), which provides for the reservation of 333,400 shares of common stock for equity awards. On June 16, 2020, the Company’s stockholders approved the 2020 Equity Incentive Plan (as amended or restated, the “2020 Plan”) which consisted of, at the time of approval, (i) 200,000 shares of common stock and (ii) additional shares of common stock (up to 325,268) as is equal to (i) the number of shares reserved under the 2018 Plan that remain available for grant under the 2018 Plan as of immediately prior to the date the 2020 Plan was approved by the Company’s stockholders and (ii) the number of shares subject to awards granted under the 2018 Plan which awards expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right.

As of the effective date of the 2020 Plan, no further awards will be made under the 2018 Plan. Any options or awards outstanding under the 2018 Plan remain outstanding and effective and are governed by their existing terms.

On June 16, 2021, the Company’s stockholders approved an amendment to the 2020 Plan to reserve an additional 466,666 shares of common stock for issuance under the plan.

On December 1, 2022, the Company’s stockholders approved an amendment and restatement of the 2020 Plan to (i) increase the number of shares of common stock reserved for issuance under the plan by 866,666 shares to 1,533,333 shares, subject to adjustment in the event of stock splits and other similar events, (ii) provide for an annual increase, to be added on the first day of each fiscal year during the term of the plan, beginning with the fiscal year ending December 31, 2023, of 5% of the number of shares of common stock outstanding on the first day of such fiscal year or a lesser number of shares determined by the Company’s Board of Directors, (iii) provide that up to 1,858,601 shares of common stock may be granted as “incentive stock options” under the plan, (iv) extend the term of the plan to December 1, 2032 and (v) revise certain provisions of the plan relating to the Company’s Board of Directors’ ability to delegate authority to make awards under the plan.

At December 31, 2023, 738,778 shares remained available for future issuance under the 2020 Plan. Under the 2020 Plan, stock options may not be granted at less than fair value on the date of grant.

The 2021 Employee Stock Purchase Plan (the “ESPP”) was adopted by the Company's Board of Directors on April 14, 2021, approved by the stockholders on June 16, 2021, and became effective on June 16, 2021. The first offering period under the ESPP commenced on September 1, 2021.

On June 6, 2023, the Company's stockholders approved an amendment and restatement of the ESPP to (i) increase the number of shares of common stock reserved for issuance under the ESPP from 73,525 to 473,525 and (ii) provide for an annual increase to be added on the first day of each fiscal year, beginning with the fiscal year ending December 31, 2024 and ending with the fiscal year ending December 31, 2033, equal to the least of (a) 293,597 shares of common stock, (b) one percent (1%) of the outstanding shares of common stock on such date and (c) the number of shares of common stock determined by the Board of Directors. The Company amended and restated the ESPP on November 12, 2023 to provide for 24-month offering periods.

The number of shares of the Company’s common stock reserved for issuance under the ESPP is 473,525 shares. At December 31, 2023, 397,546 shares remained available for future issuance under the ESPP.

The following table summarizes the Company’s stock option activity for the year ended December 31, 2023:


	Number of Options			Weighted Average Exercise Price		Remaining Contractual Life (in years)
Outstanding at December 31, 2022		1,433,968		$	36.22		8.88
Granted		1,141,449			5.38
Exercised		—			—
Expired		(54,941	)		81.84
Forfeitures		(260,804	)		15.07
Outstanding at December 31, 2023		2,259,672		$	21.93		8.20
Vested and expected to vest as of December 31, 2023		2,259,672		$	21.93		8.20
Exercisable at December 31, 2023		616,133		$	57.99		7.69

At December 31, 2023, the Company had an aggregate of $8,949 of unrecognized equity-based compensation cost related to stock options outstanding which is expected to be recognized over a weighted average period of 2.8 years. The intrinsic value of stock options outstanding as of December 31, 2023 and 2022 was $863 and $0, respectively. The intrinsic value of stock options exercisable as of December 31, 2023 and 2022 was $27 and $0, respectively. No stock options were exercised during the year ended December 31, 2023.

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions noted in the following table for the years ended December 31:



	2023	2022	2021
Expected volatility	121.8% - 129.60%	118.5% - 130.3%	115.5% - 123.9%
Expected dividends	0.0%	0.0%	0.0%
Expected term (in years)	5.31 - 6.25	5.31 - 6.25	5.10 - 6.25
Risk-free rate	3.5% - 4.9%	1.4% - 3.9%	0.4% - 1.4%

The weighted average fair value of options to purchase shares of common stock granted during the year ended December 31, 2023 and 2022 was $4.76 and $7.59, respectively.

In 2023, 2022 and 2021, the Company's Board of Directors issued restricted stock units to employees. Restricted stock unit grants typically vest over one to four years.

The following table summarizes the Company’s restricted stock unit activity for the year ended December 31, 2023:


	Units			Weighted- Average Grant Date Fair Value
Unvested at December 31, 2022		512,557		$	8.39
Granted		666,516			5.46
Vested		(183,951	)		7.64
Forfeitures		(117,941	)		7.96
Outstanding at December 31, 2023		877,181		$	6.37
Unvested as of December 31, 2023		877,181		$	6.37

At December 31, 2023, the Company had an aggregate of $4,293 of unrecognized equity-based compensation cost related to restricted stock units outstanding. The unrecognized expense for the restricted stock units is expected to be recognized over a weighted average period of 2.59 years.

In connection with the Company’s statutory corporate conversion on January 25, 2018, all restricted Series B and D common units were converted into restricted shares of common stock. As of December 31, 2021, there were no remaining restricted shares of common stock outstanding.

No restricted common shares vested during the years ended December 31, 2023 and 2022. The aggregate intrinsic value of restricted common shares that vested during the year ended December 31, 2021 was $199.

At December 31, 2023, the Company had an aggregate of $0 of unrecognized equity-based compensation related restricted shares of common stock.

The Company recorded equity-based compensation expense related to all of its share-based awards to employees and non-employees in the following captions within its consolidated statements of operations for the years ended December 31, 2023, 2022, and 2021:


	For the Year Ended December 31,
	2023		2022		2021
Research and development expenses	$	3,094	$	2,756	$	6,289
General and administrative expenses		4,531		4,781		7,084
	$	7,625	$	7,537	$	13,373