The following table presents stock-based compensation expense as reflected in the Company's consolidated statements of operations (in thousands):


	Year Ended December 31,
	2023		2022
Research and development	$	9,143	$	7,225
General and administrative		11,116		9,901
Total stock-based compensation	$	20,259	$	17,126

The following table presents stock-based compensation expense by type of award (in thousands):


	Year Ended December 31,
	2023		2022
Stock options	$	18,060	$	14,054
Restricted stock units (including performance-based RSUs)		2,023		2,878
Employee Stock Purchase Plan		176		194
Total	$	20,259	$	17,126

A summary of stock option activity for the year ended December 31, 2023 is set forth below (in thousands, except share and per share data):


	Number of Shares Underlying Outstanding Options			Weighted Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2022		6,203,020		$	14.44		8.4	$	780
Options granted		4,293,442		$	5.36
Options exercised		(946	)	$	4.76
Options forfeited or cancelled		(1,112,411	)	$	10.59
Outstanding, December 31, 2023		9,383,105		$	10.64		8.2	$	45
Options exercisable, December 31, 2023		3,984,821		$	13.65		7.4	$	6
Vested and expected to vest, December 31, 2023		9,383,105		$	10.64		8.2	$	45

The assumptions used in the Black Scholes Model to calculate stock-based compensation are as follows:


	Year Ended December 31,
	2023	2022
Fair value of common stock	$1.32 - $9.15	$8.94 - $19.97
Expected term (years)	5.5 - 6.1 years	5.5 - 6.1 years
Volatility	83.25% - 92.87%	77.4% - 81.7%
Risk free rates	3.5% - 4.86%	1.6% - 4.22%
Dividend rate	0.0%	0.0%

The fair value of each stock option was estimated at the date of grant using a Black-Scholes option-pricing model using the following assumptions:

The Company will continue to use judgment in evaluating the expected volatility, risk-free interest rates, dividend yield and expected term, utilized for stock-based compensation on a prospective basis.

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying stock options and the fair value of the Company’s common stock for stock options that were in-the-money at December 31, 2023 and 2022. The aggregate intrinsic value of stock options exercised during the years ended on December 31, 2023 and 2022 was less than $0.1 million and $1.0 million, respectively.

The total fair value of options that vested during the years ended December 31, 2023 and 2022 was $17.9 million and $13.4 million, respectively. The options granted during the years ended December 31, 2023 and 2022 had a weighted-average per share grant-date fair value of $3.94 per share and $10.07 per share, respectively.

As of December 31, 2023, the total unrecognized stock-based compensation expense related to unvested stock options was $31.7 million, which is expected to be recognized over the remaining weighted-average vesting period of 2.3 years.

The summary of RSU activity and related information for the year ended December 31, 2023 is set forth below:


	Number of Units Outstanding			Weighted Average Grant Date Fair Value
Outstanding, December 31, 2022		197,580		$	7.80
RSUs granted		513,700		$	7.47
RSUs vested		(194,455	)	$	7.43
RSUs forfeited		(62,625	)	$	7.07
Outstanding, December 31, 2023		454,200		$	7.69

The Company granted 513,700 RSU's in the year ended December 31, 2023. The Company did not grant any RSU's in the year ended December 31, 2022. The weighted-average grant date fair value of RSUs granted during the year ended December 31, 2023 was $7.69.

As of December 31, 2023, there was approximately $2.4 million of unrecognized compensation cost related to unvested RSUs that the Company expects to recognize over a remaining weighted-average period of approximately 2.1 years.

On August 8, 2023, the board of directors approved a stock option repricing (the Option Repricing) to be effective on August 14, 2023 (the Effective Date) in accordance with the terms of the Company’s 2015 Stock Incentive Plan and 2018 Plan (together, the Plans). Pursuant to the Option Repricing, and subject to a one year cliff period, the exercise price of each stock

option previously granted under the Plans, totaling 6,431,910 options, was amended to reduce the exercise price of such options to $2.14 per share, the closing price of the Company’s common stock on the Nasdaq Global Market on the Effective Date. Under the terms of the Option Repricing, a repriced option will revert to its original exercise price if, prior to the one year anniversary of the Effective Date, (a) the option holder’s employment is terminated by the Company with cause or by the option holder or (b) the option is exercised.

The repriced options otherwise retained their existing terms and conditions as set forth in the Plans and applicable award agreements. The stock option modification resulted in an incremental compensation cost of approximately $4.6 million, which was calculated based on the difference between the fair value of the stock options before the repricing and the fair value as of the Effective Date, using the Black-Scholes option-pricing model. Of the incremental compensation cost, $1.3 million was recognized in the year ended December 31, 2023, and the remaining amount, less any employee terminations, will be recognized on the straight-line basis over the remaining vesting period of the repriced options. The incremental cost is included in general and administrative expense and research and development expense on the consolidated statements of operations.

In addition, as of the Effective Date, the Company issued 1,418,042 options to purchase shares of common stock under the 2018 Plan to eligible employees who held inducement awards as of August 8, 2023. These new options were issued to eligible employees because their inducement awards granted under Nasdaq Listing Rule 5635(c)(4) are not eligible for repricing. The prior inducement awards remain outstanding under their original terms.