Stock-Based Compensation

9 Months Ended

Sep. 30, 2024

Share-Based Payment Arrangement [Abstract]

Stock-based Compensation

11. Stock-based Compensation

Stock-based Compensation Expense

Total stock-based compensation expense recognized was as follows (in thousands):

	Three Months Ended September 30,				Nine Months Ended September 30,
	2024		2023		2024		2023
Research and development	$	3,622	$	2,545	$	8,833	$	7,132
General and administrative		3,155		3,015		9,627		8,216
Total stock-based compensation	$	6,777	$	5,560	$	18,460	$	15,348

Stock Options

A summary of stock option activity for the nine months ended September 30, 2024 is set forth below:

	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, 2023		9,383,105		$	10.64		8.2	$	45
Options granted		7,474,253		$	2.48
Options exercised		(92,283	)	$	2.12
Options forfeited or cancelled		(1,066,445	)	$	10.66
Outstanding, September 30, 2024		15,698,630		$	3.12		8.4	$	15
Options exercisable, September 30, 2024		6,699,537		$	3.58		7.6	$	1
Vested and expected to vest, September 30, 2024		15,698,630		$	3.12		8.4	$	15

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

	Three Months Ended September 30,		Nine Months Ended September 30,
	2024	2023	2024	2023
Fair value of common stock	$1.44 - $1.55	$1.37 - $3.38	$1.21 - $3.12	$1.37 - $9.15
Expected term (years)	5.77 - 6.08	5.5 - 6.1	5.5 - 6.08	5.5 - 6.1
Volatility	85.32% - 86.64%	84.7% - 87.3%	82.15% - 86.64%	83.3% - 87.3%
Risk free rates	3.63% - 3.99%	4.1% - 4.6%	3.63% - 4.52%	3.5% - 4.6%
Dividend rate	0.0%	0.0%	0.0%	0.0%

Restricted Stock Units (RSUs)

The summary of RSU activity and related information for the nine months ended September 30, 2024 is set forth below:

	Number of Units Outstanding			Weighted Average Grant Date Fair Value
Outstanding, December 31, 2023		454,200		$	7.69
RSUs granted		537,400		$	2.60
RSUs vested		(184,380	)	$	7.15
RSUs forfeited		(41,364	)	$	4.21
Outstanding, September 30, 2024		765,856		$	4.40

Option repricing

On August 8, 2023, the board of directors approved a stock option repricing (the Option Repricing) effective on August 14, 2023 (the Effective Date) in accordance with the terms of the Company’s 2015 Stock Incentive Plan (the 2015 Plan) and Second Amended and Restated 2018 Stock Option and Incentive Plan (the 2018 Plan, and together with the 2015 Plan, the Plans). Pursuant to the Option Repricing, the exercise price of each stock option previously granted under the Plans, totaling 6,431,077 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.

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 $4.6 million of incremental compensation cost, which was calculated using the Black-Scholes option-pricing model. Of the incremental compensation cost, $0.6 million and $2.2 million was recognized in the three and nine months ended September 30, 2024, respectively, compared to $0.4 million recognized in the three and nine months ended September 30, 2023. Additionally, $0.8 million 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 condensed consolidated statements of operations.

Effective August 21, 2024, the board of directors approved a rescission of the Option Repricing for certain non-employee directors of the Company. All of the affected stock options have been reverted to their original exercise price as established at the time of the grant. The Company will continue to recognize the incremental fair value from the Option Repricing for the impacted options. The compensation expense associated with the Option Repricing for these options was not material.