Stock-Based Compensation

6 Months Ended

Jun. 30, 2025

Share-Based Payment Arrangement [Abstract]

12. Stock-based Compensation

Stock-based Compensation Expense

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


	Three Months Ended June 30,				Six Months Ended June 30,
	2025		2024		2025		2024
Research and development	$	1,607	$	2,671	$	3,227	$	5,211
General and administrative		1,301		3,333		2,861		6,472
Total stock-based compensation	$	2,908	$	6,004	$	6,088	$	11,683

Stock Options

A summary of stock option activity for the six months ended June 30, 2025 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, 2024		14,965,270		$	3.09		8.1	$	—
Options granted		5,040,276		$	0.87
Options exercised		—		$	—
Options forfeited or cancelled		(1,820,726	)	$	2.21
Outstanding, June 30, 2025		18,184,820		$	2.56		8.2	$	6
Options exercisable, June 30, 2025		8,306,140		$	3.32		7.3	$	—
Vested and expected to vest, June 30, 2025		18,184,820		$	2.56		8.2	$	6

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


	Three Months Ended June 30,		Six Months Ended June 30,
	2025	2024	2025	2024
Fair value of common stock	$0.53 - $0.78	$1.21 - $1.45	$0.53 - $0.90	$1.21 - $3.12
Expected term (years)	5.5 - 6.1	5.5 - 6.1	5.5 - 6.1	5.5 - 6.1
Volatility	89.9% - 94.0%	84.1% - 85.1%	88.3% - 94.0%	82.2% - 85.1%
Risk free rates	3.8% - 4.1%	4.3% - 4.5%	3.8% - 4.4%	3.9% - 4.5%
Dividend rate	0.0%	0.0%	0.0%	0.0%

Restricted Stock Units (RSUs)

The summary of RSU activity and related information for the six months ended June 30, 2025 is set forth below:


	Number of Units Outstanding			Weighted Average Grant Date Fair Value
Outstanding, December 31, 2024		719,656		$	4.40
RSUs granted		619,300		$	0.88
RSUs vested		(286,413	)	$	4.69
RSUs forfeited		(150,426	)	$	1.92
Outstanding, June 30, 2025		902,117		$	2.27

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 6,431,910 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.

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, $0.2 million and $0.8 million was recognized in the six months ended June 30, 2025 and 2024, respectively. The remaining incremental compensation cost of $0.4 million, net of the reversal of expense related to employee terminations prior to August 14, 2024, 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.

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.