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


	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
Research and development	$	2,671	$	2,381	$	5,211	$	4,587
General and administrative		3,333		2,642		6,472		5,201
Total stock-based compensation	$	6,004	$	5,023	$	11,683	$	9,788

Stock Options

A summary of stock option activity for the six months ended June 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		7,149,453		$	2.53
Options exercised		(92,283	)	$	2.12
Options forfeited or cancelled		(285,610	)	$	8.55
Outstanding, June 30, 2024		16,154,665		$	7.14		8.6	$	—
Options exercisable, June 30, 2024		5,602,588		$	11.95		7.4	$	—
Vested and expected to vest, June 30, 2024		16,154,665		$	7.14		8.6	$	—

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,
	2024	2023	2024	2023
Fair value of common stock	$1.21 - $1.45	$2.43 - $5.84	$1.21 - $3.12	$2.43 - $9.15
Expected term (years)	5.5 - 6.1	5.5 - 6.1	5.5 - 6.1	5.5 - 6.1
Volatility	84.1% - 85.1%	84.6% - 86.5%	82.2% - 85.1%	83.3% - 86.5%
Risk free rates	4.3% - 4.5%	3.5% - 4.1%	3.9% - 4.5%	3.5% - 4.2%
Dividend rate	0.0%	0.0%	0.0%	0.0%

The summary of RSU activity and related information for the six months ended June 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		529,600		$	2.62
RSUs vested		(184,380	)	$	7.15
RSUs forfeited		(16,632	)	$	4.42
Outstanding, June 30, 2024		782,788		$	4.42

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 (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. 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 $4.6 million of incremental compensation cost, which was calculated using the Black-Scholes option-pricing model. Of the incremental compensation cost, $0.8 million and $1.6 million was recognized in the three and six months ended June 30, 2024, respectively, and $1.4 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 and comprehensive loss.