Stock-Based Compensation

9 Months Ended

Sep. 30, 2023

Share-Based Payment Arrangement [Abstract]

Stock-Based Compensation

10. Stock-based Compensation

Stock Option Valuation

The assumptions that the Company used to determine the grant-date fair value of stock options granted to employees and members of the Board were as follows, presented on a weighted-average basis:

	Nine Months Ended September 30,
	2023			2022
Expected option life (years)		6.00			5.96
Risk-free interest rate		3.82	%		1.80	%
Expected volatility		83.36	%		80.22	%
Expected dividend yield		—	%		—	%

The following table summarizes the Company’s stock option activity for the nine months ended September 30, 2023:

	Number of Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Outstanding as of December 31, 2022		9,423,271		$	7.26		8.14	$	22
Granted		123,501		$	1.09
Exercised		(223,273	)	$	0.79
Cancelled or forfeited		(3,401,052	)	$	7.15
Outstanding as of September 30, 2023		5,922,447		$	7.43		6.32	$	1,068
Exercisable as of September 30, 2023		3,606,590		$	10.16		4.82	$	217

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the underlying stock options and the estimated fair value of the Company’s common stock for those stock options that had exercise prices lower than the estimated fair value of the Company’s common stock.

The aggregate intrinsic value of options exercised during the nine months ended September 30, 2023 was $85. No options were exercised during the nine months ended September 30, 2022.

The weighted-average grant-date fair value of the Company’s stock options granted during the nine months ended September 30, 2023 and 2022 was $0.79 and $1.23, respectively.

Restricted Stock Units

The following table summarizes the Company’s restricted common stock units for the nine months ended September 30, 2023:

	Number of Shares			Weighted- Average Grant Date Fair Value
Issued and unvested as of December 31, 2022		940,392		$	3.62
Granted		1,548,117		$	1.65
Vested		(283,835	)	$	4.98
Forfeited, cancelled or expired		(979,677	)	$	2.02
Issued and unvested as of September 30, 2023		1,224,997		$	2.10

The total fair value of restricted stock units vested during the nine months ended September 30, 2023 and 2022 was $1,367 and $7, respectively.

Stock-Based Compensation

Stock-based compensation expense was allocated as follows:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2023		2022		2023		2022
Research and development	$	415	$	448	$	1,705	$	2,169
General and administrative		1,804		2,208		4,579		7,098
Total stock-based compensation expense	$	2,219	$	2,656	$	6,284	$	9,267

As of September 30, 2023, total unrecognized compensation cost related to the unvested stock-based awards was $6,282, which is expected to be recognized over a weighted-average period of 2.10 years.