Stock Incentive Plans (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2023

Dec. 31, 2022

Share-Based Payment Arrangement [Abstract]

Schedule of Stock Options Valuation Assumptions Using a Black-Scholes Option Pricing Model

The assumptions used in the Black-Scholes option pricing model are as follows (note that there were no options granted during the three months ended September 30, 2023):

	Three months ended			Nine months ended September 30,
	September 30, 2022			2023			2022
Expected volatility		70.1	%		69.2%-69.7%			68.7%-70.1%
Weighted-average risk-free interest rate		3.20%-3.66%			3.45%-4.22%			1.46%-3.66%
Expected dividend yield		—	%		—	%		—	%
Expected term (in years)		6.25			5.5-6.25			5.5-6.25
Underlying common stock fair value	$	1.82-$2.82		$	0.92-$1.60		$	1.78-$4.17

The assumptions used in the Black-Scholes option pricing model are as follows:

	For the Year Ended December 31,
	2022	2021
Expected volatility	68.7% - 70.1%	64.6% - 71.7%
Weighted-average risk-free interest rate	1.46% - 4.16%	0.50% - 1.33%
Expected dividend yield	— %	— %
Expected term (in years)	5.5 - 6.25	5.5 - 6.25
Underlying common stock fair value	$1.40 - $4.17	$4.85 - $13.91

Summary of Option Activity under Plans

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 per Share		Weighted- Average Remaining Contractual Term (in Years)		Aggregate Intrinsic Value
								(in thousands)
Outstanding at January 1, 2023		9,865,734		$	10.96		7.2	$	493
Granted		3,188,150		$	1.53
Exercised		(3,366	)	$	0.47
Cancelled/Forfeited		(623,776	)	$	11.11
Outstanding at September 30, 2023		12,426,742		$	8.55		4.9	$	488
Vested and expected to vest at September 30, 2023		12,426,742		$	8.55		4.9	$	488
Exercisable at September 30, 2023		7,566,304		$	11.80		4.5	$	421

A summary of option activity under the Plans during the year ended December 31, 2022 is as follows:

	Number of Options			Weighted- Average Exercise Price per Share		Weighted- Average Remaining Contractual Term (in Years)		Aggregate Intrinsic Value
								(in thousands)
Outstanding at January 1, 2022		7,624,306		$	14.25		7.5	$	2,069
Granted		3,089,655		$	2.64
Exercised		(293	)	$	2.71
Cancelled/Forfeited		(847,934	)	$	10.25
Outstanding at December 31, 2022		9,865,734		$	10.96		7.2	$	493
Vested and expected to vest at December 31, 2022		9,865,734		$	10.96		7.2	$	493
Exercisable at December 31, 2022		6,111,596		$	13.19		6.3	$	493

Summary of Company's RSU Activity

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

	Number of Restricted Stock Units			Weighted- Average Grant Date Fair Value
Outstanding at January 1, 2023		543,179		$	6.12
Granted		483,850		$	1.60
Vested		(281,117	)	$	5.31
Forfeited		(258,184	)	$	2.54
Outstanding at September 30, 2023		487,728		$	3.04

The following table summarizes the Company’s RSU activity for the year ended December 31, 2022:

	Number of Restricted Stock Units			Weighted- Average Grant Date Fair Value
Outstanding at January 1, 2022		307,600		$	12.75
Granted		400,495		$	2.70
Vested		(106,890	)	$	12.42
Forfeited		(58,026	)	$	6.05
Outstanding at December 31, 2022		543,179		$	6.12

Schedule of Stock-Based Compensation Expense

The Company recorded stock-based compensation expense related to stock options, shares purchased under the 2018 ESPP, restricted stock units and stock award modifications as follows:

	Three months ended September 30,				Nine months ended September 30,
	2023		2022		2023		2022
	(in thousands)
Research and development	$	230	$	889	$	1,507	$	4,143
General and administrative		1,477		1,882		4,970		5,822
	$1,707		$2,771		$6,477		$9,965

The Company recorded stock-based compensation expense related to stock options, shares purchased under the 2018 ESPP and restricted stock units as follows:

	For the Year Ended December 31,
	2022		2021
	(in thousands)
General and administrative	$	7,867	$	8,450
Research and development		5,187		8,795
	$	13,054	$	17,245