Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]

Stock-based Compensation Expense Recognized

Stock‑based compensation expense recognized was as follows (in thousands):

	Year Ended December 31,
	2023		2022
Research and development	$	4,003	$	3,217
General and administrative		6,620		7,389
Total stock-based compensation expense	$	10,623	$	10,606

Weighted-Average Assumptions used in Estimating Fair Value of each Stock Options using Black-Scholes Option-Pricing Model

The fair value of each stock option was estimated using the Black‑Scholes option‑pricing model with the following weighted-average assumptions:

	Year Ended December 31,
	2023			2022
Risk-free interest rate		3.54	%		2.15	%
Expected volatility		99.18	%		92.95	%
Expected term (in years)		5.84			6.02
Expected dividend yield		—			—

Summary of Option Activity Changes in Shares Available for Grant under the 2020 Plan

Changes in shares available for grant under the 2020 Plan during the year ended December 31, 2023 were as follows:

	Shares Available for Grant
Shares available for grant at December 31, 2022		4,253,854
2020 Plan reserve increase on January 1, 2023		1,893,869
Options and restricted stock units granted		(3,834,783	)
Options and restricted stock units forfeited,    cancelled, or expired		845,154
Shares available for grant at December 31, 2023		3,158,094

Summary of Option Activity

A summary of option activity for the year ended December 31, 2023 was as follows:

	Shares			Weighted- Average Exercise price per Share		Weighted- Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Balance at December 31, 2022		4,033,522		$	11.75	8.40		$	827
Granted		3,696,783			3.34
Exercised		(128,534	)		1.29
Forfeited/expired		(780,902	)		6.72
Balance at December 31, 2023		6,820,869		$	7.97		8.24	$	387
Expected to vest		4,000,554		$	6.41		8.72	$	—
Options exercisable		2,820,315		$	10.18		7.56	$	387

Summary of Stock Options Outstanding and Exercisable

Stock options outstanding and exercisable consisted of the following at December 31, 2023:

Exercise Price ($)	Shares Outstanding		Shares Exercisable
0.16 to 4.22		3,709,593		1,005,785
8.93 to 19.94		2,759,028		1,615,758
20.55 to 29.41		352,248		198,772
Total		6,820,869		2,820,315

Restricted Stock Units (RSUs)

Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]

Summary of Restricted Stock Activity

A summary of RSU activity for the year ended December 31, 2023 is as follows:

	Share Equivalent			Weighted- Average Grant Date Fair Value
Non-vested at December 31, 2022		236,339		$	15.16
Granted		138,000			3.28
Cancelled		(64,252	)		7.63
Vested		—			—
Non-vested at December 31, 2023		310,087		$	11.44