Stock-based compensation (Tables)

A summary of stock option activity is as follows:


	Number of Shares	Weighted- Average Exercise Price
Outstanding at January 1, 2024	10,902,845	$	7.42
Granted	2,289,529	$	3.02
Exercised	(12,117)	$	3.21
Cancelled/forfeited	(469,818)	$	8.36
Outstanding at March 31, 2024	12,710,439	$	6.59

Exercisable at March 31, 2024	6,626,590	$	8.21

A summary of the RSU activity is as follows:


			Number of Shares
Unvested at January 1, 2024			3,834,108
Granted			2,290,749
Vested			(594,067)
Forfeited(a)			(244,419)
Unvested at March 31, 2024			5,286,371

(a) Includes 14,467 rescinded RSUs.

The following table presents stock-based compensation expense by award type included within the Company’s condensed consolidated statements of operations and comprehensive loss:


	Three Months Ended March 31,
(in thousands)	2024		2023
Stock options	$	2,597	$	4,219
Restricted stock units and other stock awards	1,949		1,893
Employee stock purchase plan	114		295
Stock-based compensation expense included in total operating expenses	$	4,660	$	6,407

The following table presents stock-based compensation expense as reflected in the Company’s condensed consolidated statements of operations and comprehensive loss:


	Three Months Ended March 31,
(in thousands)	2024		2023
Research and development	$	2,528	$	3,332
General and administrative	2,132		3,075
Stock-based compensation expense included in total operating expenses	$	4,660	$	6,407

The fair value of each option award is estimated on the date of grant using the Black-Scholes option pricing model with the following weighted average assumptions:


	Three Months Ended March 31,
	2024		2023
Risk-free interest rate	3.9	%	3.6	%
Expected dividend yield	—	%	—	%
Expected term (years)	6.06		6.07
Expected stock price volatility	115	%	99	%