Stock-based Compensation Expense (Tables)

6 Months Ended

Jun. 30, 2024

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity during the six months ended June 30, 2024:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2023		9,972,217		$	8.44		8.49	$	15,114,074
Granted		2,964,458			7.55
Exercised		(239,727	)		7.37
Cancelled		(149,554	)		8.89
Outstanding at June 30, 2024		12,547,394		$	8.24		8.43	$	12,343,417
Exercisable at June 30, 2024		4,705,847		$	10.54		7.47	$	3,434,971

Weighted Average Assumptions Used to Calculate Fair Value of Stock Option

The fair value of stock options granted during the three and six months ended June 30, 2024 and 2023 has been calculated on the date of grant using the following weighted average assumptions:

	Three Months Ended June 30, 2024			Three Months Ended June 30, 2023			Six Months Ended June 30, 2024			Six Months Ended June 30, 2023
Risk-free interest rate		4.3	%		3.5	%		4.1	%		3.6	%
Expected dividend yield		0.0	%		0.0	%		0.0	%		0.0	%
Expected term (years)		5.8			6.1			6.0			6.1
Expected stock price volatility		102.8	%		100.2	%		103.1	%		98.2	%

Summary of Stock-Based Compensation Expense Recognized

The total compensation cost recognized in the statements of operations and comprehensive loss associated with all stock-based compensation awards granted by the Company is as follows (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2024		2023		2024		2023
General and administrative	$	2,681	$	2,391	$	5,268	$	5,799
Research and development		1,663		972		2,992		1,817
Total stock-based compensation expense	$	4,344	$	3,363	$	8,260	$	7,616

Grants Outside of 2016 Stock Incentive Plan and 2019 Stock Incentive Plan

Summary of Restricted Stock Activity

The Company has also granted restricted stock units. The shares of common stock underlying restricted stock units typically vest over a four-year period. The shares of common stock are recorded in stockholders’ equity as they vest.

The following table summarizes the Company’s restricted stock unit activity during the six months ended June 30, 2024:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested at December 31, 2023		75,017		$	10.77
Granted		70,445			6.95
Vested		(22,326	)		10.65
Cancelled		(7,141	)		9.32
Unvested at June 30, 2024		115,995		$	8.56