Stock Option and Grant Plan (Tables)

12 Months Ended

Dec. 31, 2023

Share-Based Payment Arrangement [Abstract]

Summary of Stock-based Compensation Expense

Stock-based compensation expense recorded as research and development and general and administrative expenses in the consolidated statements of operations and other comprehensive loss is as follows (in thousands):


	Years Ended December 31,
	2023		2022		2021
Research and development	$	57,812	$	52,004	$	26,644
General and administrative		40,835		32,317		16,926
Total stock-based compensation expense	$	98,647	$	84,321	$	43,570

Schedule of Assumptions Used in Black-Scholes Option-Pricing Model for Stock Options Granted

The assumptions used in the Black-Scholes option-pricing model for stock options granted were:


	Years Ended December 31,
	2023			2022			2021
Expected volatility	76.3-78.8%			74.8-77.3%			71.4-76.7%
Weighted-average risk-free interest rate		3.73	%		2.27	%		1.11	%
Expected dividend yield		0.00	%		0.00	%		0.00	%
Expected term (in years)		6.03			6.08			6.12

Summary of Option Activity Under Equity Award Plans

The following table provides a summary of option activity under the Company’s equity award plans:


	Number of options			Weighted average exercise price		Weighted average remaining contractual life (years)		Aggregate intrinsic value (1) (in thousands)
Outstanding at December 31, 2022		7,548,392		$	41.77		7.5	$	110,990
Granted		2,356,838			39.62
Exercised		(797,709	)		7.69
Forfeitures		(831,488	)		60.18
Outstanding at December 31, 2023		8,276,033			42.59		7.2		51,653
Exercisable as of December 31, 2023		4,818,116		$	37.81		6.3	$	45,895

(1)

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money as of December 31, 2023 and 2022.

Summary of Restricted Stock Activity

The following summarizes the Company’s restricted stock activity:


	Shares			Weighted- average grant date fair value
Unvested as of December 31, 2022		1,692,819		$	65.49
Issued		2,050,025			28.69
Vested		(469,531	)		67.77
Forfeited		(346,161	)		52.49
Unvested as of December 31, 2023		2,927,152		$	40.89