Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Stock-based Compensation Expense for Employees and Non-employees

In the accompanying consolidated statement of operations, the Company recognized stock-based compensation expense for its employees and non-employees as follows:

	Year ended December 31,
	2021		2020		2019
	(in thousands)
Research and development	$	7,197	$	4,316	$	3,873
General and administrative		5,693		5,154		1,822
Total stock-based compensation	$	12,890	$	9,470	$	5,695

Schedule of Estimated Grant-date Fair Value of Stock-based Awards Using Black-Scholes Option Pricing Model Assumptions

The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions:

	Year Ended December 31,
	2021			2020			2019
Expected term (in years)	5.3 – 7.0			5.3 – 7.0			5.3 – 6.9
Expected volatility		82 – 85	%		87 – 90	%		89 – 94	%
Risk-free interest rate		0.5 – 1.3	%		0.3 – 1.2	%		1.6 – 2.6	%
Expected dividend rate		—	%		—	%		—	%

Summary of Stock-Based Compensation Activity

A summary of activity under the 2008 Plan, 2015 Plan and 2018 Plan and related information is as follows:

				Options Outstanding
	Shares Available for Grant			Number of Shares Outstanding			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value of Outstanding Options (in thousands)
Outstanding — December 31, 2020		1,117,796			4,146,928		$	19.45		8.84	$	12,227
Awards authorized		945,139
Options granted		(1,134,860	)		1,134,860			17.78
Options exercised		—			(64,847	)		13.06
Options forfeited/cancelled		279,752			(279,752	)		17.99
Outstanding — December 31, 2021		1,207,827			4,937,189		$	19.23	8.14		$	36,080
Exercisable — December 31, 2021					1,421,885		$	30.98		6.56	$	9,682
Vested and expected to vest —    December 31, 2021					4,422,699		$	20.06	8.07		$	31,146