Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2021

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

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

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
	(in thousands)
Research and development	$	1,823	$	1,174	$	5,262	$	2,653
General and administrative		1,457		900		4,177		3,956
Total stock-based compensation	$	3,280	$	2,074	$	9,439	$	6,609

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

The fair values of the Company’s stock-based awards granted during the three and nine months ended September 30, 2021 and 2020 were estimated as of the grant date using the Black-Scholes option pricing model, based on assumptions as follows:

	Three Months Ended September 30,						Nine Months Ended September 30,
	2021			2020			2021			2020
Expected term (in years)	6.0 – 6.1			5.5 – 7.0			5.3 – 7.0			5.3 – 7.0
Expected volatility		82 – 83	%		88 – 89	%		82 – 85	%		88 – 90	%
Risk-free interest rate		0.8 – 1.2	%		0.3 – 0.5	%		0.5 – 1.2	%		0.3 – 1.2	%
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		(1,048,610	)		1,048,610			17.39
Options exercised		—			(51,827	)		13.09
Options forfeited/cancelled		263,444			(263,444	)		17.66
Outstanding — September 30, 2021		1,277,769			4,880,267		$	19.17	8.36		$	37,118
Exercisable — September 30, 2021					1,249,757		$	33.16		6.54	$	8,510
Vested and expected to vest — September 30, 2021					4,364,027		$	20.00	8.29		$	32,074