Stock-Based Compensation (Tables)

6 Months Ended

Jun. 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 June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
	(in thousands)
Research and development	$	1,776	$	933	$	3,439	$	1,479
General and administrative		1,452		2,684		2,720		3,056
Total stock-based compensation	$	3,228	$	3,617	$	6,159	$	4,535

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 six months ended June 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 June 30,						Six Months Ended June 30,
	2021			2020			2021			2020
Expected term (in years)	5.3 – 6.1			5.3 – 7.0			5.3 – 7.0			5.3 – 7.0
Expected volatility		83 – 85	%		88 – 90	%		83 – 85	%		88 – 90	%
Risk-free interest rate		0.8 – 1.1	%		0.4 – 0.5	%		0.5 – 1.1	%		0.4 – 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		(869,460	)		869,460			16.87
Options exercised		—			(30,218	)		13.29
Options forfeited/cancelled		199,701			(199,701	)		17.43
Outstanding — June 30, 2021		1,393,176			4,786,469		$	19.11	8.56		$	26,418
Exercisable — June 30, 2021					955,199		$	39.28		6.10	$	4,149
Vested and expected to vest — June 30, 2021					4,261,231		$	19.96	8.48		$	22,573