Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 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 March 31,
	2021		2020
	(in thousands)
Research and development	$	1,663	$	546
General and administrative		1,268		372
Total stock-based compensation	$	2,931	$	918

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

	Three Months Ended March 31,
	2021			2020
Expected term (in years)	5.5 – 7.0			5.7 – 6.7
Expected volatility		84 – 85	%		88 – 90	%
Risk-free interest rate		0.5 – 1.1	%		0.6 – 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		(650,460	)		650,460			16.52
Options exercised	-				(21,662	)		13.28
Options forfeited/cancelled		108,553			(108,553	)		14.72
Outstanding — March 31, 2021		1,521,028			4,667,173		$	19.18	8.76		$	16,794
Exercisable — March 31, 2021					755,413		$	46.18	5.66		$	1,905
Vested and expected to vest — March 31, 2021					4,131,935		$	20.09	8.67		$	14,075