Stock-Based Compensation (Tables)

9 Months Ended

Sep. 30, 2020

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,
	2020		2019		2020		2019
	(in thousands)
Research and development	$	1,174	$	856	$	2,653	$	3,269
General and administrative		900		424		3,956		1,468
Total stock-based compensation	$	2,074	$	1,280	$	6,609	$	4,737

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

	Three Months Ended September 30,						Nine Months Ended September 30,
	2020			2019			2020			2019
Expected term (in years)	5.5 – 7.0			6.0 – 6.1			5.3 – 7.0			5.3 – 6.9
Expected volatility		88 – 89	%		91	%		88 – 90	%		90 – 94	%
Risk-free interest rate		0.3 – 0.5	%		1.6	%		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, 2019		51,514			326,023		$	102.56		7.45	$	11
Awards authorized		4,887,187
Options granted		(3,965,439	)		3,965,439			12.84
Options forfeited/cancelled		45,295			(45,295	)		45.35
Outstanding — September 30, 2020		1,018,557			4,246,167		$	19.38	8.71		$	3
Exercisable — September 30, 2020					413,241		$	70.97	3.95		$	—
Vested and expected to vest — September 30, 2020					3,216,792		$	21.53	8.83		$	3