Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2020

Share-based Payment Arrangement [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,
	2020		2019		2020		2019
	(in thousands)
Research and development	$	933	$	1,229	$	1,479	$	2,413
General and administrative		2,684		529		3,056		1,044
Total stock-based compensation	$	3,617	$	1,758	$	4,535	$	3,457

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

	Three Months Ended June  30,						Six Months Ended June  30,
	2020			2019			2020			2019
Expected term (in years)		5.3 – 7.0			5.3 – 6.1			5.3 – 7.0			5.3 – 6.9
Expected volatility		88 – 90	%		90 – 92	%		88 – 90	%		90 – 94	%
Risk-free interest rate		0.4 – 0.5	%		1.9 – 2.3	%		0.4 – 1.2	%		1.9 – 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		(2,055,130	)		2,055,130			13.51
Options forfeited/cancelled		30,098			(30,098	)		57.08
Outstanding — June 30, 2020		2,913,669			2,351,055		$	25.30		7.98	$	9
Exercisable — June 30, 2020					358,948		$	78.02		3.93	$	—
Vested and expected to vest — June 30, 2020					2,152,111		$	26.27		8.56	$	9