Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

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

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

	Year ended December 31,
	2020		2019		2018
	(in thousands)
Research and development	$	4,316	$	3,873	$	4,499
General and administrative		5,154		1,822		2,297
Total stock-based compensation	$	9,470	$	5,695	$	6,796

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

The estimated grant-date fair value of all the Company’s stock-based awards was calculated using the Black-Scholes option pricing model, based on the following assumptions:

	Year Ended December 31,
	2020			2019			2018
Expected term (in years)	5.3 – 7.0			5.3 – 6.9			5.3 – 7.0
Expected volatility		87 – 90	%		89 – 94	%		88 – 91	%
Risk-free interest rate		0.3 – 1.2	%		1.6 – 2.6	%		2.6 – 3.1	%
Expected dividend rate		—	%		—	%		—	%

Summary of Stock-Based Compensation Activity

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				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		(4,074,939	)		4,074,939			12.88
Options forfeited/cancelled		254,034			(254,034	)		20.73
Outstanding — December 31, 2020		1,117,796			4,146,928		$	19.45	8.84		$	12,227
Exercisable — December 31, 2020					466,879		$	65.48		3.90	$	555
Vested and expected to vest —    December 31, 2020					3,594,690		$	20.57	8.71		$	10,084