Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Stock Option Valuation

The following table presents, on a weighted average basis, the assumptions used in the Black-Scholes option-pricing model to determine the grant-date fair value of stock options granted:

	Year Ended December 31,
	2020		2019		2018
Risk-free interest rate	0.6	%	2.0	%	2.8	%
Expected term (in years)	6.01		5.99		5.94
Expected volatility	94.9	%	88.5	%	86.0	%
Expected dividend yield	0	%	0	%	0	%

Summary of Stock Option Activity

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2020:

	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (Years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2019	1,297,029	$	17.05	8.4	$	1,286
Granted	864,616	8.06
Exercised	(17,689)	7.25
Forfeited	(269,442)	17.43
Outstanding as of December 31, 2020	1,874,514	$	12.94	8.3	$	7
Exercisable as of December 31, 2020	682,632	$	17.42	7.0	$	—
Vested and expected to vest as of December 31, 2020	1,404,246	$	13.35	8.1	$	4

Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity

The following table summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2020:

	Number of Shares
Unvested at December 31, 2019	101,773
Granted	692,331
Vested	(134,074)
Forfeited	(87,570)
Unvested at December 31, 2020	572,460

Summary of Stock-Based Compensation Expense Classification

Stock-based compensation expense was classified in the consolidated statements of operations as follows:

	Year Ended December 31,
	2020		2019		2018
Research and development expense	$	2,316	$	909	$	258
General and administrative expense	3,112		1,141		501
Total stock-based compensation	$	5,428	$	2,050	$	759