Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2021

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,
	2021		2020		2019
Risk-free interest rate	1.0	%	0.6	%	2.0	%
Expected term (in years)	5.96		6.01		5.99
Expected volatility	97.3	%	94.9	%	88.5	%
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, 2021:

	Number of Shares	Weighted Average Exercise Price		Weighted Average Contractual Term (in Years)	Aggregate Intrinsic Value (in Thousands)
Outstanding as of December 31, 2020	1,874,514	$	12.94	8.3	$	7
Granted	388,050	6.69
Exercised	(5,860)	7.08
Forfeited	(340,653)	22.43
Outstanding as of December 31, 2021	1,916,051	$	10.01	7.8	$	—
Exercisable as of December 31, 2021	1,007,236	$	11.81	6.8	$	—
Vested and expected to vest as of December 31, 2021	1,689,992	$	10.05	7.6	$	—

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, 2021:

	Number of Shares
Unvested at December 31, 2020	572,460
Granted	674,478
Vested	(205,185)
Forfeited	(116,652)
Unvested at December 31, 2021	925,101

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,
	2021		2020		2019
Research and development expense	$	2,723	$	2,316	$	909
General and administrative expense	3,457		3,112		1,141
Total stock-based compensation	$	6,180	$	5,428	$	2,050