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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2022
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,



2022

20212020
Risk-free interest rate


3.4 %

1.0 %0.6 %
Expected term (in years)


6.06

5.966.01
Expected volatility


96.0 %

97.3 %94.9 %
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, 2022:

Number of
Shares

Weighted
Average
Exercise
Price

Weighted
Average
Contractual
Term
(in Years)

Aggregate Intrinsic
Value
(in Thousands)
Outstanding as of December 31, 20211,916,051 

$10.01 

7.8

$— 
Granted759,200 

1.76 




Forfeited(653,771)

9.75 




Outstanding as of December 31, 20222,021,480 

$6.99 

6.5

$1 
Exercisable as of December 31, 20221,114,604 

$9.93 

6.3

$— 
Vested and expected to vest as of December 31, 20221,722,110 

$7.77 

7.3

$— 
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, 2022:
Number of Shares
Unvested at December 31, 2021925,101 
Granted1,814,425 
Vested(384,814)
Forfeited(674,149)
Unvested at December 31, 20221,680,563 
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,

2022

20212020
Research and development expense$2,534 

$2,723 $2,316 
Selling, general and administrative expense2,665 

3,457 3,112 
Total stock-based compensation$5,199 

$6,180 $5,428