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Stock-based Compensation Plans (Tables)
12 Months Ended
Dec. 31, 2022
Share-Based Payment Arrangement [Abstract]  
Summary of Stock Option Activity The following table summarizes stock option activity for the twelve months ended December 31, 2022 and 2021 under the 2019 Plan:
OptionsWeighted-
Average
Exercise
Price
Weighted-
Average
Remaining
Contractual
Term (Years)
Aggregate
Intrinsic
Value
Outstanding as of January 1, 20222,157,460 $13.54 
Granted1,886,263 $8.73 
Exercised(852)$5.67 
Forfeited or expired(251,183)$10.79 
Outstanding as of December 31, 20223,791,688 $11.33 8.35$375 
Options vested and expected to vest as of December 31, 20223,791,688 $11.33 8.35$375 
Options exercisable as of December 31, 20221,398,490 $13.33 7.50$— 
OptionsWeighted-
Average
Exercise
Price
Weighted-
Average
Remaining
Contractual
Term (Years)
Aggregate
Intrinsic
Value
Outstanding as of January 1, 20211,117,160 $12.96 
Granted1,193,809 $14.08 
Exercised— $— 
Forfeited or expired(153,509)$13.50 
Outstanding as of December 31, 20212,157,460 $13.54 8.74$302,155 
Options vested and expected to vest as of December 31, 20212,157,460 $13.54 8.74$302,155 
Options exercisable as of December 31, 2021752,954 $13.48 8.15$79,431 
Schedule of Valuation Assumptions Used The following are the underlying assumptions used in the Black-Scholes-Merton option pricing model to determine the fair value of stock options granted to employees and to non-employees under this stock plan:
20222021
Risk-free interest rate2.09%0.93%
Expected dividend yield0%0%
Expected volatility97.8%93.0%
Expected term of options (years)6.06.0
Schedule of Stock-based Compensation Expense for Stock Awards Recognized
Total stock-based compensation expense for all stock awards recognized in the accompanying audited consolidated statements of operations is as follows (in thousands):
 Year
 Ended December 31,
 20222021
Research and development$3,219 $1,760 
General and administrative4,710 4,189 
Total$7,929 $5,949