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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2023
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 to employees, directors and non-employees.
Three Months Ended June 30,Six Months Ended June 30,
2023202220232022
Risk-free interest rate4.0 %3.0 %3.8 %2.1 %
Expected term (in years)6.16.16.06.1
Expected volatility93.4 %95.1 %92.1 %94.6 %
Expected dividend yield0 %0 %0 %0 %
Summary of Stock Option Activity The following table summarizes the Company’s stock option activity for the six months ended June 30, 2023:
Number of
Shares		Weighted
Average
Exercise
Price		Weighted
Average
Contractual
Term (Years)		Aggregate
Intrinsic
Value (in thousands)
Outstanding as of December 31, 20222,021,480 $6.99 6.5$1 
Granted1,597,400 1.36 
Exercised(2,031)1.50 
Forfeited and Expired(33,663)8.47 
Outstanding as of June 30, 20233,583,186 $4.47 8.3$1,062 
Exercisable as of June 30, 20231,222,051 $9.57 6.0$15 
Vested and expected to vest as of June 30, 20232,726,046 $5.37 7.9$666 
Share-based Payment Arrangement, Restricted Stock and Restricted Stock Unit, Activity
Restricted Stock Units— The following table summarizes the Company's restricted stock unit activity for the six months ended June 30, 2023:
Number of
Shares
Unvested as of December 31, 20221,680,563 
Granted5,064,691 
Vested(644,179)
Forfeited(241,843)
Unvested as of June 30, 20235,859,232 
Summary of Stock-Based Compensation Expense Classification Stock-based compensation expense was classified in the condensed consolidated statements of operations as follows:
Three Months Ended June 30,Six Months Ended June 30,
(in thousands)2023202220232022
Research and development expense$1,120 $710 $1,951 $1,414 
Selling, general and administrative expense1,022 812 1,836 1,567 
Total stock-based compensation$2,142 $1,522 3,787 2,981 
Share-Based Payment Arrangement, Option and Stock Appreciation Rights, Activity
The calculation of the fair value of the outstanding SARs as of June 30, 2023 includes the closing price of the Company’s common stock of $1.94 and the following assumptions on a weighted average basis:

June 30, 2023
Risk free rate4.1 %
Expected term (years)5.36
Expected volatility94.8 %
Expected dividend yield— %
Expected forfeiture rate22 %