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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 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 March 31,
20232022
Risk-free interest rate3.6 %2.0 %
Expected term (in years)6.06.1
Expected volatility90.8 %94.5 %
Expected dividend yield0 %0 %
Summary of Stock Option Activity The following table summarizes the Company’s stock option activity for the three months ended March 31, 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 
Granted834,900 0.90 
Forfeited and Expired(26,080)8.68 
Outstanding as of March 31, 20232,830,300 $5.18 8.0$— 
Exercisable as of March 31, 20231,163,500 $9.72 6.1$— 
Vested and expected to vest as of March 31, 20232,210,011 $6.20 7.6$— 
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 three months ended March 31, 2023:
Number of
Shares
Unvested as of December 31, 20221,680,563 
Granted4,694,691 
Vested(540,238)
Unvested as of March 31, 20235,835,016 
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 March 31,
(in thousands)20232022
Research and development expense$831 $704 
Selling, general and administrative expense814 755 
Total stock-based compensation$1,645 $1,459 
Share-Based Payment Arrangement, Option and Stock Appreciation Rights, Activity
The calculation of the fair value of the outstanding SARs as of March 31, 2023 includes the closing price of the Company’s common stock of $0.87 and the following assumptions on a weighted average basis:

March 31, 2023
Risk free rate3.6 %
Expected term (years)5.61
Expected volatility90.8 %
Expected dividend yield— %
Expected forfeiture rate22 %