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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2024
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,
2024202320242023
Risk-free interest rate4.4 %4.0 %4.2 %3.8 %
Expected term (in years)6.16.16.16.0
Expected volatility96.5 %93.4 %96.0 %92.1 %
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, 2024:
Number of
Shares		Weighted
Average
Exercise
Price		Weighted
Average
Contractual
Term (Years)		Aggregate
Intrinsic
Value (in thousands)
Outstanding as of December 31, 20236,008,541 $2.97 8.6$24 
Granted4,373,875 1.09 
Forfeited and Expired(476,118)2.12 
Outstanding as of June 30, 20249,906,298 $2.18 8.8$— 
Exercisable as of June 30, 20241,813,063 $6.55 6.2$— 
Vested and expected to vest as of June 30, 20247,672,558 $2.47 8.6$— 
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 six months ended June 30, 2024:
Number of
Shares
Unvested as of December 31, 20233,118,824 
Granted6,242,459 
Vested(829,851)
Forfeited(313,961)
Unvested as of June 30, 20248,217,471 
Summary of Stock-Based Compensation Expense Classification
Stock-based compensation expense was classified in the condensed consolidated statements of operations and comprehensive income (loss) as follows:
Three Months Ended June 30,Six Months Ended June 30,
(in thousands)2024202320242023
Research and development expense$1,182 $1,120 $1,965 $1,951 
Selling, general and administrative expense1,246 1,022 2,202 1,836 
Total stock-based compensation$2,428 $2,142 $4,167 $3,787 
Share-Based Payment Arrangement, Option and Stock Appreciation Rights, Activity Stock Appreciation Rights— On February 13, 2024, (the “Grant Date”), the compensation committee of the Board of Directors approved the grant of stock appreciation rights (“SARs”), pursuant to the 2017 Plan, to the Company’s executive officers. The SARs have a measurement price per SAR equal to $0.92, the closing price per share of the Company’s common stock on the Grant Date, and each grant of SARs will have a maximum term of ten years from the Grant Date. Unless otherwise determined by the Board of Directors, the SARs will be settled in cash upon exercise. The settlement value will be based on the difference between the closing price of the Company’s common stock on the date of settlement less $0.92 multiplied by the number of SARs exercised. The SARs will vest and become exercisable in equal annual installments on the first, second, and third anniversaries of the Grant Date, subject to the recipient remaining an employee of the Company through and including each applicable vesting date.