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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 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 to employees, directors and non-employees.
Three Months Ended March 31,
20222021
Risk-free interest rate2.0 %0.9 %
Expected term (in years)6.16.0
Expected volatility94.5 %101.4 %
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, 2022:
Number of
Shares		Weighted
Average
Exercise
Price		Weighted
Average
Contractual
Term (Years)		Aggregate
Intrinsic
Value (in thousands)
Outstanding as of December 31, 20211,916,051 $10.01 7.8$— 
Granted232,850 1.85 
Forfeited and Expired(99,743)23.52 
Outstanding as of March 31, 20222,049,158 $8.43 7.9$11 
Exercisable as of March 31, 20221,029,146 $10.44 6.8$— 
Vested and expected to vest as of March 31, 20221,798,855 $8.84 7.7$8 
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, 2022:
Number of
Shares
Unvested as of December 31, 2021925,101 
Granted1,242,756 
Vested(192,255)
Forfeited(34,814)
Unvested as of March 31, 20221,940,788 
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)20222021
Research and development expense$704 $577 
Selling, general and administrative expense755 681 
Total stock-based compensation$1,459 $1,258