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Stock Based Compensation
 3 Months Ended
Mar. 31, 2023
Stock Based Compensation [Abstract]  
Stock Based Compensation
9.
Stock Based Compensation


Stock Option Valuation



The weighted average assumptions that the Company used in the Black-Scholes pricing model to determine the fair value of the stock options granted to employees, non-employees and directors were as follows:

 
Three Months Ended March 31,
  
 
  
2023
    
2022
  
Risk-free interest rate
    
4.02
%
    
1.88
%
Expected term (in years)
    
5.88
      
5.86
  
Expected volatility
    
73.54
%
    
74.07
%
Expected dividend yield
    
0.00
%
    
0.00
%
Exercise price
  
$
20.17
    
$
17.85
  
Fair value of common stock
  
$
20.17
    
$
17.85
  


The following table summarizes stock option activity for the three months ended March 31, 2023, under the Second Amended and Restated 2014 Stock Option and Incentive Plan:

      Weighted   Weighted      
      Average   Average   Aggregate  
  Number of   Exercise   Contractual   Intrinsic  
  Shares   Price   Term (Years)   Value  
 
                
Outstanding as of December 31, 2022
    
13,138,870
    
$
14.52
      
5.46
    
$
118,767
  
Granted
    
1,792,097
      
20.36
      
6.86
          
Exercised
    
(88,429
)
    
12.59
              
631
  
Cancelled
    
(191,148
)
    
33.99
                  
Outstanding as of March 31, 2023
    
14,651,390
    
$
14.99
      
5.85
    
$
97,263
  
 
                                
Options vested and exercisable as of March 31, 2023
    
10,586,141
    
$
12.60
      
4.52
    
$
92,808
  
Options unvested as of March 31, 2023
    
4,065,249
    
$
21.24
      
9.32
     $ 4,455  



The weighted average grant-date fair value per share of stock options granted during the three months ended March 31, 2023, and 2022 was $13.50 and $11.60, respectively.


The total fair value of options vested during the three months ended March 31, 2023 and 2022 was $11.4 million and $12.5 million, respectively.


Restricted Stock Units (“RSU”)



The following table summarizes the Company’s RSU activity for the three months ended March 31, 2023:

     
Weighted
  
      Average  
 
Number of
  
Grant Date
  
  Shares  
Fair Value
  
         
Unvested as of December 31, 2022
    
992,874
    
$
16.49
  
Granted
    
764,204
      
20.23
  
Vested(1)
    
(126,145
)
    
17.37
  
Forfeited
    
(8,476
)
    
17.19
  
Unvested as of March 31, 2023
    
1,622,457
    
$
18.18
  


(1) Common stock issued is net of 85 shares related to taxes.


Stock-based Compensation


Stock-based compensation expense recognized by award type was as follows:


  
Three Months Ended March 31,
  
 
  
2023
    
2022
  
 
      
Stock options
  
$
6,985
    
$
5,961
  
Restricted stock units
    
1,930
      
309
  
Total stock-based compensation expense
  
$
8,915
    
$
6,270
  


Stock-based compensation expense by classification included within the consolidated statements of operations and comprehensive loss was as follows:


  
Three Months Ended March 31,
  
 
  
2023
    
2022
  
 
      
Research and development
  
$
3,819
    
$
2,318
  
General and administrative
    
5,096
      
3,952
  
Total stock-based compensation expense
  
$
8,915
    
$
6,270
  


As of March 31, 2023, the Company had an aggregate of $75.4 million of unrecognized stock-based compensation expense related to both stock options and RSU grants, which is expected to be recognized over the weighted average period of 1.52 years.