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Stock-based Compensation (Tables)
 12 Months Ended
Dec. 31, 2023
Stock-based Compensation [Abstract]  
Assumptions Used to Value Option Grants The assumptions used to value option grants to employees and nonemployees for the years ended December 31, 2023, 2022 and 2021 were as follows:

​
​
2023
​
2022
  
2021
Expected term (in years)
   5.0 – 6.2   5.5 – 6.4   5.5 – 6.3
Risk-free interest rate
​
3.4% – 4.4%
​
1.7% – 4.2%
  
0.9% – 1.4%
Expected volatility
  62% - 64%   58% - 64%   54% - 70%
Expected dividend yield
​
-
​
-
  
-
Weighted average fair value/share at grant date
 $
1.01
$
1.51
 $
5.25
Stock Option Activity
A summary of the stock option activity is presented in the table below:

   
Number
of Options
    
Weighted Average
Exercise Price
(per share)
    
Weighted Average
Remaining
Contractual Life
(in years)
    
Aggregate
Intrinsic Value
(in thousands)
  
Outstanding at December 31, 2020
     7,369,541     $ 2.37       6.77     $
4,094  
Granted
     3,514,510    
8.89                  
Exercised
     (2,661,252 )  
2.11                  
Forfeited
     (495,827 )  
6.84                  
Outstanding at December 31, 2021
     7,726,972     $ 5.14       7.58     $
24,511  
Granted
     14,271,330    
3.04                  
Exercised
     (1,123,249 )  
2.45                  
Forfeited
     (1,447,298 )  
5.43                  
Outstanding at December 31, 2022
    
19,427,755
    
$
3.69
      
8.68
    
$
378
  
Granted
    
10,138,730
    

1.79
                  
Exercised
    
(140,559
)
  

2.55
                  
Forfeited
    
(6,914,026
)
  

3.86
                  
Outstanding at December 31, 2023
    
22,511,900
    
$
2.79
      
8.22
    
$
3,194
  
Exercisable at December 31, 2023
    
6,799,386
    
$
3.73
      
6.45
    
$
406
  
Vested and expected to vest at December 31, 2023
    
18,629,173
    
$
2.87
      
8.06
    
$
2,505
  
RSU Activity
The number of shares and weighted average grant date fair values of restricted non-vested common stock at the beginning and end of 2023, 2022 and 2021, as well as restricted stock units granted, vested, and forfeited during the year were as follows:

   
Number of Shares
Underlying RSUs
    
Weighted Average
Grant-Date Fair
Value (per share)
  
Nonvested RSUs at December 31, 2020
     -     $ -  
Granted
     4,861,315    
8.03  
Vested
     (274,343 )  
8.53  
Forfeited
     -    
-  
Nonvested RSUs at December 31, 2021
     4,586,972     $ 8.00  
Granted
     66,666    
3.00  
Vested
     (798,575 )  
8.24  
Forfeited
     (1,836,614 )  
7.26  
Nonvested RSUs at December 31, 2022
    
2,018,449
    
$
8.41
  
Granted
    
786,938
    

1.67
  
Vested
    
(1,685,101
)
  

8.55
  
Forfeited
    
(273,117
)
  

5.89
  
Nonvested RSUs at December 31, 2023
    
847,169
    
$
2.68
  
Stock-Based Compensation
Stock-based compensation is allocated to Research and development and Selling, general and administrative operating expenses in the Consolidated Statements of Operations and Comprehensive Loss. Stock-based compensation expense for the years ended December 31, 2023, 2022 and 2021 is as follows (in thousands):

   
2023
    
2022
    
2021
  
Research and development
  
$
2,961
    
$
4,548
    
$
5,718
  
Selling, general and administrative
    
5,555
      
6,658
      
19,200
  
Total stock-based compensation
  
$
8,516
    
$
11,206
    
$
24,918