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Stock Based Compensation (Tables)
3 Months Ended
Mar. 31, 2022
Stock Based Compensation [Abstract]  
Weighted-Average Assumptions for Stock Options

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,
 
 
 
2022
   
2021
 
             
Risk-free interest rate
   
1.88
%
   
0.62
%
Expected term (in years)
   
5.86
     
5.87
 
Expected volatility
   
74.07
%
   
69.41
%
Expected dividend yield
   
0.00
%
   
0.00
%
Exercise price
 
$
17.85
   
$
36.99
 
Fair value of common stock
 
$
17.85
   
$
36.99
 
Stock Option Activity

The following table summarizes stock option activity for the three months ended March 31, 2022, under the Second Amended and Restated 2014 Stock Option and Incentive Plan:

 
 
Number of
Shares
   
Weighted
Average
Exercise
Price
   
Weighted
Average
Contractual
Term (Years)
   
Aggregate
Intrinsic
Value
 
 
                       
Outstanding as of December 31, 2021
   
11,143,761
   
$
14.51
     
5.95
   
$
128,817
 
Granted
   
1,336,206
     
17.85
     
6.93
         
Exercised
   
(6,000
)
   
12.65
            $
20
 
Forfeited
   
(426,668
)
   
34.69
                 
Outstanding as of March 31, 2022
   
12,047,299
   
$
14.17
     
5.93
   
$
84,961
 
 
                               
Options vested and exercisable as of March 31, 2022
   
9,141,380
   
$
9.12
     
4.88
   
$
84,457
 
Options unvested as of March 31, 2022
   
2,905,919
   
$
30.03
     
9.22
         
RSU Activity

The following table summarizes the Company’s RSU activity for the three months ended March 31, 2022:


 
Number of
Shares
   
Weighted
Average
Grant Date
Fair Value
 
             
Unvested as of December 31, 2021
   
23,500
   
$
30.61
 
Granted
   
467,172
    $
17.06
 
Exercised
   
(10,168
)
  $
62.32
 
Forfeited
   
(22,795
)
  $
18.03
 
Unvested as of March 31, 2022
   
457,709
    $
17.24
 
Stock-Based Compensation Expense by Award Type

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


 
Three Months Ended March 31,
 
 
 
2022
   
2021
 
 
     
Stock options
 
$
5,961
   
$
7,826
 
Restricted stock units
   
309
     
74
 
Total stock-based compensation expense
 
$
6,270
   
$
7,900
 
Stock-Based Compensation Expense

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


 
Three Months Ended March 31,
 
 
 
2022
   
2021
 
 
     
Research and development
 
$
2,318
   
$
2,916
 
General and administrative
   
3,952
     
4,984
 
Total stock-based compensation expense
 
$
6,270
   
$
7,900
 
Warrants Outstanding and Changes in Warrants to Purchase Common Stock

A summary of the warrants outstanding as of March 31, 2022 is as follows:


Price
     
Outstanding
 
Grant Date
Expiration Date
 
24.42
       
7,051
 
June 28, 2013
June 28, 2023
 
57.11
       
603,386
 
December 21, 2020
December 21, 2030
 
33.63
       
301,291
 
August 9, 2021
August 9, 2031
 
22.51
       
153,155
 
December 17, 2021
December 17, 2031
 
22.51
       
153,155
 
December 17, 2021
December 17, 2031
      Total    
1,218,038
       



The following table below is the summary of changes in warrants to purchase common stock for the three months ended March 31, 2022:


   
Number of
Warrant Shares
Outstanding and
Exercisable
   
Exercise
Price per
Share
 
             
Balance as of December 31, 2020
   
610,437
       
Granted August 2021
   
301,291
   
$
33.63
 
Granted December 2021
   
306,310
   
$
22.51
 
Exercised
   
-
   
$
-
 
Balance as of December 31, 2021
   
1,218,038
         
Granted
   
-
   
$
-
 
Exercised
   
-
   
$
-
 
Balance as of March 31, 2022
   
1,218,038