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Stockholders' equity (Tables)
 6 Months Ended 12 Months Ended
Jun. 30, 2023
Dec. 31, 2022
Equity [Abstract]    
Schedule of Stock Option Activity
A summary of the Company’s stock option activity and related information for the six months ended June 30, 2023 is as follows:
 
 
  
Number of
Shares
 
 
Weighted
Average
Exercise
Price
 
  
Weighted
Average
Remaining
Contractual
Life

(in Years)
 
  
Aggregate
Intrinsic Value

(in Thousands)
 
Outstanding at December 31, 2022
     
1,702,876
     
$
24.45
       
8.32
     
$
—
  
Options granted
     
52,100
     
$
3.50
                    
Options exercised
     
—
     
$
—
                    
Options cancelled/forfeited
     
(827,664
)
 
   
$
25.35
                    
    
 
 
                             
Outstanding at June 30, 2023
     
927,312
     
$
19.80
       
4.41
     
$
11
  
    
 
 
                             
Exercisable as of June 30, 2023
     
706,942
     
$
22.55
       
3.01
     
$
—
  
Stock options
A summary of the Company’s stock option activity and related information for the year ended December 31, 2022 is as follows:
 
 
  
Number of
Shares
 
 
Weighted
Average

Exercise
Price
 
  
Weighted

Average

Remaining

Contractual

Life

(In Years)
 
  
Aggregate

Intrinsic

Value

(In Thousands)
 
Outstanding at December 31, 2021
     
1,792,789
     
$
36.00
       
8.32
     
$
6,912
  
Options granted
     
686,210
     
$
6.60
                    
Options exercised
     
(7,300
) 
   
$
18.35
                    
Options cancelled/forfeited
     
(768,822
) 
   
$
35.50
                    
    
 
 
                             
Outstanding at December 31, 2022
     
1,702,877
     
$
24.45
       
8.32
     
$
—
  
    
 
 
                             
Exercisable as of December 31, 2022
     
778,184
     
$
30.05
       
7.18
     
$
—
  
Schedule of Weighted Average Assumptions
The fair values of stock options granted are estimated using the Black-Scholes option pricing model with the following weighted average assumptions:

 
  
Three Months Ended

June 30,
 
 
Six Months Ended

June 30,
 
 
  
 2023 
 
 
 2022 
 
 
 2023 
 
 
 2022 
 
Expected volatility
      87.37 %      82.68 %      86.77 %      83.01 % 
Expected dividends
      0 %      0 %      0 %      0 % 
Expected terms (years)
      5.27       5.73       5.60       5.96  
Risk free rate
      3.82 %      2.77 %      3.67 %      2.22 % 
The fair value of stock options granted is estimated using the Black-Scholes option pricing model with the following weighted average assumptions:
 

 
  
December 31,
 
 
  
2022
 
 
2021
 
Expected volatility
      84 %      89 % 
Expected dividends
      0 %      0 % 
Expected terms (years)
      6.04       6.03  
Risk free rate
      2.66 %      0.93 % 
Schedule of RSU Activity  
Restricted stock units
A summary of the Company’s r
es
tricted stock unit activity and related informatio
n
for the year ended December 31, 2022, after giving effect to the 1-for-5 reverse stock split of the Company’s common stock that was effected on September 25, 2023, is as follows:
 

 
  
Number of

Shares
 
  
Weighted

Average

Grant Date
Fair Value
 
Non-vested
at December 31, 2021
     
26,400
     
$
49.85
  
Restricted stock units granted
     
140,000
     
$
18.45
  
Restricted stock units vested
     
(4,950
) 
   
$
61.00
  
Restricted stock units forfeited
     
(85,750
) 
 
   
$
23.30
  
    
 
 
           
Non-vested
at December 31, 2022
     
75,700
     
$
21.10
  
    
 
 
           
Summary of Stock-based Compensation Expense
Stock-based compensation expense is classified in the condensed statements of operations and comprehensive income (loss) as follows:

(in thousands)
  
Three Months Ended

June 30,
 
  
Six Months Ended

June 30,
 
 
  
 2023 
 
  
 2022 
 
  
 2023 
 
  
2022 
 
Research and development expenses
    $ —      $ 1,051      $ 255      $ 2,315  
General and administrative expenses
      331        1,261        1,267        2,443  
    
 
 
     
 
 
     
 
 
     
 
 
  
Total stock-based compensation expense
    $ 331      $ 2,312      $ 1,522      $ 4,758  
    
 
 
     
 
 
     
 
 
     
 
 
  
Stock-based compensation expense is classified in the statements of operations as follows (in thousands):

 
 
  
December 31,
 
 
  
2022
 
  
2021
 
Research and development expenses
    $ 4,342      $ 5,095  
General and administrative expenses
      4,487  
 
      6,462  
    
 
 
     
 
 
  
Total stock-based compensation expense
    $ 8,829      $ 11,557  
    
 
 
     
 
 
  
Schedule of Unvested Restricted Stock Units Roll Forward
(d) Restricted stock units
A summary of the Company’s RSU activity and related information for the six months ended June 30, 2023 is as follows, giving effect to the 1-for-5 reverse stock split of the Company’s common stock that was effected on September 25, 2023:
 
 
  
Number of
Shares
 
 
Weighted
Average
Grant Date
Fair Value
 
Non-vested
at December 31, 2022
     
75,700
     
$
21.10
  
Restricted stock units granted
     
—
     
$
—
  
Restricted stock units vested
     
(36,000
) 
   
$
20.90
  
Restricted stock units forfeited
     
(30,450
) 
   
$
20.75
  
    
 
 
           
Non-vested
at June 30, 2023
     
9,250
     
$
22.95