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Share-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2020
Share-based Payment Arrangement [Abstract]  
Summary of Stock Option Activity

The following table summarizes stock option activity for the six months ended June 30, 2020:

 

     Number of Options        Weighted- Average Exercise Price     Weighted- Average Remaining Contractual Term (Years)     Aggregate Intrinsic Value  
Outstanding — December 31, 2019     900,003     $ 11.63       8.52     $ —  
Granted     2,249,104     $ 1.53                  
Exercised     (22,497 )   $ 1.06                  
Forfeited or expired     (146,178 )   $ 27.27                  
Outstanding — June 30, 2020     2,980,432     $ 3.32       9.21     $ 727  
Exercisable — June 30, 2020     637,993     $ 9.55       8.50     $ 134  
Schedule of Calculation of Fair Value Assumptions

The estimated fair values of employee stock options granted during the three months and six months ended June 30, 2020 and 2019, were determined on the date of grant using the Black-Scholes option-pricing model with the following assumptions:

 

    Three Months Ended June 30,     Six Months Ended June 30,  
    2020     2019     2020     2019  
Expected option life (years)     6.03       6.02       5.92       6.02  
Risk-free interest rate     0.45 %     2.22 %     1.62 %     2.22 %
Expected volatility     94.40 %     74.14 %     93.90 %     74.14 %
Expected dividend yield     0 %     0 %     0 %     0 %
Schedule of Stock-Based Compensation Expense

The following table presents total stock-based compensation expense for the three and six months ended June 30, 2020 and 2019:

 

    Three Months Ended June 30,     Six Months Ended June 30,  
    2020     2019     2020     2019  
Research and development   $ 49     $ 18     $ 96     $ 67  
General and administrative     217       1,073       513       1,483  
Total share-based compensation expense   $ 266     $ 1,091     $ 609     $ 1,550