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Stock Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2020
Share-based Payment Arrangement [Abstract]  
Schedule of stock based compensation expense
The Company recorded stock-based compensation expense in the following expense categories of its accompanying consolidated statements of operations and comprehensive loss for the three months ended June 30, 2020 and 2019 and six months ended June 30, 2020 and 2019, respectively (amounts in thousands):
Three Months Ended
June 30,		Six Months Ended
June 30,
2020201920202019
Research and development
$173  $378  $473  $801  
General and administrative807  616  1,587  1,133  
Total
$980  $994  $2,060  $1,934  
Summary of the activity related to stock option grants to employees and nonemployees
The following table summarizes the activity related to stock option grants to employees and nonemployees for the six months ended June 30, 2020:
Shares
Weighted
average
exercise price
per share		Weighted-average
remaining
contractual
life (years)
Outstanding at December 31, 20192,498,606  $17.18  7.7
Granted1,823,625  4.77  
Exercised(1,449) 1.08  
Forfeited(403,526) 15.05  
Outstanding at June 30, 20203,917,256  $11.63  7.9
Vested and exercisable at June 30, 20201,403,163  $20.58  5.4
Vested and expected to vest at June 30, 20203,917,256  $11.63  7.9
Summary of grant date fair value assumptions The grant date fair value of each option grant was estimated during the three and six months ended June 30, 2020 and 2019 using the following assumptions within the Black-Scholes option-pricing model:
Three Months Ended
June 30, 2020		Three Months Ended
June 30, 2019		Six Months Ended
June 30, 2020		Six Months Ended
June 30, 2019
Expected term (in years)5.465.935.756.01
Expected volatility77 %79 %77 %80 %
Risk-free interest rate0.43 %1.91 %0.87 %2.35 %
Expected dividend yield0 %0 %0 %0 %