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Equity-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2020
Share-based Payment Arrangement [Abstract]  
Equity-Based Compensation Expense Classification in Statement of Operations
Equity-based compensation expense is classified in the statements of operations as follows:
Three Months Ended
June 30,		Six Months Ended
June 30,
2020201920202019
Research and development expense$203  $103  $360  $224  
General and administrative expense341  336  625  704  
$544  $439  $985  $928  
Assumptions Used to Determine Fair Value of Common Stock Option Grants In addition to an assumption on the expected term of the option grants as discussed below, application of the Black-Scholes model requires additional inputs for which we have assumed the values described in the table below:
Six Months Ended
June 30,
20202019
Expected term
5.2 to 6.1 years
5.3 to 6.1 years
Risk-free interest rate
0.41% to 1.68%; weighted avg. 0.67%
1.86% to 2.56%; weighted avg. 2.34%
Expected volatility
81.0% to 85.8%; weighted avg. 83.8%
83.2% to 86.7%; weighted avg. 84.4%
Forfeiture rate5 %5 %
Expected dividend yield— %— %
The fair value of the underlying common stock and the exercise price for the common stock options granted during the six months ended June 30, 2020 and 2019 are summarized in the table below:
Common Stock Options Granted During Period Ended:Fair Value of Underlying Common StockExercise Price of Common Stock Option
Six months ended June 30, 2020
$1.19 to $2.80; weighted avg. $1.85
$1.19 to $2.80; weighted avg. $1.85
Six months ended June 30, 2019
$2.78 to $2.80:
weighted avg. $2.79
$2.78 to $2.80:
weighted avg. $2.79
Common Stock Option Activity
A summary of common stock option activity as of the periods indicated is as follows:
OptionsWeighted-Average Exercise PriceWeighted-Average Remaining Contractual Term (years)Aggregate Intrinsic Value (thousands)
Outstanding - December 31, 20195,697,714  $2.34  6.7$4,625  
Granted2,075,248  1.85  
Exercised(55,899) 0.89  
Forfeited(18,750) 2.78  
Outstanding - June 30, 20207,698,313  $2.22  7.2$4,738  
Exercisable - June 30, 20204,372,185  $2.12  5.5$3,351  
Vested and Expected to Vest -
June 30, 2020		7,442,992  $2.22  7.1$4,613