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Equity-Based Compensation (Tables)
 9 Months Ended
Sep. 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
September 30,		Nine Months Ended
September 30,
2020201920202019
Research and development expense$247 $199 $607 $423 
General and administrative expense371 312 996 1,016 
$618 $511 $1,603 $1,439 
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:
Nine Months Ended
September 30,
20202019
Expected term
5.2 to 6.1 years
5.3 to 6.1 years
Risk-free interest rate
0.31% to 1.68%; weighted avg. 0.61%
1.69% to 2.56%; weighted avg. 2.16%
Expected volatility
81.0% to 85.8%; weighted avg. 83.8%
83.2% to 86.7%; weighted avg. 84.9%
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 nine months ended September 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
Nine months ended September 30, 2020
$1.19 to $2.80; weighted avg. $1.91
$1.19 to $2.80; weighted avg. $1.94
Nine months ended September 30, 2019
$2.32 to $2.95:
weighted avg. $2.75
$2.32 to $2.95:
weighted avg. $2.75
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 
Granted2,630,248 1.94 
Exercised(78,139)0.88 
Forfeited(510,042)2.67 
Outstanding - September 30, 20207,739,781 $2.20 6.7$2,090 
Exercisable - September 30, 20204,565,863 $2.12 4.9$1,657 
Vested and Expected to Vest -
September 30, 2020		7,739,781 $2.20 6.7$2,090