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Equity-Based Compensation (Tables)
 9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [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,
 
2017
 
2016
 
2017
 
2016
Research and development expense
$
43

 
$
40

 
$
129

 
$
119

General and administrative expense
319

 
127

 
975

 
388

 
$
362

 
$
167

 
$
1,104

 
$
507
Equity-based compensation expense is classified in the statements of operations as follows:
 
Year Ended
December 31,
 
2016
 
2015
Research and development expense
$
160

 
$
68

General and administrative expense
522

 
302

 
$
682

 
$
370
Assumptions Used to Determine Fair Value of Common Stock Option Grants
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, 2017 and 2016 are summarized in the table below:
Common Stock Options Granted During Period Ended:
Fair Value of Underlying Common Stock
 
Exercise Price of Common Stock Option
Nine months ended September 30, 2017
$4.21
 
$4.21
Nine months ended September 30, 2016
$1.91 to $2,12; weighted avg. $1.98
 
$1.91 to $2,12; weighted avg. $1.98
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,
 
2017
 
2016
Expected term
5.3 to 6.5 years

 
5.0 to 6.9 years

Risk-free interest rate
1.97% to 2.17%; weighted avg. 2.07%

 
1.01% to 1.41%; weighted avg. 1.26%

Expected volatility
80.8% to 83.1%; weighted avg. 81.0%

 
79.9% to 82.4%; weighted avg. 80.9%

Forfeiture rate
5
%
 
5
%
Expected dividend yield
—
%
 
—
%
The fair value of the underlying common stock and the exercise price for the common stock options granted during the years ended December 31, 2016 and 2015 are summarized in the table below:
Common Stock Options Granted During Period Ended:
Fair Value of Underlying Common Stock
 
Exercise Price of Common Stock Option
Year ended December 31, 2016
$0.95 to $1.20; weighted avg. $1.14
 
$0.95 to $1.20; weighted avg. $1.14
Year ended December 31, 2015
$0.51 to $0.98;
weighted avg. $0.69
 
$0.51 to $0.98;
weighted avg. $0.69
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:
 
Years Ended
December 31,
 
2016
 
2015
Expected term
5.0 to 7.0 years

 
5.2 to 7.0 years

Risk-free interest rate
1.01% to 2.24%; weighted avg. 1.58%

 
1.59% to 2.08%; weighted avg. 1.73%

Expected volatility
79.2% to 83.6%; weighted avg. 79.9%

 
78.9% to 85.4%; weighted avg. 82.2%

Forfeiture rate
5
%
 
5
%
Expected dividend yield
—
%
 
—
%
Common Stock Option Activity
A summary of common stock option activity as of the periods indicated is as follows:
 
Options
 
Weighted-Average Exercise Price
 
Weighted-Average Remaining Contractual Term (years)
 
Aggregate Intrinsic Value (thousands)
Outstanding - December 31, 2016
3,089,352

 
$
1.25

 
8.2
 
$
9,143

Granted
657,843

 
4.21

 
 
 
 
Exercised
(58,440
)
 
0.75

 
 
 
 
Forfeited
(10,396
)
 
1.00

 
 
 
 
Outstanding - September 30, 2017
3,678,359

 
$
1.79

 
7.8
 
$
5,257

Exercisable - September 30, 2017
2,131,730

 
$
1.37

 
7.4
 
$
3,704

Vested and Expected to Vest - September 30, 2017
3,595,855

 
$
1.77

 
7.7
 
$
5,195
A summary of common stock option activity as of the periods indicated is as follows:
 
Options
 
Weighted-Average Exercise Price
 
Weighted-Average Remaining Contractual Term (years)
 
Aggregate Intrinsic Value (thousands)
Outstanding - December 31, 2014
2,954,418

 
$
0.32

 
 
 
 
   Granted
3,941,134

 
0.76

 
 
 
 
   Exercised
(166,875
)
 
0.32

 
 
 
 
   Forfeited
(200,625
)
 
0.35

 
 
 
 
Outstanding - December 31, 2015
6,528,052

 
$
0.59

 
9.1
 
$
2,310

Granted
175,000

 
1.14

 
 
 
 
Exercised
(98,283
)
 
0.33

 
 
 
 
Forfeited
(382,321
)
 
0.33

 
 
 
 
Outstanding - December 31, 2016
6,222,448

 
$
0.62

 
8.2
 
$
9,143

Exercisable - December 31, 2016
2,956,455

 
$
0.51

 
7.8
 
$
4,683

Vested and Expected to Vest - December 31, 2016
6,025,583

 
$
0.61

 
8.1
 
$
8,888