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Stock-based Compensation (Tables)
 9 Months Ended
Sep. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of allocated stock-based compensation expense
The Company recorded stock-based compensation expense related to stock options and shares issued under the Company's 2017 Employee Stock Purchase Plan (2017 Plan) in the following expense categories of its accompanying consolidated statements of operations for the three and nine months ended September 30, 2018 and 2017:
 
Three Months Ended September 30,
 
Nine Months Ended September 30,
 
2018
 
2017
 
2018
 
2017
Cost of product sales
$
3

 
$
—

 
$
6

 
$
—

Research and development
329

 
411

 
734

 
919

General and administrative
1,870

 
360

 
5,585

 
881

 
$
2,202

 
$
771

 
$
6,325

 
$
1,800
Schedule of stock option activity
The following table summarizes the activity related to stock option grants to employees and nonemployees for the nine months ended September 30, 2018:
 
Shares
 
Weighted
average
exercise price
per share
 
Weighted
average
remaining
contractual life
Outstanding at December 31, 2017
6,251,576

 
$
9.34

 
6.67
Granted
423,892

 
20.35

 

Exercised
(491,654
)
 
3.22

 

Expired
—

 
—

 
 
Forfeited
(45,441
)
 
8.81

 

Outstanding at September 30, 2018
6,138,373

 
$
10.60

 
6.71
Exercisable at September 30, 2018
3,228,993

 
$
6.91

 
4.70
Vested and expected to vest at September 30, 2018
6,138,373

 
$
10.60

 
6.71
Schedule of fair value options using Black-Scholes pricing model
The grant date fair value of each stock option grant was estimated at the time of grant using the Black-Scholes option-pricing model using the following weighted average assumptions:
 
Nine Months Ended September 30,
 
2018
 
2017
Risk free interest rate
2.71
%
 
2.01
%
Expected term (in years)
5.93

 
6.08

Expected volatility
74.61
%
 
74.06
%
Annual dividend yield
0.00
%
 
0.00
%
Fair value of common stock
$
13.50

 
$
3.90