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Stock-based Compensation (Tables)
 9 Months Ended
Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of allocated stock-based compensation expense
The Company recorded stock-based compensation expense in the following expense categories of its accompanying consolidated statements of operations for the three and nine months ended September 30, 2017 and 2016:
 
Three Months Ended
September 30,
Nine Months Ended September 30,
 
2017
 
2016
2017
 
2016
Research and development
$
411

 
$
25

$
919

 
$
323

General and administrative
360

 
9

881

 
216

 
$
771

 
$
34

$
1,800

 
$
539
Schedule of stock option activity
The following table summarizes the activity related to service-based stock option grants to employees and nonemployees for the nine months ended September 30, 2017:
 
Shares
 
Weighted
average
exercise price
per share
 
Weighted
average
remaining
contractual life
Outstanding at December 31, 2016
1,894,083

 
$
3.09

 
6.67
Granted
524,147

 
5.90

 

Exercised
—

 
—

 

Expired
(2,887
)
 
5.68

 
 
Forfeited
(3,955
)
 
3.05

 

Outstanding at September 30, 2017
2,411,388

 
$
3.70

 
6.67
Exercisable at September 30, 2017
1,193,791

 
$
1.99

 
4.09
Vested and expected to vest at September 30, 2017
2,411,388

 
$
3.70

 
6.67
The following table summarizes the activity related to performance-based stock option grants to employees and nonemployees for the nine months ended September 30, 2017:
 
Shares
 
Weighted
average
exercise price
per share
 
Weighted
average
remaining
contractual life
Outstanding at December 31, 2016
2,171,760

 
$
9.59

 
6.55
Granted
—

 
—

 

Exercised
—

 
—

 

Forfeited
—

 
—

 

Outstanding at September 30, 2017
2,171,760

 
$
9.59

 
5.74
Exercisable at September 30, 2017
1,016,600

 
$
1.96

 
4.08
Schedule of fair value options using Black-Scholes pricing model
The grant date fair value of each option grant was estimated at the time of grant using the Black-Scholes option-pricing model using the following weighted average assumptions:
Risk free interest rate
2.01
%
Expected term (in years)
6.08

Expected volatility
74.06
%
Annual dividend yield
0.00
%
Fair value of common stock
$
3.90