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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2019
Share-based Payment Arrangement [Abstract]  
Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions
The fair market value of these stock options at the date of grant was estimated using the Black-Scholes option-pricing model with the following weighted average assumptions for the six months ended:
 
June 30, 2019
 
June 30, 2018
Expected term
6 years, 2 months

 
6 years

Interest rate
2.29
%
 
2.61
%
Dividend rate
—

 
—

Volatility
96.3
%
 
88.0
%
Summary of Stock Option Activity
The following table summarizes stock option activity for the six months ended June 30, 2019:
 
Number of
shares
 
Weighted-
average
exercise
price
 
Weighted- average
remaining
contractual
term
 
Aggregate
intrinsic
value
Outstanding at December 31, 2018
286,712

 
$
46.01

 
7 years

 
$
—

Granted
310,500

 
1.82

 
9 years, 11 months

 
—

Exercised
—

 
—

 
—

 
—

Forfeited
—

 
—

 
—

 
—

Expired
(385
)
 
18.05

 
—

 
—

Outstanding at June 30, 2019(1)
596,827

 
$
23.04

 
8 years, 3 months

 
$
26

Exercisable at June 30, 2019
223,805

 
$
57.36

 
6 years

 
$
—


(1)
Includes both vested stock options as well as unvested stock options for which the requisite service period has not been rendered but that are expected to vest based on achievement of a service condition.