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Stock-based compensation (Tables)
 6 Months Ended
Jun. 30, 2019
Share-based Payment Arrangement [Abstract]  
Summary of Stock Option Activity
The following table summarizes stock option activity for employees and non-employees for the six months ended June 30, 2019:
 
 
Shares
 
Weighted-Average
Exercise Price
 
Weighted-Average
Remaining
Contractual
Term (in years)
 
Aggregate
Intrinsic
Value
Outstanding at December 31, 2018
2,320,981

 
$
4.10

 
7.61
 
$
—

Granted
—

 
—

 
 
 
 
Exercised
—

 
—

 
 
 
 
Forfeited
—

 
—

 
 
 
 
Expired
(23,945
)
 
4.41

 
 
 
 
Outstanding at June 30, 2019
2,297,036

 
$
4.10

 
7.11
 
$
—

Exercisable at June 30, 2019
1,346,425

 
$
5.11

 
6.05
 
$
—

Vested or expected to vest at June 30, 2019
2,297,036

 
$
4.10

 
7.11
 
$
—
Summary of Stock-based Compensation Expense
Total stock-based compensation expense recognized for employee and non-employee restricted common stock, and stock options granted to employees and non-employees is included in the Company's condensed consolidated statements of operations as follows:
 
 
Three Months Ended June 30, 2019
 
Three Months Ended June 30, 2018
 
Six Months Ended June 30, 2019
 
Six Months Ended June 30, 2018
Research and development
$
19,323

 
$
244,869

 
$
38,765

 
$
631,406

Selling, general and administrative
193,224

 
266,124

 
394,757

 
788,527

Total
$
212,547

 
$
510,993

 
$
433,522

 
$
1,419,933