Stock-based compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

The following table summarizes stock option activity for employees and non-employees for the twelve months ended December 31, 2018:



	Shares		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at December 31, 2017	2,580,491		$	6.65	7.55	$	803,600
Granted	1,517,544		2.74
Exercised	(97,310	)	1.21
Forfeited	(859,543	)	5.76
Expired	(820,201	)	8.19
Outstanding at December 31, 2018	2,320,981		$	4.10	7.61	$	—
Exercisable at December 31, 2018	1,149,927		$	5.53	6.07	$	—
Vested or expected to vest at December 31, 2018	2,320,981		$	4.10	7.53	$	—

Schedule of fair value assumptions

The Company estimates the fair value of each stock option award on the grant date using the Black-Scholes option-pricing model based on the following assumptions regarding the fair value of the underlying Common Stock on each measurement date:



	Year Ended December 31, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
Expected volatility	81.56% to 82.41%		73.87% to 81.04%		71.01% to 74.20%
Risk-free interest rate	2.45% to 2.91%		1.83% to 2.40%		1.23% to 2.40%
Expected term	5.3 - 10 years		5.3 - 9.5 years		5.3 - 10 years
Expected dividend yield	0	%	0	%	0	%

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 consolidated statements of operations and comprehensive loss as follows:



	Year Ended December 31, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
Research and development	$	705,687	$	1,545,737	$	2,435,565
Selling, general and administrative	1,233,924		2,673,428		4,137,398
Total	$	1,939,611	$	4,219,165	$	6,572,963