Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

Stock-Based Compensation

Schedule of stock-based compensation expense

	Stock Option Grants						Restricted stock awards
	2019		2018		2017		2019		2018		2017
Research and development	$	2,348,205	$	2,884,689	$	2,127,386	$	23,793	$	202,809	$	157,480
General and administrative		3,189,897		3,491,432		3,279,626		—		46,813		82,360
	$	5,538,102	$	6,376,121	$	5,407,012	$	23,793	$	249,622	$	239,840

Summary of stock option activity

		Weighted-		Weighted-
		Average		Average	Aggregate
	Number	Exercise		Contractual	Intrinsic
	of Shares	Price		Life (in Years)	Value
Outstanding as of December 31, 2017	2,386,538	$	12.53
Granted	875,416		11.34
Expired	(109,687)		9.20
Outstanding as of December 31, 2018	3,152,267		12.16
Granted	960,432		6.07
Exercised	(40,983)		4.63
Forfeited	(83,000)		9.48
Outstanding as of December 31, 2019	3,988,716		10.83	7.20	$	2,668,248
Exercisable as of December 31, 2019	2,439,217		11.98	6.29	$	1,047,743
Vested and expected to vest as of December 31, 2019	3,905,436	$	10.81

Schedule of weighted-average assumptions used to calculate fair values of stock options

	Year ended December 31,
	2019	2018	2017
Weighted-average risk-free interest rate	2.36%	2.53%	2.11%
Expected term of options (in years)	6.16	6.12	6.12
Expected stock price volatility	79.85%	78.09%	77.00%
Expected dividend yield	0%	0%	0%

Summary of restricted stock activity

		Weighted
		Average
		Grant Date
	Shares	Fair Value
Unvested as of December 31, 2017	108,730	$	1.65
Granted	10,500		11.86
Vested	(108,730)		1.65
Unvested as of December 31, 2018	10,500		11.86
Granted	8,600		4.42
Vested	(10,500)		11.86
Unvested as of December 31, 2019	8,600	$	4.42