Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award

A summary of the Company’s option activity and related information for the years ended December 31, 2017, 2016 and 2015 is as follows:

	Number of Shares		Weighted Average Exercise Price		Total Intrinsic Value
Outstanding as of December 31, 2015		3,367,784	$	7.16	$	3,971,205
Granted		1,196,500	$	7.46	$	5,676,405
Exercised		(21,200)	$	7.20		-
Forfeited		(85,833)	$	12.18		-
Outstanding as of December 31, 2016		4,457,251	$	7.14	$	23,258,604
Granted		706,800	$	24.16	$	15,792,854
Exercised		(353,612)	$	7.03		-
Forfeited		(258,620)	$	9.51		-
Outstanding as of December 31, 2017		4,551,819	$	9.66	$	162,002,439
Options vested and exercisable		3,128,885	$	6.80	$	120,316,959

Assumptions for Weighted Average Fair Value of Options Granted

The Company expects that all outstanding unvested options will vest. The fair value of the options granted for the year ended December 31, 2017, 2016 and 2015, was based on the following assumptions:

	Year Ended December 31,
	2017	2016	2015
Exercise price	$12.81 - $44.28	$5.84 - $13.21	$7.88 - $16.55
Expected stock price volatility	81.2% - 87.0%	85.8% - 91.8%	88.62% - 95.55%
Risk-free rate of interest	2.02% - 2.29%	1.36% - 2.18%	1.49% - 2.27%
Term (years)	5.5 - 7.0	5.3 - 7.0	5.0 - 8.2

Estimated Future Stock Based Expenses Relating to Unvested Stock Options

Estimated future stock-based compensation expense relating to unvested stock options is as follows:

	Future Stock Option Compensation Expenses
Year Ended December 31, 2018	$	5,755,478
Year Ended December 31, 2019		1,760,834
Year Ended December 31, 2020		634,516
Year Ended December 31, 2021		54,968
Total	$	8,205,796

Schedule of Restricted Stock Number of units

A summary of the Company's restricted stock units and related information is as follows:

	Number of units		Weighted average grant price
Unvested as of December 31, 2016	-	$	-
Granted	120,000		44.28
Unvested as of December 31, 2017	120,000	$	44.28

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs

Stock-based compensation expenses for the years ended December 31, 2017, 2016 and 2015 were as follows:

	Year Ended December 31,
	2017		2016		2015
Research and development	$	5,422,731	$	3,025,178	$	3,257,732
General and administrative		3,178,711		1,999,334		4,618,852
Total stock-based compensation expense	$	8,601,442	$	5,024,512	$	7,876,584

Warrant [Member]

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award

A summary of the Company’s warrant activity and related information is as follows:

	Warrants		Weighted Average Exercise Price
Outstanding as of December 31, 2015		16,909	$	30.81
Outstanding as of December 31, 2016		16,909	$	30.81
Expired		(1,613)		-
Outstanding as of December 31, 2017		15,296	$	30.00