Preferred Stock, Common Stock and Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Summary of Warrants Activity

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

						Weighted
				Weighted		Average
				Average		Remaining
				Exercise		Contractual
	Shares			Price		Term (Years)
Outstanding at December 31, 2016		3,323,876		$	3.29		4.96
Issued	—			—			—
Exercised		(526,315	)	$	2.41		4.07
Expired/Forfeited	—			—			—
Outstanding at December 31, 2017		2,797,561		$	3.46		3.94

Summary of Stock Option Activity

A summary of the Company’s stock option activity under the 2007 Plan and 2013 Plan is as follows:

					Weighted
			Weighted		Average
			Average		Remaining
			Exercise		Contractual		Aggregate
	Shares		Price		Term (Years)		IntrinsicValue
Outstanding at December 31, 2016		1,275,624	$	4.04		8.34	$	191,160
Granted		856,000	$	2.45		9.13	—
Exercised	—		—		—		—
Expired/Forfeited/Exchanged	—		—		—		—
Outstanding at December 31, 2017		2,131,624	$	3.40		8.06	$	219,480
Vested and expected to vest at December 31, 2017		2,131,624	$	3.40		8.06	$	219,480
Exercisable at December 31, 2017		1,111,270	$	4.08		7.38	$	219,480

Summary of Nonvested Options Activity

The Company had the following nonvested options under the 2007 Plan and 2013 Plan:

	Shares			Weighted Average Grant Date Fair Value Per Share
Nonvested at December 31, 2016		690,961		$	3.34
Granted		856,000		$	2.45
Vested		(526,607	)	$	3.22
Expired/Forfeited/Exchanged		—			—
Nonvested at December 31, 2017		1,020,354		$	2.65

Summary of Estimated Fair Value of Employee Stock Purchase Plan Award

The estimated fair value of the shares to be acquired under the ESPP was determined on the initiation date of each six month purchase period using the Black-Scholes option-pricing valuation model with the following weighted-average assumptions for ESPP shares to be purchased during the year ended December 31, 2017 and 2016:

	Year Ended December 31,
	2017	2016
Risk free interest rate	0.79% - 1.10%	0.47% - 0.50%
Expected term	6 months	6 months
Expected volatility of common stock	37.60% - 99.23%	83.83% - 212.80%
Expected dividend yield	0.0%		0.0%

Summary of Estimated Fair Value of Stock Option Award

The estimated fair value of each option award granted was determined on the date of grant using the Black-Scholes option-pricing valuation model with the following weighted-average assumptions for options grants during the two years ended December 31, 2017:

	Year Ended December 31,
	2017	2016
Risk free interest rate	1.93% - 2.16%	1.25% - 1.58%
Expected option term	5.5 - 6.0 years	5.3 - 6.0 years
Expected volatility of common stock	94.05% - 98.23%	74.44 - 75.91%
Expected dividend yield	0.0%		0.0%

Summary of Recognized Non-Cash Stock-Based Compensation Expense

The Company recognized non-cash stock-based compensation expense to employees and directors in its research and development and its general and administrative functions as follows:

	Year Ended December 31,
	2017		2016
Research and development	$	819,815	$	698,032
General and administrative		999,616		1,008,492
Total stock-based compensation expense	$	1,819,431	$	1,706,524

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance consists of the following at December 31, 2017 and 2016:

	December 31,
	2017		2016
Stock options issued and outstanding		2,131,624		1,275,624
Authorized for future option grants		72,801		628,801
Warrants to purchase common stock		2,797,561		3,323,876
Authorized for employee stock purchase plan		91,934		37,463
Total common stock reserved for future issuance		5,093,920		5,265,764