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

12 Months Ended

Dec. 31, 2018

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, 2017		2,797,561		$	3.46		3.94
Issued	—			—		—
Exercised	—			—		—
Expired/Forfeited		(84,000	)	$	21.00	—
Outstanding at December 31, 2018		2,713,561		$	2.91		3.04

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, 2017		2,131,624	$	3.40		8.06	$	219,480
Granted		886,000	$	2.41		9.15	—
Exercised	—		—		—		—
Expired/Forfeited/Exchanged	—		—		—		—
Outstanding at December 31, 2018		3,017,624	$	3.11		7.68	$	382,160
Vested and expected to vest at December 31, 2018		3,017,624	$	3.11		7.68	$	382,160
Exercisable at December 31, 2018		1,900,216	$	3.50		7.07	$	292,831

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, 2017		1,020,354		$	2.65
Granted		886,000		$	1.86
Vested		(788,947	)	$	2.69
Expired/Forfeited/Exchanged	—			—
Nonvested at December 31, 2018		1,117,407		$	2.44

Summary of Estimated Fair Value of Stock Option Award

The estimated fair value of each stock 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 option grants during the years ended December 31, 2018 and 2017:

	Year Ended December 31,
	2018	2017
Risk free interest rate	2.66% - 2.85%	1.93% - 2.16%
Expected option term	5.5 - 6.0 years	5.5 - 6.0 years
Expected volatility of common stock	90.15% - 92.30%	94.05% - 98.23%
Expected dividend yield	0.0%	0.0%

Summary of Estimated Fair Value of Shares to be Acquired under Employee Stock Purchase Plan

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 years ended December 31, 2018 and 2017 as follows:

	Year Ended December 31,
	2018	2017
Risk free interest rate	1.85% - 2.29%	0.79% - 1.10%
Expected term	6 months	6 months
Expected volatility of common stock	45.24% - 58.76%	37.60% - 99.23%
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 during the years ended December 31, 2018 and 2017 as follows:

	Year Ended December 31,
	2018		2017
Research and development	$	673,525	$	819,815
General and administrative		865,944		999,616
Total stock-based compensation expense	$	1,539,469	$	1,819,431

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance consists of the following at December 31, 2018 and 2017:

	December 31,
	2018		2017
Stock options issued and outstanding		3,017,624		2,131,624
Authorized for future option grants		986,801		72,801
Warrants to purchase common stock		2,713,561		2,797,561
Authorized for employee stock purchase plan		163,065		91,934
Total common stock reserved for future issuance		6,881,051		5,093,920