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

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Summary of Stock Option Activity

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

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)		Aggregate IntrinsicValue
Outstanding at December 31, 2018		3,017,624		$	3.11		7.68	$	382,160
Granted/Exchanged		2,672,246		$	1.20		9.36	—
Exercised	—			—		—		—
Expired/Forfeited/Exchanged		(2,575,499	)	$	2.82		7.48	—
Outstanding at December 31, 2019		3,114,371		$	1.71		8.32	$	2,143,756
Vested and expected to vest at December 31, 2019		3,114,371		$	1.71		8.32	$	2,143,756
Exercisable at December 31, 2019		1,230,605		$	3.17		6.70	$	392,604

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, 2018		1,117,407		$	2.44
Granted		2,672,246		$	1.20
Vested		(494,674	)	$	1.53
Expired/Forfeited/Exchanged		(1,411,213	)	—
Nonvested at December 31, 2019		1,883,766		$	0.75

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, 2019 and 2018:

	Year Ended December 31,
	2019	2018
Risk free interest rate	1.80% - 2.55%	2.66% - 2.85%
Expected option term	4.27 - 6.0 years	5.5 - 6.0 years
Expected volatility of common stock	90.34% - 112.58%	90.15% - 92.30%
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, 2019 and 2018 as follows:

	Year Ended December 31,
	2019	2018
Risk free interest rate	1.89% - 2.52%	1.85% - 2.29%
Expected term	0.5 years	0.5 years
Expected volatility of common stock	130.36% - 170.68%	45.24% - 58.76%
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, 2019 and 2018 as follows:

	Year Ended December 31,
	2019		2018
Research and development	$	710,155	$	673,525
General and administrative		663,803		865,944
Total stock-based compensation expense	$	1,373,958	$	1,539,469

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance consists of the following at December 31, 2019 and 2018:

	December 31,
	2019		2018
Stock options issued and outstanding		3,114,371		3,017,624
Authorized for future option grants		1,587,155		986,801
Warrants to purchase common stock		2,713,561		2,713,561
Authorized for employee stock purchase plan		263,065		163,065
Total common stock reserved for future issuance		7,678,152		6,881,051