Stockholders' Equity (Tables)

9 Months Ended

Sep. 30, 2019

Equity [Abstract]

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 nine months ended September 30, 2019 and 2018:

	Nine Months Ended September 30,
	2019	2018
Common Stock Options
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

There were no stock options granted during the three months ended September 30, 2019 and 2018.

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 three and nine months ended September 30, 2019 and 2018:

	Three Months Ended September 30,		Nine Months Ended September 30,
	2019	2018	2019	2018
Employee Stock Purchase Plan
Risk free interest rate	1.89%	2.29%	1.89% - 2.52%	1.85% - 2.29%
Expected term	0.5 years	0.5 years	0.5 years	0.5 years
Expected volatility of common stock	170.68%	45.24%	130.36% - 170.68%	45.24% - 58.76%
Expected dividend yield	0.0%	0.0%	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:

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Research and development	$	179,227	$	161,515	$	533,518	$	519,025
General and administrative		152,076		218,547		521,585		642,241
Total stock-based compensation expense	$	331,303	$	380,062	$	1,055,103	$	1,161,266