Stockholders' Equity (Tables)

3 Months Ended

Mar. 31, 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 three months ended March 31, 2019 and 2018:

	Three Months Ended March 31,
	2019	2018
Common Stock Options
Risk free interest rate	2.55%	2.66%
Expected option term	6.0 years	6.0 years
Expected volatility of common stock	90.34%	90.15%
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 three months ended March 31, 2019 and 2018:

	Three Months Ended March 31,
	2019	2018
Employee Stock Purchase Plan
Risk free interest rate	2.52%	1.85%
Expected term	0.5 years	0.5 years
Expected volatility of common stock	130.36%	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 as follows:

	Three Months Ended March 31,
	2019		2018
Research and development	$	152,174	$	188,777
General and administrative		226,785		204,998
Total stock-based compensation expense	$	378,959	$	393,775