Stockholders' Equity (Tables)

6 Months Ended

Jun. 30, 2018

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 and six months ended June 30, 2018 and 2017:

	Three Months Ended June 30,		Six Months Ended June 30,
	2018	2017	2018	2017
Common Stock Options
Risk free interest rate	2.85%	1.93%	2.66%-2.85%	1.93%-2.16%
Expected option term	5.5 years	5.5 years	5.5-6.0 years	5.5-6.0 years
Expected volatility of common stock	92.30%	98.23%	90.15%-92.30%	94.05%-98.23%
Expected dividend yield	0.0%	0.0%	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 and six months ended June 30, 2018 and 2017:

	Six Months Ended June 30,
	2018	2017
Employee Stock Purchase Plan
Risk free interest rate	1.85%	0.79%
Expected term	6.0 months	6.0 months
Expected volatility of common stock	58.76%	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 as follows:

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development	$	168,733	$	219,345	$	357,510	$	426,203
General and administrative		218,696		245,526		423,694		502,826
Total stock-based compensation expense	$	387,429	$	464,871	$	781,204	$	929,029