In November 2017, the Company filed a shelf registration with the SEC on Form S-3. The shelf registration statement includes a prospectus for the at-the-market offering to sell up to an aggregate of $16.0 million of shares of the Company’s common stock through B. Riley FBR, Inc. (“FBR”) as a sales agent (the “FBR Sales Agreement”). During the three months ended March 31, 2018, the Company sold 268,870 shares of common stock at a weighted-average price per share of $2.10 pursuant to the FBR Sales Agreement and received proceeds of approximately $545,000, net of commissions and fees. During the three months ended March 31, 2019, the Company sold 450,000 shares of common stock at a weighted-average price per share of $1.44 pursuant to the FBR Sales Agreement and received proceeds of approximately $636,000, net of commissions and fees. In April 2019, the Company sold 5,384,117 shares of common stock at a weighted-average price per share of $0.86 pursuant to the FBR Sales Agreement and received proceeds of approximately $4.5 million, net of commissions and fees.

Future sales will depend on a variety of factors including, but not limited to, market conditions, the trading price of the Company’s common stock and the Company’s capital needs. There can be no assurance that FBR will be successful in consummating future sales based on prevailing market conditions or in the quantities or at the prices that the Company deems appropriate.

In addition, the Company will not be able to make future sales of common stock pursuant to the FBR Sales Agreement unless certain conditions are met, which include the accuracy of representations and warranties made to FBR under the FBR Sales Agreement. Furthermore, FBR is permitted to terminate the FBR Sales Agreement in its sole discretion upon ten days’ notice, or at any time in certain circumstances, including the occurrence of an event that would be reasonably likely to have a material adverse effect on the Company’s assets, business, operations, earnings, properties, condition (financial or otherwise), prospects, stockholders’ equity or results of operations. The Company has no obligation to sell the remaining shares available for sale pursuant to the FBR Sales Agreement.

In March 2018, the Company entered into the Warrant Amendments with each of the holders of the Company’s outstanding Warrants acquired as a part of the Company’s financings which closed in July and August 2016. As a result of the Warrant Amendments, all of the remaining Warrants to purchase 2,449,129 shares of the Company’s common stock are no longer required to be classified as liabilities. The value of the amended Warrants was adjusted to the fair value immediately prior to the Warrant Amendments, resulting in a net gain of approximately $433,000 in the statement of operations, and approximately $3.3 million was reclassified from warrant liability to additional paid-in capital.

In September 2018, warrants to purchase 84,000 shares of the Company’s common stock, issued to representatives of the underwriters in connection with the Company’s initial public offering in September 2013, expired and were cancelled.

Stock-based compensation expense includes charges related to employee stock purchases under the ESPP and stock option grants. The Company measures stock-based compensation expense based on the grant date fair value of any awards granted to its employees. Such expense is recognized over the period of time that employees provide service and earn rights to the awards.

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%

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%

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

As of March 31, 2019, there were approximately $2.9 million of unrecognized compensation costs related to outstanding employee and board of director options, which are expected to be recognized over a weighted-average period of 1.52 years.