The Company has authorized 150,000,000 shares of common stock, $0.0001 par value per share, of which 55,603,427 shares, 44,681,745 shares, and 37,605,134 shares were issued and outstanding as of December 31, 2017, 2016, and 2015, respectively.

During the year ended December 31, 2015, the Company issued 11,666,802 shares of common stock upon the exercise of warrants and stock options to purchase common stock and the Company received net proceeds of $10,819,714 from these exercises.

In June 2016, the Company completed a sale of shares of its common stock pursuant to the terms of a securities purchase agreement under which the Company sold an aggregate of 5,960,000 shares of its common stock in a registered direct offering to investors at a purchase price of $2.50 per share with gross proceeds to the Company totaling approximately $14,900,000 less issuance costs of $25,222. On February 28, 2017, the Company entered in a securities purchase agreement providing for the issuance and sale by the Company of 3,887,815 shares of its common stock in a registered direct offering to institutional and accredited investors at a purchase price of $7.00 per share with gross proceeds to the Company totaling $27,214,705 less issuance costs of $36,291.

In November 2016, the Company entered into a sales agreement with Cantor Fitzgerald under which the Company could direct Cantor Fitzgerald as its sales agent to sell common stock under an “At the Market Offering” (“Sales Agreement”). Sales of common stock under the Sales Agreement were made pursuant to an effective registration statement for an aggregate offering of up to $35 million. Under the Sales Agreement, the Company was obligated to pay Cantor a 3% commission on gross proceeds. In 2016, the Company sold 188,695 shares of our common stock under the Sales Agreement at an average selling price of approximately $8.54 per share (net of 3% commission paid to Cantor Fitzgerald) which resulted in proceeds of approximately $1,621,182 and net proceeds of approximately $1,426,145 net of incurred issuance costs. Approximately $330,413 of these proceeds were classified in stock subscriptions receivable as of December 31, 2016 because the Company did not receive these proceeds until January 2017. During the year ended December 31, 2017, in the first quarter, the Company sold 1,413,633 shares of its common stock under the Sales Agreement at an average selling price of approximately $9.71 per share for gross proceeds of $13,724,591 and net proceeds of $13,268,208. The Company did not sell any shares of its common stock under the Sales Agreement in the second or third quarter of 2017 and terminated the Sales Agreement in connection with the October 2017 Offering discussed below. The aggregate amount sold under the Sales Agreement as described above was approximately $15.4 million.

See Note 15 for the January 2018 Sales Agreement entered into by the Company and Cantor Fitzgerald after the balance sheet date.

On October 26, 2017, the Company consummated an underwritten public offering of shares of its common stock pursuant to which the Company sold an aggregate of 4,650,000 shares of its common stock to institutional investors at a purchase price of $7.00 per share with gross proceeds to the Company totaling $32,550,000, less estimated issuance costs incurred of approximately $2,184,000. The Company also granted the underwriters a 30-day option to purchase up to an additional 697,500 shares of common stock on the same terms as the underwriters were purchasing the base number of shares, which they exercised in November 2017 with net proceeds to us totaling $4,589,550.

During the year ended December 31, 2017, the Company issued 272,734 shares of common stock upon the exercise of stock options and warrants to purchase common stock and the Company received net proceeds of $189,490 from these exercises. During the year ended December 31, 2016, the Company issued 927,916 shares of common stock upon the exercise of stock options and warrants to purchase common stock and the Company received net proceeds of $468,442 from these exercises.