UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): October 24, 2013

SORRENTO THERAPEUTICS, INC.

(Exact name of registrant as specified in its charter)

6042 Cornerstone Ct. West, Suite B

San Diego, CA 92121

(Address of principal executive offices)

Registrant’s telephone number, including area code: (858) 210-3700

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

On October 24, 2013, Sorrento Therapeutics, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Aegis Capital Corp., as representative of the several underwriters (collectively, the “Underwriters”), relating to the issuance and sale of 4,150,000 shares of common stock, par value $0.0001 per share (“Common Stock”). The public offering price for each share of Common Stock is $7.25.

The Underwriting Agreement contains customary representations, warranties and agreements by the Company, customary conditions to closing, indemnification obligations of the Company and the Underwriter, including for liabilities under the Securities Act of 1933, as amended (the “Securities Act”), other obligations of the parties and termination provisions. The representations, warranties and covenants contained in the Underwriting Agreement were made only for purposes of such agreement and as of specific dates, were solely for the benefit of the parties to such agreement, and may be subject to limitations agreed upon by the contracting parties, including being qualified by confidential disclosures exchanged between the parties in connection with the execution of the Underwriting Agreement.

Pursuant to the Underwriting Agreement, subject to certain exceptions, the Company, its directors and officers and certain significant stockholders have agreed not to sell or otherwise dispose of any of the Company’s Common Stock held by them for a period ending 90 days after the date of the Underwriting Agreement without first obtaining the written consent of Aegis Capital Corp., as representatives of the Underwriter, subject to certain exceptions.

The Common Stock is being offered and sold pursuant to the Company’s effective shelf registration statement on Form S-3 and an accompanying prospectus (Registration Statement No. 333-189538) filed with the Securities and Exchange Commission (the “SEC”) on June 24, 2013 and declared effective by the SEC on July 22, 2013, and a preliminary and final prospectus supplement filed with the SEC in connection with the Company’s takedown relating to the offering. A copy of the opinion of Sichenzia Ross Friedman Ference LLP relating to the legality of the issuance and sale of the shares of Common Stock in the offering is attached as Exhibit 5.1 hereto.

The net proceeds to the Company from the sale of the shares of Common Stock is expected to be approximately $27.2 million, after deducting underwriting discounts and commissions and other estimated offering expenses payable by the Company, assuming no exercise by the Underwriters of the 45 day option which the Company has granted the Underwriters under the terms of the Underwriting Agreement to purchase up to an additional 622,500 shares of Common Stock to cover over-allotments, if any. On October 25, 2013, the Underwriters exercised the over-allotment option in full bringing the expected total gross proceeds from the offering to approximately $34.6 million, before underwriting discounts and commissions and other offering expenses payable by the Company. The Underwriting Agreement is filed as an exhibit to this report, and the description of the terms of the Underwriting Agreement in this report is qualified in its entirety by reference to such exhibit. The offering is expected to close on or about October 30, 2013, subject to customary closing conditions.

The foregoing description of the terms of the Underwriting Agreement does not purport to be complete and are subject to, and qualified in their entirety by reference to, the Underwriting Agreement, which is filed herewith as Exhibits 1.1 and are incorporated herein by reference.

On October 24, 2013, the Company issued a press release announcing that it had priced the offering. A copy of the press release is attached hereto as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

On October 25, 2013, the Company issued a press release announcing that the Underwriters had exercised the over-allotment option in full. A copy of the press release is attached hereto as Exhibit 99.2 to this Current Report on Form 8-K and is incorporated herein by reference.

Forward-Looking Statements

Certain statements in this Current Report on Form 8-K are forward-looking statements that involve a number of risks and uncertainties. Such forward-looking statements include statements about the expected settlement of the sale and purchase of securities described herein and the Company’s receipt of net proceeds therefrom. For such statements, the Company claims the protection of the Private Securities Litigation Reform Act of 1995. Actual events or results may differ materially from the Company’s expectations. Factors that could cause actual results to differ materially from the forward-looking statements include, but are not limited to, the Company’s ability to satisfy applicable closing conditions under the Underwriting Agreement. Additional factors that could cause actual results to differ materially from those stated or implied by the Company’s forward-looking statements are disclosed in the Prospectus Supplement and accompanying prospectus and the Company’s reports filed with the Securities and Exchange Commission.

The following exhibits are filed with this Current Report on Form 8-K:

SIGNATURE

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Dated: October 25, 2013