UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K/A

(AMENDMENT NO. 4)

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): April 8, 2011

ADVENTRX Pharmaceuticals, Inc.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (858) 552-0866

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:

EXPLANATORY NOTE

On April 8, 2011, ADVENTRX Pharmaceuticals, Inc. (the “Company”) completed its acquisition of SynthRx, Inc. (“SynthRx”) pursuant to the terms of the Agreement and Plan of Merger dated February 12, 2011, by and among the Company, SRX Acquisition Corporation, a wholly owned subsidiary of the Company, SynthRx and, solely with respect to Sections 2 and 8 of the agreement, an individual who was a principal stockholder of SynthRx, and SynthRx became a wholly owned subsidiary of the Company. The Company’s acquisition of SynthRx and related matters were reported in the Company’s Current Report on Form 8-K filed on April 11, 2011 (the “Original Current Report”) and the financial statements of SynthRx and the pro forma financial information required by Item 9.01(a)(1) and (b)(1) of Form 8-K were reported in the Company’s Amendment No. 1 to the Original Current Report filed on June 3, 2011 (“Amendment No. 1”). On October 25, 2011, the Company filed Amendment No. 2 to the Original Current Report (“Amendment No. 2”) to provide the financial statements of SynthRx for the three months ended March 31, 2011 and unaudited pro forma condensed combined financial information for the six months ended June 30, 2011. On November 9, 2011, the Company filed Amendment No. 3 to the Original Current Report (“Amendment No. 3”) to provide unaudited pro forma condensed combined financial information for the nine months ended September 30, 2011.

The Company is filing this Amendment No. 4 to the Original Current Report to provide unaudited pro forma condensed combined financial information for the year ended December 31, 2011. In addition, the Company’s Annual Report on Form 10-K for the year ended December 31, 2011, which was filed on March 8, 2012, includes estimated fair values, as of December 31, 2011, of net tangible and intangible assets acquired and liabilities assumed in the SynthRx acquisition, which supersede the preliminary estimated fair values of such assets and liabilities included in Exhibit 99.3, which was filed with Amendment No. 1. Except as described above, all information in and exhibits to the Original Current Report, Amendment No. 1, Amendment No. 2 and Amendment No. 3 remain unchanged.

(b) Pro Forma Financial Information

The following unaudited pro forma financial information is attached as Exhibit 99.7 to this report and is incorporated herein by reference:

(d) Exhibits.

The list of exhibits called for by this Item is incorporated by reference to the Exhibit Index filed with this report.

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.

Exhibit Index