UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 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): August 4, 2014

CTI BIOPHARMA CORP.

(Exact name of registrant as specified in its charter)

3101 Western Avenue, Suite 600

Seattle, Washington 98121

(Address of principal executive offices)

Registrant’s telephone number, including area code: (206) 282-7100

Not applicable

(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 (see General Instruction A.2. below):

Item 2.02. Results of Operations and Financial Condition.

The information provided pursuant to this Item 2.02 shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) or otherwise subject to the liabilities of that section, and shall not be incorporated by reference into any filing or other document filed by the Company pursuant to the Exchange Act or the Securities Act of 1933, as amended (the “Securities Act”), except as shall be expressly set forth by specific reference in such filing or document. The information provided pursuant to this Item 2.02 shall instead be deemed “furnished.”

On August 4, 2014, CTI BioPharma Corp. (the “Company”) issued a press release in Italy announcing, among other things, its financial results for the quarter ended June 30, 2014 and providing certain additional information requested by CONSOB, the Italian securities regulatory authority, about the Company (the “Quarterly Italian Press Release”). An English translation of the Quarterly Italian Press Release is attached as Exhibit 99.1 and incorporated by reference herein. The particular information provided at the request of CONSOB in the Quarterly Italian Press Release is set forth under the heading “Information required by CONSOB pursuant to section 114, paragraph 5, of the Italian Legislative Decree no. 58/98”. The Company’s press release issued in the United States pertaining to its financial results for the quarter ended June 30, 2014 has been furnished to the Commission on a separate Current Report on Form 8-K, also dated August 4, 2014.

Item 7.01. Regulation FD Disclosure.

The information provided pursuant to this Item 7.01 shall not be deemed “filed” for purposes of Section 18 of the Exchange Act or otherwise subject to the liabilities of that section, and shall not be incorporated by reference into any filing or other document filed by the Company pursuant to the Exchange Act or the Securities Act except as shall be expressly set forth by specific reference in such filing or document. The information provided pursuant to this Item 7.01 shall instead be deemed “furnished.”

On August 4, 2014, the Company issued the Quarterly Italian Press Release containing the information requested by CONSOB discussed above. An English translation of the Quarterly Italian Press Release is attached hereto as Exhibit 99.1.

Item 9.01. Financial Statements and Exhibits.

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