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): July 19, 2011

AMAG PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

Delaware

(State or other jurisdiction of incorporation)

(617) 498-3300

(Registrant’s telephone number, including area code)

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:

x          Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o            Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o            Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o            Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Item 2.02. Results of Operations and Financial Condition.

On July 20, 2011, AMAG Pharmaceuticals, Inc., a Delaware corporation (“AMAG” or the “Company”), and Allos Therapeutics, Inc., a Delaware corporation (“Allos”), issued a joint press release announcing, among other things, the execution of the Merger Agreement (defined below), certain preliminary estimates of financial results of the Company for the quarter ended June 30, 2011 and its intention to hold a conference call regarding these topics. A copy of the press release is attached hereto as Exhibit 99.1 to this Current Report on Form 8-K and incorporated herein by reference.

Item 8.01 Other Events.

On July 19, 2011, the Company, Alamo Acquisition Sub, Inc., a Delaware corporation and wholly owned subsidiary of AMAG (“Merger Sub”) and Allos, entered into an Agreement and Plan of Merger and Reorganization (the “Merger Agreement”), pursuant to which, Merger Sub will, upon the terms and subject to the satisfaction or waiver of the conditions therein, merge with and into Allos, with Allos continuing as the surviving corporation and as a wholly-owned subsidiary of AMAG.  On July 20, 2011, AMAG and Allos issued a joint press release announcing, among other things, the execution of the Merger Agreement. A copy of the press release is attached hereto as Exhibit 99.1 to this Current Report on Form 8-K and incorporated herein by reference.

Additional Information and Where You Can Find It

This report does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval. The proposed merger between AMAG and Allos will be submitted to the respective stockholders of AMAG and Allos for their consideration.

AMAG will file a Registration Statement on Form S-4 containing a joint proxy statement/prospectus of Allos and AMAG and other documents concerning the proposed acquisition with the Securities and Exchange Commission (the “SEC”).  Investors are urged to read the joint proxy statement/prospectus when it becomes available and other relevant documents filed with the SEC because they will contain important information.  Security holders may obtain a free copy of the proxy statement/prospectus (when it is available) and other documents filed by Allos and AMAG with the SEC at the SEC’s website at www.sec.gov.  The joint proxy statement/prospectus and other documents may also be obtained for free by contacting Allos’ Investor Relations by e-mail at investorrelations@allos.com, by telephone at (303) 426-6262 or by mail at Investor Relations, Allos Therapeutics, Inc., 11080 CirclePoint Road, Suite 200, Westminster, CO 80020 or by contacting AMAG’s Investor Relations by e-mail at cmiceli@amagpharma.com, by telephone at (617) 498-3361 or by mail at Investor Relations, AMAG Pharmaceuticals, Inc., 100 Hayden Avenue, Lexington, MA 02421.

Allos, AMAG, certain of their respective directors, executive officers, members of management and employees may, under the rules of the SEC, be deemed to be participants in the solicitation of proxies in connection with the proposed merger.  Information regarding Allos’ directors and executive officers and their beneficial ownership of Allos’ common stock is also set forth in Allos’ annual proxy statement on Schedule 14A filed with the SEC on April 29, 2011. This document is available free of charge at the SEC’s website at www.sec.gov or by going to Allos’ Investors page on its corporate website at www.allos.com.  Information concerning AMAG’s directors and executive officers and their beneficial ownership of AMAG’s common stock is set forth in AMAG’s annual proxy statement on Schedule 14A filed with the SEC on April 18, 2011.  This document is available free of charge at the SEC’s website at www.sec.gov or by going to AMAG’s Investors page on its corporate website at www.amagpharma.com.  Additional information regarding the persons who may, under the rules of the SEC, be deemed “participants” in the solicitation of proxies in connection with the proposed merger, and a description of

2

their direct and indirect interests in the proposed merger, which may differ from the interests of Allos’ investors or AMAG’s investors generally, will be set forth in the joint proxy statement/prospectus when it is filed with the SEC.

Forward-Looking Statements

This report contains forward-looking statements that are made pursuant to the safe harbor provisions of the Private Securities Litigation Reform Act of 1995.  Terminology such as “may,” “will,” “should,” “expects,” “intends,” “plans,” “anticipates,” “believes,” “estimates,” “predicts,” “projects,” “potential,” “continue,” and other similar terminology or the negative of these terms, are intended to identify such forward-looking statements, but their absence does not mean that a particular statement is not forward-looking.  Such forward-looking statements are not guarantees of future performance and are subject to risks and uncertainties that may cause actual results to differ materially from those anticipated by the forward-looking statements.  These statements are not guarantees of future performance, involve risks, uncertainties and assumptions that are difficult to predict, and are based upon assumptions as to future events that may not prove accurate. Examples of such forward looking statements include Allos and AMAG’s expectations with respect to the synergies, costs and other anticipated financial impacts of the proposed transaction; future financial and operating results of the combined company; the combined company’s plans, objectives, expectations and intentions with respect to future operations; approval of the proposed transaction by requisite stockholders; the satisfaction of the closing conditions to the proposed transaction; and the timing of the completion of the proposed transaction. In any forward-looking statement in which AMAG or Allos expresses an expectation or belief as to future results, such expectation or belief is expressed in good faith and believed to have a reasonable basis, but there can be no assurance that the statement or expectation or belief will result or be achieved or accomplished. The following factors, among others, could cause actual results to differ materially from those described in the forward-looking statements: failure of Allos or AMAG stockholders to approve the proposed transaction; the challenges and costs of closing the proposed transaction, integrating the two companies, restructuring the combined company; the possibility that the expected synergies will not be realized, or will not be realized within the expected time period; the ability to retain key employees; and other economic, business, competitive, and/or regulatory factors affecting the businesses of Allos and AMAG generally, including those set forth in the filings of Allos and AMAG with the Securities and Exchange Commission, especially in the “Risk Factors” section of Allos’ Quarterly Report on Form 10-Q for the quarter ended March 31, 2011 filed with the SEC on May 10, 2011, the “Risk Factors” section of AMAG’s Quarterly Report on Form 10-Q for the quarter ended March 31, 2011 filed with the SEC on May 9, 2011, and in Allos’ and AMAG’s other periodic reports and filings with the SEC.  Allos and AMAG cautions investors not to place undue reliance on the forward-looking statements contained herein.  All forward-looking statements are based on information currently available to Allos and AMAG on the date hereof, and Allos and AMAG undertake no obligation to revise or update these forward-looking statements to reflect events or circumstances after the date of this press release, except as required by law.

Item 9.01.  Financial Statements and Exhibits.

(d)           Exhibits.

99.1                                                                        Joint Press Release Announcing Agreement and Plan of Merger and Reorganization, dated July 20, 2011

3

SIGNATURES

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:  July 20, 2011

4

EXHIBIT INDEX

5