UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 8-K/A

(Amendment No. 1)

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): September 4, 2012

SPECTRUM PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (702) 835-6300

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:

EXPLANATORY NOTE

This Current Report on Form 8-K/A amends and supplements Item 9.01 of the Current Report on Form 8-K filed by Spectrum Pharmaceuticals, Inc. (“Spectrum”) on September 5, 2012 to include the financial statements and pro forma financial information required by parts (a) and (b) of Item 9.01 of Form 8-K that were not available at the time of the filing of the initial report and filed by amendment no later than 71 calendar days after the initial Form 8-K was required to be filed with the Securities and Exchange Commission. Such financial statements and pro forma financial information are required as a result of Spectrum’s September 5, 2012 acquisition of Allos Therapeutics, Inc. (“Allos”) by a merger of Sapphire Acquisition Sub, Inc., a wholly-owned subsidiary of Spectrum, with and into Allos pursuant to the terms of that certain Agreement and Plan of Merger and Reorganization, dated as of April 4, 2012, by and among Spectrum, Allos and the other parties named therein (the “Merger Agreement”).

Item 9.01 Financial Statements and Exhibits.

(a) Financial Statements of Businesses Acquired

The audited financial statements of Allos and statements of operations, statements of comprehensive loss, statements of cash flows and statements of stockholders’ equity for the years ended December 31, 2011, December 31, 2010 and December 31, 2009 are incorporated herein by reference from Allos’ Annual Report on Form 10-K for the year ended December 31, 2011 filed on March 26, 2012 and Amendment No. 1 to Annual Report on Form 10-K/A for the year ended December 31, 2011 filed on April 27, 2012.

The unaudited balance sheet of Allos as of June 30, 2012 and related statements of operations and cash flows for the six months ended June 30, 2012 are incorporated herein by reference from Allos’ Quarterly Report on Form 10-Q for the three and six months ended June 30, 2012 filed on August 3, 2012 and Amendment No. 1 to Quarterly Report on Form 10-Q/A for the three and six months ended June 30, 2012 filed on August 17, 2012.

The unaudited balance sheet of Allos as of June 30, 2012 and related statements of operations and cash flows for the period ended June 30, 2012 are being filed in this Current Report on Form 8-K/A.

(b) Unaudited Pro Forma Condensed Combined Financial Information

On September 5, 2012, Spectrum completed the acquisition of Allos for cash consideration of $205.2 million pursuant to the terms of the Merger Agreement.

The following unaudited pro forma condensed combined statements of income for the year ended December 31, 2011 and for the six months ended June 30, 2012 included in this report have been prepared as if the acquisition occurred on January 1, 2011. The unaudited pro forma condensed combined balance sheet as of June 30, 2012 has been prepared as if the acquisition occurred on June 30, 2012. The historical consolidated statements of income have been adjusted in the unaudited pro forma condensed combined financial statements to only give effect to pro forma events that are (1) directly attributable to the acquisition; (2) factually supportable; and (3) expected to have a continuing impact on the combined results of Spectrum and Allos. The unaudited pro forma condensed combined financial information should be read in conjunction with the accompanying notes to the unaudited pro forma condensed combined financial statements presented below.

In addition, the unaudited pro forma condensed combined financial information was based on and should be read in conjunction with:

The unaudited pro forma condensed combined financial information is based on estimates and assumptions and has been presented for illustrative purposes only and is not necessarily indicative of what the combined company’s results of operations actually would have been had the acquisition been completed as of the dates indicated. Additionally, the unaudited pro forma condensed combined financial information is not necessarily indicative of the condensed combined financial position or results of operations in future periods or the results that actually would have been realized if the acquisition had been completed as of the dates indicated.

The unaudited pro forma adjustments related to the acquisition have been prepared using the acquisition method of accounting under existing U.S. generally accepted accounting principles, which are subject to change and interpretation. The allocation of purchase price for acquisitions requires extensive use of accounting estimates and judgments to allocate the purchase price to the identifiable tangible and intangible assets acquired and liabilities assumed based on their respective fair values. The purchase price for Allos was allocated to tangible and intangible assets acquired and liabilities assumed based on their estimated fair values at the acquisition date. Spectrum has engaged an independent third-party valuation firm to assist in determining the estimated fair values of in-process research and development, identifiable intangible assets and certain tangible assets. Such a valuation requires significant estimates and assumptions including but not limited to: determining the timing and estimated costs to complete the in-process projects, projecting regulatory approvals, estimating future cash flows, and developing appropriate discount rates. Spectrum believes the fair values assigned to the assets acquired and liabilities assumed are based on reasonable assumptions. The fair value estimates for the purchase price allocation may change if additional information becomes available. The unaudited pro forma condensed combined financial information does not reflect the realization of any potential operating synergies or other operational improvements, if any, that the combined company may achieve as a result of the acquisition, the costs to integrate the operations of Spectrum and Allos or the costs necessary to achieve potential operating synergies and revenue enhancements. However, as Spectrum integrates the acquisition it does expect to realize cost saving synergies.

UNAUDITED PRO FORMA CONDENSED COMBINED

STATEMENT OF OPERATIONS

For the year ended December 31, 2011

See notes to unaudited pro forma condensed combined financial statements.

UNAUDITED PRO FORMA CONDENSED COMBINED

STATEMENT OF OPERATIONS

For the six months ended June 30, 2012

See notes to unaudited pro forma condensed combined financial statements.

UNAUDITED PRO FORMA CONDENSED COMBINED

BALANCE SHEET

As of June 30, 2012

See notes to unaudited pro forma condensed combined financial statements.

Note 1 – Description of Transaction

On September 5, 2012, Spectrum completed the acquisition of Allos for cash consideration of $205.2 million pursuant to the terms of the Merger Agreement. Pursuant to the terms of the Merger Agreement, Spectrum acquired a total of 96,259,850 shares pursuant to a tender offer, representing approximately 89.98% of the outstanding shares of Allos common stock on September 5, 2012 for an amount equal to the price per share of $1.82 in cash plus the contingent value right described in Note 4 below. Allos develops and markets an anti-cancer therapeutic in the United States. As a result of the acquisition, Spectrum acquired an assembled sales force and an anti-cancer therapeutic which enhanced its existing product base.

Note 2 – Basis of Presentation

The unaudited pro forma condensed combined financial information was prepared using the June 30, 2012 historical financial statements of Spectrum and Allos, which were prepared under Unites States GAAP (“GAAP”). The acquisition is accounted for under the purchase method of accounting in accordance with the Financial Accounting Standards Board Accounting Standards Codification (“ASC”) Topic 805, Business Combinations. Under the purchase method of accounting, the total purchase price, calculated as described in Note 4 to these unaudited pro forma condensed combined financial statements, is allocated to the net tangible and intangible assets acquired and liabilities assumed of Allos based on their estimated fair values. The allocation of purchase price for acquisitions requires extensive use of accounting estimates and judgments to allocate the purchase price to the identifiable tangible and intangible assets acquired and liabilities assumed based on their respective fair values. The purchase price for Allos was allocated to tangible and intangible assets acquired and liabilities assumed based on their estimated fair values at the acquisition date. Spectrum has engaged an independent third-party valuation firm to assist in determining the estimated fair values of in-process research and development, identifiable intangible assets and certain tangible assets. Such a valuation requires significant estimates and assumptions including but not limited to: determining the timing and estimated costs to complete the in-process projects, projecting regulatory approvals, estimating future cash flows, and developing appropriate discount rates. Spectrum believes the fair values assigned to the assets acquired and liabilities assumed are based on reasonable assumptions. The fair value estimates for the purchase price allocation may change if additional information becomes available. The unaudited pro forma condensed combined financial information is presented after giving effect to the acquisition of Allos as if it occurred on January 1, 2011 for the year ended December 31, 2011 and six months ended June 30, 2012. Certain reclassifications have been made to the historical financial statements of Allos to conform to Spectrum’s presentation.

Note 3 – Accounting Policies

As a result of the continuing review of Allos’ accounting policies, Spectrum may identify differences between the accounting policies of the two companies that, when conformed, could have a material impact on the combined financial statements. At this time, Spectrum is not aware of any differences that would have a material impact on the combined financial statements. The unaudited pro forma condensed combined financial statements do not assume any differences in accounting policies.

Note 4 – Purchase Price

The purchase price consideration for the Allos acquisition consisted of $205.2 million in cash. As part of the purchase consideration, Spectrum agreed to pay Allos shareholders an additional $0.11 per share if FOLOTYN® receives a conditional regulatory approval in the European Union, or the EU, by December 31, 2012 and the first reimbursable commercial sale of FOLOTYN® in the lead indication in the third EU major market country is made by December 31, 2013. On June 21, 2012, Allos received a letter from the European Commission (the “EC”) stating that the EC had adopted the Committee for Medicinal Products for Human Use’s opinion recommending against approval of the Marketing Authorisation Application. The decision is final and binding. Therefore, Spectrum management does not believe that the milestones triggering the contingent value right are achievable within the specified time frame and accordingly, did not value the contingent value right. Spectrum will continue to evaluate the fair value of the contingent value right through December 31, 2013.

Fair Value Estimate of Assets Acquired and Liabilities Assumed

Under the purchase method of accounting, the total purchase consideration is allocated to Allos net tangible and intangible assets based on their estimated fair values as of the closing date. The excess of the purchase price over the fair value of assets acquired and liabilities assumed was allocated to goodwill. The goodwill acquired is not deductible for tax purposes. The following table summarizes the fair value of the net assets acquired as of September 5, 2012 ($ in 000’s):

The acquired intangible assets consisted of developed technology for approved indications of currently marketed products. The acquired intangible assets principally relate to the FOLOTYN® distribution rights in the United States and Canada. The weighted-average amortization period for such intangible assets acquired is outlined in the table below:

Purchase price adjustments recorded subsequent to this date will affect the recorded amount of goodwill. Goodwill acquired at September 5, 2012 will differ from the estimated amount included in the accompanying unaudited pro forma condensed combined financial statements prepared as of June 30, 2012.

Note 5 – Unaudited Pro Forma Adjustments

Pro forma adjustments are necessary to reflect the total purchase price, to reflect amounts related to Allos’ net tangible and intangible assets at an amount equal to the estimated fair values on the closing date, and to reflect changes in amortization expense resulting from the estimated fair value adjustments to net intangible assets. The unaudited pro forma condensed combined financial statements should be read in conjunction with the accompanying notes to the unaudited pro forma condensed combined financial statements presented below and with the separate historical financial statements of Spectrum and Allos.

Adjustments included in the column under the heading “Pro Forma Adjustments” represent the following:

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.

Date: November 19, 2012

EXHIBIT INDEX

* Filed herewith