ALTH Allos Therapeutics, Inc. (MM)

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Filed Pursuant to Rule 424(b)(5)
Registration File No. 333-143198

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(To Prospectus dated June 5, 2007)

7,750,000 Shares

Common stock

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We are offering 7,750,000 shares of our common stock, par value $0.001 per share.

Our common stock is quoted on The NASDAQ Global Market under the symbol "ALTH." On March 30, 2009, the reported last sale price of our common stock on The NASDAQ Global Market was $6.99 per share.

Investing in our common stock involves a high degree of risk. Before buying any shares, you should read the discussion of material risks of investing in our common stock under the sections captioned "Risk Factors" contained in our annual report on Form 10-K for the year ended December 31, 2008, which has been filed with the Securities and Exchange Commission and is incorporated by reference in this prospectus supplement and the accompanying prospectus.

Neither the Securities and Exchange Commission nor any other regulatory body has approved or disapproved of these securities or passed upon the accuracy or adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

The underwriter may also purchase up to an additional 1,162,500 shares of our common stock at the public offering price, less the underwriting discounts and commissions, to cover over-allotments, if any, within 30 days from the date of this prospectus supplement. If the underwriter exercises this option in full, the total underwriting discounts and commissions will be $1,604,250, and our total proceeds, before expenses, will be $54,544,500.

The underwriter is offering the common stock as set forth under "Underwriting." The underwriter expects to deliver the shares against payment on or about April 3, 2009.

UBS Investment Bank

The date of this prospectus supplement is March 31, 2009.

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You should rely only on the information contained or incorporated by reference in this prospectus supplement, the accompanying prospectus and any free writing prospectus that we have authorized for use in connection with this offering. We have not, and the underwriter has not, authorized anyone to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not, and the underwriter is not, making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference in this prospectus supplement and the accompanying prospectus, and any free writing prospectus that we have authorized for use in connection with this offering is accurate only as of the date of those respective documents. Our business, financial condition, results of operations and prospects may have changed since those dates. You should read this prospectus supplement, the accompanying prospectus, the documents incorporated by reference in this prospectus supplement and the accompanying prospectus, and any free writing prospectus that we have authorized for use in connection with this offering when making your investment decision. You should also read and consider the information in the documents we have referred you to in the sections of this prospectus supplement entitled "Where you can find more information" and "Incorporation by reference."

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About this prospectus supplement

This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering of common stock and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference into this prospectus supplement and the accompanying prospectus. The second part, the accompanying prospectus dated June 5, 2007, provides more general information. To the extent the information contained in this prospectus supplement differs or varies from the information contained in the accompanying prospectus or the documents incorporated by reference, the information in this prospectus supplement shall control.

Unless we indicate otherwise, references in this prospectus supplement and the accompanying prospectus to "Allos," "we," "us," or "our" refer to Allos Therapeutics, Inc., a Delaware corporation.

This prospectus supplement and the accompanying prospectus, including the information incorporated by reference into this prospectus supplement and the accompanying prospectus, and any free writing prospectus that we have authorized for use in connection with this offering include trademarks, service marks and trade names owned by us or others. Allos Therapeutics, Inc., the Allos Therapeutics, Inc. logo, and all other Allos names are trademarks of Allos Therapeutics, Inc. in the U.S. and in other selected countries. All other trademarks, service marks and trade names included or incorporated by reference in this prospectus supplement and the accompanying prospectus and any free-writing prospectus that we have authorized for use in connection with this offering are the property of their respective owners.

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Prospectus supplement summary

This summary highlights selected information appearing elsewhere or incorporated by reference in this prospectus supplement and the accompanying prospectus and any free-writing prospectus that we have authorized for use in connection with this offering and may not contain all of the information that is important to you. This prospectus supplement and the accompanying prospectus include information about the shares we are offering as well as information regarding our business and financial data. You should read this prospectus supplement and the accompanying prospectus, including information incorporated by reference, and any free writing prospectus that we have authorized for use in connection with this offering, in their entirety. Investors should carefully consider the information set forth under "Risk factors" in this prospectus supplement.

ABOUT ALLOS THERAPEUTICS, INC.

We are a biopharmaceutical company focused on developing and commercializing innovative small molecule drugs for the treatment of cancer. Our goal is to build a profitable company by generating income from products we develop and commercialize, either alone or together with one or more potential strategic partners. We strive to develop proprietary products that have the potential to improve the standard of care in cancer therapy. Our focus is on product opportunities for oncology that leverage our internal clinical development and regulatory expertise and address important markets with unmet medical need. We may also seek from time to time to grow our existing portfolio of product candidates through product acquisition and in-licensing efforts.

Since our inception in 1992, we have not generated any revenue from product sales and have experienced significant net losses and negative cash flows from operations. We have incurred these losses principally from costs incurred in our research and development programs, clinical manufacturing and from our marketing, general and administrative expenses. We expect to continue incurring net losses and negative cash flows from operations for the foreseeable future. Our ability to generate revenue and achieve profitability is dependent on our ability, alone or with partners, to successfully complete the development of pralatrexate, conduct clinical trials, obtain the necessary regulatory approvals, and manufacture and market pralatrexate.

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PRALATREXATE

The following table summarizes the target indications and clinical development status of our lead product candidate, pralatrexate:

Pralatrexate is a novel targeted antifolate designed to accumulate preferentially in cancer cells. Based on preclinical studies, we believe that pralatrexate selectively enters cells expressing RFC-1, a protein that is over expressed on cancer cells compared to normal cells. Once inside cancer cells, pralatrexate is efficiently polyglutamylated, which leads to high intracellular drug retention. Polyglutamylated pralatrexate essentially becomes "trapped" inside cancer cells, making it less susceptible to efflux-based drug resistance. Acting on the folate pathway, pralatrexate interferes with DNA synthesis and triggers cancer cell death. We believe pralatrexate has the potential to be delivered as a single agent or in combination therapy regimens.

In February 2009, we announced the final results from PROPEL, our pivotal Phase 2 trial of pralatrexate in patients with relapsed or refractory peripheral T-cell lymphoma, or PTCL. The trial enrolled a total of 115 patients, 109 of whom were considered evaluable for response according to the trial protocol. The results of the trial demonstrated that 29 of 109 evaluable patients, or 27%, achieved a response as assessed by central independent oncology review, which is the primary endpoint of the trial. The Kaplan-Meier estimate for the median duration of response was 287 days, or 9.4 months. Duration of response is the key secondary endpoint of the trial. The most common grade 3/4 adverse events were thrombocytopenia, which was observed in 32% of patients; mucosal inflammation in 21% of patients; neutropenia in 20% of patients; and anemia in 17% of patients.

Based on the results of this trial, we submitted a New Drug Application, or NDA, to the U.S. Food and Drug Administration, or FDA, for pralatrexate for the treatment of patients with relapsed or

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refractory PTCL in March 2009. We have requested a priority review of the application, which, if granted, would give the FDA six months from receipt of the submission to take action on the application. If it is approved for marketing, we intend to commercialize pralatrexate by building an oncology-focused U.S. sales and marketing organization that may be complemented by co-promotion arrangements with pharmaceutical or biotechnology partners, where appropriate. We intend to enter into co-promotion or out-licensing arrangements with other pharmaceutical or biotechnology partners where necessary to reach foreign market segments that are not reachable by a U.S.-based sales force or when deemed strategically and economically advisable. We currently retain exclusive worldwide commercial rights to pralatrexate for all indications.

The PROPEL trial was conducted under an agreement reached with the FDA under its special protocol assessment, or SPA, process. The SPA process allows for FDA evaluation of a clinical trial protocol intended to form the primary basis of an efficacy claim in support of an NDA, and provides an agreement that the trial design, including trial size, clinical endpoints and data analyses are acceptable to the FDA. However, the SPA agreement is not a guarantee of approval, and we cannot assure you that the design of, or data collected from, the PROPEL trial will be adequate to demonstrate the safety and efficacy of pralatrexate for the treatment of patients with relapsed or refractory PTCL, or otherwise be sufficient to support FDA or any foreign regulatory approval. For example, the response rate, duration of response and safety profile required to support FDA approval are not specified in the PROPEL trial protocol and will be subject to FDA review. In addition, the median duration of response reported above is a Kaplan-Meier estimate based on the length of follow up for all responders at the time the PROPEL trial database was locked. As a result, the median duration of response may change based on continued patient follow up.

In addition to the PROPEL trial, we are committed to evaluating pralatrexate for oncology use as a single agent and in combination with other therapies. We currently have seven ongoing clinical trials involving pralatrexate, including the PROPEL trial, and plan to initiate additional trials to evaluate pralatrexate's potential clinical utility in other hematologic malignancies and solid tumor indications. The following clinical trials involving pralatrexate are currently open for enrollment:

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a Phase 2b, randomized, multi-center study comparing pralatrexate and Tarceva (erlotinib), both with vitamin B ₁₂ and folic acid supplementation, in patients with Stage IIIB/IV non-small cell lung cancer, or NSCLC, who are, or have been, cigarette smokers who have failed treatment with at least one prior platinum-based chemotherapy regimen. We initiated patient enrollment in this study in January 2008. The study will seek to enroll approximately 160 patients in up to 50 investigative sites worldwide. Based on current enrollment rates, we expect to complete patient enrollment in this study in the third quarter of 2009.



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a Phase 2, open-label, single-arm, multi-center study of pralatrexate with vitamin B ₁₂ and folic acid supplementation in patients with advanced or metastatic relapsed transitional cell carcinoma, or TCC, of the urinary bladder. We initiated patient enrollment in this study in July 2008. The study will seek to enroll approximately 41 patients in up to 20 investigative sites worldwide.



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a Phase 1/2a, open-label, multi-center study of pralatrexate and gemcitabine with vitamin B ₁₂ and folic acid supplementation in patients with relapsed or refractory non-Hodgkin's lymphoma, or NHL, and Hodgkin's disease. We initiated patient enrollment in this study in May 2007. We plan to enroll up to 54 evaluable patients in the Phase 1 portion of the study and up to 45 additional patients with relapsed or refractory PTCL in the expanded Phase 2a portion of the study.



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a Phase 1, open-label, multi-center study of pralatrexate with vitamin B ₁₂ and folic acid supplementation in patients with relapsed or refractory cutaneous T-cell lymphoma. We initiated patient enrollment in this study in August 2007. We plan to enroll up to 56 evaluable

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patients in the study with the objective of determining the optimal dose and safety profile, including at least 20 patients at what we believe to be the optimal dose and schedule.

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a Phase 1/2, open-label, single-center study of pralatrexate with vitamin B ₁₂ and folic acid supplementation in patients with relapsed or refractory NHL and Hodgkin's disease. This study is currently focused on exploring alternate dosing and administration schedules in patients with B-cell lymphoma to further evaluate pralatrexate's potential clinical utility in this setting.

In addition to our ongoing NSCLC and bladder cancer studies, we are evaluating the potential future development of pralatrexate for other solid tumor indications, including Stage III/IV head and neck cancer and Stage III/IV breast cancer, among others. There can be no assurances that we will pursue the development of pralatrexate for one or more of these indications or that such development efforts will be ultimately successful.

The FDA has awarded orphan drug status to pralatrexate for the treatment of patients with T-cell lymphoma, follicular lymphoma and diffuse large B-cell lymphoma. Under the U.S. Orphan Drug Act, if we are the first company to receive FDA approval for pralatrexate for the designated orphan drug indication, we will obtain seven years of marketing exclusivity during which the FDA may not approve another company's application for pralatrexate for the same orphan indication. Orphan drug exclusivity would not prevent FDA approval of a different drug for the orphan indication or the same drug for a different indication. The FDA has also awarded fast track designation to pralatrexate for the treatment of patients with T-cell lymphoma. The FDA's fast track program is designed to facilitate the development and expedite the review of new drugs that are intended to treat serious or life-threatening conditions and that demonstrate the potential to address unmet medical needs.

The European Medicines Agency, or EMEA, has granted Orphan Medicinal Product Designation to pralatrexate for the treatment of PTCL and non-papillary TCC of the urinary bladder. The EMEA Orphan Medicinal Product Designation is intended to promote the development of drugs that may provide significant benefit to patients suffering from rare diseases identified as life-threatening or very serious. Under EMEA guidelines, Orphan Medicinal Product Designation provides ten years of potential market exclusivity once the product candidate is approved for marketing for the designated indication in the European Union.

In December 2002, we entered into a license agreement with Memorial Sloan-Kettering Cancer Center, SRI International and Southern Research Institute. Under the agreement, as amended, we obtained exclusive worldwide rights to a portfolio of patents and patent applications related to pralatrexate and its uses. The portfolio currently consists of two issued patents in the U.S., two allowed patent applications in Europe, and pending patent applications in the U.S., Canada, Europe, Australia, Japan, China, Brazil, Indonesia, South Korea, Mexico, Norway, New Zealand, the Philippines, Singapore, and South Africa.

COMPANY INFORMATION

We were incorporated in Delaware in 1992. Our principal executive offices are located at 11080 CirclePoint Road, Suite 200, Westminster, Colorado 80020. Our telephone number is (303) 426-6262. Our website is located at www.allos.com. We have not incorporated by reference into this prospectus supplement or the accompanying prospectus the information on our website, and you should not consider it to be a part of this document. Our website address is included in this document as an inactive textual reference only.

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The offering

The number of shares of common stock to be outstanding immediately after this offering as shown above is based on 81,238,812 shares of common stock outstanding on December 31, 2008. This number excludes, as of December 31, 2008:

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7,236,512 shares of common stock issuable upon exercise of outstanding options granted under our stock option plans at a weighted average exercise price of $5.14 per share;



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2,259,777 shares of common stock available for sale under our 2001 Employee Stock Purchase Plan; and



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5,146,701 shares of common stock available for grant under our 2008 Equity Incentive Plan.

Unless we specifically state otherwise, the information in this prospectus supplement assumes that the underwriter does not exercise its option to purchase up to 1,162,500 additional shares of our common stock within 30 days after the date of this prospectus supplement.

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Risk factors

Investment in our common stock involves a high degree of risk. Before deciding whether to invest in our common stock, you should consider carefully the risk factors discussed under the sections captioned "Risk Factors" contained in our Annual Report on Form 10-K for the year ended December 31, 2008, which are incorporated by reference in this prospectus supplement and the accompanying prospectus in their entirety. If any of these risks actually occurs, our business, financial condition, results of operations or cash flow could be seriously harmed. This could cause the trading price of our common stock to decline, resulting in a loss of all or part of your investment.

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Forward-looking statements

This prospectus supplement, the accompanying prospectus, the documents incorporated by reference and any free writing prospectus that we have authorized for use in connection with this offering contain various forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act. These statements are based on management's beliefs as well as assumptions made by, and information currently available to, management. These forward-looking statements can generally be identified as such because the context of the statement will include words such as "may," "will," "intends," "plans," "believes," "anticipates," "expects," "estimates," "predicts," "potential," "continue," or "opportunity," the negative of these words or words of similar import. Similarly, statements that describe our reserves and our future plans, strategies, intentions, expectations, objectives, goals or prospects are also forward-looking statements. Such statements are subject to certain risks, uncertainties and assumptions. Should one or more of these risks or uncertainties materialize, or should underlying assumptions prove incorrect, actual results may vary materially from those anticipated, estimated, projected or expected. Forward-looking statements include, without limitation, statements set forth in this prospectus supplement and incorporated by reference in this prospectus supplement regarding, among other things:

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the potential for the results of our PROPEL trial to support marketing approval of pralatrexate;



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any plans to develop sales and marketing capabilities, either internally or through contract relationships or strategic collaborations;



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our plans to continue development of our current product candidates, including pralatrexate, and to conduct clinical trials for the treatment of certain indications;



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the success and timing of our clinical trials, and the commencement of future clinical trials;



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the projected timelines for the announcement of results from our ongoing clinical trials;



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any plans to secure additional third-party manufacturing arrangements;



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our intention to evaluate the in-licensing or purchase of complementary products to enhance our oncology portfolio;



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the use of proceeds from this offering; and



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our liquidity.

Such forward-looking statements involve known and unknown risks, uncertainties and other factors that may cause our actual results, performance or achievements to differ materially from any future

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Forward-looking statements

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results, performance or achievements expressed or implied by such forward-looking statements. Important factors that could cause such differences include, among other things, the following:

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the inability to obtain regulatory approvals in the U.S. or Europe for our product candidates, including pralatrexate, or delays in such approvals;



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difficulties or delays we may experience in the conduct of clinical trials, and whether the results of those clinical trials will be adequate to support regulatory filings and/or approvals;



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the ability of our third-party collaborators to satisfactorily perform their functions relating to research and development activities and the manufacture of our product candidates;



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difficulties we may experience in developing and commercializing our small molecule drugs, including pralatrexate, to address the treatment of cancer and the enhancement of current therapies;



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difficulties we may experience in identifying, licensing or purchasing complementary products for our product portfolio;



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difficulties we may experience in identifying and successfully collaborating with appropriate corporate partners;



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the effect of capital market conditions and other factors on capital availability; and



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other factors referenced in this prospectus supplement and the accompanying prospectus.

Because the risks and uncertainties referred to above, as well as the risk factors incorporated by reference, could cause actual results or outcomes to differ materially from those expressed in any forward-looking statements made by us or on our behalf, you should not place undue reliance on any forward-looking statements. Further, any forward-looking statement speaks only as of the date on which it is made, and we undertake no obligation to update any forward-looking statement to reflect events or circumstances after the date on which the statement is made or to reflect the occurrence of unanticipated events. New factors emerge from time to time, and it is not possible for us to predict which factors will arise. In addition, we cannot assess the impact of each factor on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements. Except as required by law, we undertake no obligation to publicly revise our forward-looking statements to reflect events or circumstances that arise after the date of this prospectus or the supplement or the accompanying prospectus or the date of documents incorporated by reference in this prospectus supplement that include forward-looking statements.

You should rely only on the information contained, or incorporated by reference, in this prospectus supplement, the accompanying prospectus, the registration statement of which this prospectus supplement is a part, the documents incorporated by reference herein, and any applicable prospectus supplement and understand that our actual future results may be materially different from what we expect. We qualify all of the forward-looking statements in the foregoing documents by these cautionary statements. Unless required by law, we undertake no obligation to update or revise any forward-looking statements to reflect new information or future events or developments. Thus, you should not assume that our silence over time means that actual events are bearing out as expressed or implied in such forward-looking statements. Before deciding to purchase our securities, you should carefully consider the risk factors discussed herein or incorporated by reference, in addition to the other information set forth in this prospectus supplement, the accompanying prospectus and in the documents incorporated by reference.

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Use of proceeds

We estimate the net proceeds to us from the sale of the 7,750,000 shares of common stock in this offering to be approximately $46.8 million ($53.9 million if the underwriter's over-allotment option is exercised in full), after deducting the underwriting discounts and commissions and estimated offering expenses payable by us.

We currently intend to use the net proceeds from this offering primarily for activities relating to preparations for the potential commercial launch of pralatrexate, clinical and preclinical research and development of pralatrexate, working capital and general corporate purposes. We have not determined the amounts we plan to spend on any of the areas listed above or the timing of these expenditures. As a result, our management will have broad discretion to allocate the net proceeds of this offering. Pending the application of the net proceeds for these purposes, we intend to invest the net proceeds in money market funds and/or short-term, interest-bearing, investment-grade, securities.

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Dilution

If you purchase our common stock in this offering, your interest will be diluted to the extent of the difference between the public offering price per share and the net tangible book value per share of our common stock after this offering. We calculate net tangible book value per share by dividing our net tangible book value, which is equal to our total tangible assets less total liabilities, by the number of outstanding shares of our common stock.

Our net tangible book value at December 31, 2008, was approximately $79.5 million, or $0.98 per share. After giving effect to the sale of 7,750,000 shares of common stock by us at a public offering price of $6.30 per share, and after deducting underwriting discounts and commissions and estimated offering expenses payable by us, our net tangible book value at December 31, 2008, would have been approximately $126.3 million, or $1.42 per share. This represents an immediate increase in the net tangible book value of $0.44 per share to existing stockholders and an immediate dilution of $4.88 per share to investors in this offering. The following table illustrates this per share dilution without giving effect to the over-allotment option granted to the underwriter:

If the underwriter's over-allotment option is exercised in full, our pro forma net tangible book value per share after giving effect to this offering would be $1.48 per share, and the dilution in pro forma net tangible book value per share to investors in this offering would be $4.82 per share.

The information above is based on 81,238,812 shares of common stock outstanding on December 31, 2008 and does not include:

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7,236,512 shares of common stock issuable upon exercise of outstanding options granted under our stock option plans at a weighted average exercise price of $5.14 per share;



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2,259,777 shares of common stock available for sale under our 2001 Employee Stock Purchase Plan; and



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5,146,701 shares of common stock available for grant under our 2008 Equity Incentive Plan.

The exercise of outstanding options having an exercise price less than the public offering price will increase dilution to new investors.

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Underwriting

We are offering the shares of our common stock described in this prospectus supplement and the accompanying prospectus through UBS Securities LLC, the sole underwriter for this offering. We have entered into an underwriting agreement with the underwriter. Subject to the terms and conditions of the underwriting agreement, the underwriter has agreed to purchase all of the 7,750,000 shares of common stock being offered.

The underwriting agreement provides that the underwriter must buy all of the shares if it buys any of them. However, the underwriter is not required to take or pay for the shares covered by the underwriter's over-allotment option described below.

Our common stock is offered subject to a number of conditions, including:

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receipt and acceptance of our common stock by the underwriter; and



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the underwriter's right to reject orders in whole or in part.

In connection with this offering, the underwriter or securities dealers may distribute prospectuses electronically. Sales of shares made outside the U.S. may be made by affiliates of the underwriter.

OVER-ALLOTMENT OPTION

We have granted the underwriter an option to buy up to 1,162,500 additional shares of our common stock. The underwriter may exercise this option solely for the purpose of covering over-allotments, if any, made in connection with this offering. The underwriter has 30 days from the date of this prospectus supplement to exercise this option.

COMMISSIONS AND DISCOUNTS

Shares sold by the underwriter to the public will initially be offered at the initial public offering price set forth on the cover of this prospectus supplement. If all the shares are not sold at the public offering price, the underwriter may change the public offering price and the other selling terms.

The following table shows the per share and total underwriting discounts and commissions we will pay to the underwriter, assuming both no exercise and full exercise of the underwriter's option to purchase up to an additional 1,162,500 shares:

We estimate that the total expenses of this offering payable by us, not including the underwriting discounts and commissions, will be approximately $607,000.

NO SALES OF SIMILAR SECURITIES

We and our current executive officers and directors have entered into lock-up agreements with the underwriter. Under these agreements, we and each of these persons may not, without the prior written approval of UBS Securities LLC, subject to limited exceptions, sell, offer to sell, contract or agree to sell, pledge or otherwise dispose of, directly or indirectly, or hedge our common stock or securities convertible into or exchangeable or exercisable for our common stock. In our case, these limited exceptions include the issuance of common stock upon the exercise of outstanding options or warrants or convertible notes and the issuance of common stock to directors pursuant to any non-employee

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Underwriting

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director stock plan. For our current executive officers and directors, these limited exceptions include any sales made pursuant to Rule 10b5-1 sales plans, any bona fide gifts, provided that the recipient agrees in writing to be bound by the terms of the lock-up agreement, or dispositions to any trust for the direct or indirect benefit of the executive officer or director or his or her immediate family, provided that the trust agrees in writing to be bound by the terms of the lock-up agreement. These restrictions will be in effect for a period of 90 days after the date of this prospectus supplement. At any time and without public notice, UBS Securities LLC may, in its sole discretion, release some or all of the securities from these lock-up agreements.

Notwithstanding the above, if: (a) during the last 17 days of the 90-day restricted period, Allos issues an earnings release or material news or a material event relating to Allos occurs; or (b) prior to the expiration of the 90-day restricted period, Allos announces that it will release earnings results or becomes aware that material news or a material event will occur during the 16-day period beginning on the last day of the 90-day restricted period, the restrictions imposed by the lock-up agreements shall continue to apply until the expiration of the 18-day period beginning on the issuance of the earnings release or the occurrence of the material news or material event, unless UBS Securities LLC waives such extension.

INDEMNIFICATION AND CONTRIBUTION

We have agreed to indemnify the underwriter and its controlling persons against certain liabilities, including liabilities under the Securities Act. If we are unable to provide this indemnification, we will contribute to payments the underwriter and its controlling persons may be required to make in respect of those liabilities.

NASDAQ GLOBAL MARKET LISTING

Our common stock is listed on The NASDAQ Global Market under the symbol "ALTH."

PRICE STABILIZATION, SHORT PROVISIONS, PASSIVE MARKET MAKING

In connection with this offering, the underwriter may engage in activities that stabilize, maintain or otherwise affect the price of our common stock, including:

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stabilizing transactions



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short sales;



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purchases to cover positions created by short sales;



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syndicate covering transactions; and



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passive market making.

Stabilizing transactions consist of bids or purchases made for the purpose of preventing or retarding a decline in the market price of our shares of common stock while this offering is in progress. These transactions may also include making short sales of our shares, which involves the sale by the underwriter of a greater number of shares of our common stock than it is required to purchase in this offering. Short sales may be "covered short sales," which are short positions in an amount not greater than the underwriters' over-allotment option referred to above, or may be "naked short sales," which are short positions in excess of that amount.

The underwriter may close out any covered short position either by exercising its over-allotment option, in whole or in part, or by purchasing shares in the open market. In making this determination,

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Underwriting

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the underwriter will consider, among other things, the price of shares available for purchase in the open market compared to the price at which they may purchase shares through the over-allotment option. The underwriter must close out any naked short position by purchasing shares in the open market. A naked short position is more likely to be created if the underwriter is concerned that there may be downward pressure on the price of the common stock in the open market that could adversely affect investors who purchased in this offering. The underwriter also may impose a penalty bid. This occurs when a particular underwriter repays to the underwriter a portion of the underwriting discount received by it because the representatives have repurchased shares sold by or for the account of that underwriter in stabilizing or short covering transactions.

As a result of these activities, the price of our shares of common stock may be higher than the price that otherwise might exist in the open market. If these activities are commenced, they may be discontinued by the underwriter at any time. The underwriter may carry out these transactions on The NASDAQ Global Market, in the over-the-counter market or otherwise.

In addition, in connection with this offering, the underwriter (and selling group members) may engage in passive market making transactions in our common stock on The NASDAQ Global Market prior to the pricing and completion of this offering. Passive market making consists of displaying bids on The NASDAQ Global Market no higher than the bid prices of independent market makers and making purchases at prices no higher than these independent bids and effected in response to order flow. Net purchases by a passive market maker on each day are generally limited to a specified percentage of the passive market maker's average daily trading volume in the common stock during a specified period and must be discontinued when such limit is reached. Passive market making may cause the price of our common stock to be higher than the price that otherwise would exist in the open market in the absence of these transactions. If passive market making is commenced, it may be discontinued at any time.

AFFILIATIONS

UBS Securities LLC and its affiliates have provided and may provide certain commercial banking, financial advisory or investment banking services for us for which they receive fees.

UBS Securities LLC and its affiliates may from time to time in the future engage in transactions with us and perform services for us in the ordinary course of their business.

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Notice to investors

EUROPEAN ECONOMIC AREA

In relation to each Member State of the European Economic Area, or EEA, which has implemented the Prospectus Directive (each, a Relevant Member State), with effect from and including the date on which the Prospectus Directive is implemented in that Relevant Member State, or the Relevant Implementation Date, the shares may not be offered to the public in that Relevant Member State prior to the publication of a prospectus in relation to the shares that has been approved by the competent authority in that Relevant Member State or, where appropriate, approved in another Relevant Member State and notified to the competent authority in that Relevant Member State, all in accordance with the Prospectus Directive, except that, with effect from and including the Relevant Implementation Date, the shares may be offered to the public in that Relevant Member State at any time:

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to legal entities which are authorized or regulated to operate in the financial markets or, if not so authorized or regulated, whose corporate purpose is solely to invest in securities;



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to any legal entity which has two or more of (1) an average of at least 250 employees during the last financial year; (2) a total balance sheet of more than €43,000,000 and (3) an annual net turnover of more than €50,000,000, as shown in its last annual or consolidated accounts; or



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in any other circumstances which do not require the publication by us of a prospectus pursuant to Article 3 of the Prospectus Directive.

As used above, the expression "offered to the public" in relation to the shares in any Relevant Member State means the communication in any form and by any means of sufficient information on the terms of the offer and the shares to be offered so as to enable an investor to decide to purchase or subscribe for the shares, as the same may be varied in that Member State by any measure implementing the Prospectus Directive in that Member State and the expression "Prospectus Directive" means Directive 2003/71/ EC and includes any relevant implementing measure in each Relevant Member State.

The EEA selling restriction is in addition to any other selling restrictions set out below.

UNITED KINGDOM

The shares may not be offered or sold to any persons in the United Kingdom other than to persons whose ordinary activities involve them in acquiring, holding, managing or disposing of investments (as principal or as agent) for the purposes of their businesses and in compliance with all applicable provisions of the Financial Services and Markets Act 2000, or the FSMA, with respect to anything done in relation to the shares, in, from or otherwise involving the United Kingdom. In addition, each underwriter may only communicate or cause to be communicated any invitation or inducement to engage in investment activity (within the meaning of Section 21 of the FSMA) received by it in connection with the issue or sale of the shares in circumstances in which Section 21(1) of the FSMA does not apply to us. Without limitation to the other restrictions referred to herein, this prospectus supplement may only be directed at (1) persons outside the United Kingdom, (2) persons having professional experience in matters relating to investments who fall within the definition of "investment professionals" in Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005; or (3) high net worth bodies corporate, unincorporated associations and partnerships and trustees of high value trusts as described in Article 49(2) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005. Without limitation to the other restrictions referred to herein, investment or investment activity to which this prospectus supplement relates may be made available

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Notice to investors

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only to, and may be engaged only with, such persons, and persons within the United Kingdom who receive this communication (other than persons who fall within (2) or (3) above).

FRANCE

No prospectus (including any amendment, supplement or replacement thereto) has been prepared in connection with the offering of the shares that has been approved by the Autorité des marchés financiers or by the competent authority of another State that is a contracting party to the Agreement on the European Economic Area and notified to the Autorité des marchés financiers; Shares may not be offered or sold, directly or indirectly, to the public in France except to permitted investors, consisting of persons licensed to provide the investment service of portfolio management for the account of third parties, qualified investors (investisseurs qualifiés) acting for their own account and/or corporate investors meeting one of the four criteria provided in Article 1 of Decree N7 2004-1019 of September 28, 2004 and belonging to a limited circle of investors (cercle restreint d'investisseurs) acting for their own account, with "qualified investors" and "limited circle of investors" having the meaning ascribed to them in Article L. 411-2 of the French Code Monétaire et Financier and applicable regulations thereunder; none of this prospectus supplement or any other materials related to the offer or information contained therein relating to the shares has been released, issued or distributed to the public in France except to Permitted Investors; and the direct or indirect resale to the public in France of any common stock acquired by any Permitted Investors may be made only as provided by articles L. 412-1 and L. 621-8 of the French Code Monétaire et Financier and applicable regulations thereunder.

ITALY

The offering of the shares has not been cleared by the Italian Securities Exchange Commission (Commissione Nazionale per le Società e la Borsa, or the CONSOB) pursuant to Italian securities legislation and, accordingly, the shares may not be offered, sold or delivered, nor may copies of this prospectus supplement or any other documents relating to the shares or the offering be distributed in Italy other than to professional investors (operatori qualificati), as defined in Article 31, paragraph 2 of CONSOB Regulation No. 11522 of July 1, 1998, as amended, or Regulation No. 11522.

Any offer, sale or delivery of the shares or distribution of copies of this prospectus supplement or any other document relating to the shares or the offering in Italy may only be effected in accordance with all Italian securities, tax, exchange control and other applicable laws and regulations, and, in particular, may only be: (i) made by an investment firm, bank or financial intermediary permitted to conduct such activities in Italy in accordance with the Legislative Decree No. 385 of September 1, 1993, as amended, or the Italian Banking Law, Legislative Decree No. 58 of February 24, 1998, as amended, Regulation No. 11522, and any other applicable laws and regulations; (ii) in compliance with Article 129 of the Italian Banking Law and the implementing guidelines of the Bank of Italy; and (iii) in compliance with any other applicable notification requirement or limitation which may be imposed by CONSOB or the Bank of Italy.

Any investor purchasing shares in the offering is solely responsible for ensuring that any offer or resale of shares it purchased in the offering occurs in compliance with applicable laws and regulations.

This prospectus supplement and the information contained herein are intended only for the use of its recipient and are not to be distributed to any third party resident located in Italy for any reason. No person resident or located in Italy other than the original recipients of this document may rely on it or its content.

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Notice to investors

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In addition to the above (which shall continue to apply to the extent not inconsistent with the implementing measures of the Prospective Directive in Italy), after the implementation of the Prospectus Directive in Italy, the restrictions, warranties and representations set out under the heading "European Economic Area" above shall apply to Italy.

GERMANY

Shares may not be offered or sold or publicly promoted or advertised by any underwriter in the Federal Republic of Germany other than in compliance with the provisions of the German Securities Prospectus Act (Wertpapierprospektgestz—WpPG) of June 22, 2005, as amended, or of any other laws applicable in the Federal Republic of Germany governing the issue, offering and sale of securities.

SPAIN

Neither the shares nor this prospectus supplement have been approved or registered in the administrative registries of the Spanish National Securities Exchange Commission (Comisión Nacional del Mercado de Valores). Accordingly, the shares may not be offered in Spain except in circumstances which do not constitute a public offer of securities in Spain within the meaning of articles 30bis of the Spanish Securities Markets Law of 28 July 1988 (Ley 24/1988, de 28 de Julio, del Mercado de Valores), as amended and restated, and supplemental rules enacted thereunder.

SWEDEN

This is not a prospectus under, and has not been prepared in accordance with the prospectus requirements provided for in, the Swedish Financial Instruments Trading Act (lagen (1991:980) om handel med finasiella instrument) nor any other Swedish enactment. Neither the Swedish Financial Supervisory Authority nor any other Swedish public body has examined, approved, or registered this document.

SWITZERLAND

The shares may not be publicly offered, distributed or re-distributed on a professional basis in or from Switzerland and neither this prospectus supplement nor any other solicitation for investments in the shares may be communicated or distributed in Switzerland in any way that could constitute a public offering within the meaning of Articles 1156 or 652a of the Swiss Code of Obligations or of Article 2 of the Federal Act on Investment Funds of March 18, 1994. This prospectus supplement may not be copied, reproduced, distributed or passed on to others without the underwriter's prior written consent. This prospectus supplement is not a prospectus within the meaning of Articles 1156 and 652a of the Swiss Code of Obligations or a listing prospectus according to article 32 of the Listing Rules of the Swiss Exchange and may not comply with the information standards required thereunder. The shares will not be listed on any Swiss stock exchange or other Swiss regulated market and this prospectus supplement may not comply with the information required under the relevant listing rules. The shares offered hereby have not been registered with the Swiss Federal Banking Commission and have not been authorized under the Federal Act on Investment Funds of March 18, 1994. The investor protection afforded to acquirers of investment fund certificates by the Federal Act on Investment Funds of March 18, 1994 does not extend to acquirers of the shares.

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Legal matters

The validity of the securities being offered hereby will be passed upon by Cooley Godward Kronish LLP, Broomfield, Colorado. Latham & Watkins LLP, Costa Mesa, California, is counsel to the underwriter in this offering.

Experts

The financial statements and management's assessment of the effectiveness of internal control over financial reporting (which is included in Management's Report on Internal Control over Financial Reporting) incorporated in this prospectus supplement by reference to our Annual Report on Form 10-K for the year ended December 31, 2008 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in accounting and auditing.

Where you can find more information

This prospectus supplement and the accompanying prospectus are part of the registration statement on Form S-3 we filed with the Securities and Exchange Commission, or SEC, under the Securities Act and do not contain all the information set forth in the registration statement. Whenever a reference is made in this prospectus supplement or the accompanying prospectus to any of our contracts, agreements or other documents, the reference may not be complete and you should refer to the exhibits that are a part of the registration statement or the exhibits to the reports or other documents incorporated by reference in this prospectus supplement and the accompanying prospectus for a copy of such contract, agreement or other document. Because we are subject to the information and reporting requirements of the Exchange Act, we file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public over the Internet at the SEC's website at http://www.sec.gov. You may also read and copy any document we file at the SEC's Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information on the operation of the Public Reference Room.

Incorporation by reference

The SEC allows us to "incorporate by reference" information from other documents that we file with it, which means that we can disclose important information to you by referring to those documents. The information incorporated by reference is considered to be part of this prospectus supplement and the accompanying prospectus, and information that we file later with the SEC will automatically update and supersede this information. We incorporate by reference the documents listed below, and any future filings (other than current reports or portions thereof furnished under Item 2.02 or Item 7.01 of Form 8-K) we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act prior to the termination of this offering:

1.

Our Annual Report on Form 10-K for the year ended December 31, 2008.

2.

Information from our Proxy Statement for our 2008 Annual Meeting of Stockholders that is specifically incorporated by reference into our Annual Report on Form 10-K for the year ended December 31, 2007.

3.

Our Current Reports on Form 8-K filed February 3, 2009, February 27, 2009, March 3, 2009, March 24, 2009 and March 25, 2009.

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Incorporation by reference

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4.

The description of our common stock set forth in our registration statement on Form 8-A, filed with the SEC on March 6, 2000, including any amendments or reports filed for the purposes of updating this description.

To the extent that any statement in this prospectus supplement is inconsistent with any statement that is incorporated by reference and that was made on or before the date of this prospectus supplement, the statement in this prospectus supplement shall supersede such incorporated statement. The incorporated statement shall not be deemed, except as modified or superseded, to constitute a part of this prospectus supplement, the accompanying prospectus or the registration statement. Statements contained in this prospectus supplement as to the contents of any contract or other document are not necessarily complete and, in each instance, we refer you to the copy of each contract or document filed as an exhibit to the registration statement.

We will furnish without charge to you, on written or oral request, a copy of any or all of the documents incorporated by reference, including exhibits to these documents. You should direct any requests for documents to Allos Therapeutics, Inc.:

Allos Therapeutics, Inc.
Attention: Investor Relations
11080 CirclePoint Road, Suite 200
Westminster, Colorado 80020
Telephone: (303) 426-6262
Fax: (303) 426-4731
E-mail: investorrelations@allos.com

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PROSPECTUS

$150,000,000

Common Stock, Preferred Stock,
Depositary Shares, Debt Securities,
Warrants and Units

ALLOS THERAPEUTICS, INC.

        From time to time, we may offer up to $150,000,000 of any combination of the securities described in this prospectus, either individually or in units. Any preferred stock that we sell may be sold either as shares of preferred stock or represented by depositary shares.

        This prospectus provides a general description of the securities we may offer. Each time we sell securities, we will provide specific terms of the securities offered in a supplement to this prospectus. The prospectus supplement may also add, update or change information contained in this prospectus. You should read this prospectus and the applicable prospectus supplement carefully before you invest in any securities. This prospectus may not be used to consummate a sale of securities unless accompanied by the applicable prospectus supplement.

        Our common stock is traded on the Nasdaq Global Market under the symbol "ALTH." On May 22, 2007, the last reported sales price of our common stock was $6.00 per share. The applicable prospectus supplement will contain information, where applicable, as to any other listing on the Nasdaq Global Market or any securities market or other exchange of the securities, if any, covered by the prospectus supplement.

         INVESTING IN OUR SECURITIES INVOLVES RISKS. SEE THE SECTION ENTITLED "RISK FACTORS" BEGINNING ON PAGE 9 OF THIS PROSPECTUS.

        We will sell these securities directly to investors, through agents designated from time to time or to or through underwriters or dealers. For additional information on the methods of sale, you should refer to the section entitled "Plan of Distribution" in this prospectus. If any underwriters are involved in the sale of any securities with respect to which this prospectus is being delivered, the names of such underwriters and any applicable commissions or discounts will be set forth in a prospectus supplement. The price to the public of such securities and the net proceeds we expect to receive from such sale will also be set forth in a prospectus supplement.

         NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES COMMISSION HAS APPROVED OR DISAPPROVED OF THESE SECURITIES OR DETERMINED IF THIS PROSPECTUS IS TRUTHFUL OR COMPLETE. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

The date of this prospectus is June 5, 2007

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ABOUT THIS PROSPECTUS

        This prospectus is a part of a registration statement that we filed with the Securities and Exchange Commission, or SEC, utilizing a "shelf" registration process. Under this shelf registration process, we may sell any combination of the securities described in this prospectus in one or more offerings up to a total dollar amount of $150,000,000. This prospectus provides you with a general description of the securities we may offer. Each time we sell securities under this shelf registration, we will provide a prospectus supplement that will contain specific information about the terms of that offering. The prospectus supplement may also add, update or change information contained in this prospectus. You should read both this prospectus and any prospectus supplement together with additional information described under the heading "Where You Can Find More Information."

        We have not authorized any dealer, salesman or other person to give any information or to make any representation other than those contained or incorporated by reference in this prospectus and the accompanying supplement to this prospectus. You must not rely upon any information or representation not contained or incorporated by reference in this prospectus or the accompanying prospectus supplement. This prospectus and the accompanying supplement to this prospectus do not constitute an offer to sell or the solicitation of an offer to buy any securities other than the registered securities to which they relate, nor do this prospectus and the accompanying supplement to this prospectus constitute an offer to sell or the solicitation of an offer to buy securities in any jurisdiction to any person to whom it is unlawful to make such offer or solicitation in such jurisdiction. You should not assume that the information contained in this prospectus and the accompanying prospectus supplement is accurate on any date subsequent to the date set forth on the front of the document or that any information we have incorporated by reference is correct on any date subsequent to the date of the document incorporated by reference, even though this prospectus and any accompanying prospectus supplement is delivered or securities sold on a later date.

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SUMMARY

Prospectus Summary

        This summary highlights selected information from this prospectus and does not contain all of the information that you need to consider in making your investment decision. You should carefully read the entire prospectus, including the risks of investing discussed under " Risk Factors " beginning on page 9, the information incorporated by reference, including our financial statements, and the exhibits to the registration statement of which this prospectus is a part.

        Throughout this prospectus, references to "Allos," the "Company," "we," "us," and "our" refer to Allos Therapeutics, Inc.

Our Company

        We are a biopharmaceutical company focused on developing and commercializing innovative small molecule drugs for the treatment of cancer. We strive to develop proprietary products that have the potential to improve the standard of care in cancer therapy. Our goal is to build a profitable company by generating income from products we develop and commercialize, either alone or with one or more potential strategic partners. Our focus is on product opportunities that leverage our internal clinical development and regulatory expertise and address important markets with unmet medical need. We may also seek to grow our existing portfolio of product candidates through product acquisition and in-licensing efforts.

        We have three product candidates that are currently under development, EFAPROXYN (efaproxiral), PDX (pralatrexate) and RH1.

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EFAPROXYN (efaproxiral) is the first synthetic small molecule designed to sensitize hypoxic, or oxygen-deprived, areas of tumors during radiation therapy by facilitating the release of oxygen from hemoglobin, the oxygen-carrying protein contained within red blood cells, thereby increasing the level of oxygen in tumors. The presence of oxygen in tumors is an essential element for the effectiveness of radiation therapy. By increasing tumor oxygenation, we believe EFAPROXYN has the potential to enhance the efficacy of standard radiation therapy.



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PDX (pralatrexate) is a novel, small molecule chemotherapeutic agent that inhibits dihydrofolate reductase, or DHFR, a folic acid (folate)-dependent enzyme involved in the building of nucleic acid, or DNA, and other processes. PDX was rationally designed for efficient transport into tumor cells via the reduced folate carrier, or RFC-1, and effective intracellular drug retention. We believe these biochemical features, together with preclinical and clinical data in a variety of tumors, suggest that PDX may have a favorable safety and efficacy profile relative to methotrexate and other related DHFR inhibitors. We believe PDX has the potential to be delivered as a single agent or in combination therapy regimens.



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RH1 is a small molecule chemotherapeutic agent that we believe is bioactivated by the enzyme DT-diaphorase, or DTD, also known as NAD(P)H quinone oxidoreductase, or NQ01. DTD is over-expressed in many tumors relative to normal tissue, including lung, colon, breast and liver tumors. We believe that because RH1 is bioactivated in the presence of DTD, it has the potential to provide targeted drug delivery to these tumor types while limiting the amount of toxicity to normal tissue.

EFAPROXYN

        In September 2006, we completed patient enrollment in ENRICH, a Phase 3 clinical trial of EFAPROXYN plus whole brain radiation therapy, or WBRT, in women with brain metastases originating from breast cancer. A total of 368 patients were enrolled at 78 centers across North

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America, Europe and South America. The protocol for the ENRICH trial was reviewed by the United States Food and Drug Administration, or FDA, under its special protocol assessment, or SPA, process, which allows for FDA evaluation of a clinical trial protocol intended to form the primary basis of an efficacy claim in support of a new drug application, or NDA, and provides an agreement that the study design, including trial size, clinical endpoints and/or data analyses are acceptable to the FDA. The primary endpoint of the trial will measure the difference in survival between patients receiving WBRT plus supplemental oxygen, with or without EFAPROXYN. In accordance with the ENRICH trial protocol, we will conduct the final analysis of the primary endpoint following the occurrence of 282 patient deaths, which we currently expect to occur in mid-2007, although the actual timing of the final analysis may vary based on a number of factors, including patient survival rates. If the trial is deemed to be positive at the final analysis, we intend to submit an amendment to our previously filed NDA to the FDA to seek marketing approval for EFAPROXYN for use as an adjunct to WBRT for the treatment of women with brain metastases originating from breast cancer. In June 2004, the FDA issued an "approvable letter" in which it indicated that the NDA may be approved if we successfully complete our ongoing Phase 3 ENRICH trial and submit the results as an NDA amendment for the FDA's review. In the letter, the FDA stated, "if the study shows effectiveness in this population (increased survival) using the pre specified analysis, and the study is otherwise satisfactory, we believe it would, together with the subset result in [REACH, the prior Phase 3 clinical trial of EFAPROXYN in patients with brain metastases], support approval."

        In addition to the ENRICH trial, we are currently enrolling patients in a Phase 1 clinical trial of EFAPROXYN in patients with locally advanced non-small cell lung cancer, or NSCLC, receiving concurrent chemoradiotherapy (chemotherapy and radiation therapy administered at the same time), and a Phase 1b/2 clinical trial of EFAPROXYN in patients with locally advanced cancer of the cervix receiving concurrent chemoradiotherapy. In addition, in the event the ENRICH trial is positive, we intend to expand our clinical development activities involving EFAPROXYN to pursue additional indications in which there may be potential clinical benefit.

PDX

        In August 2006, we initiated PROPEL, a Phase 2 clinical trial of PDX in patients with relapsed or refractory peripheral T-cell lymphoma, or PTCL, that we believe, if positive, will be sufficient to support the filing of an NDA to seek marketing approval for PDX in this indication. In July 2006, we reached agreement with the FDA under the SPA process, on the design of this Phase 2 trial. The trial will seek to enroll a minimum of 100 evaluable patients at approximately 35 centers across the United States, Canada and Europe. The primary endpoint of the study is objective response rate (complete and partial response). Secondary endpoints include duration of response, progression-free survival and overall survival. In January 2007, an independent Data Monitoring Committee, or DMC, completed a planned interim analysis of safety data from the PROPEL trial and recommended that the trial continue per the protocol. This interim assessment was based upon an evaluation of the first 10 patients enrolled to the study who completed at least one cycle of treatment with PDX. No major patient safety concerns were identified by the DMC. According to the PROPEL trial protocol, we will conduct an interim analysis of efficacy data from the PROPEL trial after 35 patients have completed at least one cycle of treatment with PDX, and must observe at least four responses (complete or partial) out of the first 35 patients in order to continue the trial. We currently expect to conduct the 35 patient efficacy analysis in the second half of 2007, and complete patient enrollment in the trial by the third quarter of 2008, although the actual timing of the interim analysis and completion of enrollment may vary based on a number of factors, including site initiation and patient enrollment rates.

        In July 2006, the FDA awarded orphan drug status to PDX for the treatment of patients with T-cell lymphoma. Under the Orphan Drug Act, if we are the first company to receive FDA approval for PDX for this orphan drug indication, we will obtain seven years of marketing exclusivity during

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which the FDA may not approve another company's application for the same drug for the same orphan indication. In October 2006, the FDA granted fast track designation to PDX for the treatment of patients with T-cell lymphoma. The fast track program is designed to facilitate the development and expedite the review of new drugs that are intended to treat serious or life-threatening conditions and that demonstrate the potential to address unmet medical needs. In April 2007, the Commission of the European Communities, with a favorable opinion of the Committee for Orphan Medicinal Products of the European Medicines Agency, or EMEA, granted orphan drug designation to PDX for the treatment of patients with PTCL. The EMEA Orphan Medicinal Product Designation, or OMPD, is intended to promote the development of drugs that may provide significant benefit to patients suffering from rare diseases identified as life-threatening or very serious. Under EMEA guidelines, OMPD provides ten years of potential market exclusivity once the product candidate is approved for marketing for the designated indication in the European Union. OMPD also provides potential protocol assistance, advice on the conduct of clinical trials, a reduced Marketing Authorization Application filing fee for the drug's sponsor and the potential for grant funding.

        In addition to the PROPEL study, we are currently enrolling patients in a Phase 1/2 study of PDX in patients with non-Hodgkin's lymphoma, or NHL, and Hodgkin's disease, and a Phase 1 dose escalation of PDX with vitamin B12 and folic acid supplementation in patients with previously treated advanced NSCLC. During 2007, we also plan to initiate several new studies to further evaluate PDX's potential clinical utility in other hematologic and oncologic indications, including a Phase 1 study of PDX plus gemcitabine in patients with relapsed or refractory NHL and Hodgkin's disease, and a Phase ¹ / 2 study of PDX in patients with relapsed or persistent cutaneous T-cell lymphoma, or CTCL. We also plan to initiate a study of PDX in one or more solid tumors during the second half of 2007, once we have determined the maximum tolerated dose for PDX in the ongoing Phase 1 dose escalation study of PDX with vitamin B12 and folic acid supplementation in patients with previously-treated advanced NSCLC.

RH1

        RH1 is currently being evaluated in patients with advanced solid tumors refractory to other chemotherapy regimens in an open label, Phase 1 dose escalation study to determine the maximum tolerated dose by testing the safety, tolerability and pharmacokinetics of escalating doses of RH1. In 2006, we completed patient enrollment in this study. The final results of this study are scheduled to be presented at the American Society of Clinical Oncology Annual Meeting on June 4, 2007.

        Since our inception in 1992, we have not generated any revenue from product sales and have experienced significant net losses and negative cash flows from operations. We have incurred these losses principally from costs incurred in our research and development programs and from our general and administrative expenses. For the years ended December 31, 2004, 2005 and 2006, and the three months ended March 31, 2007 we had net losses attributable to common stockholders of $21.8 million, $20.8 million, $30.2 million and $8.4 million, respectively. Research and development expenses for the years ended December 31, 2004, 2005 and 2006 and the three months ended March 31, 2007 were $10.2 million, $11.2 million, $14.3 million and $3.3 million, respectively. As of March 31, 2007, we had accumulated a deficit of $217.0 million.

        Our ability to generate revenue and achieve profitability is dependent on our ability, alone or with partners, to successfully complete the development of our product candidates, conduct clinical trials, obtain the necessary regulatory approvals, and manufacture and market our product candidates. The timing and costs to complete the successful development of any of our product candidates are highly uncertain, and therefore difficult to estimate. The lengthy process of seeking regulatory approvals for our product candidates, and the subsequent compliance with applicable regulations, require the expenditure of substantial resources. Because of these risks and uncertainties, we cannot predict when or whether we will successfully complete the development of any of our product candidates or the

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ultimate costs of such efforts. Due to these same factors, we cannot be certain when, or if, we will generate any revenue or net cash inflow from any of our current product candidates.

        Even if our clinical trials demonstrate the safety and effectiveness of our product candidates in their target conditions, we do not expect to be able to record commercial sales for any of our product candidates until mid-2008 at the earliest. We expect to incur significant and growing net losses for the foreseeable future as a result of our research and development programs and the costs of preparing for the potential commercial launch of EFAPROXYN and PDX. Although the size and timing of our future net losses are subject to significant uncertainty, we expect them to increase over the next several years as we continue to fund our development programs and prepare for the potential commercial launch of our product candidates.

        We will be required to raise additional capital to support our future operations, including the potential commercialization of EFAPROXYN and/or PDX in the event we obtain regulatory approval to market such product candidates. We may seek to obtain this additional capital through arrangements with corporate partners, equity or debt financings, or from other sources. Such arrangements, if successfully consummated, may be dilutive to our existing stockholders. However, there is no assurance that we will be successful in consummating any such arrangements. In addition, in the event that additional funds are obtained through arrangements with collaborative partners or other sources, such arrangements may require us to relinquish rights to some of our technologies, product candidates or products under development that we would otherwise seek to develop or commercialize ourselves. If we are unable to generate meaningful amounts of revenue from future product sales, if any, or cannot otherwise raise sufficient additional funds to support our operations, we may be required to delay, reduce the scope of, or eliminate one or more of our development programs and our business and future prospects for revenue and profitability may be harmed.

        We are a Delaware corporation with our principal executive offices located at 11080 CirclePoint Road, Suite 200, Westminster, Colorado 80020. Our telephone number is (303) 426-6262. Our Web site is located at www.allos.com. We have not incorporated by reference into this prospectus the information on our Web site, and you should not consider it to be a part of this document. Our Web site address is included in this document as an inactive textual reference only. Our annual report on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K, as well as any amendments or exhibits to those reports, are available free of charge through our Web site as soon as reasonably practicable after we file or furnish them with or to the SEC.

The Securities We May Offer

        We may offer shares of our common stock and preferred stock, depositary shares, various series of debt securities and warrants to purchase any of such securities, either individually or in units, with a total value of up to $150 million from time to time under this prospectus at prices and on terms to be determined by market conditions at the time of offering. Any preferred stock that we may offer may be offered either as shares of preferred stock or be represented by depositary shares. This prospectus provides you with a general description of the securities we may offer. Each time we offer a type or series of securities, we will provide a prospectus supplement that will describe the specific amounts, prices and other important terms of the securities, including, to the extent applicable:

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designation or classification;



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aggregate principal amount or aggregate offering price;



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maturity, if applicable;



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original issue discount, if any;



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rates and times of payment of interest or dividends, if any;

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redemption, conversion, exchange or sinking fund terms, if any;



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conversion or exchange prices or rates, if any, and, if applicable, any provisions for changes to or adjustments in the conversion or exchange prices or rates and in the securities or other property receivable upon conversion or exchange;



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ranking;



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restrictive covenants, if any;



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voting or other rights, if any; and



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important federal income tax considerations.

        The prospectus supplement also may add, update or change information contained in this prospectus or in documents we have incorporated by reference into this prospectus.

        We may sell the securities directly to or through underwriters, dealers or agents. We, and our underwriters or agents, reserve the right to accept or reject all or part of any proposed purchase of securities. If we do offer securities through underwriters or agents, we will include in the applicable prospectus supplement:

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the names of those underwriters or agents;



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applicable fees, discounts and commissions to be paid to them;



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details regarding over-allotment options, if any; and



•

the net proceeds to us.

        Common Stock.     We may offer shares of our common stock from time to time. Holders of our common stock are entitled to one vote per share for the election of directors and on all other matters that require stockholder approval. Subject to any preferential rights of any outstanding preferred stock, in the event of our liquidation, dissolution or winding up, holders of our common stock are entitled to share ratably in the assets remaining after payment of liabilities and the liquidation preferences of any outstanding preferred stock. Our common stock does not carry any preemptive rights enabling a holder to subscribe for, or receive shares of, any class of our common stock or any other securities convertible into shares of any class of our common stock, or any redemption rights.

        Preferred Stock.     We may offer shares of our preferred stock from time to time, in one or more series. Under our certificate of incorporation, our board of directors has the authority, without further action by stockholders, to designate up to 10,000,000 shares of preferred stock in one or more series and to fix the rights, preferences, privileges, qualifications and restrictions granted to or imposed upon the preferred stock, including dividend rights, conversion rights, voting rights, rights and terms of redemption, liquidation preference and sinking fund terms, any or all of which may be greater than the rights of the common stock. To date, our board of directors has designated 1,000,000 of the 10,000,000 authorized shares of preferred stock as Series A Junior Participating Preferred Stock, which series is described in greater detail in this prospectus under "Description of Capital Stock—Preferred Stock—Share Purchase Rights Plan."

        We will fix the rights, preferences, privileges, qualifications and restrictions of the preferred stock of each series that we sell under this prospectus and applicable prospectus supplements in the certificate of designation relating to that series. We will incorporate by reference into the registration statement of which this prospectus is a part the form of any certificate of designation that describes the terms of the series of preferred stock we are offering before the issuance of the related series of preferred stock. We urge you to read the prospectus supplements related to the series of preferred stock being offered, as well as the complete certificate of designation that contains the terms of the applicable series of preferred stock.

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        Depositary Shares.     We may elect to offer fractional shares of preferred stock rather than full shares of preferred stock and, in that event, will issue receipts for depositary shares. Each of these depositary shares will represent a fraction, which will be set forth in the applicable prospectus supplement, of a share of the applicable series of preferred stock.

        Any depositary shares that we sell under this prospectus will be evidenced by depositary receipts issued under a deposit agreement between us and a depositary with whom we deposit the shares of the applicable series of preferred stock that underlie the depositary shares that are sold. A form of deposit agreement, including a form of depositary receipt, has been filed as an exhibit to the registration statement of which this prospectus is a part, and supplements to those forms containing other terms of any depositary shares that we offer under this prospectus will be incorporated by reference into the registration statement of which this prospectus is a part from reports we file with the SEC. We urge you to read the prospectus supplements related to any depositary shares being offered, as well as the complete deposit agreement and depositary receipt.

        Debt Securities.     We may offer debt securities from time to time, in one or more series, as either senior or subordinated debt or as senior or subordinated convertible debt. The senior debt securities will rank equally with any other unsubordinated debt that we may have and may be secured or unsecured. The subordinated debt securities will be subordinate and junior in right of payment, to the extent and in the manner described in the instrument governing the debt, to all or some portion of our indebtedness. Any convertible debt securities that we issue will be convertible into or exchangeable for our common stock or other securities of ours. Conversion may be mandatory or at your option and would be at prescribed conversion rates.

        The debt securities will be issued under one or more documents called indentures, which are contracts between us and a trustee for the holders of the debt securities. In this prospectus, we have summarized certain general features of the debt securities. We urge you, however, to read the prospectus supplements related to the series of debt securities being offered, as well as the complete indentures that contain the terms of the debt securities. Forms of indentures have been filed as exhibits to the registration statement of which this prospectus is a part, and supplemental indentures and forms of debt securities containing the terms of debt securities being offered will be incorporated by reference into the registration statement of which this prospectus is a part from reports we file with the SEC.

        Warrants.     We may offer warrants for the purchase of our common stock, preferred stock, depositary shares and/or debt securities in one or more series, from time to time. We may issue warrants independently or together with common stock, preferred stock, depositary shares and/or debt securities, and the warrants may be attached to or separate from those securities.

        The warrants will be evidenced by warrant certificates issued under one or more warrant agreements, which are contracts between us and an agent for the holders of the warrants. In this prospectus, we have summarized certain general features of the warrants. We urge you, however, to read the prospectus supplements related to the series of warrants being offered, as well as the complete warrant agreements and warrant certificates that contain the terms of the warrants. Forms of warrant agreements and warrant certificates relating to warrants for the purchase of common stock, preferred stock, depositary shares and debt securities have been filed as exhibits to the registration statement of which this prospectus is a part, and complete warrant agreements and warrant certificates containing the terms of the warrants being offered will be incorporated by reference into the registration statement of which this prospectus is a part from reports we file with the SEC.

        Units.     We may offer units consisting of common stock, preferred stock, depositary shares, debt securities and/or warrants to purchase any of such securities in one or more series. In this prospectus, we have summarized certain general features of the units. We urge you, however, to read the prospectus supplements related to the series of units being offered, as well as the unit agreements that

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contain the terms of the units. We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from a current report on Form 8-K that we file with the SEC, the form of unit agreement and any supplemental agreements that describe the terms of the series of units we are offering before the issuance of the related series of units.

        We will evidence each series of units by unit certificates that we will issue under a separate agreement. We will enter into the unit agreements with a unit agent. Each unit agent will be a bank or trust company that we select. We will indicate the name and address of the unit agent in the applicable prospectus supplement relating to a particular series of units.

         THIS PROSPECTUS MAY NOT BE USED TO OFFER OR SELL ANY SECURITIES UNLESS ACCOMPANIED BY A PROSPECTUS SUPPLEMENT.

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RISK FACTORS

        Except for the historical information contained in this prospectus or incorporated by reference, this prospectus (and the information incorporated by reference in this prospectus) contains forward-looking statements that involve risks and uncertainties. Our actual results could differ materially from those discussed here or incorporated by reference. Factors that could cause or contribute to such differences include, but are not limited to, those discussed in the section entitled "RISK FACTORS" contained in our most recent Annual Report on Form 10-K and Quarterly Report on Form 10-Q filed with the SEC, as well as any amendments thereto reflected in subsequent filings with the SEC, which are incorporated herein by reference in their entirety (the "Allos Risk Factors").

        Investment in our securities involves risks. Prior to making a decision about investing in our securities, you should consider carefully the Allos Risk Factors, together with all of the other information contained or incorporated by reference in this prospectus and any prospectus supplement, including any additional specific risks described in any prospectus supplement. Each of these risk factors could adversely affect our business, operating results and financial condition, which may result in the loss of all or part of your investment.

        Keep these risk factors in mind when you read forward-looking statements contained elsewhere or incorporated by reference in this prospectus and the prospectus supplement. These statements relate to our expectations about future events. Discussions containing forward-looking statements may be found, among other places, in "Business" and "Management's Discussion and Analysis of Financial Condition and Results of Operations" incorporated by reference from our Annual Reports on Form 10-K and our Quarterly Reports on Form 10-Q, as well as any amendments thereto reflected in subsequent filings with the SEC. These forward-looking statements are based largely on our expectations and projections about future events and future trends affecting our business, and so are subject to risks and uncertainties, including the risks and uncertainties described below under "Forward-Looking Information," that could cause actual results to differ materially from those anticipated in the forward-looking statements.

FORWARD-LOOKING INFORMATION

        This prospectus, including the information that we incorporate by reference, contains various forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, or the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act. These statements are based on management's beliefs as well as assumptions made by, and information currently available to, management. These forward-looking statements can generally be identified as such because the context of the statement will include words such as "may," "will," "intends," "plans," "believes," "anticipates," "expects," "estimates," "predicts," "potential," "continue," or "opportunity," the negative of these words or words of similar import. Similarly, statements that describe our reserves and our future plans, strategies, intentions, expectations, objectives, goals or prospects are also forward-looking statements. Such statements are subject to certain risks, uncertainties and assumptions. Should one or more of these risks or uncertainties materialize, or should underlying assumptions prove incorrect, actual results may vary materially from those anticipated, estimated, projected or expected. Forward-looking statements include, without limitation, statements set forth in this prospectus and incorporated by reference in this prospectus regarding, among other things:

•

our intention to seek regulatory approvals in the United States and Europe for EFAPROXYN and to undertake commercialization efforts if any such regulatory approvals are obtained;



•

our plans to continue development of our current product candidates, including EFAPROXYN and PDX, and to conduct clinical trials of EFAPROXYN, PDX and RH1 for the treatment of certain indications;

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•

the potential for EFAPROXYN to enhance the efficacy of standard radiation therapy;



•

any plans to develop sales and marketing capabilities, either internally or through contract relationships or strategic collaborations;



•

any plans to secure additional third-party manufacturing arrangements; and



•

our intention to evaluate the in-licensing or purchase of complimentary products to enhance our oncology portfolio.

        Such forward-looking statements involve known and unknown risks, uncertainties and other factors that may cause our actual results, performance or achievements to differ materially from any future results, performance or achievements expressed or implied by such forward-looking statements. Important factors that could cause such differences include, among other things, the following:

•

the inability to obtain regulatory approvals in the United States or Europe for our products or delays in such approvals;



•

difficulties or delays we may experience in the conduct of clinical trials, including ENRICH and PROPEL, and whether the results of those clinical trials will be adequate to support regulatory filings and/or approvals;



•

the ability of our third party collaborators to satisfactorily perform their functions relating to research and development activities and the manufacture of our product candidates;



•

difficulties we may experience in developing and commercializing our small molecule drugs, including EFAPROXYN and PDX, to address the treatment of cancer and the enhancement of current therapies;



•

difficulties we may experience in identifying, licensing or purchasing complimentary products for our product portfolio;



•

difficulties we may experience in identifying and successfully collaborating with appropriate corporate partners;



•

the effect of capital market conditions and other factors on capital availability; and



•

other factors referenced in this prospectus.

        Because the risks and uncertainties referred to above, as well as the risk factors incorporated by reference, could cause actual results or outcomes to differ materially from those expressed in any forward-looking statements made by us or on our behalf, you should not place undue reliance on any forward-looking statements. Further, any forward-looking statement speaks only as of the date on which it is made, and we undertake no obligation to update any forward-looking statement to reflect events or circumstances after the date on which the statement is made or to reflect the occurrence of unanticipated events. New factors emerge from time to time, and it is not possible for us to predict which factors will arise. In addition, we cannot assess the impact of each factor on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statements. Except as required by law, we undertake no obligation to publicly revise our forward-looking statements to reflect events or circumstances that arise after the date of this prospectus or the prospectus supplement or the date of documents incorporated by reference in this prospectus that include forward-looking statements.

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FINANCIAL RATIOS

        The following table sets forth our ratio of earnings to fixed charges and the ratio of our earnings to combined fixed charges and preferred stock dividends to earnings for each of the periods presented. Our net losses were insufficient to cover fixed charges and combined fixed charges and preferred stock dividends in each of the years in the five-year period ended December 31, 2006 and in the three months ended March 31, 2007. Because of these deficiencies, the ratio information is not applicable. The extent to which earnings were insufficient to cover fixed charges and combined fixed charges and preferred stock dividends is shown below. Amounts shown are in thousands.

        For purposes of computing the ratio of earnings to fixed charges and the ratio of our earnings to fixed charges and preferred stock dividends, fixed charges represent the portion of operating lease rental expense that is representative of interest. Earnings consist of net income (loss) before income taxes, plus fixed charges.

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USE OF PROCEEDS

        Except as described in any prospectus supplement, we currently intend to use the net proceeds from the sale of the securities offered hereby to fund the costs of clinical and preclinical research and development activities, both on-going and planned, to support the potential commercialization of EFAPROXYN in the event we obtain regulatory approval to market EFAPROXYN, the manufacture of preclinical and clinical drug supply, and for general corporate purposes, including working capital. We may also use a portion of the net proceeds to in-license product candidates or to invest in or acquire businesses or technologies that we believe are complementary to our own, although we have no current plans, commitments or agreements with respect to any acquisitions as of the date of this prospectus. Pending these uses, we intend to invest the net proceeds in investment-grade, interest-bearing securities.

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DESCRIPTION OF CAPITAL STOCK

        As of the date of this prospectus, our certificate of incorporation authorizes us to issue 150,000,000 shares of common stock, par value $0.001 per share and 10,000,000 shares of preferred stock, par value $0.001 per share. As of May 22, 2007, approximately 66,138,013 shares of common stock were outstanding and no shares of preferred stock were outstanding. To date, our board of directors has designated 1,000,000 of the 10,000,000 authorized shares of preferred stock as Series A Junior Participating Preferred Stock, which series is described in greater detail below under "—Preferred Stock—Share Purchase Rights Plan."

        The following summary describes the material terms of our capital stock and stockholder rights plan. The description of capital stock and stockholder rights plan is qualified by reference to our amended and restated certificate of incorporation, as amended, our bylaws, as amended, the certificate of designation for our Series A Junior Participating Preferred Stock, and our stockholder rights plan, as amended, which are incorporated by reference as exhibits into the registration statement of which this prospectus is a part.

Common Stock

        Voting.     Common stockholders are entitled to one vote per share for the election of directors and on all other matters that require stockholder approval.

        Dividends and Other Distributions.     Holders of our common stock are entitled to share in an equal amount per share in any dividends declared by our board of directors on the common stock and paid out of legally available assets.

        Distribution on Dissolution.     Subject to any preferential rights of any outstanding preferred stock, in the event of our liquidation, dissolution or winding up, holders of our common stock are entitled to share ratably in the assets remaining after payment of liabilities and the liquidation preferences of any outstanding preferred stock.

        Other Rights.     Our common stock does not carry any preemptive rights enabling a holder to subscribe for, or receive shares of, any class of our common stock or any other securities convertible into shares of any class of our common stock, or any redemption rights.

        Additional shares of authorized common stock may be issued, as authorized by our board of directors from time to time, without stockholder approval, except as may be required by applicable stock exchange requirements.

Preferred Stock

        Under our certificate of incorporation, our board of directors has the authority, without further action by stockholders, to designate up to 10,000,000 shares of preferred stock in one or more series and to fix the rights, preferences, privileges, qualifications and restrictions granted to or imposed upon the preferred stock, including dividend rights, conversion rights, voting rights, rights and terms of redemption, liquidation preference and sinking fund terms, any or all of which may be greater than the rights of the common stock. To date, our board of directors has designated 1,000,000 of the 10,000,000 authorized shares of preferred stock as Series A Junior Participating Preferred Stock, which series is described in greater detail below under "—Share Purchase Rights Plan."

        The issuance of preferred stock could adversely affect the voting power of holders of common stock and reduce the likelihood that common stockholders will receive dividend payments and payments upon liquidation. The issuance could have the effect of decreasing the market price of our common stock. The issuance of preferred stock also could have the effect of delaying, deterring or preventing a change in control of our company.

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        Our board of directors will fix the rights, preferences, privileges, qualifications and restrictions of the preferred stock of each series that we sell under this prospectus and applicable prospectus supplements in the certificate of designation relating to that series. We will incorporate by reference into the registration statement of which this prospectus is a part the form of any certificate of designation that describes the terms of the series of preferred stock we are offering before the issuance of the related series of preferred stock. This description will include:

•

the title and stated value;



•

the number of shares we are offering;



•

the liquidation preference per share;



•

the purchase price per share;



•

the dividend rate per share, dividend period and payment dates and method of calculation for dividends;



•

whether dividends will be cumulative or non-cumulative and, if cumulative, the date from which dividends will accumulate;



•

our right, if any, to defer payment of dividends and the maximum length of any such deferral period;



•

the procedures for any auction and remarketing, if any;



•

the provisions for a sinking fund, if any;



•

the provisions for redemption or repurchase, if applicable, and any restrictions on our ability to exercise those redemption and repurchase rights;



•

any listing of the preferred stock on any securities exchange or market;



•

whether the preferred stock will be convertible into our common stock or other securities of ours, including depositary shares and warrants, and, if applicable, the conversion period, the conversion price, or how it will be calculated, and under what circumstances it may be adjusted;



•

whether the preferred stock will be exchangeable into debt securities, and, if applicable, the exchange period, the exchange price, or how it will be calculated, and under what circumstances it may be adjusted;



•

voting rights, if any, of the preferred stock;



•

preemption rights, if any;



•

restrictions on transfer, sale or other assignment, if any;



•

whether interests in the preferred stock will be represented by depositary shares;



•

a discussion of any material or special United States federal income tax considerations applicable to the preferred stock;



•

the relative ranking and preferences of the preferred stock as to dividend rights and rights if we liquidate, dissolve or wind up our affairs;



•

any limitations on issuances of any class or series of preferred stock ranking senior to or on a parity with the series of preferred stock being issued as to dividend rights and rights if we liquidate, dissolve or wind up our affairs; and



•

any other specific terms, rights, preferences, privileges, qualifications or restrictions of the preferred stock.

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        When we issue shares of preferred stock under this prospectus, the shares will be fully paid and nonassessable and will not have, or be subject to, any preemptive or similar rights.

        The General Corporation Law of the State of Delaware, the state of our incorporation, provides that the holders of preferred stock will have the right to vote separately as a class on any proposal involving fundamental changes in the rights of holders of that preferred stock. This right is in addition to any voting rights that may be provided for in the applicable certificate of designation.

        Share Purchase Rights Plan.     Each outstanding share of our common stock has attached to it one preferred share purchase right, which we refer to as a right. Each right entitles the registered holder of our common stock to purchase from us one one-hundredth of a share of Series A Junior Participating Preferred Stock, which we refer to as participating preferred shares, at a price of $30.00 per one one-hundredth of a participating preferred share, subject to adjustment. Each one one-hundredth of a share of participating preferred shares has designations and powers, preferences and rights, and the qualifications, limitations and restrictions which make its value approximately equal to the value of a share of our common stock. The description and terms of the rights are set forth in a Rights Agreement, dated as of May 6, 2003, as amended March 4, 2005 and January 29, 2007, between us and Mellon Investor Services, LLC, as rights agent, which is incorporated by reference as an exhibit into the registration statement of which this prospectus is a part.

        Until the distribution date described below, we will not issue separate certificates evidencing the rights. Until that date, the rights will be evidenced, with respect to any common stock certificate, by that common stock certificate. The rights will detach from the common stock and a distribution date will occur upon the earlier of the following dates:

•

the date of a public announcement that an "acquiring person," which may include an entity or group of affiliated or associated persons, has acquired beneficial ownership of 15% or more of our outstanding common stock, or



•

the tenth business day following the commencement of, or the first public announcement by any person or group of an intention to commence, a tender offer that would result in any person or entity, including a group of affiliated or associated persons, acquiring beneficial ownership of 15% or more of our outstanding common stock.

Our board of directors may postpone the distribution date by determining a later distribution date before the time any person or group becomes an acquiring person.

        The term "acquiring person" does not include us, any of our subsidiaries, any of our or our subsidiaries' employee benefit plans or any entity holding our common stock for or under any of our or our subsidiaries' employee benefit plans. In addition, a person who would otherwise be an acquiring person will not be considered an acquiring person if our board of directors determines in good faith that such person inadvertently became the beneficial owner of 15% or more of our common stock and such person divests itself, as promptly as practicable, of beneficial ownership of a sufficient number of shares of our common stock so that it would no longer otherwise qualify as an acquiring person.

        In addition, except under limited circumstances, no person or entity shall become an acquiring person as the result of the acquisition of our common stock which, by reducing the number of shares outstanding, increases the proportionate number of shares beneficially owned by such person or entity to 15% or more of our outstanding common stock.

        The rights agreement provides that, until the distribution date, or earlier redemption or expiration of the rights, the rights will be transferred only with our common stock. The rights will be evidenced, with respect to any common stock certificate outstanding as of May 28, 2003, by that common stock certificate with a summary of the rights attached to it. Until the distribution date, or earlier redemption or expiration of the rights, new common stock certificates issued after May 28, 2003, upon transfer or

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new issuances of common stock will contain a notation incorporating the rights agreement by reference. Until the distribution date, the surrender for transfer of any certificates for common stock, even without a summary of the rights attached to it, also will constitute the transfer of the rights associated with the common stock represented by that certificate. As soon as practicable after the distribution date, separate certificates evidencing the rights will be mailed to holders of record of our common stock as of the close of business on the distribution date, and the separate right certificates alone will evidence the rights. Only our common stock issued before the distribution date will be issued with rights.

        The rights are not exercisable until the distribution date. The rights will expire on May 28, 2013, unless the rights are earlier redeemed or exchanged by us, in each case, as described below.

        The purchase price payable for the participating preferred shares, and the number of participating preferred shares or other securities or property issuable, upon exercise of the rights, as well as the number of rights outstanding, are subject to adjustment from time to time to prevent dilution in the following circumstances:

•

in the event of a stock dividend on, or a subdivision, combination or reclassification of the participating preferred shares,



•

upon the grant to holders of the participating preferred shares of rights or warrants to subscribe for or purchase participating preferred shares at a price, or securities convertible into participating preferred shares with a conversion price, less than the current market price of the participating preferred shares, or



•

upon the distribution to holders of the participating preferred shares of evidences of indebtedness, securities or assets, excluding regular quarterly cash dividends or dividends payable in participating preferred shares or of subscription rights or warrants, other than those referred to above.

        The number of outstanding rights and the number of one one-hundredths of a participating preferred share issuable upon exercise of each right are also subject to adjustment in the event of a dividend or other distribution on the common stock payable in common stock or subdivisions, consolidations or combinations of our common stock occurring, in any of those cases, before the distribution date.

        Participating preferred shares purchasable upon exercise of the rights will not be redeemable. Each participating preferred share will be entitled to a minimum preferential quarterly dividend payment of $1.00 per share, but will be entitled to an aggregate dividend of 100 times the dividend declared per share of our common stock. If there is a liquidation, the holders of the participating preferred shares will be entitled to a minimum preferential liquidation payment of $100 per share, but will be entitled to an aggregate payment of 100 times the payment made per share of our common stock. Each participating preferred share will have 100 votes, voting together with our common stock. If there is a merger, consolidation or other transaction in which our common stock is exchanged, each participating preferred share will be entitled to receive 100 times the amount received per share of our common stock. These rights are protected by customary antidilution provisions.

        Because of the nature of the dividend, liquidation and voting rights of the participating preferred shares, the value of the one one-hundredth interest in a participating preferred share purchasable upon exercise of each right should approximate the value of one share of our common stock.

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        If any person or group becomes an acquiring person, or if we are the surviving corporation in a merger with an acquiring person or any affiliate or associate of an acquiring person and our common stock is not changed or exchanged, proper provision will be made so that each holder of a right, other than rights beneficially owned by the acquiring person, which will become null and void, will have the right to receive upon exercise of the right at the then-current market price, instead of participating preferred shares, that number of shares of our common stock having a market value of two times the exercise price of the right. If we do not have sufficient common stock issued but not outstanding, or authorized but unissued, to permit the exercise in full of the rights, we will be required to take all action necessary to authorize additional common stock for issuance upon exercise of the rights.

        If, after a person or group has become an acquiring person, we are acquired in a merger or other business combination transaction or 50% or more of our consolidated assets or earning power are sold, proper provision will be made so that each holder of a right, other than rights beneficially owned by the acquiring person, which will become null and void, will have the right to receive, upon the exercise of the right at its then-current exercise price and instead of participating preferred shares, that number of shares of common stock of the acquiring company, or its parent, which at the time of the transaction will have a market value of two times the exercise price of the right.

        At any time after any person or group becomes an acquiring person and before the acquisition by that person or group of 50% or more of our outstanding common stock, our board of directors may exchange the rights, in whole or in part, for a number of shares of our common stock, per right, having an aggregate value equal to the excess of the value of the shares of our common stock issuable upon exercise of a right after a person or group becomes an acquiring person over the purchase price, subject to adjustment. Our board of directors will not exchange the rights owned by the acquiring person or group, which will have become null and void.

        With specified exceptions, no adjustments in the purchase price for the preferred shares will be required until cumulative adjustments require an adjustment of at least 1% of that purchase price. No fractional participating preferred shares will be issued, other than fractions which are integral multiples of one one-hundredth of a participating preferred share, which may, at our election, be evidenced by depositary receipts. Instead of issuing fractional participating preferred shares, we will make an adjustment in cash based on the market price of the participating preferred shares on the last trading day before the date of exercise.

        Upon approval by our board of directors, we may redeem the rights, in whole, but not in part, at a price of $.001 per right at any time until the earlier of:

•

the day of a public announcement that a person or group has become an acquiring person, or



•

May 28, 2013.

        Until a right is exercised, the holder of the right, in the capacity of a holder, will have no rights as a stockholder of ours, including, without limitation, the right to vote or to receive dividends. Although the distribution of the rights will not be taxable to stockholders or to us, stockholders may, depending upon the circumstances, recognize taxable income in the event that the rights become exercisable for our common stock or other consideration, or for common stock of the acquiring company or its parent as set forth above.

        Any of the provisions of the rights agreement may be amended or supplemented by our board of directors before the distribution date. From and after the distribution date, we and the rights agent may amend or supplement the rights agreement from time to time without the approval of any holders of rights:

•

to cure any ambiguity, to correct or supplement any defective or inconsistent provisions,

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•

to make any other provisions with respect to the rights which we and the rights agent may deem necessary or desirable.

        Notwithstanding this right to amend or supplement, from and after the distribution date, the rights agreement may not be amended in any manner which would adversely affect the interest of the holders of rights.

        On March 4, 2005, we amended the rights agreement in connection with the purchase of our Series A Exchangeable Preferred Stock (the "Exchangeable Preferred") by Warburg Pincus Private Equity VIII, L.P. ("Warburg Pincus"), described below, to provide that Warburg Pincus and its affiliates will be exempt from the rights agreement, unless Warburg Pincus and its affiliates become, without our prior consent, the beneficial owner of more than 44% of our common stock.

        On January 29, 2007, we amended the rights agreement in connection with an underwritten offering of 9,000,000 shares of our common stock (the "February 2007 Offering") of which Baker Brothers Life Sciences, L.P. and certain other affiliated funds (collectively, "Baker") purchased 3,300,000 shares. Such amendment provides that Baker and its affiliates will be exempt from the rights agreement unless Baker and its affiliates become, without our prior consent, the beneficial owner of more than 20% of our common stock.

        The rights have certain anti-takeover effects. The rights will cause substantial dilution to a person or group that attempts to acquire us on terms not approved by our board of directors. The rights should not interfere with any merger or other business combination approved by the board of directors since the rights may be amended to permit such an acquisition or they can be redeemed by us at $.001 per right prior to the earliest of the time that a person or group has acquired beneficial ownership of 15% or more of our common stock or the final expiration date of the rights.

Registration Rights

        In connection with the sale of Exchangeable Preferred, we entered into a Registration Rights Agreement between us and the purchasers of Exchangeable Preferred. Pursuant to this Registration Rights Agreement, beginning on March 4, 2007, the purchasers of Exchangeable Preferred are entitled to certain registration rights with respect to the shares of common stock that were issued upon exchange of the Exchangeable Preferred.

Anti-Takeover Provisions

        Delaware Takeover Statute.     We are subject to Section 203 of the Delaware General Corporation Law, or DGCL, which regulates acquisitions of some Delaware corporations. In general, Section 203 prohibits, with some exceptions, a publicly held Delaware corporation from engaging in a "business combination" with an "interested stockholder" for a period of three years following the date of the transaction in which the person became an interested stockholder, unless:

•

the board of directors of the corporation approved the business combination or the other transaction in which the person became an interested stockholder prior to the date of the business combination or other transaction;



•

upon consummation of the transaction that resulted in the person becoming an interested stockholder, the person owned at least 85% of the voting stock of the corporation outstanding at the time the transaction commenced, excluding shares owned by persons who are directors and also officers of the corporation and shares issued under employee stock plans under which employee participants do not have the right to determine confidentially whether shares held subject to the plan will be tendered in a tender or exchange offer; or

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•

on or subsequent to the date the person became an interested stockholder, the board of directors of the corporation approved the business combination and the stockholders of the corporation authorized the business combination at an annual or special meeting of stockholders by the affirmative vote of at least 66 ² / 3 % of the outstanding stock of the corporation not owned by the interested stockholder.

        Section 203 of the DGCL generally defines a "business combination" to include any of the following:

•

any merger or consolidation involving the corporation and the interested stockholder;



•

any sale, transfer, pledge or other disposition of 10% or more of the corporation's assets or outstanding stock involving the interested stockholder;



•

in general, any transaction that results in the issuance or transfer by the corporation of any of its stock to the interested stockholder;



•

any transaction involving the corporation that has the effect of increasing the proportionate share of its stock owned by the interested stockholder; or



•

the receipt by the interested stockholder of the benefit of any loans, advances, guarantees, pledges or other financial benefits provided by or through the corporation.

        In general, Section 203 defines an "interested stockholder" as any person who, together with the person's affiliates and associates, owns, or within three years prior to the determination of interested stockholder status did own, 15% or more of a corporation's voting stock.

        Section 203 of the DGCL could depress our stock price and delay, discourage or prohibit transactions not approved in advance by our board of directors, such as takeover attempts that might otherwise involve the payment to our stockholders of a premium over the market price of our common stock.

        Bylaw and Certificate of Incorporation Provisions.     Our bylaws provide that special meetings of our stockholders may be called only by the Chairman of the board of directors, our Chief Executive Officer or by the board of directors pursuant to a resolution adopted by a majority of the total number of authorized directors. Our certificate of incorporation also specifies that the authorized number of directors may be changed only by resolution of the board of directors and does not include a provision for cumulative voting for directors. Under cumulative voting, a minority stockholder holding a sufficient percentage of a class of shares may be able to ensure the election of one or more directors. These and other provisions contained in our certificate of incorporation and bylaws could delay or discourage transactions involving an actual or potential change in control of us or our management, including transactions in which stockholders might otherwise receive a premium for their shares over then current prices. Such provisions could also limit the ability of stockholders to remove current management or approve transactions that stockholders may deem to be in their best interests and could adversely affect the price of our common stock.

        Standstill and Voting Agreement.     On March 2, 2005, we entered into a Securities Purchase Agreement with Warburg Pincus and certain other investors pursuant to which we issued and sold an aggregate of 2,352,443 shares of our Exchangeable Preferred at a price per share of $22.10. On May 18, 2005, at our annual meeting of stockholders, our stockholders voted to approve the issuance of shares of our common stock upon exchange of shares of the Exchangeable Preferred. As a result of such approval, we issued a total of 23,524,430 shares of common stock upon exchange of 2,352,443 shares of Exchangeable Preferred. In connection with Warburg Pincus' purchase of Exchangeable Preferred, we entered into a letter agreement with Warburg Pincus and certain of its affiliates dated March 4, 2005, pursuant to which they have agreed not to pursue, for four years, certain activities, the purpose or effect of which may be to change or influence the control of the Company, without the approval of our

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board of directors. In addition, with respect to any vote to elect or remove members of our board of directors, Warburg Pincus and its affiliates have agreed to vote any shares of common stock owned by them in excess of 33% of our total outstanding common stock, at their option, either as recommended by the board of directors or in the same proportion as the votes of shares of all other common stock voted in such election.

        On January 28, 2007, in connection with the February 2007 Offering, we entered into a letter agreement with Baker and certain of its affiliates, pursuant to which they have agreed not to pursue, for four years, certain activities, the purpose or effect of which may be to change or influence the control of the Company, without the approval of our board of directors.

        As of March 31, 2007, we had approximately 65.9 million shares of common stock outstanding, of which Warburg Pincus owned 22,624,430 shares, or approximately 34% of the voting power of our outstanding common stock, and Baker owned 9,929,493 shares, or approximately 15% of the voting power of our outstanding common stock. Although each of Warburg Pincus and Baker have entered into a standstill agreement with us, they are, and will continue to be, able to exercise substantial influence over any actions requiring stockholder approval.

Transfer Agent And Registrar

        The transfer agent and registrar for our common stock is Mellon Investor Services LLC.

Listing on the Nasdaq Global Market

        Our common stock is listed on the Nasdaq Global Market under the symbol "ALTH".

DESCRIPTION OF DEPOSITARY SHARES

        We may offer fractional shares of preferred stock rather than full shares of preferred stock, and, in that event, will issue receipts for depositary shares. Each of these depositary shares will represent a fraction, which will be set forth in the applicable prospectus supplement, of a share of the applicable series of preferred stock.

        The shares of any series of preferred stock underlying any depositary shares that we may sell under this prospectus will be deposited under a deposit agreement between us and a depositary selected by us. Subject to the terms of the deposit agreement, each holder of a depositary share will be entitled, in proportion to the applicable fraction of a share of the preferred stock underlying the depositary share, to all of the rights, preferences and privileges, and be subject to the qualifications and restrictions, of the preferred stock underlying that depositary share.

        The depositary shares will be evidenced by depositary receipts issued under a deposit agreement. Depositary receipts will be distributed to the holders of the depositary shares that are sold in the applicable offering. A form of deposit agreement, including a form of depositary receipt, has been filed as an exhibit to the registration statement of which this prospectus is a part, and supplements to those forms containing other terms of any depositary shares that we sell under this prospectus will be incorporated by reference into the registration statement of which this prospectus is a part from reports we file with the SEC. The following description of the material terms of the deposit agreement, the depositary shares and the depositary receipts is only a summary. We urge you to read the prospectus supplements relating to any depositary shares that are sold under this prospectus, as well as the complete deposit agreement and depositary receipt.

        Form.     Pending the preparation of definitive depositary receipts, the depositary may, upon our written order, issue temporary depositary receipts substantially identical to the definitive depositary receipts but not in definitive form. These temporary depositary receipts entitle their holders to all of

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the rights of definitive depositary receipts. Temporary depositary receipts will then be exchangeable for definitive depositary receipts at our expense.

        Dividends and Other Distributions.     The depositary will distribute all cash dividends or other cash distributions received with respect to the underlying preferred stock to the record holders of depositary shares in proportion to the number of depositary shares owned by those holders.

        If there is a distribution other than in cash, the depositary will distribute property received by it to the record holders of depositary shares in proportion to the number of depositary shares owned by those holders, unless the depositary determines that it is not feasible to do so. If this occurs, the depositary may, with our approval, sell the property and distribute the net proceeds from the sale to those holders in proportion to the number of depositary shares owned by them.

        Withdrawal of Underlying Preferred Stock.     Except as otherwise provided in a prospectus supplement, holders may surrender depositary receipts at the principal office of the depositary and, upon payment of any unpaid amount due to the depositary, be entitled to receive the number of whole shares of underlying preferred stock and all money and other property represented by the related depositary shares. We will not issue any partial shares of preferred stock. If the holder delivers depositary receipts evidencing a number of depositary shares that represent more than a whole number of shares of preferred stock, the depositary will issue a new depositary receipt evidencing the excess number of depositary shares to the holder.

        Redemption of Depositary Shares.     If the preferred stock underlying any depositary shares we may sell under this prospectus is subject to redemption, the depositary shares will be redeemed from the proceeds received by the depositary resulting from any such redemption, in whole or in part, of that underlying preferred stock. The redemption price per depositary share will be equal to the applicable fraction of the redemption price per share payable with respect to the underlying preferred stock. Whenever we redeem shares of underlying preferred stock that are held by the depositary, the depositary will redeem, as of the same redemption date, the number of depositary shares representing the shares of underlying preferred stock so redeemed. If fewer than all of the depositary shares are to be redeemed, the depositary shares to be redeemed will be selected by lot or proportionately, as may be determined by the depositary.

        Voting.     Upon receipt of notice of any meeting at which holders of the preferred stock underlying any depositary shares that we may sell under this prospectus are entitled to vote, the depositary will mail the information contained in the notice to the record holders of the depositary shares. Each record holder of the depositary shares on the record date, which will be the same date as the record date for the underlying preferred stock, will be entitled to instruct the depositary as to the exercise of the voting rights pertaining to the amount of the underlying preferred stock represented by the holder's depositary shares. The depositary will then try, as far as practicable, to vote the number of shares of preferred stock underlying those depositary shares in accordance with those instructions, and we will agree to take all reasonable actions which may be deemed necessary by the depositary to enable the depositary to do so. The depositary will not vote the underlying preferred stock to the extent it does not receive specific instructions with respect to the depositary shares representing such preferred stock.

        Conversion of Preferred Stock.     If the prospectus supplement relating to any depositary shares that we may sell under this prospectus states that the underlying preferred stock is convertible into our common stock or other securities, the following will apply. The depositary shares, as such, will not be convertible into any of our securities. Rather, any holder of the depositary shares may surrender the related depositary receipts to the depositary with written instructions that direct us to cause conversion of the preferred stock represented by the depositary shares into or for whole shares of our common stock or other securities, as applicable. Upon receipt of those instructions and any amounts payable by the holder in connection with the conversion, we will cause the conversion using the same procedures

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as those provided for conversion of the underlying preferred stock. If only some of a holder's depositary shares are converted, a new depositary receipt or receipts will be issued to the holder for any depositary shares not converted.

        Amendment and Termination of the Deposit Agreement.     The form of depositary receipt evidencing the depositary shares and any provision of the deposit agreement may at any time be amended by agreement between us and the depositary. However, any amendment which materially and adversely alters the rights of the holders of depositary shares will not be effective until 90 days after notice of that amendment has been given to the holders. Each holder of depositary shares at the time any amendment becomes effective shall be deemed to consent and agree to that amendment and to be bound by the deposit agreement as so amended. The deposit agreement may be terminated by us or by the depositary only if all outstanding depositary shares have been redeemed or converted into any other securities into which the underlying preferred stock is convertible or there has been a final distribution, including to holders of depositary receipts, of the underlying preferred stock in connection with our liquidation, dissolution or winding up.

        Charges of Depositary.     We will pay all charges of the depositary, except for taxes and governmental charges and other charges as are expressly provided for in the deposit agreement to be for the account of the holders of depositary shares or persons other than ourselves who may deposit any underlying preferred stock with the depositary.

        Reports.     The depositary will forward to holders of depositary receipts all notices and reports from us that we deliver to the depositary and that we are required to furnish to the holders of the underlying preferred stock.

        Limitation on Liability.     Neither we nor the depositary will be liable if either of us is prevented or delayed by law or any circumstance beyond our control in performing our respective obligations under the deposit agreement. Our obligations and those of the depositary will be limited to performance of our respective duties under the deposit agreement without, in our case, negligence or bad faith or, in the case of the depositary, negligence or willful misconduct. We and the depositary may rely upon advice of counsel or accountants, or upon information provided by persons presenting the underlying preferred stock for deposit, holders of depositary receipts or other persons believed by us in good faith to be competent and on documents believed to be genuine.

        Resignation and Removal of Depositary.     The depositary may resign at any time by delivering notice to us of its election to resign. We may remove the depositary at any time. Any resignation or removal will take effect upon the appointment of a successor depositary and its acceptance of the appointment. The successor depositary must be appointed within 60 days after delivery of the notice of resignation or removal and must be a bank or trust company having its principal office in the United States and having a combined capital and surplus of at least $50,000,000.

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DESCRIPTION OF DEBT SECURITIES

        The following description, together with the additional information we include in any applicable prospectus supplements, summarizes the material terms and provisions of the debt securities that we may offer under this prospectus. While the terms we have summarized below will generally apply to any future debt securities we may offer under this prospectus, we will describe the particular terms of any debt securities that we may offer in more detail in the applicable prospectus supplement. The terms of any debt securities we offer under a prospectus supplement may differ from the terms we describe below. However, no prospectus supplement shall fundamentally change the terms that are set forth in this prospectus or offer a security that is not registered and described in this prospectus at the time of its effectiveness. As of the date of this prospectus, we have no outstanding registered debt securities.

        We will issue the senior notes under the senior indenture, which we will enter into with the trustee to be named in the senior indenture. We will issue the subordinated notes under the subordinated indenture, which we will enter into with the trustee to be named in the subordinated indenture. We have filed forms of these documents as exhibits to the registration statement of which this prospectus is a part. We use the term "indentures" to refer to both the senior indenture and the subordinated indenture.

        The indentures will be qualified under the Trust Indenture Act of 1939. We use the term "debenture trustee" to refer to either the senior trustee or the subordinated trustee, as applicable.

        The following summaries of material provisions of the senior notes, the subordinated notes and the indentures are subject to, and qualified in their entirety by reference to, all the provisions of the indenture applicable to a particular series of debt securities. We urge you to read the applicable prospectus supplements related to the debt securities that we sell under this prospectus, as well as the complete indentures that contain the terms of the debt securities. Except as we may otherwise indicate, the terms of the senior and the subordinated indentures are identical.

General

        The terms of each series of debt securities will be established by or pursuant to a resolution of our board of directors and set forth or determined in the manner provided in an officers' certificate or by a supplemental indenture. Debt securities may be issued in separate series without limitation as to aggregate principal amount. We may specify a maximum aggregate principal amount for the debt securities of any series. The particular terms of each series of debt securities will be described in a prospectus supplement relating to such series, including any pricing supplement. The prospectus supplement will set forth:

•

the title;



•

the principal amount being offered, and, if a series, the total amount authorized and the total amount outstanding;



•

any limit on the amount that may be issued;



•

whether or not we will issue the series of debt securities in global form and, if so, the terms and who the depositary will be;



•

the maturity date;



•

whether and under what circumstances, if any, we will pay additional amounts on any debt securities held by a person who is not a United States person for tax purposes, and whether we can redeem the debt securities if we have to pay such additional amounts;

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•

the annual interest rate, which may be fixed or variable, or the method for determining the rate, the date interest will begin to accrue, the dates interest will be payable and the regular record dates for interest payment dates or the method for determining such dates;



•

whether or not the debt securities will be secured or unsecured, and the terms of any secured debt;



•

the terms of the subordination of any series of subordinated debt;



•

the place where payments will be payable;



•

restrictions on transfer, sale or other assignment, if any;



•

our right, if any, to defer payment of interest and the maximum length of any such deferral period;



•

the date, if any, after which, the conditions upon which, and the price at which we may, at our option, redeem the series of debt securities pursuant to any optional or provisional redemption provisions, and any other applicable terms of those redemption provisions;



•

the date, if any, on which, and the price at which we are obligated, pursuant to any mandatory sinking fund or analogous fund provisions or otherwise, to redeem, or at the holder's option to purchase, the series of debt securities and the currency or currency unit in which the debt securities are payable;



•

whether the indenture will restrict our ability and/or the ability of our subsidiaries to:



•

incur additional indebtedness;



•

issue additional securities;



•

create liens;



•

pay dividends and make distributions in respect of our capital stock and the capital stock of our subsidiaries;



•

redeem capital stock;



•

place restrictions on our subsidiaries' ability to pay dividends, make distributions or transfer assets;



•

make investments or other restricted payments;



•

sell or otherwise dispose of assets;



•

enter into sale-leaseback transactions;



•

engage in transactions with stockholders and affiliates;



•

issue or sell stock of our subsidiaries; or



•

effect a consolidation or merger;



•

whether the indenture will require us to maintain any interest coverage, fixed charge, cash flow-based, asset-based or other financial ratios;



•

a discussion of any material or special United States federal income tax considerations applicable to the debt securities;



•

information describing any book-entry features;



•

provisions for a sinking fund purchase or other analogous fund, if any;

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•

whether the debt securities are to be offered at a price such that they will be deemed to be offered at an "original issue discount" as defined in paragraph (a) of Section 1273 of the Internal Revenue Code;



•

the procedures for any auction and remarketing, if any;



•

the denominations in which we will issue the series of debt securities, if other than denominations of $1,000 and any integral multiple thereof;



•

if other than dollars, the currency in which the series of debt securities will be denominated; and



•

any other specific terms, preferences, rights or limitations of, or restrictions on, the debt securities, including any events of default that are in addition to those described in this prospectus or any covenants provided with respect to the debt securities that are in addition to those described above, and any terms which may be required by us or advisable under applicable laws or regulations or advisable in connection with the marketing of the debt securities.

Conversion or Exchange Rights

        We will set forth in the prospectus supplement the terms on which a series of debt securities may be convertible into or exchangeable for common stock or other securities of ours or a third party, including the conversion or exchange rate, as applicable, or how it will be calculated, and the applicable conversion or exchange period. We will include provisions as to whether conversion or exchange is mandatory, at the option of the holder or at our option. We may include provisions pursuant to which the number of our securities or the securities of a third party that the holders of the series of debt securities receive upon conversion or exchange would, under the circumstances described in those provisions, be subject to adjustment, or pursuant to which those holders would, under those circumstances, receive other property upon conversion or exchange, for example in the event of our merger or consolidation with another entity.

Consolidation, Merger or Sale

        The indentures in the forms initially filed as exhibits to the registration statement of which this prospectus is a part do not contain any covenant which restricts our ability to merge or consolidate, or sell, convey, transfer or otherwise dispose of all or substantially all of our assets. However, any successor of ours or the acquiror of such assets must assume all of our obligations under the indentures and the debt securities.

        If the debt securities are convertible for our other securities, the person with whom we consolidate or merge or to whom we sell all of our property must make provisions for the conversion of the debt securities into securities that the holders of the debt securities would have received if they had converted the debt securities before the consolidation, merger or sale.

Events of Default Under the Indenture

        The following are events of default under the indentures with respect to any series of debt securities that we may issue:

•

if we fail to pay interest when due and payable and our failure continues for 90 days and the time for payment has not been extended or deferred;



•

if we fail to pay the principal, sinking fund payment or premium, if any, when due and payable and the time for payment has not been extended or delayed;

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•

if we fail to observe or perform any other covenant contained in the debt securities or the indentures, other than a covenant specifically relating to another series of debt securities, and our failure continues for 90 days after we receive notice from the debenture trustee or holders of at least 25% in aggregate principal amount of the outstanding debt securities of the applicable series; and



•

if specified events of bankruptcy, insolvency or reorganization occur.

        If an event of default with respect to debt securities of any series occurs and is continuing, other than an event of default specified in the last bullet point above, the debenture trustee or the holders of at least 25% in aggregate principal amount of the outstanding debt securities of that series, by notice to us in writing, and to the debenture trustee if notice is given by such holders, may declare the unpaid principal of, premium, if any, and accrued interest, if any, due and payable immediately. If an event of default specified in the last bullet point above occurs with respect to us, the principal amount of and accrued interest, if any, of each issue of debt securities then outstanding shall be due and payable without any notice or other action on the part of the debenture trustee or any holder.

        The holders of a majority in principal amount of the outstanding debt securities of an affected series may waive any default or event of default with respect to the series and its consequences, except defaults or events of default regarding payment of principal, premium, if any, or interest, unless we have cured the default or event of default in accordance with the indenture. Any waiver shall cure the default or event of default.

        Subject to the terms of the indentures, if an event of default under an indenture shall occur and be continuing, the debenture trustee will be under no obligation to exercise any of its rights or powers under such indenture at the request or direction of any of the holders of the applicable series of debt securities, unless such holders have offered the debenture trustee reasonable indemnity. The holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the debenture trustee, or exercising any trust or power conferred on the debenture trustee, with respect to the debt securities of that series, provided that:

•

the direction so given by the holder is not in conflict with any law or the applicable indenture; and



•

subject to its duties under the Trust Indenture Act of 1939, the debenture trustee need not take any action that might involve it in personal liability or might be unduly prejudicial to the holders not involved in the proceeding.

        A holder of the debt securities of any series will only have the right to institute a proceeding under the indentures or to appoint a receiver or trustee, or to seek other remedies if:

•

the holder has given written notice to the debenture trustee of a continuing event of default with respect to that series;



•

the holders of at least 25% in aggregate principal amount of the outstanding debt securities of that series have made written request, and such holders have offered reasonable indemnity to the debenture trustee to institute the proceeding as trustee; and



•

the debenture trustee does not institute the proceeding, and does not receive from the holders of a majority in aggregate principal amount of the outstanding debt securities of that series other conflicting directions within 90 days after the notice, request and offer.

        These limitations do not apply to a suit instituted by a holder of debt securities if we default in the payment of the principal, premium, if any, or interest on, the debt securities.

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        We will periodically file statements with the debenture trustee regarding our compliance with specified covenants in the indentures.

Modification of Indenture; Waiver

        We and the debenture trustee may change an indenture without the consent of any holders with respect to specific matters, including:

•

to fix any ambiguity, defect or inconsistency in the indenture;



•

to comply with the provisions described above under "Consolidation, Merger or Sale";



•

to comply with any requirements of the SEC in connection with the qualification of any indenture under the Trust Indenture Act of 1939;



•

to evidence and provide for the acceptance of appointment hereunder by a successor trustee;



•

to provide for uncertificated debt securities and to make all appropriate changes for such purpose;



•

to add to, delete from, or revise the conditions, limitations and restrictions on the authorized amount, terms or purposes of issuance, authorization and delivery of debt securities or any series, as set forth in the indenture;



•

to provide for the issuance of and establish the form and terms and conditions of the debt securities of any series as provided under "General" to establish the form of any certifications required to be furnished pursuant to the terms of the indenture or any series of debt securities, or to add to the rights of the holders of any series of debt securities;



•

to add to our covenants such new covenants, restrictions, conditions or provisions for the protection of the holders, to make the occurrence, or the occurrence and the continuance, of a default in any such additional covenants, restrictions, conditions or provisions an event of default, or to surrender any of our rights or powers under the indenture; or



•

to change anything that does not materially adversely affect the interests of any holder of debt securities of any series.

        In addition, under the indentures, the rights of holders of a series of debt securities may be changed by us and the debenture trustee with the written consent of the holders of at least a majority in aggregate principal amount of the outstanding debt securities of each series that is affected. However, we and the debenture trustee may only make the following changes with the consent of each holder of any outstanding debt securities affected:

•

extending the fixed maturity of the series of debt securities;



•

reducing the principal amount, reducing the rate of or extending the time of payment of interest, or reducing any premium payable upon the redemption of any debt securities; or



•

reducing the percentage of debt securities, the holders of which are required to consent to any amendment, supplement, modification or waiver.

Discharge

        Each indenture provides that we can elect to be discharged from our obligations with respect to one or more series of debt securities, except for obligations to:

•

register the transfer or exchange of debt securities of the series;



•

replace stolen, lost or mutilated debt securities of the series;

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•

maintain paying agencies;



•

hold monies for payment in trust;



•

recover excess money held by the debenture trustee;



•

compensate and indemnify the debenture trustee; and



•

appoint any successor trustee.

        In order to exercise our rights to be discharged, we must deposit with the debenture trustee money or government obligations sufficient to pay all the principal of, any premium, if any, and interest on, the debt securities of the series on the dates payments are due.

Form, Exchange and Transfer

        We will issue the debt securities of each series only in fully registered form without coupons and, unless we otherwise specify in the applicable prospectus supplement, in denominations of $1,000 and any integral multiple thereof. The indentures provide that we may issue debt securities of a series in temporary or permanent global form and as book-entry securities that will be deposited with, or on behalf of, The Depository Trust Company, New York, New York, known as DTC, or another depositary named by us and identified in a prospectus supplement with respect to that series. See "Legal Ownership of Securities" for a further description of the terms relating to any book-entry securities.

        At the option of the holder, subject to the terms of the indentures and the limitations applicable to global securities described in the applicable prospectus supplement, the holder of the debt securities of any series can exchange the debt securities for other debt securities of the same series, in any authorized denomination and of like tenor and aggregate principal amount.

        Subject to the terms of the indentures and the limitations applicable to global securities set forth in the applicable prospectus supplement, holders of the debt securities may present the debt securities for exchange or for registration of transfer, duly endorsed or with the form of transfer endorsed thereon duly executed if so required by us or the security registrar, at the office of the security registrar or at the office of any transfer agent designated by us for this purpose. Unless otherwise provided in the debt securities that the holder presents for transfer or exchange, we will make no service charge for any registration of transfer or exchange, but we may require payment of any taxes or other governmental charges.

        We will name in the applicable prospectus supplement the security registrar, and any transfer agent in addition to the security registrar, that we initially designate for any debt securities. We may at any time designate additional transfer agents or rescind the designation of any transfer agent or approve a change in the office through which any transfer agent acts, except that we will be required to maintain a transfer agent in each place of payment for the debt securities of each series.

        If we elect to redeem the debt securities of any series, we will not be required to:

•

issue, register the transfer of, or exchange any debt securities of any series being redeemed in part during a period beginning at the opening of business 15 days before the day of mailing of a notice of redemption of any debt securities that may be selected for redemption and ending at the close of business on the day of the mailing; or



•

register the transfer of or exchange any debt securities so selected for redemption, in whole or in part, except the unredeemed portion of any debt securities we are redeeming in part.

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Information Concerning the Debenture Trustee

        The debenture trustee, other than during the occurrence and continuance of an event of default under an indenture, undertakes to perform only those duties as are specifically set forth in the applicable indenture. Upon an event of default under an indenture, the debenture trustee must use the same degree of care as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, the debenture trustee is under no obligation to exercise any of the powers given it by the indentures at the request of any holder of debt securities unless it is offered reasonable security and indemnity against the costs, expenses and liabilities that it might incur.

Payment and Paying Agents

        Unless we otherwise indicate in the applicable prospectus supplement, we will make payment of the interest on any debt securities on any interest payment date to the person in whose name the debt securities, or one or more predecessor securities, are registered at the close of business on the regular record date for the interest.

        We will pay principal of and any premium and interest on the debt securities of a particular series at the office of the paying agents designated by us, except that, unless we otherwise indicate in the applicable prospectus supplement, we may make interest payments by check which we will mail to the holder or by wire transfer to certain holders. Unless we otherwise indicate in a prospectus supplement, we will designate the corporate office of the debenture trustee in the City of New York as our sole paying agent for payments with respect to debt securities of each series. We will name in the applicable prospectus supplement any other paying agents that we initially designate for the debt securities of a particular series. We will maintain a paying agent in each place of payment for the debt securities of a particular series.

        All money we pay to a paying agent or the debenture trustee for the payment of the principal of or any premium or interest on any debt securities which remains unclaimed at the end of two years after such principal, premium or interest has become due and payable will be repaid to us, and the holder of the debt security thereafter may look only to us for payment thereof.

Governing Law

        The indentures and the debt securities will be governed by and construed in accordance with the laws of the State of New York, except to the extent that the Trust Indenture Act of 1939 is applicable.

Subordination of Subordinated Debt Securities

        The subordinated debt securities will be subordinate and junior in priority of payment to certain of our other indebtedness to the extent described in a prospectus supplement. The indentures in the forms initially filed as exhibits to the registration statement of which this prospectus is a part do not limit the amount of indebtedness which we may incur, including senior indebtedness or subordinated indebtedness, and do not limit us from issuing any other debt, including secured debt or unsecured debt.

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DESCRIPTION OF WARRANTS

        The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the warrants that we may offer under this prospectus. While the terms we have summarized below will apply generally to any warrants that we may offer under this prospectus, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. The terms of any warrants offered under a prospectus supplement may differ from the terms described below. However, no prospectus supplement will fundamentally change the terms that are set forth in this prospectus or offer a security that is not registered and described in this prospectus at the time of its effectiveness.

        We have filed forms of the warrant agreements and forms of warrant certificates containing the terms of the warrants being offered as exhibits to the registration statement of which this prospectus is a part. We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from a current report on Form 8-K that we file with the SEC, the form of warrant agreement, including a form of warrant certificate, that describes the terms of the particular series of warrants we are offering before the issuance of the related series of warrants. The following summaries of material provisions of the warrants and the warrant agreements are subject to, and qualified in their entirety by reference to, all the provisions of the warrant agreement and warrant certificate applicable to a particular series of warrants. We urge you to read the applicable prospectus supplements related to the particular series of warrants that we sell under this prospectus, as well as the complete warrant agreements and warrant certificates that contain the terms of the warrants.

General

        We will describe in the applicable prospectus supplement the terms relating to a series of warrants, including:

•

the offering price and aggregate number of warrants offered;



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the currency for which the warrants may be purchased;



•

if applicable, the designation and terms of the securities with which the warrants are issued and the number of warrants issued with each such security or each principal amount of such security;



•

if applicable, the date on and after which the warrants and the related securities will be separately transferable;



•

in the case of warrants to purchase debt securities, the principal amount of debt securities purchasable upon exercise of one warrant and the price at, and currency in which, this principal amount of debt securities may be purchased upon such exercise;



•

in the case of warrants to purchase common stock or preferred stock, the number of shares of common stock or preferred stock, as the case may be, purchasable upon the exercise of one warrant and the price at which these shares may be purchased upon such exercise;



•

the effect of any merger, consolidation, sale or other disposition of our business on the warrant agreements and the warrants;



•

the terms of any rights to redeem or call the warrants;



•

any provisions for changes to or adjustments in the exercise price or number of securities issuable upon exercise of the warrants;



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the dates on which the right to exercise the warrants will commence and expire;



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the manner in which the warrant agreements and warrants may be modified;



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federal income tax consequences of holding or exercising the warrants;

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•

the terms of the securities issuable upon exercise of the warrants; and



•

any other specific terms, preferences, rights or limitations of or restrictions on the warrants.

        Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including:

•

in the case of warrants to purchase debt securities, the right to receive payments of principal of, or premium, if any, or interest on, the debt securities purchasable upon exercise or to enforce covenants in the applicable indenture; or



•

in the case of warrants to purchase common stock or preferred stock, the right to receive dividends, if any, or, payments upon our liquidation, dissolution or winding up or to exercise voting rights, if any.

Exercise of Warrants

        Each warrant will entitle the holder to purchase the securities that we specify in the applicable prospectus supplement at the exercise price that we describe in the applicable prospectus supplement. Unless we otherwise specify in the applicable prospectus supplement, holders of the warrants may exercise the warrants at any time up to the specified time on the expiration date that we set forth in the applicable prospectus supplement. After the close of business on the expiration date, unexercised warrants will become void.

        Holders of the warrants may exercise the warrants by delivering the warrant certificate representing the warrants to be exercised together with specified information, and paying the required amount to the warrant agent in immediately available funds, as provided in the applicable prospectus supplement. We will set forth on the reverse side of the warrant certificate and in the applicable prospectus supplement the information that the holder of the warrant will be required to deliver to the warrant agent.

        Upon receipt of the required payment and the warrant certificate properly completed and duly executed at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement, we will issue and deliver the securities purchasable upon such exercise. If fewer than all of the warrants represented by the warrant certificate are exercised, then we will issue a new warrant certificate for the remaining amount of warrants. If we so indicate in the applicable prospectus supplement, holders of the warrants may surrender securities as all or part of the exercise price for warrants.

Enforceability of Rights by Holders of Warrants

        Each warrant agent will act solely as our agent under the applicable warrant agreement and will not assume any obligation or relationship of agency or trust with any holder of any warrant. A single bank or trust company may act as warrant agent for more than one issue of warrants. A warrant agent will have no duty or responsibility in case of any default by us under the applicable warrant agreement or warrant, including any duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a warrant may, without the consent of the related warrant agent or the holder of any other warrant, enforce by appropriate legal action its right to exercise, and receive the securities purchasable upon exercise of, its warrants.

Outstanding Warrants

        As of May 22, 2007, there were outstanding warrants to purchase 605,946 shares of our common stock. The exercise price of these warrants is $3.14 per share. The warrants will expire in November 2007, unless earlier exercised.

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DESCRIPTION OF UNITS

        The following description, together with the additional information we may include in any applicable prospectus supplements, summarizes the material terms and provisions of the units that we may offer under this prospectus. While the terms we have summarized below will apply generally to any units that we may offer under this prospectus, we will describe the particular terms of any series of units in more detail in the applicable prospectus supplement. The terms of any units offered under a prospectus supplement may differ from the terms described below. However, no prospectus supplement will fundamentally change the terms that are set forth in this prospectus or offer a security that is not registered and described in this prospectus at the time of its effectiveness.

        We will file as exhibits to the registration statement of which this prospectus is a part, or will incorporate by reference from a current report on Form 8-K that we file with the SEC, the form of unit agreement that describes the terms of the series of units we are offering, and any supplemental agreements, before the issuance of the related series of units. The following summaries of material terms and provisions of the units are subject to, and qualified in their entirety by reference to, all the provisions of the unit agreement and any supplemental agreements applicable to a particular series of units. We urge you to read the applicable prospectus supplements related to the particular series of units that we sell under this prospectus, as well as the complete unit agreement and any supplemental agreements that contain the terms of the units.

General

        We may issue units comprised of one or more debt securities, shares of common stock, shares of preferred stock and warrants in any combination. Each unit will be issued so that the holder of the unit is also the holder of each security included in the unit. Thus, the holder of a unit will have the rights and obligations of a holder of each included security. The unit agreement under which a unit is issued may provide that the securities included in the unit may not be held or transferred separately, at any time or at any time before a specified date.

        We will describe in the applicable prospectus supplement the terms of the series of units, including:

•

the designation and terms of the units and of the securities comprising the units, including whether and under what circumstances those securities may be held or transferred separately;



•

any provisions of the governing unit agreement that differ from those described below; and



•

any provisions for the issuance, payment, settlement, transfer or exchange of the units or of the securities comprising the units.

        The provisions described in this section, as well as those described under "Description of Capital Stock," "Description of Depositary Shares," "Description of Debt Securities" and "Description of Warrants" will apply to each unit and to any common stock, preferred stock, debt security or warrant included in each unit, respectively.

Issuance in Series

        We may issue units in such amounts and in numerous distinct series as we determine.

Enforceability of Rights by Holders of Units

        Each unit agent will act solely as our agent under the applicable unit agreement and will not assume any obligation or relationship of agency or trust with any holder of any unit. A single bank or trust company may act as unit agent for more than one series of units. A unit agent will have no duty or responsibility in case of any default by us under the applicable unit agreement or unit, including any

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duty or responsibility to initiate any proceedings at law or otherwise, or to make any demand upon us. Any holder of a unit may, without the consent of the related unit agent or the holder of any other unit, enforce by appropriate legal action its rights as holder under any security included in the unit.

Title

        We, the unit agents and any of their agents may treat the registered holder of any unit certificate as an absolute owner of the units evidenced by that certificate for any purpose and as the person entitled to exercise the rights attaching to the units so requested, despite any notice to the contrary. See "Legal Ownership of Securities."

LEGAL OWNERSHIP OF SECURITIES

        We can issue securities in registered form or in the form of one or more global securities. We describe global securities in greater detail below. We refer to those persons who have securities registered in their own names on the books that we or any applicable trustee or depositary or warrant agent maintain for this purpose as the "holders" of those securities. These persons are the legal holders of the securities. We refer to those persons who, indirectly through others, own beneficial interests in securities that are not registered in their own names, as "indirect holders" of those securities. As we discuss below, indirect holders are not legal holders, and investors in securities issued in book-entry form or in street name will be indirect holders.

Book-Entry Holders

        We may issue securities in book-entry form only, as we will specify in the applicable prospectus supplement. This means securities may be represented by one or more global securities registered in the name of a financial institution that holds them as depositary on behalf of other financial institutions that participate in the depositary's book-entry system. These participating institutions, which are referred to as participants, in turn, hold beneficial interests in the securities on behalf of themselves or their customers.

        Only the person in whose name a security is registered is recognized as the holder of that security. Global securities will be registered in the name of the depositary. Consequently, for global securities, we will recognize only the depositary as the holder of the securities, and we will make all payments on the securities to the depositary. The depositary passes along the payments it receives to its participants, which in turn pass the payments along to their customers who are the beneficial owners. The depositary and its participants do so under agreements they have made with one another or with their customers; they are not obligated to do so under the terms of the securities.

        As a result, investors in a global security will not own securities directly. Instead, they will own beneficial interests in a global security, through a bank, broker or other financial institution that participates in the depositary's book-entry system or holds an interest through a participant. As long as the securities are issued in global form, investors will be indirect holders, and not holders, of the securities.

Street Name Holders

        We may terminate global securities or issue securities that are not issued in global form. In these cases, investors may choose to hold their securities in their own names or in "street name." Securities held by an investor in street name would be registered in the name of a bank, broker or other financial institution that the investor chooses, and the investor would hold only a beneficial interest in those securities through an account he or she maintains at that institution.

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        For securities held in street name, we or any applicable trustee or depositary will recognize only the intermediary banks, brokers and other financial institutions in whose names the securities are registered as the holders of those securities, and we or any such trustee or depositary will make all payments on those securities to them. These institutions pass along the payments they receive to their customers who are the beneficial owners, but only because they agree to do so in their customer agreements or because they are legally required to do so. Investors who hold securities in street name will be indirect holders, not holders, of those securities.

Legal Holders

        Our obligations, as well as the obligations of any applicable trustee or third party employed by us or a trustee, run only to the legal holders of the securities. We do not have obligations to investors who hold beneficial interests in global securities, in street name or by any other indirect means. This will be the case whether an investor chooses to be an indirect holder of a security or has no choice because we are issuing the securities only in global form.

        For example, once we make a payment or give a notice to the holder, we have no further responsibility for the payment or notice even if that holder is required, under agreements with its participants or customers or by law, to pass it along to the indirect holders but does not do so. Similarly, we may want to obtain the approval of the holders to amend an indenture, to relieve us of the consequences of a default or of our obligation to comply with a particular provision of an indenture, or for other purposes. In such an event, we would seek approval only from the holders, and not the indirect holders, of the securities. Whether and how the holders contact the indirect holders is up to the holders.

Special Considerations for Indirect Holders

        If you hold securities through a bank, broker or other financial institution, either in book-entry form because the securities are represented by one or more global securities or in street name, you should check with your own institution to find out:

•

how it handles securities payments and notices;



•

whether it imposes fees or charges;



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how it would handle a request for the holders' consent, if ever required;



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whether and how you can instruct it to send you securities registered in your own name so you can be a holder, if that is permitted in the future;



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how it would exercise rights under the securities if there were a default or other event triggering the need for holders to act to protect their interests; and



•

if the securities are in book-entry form, how the depositary's rules and procedures will affect these matters.

Global Securities

        A global security is a security that represents one or any other number of individual securities held by a depositary. Generally, all securities represented by the same global securities will have the same terms.

        Each security issued in book-entry form will be represented by a global security that we issue to, deposit with and register in the name of a financial institution or its nominee that we select. The financial institution that we select for this purpose is called the depositary. Unless we specify otherwise

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in the applicable prospectus supplement, The Depository Trust Company, New York, New York, known as DTC, will be the depositary for all securities issued in book-entry form.

        A global security may not be transferred to or registered in the name of anyone other than the depositary, its nominee or a successor depositary, unless special termination situations arise. We describe those situations below under "—Special Situations When a Global Security Will Be Terminated." As a result of these arrangements, the depositary, or its nominee, will be the sole registered owner and holder of all securities represented by a global security, and investors will be permitted to own only beneficial interests in a global security. Beneficial interests must be held by means of an account with a broker, bank or other financial institution that in turn has an account with the depositary or with another institution that does. Thus, an investor whose security is represented by a global security will not be a holder of the security, but only an indirect holder of a beneficial interest in the global security.

        If the prospectus supplement for a particular security indicates that the security will be issued as a global security, then the security will be represented by a global security at all times unless and until the global security is terminated. If termination occurs, we may issue the securities through another book-entry clearing system or decide that the securities may no longer be held through any book-entry clearing system.

Special Considerations For Global Securities

        As an indirect holder, an investor's rights relating to a global security will be governed by the account rules of the investor's financial institution and of the depositary, as well as general laws relating to securities transfers. We do not recognize an indirect holder as a holder of securities and instead deal only with the depositary that holds the global security.

        If securities are issued only as global securities, an investor should be aware of the following:

•

an investor cannot cause the securities to be registered in his or her name, and cannot obtain non-global certificates for his or her interest in the securities, except in the special situations we describe below;



•

an investor will be an indirect holder and must look to his or her own bank or broker for payments on the securities and protection of his or her legal rights relating to the securities, as we describe above;



•

an investor may not be able to sell interests in the securities to some insurance companies and to other institutions that are required by law to own their securities in non-book-entry form;



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an investor may not be able to pledge his or her interest in the global security in circumstances where certificates representing the securities must be delivered to the lender or other beneficiary of the pledge in order for the pledge to be effective;



•

the depositary's policies, which may change from time to time, will govern payments, transfers, exchanges and other matters relating to an investor's interest in the global security. We and any applicable trustee have no responsibility for any aspect of the depositary's actions or for its records of ownership interests in the global security. We and the trustee also do not supervise the depositary in any way;



•

the depositary may, and we understand that DTC will, require that those who purchase and sell interests in the global security within its book-entry system use immediately available funds, and your broker or bank may require you to do so as well; and



•

financial institutions that participate in the depositary's book-entry system, and through which an investor holds its interest in the global security, may also have their own policies affecting

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payments, notices and other matters relating to the securities. There may be more than one financial intermediary in the chain of ownership for an investor. We do not monitor and are not responsible for the actions of any of those intermediaries.

Special Situations When A Global Security Will Be Terminated

        In a few special situations described below, a global security will terminate and interests in it will be exchanged for physical certificates representing those interests. After that exchange, the choice of whether to hold securities directly or in street name will be up to the investor. Investors must consult their own banks or brokers to find out how to have their interests in securities transferred to their own names, so that they will be direct holders. We have described the rights of holders and street name investors above.

        A global security will terminate when the following special situations occur:

•

if the depositary notifies us that it is unwilling, unable or no longer qualified to continue as depositary for that global security and we do not appoint another institution to act as depositary within 90 days;



•

if we notify any applicable trustee that we wish to terminate that global security; or



•

if an event of default has occurred with regard to securities represented by that global security and has not been cured or waived.

        The prospectus supplement may also list additional situations for terminating a global security that would apply only to the particular series of securities covered by the prospectus supplement. When a global security terminates, the depositary, and not we or any applicable trustee, is responsible for deciding the names of the institutions that will be the initial direct holders.

PLAN OF DISTRIBUTION

        We may sell the securities through underwriters or dealers, through agents, or directly to one or more purchasers. A prospectus supplement or supplements will describe the terms of the offering of the securities, including:

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the name or names of any underwriters, if any;



•

the purchase price of the securities and the proceeds we will receive from the sale;



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any over-allotment options under which underwriters may purchase additional securities from us;



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any agency fees or underwriting discounts and other items constituting agents' or underwriters' compensation;



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any public offering price;



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any discounts or concessions allowed or reallowed or paid to dealers; and



•

any securities exchange or market on which the securities may be listed.

        Only underwriters named in the prospectus supplement are underwriters of the securities offered by the prospectus supplement.

        If underwriters are used in the sale, they will acquire the securities for their own account and may resell the securities from time to time in one or more transactions at a fixed public offering price or at varying prices determined at the time of sale. The obligations of the underwriters to purchase the securities will be subject to the conditions set forth in the applicable underwriting agreement. We may offer the securities to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. Subject to certain conditions, the underwriters will be obligated

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to purchase all of the securities offered by the prospectus supplement. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealers may change from time to time. We may use underwriters with whom we have a material relationship. We will describe in the prospectus supplement, naming the underwriter, the nature of any such relationship.

        We may sell securities directly or through agents we designate from time to time. We will name any agent involved in the offering and sale of securities and we will describe any commissions we will pay the agent in the prospectus supplement. Unless the prospectus supplement states otherwise, our agent will act on a best-efforts basis for the period of its appointment.

        We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the prospectus supplement.

        We may provide agents and underwriters with indemnification against civil liabilities related to this offering, including liabilities under the Securities Act, or contribution with respect to payments that the agents or underwriters may make with respect to these liabilities. Agents and underwriters may engage in transactions with, or perform services for, us in the ordinary course of business.

        All securities we offer, other than common stock, will be new issues of securities with no established trading market. Any underwriters may make a market in these securities, but will not be obligated to do so and may discontinue any market making at any time without notice. We cannot guarantee the liquidity of the trading markets for any securities.

        Any underwriter may engage in overallotment, stabilizing transactions, short covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Overallotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

        Any underwriters who are qualified market makers on the Nasdaq Global Market may engage in passive market making transactions in the securities on the Nasdaq Global Market in accordance with Rule 103 of Regulation M, during the business day prior to the pricing of the offering, before the commencement of offers or sales of the securities. Passive market makers must comply with applicable volume and price limitations and must be identified as passive market makers. In general, a passive market maker must display its bid at a price not in excess of the highest independent bid for such security; if all independent bids are lowered below the passive market maker's bid, however, the passive market maker's bid must then be lowered when certain purchase limits are exceeded.

        In compliance with guidelines of the National Association of Securities Dealers, or NASD, the maximum consideration or discount to be received by any NASD member or independent broker dealer may not exceed 8% of the aggregate amount of the securities offered pursuant to this prospectus and any applicable prospectus supplement.

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LEGAL MATTERS

        The validity of the securities being offered hereby will be passed upon by Cooley Godward Kronish LLP, Broomfield, Colorado.

EXPERTS

        The financial statements and management's assessment of the effectiveness of internal control over financial reporting (which is included in Management's Report on Internal Control over Financial Reporting) incorporated in this prospectus by reference to our Annual Report on Form 10-K for the year ended December 31, 2006 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

WHERE YOU CAN FIND MORE INFORMATION

        We are a reporting company and file annual, quarterly and current reports, proxy statements and other information with the SEC. We have filed with the SEC a registration statement on Form S-3 under the Securities Act with respect to the securities we are offering under this prospectus. This prospectus does not contain all of the information set forth in the registration statement and the exhibits to the registration statement. For further information with respect to us and the securities we are offering under this prospectus, we refer you to the registration statement and the exhibits and schedules filed as a part of the registration statement. You may read and copy the registration statement, as well as our reports, proxy statements and other information, at the SEC's public reference rooms at 100 F Street, N.E., Washington, D.C. 20549, as well as at the SEC's regional offices at 500 West Madison Street, Suite 1400, Chicago, IL 60661 and at 233 Broadway, New York, NY 10279. You can request copies of these documents by writing to the SEC and paying a fee for the copying cost. Please call the SEC at 1-800-SEC-0330 for more information about the operation of the public reference rooms. Our SEC filings are also available at the SEC's web site at "http://www.sec.gov." In addition, you can read and copy our SEC filings at the office of the National Association of Securities Dealers, Inc. at 1735 K Street, N.W., Washington, D.C. 20006.

INCORPORATION BY REFERENCE

        The SEC allows us to "incorporate by reference" information that we file with it, which means that we can disclose important information to you by referring you to those documents. The information incorporated by reference is an important part of this prospectus. Information in this prospectus supersedes information incorporated by reference that we filed with the SEC prior to the date of this prospectus, while information that we file later with the SEC will automatically update and supersede this information. We incorporate by reference into this registration statement and prospectus the documents listed below, and any future filings we will make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of the initial registration statement but prior to effectiveness of the registration statement and after the date of this prospectus but prior to the termination of the offering of the securities covered by this prospectus (other than current reports or portions thereof furnished under Item 2.02 or Item 7.01 of Form 8-K):

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Our Form 10-Q for the three months ended March 31, 2007;



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Our Annual Report on Form 10-K for the year ended December 31, 2006;



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Our Current Reports on Form 8-K filed January 9, 2007, January 29, 2007, January 30, 2007, February 23, 2007, March 21, 2007, April 19, 2007 and May 1, 2007;



•

Our definitive proxy statement filed pursuant to Section 14 of the Exchange Act in connection with our 2007 Annual Meeting of Stockholders filed with the SEC on April 30, 2007; and

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•

The description of our common stock set forth in our registration statement on Form 8-A, filed with the SEC on March 6, 2000.

        We will provide to each person, including any beneficial owner, to whom a prospectus is delivered, without charge upon written or oral request, a copy of any or all of the documents that are incorporated by reference into this prospectus but not delivered with the prospectus, including exhibits that are specifically incorporated by reference into such documents. Requests should be directed to: Allos Therapeutics, Inc., Attention: Chief Executive Officer, 11080 CirclePoint Road, Suite 200, Westminster, Colorado 80020, telephone: (303) 426-6262.

DISCLOSURE OF COMMISSION POSITION ON INDEMNIFICATION FOR
SECURITIES ACT LIABILITY

        Insofar as indemnification for liabilities arising under the Securities Act may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

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