Use these links to rapidly review the document
TABLE OF CONTENTS
TABLE OF CONTENTS

Filed Pursuant to Rule 424(b)(5)
Reg. No. 333-140251

PROSPECTUS SUPPLEMENT
(To Prospectus dated February 8, 2007)

4,000,000 Shares

GRAPHIC

Common Stock



         We are offering 4,000,000 shares of our common stock pursuant to this prospectus supplement and the accompanying prospectus.
         Our common stock is listed on the NASDAQ Global Market under the symbol "MNTA." On September 22, 2009, the last reported sale price for our common stock on the NASDAQ Global Market was $11.60 per share.
        
Investing in our common stock involves significant risks. You should read this prospectus supplement and the accompanying prospectus carefully before you make your investment decision. See "Risk Factors," beginning on page S-5 of this prospectus supplement and those contained in our incorporated documents, to read about factors you should consider before buying shares of our common stock.



 
  Per Share   Total  
Public offering price   $ 10.7500   $ 43,000,000  
Underwriting discounts and commissions   $ .5375   $ 2,150,000  
Proceeds, before expenses, to us   $ 10.2125   $ 40,850,000  

         The underwriter may also purchase up to an additional 600,000 shares of our common stock at the public offering price, less the underwriting discounts and commissions, within 30 days from the date of this prospectus supplement to cover overallotments.
         You should carefully read this prospectus supplement and the accompanying prospectus, together with the documents we incorporate by reference, before you invest in our common stock.
         Neither the Securities and Exchange Commission nor any state securities commission 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 shares of common stock will be ready for delivery on or about September 28, 2009.



Leerink Swann
Sole Book-Running Manager



The date of this prospectus supplement is September 22, 2009


Table of Contents


TABLE OF CONTENTS

Prospectus Supplement

 
  Page

About this Prospectus Supplement

 
ii

Prospectus Supplement Summary

  S-1

Risk Factors

  S-5

Cautionary Note Regarding Forward-Looking Statements

  S-6

Use of Proceeds

  S-6

Dilution

  S-7

Underwriting

  S-8

Legal Matters

  S-11

Experts

  S-11

Incorporation of Documents by Reference

  S-11

Where You Can Find More Information

  S-12

Prospectus

About this Prospectus

 
1

Momenta Pharmaceuticals, Inc. 

  1

Risk Factors

  2

Special note Regarding Forward-Looking Information

  2

Use of Proceeds

  3

Ratio of Earnings to Fixed Charges

  3

Dilution

  3

The Securities We May Offer

  4

Description of Capital Stock

  5

Description of Debt Securities

  10

Description of Warrants

  15

Legal Ownership of Securities

  17

Plan of Distribution

  20

Validity of Securities

  21

Experts

  22

Where You Can Find More Information

  22

Incorporation of Documents by Reference

  22

i


Table of Contents


ABOUT THIS PROSPECTUS SUPPLEMENT

              This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this common stock offering and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference herein. The second part, the accompanying prospectus, provides more general information. Generally, when we refer to this prospectus, we are referring to both parts of this document combined. To the extent there is a conflict between the information contained in this prospectus supplement and the information contained in the accompanying prospectus or any document incorporated by reference therein filed prior to the date of this prospectus supplement, you should rely on the information in this prospectus supplement, provided that, if any statement in one of these documents is inconsistent with a statement in another document having a later date—for example, a document incorporated by reference in the accompanying prospectus—the statement in the document having the later date modifies or supersedes the earlier statement.

              We further note that the representations, warranties and covenants made by us in any agreement that is filed as an exhibit to any document that is incorporated by reference herein were made solely for the benefit of the parties to such agreement, including, in some cases, for the purpose of allocating risk among the parties to such agreements, and should not be deemed to be a representation, warranty or covenant to you. Moreover, such representations, warranties or covenants were accurate only as of the date when made. Accordingly, such representations, warranties and covenants should not be relied on as accurately representing the current state of our affairs.

              You should rely only on the information contained in this prospectus supplement or the accompanying prospectus, or incorporated by reference herein. We have not authorized, and the underwriter has not authorized, anyone to provide you with information that is different. The information contained in this prospectus supplement or the accompanying prospectus, or incorporated by reference herein is accurate only as of the respective dates thereof, regardless of the time of delivery of this prospectus supplement and the accompanying prospectus or of any sale of our common stock. It is important for you to read and consider all information contained in this prospectus supplement and the accompanying prospectus, including the documents incorporated by reference herein and therein, in making your investment decision. You should also read and consider the information in the documents to which we have referred you in the sections entitled "Where You Can Find More Information" and "Incorporation of Documents by Reference" in this prospectus supplement and in the accompanying prospectus.

              We are offering to sell, and seeking offers to buy, shares of our common stock only in jurisdictions where offers and sales are permitted. The distribution of this prospectus supplement and the accompanying prospectus and the offering of the common stock in certain jurisdictions may be restricted by law. Persons outside the United States who come into possession of this prospectus supplement and the accompanying prospectus must inform themselves about, and observe any restrictions relating to, the offering of the common stock and the distribution of this prospectus supplement and the accompanying prospectus outside the United States. This prospectus supplement and the accompanying prospectus do not constitute, and may not be used in connection with, an offer to sell, or a solicitation of an offer to buy, any securities offered by this prospectus supplement and the accompanying prospectus by any person in any jurisdiction in which it is unlawful for such person to make such an offer or solicitation.

              Unless otherwise stated, all references in this prospectus to "we," "us," "our," "Momenta," the "Company" and similar designations refer to Momenta Pharmaceuticals, Inc. Our logo, trademarks and service marks are the property of Momenta. Other trademarks or service marks appearing in this prospectus supplement are the property of their respective holders.

ii


Table of Contents


PROSPECTUS SUPPLEMENT SUMMARY

              This summary highlights selected information about us and this offering. This information is not complete and does not contain all the information you should consider before investing in our common stock. You should carefully read this entire prospectus supplement and the accompanying prospectus, including the "Risk Factors" section contained in this prospectus supplement and the financial statements and the other information incorporated by reference herein, before making an investment decision.


Momenta Pharmaceuticals, Inc.

Our Business

              Momenta is a biotechnology company with a product pipeline of both complex generic and novel drugs. This pipeline is derived from our proprietary, innovative technology platform for the detailed structural analysis of complex mixture drugs. We use this platform to study the structure (thorough characterization of chemical components), structure-process (design and control of manufacturing process), and structure-activity (relating structure to biological and clinical activity) of complex mixture drugs.

Complex Mixture Generics Portfolio

              Our complex mixture generics and follow-on biologics effort is focused on building a thorough understanding of the structure-process-activity of complex mixture drugs to develop generic versions of marketed products. While we use a similar analytical and development approach across all of our product candidates, we tailor that approach for each specific product candidate. Our first objective is to apply our core analytical technology to thoroughly characterize the structure of the marketed product. By defining the chemical composition of multiple batches of the marketed product, we are able to develop an equivalence window, which captures the inherent variability of the innovator's manufacturing process. Using this information we then build an extensive understanding of the structure-process relationship to design and control our manufacturing process to reproducibly manufacture an equivalent version of the marketed product. Where necessary, and as required by the U.S. Food and Drug Administration, or FDA, we will supplement an application with additional supportive structure-activity data (e.g., immunogenicity, pharmacodynamics). Our goal is to obtain FDA approval for and commercialize generic or follow-on versions of complex mixture products, thereby providing high quality, safe and affordable medicines to patients in need.

              In November 2003, we entered into a collaboration and license agreement, or the 2003 Sandoz Collaboration, with Sandoz N.V. and Sandoz Inc. to jointly develop and commercialize M-Enoxaparin. Sandoz N.V. later assigned its rights in the 2003 Sandoz Collaboration to Sandoz AG. We refer to Sandoz AG and Sandoz Inc. together as Sandoz.

              Our most advanced product candidate, M-Enoxaparin, is designed to be a generic version of Lovenox®, a low molecular weight heparin, or LMWH, used to prevent and treat deep vein thrombosis, or DVT, and to support the treatment of acute coronary syndromes, or ACS. This drug is a complex mixture of polysaccharide chains derived from naturally sourced heparin. Under our 2003 Sandoz Collaboration, we work with Sandoz exclusively to develop, manufacture and commercialize M-Enoxaparin in the U.S. and Sandoz is responsible for funding substantially all of the U.S.-related M-Enoxaparin development, regulatory, legal and commercialization costs. The total cost of development and commercialization, and the timing of M-Enoxaparin product launch, are subject to uncertainties relating to the development, regulatory approval and legal processes. Sandoz submitted abbreviated new drug applications, or ANDAs, in its name to the FDA for M-Enoxaparin in syringe and vial forms seeking approval to market M-Enoxaparin in the United States.

S-1


Table of Contents

              The FDA is currently reviewing both Sandoz's M-Enoxaparin ANDAs, including our manufacturing data and technology and characterization methodology. In November 2007, Sandoz received a letter from the FDA stating that the syringe ANDA for M-Enoxaparin was not approvable in its current form because the ANDA did not adequately address the potential for immunogenicity of the drug product. Starting in early 2008, we and Sandoz conferred with the FDA concerning the design of studies to address the FDA's concerns in this area. These interactions led to the FDA's general concurrence with our proposed approach and to the submission of an immunogenicity amendment to the M-Enoxaparin ANDA in September 2008. The review of Sandoz's ANDA is ongoing. We and Sandoz are in regular communication with the FDA to address any additional questions or requests that it may have as it continues the review of Sandoz's application. The FDA has not requested human clinical trials at this time. However, there can be no assurances that the FDA will not require additional studies, including clinical studies, in the future and we cannot predict with a high degree of certainty the timing of any potential approval of the M-Enoxaparin ANDA by the FDA. We and Sandoz are also in active dialogue with the FDA regarding the sourcing and processing of our heparin supply. The FDA has inspected the Chinese facilities that supply heparin to Sandoz for the manufacture of M-Enoxaparin and inspection activities are ongoing. Sandoz has not yet received the FDA's final inspection reports. We and Sandoz are working together to prepare for the commercialization of M-Enoxaparin, if and when approved, by advancing manufacturing, supply chain, and sales and marketing objectives.

              Our second major generic product candidate is M356, a technology-enabled generic version of Copaxone® (glatiramer acetate injection), a drug that is indicated for the reduction of the frequency of relapses in patients with Relapsing-Remitting Multiple Sclerosis. Copaxone consists of a complex mixture of polypeptide chains. With M356, we have extended our core characterization capabilities from the characterization of complex polysaccharide mixtures to include the characterization of complex polypeptide mixtures.

              In 2006 and 2007, we entered into a series of agreements, including a Stock Purchase Agreement and an Investor Rights Agreement, each with Novartis Pharma AG, and a collaboration and license agreement, or the Definitive Agreement, with Sandoz AG, an affiliate of Novartis Pharma AG. Together, this series of agreements is referred to as the 2006 Sandoz Collaboration. Under the Definitive Agreement, we and Sandoz AG jointly develop, manufacture and commercialize M356.

              In December 2007, our collaborative partner, Sandoz AG, submitted to the FDA an ANDA in its name containing a Paragraph IV certification seeking approval to market M356 in the United States. In July 2008, the FDA notified Sandoz AG that it had accepted the ANDA for review as of December 27, 2007. In addition, the FDA's published database indicates that the first substantially complete ANDA submitted for glatiramer acetate injection containing a Paragraph IV certification was filed on December 27, 2007, making Sandoz AG's ANDA eligible for the grant of a 180-day generic exclusivity period upon approval. The review of Sandoz's ANDA is ongoing. We and Sandoz are in regular communication with the FDA to address any additional questions or requests that it may have as it continues the review of Sandoz's application.

Novel Drugs Portfolio

              Our complex mixture novel drug research and development efforts leverage our analytical technology platform and structure-process knowledge to develop novel drugs by studying the structure-activity of complex mixtures. With our capabilities to thoroughly characterize complex mixtures, we are targeting our efforts to understand the relationship between structure and the biological and therapeutic activity of various complex mixture drug candidates. Our goal is to capitalize on the structural diversity and multitargeting potential of these complex mixtures to engineer novel drug candidates that we believe will meet key unmet medical needs in various diseases. While we believe

S-2


Table of Contents


that our capabilities to engineer improved and novel complex mixture drug candidates can be applied across several product categories with significant therapeutic potential, such as polysaccharides, polypeptides and glycoproteins, our initial focus has been in the area of complex polysaccharide mixtures.

              Our lead novel drug candidate, M118, has been engineered to possess what we believe will be an improved therapeutic profile compared with other currently marketed products to support the treatment of ACS. M402, our second novel drug candidate, is in early development as a potential inhibitor of angiogenesis and tumor metastasis. We also are seeking to discover and develop additional novel drug candidates by applying our technology to better understand the function of these complex polysaccharide and glycoprotein mixtures in biological processes.

Company Background

              We were incorporated in Delaware in May 2001 under the name Mimeon, Inc. In September 2002, we changed our name to Momenta Pharmaceuticals, Inc. Since our inception in May 2001, we have incurred annual net losses. As of June 30, 2009, we had an accumulated deficit of $291.7 million. We expect to incur substantial and increasing losses for the next several years as we develop our product candidates, expand our research and development activities and prepare for the commercial launch of our product candidates. Additionally, we plan to continue to evaluate possible acquisitions or licensing of rights to additional technologies, products or assets that fit within our growth strategy. Accordingly, we will need to generate significant revenues to achieve and then maintain profitability.

              Since our inception, we have had no revenues from product sales. Our revenues have all been derived from our 2003 Sandoz Collaboration and 2006 Sandoz Collaboration and primarily consist of amounts earned by us for reimbursement by Sandoz of research and development services and development costs for certain programs. In June 2004, we completed an initial public offering of 6,152,500 shares of common stock, the net proceeds of which were $35.3 million after deducting underwriters' discounts and expenses. In July 2005, we raised $122.3 million in a follow-on public offering, net of expenses, from the sale and issuance of 4,827,300 shares of our common stock. In September 2006, in connection with the 2006 Sandoz Collaboration, we sold 4,708,679 shares of common stock to Novartis Pharma AG at a per share price of $15.93 (the closing price of our common stock on the NASDAQ Global Market was $13.05 on the date of purchase) for an aggregate purchase price of $75.0 million, resulting in an equity premium of $13.6 million. In December 2008, we raised $24.1 million in a public offering, net of expenses, from the sale and issuance of 2,800,000 shares of our common stock. To date, we have devoted substantially all of our capital resource expenditures to the research and development of our product candidates.

              Our principal executive offices are located at 675 West Kendall Street, Cambridge, Massachusetts 02142, and our telephone number is (617) 491-9700. Our Internet address is www.momentapharma.com. The information on our website is not incorporated by reference into this prospectus supplement and should not be considered to be a part of this prospectus. We have included our website address as an inactive textural reference only.

S-3


Table of Contents


THE OFFERING

Common stock offered by us in this offering

  4,000,000 shares

Common stock to be outstanding after the offering

 

43,895,081 shares

Use of Proceeds

 

We intend to use the net proceeds from this offering for general corporate and working capital purposes, including research and development expenses, manufacturing expenses, clinical trial costs, general and administrative expenses, and potential acquisitions of, or investments in, companies, products and technologies that complement our business. See "Use of Proceeds."

Risk Factors

 

You should read the description of risks set forth in the "Risk Factors" section of this prospectus supplement or otherwise incorporated by reference in this prospectus supplement for a discussion of factors to consider before deciding to purchase shares of our common stock.

NASDAQ Global Market symbol

 

MNTA

              The number of shares of our common stock to be outstanding after this offering is based on 39,895,081 shares of our common stock outstanding as of June 30, 2009, and excludes:

              Except as otherwise noted, we have presented the information in this prospectus supplement assuming no exercise by the underwriter of its option to purchase up to 600,000 additional shares of our common stock to cover overallotments.

S-4


Table of Contents


RISK FACTORS

              Investing in our common stock involves significant risks. In addition to the other information included or incorporated by reference in this prospectus supplement and the accompanying prospectus, you should carefully consider the risks described under the heading "Risk Factors" in our most recent annual report on Form 10-K, as revised or supplemented by our quarterly reports on Form 10-Q filed with the SEC since the filing of our most recent annual report on Form 10-K, each of which are on file with the SEC and are incorporated herein by reference, before making an investment decision. The risks and uncertainties we have described are not the only ones facing our company. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business operations.


Risks Relating to this Offering

Investors in this offering will pay a much higher price than the book value of our stock.

              If you purchase common stock in this offering, you will incur an immediate and substantial dilution in net tangible book value of $8.29 per share, after giving effect to the sale by us of 4,000,000 shares in this offering at a public offering price of $10.75 per share. In the past, we have issued options to acquire common stock at prices significantly below this offering price. To the extent these outstanding options are ultimately exercised, you will incur additional dilution. See "Dilution" for a more detailed discussion of the dilution investors in this offering will incur.

Because our management will have broad discretion over the use of the net proceeds from this offering, you may not agree with how we use them and the proceeds may not be invested successfully.

              The net proceeds from this offering will be available for, among other purposes, general corporate purposes, and our management will have broad discretion as to the use of such proceeds. Accordingly, investors will be relying on the judgment of our management with regard to the use of net proceeds we receive from this offering, and they will not have the opportunity to assess whether the proceeds are being used appropriately. It is possible that the proceeds we receive will be invested in a way that does not yield a favorable, or any, return for us.

If securities analysts downgrade our stock or cease coverage of us, the price of our stock could decline.

              The trading market for our common stock relies in part on the research and reports that industry or financial analysts publish about us or our business. Currently, six financial analysts publish reports about us and our business. We do not control these or any other analysts. Furthermore, there are many large, well-established, publicly traded companies active in our industry and market, which may mean that it is less likely that we will receive widespread analyst coverage. If any of the analysts who cover us downgrade our stock, our stock price would likely decline rapidly. If these analysts cease coverage of our company, we could lose visibility in the market, which in turn could cause our stock price to decline.

Future sales of additional shares of our common stock could cause the price of our common stock to decline.

              Sales of substantial amounts of our common stock in the public market following this offering, or the perception by the market that those sales could occur, may lower our stock price or make it difficult for us to raise additional equity capital in the future. For example, one of our large stockholders may decide to sell a substantial portion of its shares of our common stock. In addition, the issuance of common stock upon exercise of outstanding options could be dilutive, and may cause the market price for a share of our common stock to decline. As of June 30, 2009, we had 39,895,081 shares of common stock issued and outstanding, together with outstanding options to purchase 4,575,984 shares of common stock with a weighted average exercise price of $11.08 per share.

S-5


Table of Contents


CAUTIONARY NOTE REGARDING FORWARD LOOKING STATEMENTS

              Statements, other than statements of historical facts, that we include in this prospectus supplement, the accompanying prospectus and in the documents we incorporate by reference herein, may be deemed "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. We use the words "anticipate," "believe," "could," "could increase the likelihood," "hope," "target," "project," "goals," "potential," "predict," "might," "estimate," "expect," "intend," "is planned," "may," "should," "will," "will enable," "would be expected," "look forward," "may provide," "would" or similar terms, variations of such terms or the negative of those terms, to identify forward looking statements, although not all forward looking statements contain these identifying words. We cannot guarantee that we actually will achieve the plans, intentions or expectations disclosed in our forward looking statements and, accordingly, you should not place undue reliance on our forward looking statements. There are a number of important factors that could cause actual results or events to differ materially from the forward looking statements that we make, including the factors included in the documents we incorporate by reference herein. You should read these factors and the other cautionary statements made in the documents we incorporate by reference as being applicable to all related forward looking statements wherever they appear in this prospectus supplement, the accompanying prospectus and any document incorporated by reference. We caution you that we do not undertake any obligation to update or revise forward looking statements made by us, whether as a result of new information, future events or otherwise.


USE OF PROCEEDS

              We estimate that the net proceeds we will receive from this offering will be approximately $40.6 million (or $46.7 million if the underwriter exercises its option to purchase 600,000 additional shares in full), after deducting the underwriting discount and estimated offering expenses.

              We intend to use the net proceeds of this offering for general corporate and working capital purposes. Although we have not yet identified any specific uses for these proceeds, we currently anticipate using the proceeds for some or all of the following purposes: research and development expenses, manufacturing expenses, clinical trial costs, general and administrative expenses and potential acquisitions of, or investments in, companies, products and technologies that complement our business. Pending the application of the net proceeds, we intend to invest the net proceeds in investment grade, interest bearing securities.

S-6


Table of Contents


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 subtracting our total liabilities from our total tangible assets and dividing the difference by the number of outstanding shares of our common stock.

              Our net tangible book value at June 30, 2009 was $67.4 million, or $1.69 per share, based on 39,895,081 shares of our common stock outstanding on June 30, 2009. After giving effect to the sale of 4,000,000 million shares of common stock by us at a public offering price of $10.75 per share, less the underwriting discount and estimated offering expenses, our net tangible book value at June 30, 2009 would be $108.0 million, or $2.46 per share. This represents an immediate increase in net tangible book value of $0.77 per share to existing stockholders and an immediate dilution of $8.29 per share to investors in this offering. The following table illustrates this per share dilution:

Public offering price per share

        $ 10.75  
             
   

Net tangible book value per share as of June 30, 2009

  $ 1.69        
   

Increase per share attributable to new investors

  $ .77        
             

Net tangible book value per share after this offering

          2.46  
             

Dilution per share to new investors

        $ 8.29  
             

              If the underwriter exercises its option to purchase 600,000 additional shares in full, the net tangible book value per share after this offering would be $2.56 per share, representing an increase to existing stockholders of $0.88 per share, and there would be an immediate dilution of $8.19 per share to new investors.

              In the discussion and table above, we assume no exercise of outstanding options. As of June 30, 2009, there were 4,575,984 shares of common stock issuable upon exercise of outstanding options with a weighted average exercise price of $11.08 per share. To the extent that any of these outstanding options are exercised, there will be further dilution to new investors.

S-7


Table of Contents


UNDERWRITING

              Subject to the terms and conditions set forth in an underwriting agreement between us and Leerink Swann LLC, which is acting as sole underwriter for this offering, we have agreed to sell to the underwriter, and the underwriter has agreed to purchase from us all of the shares offered by us in this offering.

              We have agreed to indemnify the underwriter against certain liabilities, including liabilities under the Securities Act, or to contribute to payments the underwriter may be required to make in respect of those liabilities.

              The underwriter is offering the shares, subject to prior sale, when, as and if issued to and accepted by it, subject to approval of legal matters by its counsel, including the validity of the shares, and other conditions contained in the underwriting agreement, such as the receipt by the underwriter of officer's certificates and legal opinions. The underwriter reserves the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part.

Commissions and Discounts

              The underwriter proposes initially to offer the shares to the public at the public offering price set forth on the cover page of this prospectus supplement and to dealers at that price less a concession not in excess of $0.3225 per share. After the public offering, the public offering price, concession and discount may be changed.

              The following table shows the public offering price, underwriting discount and proceeds before expenses to us.

 
  Per Share   Without Option   With Option  

Public offering price

  $ 10.7500   $ 43,000,000   $ 49,450,000  

Underwriting discount

  $ .5375   $ 2,150,000   $ 2,472,500  

Proceeds, before expenses, to Momenta

  $ 10.2125   $ 40,850,000   $ 46,977,500  

              The expenses of the offering, not including the underwriting discount, are estimated at approximately $235,000 and are payable by us.

Overallotment Option

              We have granted an option to the underwriter to purchase up to 600,000 additional shares at the public offering price, less the underwriting discount. The underwriter may exercise this option for 30 days from the date of this prospectus solely to cover any overallotments. If the underwriter exercises this option, it will be obligated, subject to conditions contained in the purchase agreement, to purchase a number of additional shares.

No Sales of Similar Securities

              We and our officers and directors and a certain stockholder have agreed, subject to certain exceptions, not to sell or transfer any common stock or securities convertible into, exchangeable for, exercisable for, or repayable with common stock, other than the shares which we may sell in this offering, for 60 days after the date of this prospectus supplement without first obtaining the written consent of Leerink Swann LLC. Specifically, we and our officers and directors and such stockholder have agreed not to directly or indirectly:

S-8


Table of Contents

              The restrictions applicable to us described in the immediately preceding paragraph do not apply to: the shares of common stock sold pursuant to the underwriting agreement, the issuance by us of shares of common stock upon the exercise of an option or warrant or the conversion of a security outstanding on the date we enter into the underwriting agreement or the issuance of stock options and other stock-based awards (or the issuance of shares of common stock upon exercise thereof) to eligible participants pursuant to our stock incentive plans. The restrictions applicable to our officers and directors and such stockholder described in the immediately preceding paragraph do not apply to:

NASDAQ Global Market Listing

              Our common stock is traded on the NASDAQ Global Market under the symbol "MNTA."

Price Stabilization and Short Positions

              Until the distribution of the shares is completed, SEC rules may limit the underwriter from bidding for and purchasing our common stock. However, the underwriter may engage in transactions that stabilize the price of the common stock, such as bids or purchases to peg, fix or maintain that price.

              In connection with the offering, the underwriter may purchase and sell our common stock in the open market. These transactions may include short sales, purchases on the open market to cover positions created by short sales and stabilizing transactions. Short sales involve the sale by the underwriter of a greater number of shares than they are required to purchase in the offering.

S-9


Table of Contents


"Covered" short sales are sales made in an amount not greater than the underwriter's option to purchase additional shares in the offering. The underwriter may close out any covered short position by either exercising its overallotment option or purchasing shares in the open market. In determining the source of shares to close out the covered short position, the underwriter will consider, among other things, the price of shares available for purchase in the open market as compared to the price at which they may purchase shares through the overallotment option. "Naked" short sales are sales in excess of the overallotment 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 our common stock in the open market after pricing that could adversely affect investors who purchase in the offering. Stabilizing transactions consist of various bids for or purchases of shares of common stock made by the underwriter in the open market prior to the completion of the offering.

              Similar to other purchase transactions, the underwriter's purchases to cover the syndicate short sales may have the effect of raising or maintaining the market price of our common stock or preventing or retarding a decline in the market price of our common stock. As a result, the price of our common stock may be higher than the price that might otherwise exist in the open market.

              The underwriter makes no representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of our common stock. In addition, neither we nor the underwriter make any representation that the representatives will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

Passive Market Making

              In connection with the offering, the underwriter may engage in passive market-making transactions in the common stock on the NASDAQ Global Market in accordance with Rule 103 of Regulation M under the Exchange Act during the period before the commencement of offers or sales of common stock and extending through the completion and distribution. A passive market-maker must display its bids at a price not in excess of the highest independent bid of the security. However, if all independent bids are lowered below the passive market-maker's bid, that bid must be lowered when specified purchase limits are exceeded.

Electronic Offer, Sale and Distribution of Shares

              A prospectus supplement in electronic format will be made available on the website maintained by the underwriter.

              Other than the prospectus supplement in electronic format, the information on the underwriter's website and any information contained in any other website maintained by the underwriter is not part of this prospectus supplement or the registration statement of which this prospectus supplement forms a part.

Other Relationships

              In addition, the underwriter and its affiliates have provided from time to time, and may provide in the future, investment and commercial banking and financial advisory services to us in the ordinary course of business, for which they have received and may continue to receive customary fees and commissions.

S-10


Table of Contents


LEGAL MATTERS

              The validity of the shares of common stock offered hereby will be passed upon for us by Wilmer Cutler Pickering Hale and Dorr LLP, Boston, Massachusetts. The underwriter is being represented in connection with this offering by Ropes & Gray LLP, Boston, Massachusetts.


EXPERTS

              The consolidated financial statements of Momenta Pharmaceuticals, Inc. appearing in Momenta Pharmaceuticals, Inc.'s Annual Report (Form 10-K) for the year ended December 31, 2008, and the effectiveness of Momenta Pharmaceuticals, Inc.'s internal control over financial reporting as of December 31, 2008 have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon, included therein, and incorporated herein by reference. Such consolidated financial statements are incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.


INCORPORATION OF DOCUMENTS BY REFERENCE

              We are "incorporating by reference" in this prospectus supplement and the accompanying prospectus some of the documents we file with the Securities and Exchange Commission, or SEC. This means that we can disclose important information to you by referring you to those documents. The information in the documents incorporated by reference is considered to be part of this prospectus supplement and the accompanying prospectus. Statements contained in documents that we file with the SEC and that are incorporated by reference in this prospectus supplement and the accompanying prospectus will automatically update and supersede information contained herein, including information in previously filed documents or reports that have been incorporated by reference herein, to the extent the new information differs from or is inconsistent with the old information.

              We have filed or may file the following documents with the SEC. These documents are incorporated herein by reference as of their respective dates of filing (other than, in each case, documents or information deemed to have been furnished and not filed in accordance with SEC rules):

              You may request, orally or in writing, a copy of these documents, which will be provided to you at no cost, by contacting Momenta Pharmaceuticals, Inc., 675 West Kendall Street, Cambridge, Massachusetts 02142, Attention: Bruce A. Leicher, Corporate Secretary, telephone: (617) 491-9700.

S-11


Table of Contents


WHERE YOU CAN FIND MORE INFORMATION

              We file reports, proxy statements and other documents with the SEC. You may read and copy any document we file with the SEC 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 public reference room.

              The SEC also maintains an Internet site that contains reports, proxy and information statements, and other information regarding registrants, like us, that file electronically with the SEC. The address of the SEC's Internet site is www.sec.gov. Our website is www.momentapharma.com.

              Our annual reports on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K and other filings with the SEC are available, free of charge, through our website, shortly after those reports or filings are electronically filed with or furnished to the SEC. Information on our website or any other website is not incorporated by reference into this prospectus and does not constitute a part of this prospectus.

              This prospectus supplement is part of a registration statement we filed with the SEC relating to the securities we may offer. As permitted by SEC rules, this prospectus supplement does not contain all of the information we have included in the registration statement and the accompanying exhibits and schedules we filed with the SEC. You may refer to the registration statement, exhibits and schedules for more information about us and the securities. The registration statement, exhibits and schedules are available at the SEC's Public Reference Room or through its website.

S-12


Table of Contents

PROSPECTUS

$150,000,000

MOMENTA PHARMACEUTICALS, INC.

Common Stock
Preferred Stock
Debt Securities
Warrants



        We may from time to time issue up to $150,000,000 aggregate principal amount of common stock, preferred stock, debt securities and warrants. We may sell these securities to or through underwriters, directly to investors or through agents. We will specify the terms of the securities, and the names of any underwriters or agents, in supplements to this prospectus.

        Our common stock is listed on the NASDAQ Global Market and traded under the symbol "MNTA." The closing bid price of our common stock on the NASDAQ Global Market on January 24, 2007 was $18.15 per share.



        Investing in our securities involves risks. See "Risk Factors" beginning on page 2 of this prospectus.



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



        This prospectus may not be used to consummate sales of securities unless it is accompanied by a prospectus supplement.



Prospectus dated February 8, 2007.


Table of Contents


TABLE OF CONTENTS

 
  Page

ABOUT THIS PROSPECTUS

  1

MOMENTA PHARMACEUTICALS, INC. 

  1

RISK FACTORS

  2

SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION

  2

USE OF PROCEEDS

  3

RATIO OF EARNINGS TO FIXED CHARGES

  3

DILUTION

  3

THE SECURITIES WE MAY OFFER

  4

DESCRIPTION OF CAPITAL STOCK

  5

DESCRIPTION OF DEBT SECURITIES

  10

DESCRIPTION OF WARRANTS

  15

LEGAL OWNERSHIP OF SECURITIES

  17

PLAN OF DISTRIBUTION

  20

VALIDITY OF SECURITIES

  21

EXPERTS

  22

WHERE YOU CAN FIND MORE INFORMATION

  22

INCORPORATION OF DOCUMENTS BY REFERENCE

  22



        No dealer, salesperson or other person has been authorized to give any information or to make any representations other than those contained or incorporated by reference in this prospectus or any accompanying prospectus supplement in connection with the offer made by this prospectus or any accompanying prospectus supplement and, if given or made, such information or representations must not be relied upon as having been authorized by Momenta Pharmaceuticals, Inc. or any such person. Neither the delivery of this prospectus or any accompanying prospectus supplement nor any sale made hereunder and thereunder shall under any circumstances create an implication that there has been no change in the affairs of Momenta Pharmaceuticals, Inc. since the date hereof. This prospectus or any accompanying prospectus supplement does not constitute an offer or solicitation by anyone in any state in which such offer or solicitation is not authorized or in which the person making such offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make such offer or solicitation.


Table of Contents


ABOUT THIS PROSPECTUS

        This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission, using a "shelf" registration process. Under this shelf 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. We have provided to you in this prospectus a general description of the securities we may offer. Each time we sell securities under this shelf registration process, we will provide a prospectus supplement that will contain specific information about the terms of the offering. We may also add, update or change in the prospectus supplement or any "free writing prospectus" we may authorize to be delivered to you any of the information contained in this prospectus. To the extent there is a conflict between the information contained in this prospectus and the prospectus supplement or any free writing prospectus we may authorize to be delivered to you, you should rely on the information in the prospectus supplement or free writing prospectus, as the case may be, provided that if any statement in one of these documents is inconsistent with a statement in another document having a later date—for example, a document incorporated by reference in this prospectus or any prospectus supplement—the statement in the document having the later date modifies or supersedes the earlier statement. This prospectus, together with the applicable prospectus supplements and any free writing prospectus we may authorize to be delivered to you, includes all material information relating to this offering.


MOMENTA PHARMACEUTICALS, INC.

        Momenta is a biotechnology company specializing in the sequencing, or detailed structural analysis, and design of complex drugs for the development of technology-enabled generic versions of complex drug products and improved versions of existing drugs, as well as the discovery of novel drugs and biological processes. Through detailed analysis of the molecular structure of complex sugars and other complex mixtures, we believe our proprietary technology enables us to define the specific sequences contained in complex drugs, including those structures that had previously not been described due to a lack of available technology. In addition, we are able to derive a more complete understanding of the roles that complex sugars play in cellular function, disease and drug action based on our analytical capabilities. With our capabilities, we have developed a diversified pipeline of near-term product opportunities and novel discovery and development candidates.

        We were incorporated in Delaware in May 2001 under the name Mimeon, Inc. In September 2002, we changed our name to Momenta Pharmaceuticals, Inc. Our principal executive offices are located at 675 West Kendall Street, Cambridge, MA 02142, and our telephone number is (617) 491-9700. Our website address is www.momentapharma.com. The information on our website is not incorporated by reference into this prospectus and should not be considered to be a part of this prospectus. We have included our website address as an inactive technical reference only.

        Unless otherwise stated, all references to "us," "our," "Momenta," "we," the "Company" and similar designations refer to Momenta Pharmaceuticals, Inc. Our logo, trademarks and service marks are the property of Momenta. Other trademarks or service marks appearing in this prospectus are the property of their respective holders.

1


Table of Contents


RISK FACTORS

        An investment in our securities involves significant risks. You should carefully consider the risks, including the forward-looking statements made in this prospectus and any prospectus supplement, as well as all of the risk factors incorporated herein by reference before you make an investment decision pursuant to this prospectus and any prospectus supplement. The risks and uncertainties we have described are not the only ones we face. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also affect our business operations.


SPECIAL NOTE REGARDING FORWARD-LOOKING INFORMATION

        This prospectus, any prospectus supplement and the documents we incorporate by reference in this prospectus contain forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, and Section 21E of the Securities Exchange Act of 1934. All statements, other than statements of historical facts, that we include in this prospectus, any prospectus supplement, and in the documents we incorporate by reference in this prospectus, may be deemed forward-looking statements for purposes of the Securities Act and the Securities Exchange Act. We use the words "anticipate," "believe," "estimate," "expect," "intend," "may," "plan," "project," "will," "would" and similar expressions to identify forward-looking statements, although not all forward-looking statements contain these identifying words. We cannot guarantee that we actually will achieve the plans, intentions or expectations disclosed in our forward-looking statements and, accordingly, you should not place undue reliance on our forward-looking statements. There are a number of important factors that could cause actual results or events to differ materially from the forward-looking statements that we make, including the factors included in the documents we incorporate by reference in this prospectus. You should read these factors and the other cautionary statements made in the documents we incorporate by reference as being applicable to all related forward-looking statements wherever they appear in this prospectus, any prospectus supplement, and any document incorporated by reference. We caution you that we do not undertake any obligation to update forward-looking statements made by us.

2


Table of Contents


USE OF PROCEEDS

        Unless otherwise provided in the applicable prospectus supplement, we intend to use the net proceeds from the sale of the securities under this prospectus for general corporate purposes, including clinical trial costs, research and development expenses, manufacturing expenses, general and administrative expenses, and potential acquisitions of companies, products and technologies that complement our business. We will set forth in the prospectus supplement our intended use for the net proceeds received from the sale of any securities. Pending the application of the net proceeds, we intend to invest the net proceeds in investment-grade, interest-bearing securities.


RATIO OF EARNINGS TO FIXED CHARGES

        We did not record earnings for any of the years ended December 31, 2005, 2004, 2003, 2002 or 2001 or for the nine-month period ended September 30, 2006. Accordingly, our earnings were insufficient to cover fixed charges in such periods and we are unable to disclose a ratio of earnings to fixed charges. The following table sets forth, for each of the periods presented, the dollar amount of the deficiency of earnings available to cover fixed charges. For purposes of computing the deficiency of earnings available to cover fixed charges, fixed charges represent interest expense, an estimate of the interest expense within rental expense, the non-cash dividend related to the beneficial conversion feature of our Series C redeemable convertible preferred stock and the accretion to redemption value of the redeemable convertible preferred stock.

 
  Nine Month
Period
Ended,
September 30,
2006
   
   
   
   
   
 
 
  Year Ended December 31,  
In thousands
  2005   2004   2003   2002   2001  

Deficiency of Earnings to Cover Fixed Charges

  $ 35,930   $ 21,662   $ 36,316   $ 9,843   $ 5,389   $ 393  
                           


DILUTION

        We will set forth in a prospectus supplement the following information regarding any material dilution of the equity interests of investors purchasing securities in an offering under this prospectus:

3


Table of Contents


THE SECURITIES WE MAY OFFER

        The descriptions of the securities contained in this prospectus, together with the applicable prospectus supplements, summarize all the material terms and provisions of the various types of securities that we may offer. We will describe in the applicable prospectus supplement relating to any securities the particular terms of the securities offered by that prospectus supplement. If we indicate in the applicable prospectus supplement, the terms of the securities may differ from the terms we have summarized below. We will also include in the prospectus supplement information, where applicable, about material United States federal income tax considerations relating to the securities, and the securities exchange, if any, on which the securities will be listed.

        We may sell from time to time, in one or more offerings:

        In this prospectus, we refer to the common stock, preferred stock, debt securities and warrants collectively as "securities." The total dollar amount of all securities that we may sell will not exceed $150,000,000.

        If we issue debt securities at a discount from their original stated principal amount, then, for purposes of calculating the total dollar amount of all securities issued under this prospectus, we will treat the initial offering price of the debt securities as the total original principal amount of the debt securities.

        This prospectus may not be used to consummate a sale of securities unless it is accompanied by a prospectus supplement.

4


Table of Contents


DESCRIPTION OF CAPITAL STOCK

        Under our certificate of incorporation, we are authorized to issue 100,000,000 shares of common stock, par value $0.0001 per share, and 5,000,000 shares of preferred stock, par value $0.01 per share. The following description summarizes information about our capital stock. You can obtain more comprehensive information about our capital stock by consulting our certificate of incorporation and by-laws, each as amended to date, as well as the Delaware General Corporation Law.

Common Stock

        Holders of our common stock are entitled to one vote for each share held on all matters submitted to a vote of stockholders and do not have cumulative voting rights. Accordingly, holders of a majority of the shares of common stock entitled to vote in any election of directors may elect all of the directors standing for election. Holders of common stock are entitled to receive proportionately any dividends as may be declared by our board of directors, subject to any preferential dividend rights of outstanding preferred stock.

        In the event of our liquidation, dissolution or winding up, the holders of our common stock are entitled to receive proportionately our net assets available after the payment of all debts and other liabilities and subject to the prior rights of any outstanding preferred stock. Holders of our common stock have no preemptive, subscription, redemption or conversion rights. The rights, preferences and privileges of holders of common stock are subject to and may be adversely affected by, the rights of the holders of shares of any series of preferred stock that we may designate and issue in the future.

        The transfer agent and registrar for our common stock is American Stock Transfer & Trust Co.

Preferred Stock

        Under the terms of our certificate of incorporation, our board of directors is authorized to direct us to issue shares of preferred stock in one or more series without stockholder approval. Our board of directors has the discretion to determine the rights, preferences, privileges and restrictions, including voting rights, dividend rights, conversion rights, redemption privileges and liquidation preferences of each series of preferred stock. We have designated one series of preferred stock. See "—Series A Preferred Stock" below.

        The purpose of authorizing our board of directors to issue preferred stock and determine its rights and preferences is to eliminate delays associated with a stockholder vote on specific issuances. The issuance of preferred stock, while providing flexibility in connection with possible acquisitions, future financings and other corporate purposes, could have the effect of making it more difficult for a third party to acquire, or could discourage a third party from seeking to acquire, a majority of our outstanding voting stock.

        The preferred stock of each series will rank senior to the common stock and the Series A Junior Participating Preferred Stock in priority of payment of dividends and in the distribution of assets in the event of our liquidation, dissolution or winding up, to the extent of the preferential amounts to which the preferred stock of the respective series will be entitled. Upon issuance, the shares of preferred stock will be fully paid and nonassessable, which means that its holders will have paid their purchase price in full and we may not require them to pay additional funds.

        On October 31, 2005, our board of directors designated 100,000 shares of preferred stock as Series A Junior Participating Preferred Stock, $0.01 par value per share, or Series A preferred stock, as set forth in the Certificate of Designations of Series A Junior Participating Preferred Stock filed with the Secretary of State of the State of Delaware on November 8, 2005. Also on October 31, 2005, our board of directors

5


Table of Contents

approved a Rights Agreement between us and American Stock Transfer & Trust Company, as rights agent thereunder, referred to as the Rights Agreement. Effective November, 7, 2005, our board of directors declared a dividend of one Right for each outstanding share of our common stock, to stockholders of record as of the close of business on November 18, 2005. Each Right entitles the registered holder to purchase from us one one-thousandth of a share of Series A preferred stock, at a purchase price of $125.00 in cash, subject to adjustment.

        Initially, the Rights are not exercisable and will be attached to all certificates representing outstanding shares of common stock, and no separate Rights Certificates will be distributed. The Rights will separate from the common stock, and the Distribution Date will occur, upon the earlier of (i) ten business days following the later of (a) the first date of a public announcement that a person or group of affiliated or associated persons, or an Acquiring Person, has acquired, or obtained the right to acquire, beneficial ownership of 20% or more of the outstanding shares of our common stock or (b) the first date on which an executive officer of the Company has actual knowledge that an Acquiring Person has become such, with such date being a Stock Acquisition Date, or (ii) ten business days following the commencement of a tender offer or exchange offer that would result in a person or group beneficially owning 20% or more of the outstanding shares of our common stock. The Distribution Date may be deferred in circumstances determined by our board of directors. In addition, certain inadvertent acquisitions will not trigger the occurrence of the Distribution Date. Until the Distribution Date, or earlier redemption or expiration of the Rights, (i) the Rights will be evidenced by the common stock certificates outstanding on the record date of November 18, 2005, together with a summary of rights to be mailed to record holders, or by new common stock certificates issued after such record date which contain a notation incorporating the Rights Agreement by reference, (ii) the Rights will be transferred with and only with such common stock certificates and (iii) the surrender for transfer of any certificates for common stock outstanding will also constitute the transfer of the Rights associated with the common stock represented by such certificate.

        The Rights are not exercisable until the Distribution Date and will expire upon the close of business on November 6, 2008, referred to as the Final Expiration Date, unless earlier redeemed or exchanged as described below. As soon as practicable after the Distribution Date, separate Rights Certificates will be mailed to holders of record of the common stock as of the close of business on the Distribution Date and, thereafter, the separate Rights Certificates alone will represent the Rights. Except as otherwise determined by our board of directors, and except for shares of common stock issued upon exercise, conversion or exchange of then outstanding options, convertible or exchangeable securities or other contingent obligations to issue shares or pursuant to any employee benefit plan or arrangement, only shares of common stock issued prior to the Distribution Date will be issued with Rights.

        In the event that any person becomes an Acquiring Person, unless the event causing the 20% threshold to be crossed is a Permitted Offer, as defined in the Rights Agreement, then, promptly following the first occurrence of such event, each holder of a Right (except as provided in the Rights Agreement) shall thereafter have the right to receive, upon exercise, that number of shares of our common stock, or, in certain circumstances, cash, property or other securities, which equals the exercise price of the Right divided by 50% of the current market price, as defined in the Rights Agreement, per share of common stock at the date of the occurrence of such event. However, Rights are not exercisable following such event until such time as the Rights are no longer redeemable by us. Notwithstanding any of the foregoing, following the occurrence of such event, all Rights that are, or, under certain circumstances specified in the Rights Agreement, were, beneficially owned by any Acquiring Person will be null and void. The event summarized in this paragraph is referred to as an A Trigger Event.

        For example, at an exercise price of $125.00 per Right, each Right not owned by an Acquiring Person, or by certain related parties, following an A Trigger Event would entitle its holder to purchase for $125.00 such number of shares of common stock, or other consideration, as noted above, as equals $125.00 divided by one-half of the current market price, as defined in the Rights Agreement, of the common stock. Assuming that the common stock had a market price of $25.00 per share at such time, the holder of each

6


Table of Contents


valid Right would be entitled to purchase ten shares of common stock, having a market value of 10 × $25.00, or $250.00, for $125.00.

        In the event that, at any time after any person becomes an Acquiring Person, (i) we are consolidated with, or merged with and into, another entity and we are not the surviving entity of such consolidation or merger (other than a consolidation or merger that follows a Permitted Offer) or if we are the surviving entity, but shares of our outstanding common stock are changed or exchanged for stock or securities, of any other person, or cash or any other property, or (ii) more than 50% of our assets or earning power is sold or transferred, each holder of a Right, except Rights which previously have been voided as set forth above, shall thereafter have the right to receive, upon exercise, that number of shares of common stock of the acquiring company which equals the exercise price of the Right divided by 50% of the current market price, as defined in the Rights Agreement, of such common stock at the date of the occurrence of the event. The events summarized in this paragraph are referred to as a B Trigger Event. An A Trigger Event and a B Trigger Event are collectively referred to as Triggering Events.

        For example, at an exercise price of $125.00 per Right, each valid Right following a B Trigger Event would entitle its holder to purchase for $125.00 such number of shares of common stock of the acquiring company as equals $125.00 divided by one-half of the current market price, as defined in the Rights Agreement, of such common stock. Assuming that such common stock had a market price of $25.00 per share at such time, the holder of each valid Right would be entitled to purchase ten shares of common stock of the acquiring company, having a market value of 10 × $25.00, or $250.00, for $125.00.

        At any time after the occurrence of an A Trigger Event, when no person owns a majority of our common stock, our board of directors may exchange the Rights, other than Rights owned by such Acquiring Person which have become void, in whole or in part, at an exchange ratio of one share of common stock, or one one-thousandth of a share of Series A preferred stock, or of a share of a class or series of our preferred stock having equivalent rights, preferences and privileges, per Right, subject to adjustment.

        The purchase price payable, and the number of units of Series A preferred stock or other securities or property issuable, upon exercise of the Rights are subject to adjustment from time to time to prevent dilution (i) in the event of a stock dividend on, or a subdivision, combination or reclassification of, our Series A preferred stock, (ii) if holders of the Series A preferred stock are granted certain rights or warrants to subscribe for Series A preferred stock or convertible securities at less than the then-current market price, as defined in the Rights Agreement, of the Series A preferred stock, or (iii) upon the distribution to holders of the Series A preferred stock of evidences of indebtedness or assets, excluding regular periodic cash dividends paid out of earnings or retained earnings, or of subscription rights or warrants, other than those referred to above. The number of Rights associated with each share of common stock is also subject to adjustment in the event of a stock split of the common stock or a stock dividend on the common stock payable in common stock or subdivisions, consolidations or combinations of the common stock occurring, in any such case, prior to the Distribution Date.

        With certain exceptions, no adjustment in the purchase price will be required until cumulative adjustments amount to at least 1% of the purchase price. No fractional shares of Series A preferred stock, other than fractions which are integral multiples of one one-thousandth of a share of Series A preferred stock, will be issued and, in lieu thereof, an adjustment in cash will be made based on the market price of the Series A preferred stock on the last trading date prior to the date of exercise.

        Series A preferred stock purchasable upon exercise of the Rights will not be redeemable. Each share of Series A preferred stock will be entitled to receive, when, as and if declared by our board of directors, a minimum preferential quarterly dividend payment of $10.00 per share or, if greater, an aggregate dividend of 1,000 times the dividend declared per share of common stock. In the event of liquidation, the holders of the Series A preferred stock will be entitled to a minimum preferential liquidation payment of $1,000.00 per share, plus an amount equal to accrued and unpaid dividends, and will be entitled to an aggregate

7


Table of Contents


payment of 1,000 times the payment made per share of common stock. Each share of Series A preferred stock will have 1,000 votes, voting together with the common stock. In the event of any merger, consolidation or other transaction in which common stock is changed or exchanged, each share of Series A preferred stock will be entitled to receive 1,000 times the amount received per share of common stock. These rights are protected by customary antidilution provisions. Because of the nature of the Series A preferred stock's dividend, liquidation and voting rights, the value of one one-thousandth of a share of Series A preferred stock purchasable upon exercise of each Right should approximate the value of one share of common stock.

        At any time prior to the earlier of the tenth business day, or such later date as may be determined by the board of directors, after the Stock Acquisition Date and the Final Expiration Date, we may redeem the Rights in whole, but not in part, at a price of $0.001 per Right, referred to as the Redemption Price, payable in cash or stock. Immediately upon the redemption of the Rights or such earlier time as established by our board of directors in the resolution ordering the redemption of the Rights, the Rights will terminate and the only right of the holders of Rights will be to receive the Redemption Price.

        Until a Right is exercised, the holder thereof, as such, will have no rights as a stockholder of the Company, including, without limitation, the right to vote or to receive dividends. Although the distribution of the Rights should 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 as set forth above.

        Any provision of the Rights Agreement, other than the redemption price, may be amended by our board of directors prior to such time as the Rights are no longer redeemable. Once the Rights are no longer redeemable, our board of directors' authority to amend the Rights is limited to correcting ambiguities or defective or inconsistent provisions in a manner that does not adversely affect the interest of holders of Rights.

        The Rights are intended to protect our stockholders in the event of an unfair or coercive offer to acquire us and to provide our board of directors with adequate time to evaluate unsolicited offers. The Rights may have anti-takeover effects. The Rights will cause substantial dilution to a person or group that attempts to acquire us without conditioning the offer on a substantial number of Rights being acquired. The Rights, however, should not affect any prospective offeror willing to make an offer at a fair price and otherwise in our best interests and our stockholders, as determined by a majority of our board of directors. The Rights should not interfere with any merger or other business combination approved by our board of directors.

Anti-Takeover Effects of Provisions of Delaware Law and Our Charter and By-Laws

        We are subject to Section 203 of the Delaware General Corporation Law. Subject to certain exceptions, Section 203 prevents a publicly-held Delaware corporation from engaging in a "business combination" with any "interested stockholder" for three years following the date that the person became an interested stockholder, unless the interested stockholder attained such status with the approval of our board of directors or unless the business combination is approved in a prescribed manner. A "business combination" includes, among other things, a merger or consolidation involving us, and the interested stockholder and the sale of more than 10% of our assets. In general, an "interested stockholder" is any entity or person beneficially owning 15% or more of our outstanding voting stock and any entity or person affiliated with or controlling or controlled by such entity or person.

        Our certificate of incorporation provides that the directors elected may be removed only for cause and only by the affirmative vote of the holders of at least 75% of the votes which all our stockholders would be entitled to cast in any annual election of directors. Under our certificate of incorporation, any vacancy or newly created directorships on the board of directors can be filled only by vote of a majority of our directors then in office. The limitations on the ability of our stockholders to remove directors and fill

8


Table of Contents


vacancies could make it more difficult for a third party to acquire, or discourage a third party from seeking to acquire, control of our company.

        Our certificate of incorporation provides that any action required or permitted to be taken by our stockholders at an annual meeting or special meeting of stockholders may only be taken if it is properly brought before such meeting and may not be taken by written action in lieu of a meeting. Our certificate of incorporation and our by-laws also provide that, except as otherwise required by law, special meetings of the stockholders can only be called by our chief executive officer, president, our chairman of the board, or a majority of our board of directors. In addition, our by-laws establish an advance notice procedure for stockholder proposals to be brought before an annual meeting of stockholders, including proposed nominations of candidates for election to our board of directors. Stockholders at an annual meeting may only consider proposals or nominations specified in the notice of meeting or brought before the meeting by or at the direction of our board of directors or by a stockholder of record on the record date for the meeting, who is entitled to vote at the meeting and who has delivered timely written notice in proper form to our secretary of the stockholder's intention to bring such business before the meeting. These provisions could have the effect of delaying stockholder actions that are favored by the holders of a majority of our outstanding voting securities until the next stockholder meeting.

        The Delaware General Corporation Law provides generally that the affirmative vote of a majority of the shares entitled to vote on any matter is required to amend a corporation's certificate of incorporation or by-laws, unless a corporation's certificate of incorporation or by-laws, as the case may be, requires a greater percentage. Our by-laws may be amended or repealed by a majority vote of our board of directors or by the affirmative vote of the holders of at least 75% of the votes which all our stockholders would be entitled to cast in any annual election of directors.

Potential Effects of Authorized but Unissued Stock

        We have shares of common stock and preferred stock available for future issuance without stockholder approval. We may utilize these additional shares for a variety of corporate purposes, including future public offerings to raise additional capital, to facilitate corporate acquisitions or payment as a dividend on the capital stock.

        The existence of unissued and unreserved common stock and preferred stock may enable the board of directors to issue shares to persons friendly to current management or to issue preferred stock with terms that could render more difficult or discourage a third-party attempt to obtain control of us by means of a merger, tender offer, proxy contest or otherwise, thereby protecting the continuity of our management. In addition, the board has the discretion to determine designations, rights, preferences, privileges and restrictions, including voting rights, dividend rights, conversion rights, redemption privileges and liquidation preferences of each series of preferred stock, all to the fullest extent permissible under the Delaware General Corporation Law and subject to any limitations set forth in our certificate of incorporation. The purpose of authorizing the board to issue preferred stock and to determine the rights and preferences applicable to such preferred stock is to eliminate delays associated with a stockholder vote on specific issuances. The issuance of preferred stock, while providing desirable flexibility in connection with possible financings, acquisitions and other corporate purposes, could have the effect of making it more difficult for a third party to acquire, or could discourage a third party from acquiring, a majority of our outstanding voting stock.

9


Table of Contents


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 apply generally to any future debt securities we may offer, we will describe the particular terms of any debt securities that we may offer in more detail in the applicable prospectus supplement. If we indicate in a prospectus supplement, the terms of any debt securities we offer under that prospectus supplement may differ from the terms we describe below.

        We will issue senior notes under a senior indenture which we will enter into with a trustee to be named in the senior indenture. We will issue subordinated notes under a subordinated indenture which we will enter into with a trustee to be named in the subordinated indenture. We have filed forms of these documents as exhibits to the registration statement which includes this prospectus. 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, or the Trust Indenture Act. We use the term "trustee" to refer to either the senior trustee or the subordinated trustee, as applicable.

        The following summaries of material provisions of senior notes, subordinated notes and the indentures are subject to, and qualified in their entirety by reference to, the provisions of the indenture applicable to a particular series of debt securities. Except as we may otherwise indicate, the terms of the senior indenture and the subordinated indenture are identical.

General

        We will describe in the applicable prospectus supplement the following terms relating to a series of notes:

10


Table of Contents

Conversion or Exchange Rights

        We will set forth in the applicable prospectus supplement the terms on which a series of notes may be convertible into or exchangeable for common stock or other securities of ours. 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 shares of common stock or other securities of ours that the holders of the series of notes receive would be subject to adjustment.

Consolidation, Merger or Sale

        The indentures 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 to or acquirer of such assets must assume all of our obligations under the indentures or the notes, as appropriate.

Events of Default Under the Indenture

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

        If an event of default with respect to notes of any series occurs and is continuing, the trustee or the holders of at least 25% in aggregate principal amount of the outstanding notes of that series, by notice to us in writing, and to the trustee if notice is given by such holders, may declare the unpaid principal of, or premium, if any, on and accrued interest, if any, on the notes due and payable immediately.

        The holders of a majority in principal amount of the outstanding notes of an affected series may waive any default or event of default with respect to the series and its consequences, except uncured defaults or events of default regarding payment of principal, or 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 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 notes, unless such

11


Table of Contents


holders have offered the trustee reasonable indemnity. The holders of a majority in principal amount of the outstanding notes of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee, or exercising any trust or power conferred on the trustee, with respect to the notes of that series, provided that:

        A holder of the notes 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:

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

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

Modification of Indenture; Waiver

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

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

12


Table of Contents

Discharge

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

        In order to exercise our rights to be discharged, we must deposit with the 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 notes 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 notes 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, or DTC, or another depository 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 notes of any series can exchange the notes for other notes 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 notes may present the notes 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 notes that the holder presents for transfer or exchange, we will not require any payment 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 notes. 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 notes of each series.

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

13


Table of Contents

Information Concerning the Trustee

        The 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 trustee must use the same degree of care and skill as a prudent person would exercise or use in the conduct of his or her own affairs. Subject to this provision, the trustee is under no obligation to exercise any of the powers given to it by the indentures at the request of any holder of notes 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 notes on any interest payment date to the person in whose name the notes, or one or more predecessor securities, are registered at the close of business on the regular record date for the interest payment.

        We will pay principal of and any premium and interest on the notes 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 will make interest payments by check which we will mail to the holder. Unless we otherwise indicate in a prospectus supplement, we will designate the corporate trust office of the trustee in The City of New York as our sole paying agent for payments with respect to notes of each series. We will name in the applicable prospectus supplement any other paying agents that we initially designate for the notes of a particular series. We will maintain a paying agent in each place of payment for the notes of a particular series.

        All money we pay to a paying agent or the trustee for the payment of the principal of or any premium or interest on any notes 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 security thereafter may look only to us for payment thereof.

Governing Law

        The indentures and the notes 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 is applicable.

Subordination of Subordinated Notes

        The subordinated notes will be unsecured and will be subordinate and junior in priority of payment to certain of our other indebtedness to the extent described in a prospectus supplement. The subordinated indenture does not limit the amount of subordinated notes which we may issue. It also does not limit us from issuing any other secured or unsecured debt.

14


Table of Contents


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 and the related warrant agreements and warrant certificates. While the terms summarized below will apply generally to any warrants that we may offer, we will describe the particular terms of any series of warrants in more detail in the applicable prospectus supplement. If we indicate in the prospectus supplement, the terms of any warrants offered under that prospectus supplement may differ from the terms described below. Specific warrant agreements will contain additional important terms and provisions and will be incorporated by reference as an exhibit to the registration statement that includes this prospectus.

General

        We may issue warrants for the purchase of common stock, preferred stock or debt securities in one or more series. We may issue warrants independently or together with common stock, preferred stock and debt securities, and the warrants may be attached to or separate from these securities.

        We will evidence each series of warrants by warrant certificates that we will issue under a separate agreement. We will enter into the warrant agreement with a warrant agent. Each warrant agent will be a bank that we select which has its principal office in the United States and a combined capital and surplus of at least $50,000,000. We will indicate the name and address of the warrant agent in the applicable prospectus supplement relating to a particular series of 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:

Additional Information

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

15


Table of Contents

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 5 p.m., Eastern 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.

16


Table of Contents


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 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. Securities issued in global form will be registered in the name of the depositary or its nominee. Consequently, for securities issued in global form, 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 will 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 book-entry 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 a global security or issue securities in non-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.

        For securities held in street name, we 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 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 and of any third parties 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.

17


Table of Contents

        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 depositary 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 the 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 or in street name, you should check with your own institution to find out:

Global Securities

        A global security is a security held by a depositary that represents one or any other number of individual securities. 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 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 in the applicable prospectus supplement, 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 in global form only, 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.

18


Table of Contents

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 in the form of a global security, an investor should be aware of the following:

Special Situations When a Global Security will be Terminated

        In a few special situations described below, the 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 name, so that they will be direct holders. We have described the rights of holders and street name investors above.

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

19


Table of Contents

        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. The accompanying prospectus supplement will describe the terms of the offering of the securities, including:

        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 the 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 to purchase all the securities offered by the prospectus supplement. We may change from time to time the public offering price and any discounts or concessions allowed or reallowed or paid to dealers. We may use underwriters with whom we have a material relationship. We will describe such relationships in the prospectus supplement naming the underwriter and 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 the 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 enter into derivative transactions with third parties, or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If the applicable prospectus supplement indicates, in connection with those derivatives, the third parties may sell securities covered by this prospectus and the applicable prospectus supplement, including short sale transactions. If so, the third party may use securities pledged by us or borrowed from us or others to settle those sales or to close out any related open borrowings of common shares, and may use securities received from us in settlement of those derivatives to close out any related open borrowings of common shares. The third party in such sale transactions will be an underwriter and, if not identified in this prospectus, will be identified in the applicable prospectus supplement or a post-effective amendment to this registration statement.

        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.

20


Table of Contents

        Rules of the Securities and Exchange Commission may limit the ability of any underwriters to bid for or purchase securities before the distribution of the securities is completed. However, underwriters may engage in the following activities in accordance with the rules:

        Similar to other purchase transactions, an underwriter's purchases to cover the syndicate short sales or to stabilize the market price of our securities may have the effect of raising or maintaining the market price of our securities or preventing or mitigating a decline in the market price of our securities. As a result, the price of the securities may be higher than the price that might otherwise exist in the open market. The imposition of a penalty bid might also have an effect on the price of shares if it discourages resales of the securities.

        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.


VALIDITY OF SECURITIES

        The validity of the issuance of the securities offered by this prospectus will be passed upon for us by Wilmer Cutler Pickering Hale and Dorr LLP, Boston, Massachusetts.

21


Table of Contents


EXPERTS

        The consolidated financial statements of Momenta Pharmaceuticals, Inc. appearing in Momenta Pharmaceuticals, Inc.'s Annual Report (Form 10-K) for the year ended December 31, 2005 and Momenta Pharmaceuticals, Inc. management's assessment of the effectiveness of internal control over financial reporting as of December 31, 2005 included therein, have been audited by Ernst & Young LLP, independent registered public accounting firm, as set forth in their reports thereon included therein, and incorporated herein by reference. Such financial statements and management's assessment have been incorporated herein by reference in reliance upon such reports given on the authority of such firm as experts in accounting and auditing.


WHERE YOU CAN FIND MORE INFORMATION

        We file reports, proxy statements and other documents with the Securities and Exchange Commission. You may read and copy any document we file with the SEC at the SEC's Public Reference Room at 100 F. Street, N.E., Room 1580, Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information on the public reference room.

        The SEC also maintains an Internet site, the address of which is www.sec.gov. That site also contains our annual, quarterly and current reports, proxy statements, information statements and other information.

        We have filed this prospectus with the SEC as part of a registration statement on Form S-3 under the Securities Act. This prospectus does not contain all of the information set forth in the registration statement because some parts of the registration statement are omitted in accordance with the rules and regulations of the SEC. You can obtain a copy of the registration statement from the SEC at any address listed above or from the SEC's web site.

        This prospectus is part of a registration statement that we filed with the SEC. The registration statement contains more information than this prospectus regarding us and the securities being offered by us, including exhibits and schedules. We also maintain an Internet site at www.momentapharma.com, which provides additional information about our company and through which you can also access our SEC filings. The information set forth on our Internet site is not part of this prospectus.


INCORPORATION OF DOCUMENTS BY REFERENCE

        We are "incorporating by reference" in this prospectus some of the documents we file with the SEC. This means that we can disclose important information to you by referring you to those documents. The information in the documents incorporated by reference is considered to be part of this prospectus. Statements contained in documents that we file with the SEC and that are incorporated by reference in this prospectus will automatically update and supersede information contained in this prospectus, including information in previously filed documents or reports that have been incorporated by reference in this prospectus, to the extent the new information differs from or is inconsistent with the old information.

        We have filed or may file the following documents with the SEC. These documents are incorporated herein by reference as of their respective dates of filing:

22


Table of Contents

        You may request, orally or in writing, a copy of these documents, which will be provided to you at no cost, by contacting Momenta Pharmaceuticals, Inc., 675 West Kendall Street, Cambridge, Massachusetts 02142, Attention: Michael A. Lawless, Senior Director, Investor Relations, telephone: (617) 491-9700.

        You should rely only on the information contained in this prospectus, including information incorporated by reference as described above, or any prospectus supplement that we have specifically referred you to. We have not authorized anyone else to provide you with different information. You should not assume that the information in this prospectus or any prospectus supplement is accurate as of any date other than the date on the front of those documents or that any document incorporated by reference is accurate as of any date other than its filing date. You should not consider this prospectus to be an offer or solicitation relating to the securities in any jurisdiction in which such an offer or solicitation relating to the securities is not authorized. Furthermore, you should not consider this prospectus to be an offer or solicitation relating to the securities if the person making the offer or solicitation is not qualified to do so, or if it is unlawful for you to receive such an offer or solicitation.

23


Table of Contents

4,000,000 Shares

GRAPHIC

Common Stock



Leerink Swann

September 22, 2009