<Page>

                                                                  Exhibit 10.3.3

                       THIRD AMENDMENT TO LEASE AGREEMENT

     THIRD AMENDMENT TO LEASE AGREEMENT (the "Third Amendment") dated as of
January 14, 2004 among and between Wells Fargo Bank Northwest, N.A. (formerly
known as First Security Bank, N.A.), not in its individual capacity except as
expressly set forth in the Agency Agreement, but solely as Trustee under the
Owner Trust Agreement dated as of October 21, 1998 (the "Lessor") and Genzyme
Corporation, as successor-by-merger to GelTex Pharmaceuticals, Inc. (the
"Lessee").

     Reference is made to the Lease Agreement dated as of October 21, 1998,
among and between the Lessor and GelTex Pharmaceuticals, Inc., pursuant to which
the Lessee leases from the Lessor a certain parcel of land located in Waltham,
Massachusetts, described in Exhibit A to the Lease Agreement, together with all
buildings, structures, improvements, fixtures, other real or personal property
described in the Lease Agreement and all easements, rights and appurtenances
thereto, as amended by that certain First Amendment to the Lease Agreement dated
March 31, 1999 and that certain Second Amendment to the Lease Agreement dated
December 31, 2002 (the "Lease Agreement"). Capitalized terms used in this Third
Amendment have the meanings given such terms in the Lease Agreement, as amended
hereby, except as provided otherwise herein.

     The Lessee has requested that Lessor, Fleet Real Estate, Inc. and Fleet
National Bank (together, with Fleet Real Estate, Inc., "Fleet") agree to certain
modifications in the financial covenants of the Lessee, and to certain other
changes in the terms of the Lease Agreement.

1.   Amendments

     1.1.  SECTION 14(a) - FINANCIAL STATEMENTS; OTHER INFORMATION. Section 14
of the Lease Agreement is hereby amended to strike and delete Section 14
therefrom and to replace it with the following:

           (a)   Reporting Requirements. Lessee shall deliver to Lessor and
     Fleet (either by delivering the following directly to the Lessor or Fleet
     or by providing access to copies of each of the following through a
     web-site, e-mail, or other service in a manner acceptable to Fleet):

                 (i)    as soon as available and in any event within the later
           of 45 days after the end of the first three quarterly fiscal periods
           of each fiscal year of the Lessee or the deadline for the Lessee's
           Form 10-Q filing for public record with the U.S. Securities and
           Exchange Commission, Consolidated statements of earnings,
           shareholders' equity and cash flows of the Lessee and its
           Subsidiaries for such period and for the period from the beginning of
           the respective fiscal year to the end of such period, and the related
           Consolidated balance sheets of the Lessee and its Subsidiaries as at
           the end of such period, setting forth in each case in comparative
           form, to the extent such figures appear therein, the corresponding
           Consolidated figures for the corresponding periods in the preceding
           fiscal year accompanied by a certificate of a senior financial
           officer of the Lessee, which certificate shall state that said
           Consolidated financial statements present fairly in all material
           respects the

<Page>

           Consolidated financial position and results of operations of the
           Lessee and its Subsidiaries, in accordance with GAAP, consistently
           applied, as at the end of, and for, such period (subject to normal
           year-end audit adjustments);

                 (ii)   as soon as available and in any event within the later
           of 90 days after the end of each fiscal year of the Lessee or the
           deadline for the Lessee's Form 10-K filing for public record with the
           U.S. Securities and Exchange Commission, Consolidated statements of
           earnings, shareholders' equity and cash flows of the Lessee and its
           Subsidiaries for such fiscal year and the related Consolidated
           balance sheets of the Lessee and its Subsidiaries as at the end of
           such fiscal year, setting forth in each case in comparative form, to
           the extent such figures appear therein, the corresponding
           Consolidated figures for the preceding fiscal year, and accompanied
           by a report thereon of independent certified public accountants of
           recognized national standing, which report shall state that said
           Consolidated financial statements present fairly in all material
           respects the Consolidated financial position and results of
           operations of the Lessee and its Subsidiaries as at the end of, and
           for, such fiscal year in accordance with GAAP, consistently applied;

                 (iii)  simultaneously with the delivery of the financial
           statements required under Section 14(a)(i) and (ii) above, (i) a copy
           of the Lessee's Form 10-Q or 10-K filing made for the periods covered
           by such financial statements or, if such filings are not available, a
           brief narrative description of material businesses and financial
           trends and developments and significant transactions that have
           occurred in the period or periods covered thereby, together with (ii)
           a Compliance Certificate as of the date of such financial statements,
           in the form attached as EXHIBIT A to the Third Amendment;

                 (iv)   promptly upon their becoming available, copies of all
           (i) regular, periodic and special reports that the Lessee shall have
           filed with the Securities and Exchange Commission (or any
           governmental agency substituted therefor) pursuant to the Securities
           Exchange Act of 1934, as amended (ii) financial statements, reports,
           notices or proxy or other statements sent to shareholders of the
           Lessee, (iii) business plans and financial statement forecasts, when
           and to the extent prepared and distributed to other creditors of the
           Lessee; and (iv) press releases and other statements generally made
           available by the Lessee to the public concerning material
           developments in the business of the Lessee;

                 (v)    from time to time such other information regarding the
           Property, operations, business, financial condition or prospects of
           the Lessee or any of its Subsidiaries as Lessor or Fleet may
           reasonably request.

           (b)   RECORDS; ACCESS. Lessee will keep adequate records and books of
     account reflecting all its financial transactions with respect to the Lease
     and the transactions contemplated hereby. In addition, Lessee agrees upon
     prior written request to meet with

                                        2
<Page>

     Lessor or Fleet, during normal business hours at mutually convenient times,
     from time to time, to discuss this transaction and Lessee's business and
     financial condition generally.

     1.2.  SECTION 15A(a) - LIQUIDITY RATIO/CONSOLIDATED QUICK RATIO. Section
15A(a) of the Lease Agreement is hereby amended to strike and delete Section
15A(a) therefrom and to replace it with the following:

           (a)   CONSOLIDATED QUICK RATIO. The Lessee will not permit the
     Consolidated Quick Ratio on the last day of any fiscal quarter of the
     Lessee to be less than 1.50 to 1.00 (commencing with the fiscal quarter of
     the Lessee ending in December, 2003).

     1.3.  SECTION 15A(b) - TANGIBLE CAPITAL BASE/CONSOLIDATED INTEREST
COVERAGE RATIO. Section 15A(b) of the Lease Agreement is hereby amended to
strike and delete Section 15A(b) therefrom and to replace it with the following:

           (b)   CONSOLIDATED INTEREST COVERAGE RATIO. The Lessee will not
     permit the Consolidated Interest Coverage Ratio for any period of four
     consecutive fiscal quarters of the Lessee (commencing with the period of
     four consecutive fiscal quarters of the Lessee ending in December, 2003) to
     be less than 5.00 to 1.00.

     1.4.  SECTION 15A(c) - DEBT SERVICE COVERAGE/CONSOLIDATED LEVERAGE RATIO.
Section 15A(c) of the Lease Agreement is hereby amended to strike and delete
Section 15A(c) therefrom and to replace it with the following:

           (c)   CONSOLIDATED LEVERAGE RATIO. The Lessee will not permit, as of
     the last day of any fiscal quarter (commencing with the fiscal quarter of
     the Lessee ending in December, 2003), the ratio of (a) Consolidated Funded
     Debt less all Unrestricted Cash and Marketable Investments of the Lessee
     and its Consolidated Subsidiaries in excess of $200,000,000; to (b) EBITDA
     of the Lessee for the period of four consecutive fiscal quarters then ended
     to exceed 2.00 to 1.00.

     1.5.  SECTION 15A(d). Section 15A(d) of the Lease Agreement is hereby
amended to strike and delete Section 15A(d) therefrom and to replace it with the
following:

           (d)   FINANCIAL DEFINITIONS. Notwithstanding any other section of the
     Lease Agreement, including, but not limited to, Section 31, as used in
     Section 14, Sections 15A(a) through 15A(c), Section 15B, and Section
     22(a)(x) the following terms have the meaning set forth below:

                 "ACCOUNTS RECEIVABLE" shall mean, on any date, the net amount
           of accounts receivable of Lessee and its Consolidated Subsidiaries,
           excluding any such accounts which are more than 120 days old, after
           deducting all returns, discounts and allowances thereon and reserves
           relating thereto, determined in accordance with GAAP.

                 "AFFILIATE" shall mean any Person that directly or indirectly
           controls, or is under common control with, or is controlled by a
           designated Person and, if such

                                        3
<Page>

           designated Person is an individual, any member of the immediate
           family (including parents, spouse, children and siblings) of such
           individual and any trust whose principal beneficiary is such
           individual or one or more members of such immediate family and any
           Person who is controlled by any such member or trust. As used in this
           definition, "CONTROL" (including, with its correlative meanings,
           "CONTROLLED BY" and "UNDER COMMON CONTROL WITH") shall mean
           possession, directly or indirectly, of power to direct or cause the
           direction of management or policies (whether through ownership of
           securities or partnership or other ownership interests, by contract
           or otherwise), or that, in any event, any Person that owns directly
           or indirectly securities having 33% or more of the voting power for
           the election of directors or other governing body of a corporation or
           33% or more of the partnership or other ownership interests of any
           other Person will be deemed to control such corporation, partnership
           or other Person. Notwithstanding the foregoing, (a) the Lessee and
           its Subsidiaries shall not be Affiliates of each other and (b)
           neither Lessor nor Fleet shall be an Affiliate of the Lessee or any
           of its Subsidiaries.

                 "CAPITAL EXPENDITURES" shall mean, for any period, expenditures
           (including, without limitation, the aggregate amount of Capital Lease
           Obligations incurred during such period) made by the Lessee or any of
           its Subsidiaries to acquire or construct fixed assets, plant and
           equipment (including renewals, improvements and replacements, but
           excluding repairs) during such period computed in accordance with
           GAAP.

                 "CAPITAL LEASE OBLIGATIONS" shall mean, for any Person, all
           obligations of such Person to pay rent or other amounts under a lease
           of (or other agreement conveying the right to use) property to the
           extent such obligations are required to be classified and accounted
           for as a capital lease on a balance sheet of such Person under GAAP.

                 "CASH EQUIVALENTS" shall mean any interest bearing investment
           of Lessee and its Wholly Owned Subsidiaries that meets the definition
           of a "cash equivalent" under GAAP (i.e., purchased with a remaining
           maturity of 90 days or less). Such investments shall be at least
           investment grade (A1/P1 for commercial paper, BBB or better for bonds
           and similar investments).

                 "COMPLIANCE CERTIFICATE" shall mean the compliance certificate
           provided for under Section 14(a)(iii) in substantially the form of
           EXHIBIT A to the Third Amendment.

                 "CONSOLIDATED" shall mean, when used with reference to any
           term, that term (or the term "combined" in the case of partnerships,
           joint ventures and Affiliates of the Lessee that are not
           Subsidiaries) as applied to the accounts of Lessee (or any other
           specified Person) and all of its Subsidiaries (or other specified
           Persons) or such of its Subsidiaries as may be specified,
           consolidated (or

                                        4
<Page>

           combined) in accordance with GAAP and with appropriate deductions for
           minority interests in Subsidiaries, if required by GAAP.

                 "CONSOLIDATED FUNDED DEBT" shall mean, at any time, the
           outstanding balance of all Indebtedness in respect of borrowed money,
           Capital Lease Obligations, Synthetic Lease Obligations, letters of
           credit and trade acceptances for the Lessee and its Consolidated
           Subsidiaries.

                 "CONSOLIDATED INTEREST COVERAGE RATIO" shall mean, for any
           period, the ratio of (a) EBITDA for such period to (b) all Interest
           Expense for such period.

                 "CONSOLIDATED NET INCOME" shall mean, for any period, net
           income (or loss) for the Lessee and its Consolidated Subsidiaries
           (determined in accordance with GAAP), PROVIDED, HOWEVER, that
           Consolidated Net Income shall not include amounts included in
           computing net income (or loss) in respect of extraordinary and
           non-recurring gains or losses.

                 "CONSOLIDATED QUICK RATIO" shall mean, at any date, the ratio,
           for the Lessee and its Consolidated Subsidiaries, of (a) the sum of
           (i) Unrestricted Cash on such date PLUS (ii) Marketable Investments
           on such date PLUS (iii) Accounts Receivable on such date to (b) the
           sum of (i) Current Liabilities on such date, PLUS (ii) current
           Synthetic Lease Obligations.

                 "CURRENT LIABILITIES" shall mean any liability that in
           accordance with GAAP would be classified as such.

                 "EBITDA" shall mean Consolidated Net Income plus Interest
           Expense, plus Taxes, plus depreciation, plus amortization, plus non
           cash non-recurring charges for acquired research and development (to
           the extent deducted in calculating Consolidated Net Income), plus
           non-cash non-recurring charges for asset impairments (to the extent
           deducted in calculating Consolidated Net Income), each as calculated
           in accordance with GAAP.

                 "GAAP" shall mean generally accepted accounting principles
           applied on a basis consistent with those that are to be used in
           making the calculations for purposes of determining compliance with
           the Lease Agreement.

                 "GUARANTEE" shall mean a guarantee, an endorsement, a
           contingent agreement to purchase or to furnish funds for the payment
           or maintenance of, or otherwise to be or become contingently liable
           under or with respect to, the Indebtedness, other obligations, net
           worth, working capital or earnings of any Person, or a guarantee of
           the payment of dividends or other distributions upon the stock or
           equity interests of any Person, or an agreement to purchase, sell or
           lease (as lessee or lessor) Property, products, materials, supplies
           or services primarily for the purpose of enabling a debtor to make
           payment of such debtor's obligations or an agreement to assure a
           creditor against loss, and including, without

                                        5
<Page>

           limitation, causing a bank or other financial institution to issue a
           letter of credit or other similar instrument for the benefit of
           another Person, but excluding endorsements for collection or deposit
           in the ordinary course of business. The terms "Guarantee" and
           "Guaranteed" used as a verb shall have a correlative meaning.

                 "INDEBTEDNESS" shall mean, for the Lessee and its Consolidated
           Subsidiaries: (a) obligations created, issued or incurred by such
           Person for borrowed money (whether by loan, the issuance and sale of
           debt securities or the sale of Property to another Person subject to
           an understanding or agreement, contingent or otherwise, to repurchase
           such Property from such Person); (b) obligations of such Person to
           pay the deferred purchase or acquisition price of Property or
           services, other than trade accounts payable (other than for borrowed
           money) arising, and accrued expenses incurred, in the ordinary course
           of business so long as such trade accounts payable are payable within
           120 days of the date the respective goods or services are delivered
           or rendered; (c) Indebtedness of others secured by a Lien on the
           Property of such Person, whether or not the respective Indebtedness
           so secured has been assumed by such Person; (d) obligations of such
           Person, contingent or otherwise, in respect of letters of credit,
           bankers' acceptances or similar instruments issued or accepted by
           banks and other financial institutions for account of such person;
           (e) Capital Lease Obligations of such Person; (f) Synthetic Lease
           Obligations of such Person; and (g) Guarantees by such Person of
           Indebtedness of others.

                 "INTEREST EXPENSE" shall mean, for any period, the sum, for the
           Lessee and its Consolidated Subsidiaries, of the following: (a) all
           interest in respect of Indebtedness (including, without limitation,
           the interest component of any payments in respect of Capital Lease
           Obligations) accrued or capitalized during such period (whether or
           not actually paid during such period); and (b) all other amounts that
           would be accrued or capitalized during such period as "interest
           expense" in accordance with GAAP.

                 "INVESTMENT" shall mean, for any Person: (a) the acquisition
           (whether for cash, Property, services or securities or otherwise) of
           capital stock, bonds, notes, debentures, partnership or other
           ownership interests or other securities of any other Person or any
           agreement to make any such acquisition (including, without
           limitation, any "short sale" or any sale of any securities at a time
           when such securities are not owned by the Person entering into such
           sale); and (b) the making of any deposit with, or advance, loan or
           other extension of credit to, any other Person (including the
           purchase of Property from another Person subject to an understanding
           or agreement, contingent or otherwise, to resell such Property to
           such Person), but excluding any such advance, loan or extension of
           credit arising in connection with the sale of inventory or supplies
           by such Person in the ordinary course of business so long as such
           advance, loan or extension of credit is made on terms (including as
           to maturity) consistent with those terms offered by the Lessee on the
           date hereof.

                                        6
<Page>

                 "LIEN" shall mean, with respect to any Property, any mortgage,
           lien, pledge, charge, security interest or encumbrance of any kind in
           respect of such property.

                 "MARKETABLE INVESTMENTS" shall mean any interest-bearing debt
           obligations owned by Lessee and its Wholly-Owned Subsidiaries
           (excluding directors' qualifying shares and items included as Cash
           Equivalents) that meet the definition of marketable securities under
           GAAP. Such amounts shall exclude common or preferred stock. Such
           securities shall include obligations issued by the U.S. Treasury and
           other agencies of the U.S. government, corporate bonds, bank notes,
           mortgage and asset backed securities, finance company securities and
           auction rate preferred stocks. Such securities shall be rated
           investment grade (BBB or better for bonds or similar securities,
           A1/P1 for commercial paper and notes) and shall otherwise be
           reasonably liquid investments.

                 "PERMITTED LINE OF BUSINESS" shall mean any and all of the
           following lines of business: biotechnology, pharmaceutical, medical
           devices, therapeutic products, medical products, and medical services
           and diagnostic services businesses.

                 "PERSON" shall mean any individual, corporation, company,
           voluntary association, partnership, joint venture, trust,
           unincorporated organization or government (or any agency,
           instrumentality or political subdivision thereof) and shall include
           any syndicate or group which would be deemed to be a "person" under
           Section 13(d)(3) of the Securities Exchange Act of 1934.

                 "PROPERTY" shall mean any right or interest in or to property
           of any kind whatsoever, whether real, personal or mixed and whether
           tangible or intangible.

                 "SUBSIDIARY" shall mean, with respect to any Person, any
           corporation, partnership or other entity of which at least a majority
           of the securities or other ownership interests having by the terms
           thereof ordinary voting power to elect a majority of the board of
           directors or other persons performing similar functions of such
           corporation, partnership or other entity (irrespective of whether or
           not at the time securities or other ownership interests of any other
           class or classes of such corporation, partnership or other entity
           shall have or might have voting power by reason of the happening of
           any contingency), or of which at least a majority of the limited
           partnership interests or other similar ownership interests issued by
           any limited partnership or other similar entity, is at the time
           directly or indirectly owned or controlled by such Person or one or
           more Subsidiaries of such Person or by such Person and one or more
           Subsidiaries of such Person.

                 "SYNTHETIC LEASE OBLIGATIONS" shall mean, for any Person, all
           obligations of such Person to pay rent or other amounts under or in
           connection with any so-called "synthetic lease" (i.e., a lease (or
           other agreement conveying the right to use) of property that
           constitutes a lease in accordance with GAAP but that does

                                        7
<Page>

           not constitute a lease for Federal income tax purposes), including,
           without limitation, obligations under: (i) the Agency Agreement,
           dated as of October 21, 1998 (the "Agency Agreement") between the
           Lessee, as successor-by-merger to GelTex Pharmaceuticals, Inc. and
           Wells Fargo Bank Northwest, N.A. (formerly known as First Security
           Bank, N.A.), not in its individual capacity except as expressly set
           forth in the Agency Agreement, but solely as Trustee under the Owner
           Trust Agreement dated as of October 21, 1998 (the "Owner Trust
           Agreement") between Fleet Real Estate, Inc. and Wells Fargo Bank
           Northwest, N.A. (formerly known as First Security Bank, N.A.), not in
           its individual capacity except as expressly set forth in the Agency
           Agreement, but solely as Trustee under the Owner Trust Agreement; and
           (ii) the Lease Agreement.

                 "TAXES" shall mean any present tax (including, without
           limitation, any income, documentary, sales, stamp, registration,
           property or excise tax), assessment or other charge, levy, impost,
           fee, compulsory loan, charge or withholding.

                 "UNRESTRICTED CASH" shall mean cash and Cash Equivalents of the
           Lessee and its Wholly Owned Subsidiaries that are readily available
           to Lessee and not subject to any limitation or restriction on their
           use by the Lessee.

                 "WHOLLY OWNED SUBSIDIARY" shall mean, with respect to any
           Person, any corporation, partnership or other entity to which all of
           the equity securities or other ownership interests (other than, in
           the case of a corporation, directors' qualifying shares or, in the
           case of a limited partnership, not more than 1% of the aggregate
           partnership interests issued by such limited partnership) are
           directly or indirectly owned or controlled by such Person or one or
           more Wholly Owned Subsidiaries of such Person or by such Person and
           one or more Wholly Owned Subsidiaries of such Person.

     1.6   SECTION 15B. Section 15B of the Lease Agreement is hereby amended
to strike and delete Section 15B therefrom and to replace it with the following:

     15B.  At all times during the Term:

           (a)   MERGER, CONSOLIDATION, ETC. The Lessee will not, nor will it
     permit any of its Subsidiaries to, enter into any transaction of merger or
     consolidation or amalgamation, or liquidate, wind up or dissolve itself (or
     suffer any liquidation or dissolution).

           (b)   ACQUISITIONS. The Lessee will not, nor will it permit any of
     its Subsidiaries to, acquire any business or Property from, or capital
     stock of, or be a party to any acquisition of, any Person.

           (c)   SALE OF ASSETS. The Lessee will not, nor will it permit any of
     its Subsidiaries to, convey, sell, lease, transfer or otherwise dispose of,
     in one transaction or a series of transactions, any part of its business or
     Property, whether now owned or

                                        8
<Page>

     hereafter acquired (including, without limitation, receivables and
     leasehold interests) other than the sale of inventory in the course of
     business.

           (d)   EXCEPTIONS. Notwithstanding the foregoing provisions of this
     Section 15B:

                 (i)    any Subsidiary of the Lessee may be merged or
           consolidated with or into, or have its assets liquidated and
           distributed to, the Lessee or any other Subsidiary of the Lessee;
           PROVIDED that (x) if any such merger or consolidation shall be with
           the Lessee, the Lessee shall be the Person surviving such merger or
           consolidation and (y) if any such merger or consolidation shall be
           between any Subsidiary of the Lessee and a Wholly Owned Subsidiary of
           the Lessee, such Wholly Owned Subsidiary shall be the Person
           surviving such merger or consolidation;

                 (ii)   the Lessee and its Subsidiaries may acquire any assets
           used or useful in the Permitted Line of Business or the stock or
           other equity interests or rights as a holder of indebtedness of any
           Person that is engaged in a Permitted Line of Business or merge any
           Person that is in a Permitted Line of Business with the Lessee or a
           Subsidiary or the Lessee or a Subsidiary with any such Person
           (provided that the conditions in the provisos in Section 15B(d)(i)
           are satisfied with respect to such merger) provided that at the time
           of the consummation of any such transaction and after giving effect
           thereto, the Lessee shall be in compliance with the covenants in
           Sections 15A(a), (b) and (c) as of the end of the most recent fiscal
           quarter or annual period of the Lessee and the transaction will not
           be reasonably likely to result in the noncompliance with such
           financial covenants;

                 (iii)  the Lessee or any of its Subsidiaries may purchase
          inventory and other Property to be sold or used in the ordinary course
          of business, make Investments permitted by Section 15B(e) hereof and
          make Capital Expenditures in the ordinary course of its business;

                 (iv)   the Lessee or any Subsidiary of the Lessee may convey,
           sell, lease, loan, transfer or otherwise dispose of any or all of its
           Property to the Lessee or any other Subsidiary of the Lessee (and the
           Lessee or such other Subsidiary may acquire such Property);

                 (v)    the Lessee or any Subsidiary may convey, sell, lease,
          transfer or otherwise dispose of any non-material Property (of the
          Lessee and its Subsidiaries, taken as a whole) including equity
          interests in any Person and the licensing of patents and product
          rights; and

                 (vi)   the Lessee or any Subsidiary may lease or sublease any
          of its real Property.

                                        9
<Page>

                 (e)    The Lessee will not, nor will it permit any of its
          Subsidiaries to, make or permit to remain outstanding any Investments
          except Investments made in the ordinary course of business of the
          Lessee or its Subsidiaries or that would be permitted under the terms
          of this Section 15B.

     1.7.  SECTION 22(a)(x). Section 22(a)(x) of the Lease Agreement is hereby
amended to strike and delete Section 22(a)(x) therefrom and to replace it with
the following:

           (x)   if the Lessee or any of its Subsidiaries shall default in the
           payment when due of any principal of or interest on any of its other
           Indebtedness aggregating to at least $5,000,000 (after the expiration
           of any grace period originally provided for); or any event specified
           in any note, agreement, indenture or other document evidencing or
           relating to any such Indebtedness shall occur if the effect of such
           event is to cause, or (with the giving of any notice or the lapse of
           time or both) to permit the holder or holders of such Indebtedness
           (or a trustee or agent on behalf of such holder or holders) to cause,
           such Indebtedness to become due, or to be prepaid in full (whether by
           redemption, purchase, offer to purchase or otherwise), prior to its
           stated maturity; or

2.   REPRESENTATIONS AND WARRANTIES. In order to induce the Lessor to enter into
and Fleet to consent to this Third Amendment, the Lessee makes the following
representations and warranties, all of which shall survive the execution and
delivery of this Third Amendment:

           (a)   The Lessee has all requisite corporate power and authority to
     execute, deliver and perform its obligations under this Third Amendment and
     under the Lease Agreement, as amended hereby. This Third Amendment has been
     duly authorized, executed and delivered by the Lessee, and does not
     conflict with, violate or result in a breach of or require any consent
     under (i) any applicable law or regulation or any of the terms of the
     charter or by-laws of the Lessee, or (ii) any agreement or instrument to
     which the Lessee or any Subsidiary is a party or to which any of them or
     their Property is bound or to which any of them is subject; except to the
     extent, with respect to the foregoing clause (ii), any such conflict,
     violation, or breach, or the failure to have any such consent, (x) could
     not reasonably be expected (either individually or in the aggregate) to
     have a material adverse effect and (y) does not and will not result in any
     liability of the Lessor. This Third Amendment and the Lease Agreement, as
     amended hereby, constitute the legal, valid and binding obligation of the
     Lessee enforceable against the Lessee in accordance with its terms.

           (b)   On the date hereof each of the representations and warranties
     in the Lease Agreement are true, accurate and complete in all material
     respects (other than those representations and warranties made as of a
     specific date, which were true, accurate and complete in all material
     respects as of such specific date).

           (c)   Upon the execution and delivery of this Third Amendment, and
     the satisfaction of each of the conditions precedent set forth in Section 3
     of this Third Amendment, no Default or Event of Default shall exist and be
     continuing.

                                       10
<Page>

3.   CONDITIONS PRECEDENT. The agreements contained herein and the amendments
contemplated hereby shall become effective when the Lessee and Lessor shall have
executed this Third Amendment and when each of the following conditions shall
have been fulfilled (the "Effective Date"):

           (a)   EXECUTION OF DOCUMENTS, ETC. This Third Amendment and any other
     agreements, documents and instruments to be executed and/or delivered in
     connection herewith (collectively the "Third Amendment Documents") shall
     have been duly and properly authorized and executed by: the Lessee, Lessor
     and Fleet and shall be in full force and effect on and as of the Effective
     Date of this Third Amendment and all representations and warranties of the
     Lessee hereunder shall continue to be true, accurate and complete.

           (b)   PROCEEDINGS: RECEIPT OF DOCUMENTS. All requisite corporate
     action and proceedings of the Lessee in connection with the execution and
     delivery of this Third Amendment shall be satisfactory in form and
     substance to the Lessor and Fleet and their respective counsel, and Lessor
     and Fleet and their respective counsel shall have received all information
     and copies of all documents, including without limitation, records of
     requisite corporate action and proceedings that the Lessor or Fleet or
     their respective counsel may have requested in connection therewith, such
     documents where requested by the Lessor or Fleet or their respective
     counsel to be certified by appropriate persons or governmental authorities.

           (c)   MATERIAL LITIGATION. There shall be no pending or, to the best
     knowledge of the Lessee, threatened litigation with respect to the Lessee
     before any court, arbitrator or governmental or administrative body or
     agency that challenges or relates to (i) the lending transactions
     contemplated hereby or (ii) the Operative Documents.

4.   REAFFIRMATION AND RATIFICATION OF EXISTING AGREEMENTS, ETC. The Lessee is
the successor-by-merger to GelTex Pharmaceuticals, Inc. pursuant to a Merger
Agreement between GelTex Pharmaceuticals, Inc. and the Lessee effective as of
April 1, 2003 and as a result thereof: (i) reaffirms and ratifies all the
obligations to the Lessor and Fleet, in respect of the Lease Agreement, as
hereby amended and the other Operative Documents; (ii) certifies that there are
no defenses, offsets or counterclaims to such obligations as of the date hereof;
(iii) expressly acknowledges its continuing liability pursuant thereto; (iv)
agrees that each of the Lease Agreement, as hereby amended and the other
Operative Documents shall remain in full force and effect, enforceable against
the Lessee in accordance with its terms; and (v) Lessee, Lessor and Fleet
acknowledge and confirm that that certain Guaranty executed by Lessee on
December 14, 2000, as amended by that certain First Amendment to Guaranty dated
as of June 30, 2001, is no longer in effect.

5.   MISCELLANEOUS.

           (a)   This Third Amendment may be executed on separate counterparts
     by the parties hereto, each of which when so executed and delivered shall
     be an original, but all of which shall constitute one and the same
     agreement.

                                       11
<Page>

           (b)   This Third Amendment and the rights and obligations of the
     parties hereunder shall be construed in accordance with and be governed by
     the laws of the Commonwealth of Massachusetts (without giving effect to the
     conflict of law principles thereof).

           (c)   The headings of the several sections of this Third Amendment
     are inserted for convenience only and shall not in any way affect the
     meaning or construction of any provision of this Third Amendment.

           (d)   This Third Amendment embodies the entire agreement and
     understanding among the parties relating to the subject matter hereof and
     supersedes all prior proposals, negotiation, agreements and understandings
     relating to such subject matter.

           (e)   This Third Amendment shall be deemed to be an Operative
     Document under the Lease Agreement.

           (f)   EACH OF THE LESSEE AND LESSOR HEREBY IRREVOCABLY WAIVES, TO THE
     FULLEST EXTENT PERMITTED BY APPLICABLE LAW, ANY AND ALL RIGHT TO TRIAL BY
     JURY IN ANY LEGAL PROCEEDING ARISING OUT OF OR RELATING TO THIS THIRD
     AMENDMENT, OR THE TRANSACTIONS CONTEMPLATED HEREBY.

           (g)   The Lessee shall pay on demand the reasonable costs and
     expenses, including, without limitation, reasonable attorneys' fees and
     expenses incurred, or which may be incurred by the Lessor or Fleet in
     connection with the negotiation, documentation, administration and
     enforcement of this Third Amendment.


            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK]

                                       12
<Page>

     IN WITNESS WHEREOF, this Third Amendment has been duly executed and
delivered as a sealed instrument at Boston, Massachusetts as of the date first
above written.

                                        LESSOR:


                                        WELLS FARGO BANK NORTHWEST, N.A.
                                        (formerly known as FIRST SECURITY BANK,
                                        N.A.), not in its individual capacity
                                        except as expressly set forth herein,
                                        but solely as Trustee under the Owner
                                        Trust Agreement dated as of October 21,
                                        1998

                                        By: /s/ Nancy M. Dahl
                                           ----------------------------
                                           Name: Nancy M. Dahl
                                           Title: Vice President


                                        LESSEE:


                                        GENZYME CORPORATION


                                        By: /s/ Evan M. Lebson
                                           ------------------------------------
                                           Name:   Evan M. Lebson
                                           Title:  Assistant Treasurer

                                       13
<Page>

                                        AGREED AND ACCEPTED:


                                        FLEET NATIONAL BANK


                                        By: /s/ David E. Meagher
                                           ------------------------------------
                                           Name: DAVID E. MEAGHER
                                           Title: VICE PRESIDENT


                                        FLEET REAL ESTATE, INC.


                                        By: /s/ Francis E. Fitzgerald
                                           ------------------------------------
                                           Name: FRANCIS E. FITZGERALD
                                           Title: PRESIDENT

                                       14
<Page>

                         EXHIBIT A TO THIRD AMENDMENT TO
                                 LEASE AGREEMENT

                         FORM OF COMPLIANCE CERTIFICATE

                             COMPLIANCE CERTIFICATE

     This Compliance Certificate is provided pursuant to 15A(d) of that certain
Lease Agreement (the "Agreement") dated as of October 21, 1998, between Wells
Fargo Bank Northwest, N.A. (formerly known as First Security Bank, N.A.), not in
its individual capacity except as expressly set forth in the Lease Agreement,
but solely as Trustee under the Owner Trust Agreement dated as of October 21,
1998 and Genzyme Corporation, as successor-by-merger to GelTex Pharmaceuticals,
Inc. (the "Lessee"), as amended. The capitalized terms used herein shall have
the meanings ascribed to such terms in the Agreement. The undersigned hereby
certifies as follows as of this date:

     1.    The representations and warranties made by the Lessee in the
Agreement and by the Lessee in each certificate, document or financial or other
statement furnished under or in connection therewith are true and accurate in
all material respects.

     2.    The financial information and calculations shown on the attached
Schedule A are true and accurate as of the date hereof and the Lessee is in
compliance with the financial covenants set forth in Sections 15A(a) through (c)
of the Agreement.

     3.    No Default or Event of Default under the Agreement or the Agency
Agreement has occurred.

     IN WITNESS WHEREOF, this Certificate has been duly executed and delivered
as a sealed instrument at Boston, Massachusetts on this ______ day of ________,
____.

                                        GENZYME CORPORATION


                                        By:
                                           ------------------------------
                                           Name:
                                                 ------------------------
                                           Title:
                                                 ------------------------

                                       A-1
<Page>

                                   SCHEDULE A
                                  TO EXHIBIT A

SECTION 15A(a) CONSOLIDATED QUICK RATIO

cash -- $_________

Cash Equivalents -- $________

Marketable Investments -- $________

Accounts Receivable -- $________

Current Liabilities -- $________

Current Synthetic Lease Obligations -- $________

Actual Ratio -- _________

Minimum Ratio -- 1.50 to 1.00

SECTION 15A(b) CONSOLIDATED INTEREST COVERAGE RATIO

EBITDA -- $__________

Interest Expense -- $_________

Actual Ratio -- ________

Minimum Ratio -- 5.00 to 1.00

SECTION 15A(c) CONSOLIDATED LEVERAGE RATIO

Consolidated Funded Debt -- $________

Unrestricted Cash and Marketable Investments in excess of $200,000,000 -- $____

EBITDA -- $________

Actual Ratio -- ________

Maximum Ratio -- 2.00 to 1.00

                                       A-2