Exhibit 10.27

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STOCK PURCHASE AGREEMENT

By and Between

AVENTIS PHARMACEUTICALS INC.

AND

REGENERON PHARMACEUTICALS, INC.

Dated as of September 5, 2003

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STOCK PURCHASE AGREEMENT

     THIS STOCK PURCHASE AGREEMENT (“Agreement”), dated as of September 5, 2003,
by and between AVENTIS PHARMACEUTICALS INC. (the “Investor”), a corporation
organized under the laws of Delaware, with its principal place of business in
the United States at 200 Crossing Boulevard, Bridgewater, New Jersey 08807, and
REGENERON PHARMACEUTICALS, INC. (the “Company”), a corporation organized under
the laws of New York with its principal place of business at 777 Old Saw Mill
Road, Tarrytown, New York.

     WHEREAS, concurrently with the execution of this Agreement, the Investor
and the Company have entered into a Collaboration Agreement (the “Collaboration
Agreement” and together with this Agreement, the “Transaction Agreements”); and

     WHEREAS, it is contemplated by the Collaboration Agreement that the
Investor purchase, and the Company issue and sell to the Investor, shares of
common stock of the Company pursuant to the terms and conditions of this
Agreement.

     NOW, THEREFORE, in consideration of the following mutual promises and
obligations, and for good and valuable consideration, the adequacy and
sufficiency of which are hereby acknowledged, the Investor and the Company agree
as follows:

ARTICLE 1
DEFINITIONS

     1.1 Defined Terms. When used in this Agreement, the following terms shall
have the respective meanings specified therefor below:

     “Affiliate” shall mean, with respect to any Person, any other Person which
controls, is controlled by or is under common control with such Person. A Person
shall be deemed to control another Person if such Person possesses, directly or
indirectly, the power to direct or cause the direction of the management and
policies of such Person, whether through the ownership of voting securities, by
contract or otherwise.

     “Business Day” shall mean any day other than a Saturday or Sunday or a day
on which banks located in New York, New York are authorized or required by law
to close.

     “Common Stock” shall mean the Company’s Common Stock, par value $0.001 per
share. Common Stock shall not include shares of the Company’s class A shares.

     “Company Intellectual Property” shall mean the Intellectual Property that
is owned by Company and the Intellectual Property subject to an Intellectual
Property License pursuant to which its use by the Company is permitted by any
third party.

     “Disposition” or “Dispose of” shall mean any (i) offer, pledge, sale,
contract to sell, sale of any option or contract to purchase, purchase of any
option or contract to sell, grant of any option, right or warrant for the sale
of, or other disposition of or transfer of any shares of the

 

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     Company’s common stock, par value $.001 per share, or any securities
convertible into or exchangeable or exercisable for such common stock or (ii)
swap or any other agreement or any transaction that transfers, in whole or in
part, directly or indirectly, the economic consequence of ownership of the
Company’s common stock, whether any such swap or transaction is to be settled by
delivery of securities, in cash or otherwise.

     “Governmental Authority” shall mean any federal, state, municipal, local,
provincial or regional governmental authority in the United States or other
political subdivision thereof and any Person exercising executive, legislative,
judicial regulatory or administrative functions of or pertaining to government.

     “Intellectual Property” shall mean trademarks, trade names, trade dress,
service marks, copyrights, and similar rights (including registrations and
applications to register or renew the registration of any of the foregoing),
patents and patent applications, trade secrets, and any other similar
intellectual property rights.

     “Intellectual Property License” shall mean any license, permit,
authorization, approval, contract or consent granted, issued by or with any
Person relating to the use of Intellectual Property.

     “Material Adverse Effect” shall mean any events, occurrences or
circumstances which give rise to or would reasonably be expected to give rise
to, individually or in the aggregate, a material adverse effect on the business,
business prospects, properties, condition (financial or otherwise) or results of
operations of the Company.

     “Organizational Documents” shall mean the Company’s Certificate of
Incorporation as in effect on the date hereof and the Company’s Bylaws as in
effect on the date hereof.

     “Person” shall mean and include an individual, a partnership, a joint
venture, a corporation, a limited liability company, a limited liability
partnership, a trust, an incorporated organization and a Governmental Authority.

     1.2 Additional Defined Terms. In addition to the terms defined in Section
1.1, the following terms shall have the respective meanings assigned thereto in
the sections indicated below:

      Defined Term   Section

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Agreement   Preamble Closing   2.2 Closing Date   2.2 Collaboration Agreement  
Preamble Company   Preamble Company SEC Documents   3.10(a) Exchange Act  
3.10(a) Investor   Preamble Purchase Price   2.1 Purchased Stock   2.1

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      Defined Term   Section

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Rule 144   5.2 SEC   3.6 Securities Act   3.10(a) Severed Clause   6.10
Transaction Agreements   Preamble

     1.3 Construction. In this Agreement, unless the context otherwise requires:

          (a) any reference in this Agreement to “writing” or comparable
expressions includes a reference to facsimile transmission or comparable means
of communication;

          (b) words expressed in the singular number shall include the plural
and vice versa, words expressed in the masculine shall include the feminine and
neuter gender and vice versa;

          (c) references to Articles and Sections are references to articles and
sections of this Agreement;

          (d) reference to “day” or “days” are to calendar days; and

          (e) “include,” “includes,” and “including” are deemed to be followed
by “without limitation” whether or not they are in fact followed by such words
or words of similar import.

     1.4 Knowledge. Where any representation or warranty contained in this
Agreement is expressly qualified by reference to the knowledge of the Company,
the Company confirms that it has made reasonable inquiry or investigation as to
the matters that are the subject of such representations and warranty.

ARTICLE 2
PURCHASE AND SALE OF COMMON STOCK

     2.1 Issuance of Common Stock. Subject to the terms and conditions hereof,
on the Closing Date, the Company agrees to issue and sell to the Investor, and
the Investor agrees to purchase, 2,799,552 shares of Common Stock (the
“Purchased Stock”) for an aggregate purchase price of $45,000,000 (the “Purchase
Price”). The price per share of the Purchased Stock shall be $16.074, which
amount represents the average daily closing price per share on the Nasdaq Stock
Market during the five (5) trading days immediately preceding, but not
including, the date of this Agreement.

     2.2 Closing. The purchase and sale of the Purchased Stock (the “Closing”)
shall occur on the date hereof at the offices of Skadden, Arps, Slate, Meagher &
Flom LLP, at Four Times Square, New York, New York. The date on which the
Closing occurs is referred to herein as the “Closing Date”.

     2.3 Delivery. Within three (3) Business Days of the Closing Date, the
Company shall deliver to the Investor a stock certificate, registered in the
Investor’s name, representing the Purchased Stock. Within one (1) Business Day
following the Closing Date, Investor shall

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deliver to the Company the Purchase Price by wire transfer of same day funds to
the Company’s bank account as follows:

      Beneficiary Name:   Regeneron Pharmaceuticals, Inc. Beneficiary Address:  
777 Old Saw Mill River Road     Tarrytown, New York 10591 Account Number:  
[********************] Bank Name:   [********************] Bank Address:  
[********************]     [********************] Bank Clearing Number:  
[********************]

ARTICLE 3
REPRESENTATIONS AND WARRANTIES OF THE COMPANY

     The Company hereby represents and warrants to the Investor as of the date
hereof as follows:

     3.1 Organization, Good Standing and Qualification. The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of New York. The Company has all requisite corporate power and
corporate authority to own and operate its properties and assets, to carry on
its business as now conducted and as proposed to be conducted in the Company SEC
Documents, to enter into the Transaction Agreements, to issue and sell the
Purchased Stock and to carry out the other transactions contemplated under the
Transaction Agreements. The Company is qualified to transact business and is in
good standing in each jurisdiction in which the character of the properties
owned, leased or operated by the Company or the nature of the business conducted
by the Company makes such qualification necessary, except where the failure to
be so qualified would not have a Material Adverse Effect.

     3.2 Capitalization and Voting Rights.

          (a) The authorized capital of the Company as of September 4, 2003
consisted of: (i) 160,000,000 shares of Common Stock, par value $0.001 per
share, of which (w) 49,829,556 shares were issued and outstanding, (x) 2,403,848
shares were reserved for issuance upon conversion of the Company’s class A
stock, each share of class A stock being convertible into one share of Common
Stock, (y) 15,265,493 shares were reserved for issuance pursuant to the
Company’s 1990 Long-Term Incentive Plan and 2000 Long-Term Incentive Plan and
(z) 6,611,300 shares were reserved for issuance upon conversion of the Company’s
5½% Convertible Senior Subordinated Notes due 2008, (ii) 40,000,000 shares of
class A stock, par value $0.001 per share, of which 2,403,848 shares were issued
and outstanding, and (iii) 30,000,000 shares of preferred stock, par value $0.01
per share, of which no shares were issued and outstanding. All of the issued and
outstanding shares of Common Stock and class A stock have been duly authorized,
and all of the issued and outstanding shares of Common Stock and class A stock
have been validly issued, are fully paid and non-assessable, and were issued in
compliance with all applicable federal and state securities laws.

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          (b) All of the authorized shares of Common Stock are entitled to one
(1) vote per share. All of the authorized shares of class A stock are entitled
to ten (10) votes per share.

          (c) Except as set forth in the Company SEC Documents filed at least
seventy-two (72) hours prior to the date of this Agreement, or as provided in
the Transaction Agreements, there are not, nor upon the consummation of the
transactions contemplated hereby shall there be: (i) any outstanding options,
warrants, rights (including conversion or preemptive rights) or agreements
pursuant to which the Company is or may become obligated to issue, sell or
repurchase any shares of its capital stock or any other securities of the
Company and (ii) any restrictions on the transfer of capital stock of the
Company other than pursuant to state and federal securities laws.

          (d) Except as set forth in the Company SEC Documents filed prior to
the date of this Agreement or as provided in the Transaction Agreements, the
Company is not a party to or subject to any agreement or understanding relating
to the voting of shares of capital stock of the Company or the giving of written
consents by a shareholder or director of the Company.

     3.3 Subsidiaries. The Company does not have any subsidiaries required to be
disclosed in Exhibit 21 to its annual report on Form 10-K.

     3.4 Authorization. All corporate action on the part of the Company, its
directors and stockholders necessary for the authorization, execution and
delivery of the Transaction Agreements and the performance of all obligations of
the Company thereunder, including the authorization, issuance and delivery of
the Purchased Stock, has been taken. This Agreement has been duly executed and
delivered by the Company and, upon due execution and delivery by Investor,
constitutes a valid and legally binding obligation of the Company, enforceable
against the Company in accordance with its terms (except as such enforceability
may be limited by (i) applicable bankruptcy, insolvency, reorganization,
moratorium or other laws of general application relating to or affecting
enforcement of creditors’ rights and (ii) rules of law governing specific
performance, injunctive relief or other equitable remedies and limitations of
public policy).

     3.5 No Conflicts. The execution and performance of the Transaction
Agreements and compliance with the provisions thereof by the Company, do not and
shall not: (a) violate any provision of law, statute, ordinance, rule or
regulation or any ruling, writ, injunction, order, permit, judgment or decree of
any Governmental Authority, (b) constitute a breach of, or default (or an event
which, with notice or lapse of time or both, would become a default) or conflict
with, or give rise to any right of termination, cancellation or acceleration of,
any agreement, arrangement or instrument, whether written or oral, by which the
Company or any of its assets are bound, or (c) violate or conflict with any of
the provisions of the Company’s Organizational Documents; except, in the case of
subsections (a) and (b) as would not have a Material Adverse Effect.

     3.6 No Governmental Authority or Third Party Consents. No consent,
approval, authorization or other order of any Governmental Authority or other
third party is required to be obtained by the Company in connection with the
authorization, execution and delivery of this Agreement or with the
authorization, issue and sale of the Purchased Stock, except such filings

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as may be required to be made with the Securities and Exchange Commission (the
“SEC”) and with any state blue sky or securities regulatory authority, which
filings shall be made in a timely manner in accordance with all applicable laws,
rules, regulations, statutes, ordinances and orders.

     3.7 Valid Issuance of Purchased Stock. When issued, sold and delivered in
accordance with the terms hereof for the consideration expressed herein, the
Purchased Stock shall be duly authorized, validly issued, fully paid and
nonassessable, free from any encumbrances or restrictions on transfer, including
preemptive rights, rights of first refusal or other similar rights, other than
restrictions on transfer under the Transaction Agreements and under federal and
state securities laws.

     3.8 Litigation. There is no action, suit, proceeding or investigation
pending or threatened against the Company or which the Company intends to
initiate which (a) questions the validity of the Transaction Agreements or the
right of the Company to enter into them, or to consummate the transactions
contemplated thereby, or (b) except as set forth in the Company SEC Documents
filed prior to the date of this Agreement, if determined adversely would have a
Material Adverse Effect.

     3.9 Licenses and Other Rights; Compliance with Laws. The Company has all
franchises, permits, licenses and other rights and privileges necessary to
permit it to own its properties and to conduct its business as presently
conducted and is in compliance thereunder, except where the failure to be in
compliance does not and would not have a Material Adverse Effect. The Company is
and has been in compliance with all laws and governmental rules and regulations
applicable to its business, properties and assets, and to the products and
services sold by it, including, without limitation, all such rules, laws and
regulations relating to fair employment practices, occupational safety and
health and public safety, except where the failure to be in compliance does not
and would not have a Material Adverse Effect.

     3.10 Company SEC Documents; Financial Statements; Nasdaq Stock Market.

          (a) Since December 31, 2002, the Company has filed all required
reports, schedules, forms, statements and other documents (including exhibits
and all other information incorporated therein) with the SEC (“Company SEC
Documents”). As of their respective filing dates, each of the Company SEC
Documents complied in all material respects with the requirements of the
Securities Act of 1933, as amended (the “Securities Act”), or the Securities
Exchange Act of 1934, as amended (the “Exchange Act”), and the rules and
regulations of the SEC promulgated thereunder applicable to such Company SEC
Documents, and no Company SEC Documents when filed contained any untrue
statement of a material fact or omitted to state a material fact required to be
stated therein or necessary in order to make the statements therein, in light of
the circumstances under which they were made, not misleading.

          (b) The financial statements of the Company included in its annual
report on Form 10-K for the fiscal year ended December 31, 2002 and in its
quarterly report on Form 10-Q for the quarterly period ended June 30, 2003
comply as to form in all material respects with applicable accounting
requirements and the published rules and regulations of the SEC with respect
thereto, have been prepared in accordance with United States generally accepted
accounting principles applied on a consistent basis during the periods involved
(except as may be

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indicated in the notes thereto) and fairly present the financial position of the
Company as of the dates thereof and the results of its operations and cash flows
for the periods then ended. Except as set forth in the Company SEC Documents,
the Company has no liabilities, whether absolute or accrued, contingent or
otherwise, other than liabilities incurred in the ordinary course of business
subsequent to the date of the most recent balance sheet contained in the Company
SEC Documents.

          (c) The Common Stock is listed on the Nasdaq Stock Market and the
Company has taken no action designed to, or which to its knowledge is likely to
have the effect of, terminating the registration of the Common Stock under the
Exchange Act or delisting the Common Stock from the Nasdaq Stock Market. The
Company has not received any notification that, and has no knowledge that, the
SEC or the National Association of Securities Dealers Inc. is contemplating
terminating such listing or registration. The issuance of the shares of
Purchased Stock pursuant to this Agreement does not require shareholder
approval, including, without limitation, pursuant to the rules of the National
Association of Securities Dealers Inc.

     3.11 Absence of Changes. Since December 31, 2002, there has been no change
or development which, individually or in the aggregate, has had or would have a
Material Adverse Effect.

     3.12 Disclosure Controls and Procedures. The Company has implemented the
“disclosure controls and procedures” (as defined in Rules 13a-15(e) and
15d-15(e) of the Exchange Act) required in order for the Chief Executive Officer
and Chief Financial Officer of the Company to engage in the review and
evaluation process mandated by the Exchange Act, and is in compliance with such
disclosure controls and procedures in all material respects.

     3.13 Intellectual Property. The Intellectual Property that is owned by the
Company is owned free from any liens or restrictions, and all of the Company’s
material Intellectual Property Licenses are in full force and effect in
accordance with their terms, and are free of any liens or restrictions, except
(a) where the failure to be free from such liens or restrictions would not have
a Material Adverse Effect or (b) as set forth in any such Intellectual Property
License. Except as set forth in the Company SEC Documents, there is no legal
claim or demand of any Person pertaining to, or any proceeding which is pending
or, to the knowledge of Company threatened, (i) challenging the right of the
Company in respect of any Company Intellectual Property, or (ii) that claims
that any default exists under any Intellectual Property License, except, in the
case of (i) and (ii) above, where such legal claims would not have a Material
Adverse Effect.

     3.14 Offering. Subject to the accuracy of the Investor’s representations
set forth in Section 4.3 and 4.4, the offer, sale and issuance of the Purchased
Stock to be issued in conformity with the terms of this Agreement constitute
transactions which are exempt from the registration requirements of the
Securities Act and from all applicable state registration or qualification
requirements.

     3.15 No Integration. The Company has not, directly or through any agent,
sold, offered for sale, solicited offers to buy or otherwise negotiated in
respect of, any security (as defined in the Securities Act) which is or will be
integrated with the Purchased Stock sold

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pursuant to this Agreement in a manner that would require the registration of
the Purchased Stock under the Securities Act.

     3.16 Brokers’ or Finders’ Fees. No broker, finder, investment banker or
other Person is entitled to any brokerage, finder’s or other fee or commission
in connection with the transactions contemplated by this Agreement.

ARTICLE 4
REPRESENTATIONS AND WARRANTIES OF THE INVESTOR

     The Investor hereby represents and warrants as of the date hereof as
follows:

     4.1 Organization; Good Standing. The Investor is a corporation duly
organized, validly existing and in good standing under the laws of Delaware. The
Investor has all requisite corporate power and corporate authority to enter into
the Transaction Agreements, to purchase the Purchased Stock and to carry out the
other transactions contemplated under the Transaction Agreements.

     4.2 Authorization. All corporate action on the part of the Investor, and
its directors and stockholders necessary for the authorization, execution and
delivery of the Transaction Agreements, the performance of all obligations of
the Investor thereunder, including the subscription and purchase of the
Purchased Stock, has been taken. This Agreement has been duly executed and
delivered by the Investor and, upon due execution and delivery by the Company,
constitutes a valid and legally binding obligation of the Investor, enforceable
against the Investor in accordance with its terms (except as such enforceability
may be limited by (i) applicable bankruptcy, insolvency, reorganization,
moratorium or other laws of general application relating to or affecting
enforcement of creditors’ rights and (ii) rules of law governing specific
performance, injunctive relief or other equitable remedies and limitations of
public policy).

     4.3 Purchase Entirely for Own Account. The Purchased Stock shall be
acquired for investment for the Investor’s own account, not as a nominee or
agent, and not with a view to the resale or distribution of any part thereof,
and the Investor has no present intention of selling, granting any
participation, or otherwise Distributing the Purchased Stock. The Investor does
not have any contract, undertaking, agreement or arrangement with any Person to
sell, transfer or grant participation to a Person any of the Purchased Stock.

     4.4 Investment Experience and Accredited Investor Status. The Investor is
an “accredited investor” (as defined in Regulation D under the Securities Act).
The Investor has such knowledge and experience in financial or business matters
that it is capable of evaluating the merits and risks of the investment in the
Purchased Stock to be purchased hereunder.

ARTICLE 5
FURTHER ASSURANCES; SECURITIES LAW MATTERS

     5.1 Further Assurances. The parties agree to take such reasonable steps and
execute such other and further documents as may be necessary or appropriate to
cause the terms and conditions contained herein to be carried into effect.

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     5.2 Restricted Securities. The Investor understands that the Purchased
Stock, when issued, shall be “restricted securities” under the federal
securities laws inasmuch as they are being acquired from the Company in a
transaction not involving a public offering and that under such laws and
applicable regulations such securities may be resold without registration under
the Securities Act only in certain limited circumstances. In this connection,
the Investor represents that it is familiar with Rule 144 of the Securities Act,
as presently in effect (“Rule 144”).

     5.3 Limitations on Dispositions.

          (a) For a period of two (2) years from the Closing Date, neither the
Investor nor any of its Affiliates shall make any Disposition, except, upon
prior written notice to the Company, to an Affiliate of the Investor, which
Affiliate shall then be subject to the same restrictions on Disposition as set
forth in this Section 5.3. The Investor or its Affiliate, as the case may be,
(i) during the period from the day after the second anniversary of the Closing
Date until the third anniversary of the Closing Date, may Dispose of no more
than 250,000 shares (subject to stock splits, reverse stock splits,
combinations, recapitalizations and similar events) of the Purchased Stock in
the aggregate per calendar quarter, and (ii) after the third anniversary of the
Closing Date, may Dispose of no more than 500,000 shares (subject to stock
splits, reverse stock splits, combinations, recapitalizations and similar
events) of the Purchased Stock in the aggregate per calendar quarter.

          (b) With respect to any sale or transfer of the Purchased Stock, the
Investor or its Affiliate, as the case may be, shall not make any such sale or
transfer unless the sale or transfer is made pursuant to Rule 144 or similar
provisions of federal securities laws as in effect from time to time.

          (c) In the event the Company proposes to sell securities in an
underwritten offering, the Investor shall, if requested by the Company and an
underwriter of Common Stock of the Company, agree not to sell or otherwise
transfer or dispose of any Common Stock of the Company held by the Investor for
a specified period of time, such period of time not to exceed ninety (90) days.
Such agreement shall be in writing in a form satisfactory to the Company and
underwriter in such offering. The Company may impose stop transfer instructions
with respect to the Common Stock subject to the foregoing restrictions until the
end of the lock-up period. The Company may request no more than one (1) lock-up
period per calendar year.

     5.4 Legends. It is understood that the certificates representing the
Purchased Stock shall bear the following legends:

          (a) “These securities have not been registered under the Securities
Act of 1933. They may not be sold, offered for sale, pledged or hypothecated in
the absence of a registration statement in effect with respect to the securities
under the Securities Act or an opinion of counsel (which counsel shall be
reasonably satisfactory to the Company) that such registration is not required
or unless sold pursuant to Rule 144 of the Securities Act.”; and

          (b) any legend required by applicable state securities laws.

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     5.5 Current Public Information. For so long as the Company remains a public
company or, if earlier, until such time as the Investor no longer holds any
shares of Purchased Stock, the Company shall:

          (a) make and keep available, at all times, adequate public information
as required under paragraph (c) of Rule 144 under the Securities Act; and

          (b) file with the SEC in a timely manner all reports and other
documents required of the Company under Sections 13, 14, and 15(d) of the
Exchange Act.

     5.6 Form D; Blue Sky Laws. The Company agrees to timely file a Form D with
respect to the Purchased Stock as required under Regulation D after the Closing
and to provide a copy thereof to the Investor promptly after such filing. The
Company shall, on or before the Closing Date, take such action as the Company
shall reasonably determine is necessary to qualify the Purchased Stock for sale
pursuant to this Agreement under applicable securities or “blue sky” laws of the
states of the United States or obtain exemption therefrom, and shall provide
evidence of any such action so taken to the Purchaser on or prior to the Closing
Date.

     5.7 Listing. Prior to any sale of the Purchased Stock by the Investor, the
Company will take all action necessary to enable the Purchased Stock to trade on
the Nasdaq Stock Market.

ARTICLE 6
MISCELLANEOUS

     6.1 Remedies. In case any one or more of the representations, warranties,
covenants or agreements set forth in this Agreement shall have been breached by
any party hereto, the party or parties entitled to the benefit of such covenants
or agreements may proceed to protect and enforce their rights either by suit in
equity or action at law, including, but not limited to, an action for damages as
a result of any such breach or an action for specific performance of any such
covenant or agreement contained in this Agreement. The rights, powers and
remedies of the parties under this Agreement are cumulative and not exclusive of
any other right, power or remedy which such parties may have under any other
agreement or law. No single or partial assertion or exercise of any right, power
or remedy of a party hereunder shall preclude any other or further assertion or
exercise thereof.

     6.2 Successors and Assigns. Except as otherwise expressly provided herein,
the terms and conditions of this Agreement shall inure to the benefit of and be
binding upon the respective successors and assigns of the parties. Except as
otherwise expressly provided herein, neither this Agreement nor any of the
rights or obligations hereunder may be assigned by either party without (a) the
prior written consent of the Company in the case of any assignment by the
Investor, or (b) the prior written consent of the Investor in the case of any
assignment by the Company, except in each case to any third party who acquires
all or substantially all of the business of the assigning party by merger, sale
of assets or otherwise. Notwithstanding the foregoing, upon prior written notice
to the Company, the Investor may assign the right and obligation to purchase the
Purchased Stock for the Purchase Price, and all of its other rights and
obligations hereunder, to any of its Affiliates; provided that the Investor
shall remain liable for the performance of the obligations such Affiliate.

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     6.3 Entire Agreement. This Agreement contains the complete understanding of
the parties to this Agreement with respect to the subject matter hereof and
supersedes all prior understandings and writings relating to the subject matter
hereof.

     6.4 Governing Law; Submission to Jurisdiction. This Agreement shall be
governed by and construed in accordance with the laws of the State of New York,
without regard to conflict of laws principles. Each of the parties irrevocably
and unconditionally consents to the exclusive jurisdiction of the courts of the
State of New York, and the United States District Court for the Southern
District of New York for any action, suit, or proceeding arising out of or
relating to this Agreement, waives any objections to such jurisdiction and venue
and agrees not to commence any action, suit or proceeding relating to this
Agreement except in such courts.

     6.5 Counterparts. This Agreement may be executed in counterparts, each of
which shall be deemed an original but which together shall constitute one and
the same instrument.

     6.6 Titles and Subtitles. The titles and subtitles used in this Agreement
are used for convenience only and are not to be considered in construing or
interpreting this Agreement.

     6.7 Notices. All notices, instructions and other communications hereunder
or in connection herewith shall be in writing, shall be sent to the address of
the relevant party set forth below and shall be (a) delivered personally, (b)
sent by registered or certified mail, return receipt requested, postage prepaid,
(c) sent via a reputable nationwide overnight courier service, or (d) sent by
facsimile transmission, with a confirmation copy to be sent by registered or
certified mail, return receipt requested, postage prepaid. Any such notice,
instruction or communication shall be deemed to have been delivered upon receipt
if delivered by hand, three (3) Business Days after it is sent by registered or
certified mail, return receipt requested, postage prepaid, one (1) Business Day
after it is sent via a reputable nationwide overnight courier service, or when
transmitted with electronic confirmation of receipt, if transmitted by facsimile
(if such transmission is made during regular business hours of the recipient on
a Business Day; or otherwise, on the next Business Day following such
transmission). Either party may change its address by giving notice to the other
party in the manner provided above.

      To the Company:   Regeneron Pharmaceuticals, Inc.     777 Old Saw Mill
Road     Tarrytown, NY 10591     Attention: General Counsel       With a copy
(which shall not constitute notice) to:   Skadden, Arps, Slate, Meagher & Flom
LLP
4 Times Square
New York, NY 10036
Attention: David J. Goldschmidt, Esq.

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      To the Investor:   Aventis Inc.     Somerset Corporate Center     300
Somerset Corporate Boulevard     Bridgewater, NJ 08807     Attention: Owen Ball,
                     Senior Corporate Counsel           and:           Aventis
Pharmaceuticals Inc.     200 Crossing Boulevard     Bridgewater, New Jersey
08807     Attention: Vice President,                       Legal Corporate
Development       With a copy (which shall   Morgan, Lewis & Bockius, LLP not
constitute notice) to:   502 Carnegie Center     Princeton, New Jersey 08540    
Attention: Randall B. Sunberg, Esq.

     6.8 Expenses. Each party shall pay its own fees and expenses with respect
to this Agreement and the transactions contemplated hereby.

     6.9 Amendments and Waivers. Any term of this Agreement may be amended and
the observance of any term of this Agreement may be waived (either generally or
in a particular instance and either retroactively or prospectively), only with
the written consent of the Company and the Investor.

     6.10 Severability. If, under applicable laws, any provision hereof is
invalid or unenforceable, or otherwise directly or indirectly affects the
validity of any other material provision(s) of this Agreement (“Severed
Clause”), then, it is mutually agreed that this Agreement shall endure except
for the Severed Clause. The parties to this Agreement shall consult and use
their reasonable best efforts to agree upon a valid and enforceable provision
which shall be a reasonable substitute for such Severed Clause in light of the
intent of this Agreement.

[Signature Page Follows]

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     IN WITNESS WHEREOF, the parties have executed and delivered this Agreement
as of the date first above written.

              AVENTIS PHARMACEUTICALS INC.               By:   /s/ Gerald P.
Belle        

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              Name: Gerald P. Belle               Title:   Authorized Signatory
              REGENERON PHARMACEUTICALS, INC.               By:   /s/ Stuart
Kolinski        

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      Name: Stuart Kolinski               Title:   General Counsel