Exhibit 10.17.1  
 
First Amendment to the Investor Agreement 
dated as of December 20, 2007 
 
     This Amendment (“Amendment”), dated as of November 10, 2009, is by and
among sanofi-aventis (“sanofi-aventis”), a company organized under the laws of
France, with its principal headquarters at 174, avenue de France, 75013 Paris,
France, sanofi-aventis US LLC (“Sanofi US”), a Delaware limited liability
company with its headquarters at 55 Corporate Drive, Bridgewater, New Jersey
08807, Aventis Pharmaceuticals Inc. (“Aventis”), a Delaware corporation with its
headquarters at 55 Corporate Drive, Bridgewater, New Jersey 08807,
sanofi-aventis Amérique du Nord (the “Investor”), a société en nom collectif
organized under the laws of France, with its principal headquarters at 174,
avenue de France, 75013 Paris, France, and Regeneron Pharmaceuticals, Inc. (the
“Company”), a New York corporation with its principal place of business at 777
Old Saw Mill River Road, Tarrytown, New York 10591.
 
     WHEREAS, sanofi-aventis, Sanofi US, Aventis, the Investor, and the Company
(collectively, the “Parties”) entered into an Investor Agreement, dated as of
December 20, 2007 (the “Investor Agreement”); and 
 
     WHEREAS, the Parties now desire to amend the Investor Agreement as set
forth in this Amendment. 
 
     NOW, THEREFORE, in consideration of the premises and mutual agreements set
forth in the Investor Agreement and this Amendment and for other valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
Parties agree as follows:
 

1.      Definitions. Capitalized terms used herein and not otherwise defined in
this Amendment shall have the meanings ascribed to them in the Investor
Agreement.       2.   Changes to the definition of “Lock-Up Term”. The
definition of “Lock-Up Term” in the first sentence of Section 4.1 of the
Investor Agreement is hereby amended by replacing the words “fifth (5th)
anniversary” therein with the words “tenth (10th) anniversary.”       3.
Continuing Effect. Except as specifically modified in this Amendment, all of the
provisions of the Investor Agreement shall remain in full force and effect.   4.
Counterparts. This Amendment may be executed in counterparts, each of which
shall be deemed an original but which together shall constitute one and the same
instrument.

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Exhibit 10.17.1
 
     IN WITNESS WHEREOF, the Parties have executed and delivered this Amendment
in accordance with Section 7.5 of the Investor Agreement as of the date first
above written. 
 

SANOFI-AVENTIS     By:   /s/ José Ferrer Name: José Ferrer Title: VP, Legal
Operations     By: /s/ Christian Blin Name: Christian Blin Title: VP, R&D
Finance       SANOFI-AVENTIS US LLC     By: /s/ John M. Spinnato Name: John M.
Spinnato Title: VP & General Counsel, US Legal     By: /s/ Christian Blin Name:
Christian Blin Title: VP, R&D Finance     AVENTIS PHARMACEUTICALS INC.     By:
/s/ John M. Spinnato Name: John M. Spinnato Title: VP & General Counsel, US
Legal     By: /s/ Christian Blin Name: Christian Blin Title: VP, R&D Finance

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Exhibit 10.17.1
 

SANOFI-AVENTIS AMÉRIQUE DU NORD       By:   /s/ José Ferrer Name: José Ferrer
Title: VP, Legal Operations     By: /s/ Christian Blin Name: Christian Blin
Title: VP, R&D Finance     REGENERON PHARMACEUTICALS, INC.     By: /s/ Murray A.
Goldberg Name: Murray A. Goldberg Title: Senior Vice President, Finance &    
Administration and Chief Financial Officer

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