AMENDMENT AGREEMENT
  IDENIX PHARMACEUTICALS, INC.
  IDENIX SARL
  SmithKline Beccham Corporation doing business as GLAXOSMITHKLINE
  UNIVERSITA DEGLI STUDI DI CAGLIARI
  PROF. PAOLO LA COLLA
  A)
    
    on 4 January 1999, the University and Idenix SARL, this last on behalf and for the benefit of
Idenix, entered into an agreement entitled Co-operative Antiviral Research Activity
Agreement (hereinafter referred to as the Co-operative Agreement) aimed at performing a
joint research activity in the antiviral substances field;B)
    
    in accordance with the Co-operative Agreement, on 14 December 2000, the University and Idenix
entered into a license agreement (hereinafter the License Agreement) according to which the
University grants to Idenix the exclusive license for the exploitation, whether direct or
indirect, of the results obtained and that will be obtained from the performance of the joint
research activity under the Co-operative Agreement;
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C)
    
    the above mentioned Cooperative Agreement and License Agreement (together, the Original
Agreements) have been initially amended on 10 April 2002 (the April 10, 2002 Amendment);D)
    
    the Original Agreements, as so amended, have been afterwards amended with the deed
undersigned by the parties on 8 May 2003, also in consideration of the transactions at that
time pending between Idenix and Novartis Pharma AG for the acquisition, by this last, of the
majority of the shareholding of Idenix (the amendments indicated in this point have become
effective on 8 May 2003, following to the occurred execution of a Development, License and
Commercialization Agreement and of a Manufacturing and Supply Agreement by and among
Idenix, Idenix (Cayman) Limited and Novartis) (together, the Novartis Agreements); the deed
undersigned by the parties on 8 May 2003 provided, in favour of Novartis, the right to cure
possible breaches of the Original Agreements by Idenix;E)
    
    the parties further amended the Original Agreements, as previously amended, on 30 June 2004
(the June 30, 2004 Letter Amendment) and on 24 October 2005 (the October 24, 2005
Amendment). The Original Agreements, as amended through and including the April 10, 2002
Amendment, the June 30, 2004 Amendment, and the October 24, 2005 Amendment shall be
collectively defined the Cagliari Agreements, provided that the Cagliari Agreements do not
include the Novartis Agreements;F)
    
    on 30 June 2004 the University and Prof. La Colla executed an agreement (the June 30, 2004
Assignment Agreement) called assignment of the intellectual property rights deriving from
the execution of the Co-operative Antiviral Research Activity Agreement;G)
    
    on 22 December 2006 the parties have extended the validity of the Co-operative Agreement up
to 22 January 2011 (the December 22, 2006 Amendment);H)
    
    on 03 December, 2008 Idenix and Prof. Paolo La Colla executed an agreement (hereinafter
[**] Agreement, which is attached herewith under Enclosure B);I)
    
    Idenix is currently carrying on negotiations with GSK in order to define an agreement
relating to the granting of exclusive licenses to non-nucleoside reverse transcriptase
inhibitor compounds owned and/or controlled by Idenix and namely:
 (i)
    
    an exclusive license, from Idenix to GSK, of the [**]
patents covering inventions developed within the Co-operative Agreement and
listed in Enclosure A to this agreement, including future patents related to
the family of patents or compounds mentioned under such Enclosure A;(ii)
    
    an exclusive license, from Idenix to GSK, of the [**]
patents listed in Enclosure B to this agreement and as the subject of the
[**] Agreement, including future patents related to the family of
patents or compounds mentioned under such Enclosure B; (collectively the GSK
Agreement);
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J)
    
    art. 2.2 of the Licence Agreement allows Idenix (formerly Novirio) to grant sublicenses
under the license granted pursuant to Section 2.1 above to parties with whom Novirio or its
Affiliates has agreed to jointly develop or commercialise Licensed
Products;K)
    
    in the light of the negotiations currently pending between Idenix and GSK;
 (i)
    
    Idenix, Idenix SARL and the University hereby execute this
amendment to the License Agreement and the Co-operative Agreement (to the
extent that this latter refers to the licence granted by the University under
art. 9.1 to Idenix) which shall be effective only if and when Idenix and GSK
shall have executed the agreement mentioned under previous recital (I) and;(ii)
    
    Idenix and Prof. La Colla hereby execute an amendment to the
[**] Agreement which shall be effective only if and when Fdenix and
GSK shall have executed the agreement mentioned under previous recital (I),
   1)
    
    The recitals and the Enclosures to this agreement shall represent an integral and substantial
part of this agreement (hereinafter the Amendment).2)
    
    This Amendment shall represent an amendment to the License Agreement (as amended), the
Co-operative Agreement (as amended) and to the [**] Agreement and shall become
automatically effective among the parties hereof, if and when Idenix and GSK shall have
executed the agreement mentioned under previous recital I, upon the Effective Date.3a)
    
    In relation to the provision mentioned under art. 2.2 of the License Agreement and article
9.1 of the Co-operative Agreement, the University hereby acknowledges and accepts that
 (i)
    
    Idenix is permitted to sublicense to GSK the patents, arising
under the Cooperative Agreement and the License Agreement, listed in Enclosure
A, including future patents related to the family of patents or compounds
mentioned under such Enclosure A, for development or commercialization of the
Licensed Products;(ii)
    
    such sublicense shall no longer be conditioned upon an
agreement between Idenix (formerly Novirio) or its Affiliates and GSK to
jointly develop or commercialize Licensed Products, and GSK shall be entitled
to develop and commercialize Licensed Products solely and on its own behalf in
accordance with the License Agreement;
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(iii)
    
    GSK is, in its turn, permitted to sublicense the patents
mentioned under the Enclosure A, including future patents related to the family
of patents or compounds mentioned under such Enclosure A, to its affiliates or
to third parties, solely in accordance with the agreement to be executed
between Idenix and GSK, provided that, for purposes of this art. 3a),
affiliates shall mean any corporation, company, partnership, joint venture
and/or firm that controls, is controlled by, or is under common control with
GSK.
    (i)
    
    the License Agreement or the Co-operative agreement will not
terminate in case Idenix or GSK demonstrate that the obligations have been
contracted or proven the fault was a case of force majeure or the same is
demonstrated by GSK within the Grace Term;(ii)
    
    the University of Cagliari shall provide written notice to GSK
by registered mail (in addition to Idenix and Novartis) of any breach or
default under the License Agreement or the Co-operative Agreement and with any
related correspondence that is delivered to Idenix and at the same time that of
delivery to Idenix.
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5)
    
    It is understood that, should the agreement between Idenix and GSK state the right of Idenix
to receive any up-front payment, milestone or other fixed amount (Fixed Amount) related to
such agreement and pertaining to both the [**] patents and
the [**] patents
licensed by Idenix to GSK, the University and Professor La Colla shall, with specific
reference to such Fixed Amount, receive from Idenix only the percentage provided under the
Cagliari Agreements, provided that Prof. La Colla shall not be entitled also to the
percentages provided under the [**] Agreement in relation to the Fixed Amount For
purposes of clarity, it is understood by the parties that if Idenix and GSK enter into the
agreement referenced herein and a payment is made by GSK to Idenix at the time of licensing
both the [**] patents and the [**] patents, only one payment will be made
to the University and Professor LaColla in relation to the [**] patents and the
[**] patents. If future milestone or royalty payments are made based on a specific
patent family, the respective agreement shall govern any such payment to be made to the
University and Professor La Colla.6)
    
    The provisions of article 8.2 of the Co-operative Agreement and article 5.1 of the License
Agreement are hereby expressly amended to permit GSK to file, prosecute and maintain patents
licensed to GSK by Idenix and mentioned under Enclosure A (including future patents related to
the family of patents or compounds mentioned under such Enclosure A) in lieu of Idenixs first
right to file, prosecute and maintain such patents, provided that the relevant expenses shall
be borne by Idenix or by GSK as may be agreed between these last two entities and the
University of Cagliari hereby permits and acknowledges such actions by GSK. It is understood
that, with reference to the patents listed under Enclosure A, such filing, prosecution and
maintenance shall be performed in the name and on behalf of Idenix and of University of
Cagliari.7)
    
    Consistently with the amendment of article 5.1 of the License Agreement above, GSK or any of
its affiliates or permitted sublicensees shall be permitted to exercise Idenix right to
institute an action against a third party for infringement of the patents licensed to GSK by
Idenix, including those mentioned under Enclosure A as Idenix designee and as may be agreed
between Idenix and GSK.8)
    
    To this aim, with the signature of the present document, the parties acknowledge that as of
the execution date, ao breaches of the License Agreement and of the [**] Agreements
have occurred.
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Idenix Pharmaceuticals, Inc.
 By:
    
    /s/ John F. WeidenbruchName: John F. WeidenbruchTitle: Executive Vice President and General Counsel
 21 January 2009Date
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Idenix SARL
 By:
    
    /s/ Jean-Pierre SommadossiName: Jean-Pierre SommadossiTitle: Gerant
 21 January 2009Date
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SmithKline Beecham Corporation
Doing business as GLAXOSMITHKLINE
 By:
    
    /s/ William J. MosherName: William J. MosherTitle: Vice President and Secretary
 21 January 2009Date
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Università degli Studi di Cagliari
 By:
    
    /s/ Adolfo LaiName: Prof. Adolfo LaiTitle:
 8 Gen. 2009Date
 Certifico io Ufficiale Rogante Sostituto
dellUniversità degli Studi di Cagliari che la firma,
apposta in mia presenza dal Prof. Adolfo Lai nuto a
Ottana il 28.07.1943, Pro Rettore dellUniversità di
Cagliari e della cui identità personale sono certo, è
autenticaLUfficiale Rogante Sostituto
Dr. Achile Filiberto Putzu/s/ Dr. Achile Filiberto Putzu                SEAL
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Prof. Paolo La Colla
 By:
    
    /s/ Paolo La CollaProf. Paolo La CollaTitle:Date:
    
    8 Gen. 2009
 Certifico io Ufficiale Rogante Sostituto
dellUniversità degli Studi di Cagliari che la firma,
apposta in mia presenza dal Prof. Paolo La Colla,
nato a La Maddalena il 14.08.1944, Professore
Orinario presso lUniversità di Cagliari e della cui
identità personale sono certo, è autenticaLUfficiale Rogante Sostituto
Dr. Achile Filiberto Putzu/s/ Dr. Achile Filiberto Putzu                 SEAL
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ENCLOSURE A
 [**]
 TypeIndenix Ref. No.
    
    Related/Country
    
    Serial No.
    
    Filed
    
    Title




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ENCLOSURE B
 [**]
 TypeIndenix Ref. No.
    
    Related/Country
    
    Serial No.
    
    Filed
    
    Title