SIXTH AMENDMENT AGREEMENT

Exhibit 10.3

SIXTH AMENDMENT AGREEMENT

This Sixth Amendment Agreement, effective as of August 18, 2006 (the “Effective Date”), is entered into by and between the Institute of Organic Chemistry and Biochemistry of the Academy of Sciences of the Czech Republic, having offices at Flemingovo nam. 2, 166 10 Praha 6, Czech Republic (“IOCB”); and the K. U. Leuven Research and Development (representing the REGA Institute for Medical Research, Leuven), having offices at Groot Begijnhof 59, B-3000 Leuven, Belgium (“REGA”) (IOCB and REGA hereinafter collectively referred to as “IOCB/REGA”) on one side and Gilead Sciences, Inc., a Delaware, USA corporation, having offices at 333 Lakeside Drive, Foster City, California 94404, U.S.A. (“Gilead”), on the other side. In this Sixth Amendment Agreement IOCB, REGA and GILEAD are sometimes referred to individually as a “Party” and collectively as the “Parties”.

WITNESSETH:

WHEREAS, the Parties have entered into the License Agreement dated effective November 15, 1991 (the “1991 License Agreement”), relating to N-phosphonylmethoxyalkyl purines and pyrimidines and analogues including HPMPC as well as acyclic nucleotide analogues and other compounds; the License Agreement dated effective October 15, 1992 (the “1992 License Agreement”), relating to N-(3-fluoro-2-phosphonomethoxypropyl) derivatives of heterocyclic bases; and the License Agreement dated effective December 1, 1992 (the “1992 PMPA License Agreement”), relating to (R) PMPA and (R) PMPDAP and other compounds. The 1991 License Agreement, the 1992 License Agreement, and the 1992 PMPA License Agreement are referred to collectively as the “License Agreements”; and

WHEREAS, the License Agreements were first amended by the Amendment Agreement dated effective October 25, 1993; and further amended by Annex 1 on April 1, 1997, by Annex 2 on March 20, 2002, and by the Letter Agreement dated April 8, 2002 (the “First Amendment Agreement”); and

WHEREAS, the 1991 License Agreement was subsequently amended by the Second Amendment Agreement dated effective January 1, 1996 (the “Second Amendment Agreement”); and further amended by the Third Amendment Agreement dated effective April 23, 2002 (the “Third Amendment Agreement”); and the License Agreements were further amended by the Fourth Amendment Agreement dated effective August 1, 2004 (the “Fourth Amendment Agreement”), and by the Fifth Amendment Agreement dated effective January 1, 2006 (the “Fifth Amendment Agreement”);

WHEREAS, the 1991 License Agreement and the 1992 PMPA License Agreement were further amended by the Letter Agreement dated effective December 26, 2000, as amended on December 27, 2000 (the “December 2000 Letter Agreement”); and

WHEREAS, the Parties now desire to amend the License Agreements, as previously amended, in order to further clarify certain payment provisions related thereto;

NOW, THEREFORE, the Parties agree as follows:

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IN WITNESS WHEREOF, the parties hereto have caused this Sixth Amendment Agreement to be executed by their respective duly authorized representatives as of the Effective Date.

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Exhibit A

The HIGH-INCOME TERRITORY

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Exhibit B

The UPPER MIDDLE-INCOME TERRITORY

The LOWER MIDDLE-INCOME TERRITORY

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Exhibit C

The ACCESS COUNTRIES

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