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GS-9160 is a novel and potent inhibitor of human immunodeficiency virus type 1 (HIV-1) integrase (IN) thatspecifically targets the process of strand transfer. It is an authentic inhibitor of HIV-1 integration, since treatment of infected cells results in an elevation of two-long terminal repeat circles and a decrease of integration junctions. GS-9160 has potent and selective antiviral activity in primary human T lymphocytes producing a 50% effective concentration (EC 50 ) of ϳ2 nM, with a selectivity index (50% cytotoxic concentration/ EC 50 ) of ϳ2,000. The antiviral potency of GS-9160 decreased by 6-to 10-fold in the presence of human serum. The antiviral activity of GS-9160 is synergistic in combination with representatives from three different classes of antiviral drugs, namely HIV-1 protease inhibitors, nonnucleoside reverse transcriptase inhibitors, and nucleotide reverse transcriptase inhibitors. Viral resistance selections performed with GS-9160 yielded a novel pattern of mutations within the catalytic core domain of IN; E92V emerged initially, followed by L74M. While E92V as a single mutant conferred 12-fold resistance against GS-9160, L74M had no effect as a single mutant. Together, these mutations conferred 67-fold resistance to GS-9160, indicating that L74M may potentiate the resistance caused by E92V. The pharmacokinetic profile of GS-9160 in healthy human volunteers revealed that once-daily dosing was not likely to achieve antiviral efficacy; hence, the clinical development of this compound was discontinued.After human immunodeficiency virus type 1 (HIV-1) entry and uncoating, the viral RNA is reverse transcribed by the viral reverse transcriptase into a double-stranded linear DNA. Both ends of this linear DNA are then processed at the 3Ј termini by the integrase (IN) enzyme. Specifically, IN removes a dinucleotide from each 3Ј terminus through a reaction referred to as 3Ј processing. The IN-DNA complex is then transported into the nucleus where IN performs concerted integration of both viral DNA ends into host chromosomal DNA by a reaction referred to as strand transfer. The integration of viral DNA into host chromosomal DNA is essential for HIV-1 replication, making the inhibition of HIV-1 IN function an attractive antiviral strategy (9,35,36,42).Historically, treatment of individuals infected with HIV-1 has relied on agents targeting two of the viral enzymes, reverse transcriptase and protease. Despite important clinical results achieved through the use of combinations of these agents, the continuous emergence of drug resistance remains a significant problem which fuels the need to discover novel drugs targeting other steps of the HIV-1 life cycle.

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