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13th International HIV Drug Resistance Workshop8–12 June 2004, Tenerife Sur-Costa Adeje, Canary Islands, Spain |
BACKGROUND: HIV-1 entry inhibitors represent a diverse new class of antiretroviral agents. Virus entry is a multi-step process involving several virus envelope proteins (gp120, gp41) and host cell receptors (CD4, CCR5, CXCR4). The cascade of protein–protein interactions and conformational changes that mediate virus entry represent novel targets that are functionally distinct from conventional enzymatic targets, i.e. reverse transcriptase and protease. Consequently, one might presume that resistance to entry inhibitors may differ significantly from that of protease and reverse transcriptase inhibitors, and may emerge via alternative mechanisms depending on the specific molecular interaction that is targeted.
METHODS: Phenotypic drug susceptibility testing was performed using an envelope pseudo-virus infectivity assay. Drug susceptibility was plotted as percent inhibition vs log10 drug concentration and defined based on the IC50 and percent inhibition at the highest drug concentration (max % inhibition).
RESULTS: We have formulated a model that describes different molecular mechanisms of entry inhibitor escape. Susceptibility to fusion inhibitors is best described by changes in the IC50. Resistant viruses display log-sigmoid inhibition curves with increased IC50 compared to susceptible viruses, analogous to protease and reverse transcriptase inhibitor resistance. The ability to inhibit 100% of virus replication at high concentrations of fusion inhibitors is consistent with a competitive mechanism of inhibition and escape. In contrast, resistance to inhibitors that block receptor or co-receptor attachment (competitively or allosterically) is often associated with a decreased max % inhibition (plateau) that reflects an uninhibited fraction. The inability to inhibit 100% of virus replication at high drug concentrations is consistent with a non-competitive mechanism of inhibition, that is, binding in the presence of drug. To support this model we will present susceptibility curves generated using envelope proteins derived from naturally occurring viruses that exhibit variation in max % inhibition as well as viruses that have been selected by exposure to entry inhibitors.
CONCLUSIONS: Resistance to entry inhibitors can occur by multiple molecular mechanisms that may be dependent on the mode of inhibition. Our preliminary data suggest that escape from inhibitors that block receptor or co-receptor binding may occur by acquiring the ability to bind and utilize receptor–inhibitor complexes.
PRESENTING AUTHOR: CJ Petropoulos
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2004-06-08
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