SYNERGISTIC INHIBITION OF HIV-1 REVERSE-TRANSCRIPTASE BY COMBINATIONSOF CHAIN-TERMINATING NUCLEOTIDES

Synergistic inhibition of HIV replication in cell culture has been rep orted for many combinations of reverse transcriptase inhibitors. Howev er, the biochemical basis underlying this interaction is in most cases unknown. It has been previously shown that combinations of L-697,661 or U-90152s with AZT or ddC synergistically inhibit HIV-1 replication in cell culture. The combination of AZT with ddC is also favorable wit h respect to the inhibition of viral replication. However, the corresp onding combinations showed no synergy in inhibiting enzyme activity wh en tested on conventional polymerase assays using home-or heteropolyme ric RNA and DNA as template. Data obtained suggest that amplification of the effect of chain terminators, a consequence of the high potentia l number of termination sites present on the template, override the sy nergistic effect expected for the combination of two independent nucle otide analogs. When a saturating amount of enzyme over template:primer was used, and a single site on the template was available for each ch ain terminator, the combination of AZTTP and ddCTP synergistically inh ibited enzyme activity, whereas, as expected, the combination of AZTTP and ddTTP behaved as merely additive. Under similar conditions the co mbination of U-90152s and AZTTP was also synergistic. These results su ggest that synergy found in antiviral assays with combinations having nucleosidic inhibitors is not related to the synergistic inhibition of reverse transcriptase and might be due to the presence in the viral p opulation of virus strains with different sensitivity to the inhibitor s.