Lack of synergy in the inhibition of HIV-1 reverse transcriptase by combinations of the 5'-triphosphates of various anti-HIV nucleoside analogs.

3'-Deoxy-3'-azidothymidine (AZT) has been shown to synergistically inhibit the replication of human immunodeficiency virus type 1 (HIV-1) in cell culture when combined with several other 2',3'-dideoxynucleoside analogs. In an effort to understand the biochemical mechanism of this synergy, we have examined the effect of combinations of the 5'-triphosphate of AZT (AZT-TP) with either ddCTP, ddATP, or the 5'-triphosphate of the carbocyclic analog of 2',3'-didehydro-2',3'-dideoxyguanosine (carbovir) on both the RNA-directed and DNA-directed DNA polymerase activity of HIV-1 reverse transcriptase. Kinetic studies, which evaluated the ability of these combinations to competitively inhibit the enzyme, showed that AZT-TP could not bind to the enzyme with either the RNA or DNA template at the same time as either of the other three inhibitors. None of these analogs could affect the incorporation of another analog into the DNA chain by the HIV-1 reverse transcriptase. These results indicated that synergistic inhibition of the HIV-1 reverse transcriptase is not responsible for the synergistic antiviral activity seen in cell culture with combinations of these nucleoside analogs.