Synergistic Inhibition of Human Immunodeficiency Virus Replication In Vitro by Combinations of 3′-Azido-3′-Deoxythymidine and 3′-Fluoro-3′-Deoxythymidine

Abstract

The antiviral activity against human immunodeficiency virus type 1 of the two structurally related thymidine analogs azidothymidine and fluorothymidine, both alone and in combination, was tested. Fluorothymidine was tenfold more active than azidothymidine. The selectivity indices of the two compounds were similar. The combination of azidothymidine and fluorothymidine showed clearly synergistic antiviral activity, and diminished cytotoxicity. The inhibition of reverse transcriptase from human immunodeficiency virus type 1 by the triphosphates of azidothymidine and fluorothymidine, both alone and in combination was also tested. Azidothymidine triphosphate was a fourfold stronger inhibitor than fluorothymidine triphosphate. The combination of the two showed only additive (and not synergistic) effects upon reverse transcriptase. The combination of azidothymidine and fluorothymidine showed both synergistic antiviral activity and diminished cytotoxicity, and may therefore represent a promising therapeutic strategy. The additive (and not synergistic) inhibition of reverse transcriptase by the combination of the triphosphates indicates that in cell culture additional factors other than inhibition of the reverse transcriptase by the triphosphates influence the antiviral activity of the combination. Such factors might include effects upon normal nucleoside metabolism or metabolism of the analogs. Alternatively, one of the nucleosides might have an additional mechanism of action besides inhibition of the reverse transcriptase.

Metabolism of 3′-Azido-3′-Deoxythymidine and 3′-Fluoro-3′-Deoxythymidine in Combination in Human Immunodeficiency Virus Infected Lymphoblastoid Cells

“…We previously showed that combinations of AZT-and FLT triphosphates give only additive inhibition of RT (Harmenberg et al, 1990a). This, together with the results in this paper showing unchanged or reduced levels of intracellular triphosphates upon combination of AZT and FLT, suggests that the synergistic mechanism of the combination of AZT and FLT is not at the level of the RT/triphosphates.…”

“…We also showed the combination of AZT and FLT to be synergistic against HIV replication in cell culture (Harmenberg et al, 1990a). In this case the cytotoxicity of the combination was also greatly reduced, making it a most promising combination.…”

“…These results have bearing upon the observed synergy between AZT and FLT (Harmenberg et al, 1990a). In infected H9 cells, combining the compounds together in this ratio does not appear to significantly affect the metabolism of either compound.…”

“…This method of preparing cell extracts for HPLC has previously been shown to give excellent recovery of nucleotides (Harmenberg et al, 1990b). AZT and FLT were used at therapeutically relevant concentrations of about two-to three-fold the IC so value, The combination of AZT and FLT used was at a ratio and concentration previously shown to give synergistic inhibition of HIV replication (Harmenberg et al, 1990a).…”

Synergistic Inhibition of Primary Isolates of Human Immunodeficiency Virus Type 1 by Combinations of 3′-Fluoro-3′-Deoxythymidine and 2′,3′-Dideoxyinosine

“…The aim of combination therapy is to use a synergistic combination of drugs which can be given in low doses, thus reducing the risk of toxicity Whilst maintaining or even improving the antiviral effect compared to the separate drugs. Certain combinations of antiviral drugs have been shown to give synergistic inhibition of HIV-1 replication in vitro, among them: AZT and phosphonoformate (Koshida et al, 1989b); 3' -fluoro-3/-deoxythymidine (FL1) and phosphonoformate (Koshida et al, 1989a); AZT and FLT (Harmenberg et al, 1990); 2' ,3' -dideoxyinosine (ddl) and AZT (Dornsife et al, 1991); ribavirin and ddl (Saba et al, 1987); and 2,',3'-dideoxythymidine and AZT (Kong et al, 1991). In the case of AZT and FLT, the combination also showed reduced cytotoxicity compared to the separate drugs, which is an added advantage (Harmenberg et aI., 1990). However, these studies have been performed using laboratory strains of virus adapted for growth in established cell lines, which may not mirror the clinical situation closely.…”