Synergistic Inhibition of HIV-1 Reverse Transcriptase and HIV-1 Replication by Combining Trovirdine with AZT,ddl and ddC in Vitro

Trovirdine (LY300046·HCI) is a potent and selective non-nucleoside human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) inhibitor (Åhgren et al., Antimicrob Ag Chemother 39: 1329, 1995). Combinations of trovirdine with other RT inhibitors, AZT, ddC., ddl and their triphosphates, were studied as well as the pyrophosphate analogue PFA in both cell-free HIV-1 polymerase assays and HIV-1-infected MT-4 cell cultures. Synergistic effects and weak synergism were observed both using RT and HIV-1 - infected cells and using different HIV-1 RT mutants and HIV-1 drug-resistant variants known to be resistant to the inhibitory effects of trovirdine. The best combination with substantial synergism was ddC-TP and trovirdine at a 20:1 molar ratio combination in a cell-free enzyme assay. This combination showed the weak synergy in MT-4 cells. Synergism was judged by the median-effect method. The inhibitory effect of trovirdine was independent of increased concentrations of AZT triphosphate and ddC triphosphate implying that trovirdine acts in a mutually exclusive manner with AZT-TP and ddC-TP as determined by the Dixon plot. The combination effects were expressed by the combination index (Cl) using end points of 50%, 70% and 90% inhibition of HIV-1 RT activity and HIV-1 replication in MT-4 cells.