Synergistic inhibition of anatid herpesvirus replication by acyclovir and phosphonocompounds

Plaque reduction assays were used to evaluate the inhibitory effects of acyclovir (ACV), phosphonoacetate (PAA), and phosphonoformate (PFA) with a plaque-purified isolate of anatid herpesvirus (AHV-ppc3). A plaque assay employing a liquid overlay medium was developed to facilitate the drug inhibition studies. From dose-response curves, the ID50 for PAA, PFA, and ACV were 20, 12, and 0.14 micrograms/ml, respectively. From data obtained from combination dose-response curves, dose isobolograms were prepared and used to determine the types of interactions between drug pairs. Whereas the interaction between PFA and PAA was additive, synergism occurred with ACV and either PAA or PFA. Drug-resistant mutants of AHV-ppc3 resistant to 8.0 micrograms/ml ACV, 250 micrograms/ml PAA, 180 micrograms/ml PFA or 6.0 micrograms/ml AHV, and 220 micrograms/ml PAA were isolated.