Effect of combinations of acyclovir with vidarabine or its 5'-monophosphate on herpes simplex viruses in cell culture and in mice.

The combination of various concentrations of acyclovir and vidarabine or its 5'-monophosphate usually produced an additive interaction with various strains of herpes simplex virus types 1 and 2 in Vero cells. Similarly, certain combinations of these drugs were more effective than the individual drugs in decreasing the mortality and increasing the mean day of death of mice inoculated intracerebrally with herpes simplex virus type 2. Neither antagonism nor interference was noted for any of the in vitro or in vivo combinations. The increased antiviral activity was determined not to be secondary to toxic effects of the drugs. Although viruses resistant to either vidarabine or acyclovir developed readily in cell culture, no evidence of cross-resistance was obtained. Furthermore, in the presence of the two drugs, mutants resistant to vidarabine, acyclovir, or vidarabine/acyclovir could not be isolated. These findings suggest that combinations with these antivirals, which are currently being evaluated singly for the therapy of severe forms of herpetic infection, could prove clinically useful if increasing numbers of resistant viral strains are observed.