Delayed treatment with combinations of antiviral drugs in mice infected with herpes simplex virus and application of the median effect method of analysis

Mice were inoculated intracerebrally with a lethal dose of herpes simplex virus type 2. Three days later, the mice were treated intraperitoneally, twice daily for 4 days, with the following drugs alone or in combination: acyclovir (ACV), vidarabine (ara-A), 2'-fluoro-5-iodoaracytosine (FIAC), and 2'-fluoro-5-methylarauracil (FMAU). Despite delayed treatment, most of the animals receiving low doses of FMAU alone or in combination with ACV or ara-A survived. In contrast, significantly higher mortality rates were noted in mice receiving ara-A, ACV, or FIAC alone. The data were analyzed for quantitation of synergism, additivity, and antagonism of multiple drug effect by the median effect method. The median effective doses (in nanomoles per kilogram per day) calculated in this manner were: FMAU, 22.5; FIAC, 510; ara-A, 901; ACV, 7,587; ACV-ara-A (drug ratio, 1:1), 550; FIAC-ara-A (1:1), 376; FIAC-ACV (1:1), 133; FMAU-ACV (1:8), 60.3; and FMAU-ara-A (1:8), 65.2. Marked synergy was found throughout a wide range of effect levels with the five different combinations, with no increased toxicity over the single-drug treatments. Similar results were obtained when the data were analyzed by the isobologram method. Since many patients with severe herpetic infections, such as herpes encephalitis, have a poor prognosis despite single-drug therapy, the possible use of combinations including low doses of FMAU deserves further investigation.