A novel
carbocyclic thymidine analog,
N-methanocarbathymidine [(
N)-MCT], was evaluated for inhibition of
orthopoxvirus infections. Efficacy in vitro was assessed by plaque reduction assays against wild-type and
cidofovir-resistant strains of
cowpox and vaccinia viruses in nine different cell lines. Minimal differences were seen in
antiviral activity against wild-type and
cidofovir-resistant viruses. (
N)-MCT's efficacy was affected by the cell line used for assay, with 50% poxvirus-inhibitory concentrations in cells as follows: mouse=0.6-2.2 microM, rabbit=52-90 microM, monkey=87 to >1000 microM, and human=39-220 microM. Limited studies performed with
carbocyclic thymidine indicated a similar cell line dependency for
antiviral activity. (
N)-MCT did not inhibit actively dividing uninfected cells at 1000 microM. The potency of (
N)-MCT against an S-variant
thymidine kinase-deficient vaccinia virus was similar to that seen against S-variant and wild-type viruses in mouse, monkey, and human cells, implicating a cellular
enzyme in the phosphorylation of the compound. Mice were intranasally infected with
cowpox and vaccinia viruses followed 24h later by intraperitoneal treatment with (
N)-MCT (twice a day for 7 days) or
cidofovir (once a day for 2 days). (
N)-MCT treatment at 100 and 30 mg/kg/day resulted in 90 and 20% survival from cowpox virus
infection, respectively, compared to 0% survival in the placebo group. Statistically significant reductions in lung virus titers on day 5 occurred in 10, 30, and 100mg/kg/day treated mice. These same doses were also active against a lethal vaccinia virus (WR strain) challenge, and protection was seen down to 10mg/kg/day against a lethal vaccinia virus (IHD strain)
infection.
Cidofovir (100mg/kg/day) protected animals from death in all three
infections.