Antimicrobial Agents and Chemotherapy vol:58 pages:27-37
The availability of adequate treatments for poxvirus infections would be valuable not only for human, but also for veterinary use. In the search for novel antiviral agents, the 1' -methyl-substituted 4' -thiothymidine nucleoside, designated KAY-2-41, emerged as a potent inhibitor of poxviruses. In vitro, KAY-2-41 was active in the micromolar range against orthopoxviruses (OPVs) and against the parapoxvirus orf. The compound preserved its antiviral potency against OPVs resistant to the reference molecule cidofovir. KAY-2-41 had no noticeable toxicity on confluent monolayers, but a cytostatic effect was seen on growing cells. Genotyping of vaccinia (VACV), cowpox and camelpox viruses selected for resistance to KAY-2-41 revealed nucleotide deletion(s), close to the ATP binding site, or nucleotide substitution, close to the substrate binding site, in the viral thymidine kinase (TK, J2R) gene. These mutations resulted in low levels of resistance towards KAY-2-41, ranging from 2.7 to 6.0-fold, and cross-resistance to 5-bromo-deoxyuridine (5-BrdU) but not to cidofovir. The antiviral effect of KAY-2-41 relied, at least in part, on the activation (phosphorylation) by the viral TK as shown through enzymatic assays. The compound protected animals from disease and mortality after a lethal challenge with VACV, reduced viral loads in the serum and abolished virus replication in tissues. In conclusion, KAY-2-41 is a promising nucleoside analogue for the treatment of poxvirus-induced diseases. Our findings warrant the evaluation of additional 1' -carbon-substituted 4' -thiothymidine derivatives as broad spectrum antiviral agents, since this molecule also showed antiviral potency against herpes simplex virus type 1 in earlier studies.