KAY-2-41, a Novel Nucleoside Analogue Inhibitor of OrthopoxvirusesIn VitroandIn Vivo

Abstract

The availability of adequate treatments for poxvirus infections would be valuable not only for human use but also for veterinary use. In the search for novel antiviral agents, a 1=-methyl-substituted 4=-thiothymidine nucleoside, designated KAY-2-41, emerged as an efficient 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 virus (VACV), cowpox virus, and camelpox virus selected for resistance to KAY-2-41 revealed a nucleotide deletion(s) close to the ATP binding site or a nucleotide substitution close to the substrate binding site in the viral thymidine kinase (TK; J2R) gene. These mutations resulted in low levels of resistance to KAY-2-41 ranging from 2.7-to 6.0-fold and cross-resistance to 5-bromo-2=-deoxyuridine (5-BrdU) but not to cidofovir. The antiviral effect of KAY-2-41 relied, at least in part, on 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 1 in earlier studies.

Drug Development against Smallpox: Present and Future

“…However, only a mild resistance level was observed. In vivo studies showed protection (100% survival and no morbidity) in mice inoculated intranasally with VACV at a dose of 50 mg/kg once per day intraperitoneally (91).…”

“…The drug was highly effective in preventing mortality in mice inoculated intranasally with VACV or CPXV and treated intraperitoneally (87% survival for a dose of 1.5 mg/kg twice a day beginning 3 days postinfection) (90). More recently, KAY-2-41 (1=-carbon-substituted 4=-thiothymidine derivative) was shown to be efficient against VACV, CPXV, and CMLV in vitro (91). Its efficacy was greater than that of CDV but lower than that of BCV or tecovirimat.…”

An Overview of Antivirals against Monkeypox Virus and Other Orthopoxviruses

“…85 1′-Methyl-substituted 4′-thiothymidine nucleoside, designated KAY-2-41 (12), inhibited orthopoxviruses, including VACV, CPXV, and CMLV in the micromolar range (Figure 4). 86 KAY-2-41 (12) was also active against cidofovir (3)-or tecovirimat (1)-resistant orthopoxviruses. Resistance selection identified deletion/mutation in the viral thymidine kinase gene.…”

Efficacy of three key antiviral drugs used to treat orthopoxvirus infections: a systematic review

“…However, due to CDV's poor oral availability and nephrotoxicity, BCV has been of interest as a safer alternative. BCV consistently demonstrates higher potency and selectivity in vitro, exceeding CDV in cowpox virus (CPXV), vaccinia virus (VV), MPXV, VARV, ectromelia virus (ECTV) and camelpox virus (CMLV) challenges (33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43). BCV appears to be particularly efficacious against VARV, with EC50 values approximately 271-fold higher than CDV (43).…”