Viral RNA-dependent RNA polymerase inhibitor 7-deaza-2′-C-methyladenosine prevents death in a mouse model of West Nile virus infection

West Nile virus (WNV) is a medically important emerging arbovirus causing serious neuroinfections in humans against which no approved antiviral therapy is currently available. In this study, we demonstrate that 2'-C- methyl- or 4'-azido-modified nucleosides are highly effective inhibitors of WNV replication, showing nanomolar or low micromolar anti-WNV activity and negligible cytotoxicity in cell culture. One representative of C2'-methylated nucleosides, 7-deaza-2'-C- methyladenosine, significantly protected WNV-infected mice from disease progression and mortality. Twice daily treatment at 25 mg/kg starting at the time of infection resulted in 100% survival of the mice. This compound was highly effective, even if the treatment was initiated 3 days post-infection, at the time of a peak of viremia, which resulted in a 90% survival rate. However, the antiviral effect of 7-deaza-2'-C- methyladenosine was absent or negligible when the treatment was started 8 days post-infection (i.e., at the time of extensive brain infection). The 4'-azido moiety appears to be another important determinant for highly efficient inhibition of WNV replication in vitro. However, the strong anti-WNV effect of 4'-azidocytidine and 4'-azido-aracytidine was cell type-dependent and observed predominantly in PS cells. The effect was much less pronounced in Vero cells. Our results indicate that 2'-C- methylated or 4'-azidated nucleosides merit further investigation as potential therapeutic agents for treating WNV infections, as well as infections caused by other medically important flaviviruses.