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Potency of nucleoside analogs against Powassan virus replication and identification of 4'-fluorouridine as a therapeutic candidate for tick-borne orthoflavivirus encephalitis

Lewis, D. S. M.; Moore, T.; Patel, R.; Cox, R.

2026-09-25 microbiology
10.64898/2026.09.24.754293 bioRxiv
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Powassan virus (POWV) is an emerging tick-borne orthoflavivirus that causes severe encephalitis, long-term neurologic sequelae, and death, yet no approved antiviral therapies exist. Here, we evaluated several previously characterized nucleoside analogs as candidate countermeasures for POWV encephalitis. Favipiravir, molnupiravir, 4'-fluorouridine (4'-FlU), and GS-441524 inhibited POWV replication in vitro and demonstrated comparable activity against West Nile virus (WNV). We established C3H/HeNCrl mice as a stringent POWV disease model, showing increased susceptibility, higher day 7 brain titers, and more pronounced neurologic signs resembling severe human disease, compared with C57BL/6 mice. In prophylactic and therapeutic studies, 4'-fluorouridine was the most efficacious compound tested, significantly reducing brain viral burden even when treatment was delayed. Direct comparison with molnupiravir, favipiravir, and GS-441524 in mice confirmed superior survival benefit and antiviral activity for 4'-FlU against both POWV and WNV. These data identify 4'-fluorouridine as a promising therapeutic candidate for neurotropic flavivirus disease.

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