Favipiravir (T-705) Protects IFNAR^-/- Mice against Lethal Zika Virus Infection in a Sex-Dependent Manner

Zika virus (ZIKV), a member of the Flaviviridae family, is an important human pathogen that has caused epidemics in Africa, Southeast Asia, and the Americas. No licensed treatments for ZIKV disease are currently available. Favipiravir (T-705; 6-fluoro-3-hydroxy-2-pyrazinecarboxamide) and ribavirin (1-(β-D-Ribofuranosyl)-1H-1,2,4-triazole-3-carboxamide) are nucleoside analogs that have exhibited antiviral activity against a broad spectrum of RNA viruses, including some flaviviruses. In this study, we strengthened evidence for favipiravir and ribavirin inhibition of ZIKV replication in vitro. Testing in IFNAR^-/- mice revealed that daily treatments of favipiravir were sufficient to provide protection against lethal ZIKV challenge in a dose-dependent manner but did not completely abrogate disease. Ribavirin, on the other hand, had no beneficial effect against ZIKV infection in this model and under the conditions examined. Combined treatment of ribavirin and favipiravir did not show improved outcomes over ribavirin alone. Surprisingly, outcome of favipiravir treatment was sex-dependent, with 87% of female but only 25% of male mice surviving lethal ZIKV infection. Since virus mutations were not associated with outcome, a sex-specific host response likely explains the observed sex difference.