Molnupiravir inhibits Bourbon virus infection and disease-associated pathology in mice

Bourbon virus (BRBV) is an emerging tick-borne virus that can cause severe and fatal disease in humans. BRBV is vectored via the Amblyomma americanum tick, which is widely distributed throughout the central, eastern, and southern United States. Serosurveillance studies in Missouri and North Carolina identified BRBV-neutralizing antibodies in approximately 0.6% of tested individuals. To date, no specific antiviral therapy exists. Molnupiravir, an antiviral drug with oral bioavailability, has shown broad-spectrum antiviral activity against RNA viruses, including SARS-CoV-2. Here, we investigated the antiviral activity of molnupiravir against BRBV infection in cell culture and a mouse model of BRBV disease. Molnupiravir suppressed BRBV production in cells. In vivo, pre-exposure administration of molnupiravir protected susceptible type I interferon receptor knockout (Ifnar1-/-) mice against lethal BRBV infection. The protection by molnupiravir was associated with lower virus burden in mouse tissues, improvement of T cell (CD4+, CD8+) and B cell (follicular) profiles in the spleen, improvement of severe thrombocytopenia, and reduced pathology in the spleen and liver of BRBV-infected animals. Finally, therapeutic administration of molnupiravir starting 24 or 48 hours after infection ameliorated weight loss, clinical signs of disease, and lethality associated with BRBV infection. Overall, our experiments suggest that molnupiravir as a potential antiviral therapy for evaluation in humans with BRBV infections.