Oral Administration of the carboxylic acid ester prodrug NHC SN_9 protects mice from lethal Orthopoxvirus challenge: Implications for the Treatment of Monkeypox

Abstract The global resurgence of monkeypox (Mpox) since 2022 has highlighted an urgent need for effective antiviral therapeutics against orthopoxvirus infections. While vaccines provide prophylactic protection, therapeutic options remain limited, particularly for immunocompromised individuals. Previously, we demonstrated potent antiviral activity of N4-hydroxycytidine (NHC) analogs against coronaviruses, including SARS-CoV-2. In the present study, we evaluated the antiviral efficacy of one such analog, NHC conjugate with phenylpropionic acid – SN_9, against orthopoxviruses using Vaccinia virus (VACV) as a model. SN_9 demonstrated superior in vitro antiviral activity compared with EIDD-2801 (molnupiravir), significantly inhibiting VACV replication. In vivo, oral administration of SN_9 provided substantial protection in murine models of orthopoxvirus infection, achieving 90% survival in a sublethal infection model and 70% survival in a lethal challenge model. Animals receiving treatment showed a reduction in disease severity and faster weight recovery. Given the high genetic and antigenic similarity among orthopoxviruses, including VACV, variola virus, and monkeypox virus (MPXV), these findings suggest that SN_9 represents a promising candidate for the treatment and prophylaxis of Mpox. Competing Interest Statement The authors have declared no competing interest.