BCX-4430 hydrochloride is a salt of an antiviral adenosine analog BCX4430 (Immucillin-A) that acts as a viral RNA-dependent RNA polymerase (RdRp) inhibitor. It was developed as a potential treatment for deadly filovirus infections such as Ebola virus disease and Marburg virus disease but also demonstrated broad-spectrum antiviral effectiveness against a range of other RNA virus families, including, bunyaviruses, arenaviruses, paramyxoviruses, and coronaviruses. Biochemical, reporter-based and primer-extension assays indicate that BCX4430 inhibits viral RNA polymerase function, acting as a non-obligate RNA chain terminator. BCX4430 inhibits infection of distinct filoviruses in human cells. Post-exposure administration of BCX4430 protects rodents against Ebola and Marburg virus disease and cynomolgus macaques from Marburg virus when administered as late as 48 hours after infection. BCX4430 is highly active in a Syrian golden hamster model of yellow fever, even when treatment is initiated at the peak of viral replication. BCX4430 also showed efficacy against Zika virus in a mouse model.