Mpox Virus is Inhibited By Nucleoside Analogues Including the Acyclic Phosphonates Tenofovir and Adefovir

Mpox virus (MPXV) is an orthopoxvirus that causes the human disease mpox, which is characterized by fever, myalgia, and formation of rashes and lesions, and which garnered worldwide attention due to a global outbreak in 2022. In response to the outbreak, the antivirals tecovirimat, cidofovir, and brincidofovir have been used as emergency treatment for mpox. However, because of drug resistance and toxicity risks with those compounds, there is still a need for additional antivirals to treat orthopoxvirus diseases. Since cidofovir is a nucleoside analogue, we investigated a selection of other such compounds for antiviral activity against orthopoxviruses. We developed in vitro screening assays using fluorescent strains of vaccinia virus (VACV) and modified vaccinia Ankara (MVA) to measure the antiviral potency of test compounds. We found that tenofovir alafenamide and adefovir dipixovil, both acyclic phosphonates, had strong potential combinations of anti-orthopoxvirus activity and low toxicity after testing them against MVA and VACV, with EC50 values in the single digit micromolar and nanomolar range, while other potential hits included trifluridine and two arabinosides. We then recapitulated the results with MPXV using a luciferase-based assay. These data reinforce the interest of repurposing nucleoside analogues as antivirals to treat poxvirus infections and provide a basis for high throughput screening and mechanistic and antiviral resistance studies.