Mark Deninson - May 20, 2020

Mark Denison, MD, Vanderbilt University, "Targeting the coronavirus replicase for antivirals including remdesivir"

They’ve been working on Coronavirus biology for the last 20 years. The phylogeny tree shows the diversity; Bt-SHC014 and Bt-WIV1 are capable of infecting human cells. Broad diversity with capability to infect humans, and time to develop vaccines brings the need for antivirals. Their goals: Broadly active against diverse coronaviruses, high barrier resistance, extended therapeutic window for prevention and decrease transmission/oral administration. They are interested in direct acting antivirals (DAA’s) (other alternatives are monoclonal antibodies, host-directed therapies and vaccines). Coronaviruses are RNA viruses and replicate in host cytoplasm (obligate intracellular replication). The virus binds to ACE2 (different viruses, different entry mechanisms). The RNA genome is uncoated inside the cell, the genome is translated, and viral proteins are made. Then there is replicase assembly and the virus assembles, exits without damaging the cell. Endosomal proteases, entry, replication and exit mechanisms are all potential targets for antivirals. One important potential target is viral RNA-polymerases; they are structurally conserved, and they’re already known targets. Nucleosides and base analogs are efficacious antivirals against DNA and RNA polymerases. In their experiences, they didn’t really work: Coronaviruses are resistant to many nucleoside analog inhibitors. Coronaviruses encode Proofreading ExoN and remove mis-incorporated nucleotides and inactivating ExoN increases mutations and increases sensitivity to nucleosides. They discovered that Remdesivir potently inhibits CoV replication. It is a phosphoramidate prodrug of parental drug GS-441524. It inhibits RNA synthesis as a non-obligate chain terminator (terminates RNA synthesis). Then the issue is resistance; whether it selects for resistance and what the consequence of resistance would be. It looks like resistance is very unlikely. In an in vitro study, they showed that only two mutations were selected in the polymerase after 23 passages with Remdesivir. Remdesivir resistance mutations are less fit than WT in vitro and attenuated in vivo. Also, Remdesivir inhibits other CoV, both human and potential bat CoV. IV administration in early or before infection in mice, it decreases virus titer and improves lung pathologies; however, in later infection stage, it doesn’t improve lung pathologies or decrease mortality. They also show in non-peer reviewed data showing that it is also effective in macaques. The ultimate goal is combination therapies to increase efficacy and utilize oral administration.