TMPRSS2 and COVID-19: The primary mode of action of umifenovir is to inhibit viral attachment by binding to envelope protein.414 Camostat mesylate, an orally available serine protease inhibitor, is a potent inhibitor of TMPRSS2 and has been hypothesized as a potential antiviral drug against COVID-19, by inhibiting virus-cell membrane fusion and hence SARS-CoV-2 replication.259,415,416 Nafamostat mesylate, which is FDA-approved for indications unrelated to coronavirus infection, inhibits viral entry with roughly 15-fold higher efficiency than camostat mesylate, but requires intravenous dosing.417