GC376 is an inhibitor of the main protease Mpro (3CLpro) in coronaviruses as well as picornaviruses. This broad-spectrum antiviral compound has been used as investigational veterinary drug for treatment of feline infectious peritonitis virus (FIPV) infections. Recent studies also showed that GC376 inhibits the main protease Mpro (3CLpro) from SARS-CoV-2 with IC50 of 0.03 μM and EC50 of 3.37 μM.