– K777 is a potent, orally active, and irreversible inhibitor of cysteine protease, functioning as a potent CYP3A4 inhibitor (IC50 = 60 nM) and a selective CCR4 antagonist, which inhibits chemotaxis. K777 irreversibly inhibits Cruzain, the major cysteine protease of Trypanosoma cruzi, and cathepsins B and L, targeting cathepsin-mediated cell entry and exhibiting broad-spectrum antiviral activity. It inhibits EBOV and SARS-CoV pseudovirus entry with IC50 values of 0.87 nM and 0.68 nM, respectively.