Potent, cell-permeable inhibitor of the Ca2+-dependent protease, calpain. Prevents collagen- and thrombin-induced platelet aggregation, probably by blocking calpain induced phospholipase C and thromboxane synthase activation. Potent cathepsin L inhibitor. Recently shown to preferentially inhibit a subset of protein-tyrosine phosphatases. Also inhibits SARS-CoV-2 Mproin vitro (IC50 = 10.7 μM) and inhibits viral replication.