A cell-permeable calpain inhibitor. Inactivates calpain I (ID50 = 52 nM), calpain II (ID50 = 34 nM), and papain (ID50 = 138 nM). Promotes neurite elongation in differentiating PC12 cells. Prevents Ca2+-ionophore induced degradation of actin binding protein and P235 in platelets. Inhibits myosin light chain phosphorylation in platelets stimulated by collagen, ionomycin, or thrombin. Inhibits the growth of estrogen receptor positive breast cancer cells.