A potent cathepsin L inhibitor (IC50 = 0.85 nM); selective for cathepsin L over calpain II and cathepsin B (IC50s = 184, and 85.1 nM, respectively); trypanocidal against T. brucei (ED50 = 45 nM); completely suppresses osteoclastic pit formation on femur slices isolated from bovine cortical bone at a concentration of 1 μg/ml; inhibits bone loss in a mouse model of osteoporosis in a dose-dependent manner