Proteases regulate a broad spectrum of physiological functions by the specific processing of proteins and peptides. Elevated levels of active proteases can result in an array of physiological processes ultimately leading to disease states. Therefore, compounds designed to restore the natural equilibrium of proteases present excellent opportunities for drug candidates. Cysteine proteases, for example cathepsins B, L, and S, have been implicated in a number of diseases, including progressive cartilage and bone degradation associated with arthritis.1 Inhibitors of these cathepsins have reduced inflammation and prevented joint destruction in animal models of arthritis.2 Recently, human cathepsin 02 has been found to be highly expressed in osteoclastoma tissue.3 Cathepsins B and L have been linked to metastasis and invasion by cancer cells.4 The calcium-associated cysteine proteases calpains I and II have been associated with ischemia and hypoxia,5 Alzheimer's disease,6 and cataracts.7 Inhibition of the trypanosomal cysteine protease cruzain has proven effective in models of Chagas's disease.8Reversible inhibitors of cysteine proteases include peptide aldehydes,9 nitriles,10 and a-ketocarbonyl compounds.11 Irreversible inhibitors include peptide halomethyl ketones,12 diazomethyl ketones,13 (acyloxy)methyl ketones,14 and ketomethylsulfonium salts,15 believed to alkylate the active site thiol by formal Sn2 displacement, either directly or through a proposed hemithioketal-episulfonium ion pathway. Other irreversible inhibitors include various epoxysuccinyl compounds,16 whose oxiranes are opened through nucleophilic attack by the thiol. Previous work by Hanzlik and co-workers17 demonstrated peptide Michael acceptors as inactivators of the plant protease papain, which displayed second-order rate constants of inactivation from 0.05 to 70 M'1 s'1. However, to serve as disease modifying agents, inhibitors must inactivate target enzymes sufficiently in vivo as well as in vitro. Therefore, we designed potent inhibitors against disease-associated cysteine proteases such as the cathepsins B, L, S, and 02, calpains, and cruzain, concentrating on structures or scaffolds suitable for pharmaceutical development.
