Thiol-dependent cathepsins: pathophysiological implications and recent advances in inhibitor design

Thirteen papain-like cysteine proteases (cathepsins) are coded in the human genome from which two represent pseudogenes. Initially it was believed that lysosomal cysteine proteases primarily fulfill house-keeping functions which would exclude them as potential drug targets. Within the last decade, this view has dramatically changed and highly specific and therapeutically relevant functions have been assigned to individual cathepsins. Cathepsins are critical for osteoclast-mediated bone resorption and cartilage erosion in arthritis. They are involved in various aspects of immune responses, in the development and proliferation of various cell types, as well as in tumor invasion and metastasis. Cathepsins qualify as pharmaceutical targets for the treatment of osteoporosis, arthritis, asthma, autoimmune diseases, and potentially for certain forms of cancer. The major challenge in using cysteine protease inhibitors will be the design of highly selective, potent, and bioavailable compounds. Emerging novel functions of long-known and recently discovered cathepsins will require more emphasis on the selectivity of drug candidates to avoid adverse side effects. This review will focus on the discussion of presently known functions of papain-like cathepsins and on recent advances in the design of cysteine protease inhibitors.