Abstract
Cathepsin K is a cysteine protease enzyme with high and specific expression in osteoclasts, the cells responsible for resorbing bone. Its discovery and the numerous lines of evidence that support a specialised role for cathepsin K in bone resorption are discussed. Inhibition of this enzyme would be anticipated to prevent bone resorption, and be of potential utility in diseases involving excess bone loss, such as osteoporosis. The pharmaceutical companies known to be involved in developing inhibitors of cathepsin K, and their progress to date, are described.