Cathepsin K is a member of the papain cysteine proteinase family and has been identified as the predominant proteinase responsible for the resorption of the bone matrix. The enzyme cleaves proteins such as collagen type I, collagen type II and osteonectin and therefore plays a role in bone remodeling and resorption in diseases such as osteoporosis, osteolytic bone metastasis and rheumatoid arthritis (Bromme and Okamoto, 1995; Drake, F. et al 1996; Bossard et al, 1996).