The protein encoded by this gene is a lysosomal cysteine protease involved in bone remodeling and resorption. This protein, which is a member of the peptidase C1 protein family, is expressed predominantly in osteoclasts.
Cathepsin K is a protease, which is defined by its high specificity for kinins, that is involved in bone resorption. The enzyme's ability to catabolize elastin, collagen, and gelatin allow it to break down bone and cartilage. This catabolic activity is also partially responsible for the loss of lung elasticity and recoil in emphysema. Cathepsin K inhibitors show great potential in the treatment of osteoporosis. Cathepsin K is degraded by Cathepsin S, called Controlled Cathepsin Cannibalism.
Cathepsin K is also expressed in a significant fraction of human breast cancers, where it could contribute to tumor invasiveness. Mutations in this gene are the cause of pycnodysostosis, an autosomal recessive disease characterized by osteosclerosis and short stature. Cathepsin K has also been found to be over-expressed in glioblastoma.
That the expression of Cathepsin K is characteristic for some cancers and not others has been documented. Cathepsin K antibodies are marketed for research into expression of this enyzme by various cells.
Merck had a Cathepsin K inhibitor in Phase 3 clinical trials by the name of odanacatib for an expected indication of osteoporosis. On September 2, 2016, Merck announced they were discontinuing development of odanacatib after their own assessment of adverse events and an independent assessment showed increased risk of stroke in patients receiving odanacatib in Merck's Phase 3 clinical trials.
Velcura Therapeutics, Inc. has also developed a highly selective cathepsin K inhibitor named VEL-0230 and has been tested in human, rat, equine clinical trials. 
Medvir also has a highly selective cathepsin K inhibitor, named MIV-711, in phase I trial, targeting osteoarthritis, in the specific class of Disease Modifying Osteoarthritis Drugs (DMOADs).
^Asagiri M, Hirai T, Kunigami T, Kamano S, Gober HJ, Okamoto K, Nishikawa K, Latz E, Golenbock DT, Aoki K, Ohya K, Imai Y, Morishita Y, Miyazono K, Kato S, Saftig P, Takayanagi H,. (2008). Cathepsin K-dependent toll-like receptor 9 signaling revealed in experimental arthritis. Science, 319(5863), 624-627.
^Hussein, H., Ishihara, A., Menendez, M., & Bertone, A. (2014). Pharmacokinetics and bone resorption evaluation of a novel Cathepsin K inhibitor (VEL‐0230) in healthy adult horses. Journal of veterinary pharmacology and therapeutics.
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