Potent and selective cathepsin L inhibitors do not inhibit human osteoclast resorption in vitro

Cathepsins K and L are related cysteine proteases that have been proposed to play important roles in osteoclast-mediated bone resorption, To further examine the putative role of cathepsin L in bone resorption, we have evaluated selective and potent inhibitors of human cathepsin L and cathepsin K in an in vitro assay of human osteoclastic resorption and an in situ assay of osteoclast cathepsin activity, The potent selective cathepsin L inhibitors (K-i = 0.0099, 0.034, and 0.27 nM) were inactive in both the in situ cytochemical assay (IC50 > 1 muM) and the osteoclast-mediated bone resorption assay (IC50 > 300 nM). Conversely, the cathepsin K selective inhibitor was potently active in both the cytochemical (IC50 = 63 nM) and resorption (IC50 = 71 nM) assays. A recently reported dipeptide aldehyde with activity against cathepsins L (K-i = 0.052 mM) and K (K-i = 1.57 nM) was also active in both assays (IC50 = 110 and 115 nM, respectively) These data confirm that cathepsin K and not cathepsin L is the major protease responsible for human osteoclastic bone resorption.