Cathepsin K (CatK) is a major drug target for the treatment of osteoporosis. Potent active site‐directed inhibitors have been developed and showed variable success in clinical trials. These inhibitors block the entire activity of CatK and thus may interfere with other pathways. The present study investigates the antiresorptive effect of an exosite inhibitor that selectively inhibits only the therapeutically relevant collagenase activity of CatK.

Human osteoclasts and fibroblasts were used to analyse the effect of the exosite inhibitor, ortho‐dihydrotanshinone (DHT1), and the active site inhibitor, odanacatib (ODN), on bone resorption and TGF‐ß1 degradation. Cell cultures, Western blot, light and scanning electron microscopy as well as energy dispersive X‐ray spectroscopy, molecular modelling and enzymatic assays were used to evaluate the inhibitors.

DHT1 selectively inhibited the collagenase activity of CatK, without affecting the viability of osteoclasts. Both inhibitors abolished the formation of resorption trenches, with DHT1 having a slightly higher IC₅₀ value than ODN. Maximal reductions of other resorption parameters by DHT1 and ODN were comparable, respectively 41% and 33% for total resorption surface, 46% and 48% for resorption depths, and 83% and 61% for C‐terminal telopetide fragment (CTX) release. DHT1 did not affect the turnover of fibrosis‐associated TGF‐ß1 in fibroblasts, whereas 500 nM ODN was inhibitory.

Our study shows that an exosite inhibitor of CatK can specifically block bone resorption without interfering with other pathways.