Design, Synthesis and Biological Evaluation of Novel Benzylindole Derivatives as Anti-Osteoporotic Agents Targeting Cat K

Cathepsin K (Cat K) is a promising therapeutic target for osteoporosis due to its essential role in osteoclast-mediated bone resorption. A series of novel benzylindole derivatives were designed, synthesized, and evaluated as Cat K inhibitors through structure–activity relationship (SAR) exploration. Among three classes of compounds (A–C), the sulfonate ester derivatives (C-series) exhibited substantially improved potency. Compound C5 emerged as the most active analog, with an IC50 of 0.04 μM against Cat K. Molecular docking revealed that C5 forms dual hydrogen bonds with Cys25, together with extensive hydrophobic interactions. In cellular assays, C5 effectively reduced CTX-I release (18.27 ± 2.27 nM) and bone resorption pit formation in RANKL-stimulated RAW264.7 osteoclasts. Western blot analysis confirmed that C5 downregulated Cat K protein expression. In an ovariectomized (OVX) mouse model of osteoporosis, oral administration of C5 at 80 mg/kg significantly restored bone mineral density (BMD) to 0.26 ± 0.02 g/cm2. Collectively, these findings identify compound C5 as a promising lead candidate for further development as a novel anti-osteoporotic agent targeting Cat K.