A Novel RANKL/RANK Inhibitor IMB-R38 Inhibits Osteoporosis Through Regulating Bone Metabolism

Osteoporosis is a systemic skeletal disease that severely impairs the health of the elderly population. The interaction between the receptor activator of the NF-κB ligand (RANKL) and its receptor RANK is critical for osteoclast differentiation and function. Therefore, targeting the RANKL/RANK interaction represents a promising strategy for osteoporosis. In this study, we employed a newly established yeast two-hybrid system based on RANKL/RANK interaction and identified IMB-R38, a novel benzamide compound that dose-dependently blocked RANKL/RANK interaction by inhibiting the growth of AH109 cells harboring pAD-RANKL/pBD-RANK plasmids in quadruple-dropout medium. IMB-R38 significantly suppressed osteoclast differentiation, disrupted F-actin ring formation, and downregulated the expression of osteoclast-specific genes, including NFATc1 and MMP9 in RANKL-induced RAW264.7 macrophages. IMB-R38 also promoted osteoblast differentiation by upregulating the expression of osteogenic genes. Importantly, in a dexamethasone (DXM)-induced osteoporotic zebrafish model, IMB-R38 significantly increased bone mineralization, with anti-osteoporosis efficacy superior to that of alendronate sodium (Alen). RT-qPCR assays showed that IMB-R38 significantly upregulated the mRNA expression of osteogenesis genes (Bmp2, Runx2a, Runx2b, Sp7, Alp, and Oc) while markedly downregulating that of the osteoclastogenesis genes (Mmp9, Mmp13, and Mmp2) compared with the DXM group. Mechanistically, an SPR assay confirmed that IMB-R38 directly binds with RANK but not RANKL to disrupt RANKL/RANK interaction. Furthermore, Asp168 of RANK was identified as a key amino acid that mediates both RANKL interaction and IMB-R38 binding. The inhibition of RANKL/RANK by IMB-R38 suppressed JNK phosphorylation and, consequently, osteoclast differentiation and function. Collectively, our findings identify IMB-R38 as a novel RANKL/RANK inhibitor with therapeutic potential for osteoporosis through its regulation of bone metabolism.