Synthesis and Biological Evaluation of Heterocycle-Fused Pyxinol Derivatives as Anti-osteoporosis Agents

A series of novel heterocycle-fused pyxinol compounds was designed and synthesized through structure-activity relationship (SAR)-guided optimization to develop potent inhibitors of RANKL-induced osteoclastogenesis. Among the synthesized derivatives, compound 36 (SH543) demonstrated the most potent inhibitory activity with an IC₅₀ value of 3.3 nM, representing an approximately 848-fold increase in potency compared to the hit compound pyxinol (IC₅₀ = 2.8 μM). Mechanistic investigations revealed that SH543 effectively downregulated key osteoclastogenesis-related marker genes (Atp6v0d2, Trap, Ctsk, Mmp9) and proteins (TRAP, CTSK, and MMP9). Furthermore, SH543 directly bound to KEAP1, activated the Nrf2-HO-1 antioxidant pathway, reduced ROS levels, and inhibited PI3K-AKT and MAPK signaling pathways. In ovariectomized mice, SH543 administration significantly attenuated pathological bone loss by preserving trabecular microarchitecture and improving biomechanical strength. These results establish SH543 as a promising lead compound for the development of novel antiosteoporosis agents, acting through multiple mechanisms.