Synthesis and Biological Evaluation of Heterocyclic Ring-Fused 20(S)-Protopanaxadiol Derivatives as Potent Antiosteoporosis Agents

A series of heterocyclic ring-fused derivatives of 20(S)-protopanaxadiol (PPD) were synthesized and evaluated for their inhibitory effects on RANKL-induced osteoclastogenesis. Among these compounds, 33 (SH491, IC50 = 11.8 nM) showed the highest potency with 100% inhibition at 0.1 μM and 44.4% inhibition at an even lower concentration of 0.01 μM, which was much more potent than the lead compound PPD (IC50 = 10.3 μM). Cytotoxicity tests indicated that the inhibitory effect of these compounds on RANKL-induced osteoclast differentiation was not due to their cytotoxicity. Interestingly, SH491 also exhibited a notable impact on the osteoblastogenesis of MC3T3-E1 preosteoblasts. Mechanistic studies revealed that SH491 inhibits the expression of osteoclastogenesis-related marker genes and proteins, including TRAP, CTSK, MMP-9, and ATPase v0d2. In vivo, SH491 could dramatically decrease the ovariectomy-induced osteoclast activity and relieve osteoporosis obviously. Thus, these PPD derivatives could be served as promising leads for the development of novel antiosteoporosis agents.