Daily administration of eldecalcitol (ED-71), an active vitamin D analog, increases bone mineral density by suppressing RANKL expression in mouse trabecular bone.

Eldecalcitol (ED-71) is a new vitamin D₃ derivative recently approved for the treatment of osteoporosis in Japan. Previous studies have shown that the daily administration of ED-71 increases bone mineral density (BMD) by suppressing bone resorption in various animal models. In this study, we examined how ED-71 suppresses bone resorption in vivo, by analyzing bone histomorphometry and ex vivo osteoclastogenesis assays. Daily administration of ED-71 (50 ng/kg body weight) to 8-week-old male mice for 2 and 4 weeks increased BMD in the femoral metaphysis without causing hypercalcemia. Bone and serum analyses revealed that ED-71 inhibited bone resorption and formation, indicating that the increase in BMD is the result of the suppression of bone resorption. This suppression was associated with a decrease in the number of osteoclasts in trabecular bone. We previously identified cell cycle-arrested receptor activator of NF-κB (RANK)-positive bone marrow cells as quiescent osteoclast precursors (QOPs) in vivo. Daily administration of ED-71 affected neither the number of RANK-positive cells in vivo nor the number of osteoclasts formed from QOPs in ex vivo cultures. In contrast, ED-71 suppressed the expression of RANK ligand (RANKL) mRNA in femurs. Immunohistochemical experiments also showed that the perimeter of the RANKL-positive cell surface around the trabecular bone was significantly reduced in ED-71-treated mice than in the control mice. ED-71 administration also increased BMD in 12-week-old ovariectomized mice, through the suppression of RANKL expression in the trabecular bone. These results suggest that the daily administration of ED-71 increases BMD by suppressing RANKL expression in trabecular bone in vivo.