Permalink : http://hdl.handle.net/2115/68792

KEYWORDS : minodronate;isotope microscopy;cell coupling;osteoclast;osteoblast

Minodronate has been highlighted for its sustained effects on osteoporotic treatment. To determine the cellular mechanism of its sustained effects, we have assessed the localization of minodronate in mouse bones through isotope microscopy, by labeling it with a stable and rare nitrogen isotope (15N-minodronate). Eight-weeks-old male mice intravenously received 15N-minodronate (1 mg/kg) were fixed after three hours, 24 hrs, one week and one month. Isotope microscopy localized 15N-minodronate predominantly beneath osteoblasts (bone forming surface) rather than nearby osteoclasts (bone-resorbing surface). Literally, alendronate, another nitrogen-containing bisphosphonate, has been reported to accumulate on the bone-resorbing surface, and suddenly inhibit the osteoclasts. In contrast, minodronate appears to coat the bone-forming surface, without immediate inhibition of osteoclasts. A single injection of minodronate chronologically increased metaphyseal trabeculae, whereas the numbers of tartrate resistant acid phosphatase (TRAP)-positive osteoclasts and alkaline phosphatase (ALP)-reactive osteoblastic area were not reduced. Apoptotic osteoclasts were not apparent, but, finally being observed in the later stage of the experiments, while ALP-reactive osteoblasts were persisted on the trabeculae. Osteoclasts have developed ruffled borders at 3 hrs after minodronate administration; however, osteoclasts were roughly attached to the bone surfaces and did not form ruffled borders at 24 hrs after the administration. Von Kossa staining clearly demonstrated that osteoclasts did not incorporate the minodronate-treated bone matrix, while osteoclasts included abundant bone minerals inside in the control specimens. Taken together, minodronate accumulates in bone underneath osteoblasts rather than under bone-resorbing osteoclasts ; therefore, it is likely that the osteoclasts are not able to resorb and incorporate the minodronate-coated bone matrix, which may result in osteoclastic survival, avoiding osteoclastic apoptosis and consequently inducing cell coupling with osteoblasts. In conclusion, the resistance of miniodronate-coating bone from osteoclastic resorption, and the consequent cell coupling with osteoblasts appear to produce a long-lasting and bone-preserving effect.