1. In the present study, we investigated the effects of technetium-99 conjugated with methylene diphosphonate ( 99 Tc-MDP), an agent used in radionuclide therapy, on receptor activator of nuclear factor-jB ligand (RANKL)-induced osteoclastogenesis and explored the underlying mechanisms.2. The murine macrophage cell line RAW264.7 and bone marrow-derived-macrophages from C57BL/6 mice (BMM) were used as models for osteoclastogenesis in vitro. The expression of some key factors in RANKL (50 ng/mL)induced osteoclastogenesis in RAW264.7 cells was investigated by flow cytometry and real-time reverse transcription-polymerase chain reaction (RT-PCR). To detect multinucleated osteoclast formation, RAW264.7 cells were induced with RANKL for 4 days, whereas BMM were induced by 50 ng/mL RANKL and 20 ng/mL macrophage colony-stimulating factor for 7 days, before being stained with tartrate-resistant acid phosphatase.3. Osteoclastogenesis was evaluated using the osteoclast markers CD51, matrix metalloproteinase (MMP)-9 and cathepsin K. At 0.01 lg/mL, 99 Tc-MDP significantly inhibited RANKL-induced osteoclastogenesis without any cytotoxicity. In addition, 99 Tc-MDP abolished the appearance of multinucleated osteoclasts.4. Real-time RT-PCR analysis of transcription factor expression revealed that 99 Tc-MDP inhibited the expression of c-Fos and nuclear factor of activated T cells. In addition, 99 Tc-MDP inhibited the expression of the inflammatory factors interleukin (IL)-6, tumour necrosis factor-a and IL-1b. Finally, 99 Tc-MDP inhibited the activation of mitogen-activated protein kinases in RAW264.7 cells following RANKL stimulation.5. In conclusion, 99 Tc-MDP possesses anti-osteoclastogenic activity against RANKL-induced osteoclast formation.

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