Zoledronate (ZOL) is the most potent nitrogen-containing bisphosphonate and is effective at preventing osteolytic bone disease in patients with multiple myeloma (MM) and solid tumors. ZOL inhibits the enzyme farnesylpyrophosphate synthase and thus blocks the prenylation of small GTPases. In vitro studies have demonstrated that ZOL can also directly affect the growth and viability of myeloma cells, however, the molecular mechanisms underlying this activity have not been fully elucidated. The goal of our study was to investigate direct antimyeloma effects of ZOL in vitro and in vivo. In five myeloma cell lines (RPMI8226, L363, U266, JK-6L, and the IL-6 dependent INA-6), growth was inhibited and apoptosis induced by ZOL in a dose-dependent manner (IC50’s between 30 μM and 285 μM). Similar results were obtained in the presence of bone marrow stromal cells, IL-6 (20 ng/mL), IGF-1 (200 ng/mL), or a combination of both cytokines. The potential antitumor effect of ZOL on myeloma cells in vivo was studied in the INA-6 SCID model, in which mice are injected intraperitoneally with INA-6 cells and subsequently develop plasmacytomas. Mice treated with ZOL had reduced tumor burden and a significant survival benefit compared to the control group (p=0.002). Histological examination of plasmacytomas explanted 72 hours after a single injection of 8 μg ZOL revealed extensive apoptotic/necrotic areas while no such areas were found in tumors of untreated animals. Induction of apoptosis was confirmed by Western blot analysis of tumor lysates, which revealed increased levels of cleaved poly (ADP-ribose) polymerase (PARP) in tumors of ZOL treated vs. untreated animals. This correlated with an accumulation of the unprenylated form of the small GTPase Rap1A, which was virtually absent in tumors of untreated mice. Our findings demonstrate a direct and specific effect of ZOL in plasmacytomas in vitro and in vivo and point to a therapeutic potential in MM beyond the prevention of osteolytic lesions.

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