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M. W.-H. Wang, S. L. Teitelbaum, P. Tebas, W. G. Powderly, and F. P. Ross.
Washington Univ. Sch. of Med., St. Louis, MO.
Background:Potent antiretroviral therapy that includes a protease inhibitor may be associated with a decreased bone mineral density (BMD). Both osteoblast (OB) and osteoclast (OC) play an important role in regulating BMD.
Methods:We tested ritonavir and indinavir for alterations in OB and OC differentiation and function in vitro and in vivo.
Results:Indinavir (at 5mg/mL) inhibits in vitro bone nodule formation without cell toxicity, while ritonavir and nelfinavir have no inhibitory effect. The effect of indinavir occurs during the first half of the culture period, but not the second half, suggesting its action during early osteoblastogenesis. MC3T3-E1 cells, a murine osteoblastic cell line, treated with indinavir exhibit a dose dependent decrease in alkaline phosphatase activity, an early marker of OB differentiation. The in vitro effect holds true in an ex vivo calvarial organ culture system. We observed a decrease in both basal and stimulated bone formation with indinavir. To test the effect of indinavir on bone in vivo, we injected indinavir intraperitoneally into mice for two weeks and assayed for the potential of bone marrow stromal cells to differentiate into OB ex vivo. We found a tenfold decrease in OB colony forming units with indinavir. To explore the possibility that an inhibition of OB differentiation diverts the stromal cell precursors to an adipocytic lineage, we used an in vitro culturing system that promotes differentiation of murine calvarial stromal cell into both OB's and adipocytes. Surprisingly, both adipocytic and OB differentiation were inhibited by indinavir, suggesting inhibition of stromal cell differentiation at an early stage. Opposite inhibitory effects were observed when the drugs were tested with OC's. Ritonavir reversibly suppresses OC differentiation with an inhibitory dose 50% of 10mg/mL. When ritonavir is applied to mature OC's on bone slices, OC's fail to resorb bone even though OC number is unchanged. This inhibition holds true in vivo, as systemic ritonavir administration to mouse blocks parathyroid hormone induced osteoclastogenesis. These inhibitory effects were not observed with indinavir.
Conclusions:Even though indinavir and ritonavir target the same viral protease, they have different effects on OB and OC differentiation and function in vitro and in vivo, suggesting that they have different mechanisms of action on mammalian cells. Further studies are needed to address the clinical significance of the opposing effects on bone of these two drugs.
© 8th Conference on Retroviruses and Opportunistic Infections