The molecular mechanism of action of the antiresorptive and antiinflammatory drug clodronate - Evidence for the formation in vivo of a metabolite that inhibits bone resorption and causes osteoclast and macrophage apoptosis

Objective. The primary aims of this study were to determine whether clodron ate and liposome-encapsulated clodronate are metabolized to adenosine 5'-(b eta,gamma -dichloromethylene) triphosphate (AppCCl(2)p) by osteoclasts and macrophages in vivo, and to determine whether intracellular accumulation of this metabolite accounts for the antiresorptive and antimacrophage effects of clodronate. To compare the mechanism of action of clodronate and alendr onate, effects on protein prenylation in osteoclasts and macrophages in viv o were also assessed. Methods. High-performance liquid chromatography-mass spectrometry was used to determine whether rabbit osteoclasts (purified ex vivo with immunomagnet ic beads) metabolize clodronate, and whether rat peritoneal macrophages met abolize liposome-encapsulated clodronate, following in vivo administration. The effects of clodronate and AppCCl(2)p on bone resorption, osteoclast nu mber, and apoptosis in vitro were compared. Using an antibody to the unpren ylated form of Rap1A, effects on protein prenylation were assessed by Weste rn blot analysis of osteoclast and peritoneal macrophage lysates from bisph osphonate-treated animals. Results. AppCCl(2)p could be detected in extracts from osteoclasts purified from clodronate-treated rabbits. Intracellular accumulation of AppCCl(2)p caused a reduction in the number of osteoclasts, increased osteoclast apopt osis, and inhibited bone resorption in vitro. These effects were indistingu ishable from those of clodronate. Liposome-encapsulated clodronate was also metabolized to AppCCl(2)p by rat peritoneal macrophages in vivo. Liposome- encapsulated clodronate caused an increase in peritoneal macrophage apoptos is in ex vivo cultures that was indistinguishable from the increase in apop tosis caused by liposome-encapsulated AppCCl(2)p. Unlike alendronate, clodr onate and its metabolite did not affect prenylation of the small GTPase Rap 1A in osteoclasts or macrophages in vivo. Conclusion. These results provide the first direct evidence that the antiin flammatory and antiresorptive effects of clodronate on macrophages and oste oclasts in vivo occur via the intracellular formation of AppCCl(2)p.