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 clodronate and liposome‐encapsulated clodronate are metabolized to adenosine 5′‐(β,γ‐dichloromethylene) triphosphate (AppCCl2p) 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 alendronate, effects on protein prenylation in osteoclasts and macrophages in vivo were also assessed. Methods High‐performance liquid chromatography–mass spectrometry was used to determine whether rabbit osteoclasts (purified ex vivo with immunomagnetic beads) metabolize clodronate, and whether rat peritoneal macrophages metabolize liposome‐encapsulated clodronate, following in vivo administration. The effects of clodronate and AppCCl2p on bone resorption, osteoclast number, and apoptosis in vitro were compared. Using an antibody to the unprenylated form of Rap1A, effects on protein prenylation were assessed by Western blot analysis of osteoclast and peritoneal macrophage lysates from bisphosphonate‐treated animals. Results AppCCl2p could be detected in extracts from osteoclasts purified from clodronate‐treated rabbits. Intracellular accumulation of AppCCl2p caused a reduction in the number of osteoclasts, increased osteoclast apoptosis, and inhibited bone resorption in vitro. These effects were indistinguishable from those of clodronate. Liposome‐encapsulated clodronate was also metabolized to AppCCl2p by rat peritoneal macrophages in vivo. Liposome‐encapsulated clodronate caused an increase in peritoneal macrophage apoptosis in ex vivo cultures that was indistinguishable from the increase in apoptosis caused by liposome‐encapsulated AppCCl2p. Unlike alendronate, clodronate and its metabolite did not affect prenylation of the small GTPase Rap1A in osteoclasts or macrophages in vivo. Conclusion These results provide the first direct evidence that the antiinflammatory and antiresorptive effects of clodronate on macrophages and osteoclasts in vivo occur via the intracellular formation of AppCCl2p.