CLODRONATE AND LIPOSOME-ENCAPSULATED CLODRONATE ARE METABOLIZED TO A TOXIC ATP ANALOG, ADENOSINE 5'-(BETA,GAMMA-DICHLOROMETHYLENE) TRIPHOSPHATE, BY MAMMALIAN-CELLS IN-VITRO

Clodronate, alendronate, and other bisphosphonates are widely used in the treatment of bone diseases characterized by excessive osteoclastic bone resorption, The exact mechanisms of action of bisphosphonates ha ve not been identified but may involve a toxic effect on mature osteoc lasts due to the induction of apoptosis, Clodronate encapsulated in li posomes is also toxic to macrophages in vivo and may therefore be of u se in the treatment of inflammatory diseases, It is generally believed that bisphosphonates are not metabolized. However, we have found that mammalian cells in vitro (murine J774 macrophage-like cells and human MG63 osteosarcoma cells) can metabolize clodronate (dichloromethylene bisphosphonate) to a nonhydrolyzable adenosine triphosphate (ATP) anal og, adenosine 5'-(beta,gamma-dichloromethylene) triphosphate, which co uld be detected in cell extracts by using fast protein liquid chromato graphy, J774 cells could also metabolize liposome-encapsulated clodron ate to the same ATP analog, Liposome-encapsulated adenosine 5'-(beta,g amma-dichloromethylene) triphosphate was more potent than liposome-enc apsulated clodronate at reducing the viability of cultures of J774 cel ls and caused both necrotic and apoptotic cell death, Neither alendron ate nor liposome-encapsulated alendronate were metabolized, These resu lts demonstrate that the toxic effect of clodronate on J774 macrophage s, and probably on osteoclasts, is due to the metabolism of clodronate to a nonhydrolyzable ATP analog, Alendronate appears to act by a diff erent mechanism.