Bisphosphonates directly inhibit the bone resorption activity of isolated avian osteoclasts in vitro

Bisphosphonates are useful in treatment of disorders with increased osteoclastic activity, but the mechanism by which bisphosphonates act is unknown. We used cultures of chicken osteoclasts to address this issue, and found that 1-hydroxyethylidenediphosphonic acid (EHDP), dichloromethylidenediphosphonic acid (Cl2MDP), or 3-amino-1-hydroxypropylidene-1,1-diphosphonic acid (APD) all cause direct dose-dependent suppression of osteoclastic activity. Effects are mediated by bone-bound drugs, with 50% reduction of bone degradation occurring at 500 nM to 5 microM of the different agents. Osteoclastic bone-binding capacity decreased by 30-40% after 72 h of bisphosphonate treatment, despite maintenance of cell viability. Significant inhibition of bone resorption in each case is seen only after 24-72 h of treatment. Osteoclast activity depends on ATP-dependent proton transport. Using acridine orange as an indicator, we found that EHDP reduces proton accumulation by osteoclasts. However, inside-out plasma membrane vesicles from osteoclasts transport H+ normally in response to ATP in high concentrations of EHDP, Cl2MDP, or APD. This suggests that the bisphosphonates act as metabolic inhibitors. Bisphosphonates reduce osteoclastic protein synthesis, supporting this hypothesis. Furthermore, [3H]leucine incorporation by the fibroblast, which does not resorb bone, is also diminished by EHDP, Cl2MDP and APD except when co-cultured with bisphosphonate-binding bone particles. Thus, the resorption-antagonizing capacities of EHDP, Cl2MDP and APD reflect metabolic inhibition, with selectivity for the osteoclast resulting from high affinity binding to bone mineral.