The uptake of clodronate (dichloromethylene bisphosphonate) by macrophages in vivo and in vitro

Clodronate (dichloromethylene bisphosphonate) accumualtes extensively in the bone by binding to apatite crystals. We found recently that the drug also accumulates in the spleen and, to a lesser extent, in the liver of mice and rats. In the present study, the role of macrophages in soft tissue accumulation was studied in mice by autoradiography and macrophage-depleting techniques, and also by isolated rat peritoneal macrophages. The localization of [14C]clodronate in mouse spleen showed that the drug concentrates in the marginal zone between the white and red pulp, which is known to be rich in macrophages. Pretreatment of mice with pure clodronate did not change the accumulation of [14C]clodronate in the spleen. However, when splenic and hepatic macrophages were eliminated by the liposome-encapsulated clodronate, only a weak [14C]clodronate accumulation occurred in these organs. Isolated macrophages did not take up free [14C]clodronate, but the addition of ferrous iron to the incubate resulted in the uptake of 14C activity by macrophages. They were probably stimulated by the insoluble clodronate-iron complex. The results suggest that 1) macrophages are involved in the accumulation of clodronate in the spleen and liver, and 2) combination of clodronate with extracellular iron is a prerequisite for the activation of macrophages to take up the drug complex. Since the spleen and, to a lesser extent, the liver are rich in iron released from destroyed red cells, accumulation of clodronate takes place in these organs.