Renal handling of alendronate in rats. An uncharacterized renal transport system

Alendronate (4-amino-1-hydroxybutylidene-1,1-bisphosphonate), an antiosteolytic agent, is currently under investigation in the treatment of a variety of bone diseases. Earlier studies from this laboratory have demonstrated that systemically administered alendronate is rapidly either taken up by bone tissues or excreted by the kidney, and that renal excretion is the only route of elimination. The purpose of this study is to characterize the renal handling of alendronate in rats by standard clearance procedures with inulin as a marker of glomerular filtration rate. Alendronate is highly bound to rat serum protein. The excretion of alendronate by the kidney is concentration-and dose-dependent, and saturable, indicating that it is secreted by an active transport mechanism. The secretory mechanism exhibits limitation of transport, with an apparent Tm of approximately 25 micrograms/min/kg. However, high doses of cimetidine, quinine, probenecid, and p-aminohippuric acid had no effect on the renal excretion of alendronate, suggesting that alendronate is not secreted by anionic or cationic transport systems. In contrast, alendronate clearance is inhibited by etidronate, another bisphosphonate, in a dose-dependent manner, implying that these two bisphosphonates compete for an as yet uncharacterized renal transport system. As expected, the renal excretion of alendronate is drastically reduced in rats with acute renal failure. As a consequence of renal impairment, alendronate accumulates in plasma, and the concentration of the drug in bone tissues increases significantly.