Role of Calcium in Plasma Protein Binding and Renal Handling of Alendronate in Hypo- and Hypercalcemic Rats

Alendronate (4-amino-1-hydroxybutylidine-1,1-bisphosphonate), an antiosteolytic agent, is currently under investigation for the treatment of osteoporosis. Earlier studies in animals from this laboratory disclosed that systemically administered alendronate is rapidly taken up by bone tissues to the extent of 60% to 70% of the dose and excreted by the kidney, 30% to 40% in 24 hr, and that renal excretion is the only route of elimination. This study was designed to explore the effect of calcium on plasma protein binding and the renal handling of alendronate. The binding of alendronate to rat plasma was concentration, pH and calcium dependent. The fraction of unbound drug in rat plasma increased from about 3% to 9% over a drug concentration range of 0.2 to 10 micrograms/ml. Supplementation of calcium strongly augmented the binding to serum albumin. The binding of alendronate in plasma increased with increasing pH from about 50% at pH 6.6 to 98% at pH 8.6. The effects of pH on the binding of calcium and of alendronate to serum albumin were qualitatively similar. Under steady-state conditions, the binding of alendronate was substantially lower in hypocalcemic rats but unchanged in hypercalcemic rats. Although hypocalcemia caused a significant decrease in the renal secretion of alendronate, there was no effect on the renal secretion of tetraethylammonium bromide and p-aminohippuric acid. The differential effect of hypocalcemia suggests that calcium may play an important role in the renal handling of alendronate. However, hypercalcemia resulted in a substantial decrease of renal secretion of all three compounds and the decreased renal secretion was associated with a marked decrease in the glomerular filtration rate.(ABSTRACT TRUNCATED AT 250 WORDS)