Clinical pharmacology of alendronate sodium

Clinical studies have been performed to investigate the pharmacokinetics and pharmacodynamics of alendronate, an inhibitor of bone resorption for the treatment of osteoporosis. Alendronate is one of the most potent bisphosphonates currently undergoing clinical investigation (>100-fold more potent than etidronate in vivo). The pharmacokinetics of alendronate are similar to those of other bisphosphonates. After a 2-h intravenous infusion, plasma concentrations of alendronate decline rapidly to ∼5% of initial values within 6 h. About 50% of a systemic dose is excreted unchanged in the urine in the 72 h following administration. By analogy to its behavior in animals the remainder is assumed to be taken up by the skeleton. After sequestration into bone, the elimination of alendronate is very prolonged. The terminal half-life was estimated to be greater than 10 years. Despite prolonged skeletal residence, the biological effects of alendronate begin to diminish post-treatment, since the duration of effect reflects factors besides dose and cumulative drug exposure. When taken after an overnight fast, 2 h before breakfast, the oral bioavailability of alendronate averages ∼0.75% of dose with substantial variability (coefficient of variation 55%–75%) both between and within subjects. Reducing the wait before food from 2 h to 1 h, or even 30 min, produces a mean reduction in absorption of 40%. Since the clinical efficacy of alendronate is indistinguishable whether it is given 30 min, 1h, or 3 h before a meal, the observed variability in bioavailability within this range is of little consequence. Dosing up to at least 2 h after a meal dramatically reduces absorption (80%–90%).