Effect of concomitant colestipol hydrochloride administration on the bioavailability of diltiazem from immediate- and sustained-release formulations.

Effects of concomitant colestipol administration on plasma concentrations of diltiazem and desacetyldiltiazem from immediate-release (IR) and sustained-release (SR) formulations were assessed in two studies. In the first study, 12 subjects received 120-mg diltiazem hydrochloride (diltiazem) SR capsules or 120-mg diltiazem IR tablets administered alone and in combination with colestipol hydrochloride (colestipol). Following concomitant administration of SR diltiazem with colestipol, AUC(0-infinity ) and C(max), respectively, were 22 and 36% less, and were 27 and 33% lower for IR diltiazem. In the second study, subjects received 120-mg diltiazem SR capsules at staggered times, without colestipol, 1 h prior to or 4 h following multiple doses of colestipol. A 17% decrease in AUC(0-infinity ) was observed when diltiazem was taken 1 h before colestipol was given, and a 22% decrease when diltiazem was taken 4 h after colestipol, relative to diltiazem SR alone. C(max) values were similarly decreased. Results from these two studies show that colestipol can cause a significant decrease in diltiazem absorption from both IR and SR dosage forms. Staggering the administration of colestipol and diltiazem SR did not blunt this effect, indicating that concomitant administration of diltiazem and colestipol should be used with caution, and that the efficacy of diltiazem should be monitored to assure adequate dosing.