Pharmacokinetic interactions of nifedipine and quinidine.

Several clinical investigations have been published regarding the interaction of nifedipine and quinidine. The results of these studies are contradictory. In vitro studies indicate that the 3-hydroxylation and N-oxigenation of quinidine appear to involve the P4503A4 family, a form of cytochrome that predominantly catalyzes the aromatization of nifedipine, too. The aim of our study was to investigate the effect of oral intake of 200 mg quinidine on the kinetics of 20 mg nifedipine as a retarded formulation and vice versa. Twelve healthy male volunteers between 18 and 40 years were treated. Each subject was studied on three occasions each separated by a one week washout period. Drug administration consisted of one oral dose of nifedipine (Adalat retard 20 mg), one oral dose of quinidine (Chinidin sulfuricum "Buchler" 200 mg) or a combination of both (20 mg nifedipine and 200 mg quinidine) in a randomised 3 way crossover. Administration of the test drugs in combination slightly increased the bioavailability of both--nifedipine [N] to 18% and quinidine [Q] to 16%--and decreased the clearance of both drugs. The results were not statistically significant. Based on our data, the combination of nifedipine and quinidine seems to lack a clinically relevant interaction.