Elucidation of the nifedipine-quinidine interaction.

Four case reports have been published that document a clinically significant drug interaction between nifedipine and quinidine in patients with left ventricular dysfunction. To define the population at risk and the mechanisms involved in manifestations of this interaction, 12 patients currently treated with quinidine for either ventricular or supraventricular arrhythmias were stratified into two groups based on left ventricular ejection fraction (EF) measurements (group A greater than 35%; group B less than 35%). The interaction was conducted through two phases: oral quinidine (Q) and oral quinidine plus nifedipine (N + Q). Pharmacokinetic modeling of total body clearance (CL) and AUC were assessed for each phase. One patient (group A, 70% EF) exhibited the interaction with a 41% decrease in steady-state serum quinidine concentrations. The patient's AUC and CL were 48.2 micrograms/ml.hr (Q) versus 28.6 micrograms/ml.hr (N + Q) and 94.4 ml/min (Q) versus 159.1 ml/min (N + Q), respectively. There was no difference in AUC or CL between the Q and N + Q phases or between groups A and B for the entire population. The N + Q interaction is not hemodynamically mediated. Clinical consideration of the possibility of this low-frequency interaction should be noted.