The effect of quinidine on digoxin kinetics in cardiac patients.

The pharmacokinetics of digoxin was studied in 11 subjects before and during quinidine treatment. Renal clearances of digoxin and creatinine were calculated from plasma concentrations and urinary excretions of digoxin and creatinine in subjects on long-term digoxin treatment. The investigations were repeated in the same subjects during administration of quinidine. Renal clearance of digoxin decreased, while plasma concentration of digoxin and renal excretion of digoxin increased in the presence of quididine, indicating substantial changes of digoxin kinetics, induced by quinidine. The reduction in renal clearance of digoxin may be due to a specific inhibition of tubular secretion of digoxin. The considerable rise of the plasma digoxin level and the increased excretion of digoxin support the assumption of a major extrarenal mechanism of interaction.