Pharmacokinetic and pharmacodynamic aspects of concomitant mibefradil-digoxin therapy at therapeutic doses

This study investigated the effect of mibefradil on digoxin pharmacokinetics an pharmacodynamics. Following a loading dose of digoxin (0.375 mg, three times, day 1), 0.375 mg was administered once daily to 40 healthy subjects (days 2-15). Mibefradil was administered daily at 50 mg, 100 mg, or 150 mg (days 9-15). With co-administration of 50 mg or 100 mg mibefradil (the recommended doses), mean digoxin Cmax values increased 1.19- and 1.32-fold, respectively; Cmin values were 0.95- and 1.04-fold, respectively; mean AUC0-24 h increased 1.05- and 1.11-fold, respectively; and the total amount of digoxin excreted in urine remained unchanged. Digoxin monotherapy produced modest but transient prolongations of PQ interval, small decreases in heart rate, and no changes in blood pressure. With the addition of mibefradil, no effects on trough blood pressure or cardiac index were observed, but there was a further increase in PQ interval and decrease in heart rate. In a previous study, mibefradil had no significant effect on trough plasma digoxin concentration in patients with congestive heart failure and ischemia. Therefore, while the vast majority of patients should not need their digoxin dosages adjusted when given mibefradil, an occasional patient may require dose reductions based on clinical response and plasma digoxin.