Single- and Multiple-dose Mibefradil Pharmacokinetics in Normal and Hypertensive Subjects

Mibefradil is a single-enantiomer calcium antagonist belonging to a new class, the tetralol derivatives. The recommended doses for treatment of hypertension and chronic stable angina pectoris are 50 or 100 mg. Mibefradil is metabolized via parallel pathways of esterase-catalysed hydrolysis and cytochrome P450 3A4-mediated oxidation. Mibefradil also inhibits cytochrome P450 3A4 and consequently inhibits its own metabolism, a property illustrated by three studies performed early in the drug's development. After single intravenous doses of 2.5 to 80 mg to healthy male subjects, pharmacokinetics are linear. At the representative 40 mg intravenous dose mean pharmacokinetic parameters were clearance 241 ± 76 mL min^?1, terminal exponential half-life 15.0h and volume of distribution at steady state 213 L. After single oral doses of 10 to 320 mg, reduced first-pass metabolism occurs with increasing dose. This effect is accompanied by increasing absolute bioavailability as the dose is increased. In an oral multiple-dose study of healthy male volunteers mibefradil doses of 100, 150 or 250 mg (from tablets) were administered once daily for 28 days. Reduction of the first-pass effect was noted, although the data suggested that a maximum was reached for doses of 150 mg or more. In a study of the effect of hypertension on mibefradil pharmacokinetics, 12 patients received oral mibefradil once daily at doses of 50, 100, 150, or 200 mg in 100 mL orange juice for 8 days. Steady state was reached within 3 days and accumulation generally ranged from three- to sevenfold. Single-dose non-linearity was observed in the first-pass effect, although for multiple dosing oral clearance values were dose-independent and lower than for single doses. After multiple dosing at the recommended dosage of 50 and 100 mg once daily, oral clearance of mibefradil stabilizes to approximately the same value for both doses. Hence, the single-dose non-linearity has little clinical relevance but demonstrates the self-inhibition of metabolism seen with mibefradil. Studies so far suggest that self-inhibition of its oxidative metabolic pathway leads to a low clearance and long half-life, enabling once-daily dosing and conferring low intra- and inter-patient variability in pharmacokinetics.