Haemodynamics, cardiac conduction and pharmacokinetics of mibefradil (Ro 40-5967), a novel calcium antagonist

Objectives : Mibefradil (Ro40-5967) is a chemically novel non-dihydropyridine calcium antagonist. In this phase II study we compared its acute and chronic effects on blood pressure, heart rate and atrioventricular conduction (electrocardiographic PQ interval) with those of verapamil and diltiazem. Patients and methods : After a 4-week placebo run-in, 18 patients with mild to moderate essential hypertension were given single doses of mibefradil (150mg), slow-release (SR) verapamil (240mg), diltiazem (240mg) and placebo at weekly intervals ; pharmacokinetics and the effects on blood pressure, heart rate and PQ interval were studied on four 10-h study days. Seventeen of the same patients subsequently underwent 4 weeks of treatment with either mibefradil (100 mg daily ; n = 10) or verapamil SR (240 mg daily ; n=7), and on the last day, they attended a further 10-h study day. Two studies were conducted : an acute, single-dose, double-blind, randomly allocated, placebo-controlled, crossover study and a chronic, open-label, randomly allocated, parallel-group study. Results : Mibefradil was well tolerated. In the acute study, the antihypertensive effect (difference from placebo) of mibefradil 150 mg was of slower onset than that of verapamil or diltiazem, but comparable blood pressure reductions had been achieved by 6h. The mean±SD maximal PQ prolongation (difference from placebo) was 15.6±16.1 ms, compared with 44.0±22.6 ms for verapamil and 56.0±48.9 ms for diltiazem (P<0.05 mibefradil versus verapamil ; P<0.01 mibefradil versus diltiazem). In the chronic study there were no significant differences during steady-state conditions between mibefradil at 100 mg and verapamil SR at 240 mg in their effects on blood pressure, PQ and heart rate. The mean±SD elimination half-life (t 1/2 ) of mibefradil under steady-state conditions was 26.8±5.5 h (versus an apparent t 1/2 of 16.9±11.1 h for verapamil SR, P<0.05). Conclusions : Mibefradil is a well-tolerated and efficacious antihypertensive agent well suited to single daily dosing because of its intrinsic long plasma half-life. The effects on both blood pressure and PQ interval are of more gradual onset than those of unmodified verapamil and diltiazem after single doses.