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Preclinical and initial clinical studies suggest that the novel calcium antagonist mibefradil has a unique combination of properties. Mibefredil was evaluated in a multicenter, double-blind, placebo-controlled, parallel group trial. After 4 weeks of a placebo run-in period, 202 eligible patients with mild to moderate hypertension were randomized to receive doses of 25, 50, 100, or 150 mg mibefradil or placebo once a day for 4 weeks. Blood pressure and heart rate were measured repeatedly at trough and peak (24 and 2 to 6 hours postdose, respectively) at the end of each period. Concentration-effect relationships were evaluated at trough on the last treatment day. A significant (P<.01 versus placebo) drop in blood pressure (diastolic and systolic) was observed at trough and peak in all mibefradil groups, with a trough-peak ratio greater than 0.8, high response rate, and a significant dose-response relationship (P<.001). The full antihypertensive effect of mibefradil was achieved within 1 to 2 weeks and was associated with a slight dose-dependent decrease in heart rate and increase in PQ time. Clear dissociation was observed between the effect on blood pressure and PQ time when concentration-effect relationships were evaluated. These results indicate that mibefradil is an effective and well-tolerated antihypertensive compound at doses of 25, 50, and 100 mg once daily. The incidence of treatment-related adverse events observed in the 25-, 50-, and 100-mg dose groups was lower than in the placebo group, but it was slightly higher in the 150-mg dose group, and three patients from this group were prematurely withdrawn because of an adverse event.