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      Comparative Pharmacological Properties among Calcium Channel Blockers: T-Channel versus L-Channel Blockade

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          Calcium antagonists are potent vasodilators and are widely used in the treatment of hypertension and angina pectoris. The currently available compounds belong to three classes: (1) dihydropyridines (e.g. nifedipine, amlodipine and felodipine), (2) phenylalkylamines (e.g. verapamil) and (3) benzothiazepines (e.g. diltiazem). The three classes differ in their pharmacological profile and safety. For example, verapamil and diltiazem lower heart rate, while dihydropyridines increase it or leave it unchanged. With most of the latter compounds, a marked activation of the sympathetic nervous system has been noted. Most compounds exhibit negative inotropic effects, particularly the first-generation molecules, which is disadvantageous in patients with impaired left-ventricular function. The most common side effects of these drugs are flushing, headache and edema. With verapamil, constipation may represent a problem in certain patients. Hence, in spite of a large number of calcium antagonists available, there remains a need for new compounds with enhanced efficacy and improved tolerability. A new compound should lack any negative inotropism, avoid any increase in sympathetic outflow or heart rate and exhibit a high degree of vascular selectivity. Furthermore, a low incidence of side effects, particularly ankle edema and optimal pharmacokinetics allowing once-daily dosing would be desirable. Mibefradil is a new calcium antagonist with promising pharmacological and clinical properties. The compound has a high bioavailability, lacks negative inotropic effects at therapeutic concentrations, does not exhibit reflex tachycardia during vasodilation and actually slightly decreases heart rate. It is a potent direct vasodilator efficacious in hypertension and chronic angina pectoris, elicits endothelium-dependent relaxations and facilitates the effects of nitric oxide in vascular smooth muscle. The drug is a particularly efficacious vasodilator in intramyocardial coronary arteries which may be important for its anti-ischemic effects and the lack of steal in the coronary circulation. Furthermore, mibefradil has antiproliferative properties in human vascular smooth muscle cells in culture. As a unique property, mibefradil blocks T-type calcium channels and hence represents a new class of calcium channel blockers. In patients with hypertension, mibefradil has a high efficacy in controlling blood pressure. The drug does not cause constipation and has a low incidence of ankle edema. A large trial is under way to further delineate the properties of this new calcium antagonist in patients with heart failure.

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          Most cited references 3

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          Effect of amlodipine on morbidity and mortality in severe chronic heart failure. Prospective Randomized Amlodipine Survival Evaluation Study Group.

          Previous studies have shown that calcium-channel blockers increase morbidity and mortality in patients with chronic heart failure. We studied the effect of a new calcium-channel blocker, amlodipine, in patients with severe chronic heart failure. We randomly assigned 1153 patients with severe chronic heart failure and ejection fractions of less than 30 percent to double-blind treatment with either placebo (582 patients) or amlodipine (571 patients) for 6 to 33 months, while their usual therapy was continued. The randomization was stratified on the basis of whether patients had ischemic or nonischemic causes of heart failure. The primary end point of the study was death from any cause and hospitalization for major cardiovascular events. Primary end points were reached in 42 percent of the placebo group and 39 percent of the amlodipine group, representing a 9 percent reduction in the combined risk of fatal and nonfatal events with amlodipine (95 percent confidence interval, 24 percent reduction to 10 percent increase; P=0.31). A total of 38 percent of the patients in the placebo group died, as compared with 33 percent of those in the amlodipine group, representing a 16 percent reduction in the risk of death with amlodipine (95 percent confidence interval, 31 percent reduction to 2 percent increase; P=0.07). Among patients with ischemic heart disease, there was no difference between the amlodipine and placebo groups in the occurrence of either end point. In contrast, among patients with nonischemic cardiomyopathy, amlodipine reduced the combined risk of fatal and nonfatal events by 31 percent (P=0.04) and decreased the risk of death by 46 percent (P<0.001). Amlodipine did not increase cardiovascular morbidity or mortality in patients with severe heart failure. The possibility that amlodipine prolongs survival in patients with nonischemic dilated cardiomyopathy requires further study.
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            Effect of verapamil on mortality and major events after acute myocardial infarction (the Danish Verapamil Infarction Trial II--DAVIT II)

            The effect of verapamil on death and major events (i.e., death or reinfarction) after an acute myocardial infarction was studied in a double-blind, randomized, placebo-controlled multicenter trial. Eight hundred seventy-eight patients started treatment with verapamil, 360 mg/day, and 897 patients with placebo. Treatment started in the second week after admission and continued for up to 18 months (mean 16 months). Ninety-five deaths and 146 major events occurred in the verapamil group and 119 deaths and 180 major events in the placebo group. The 18-month mortality rates were 11.1 and 13.8% (p = 0.11, hazard ratio, 0.80; 95% confidence limits, 0.61 to 1.05), and major event rates 18.0 and 21.6% (p = 0.03, hazard ratio, 0.80; 95% confidence limits, 0.64 to 0.99) in the verapamil and placebo groups, respectively. In patients without heart failure in the coronary care unit the mortality rates were 7.7% in the verapamil group and 11.8% in the placebo group (p = 0.02, hazard ratio, 0.64; 95% confidence limits, 0.44 to 0.94), and major event rates 14.6 and 19.7% (p = 0.01, hazard ratio 0.70; 95% confidence limits (0.52 to 0.93). In patients with heart failure the mortality rates were 17.9 and 17.5% (p = 0.79, hazard ratio, 1.05; 95% confidence limits, 0.72 to 1.54), and major event rates 24.9 and 24.9% (p = 1.0, hazard ratio 0.98; 95% confidence limits 0.72 to 1.39). Long-term treatment with verapamil after an acute myocardial infarction caused a significant reduction in major events, and the positive effect was found in patients without heart failure.
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              Ro 40-5967, a novel calcium channel antagonist, protects against ventricular fibrillation.

              Ro 40-5967 is a new calcium channel antagonist that binds at the same membrane sites as verapamil, yet has minimal negative inotropic effects. The effects of Ro 40-5967 on the susceptibility to ventricular fibrillation were investigated and compared to diltiazem. Ventricular fibrillation (VF) was induced in 40 mongrel dogs with healed myocardial infarctions by a 2-min coronary occlusion during exercise. Twenty-four animals were found to be susceptible to VF and were given the treatments described below. Pretreatment with Ro 40-5967 (n = 17, 1000 micrograms/kg i.v.) significantly (P < 0.001) reduced the incidence of VF (13 of 17 protected) during the exercise plus ischemia test. Diltiazem (n = 8, 1000 micrograms/kg) completely suppressed VF. Lower doses of diltiazem and Ro 40-5967 did not prevent VF. The hemodynamic effects of Ro 40-5967 were also compared to diltiazem and verapamil. Diltiazem and verapamil, but not Ro 40-5967, increased P-R interval in a dose-dependent manner. Even when reflex tachycardia was controlled by beta-adrenoceptor blockade, Ro 40-5967 still exerted only minimal effects on P-R interval. Verapamil, but neither Ro 40-5967 nor diltiazem, provoked a dose-dependent negative inotropic response. All three drugs elicited large increases in coronary blood flow. These data support the hypothesis that calcium entry may play a critical role in the development of malignant arrhythmias during ischemia. Further, Ro 40-5967 can protect against ventricular fibrillation without significant negative inotropic or dromotropic effects.

                Author and article information

                S. Karger AG
                February 1998
                03 March 1998
                : 89
                : Suppl 1
                : 10-15
                Cardiology, Cardiovascular Research, University Hospital, Zurich, Switzerland
                47274 Cardiology 1998;89(suppl 1):10–15
                © 1998 S. Karger AG, Basel

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                Page count
                Figures: 3, Tables: 1, References: 50, Pages: 6


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