1 The metabolism of metoprolol was studied in 143 unselected hypertensive patients and in 10 families. 2 The loglo metoprolol to a-hydroxymetoprolol urinary ratio was bimodally distributed and was correlated with the debrisoquine oxidation phenotype (rs = 0.81, P < 0.001).3 The results of the pedigree study were compatible with poor hydroxylation of metoprolol being inherited as an autosomal recessive trait. 4 The major urinary metabolite of metoprolol metabolism was H117-04, the endproduct of O-dealkylation. The distribution of the log1o metoprolol to H117-04 (M/H117-04) urinary ratio was unimodal. However, there was a significant correlation between this ratio and the debrisoquine oxidation phenotype (rs = 0.68, P < 0.001) and poor metabolisers of debrisoquine (PMs) were concentrated at the upper end of the range of M/H117-04 values. 5 These results indicate that both the a-hydroxylation and O-dealkylation of metoprolol are under polymorphic control of the debrisoquine type. 6 Plasma concentrations of metoprolol were about three times higher in PMs than in extensive metabolisers of debrisoquine (EMs) at 3 h after dosing. 7 In a sub-group of 24 subjects, all seven PMs but only two EMs showed more than a 10% reduction in post-exercise heart rate at 24 h after dosing.
After a single 200 mg oral dose of metoprolol tartrate the mean metoprolol AUC was found to be six-fold higher in poor metabolizers (PMs) of debrisoquine than in extensive metabolizers (EMs). This was associated with impaired metabolic clearance via alpha-hydroxylation and O-dealkylation. A population study (n = 143) has shown a bimodal distribution in the ratio of metoprolol: alpha-hydroxymetoprolol recovered in urine which was correlated highly with the debrisoquine metabolic ratio. Nine per cent of the population were PMs. Plasma metoprolol concentrations three hours after a 100 mg oral dose of metoprolol were greater than 200 ng/ml in PMs but were lower than this in most EMs. This dose of metoprolol given once daily provided a clinically significant reduction (16%) in exercise heart rate in PMs after 24 hours. EMs require conventional doses (100 mg b.d.) to achieve the same degree of beta-blockade. Preliminary data from family studies support the view that the defect in metoprolol oxidation is inherited. In 12 hypertensive patients who were EMs we compared the beta-blocking activity and antihypertensive effect of chronic treatment with metoprolol 200 mg once daily (conventional and long-acting formulations), with those of atenolol 100 mg once daily and placebo. The effects of all active preparations were similar at 3.5 hours but atenolol was superior to all metoprolol formulations at 24 hours after dosing. It is concluded that for the majority of patients metoprolol should be prescribed twice daily when using currently available dosage forms. Relationships between oxidation phenotype and side-effects should be examined.
1 We have completed a double-blind parallel group comparison of the 5-HT2 receptor antagonist ketanserin with placebo in 22 hypertensive patients. 2 Ketanserin (20-40 mg twice daily) lowered sitting blood pressure more than placebo by 6.9 mm Hg systolic (NS), 13.1 mm Hg diastolic (P < 0.05) and by 11.4 mm Hg mean arterial pressure (P < 0.02). The fall in standing blood pressure was similar and we observed no first dose hypotensive effect. 3 Ketanserin lowered the sitting heart rate by 11.1 beats/min (P < 0.01). 4 The drug was well tolerated.
Summary:We have used a combination of a beta-blocker and verapamil to treat 42 consecutive patients with angina resistant to either agent alone. Patients with heart failure, heart block or uncontrolled hypertension were excluded. The mean duration of follow-up was 6.5 months. Thirty-six patients (81%) reported an improvement and the number of angina attacks was reduced from 17/week to 5/week. Side effects necessitated withdrawal of one or both drugs in 6 patients, 2 of whom developed bradyarrhythmias not solely related to drug treatment. The most common complication was mild left ventricular failure (6) treated by reducing or stopping the beta-blocker. The data suggest that the combination ofverapamil and a beta-blocker may be used in a relatively unselected group of patients with difficult angina. However, as dosage adjustment and close observation may be necessary to minimise side effects, the use of this combination should be limited to hospital practice.
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