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Pharmacology: Metoprolol tartrate is a beta-adrenergic receptor blocking agent. In vitro and in vivo animal studies have shown that it has a preferential effect on beta1 adrenoreceptors, chiefly located in cardiac muscle. This preferential effect is not absolute, however, and at higher doses, Metoprolol tartrate also inhibits beta2 adrenoreceptors, chiefly located in the bronchial and vascular musculature.
Clinical pharmacology studies have confirmed the beta-blocking activity of metoprolol in man, as shown by (1) reduction in heart rate and cardiac output at rest and upon exercise, (2) reduction of systolic blood pressure upon exercise, (3) inhibition of isoproterenol-induced tachycardia, and (4) reduction of reflex orthostatic tachycardia.
Relative beta1 selectivity has been confirmed by the following: (1) In normal subjects, Metoprolol tartrate is unable to reverse the beta2-mediated vasodilating effects of epinephrine. This contracts with the effect of nonselective (beta1 plus beta2) beta blockers, which completely reverse the vasodilating effects of epinephrine. (2) In asthmatic patients, Metoprolol tartrate reduces FEV and FVC significantly less than a nonselective beta blocker, propanolol, at equivalent beta1-receptor blocking doses.
Metoprolol tartrate has no intrinsic sympathomimetic activity, and membrane-stabilizing activity is detectable only at doses much greater than required for beta blockade. Metoprolol tartrate crosses the blood-brain barrier and has been reported in the CSF in a concentration 78% of the simultaneous plasma concentration. Animal and human experiments indicate that Metoprolol tartrate slows the sinus rate and decreases AV nodal conduction.
In a large (1,395 patients randomized), double-blind, placebo-controlled clinical study, Metoprolol tartrate was shown to reduce 3-month mortality by 36% in patients with suspected or definite myocardial infarction.
Patients were randomized and treated as soon as possible after their arrival in the hospital, once their clinical condition had stabilized and their hemodynamic status had been carefully evaluated. Subjects were ineligible if they had hypotension, bradycardia, peripheral signs of shock, and/or more than animal basal rales as signs of congestive heart failure. Initial treatment consisted of intravenous followed by oral administration of Metoprolol tartrate or placebo, given in a coronary care or comparable unit. Oral maintenance therapy with Metoprolol tartrate or placebo was then continued for 3 months. After this double-blind period, all patients were given Metoprolol tartrate and followed up to 1 year.
The median delay from the onset of symptoms to the initiation of therapy was 8 hours in both the Metoprolol tartrate and placebo treatment groups. Among patients treated with Metoprolol tartrate, there were comparable reductions in 3-month mortality for those treated early (≤8 hours) and those in whom treatment was started later. Significant reductions in the incidence of ventricular fibrillation and in chest pain following initial intravenous therapy were also observed with Metoprolol tartrate and were independent of the interval between onset of symptoms and initiation of therapy.
The precise mechanism of action of Metoprolol tartrate in patients with suspected or definite myocardial infarction is not known.
In this study, patients treated with metoprolol received the drug both very early (intravenously) and during a subsequent 3-month period, while placebo patients received no beta-blocker treatment for this period. The study thus was able to show a benefit from the overall metoprolol regimen but cannot separate the benefit of very early intravenous treatment from the benefit of later beta-blocker therapy. Nonetheless, because the overall regimen showed a clear beneficial effect on survival without evidence of an early adverse effect on survival, one acceptable dosage regimen is the precise regimen used in the trial. Because the specific benefit of very early treatment remains to be defined however, it is also reasonable to administer the drug orally to patients at a later time as is recommended for certain other beta blockers.
Pharmacokinetics: Only a small fraction of the drug (about 12%) is bound to human serum albumin. Less than 5% of an oral dose of Metoprolol tartrate is recovered unchanged in the urine; the rest is excreted by the kidneys as metabolites that appear to have no clinical significance. The systemic available and half-life of Metoprolol tartrate in patients with renal failure do not differ to a clinically significant degree from those in normal subjects. Consequently, no reduction in dosage is usually needed in patients with chronic renal failure.
Following intravenous administration of Metoprolol tartrate, the urinary recovery of unchanged drug is approximately 10%. When the drug was infused over a 10-minute period, in normal volunteers, maximum beta blockade was achieved at approximately 20 minutes. Doses of 5 mg and 15 mg yielded a maximal reduction in exercise-induced heart rate of approximately 10% and 15%, respectively. The effect on exercise heart rate decreased linearly with time at the same rate for both doses, and disappeared at approximately 5 hours and 8 hours for the 5 mg and 15 mg doses, respectively.
Equivalent maximal beta-blocking effect is achieved with oral and intravenous doses in the ratio of approximately 2.5:1.
There is a linear relationship between the log of plasma levels and reduction of exercise heart rate. However, antihypertensive activity does not appear to be related to plasma levels. Because of variable plasma levels attained with a given dose and lack of a consistent relationship of antihypertensive activity to dose, selection of proper dosage requires individual titration.
In several studies of patients with acute myocardial infarction, intravenous followed by oral administration of Metoprolol tartrate caused a reduction in heart rate, systolic blood pressure, and cardiac output. Stroke volume, diastolic blood pressure, and pulmonary artery end diastolic pressure remained unchanged.
In elderly subjects with clinically normal renal and hepatic function, there are no significant differences in Metoprolol pharmacokinetics compared to younger subjects.
