Metoprolol Tartrate

METOPROLOL TARTRATE- metoprolol tartrate tablet, film coated
Teva Pharmaceuticals USA Inc

Rx only

DESCRIPTION

Metoprolol tartrate, USP is a selective beta1 -adrenoreceptor blocking agent, available as 50 and 100 mg tablets for oral administration. Metoprolol tartrate, USP is 1-(isopropylamino)-3-[p -(2-methoxyethyl) phenoxy]-2-propanol (2:1) dextro -tartrate salt, and its structural formula is:

Structural formula for metoprolol tartrate
(click image for full-size original)

(C15 H25 NO3 )2 •C4 H6 O6 M.W. 684.82

Metoprolol tartrate, USP is a white, practically odorless, crystalline powder. It is very soluble in water; freely soluble in methylene chloride, in chloroform, and in alcohol; slightly soluble in acetone; and insoluble in ether.

Metoprolol Tartrate Tablets USP, for oral administration, contain 50 mg or 100 mg of metoprolol tartrate, USP. In addition, each tablet contains the inactive ingredients: colloidal silicon dioxide, hypromellose, lactose monohydrate, magnesium stearate, polyethylene glycol, povidone, propylene glycol, sodium starch glycolate, talc, and titanium dioxide. The 50 mg tablets contain D&C Red No. 30 Aluminum Lake, and the 100 mg tablets contain FD&C Blue No. 1 Lake and FD&C Blue No. 2 Aluminum Lake.

CLINICAL PHARMACOLOGY

Mechanism of Action

Metoprolol tartrate is a beta1 -selective (cardioselective) adrenergic receptor blocker. This preferential effect is not absolute, however, and at higher plasma concentrations, metoprolol tartrate also inhibits beta2 -adrenoreceptors, chiefly located in the bronchial and vascular musculature.

Clinical pharmacology studies have demonstrated the beta-blocking activity of metoprolol, 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.

Hypertension

The mechanism of the antihypertensive effects of beta-blocking agents has not been fully elucidated. However, several possible mechanisms have been proposed: (1) competitive antagonism of catecholamines at peripheral (especially cardiac) adrenergic neuron sites, leading to decreased cardiac output; (2) a central effect leading to reduced sympathetic outflow to the periphery; and (3) suppression of renin activity.

Angina Pectoris

By blocking catecholamine-induced increases in heart rate, in velocity and extent of myocardial contraction, and in blood pressure, metoprolol tartrate reduces the oxygen requirements of the heart at any given level of effort, thus making it useful in the long-term management of angina pectoris.

Myocardial Infarction

The precise mechanism of action of metoprolol tartrate in patients with suspected or definite myocardial infarction is not known.

Pharmacodynamics

Relative beta1 selectivity is demonstrated by the following: (1) In healthy subjects, metoprolol tartrate is unable to reverse the beta2 -mediated vasodilating effects of epinephrine. This contrasts 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 FEV1 and FVC significantly less than a nonselective beta blocker, propranolol, 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. Animal and human experiments indicate that metoprolol tartrate slows the sinus rate and decreases AV nodal conduction.

Significant beta-blocking effect (as measured by reduction of exercise heart rate) occurs within 1 hour after oral administration, and its duration is dose-related. For example, a 50% reduction of the maximum effect after single oral doses of 20, 50, and 100 mg occurred at 3.3, 5, and 6.4 hours, respectively, in normal subjects. After repeated oral dosages of 100 mg twice daily, a significant reduction in exercise systolic blood pressure was evident at 12 hours. When the drug was infused over a 10 minute period, in normal volunteers, maximum beta blockade was achieved at approximately 20 minutes. 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 patients with angina pectoris, plasma concentration measured at 1 hour is linearly related to the oral dose within the range of 50 to 400 mg. Exercise heart rate and systolic blood pressure are reduced in relation to the logarithm of the oral dose of metoprolol. The increase in exercise capacity and the reduction in left ventricular ischemia are also significantly related to the logarithm of the oral dose.

Pharmacokinetics

Absorption

The estimated oral bioavailability of immediate release metoprolol is about 50% because of pre-systemic metabolism which is saturable leading to non-proportionate increase in the exposure with increased dose.

Distribution

Metoprolol is extensively distributed with a reported volume of distribution of 3.2 to 5.6 L/kg. About 10% of metoprolol in plasma is bound to serum albumin. Metoprolol is known to cross the placenta and is found in breast milk. Metoprolol is also known to cross the blood brain barrier following oral administration and CSF concentrations close to that observed in plasma have been reported. Metoprolol is not a significant P-glycoprotein substrate.

Metabolism

Metoprolol tartrate is primarily metabolized by CYP2D6. Metoprolol is a racemic mixture of R- and S- enantiomers, and when administered orally, it exhibits stereoselective metabolism that is dependent on oxidation phenotype. CYP2D6 is absent (poor metabolizers) in about 8% of Caucasians and about 2% of most other populations. Poor CYP2D6 metabolizers exhibit several-fold higher plasma concentrations of metoprolol tartrate than extensive metabolizers with normal CYP2D6 activity thereby decreasing metoprolol tartrate’s cardioselectivity.

Elimination

Elimination of metoprolol tartrate is mainly by biotransformation in the liver. The mean elimination half-life of metoprolol is 3 to 4 hours; in poor CYP2D6 metabolizers the half-life may be 7 to 9 hours. Approximately 95% of the dose can be recovered in urine. In most subjects (extensive metabolizers), less than 5% of an oral dose and less than 10% of an intravenous dose are excreted as unchanged drug in the urine. In poor metabolizers, up to 30% or 40% of oral or intravenous doses, respectively, may be excreted unchanged; the rest is excreted by the kidneys as metabolites that appear to have no beta blocking activity. The renal clearance of the stereo-isomers does not exhibit stereo-selectivity in renal excretion.

Special Populations

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