Hydrochlorothiazide 12.5 mg, Metoprolol 50 mg Tablets

Metoprolol is a beta1-selective (cardioselective) adrenergic receptor blocker. This preferential effect is not absolute however, and at higher plasma concentrations, metoprolol also inhibits beta2-adrenoreceptors, chiefly located in the bronchial and vascular musculature. Metoprolol has no intrinsic sympathomimetic activity, and membrane-stabilizing activity is detectable only at plasma concentrations much greater than required for beta-blockade. Animal and human experiments indicate that metoprolol slows the sinus rate and decreases AV nodal conduction.

The mechanism of the antihypertensive effects of beta-blocking agents has not been 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.

The mechanism of antihypertensive effect of thiazide diuretics is unknown.

Absorption — In man, absorption of metoprolol HCT is rapid and complete. Plasma levels following oral administration, however, approximate 50% of levels following intravenous administration, indicating about 50% first-pass metabolism.

Hydrochlorothiazide is rapidly absorbed, as indicated by peak plasma concentrations 1 to 2.5 hours after oral administration. Plasma levels of the drug are proportional to dose; the concentration in whole blood is 1.6 to 1.8 times higher than in plasma.

Thiazides affect the renal tubular mechanism of electrolyte reabsorption. At maximal therapeutic dosage, all thiazides are approximately equal in their diuretic potency. Thiazides increase excretion of sodium and chloride in approximately equivalent amounts. Natriuresis causes a secondary loss of potassium.

The mechanism of the antihypertensive effect of thiazides is unknown. Thiazides do not affect normal blood pressure.

Effect of Food — Gastrointestinal absorption of hydrochlorothiazide is enhanced when administered with food. Absorption is decreased in patients with congestive heart failure, and the pharmacokinetics are considerably different in these patients.

Distribution — Plasma levels achieved are highly variable after oral administration. Only a small fraction of the drug (about 12%) is bound to human serum albumin. Metoprolol is a racemic mixture of R- and S-enantiomers. Less than 5% of an oral dose of metoprolol is recovered unchanged in the urine; the rest is excreted by the kidneys as metabolites that appear to have no clinical significance.

The systemic availability and half-life of metoprolol 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.

Elimination — Thiazides are eliminated rapidly by the kidney. After oral administration of 25 mg to 100 mg doses, 72% to 97% of the dose is excreted in the urine, indicating dose-independent absorption.

Hydrochlorothiazide is eliminated from plasma in a biphasic fashion with a terminal half-life of 10 to 17 hours. Plasma protein binding is 67.9%. Plasma clearance is 15.9 to 30.0 L/hr; volume of distribution is 3.6 to 7.8 L/kg.

Geriatric Patients — In elderly subjects with clinically normal renal function, there are no significant differences in metoprolol pharmacokinetics compared to young subjects.

Racial or Ethnic Groups — Metoprolol is extensively metabolized by the cytochrome P450 enzyme system in the liver. The oxidative metabolism of metoprolol is under genetic control with a major contribution of the polymorphic cytochrome P450 isoform 2D6 (CYP2D6). There are marked ethnic differences in the prevalence of the poor metabolizers (PM) phenotype. Approximately 7% of Caucasians and less than 1% Asian are poor metabolizers.