Metaproterenol (Metaproterenol Sulfate) - Description and Clinical Pharmacology

DESCRIPTION

Metaproterenol sulfate in tablet form is an oral bronchodilator.

Each tablet, for oral administration, contains 10 mg or 20 mg of metaproterenol sulfate. In addition, each tablet contains the following inactive ingredients: colloidal silicon dioxide, corn starch, lactose anhydrous, magnesium stearate, microcrystalline cellulose and pregelatinized starch.

Metaproterenol sulfate, 1-(3,5 dihydroxyphenyl) -2-isopropyl-aminoethanol sulfate, is a white, crystalline, racemic mixture of two optically active isomers. It has the following structural formula:

CLINICAL PHARMACOLOGY

Metaproterenol sulfate is a beta adrenergic agonist bronchodilator.

The pharmacologic effects of beta adrenergic agonist drugs, including metaproterenol, are at least in part attributable to stimulation through beta adrenergic receptors of intracellular adenyl cyclase, the enzyme which catalyzes the conversion of adenosine triphosphate (ATP) to cyclic-3',5'-adenosine monophosphate (c-AMP). Increased c-AMP levels are associated with relaxation of bronchial smooth muscle and inhibition of release mediators of immediate hypersensitivity from cells, especially from mast cells.

Pharmacokinetics: Absorption, biotransformation and excretion studies in humans following oral administration have indicated that an average of less than 10% of the drug is absorbed intact; it is not metabolized by catechol-O-methyltransferase nor converted to glucuronide conjugates but is excreted primarily as the polar sulfate conjugate, metaproterenol-3-O-sulfate, formed in the gut.

Pulmonary function tests performed after the administration of metaproterenol usually show improvement, e.g., an increase in the one-second forced expiratory volume (FEV₁), maximum expiratory flow rate, peak expiratory flow rate, forced vital capacity, and/or a decrease in airway resistance. The resultant decrease in airway obstruction may relieve the dyspnea associated with bronchospasm.

In controlled single- and multiple-dose studies in which 319 patients were treated with metaproterenol sulfate tablets (89 patients with 10 mg and 230 patients with 20 mg), a majority (65%) demonstrated improvements in pulmonary function defined as an increase of at least 15% in the one-second forced expiratory volume (FEV₁). For 54% the onset was within 30 minutes. The duration of effect persisted for at least four hours in 51% of those patients who demonstrated a response.

Recent studies in laboratory animals (minipigs, rodents and dogs) recorded the occurrence of cardiac arrhythmias and sudden death (with histologic evidence of myocardial necrosis) when beta agonists and methylxanthines were administered concurrently. The significance of these findings when applied to humans is currently unknown.