Metoprolol

Manufacturing Process

The starting material 1,2-epoxy-3-[p-(β-methoxyethyl)phenoxy]-propane was obtained from p-(β-methoxyethyl)-phenol which was reacted with epichlorohydrin whereafter the reaction product was distilled at 118°C to 128°C at a pressure of 0.35mm Hg.1,2-Epoxy-3-[p-(β-methoxyethyl)-phenoxy]-propane (16.7g) was dissolved in 50 ml isopropanol and mixed with 20 ml isopropylamine. The mixture was heated in an autoclave on boiling water-bath overnight, whereafter it was evaporated and the remainder dissolved in 2 N HCI. The solution was extracted first with ether and thereafter with methylene chloride. After evaporating the methylene chloride phase, the hydrochloride of 1- isopropylamino-3-[p(β-methoxyethyl)-phenoxy] -propanol-2 was obtained which, after recrystallization from ethyl acetate, weighed 10.4 g. Melting point 83°C. Equivalent weight: found 304.0, calculated 303.8.The hydrochloride is then converted to the tartrate.

Therapeutic Function

Beta-adrenergic blocker

Mechanism of action

Unlike propranolol, which blocks both β1 and β2-adrenoreceptors, metroprolol exhibits cardioselective action, i.e. in therapeutic doses, it blocks β1-adrenoreceptors with insignificant effects on β2-adrenoreceptors.

Synthesis

Metoprolol, 1-(iso-propylamino)-3-[4′(2-methoxyethyl)phenoxy]-2- propanol (12.1.5), is synthesized by reacting 4-(2-methoxyethyl)phenol with epichlorhydride in the presence of a base, isolating 1,2-epoxy-3-[4′(2-methoxyethyl)phenoxy] propane (12.1.4), the subsequent reaction of which, analogous to that described before, with iso-propylamine, gives an opening of the epoxide ring and leads to the formation of metoprolol (12.1.5) [7,8].

Metabolism

The pharmacokinetic profile of metoprolol (Lopressor) is similar to that of propranolol. Metoprolol is readily and rapidly absorbed after oral administration and is subject to a significant amount of first-pass metabolism by the liver. Curiously, the duration of metoprolol’s action is longer than one would predict from its plasma half-life, which ranges from 0.5 to 2.5 hours. The degree of binding of metoprolol to plasma proteins is modest (10%). The extensive distribution of metoprolol to the lungs and kidney is typical of a moderately lipophilic drug. Metoprolol undergoes considerable metabolism;only 3 to 10% of an administered dose is recovered as unchanged drug.The metabolites are essentially inactive as -receptor blocking agents and are eliminated primarily by renal excretion. Small amounts of the drug are present in the feces.

Brand name

Lopressor (Novartis).