Plasma levels and β-blocking effect of α-hydroxymetoprolol—Metabolite of metoprolol—in the dog

Abstract

The plasma levels and the β- blocking effect of metoprolol and its active metabolite α- hydroxymetoprolol have been studied after i.v. bolus injections of the substances to dogs. For both substances the β- blockade increased with the dose, and there was a linear relationship between percent reduction in exercise heart rate and the logarithm of plasma concentration. The dose of the metabolite, however, had to be 5 times higher than that of metoprolol to induce the same degree of β- blockade. Because of differences in the volume of distribution, 2.0 liters/kg for α- OH-metoprolol and 3.5 liters/kg for metoprolol, the 5 times higher dose of α- OH-metoprolol resulted in 10 times higher plasma levels of the metabolite than of metoprolol. α- OH-Metoprolol was more slowly eliminated (t_1/2∼7.0 hr, total body clearance ∼3.5 ml-kg⁻¹-min⁻¹) than metoprolol (t_1/2∼2.0 hr, total body clearance ∼20.0 ml-kg⁻¹-min⁻¹). Approximately 5% of an i.v. dose of metoprolol was metabolized to α- OH-metoprolol. The half-life of the endogenously formed metabolite was the same as after an i.v. dose of the compound.