Inhibition of Isoproterenol-Induced Tachycardia by Azimilide in the Isolated Perfused Guinea Pig Heart

The class III antiarrhythmic agent, azimilide, has been shown to inhibit dihydroalprenolol binding to the beta-adrenergic receptor of rat brain and heart in an in-vitro ligand-binding assay. Azimilide, was assessed for beta-adrenergic activity, either agonist or antagonist, in the isolated perfused guinea pig heart in comparison with class III reference agents and the class II agent, propranolol. Varying concentrations of compound (0.03–100 μM) were retrogradely perfused and the effects on corrected QT interval, baseline heart rate, and isoproterenol-stimulated heart rate were measured. Propranolol, dl-sotalol, azimilide, and d-sotalol inhibited isoproterenol-induced tachycardia with IC50 values (the concentration giving 50% inhibition of isoproterenol-stimulated heart rate) of 0.12, 1.4, 14.6, and 38.0 μM, respectively. Clofilium, dofetilide, and sematilide did not affect the action of isoproterenol. Dofetilide, clofilium, azimilide, sematilide, dl-sotalol, and d-sotalol increased the QTc interval approximately 20 ms at concentrations of 0.1, 0.3, 1.0, 3.0, 30.0, and 100.0 μM, respectively. The class III antiarrhythmic agents also slowed baseline heart rate and exhibited linear R-R and QT-interval relationships of similar slope. Azimilide’s antagonism of isoproterenol in this isolated heart model may reflect a direct receptor interaction or a contribution from the bradycardic action of the compound, which distinguishes it from several other pure IKr-blocking class III antiarrhythmic agents.