Plasma Levels and Negative Chronotropic Effect of Metoprolol Following Single Doses of a Conventional and Sustained-release Formulation

Plasma levels and associated reduction in exercise-induced tachycardia have been examined following the administration of single doses of metoprolol in conventional and slow-release tablets at different times to six healthy male subjects. The study was carried out in two parts. Initially, the tablets were given at 9 a.m. and the subjects were studied up to 14 h and then at 24 h. Subsequently, the same doses were given at 9 p.m. and the subjects were studied 12-24 h after drug administration (i.e. 9 a.m.-9 p.m. the next day). After giving the slow-release tablets the peak plasma levels were significantly lower but the drug persisted in the plasma at higher levels than after the conventional tablet. However, the beta-blocking effect was comparable from the two dosages. The results obtained for the period 12-24 h after the evening dose differed from the corresponding values after morning administration in that the plasma levels were higher and the beta-blocking effects more marked. Furthermore, the half-life values calculated from these data were significantly longer.