ALBERT

Notes: Summary β-adrenoceptor binding of (—) penbutolol and its active metabolite 4-hydroxy-penbutolol to rat reticulocyte membranes was shown in the presence of native human plasma. Due to the high plasma protein binding (∼90%) the apparent Ki-values of penbutolol were shifted 100-fold to the right after inclusion of plasma in the assay; the Ki was ∼40–70 ng/ml. That value is comparable to the IC50-values calculated from clinical studies. The interaction of 4-hydroxy-penbutolol with β-adrenoceptors was not affected to the same extent by inclusion of plasma protein binding ∼80%, apparent Ki-value ∼7 ng/ml. Thus, the active metabolite of penbutolol displays higher potency at β-adrenoceptors in vitro due to its lesser degree of plasma protein binding. A prediction procedure for antagonist activity after penbutolol administration using β-adrenoceptor interaction and plasma concentration kinetics suggests that, in addition to a rapid elimination process from human plasma, a slow elimination phase of penbutolol (or an active metabolite) is necessary to explain the long duration of action observed in clinical studies after a single oral dose. Inhibition in vitro of β-adrenoceptor binding by plasma samples obtained after oral administration of 40 mg penbutolol to 3 healthy volunteers indicated a biphasic concentration-time profile of the antagonist in plasma and was in accordance with the time course of the reported reduction in exercise tachycardia. Finally, plasma concentrations of penbutolol equivalents derived from the receptor assay were in the range of penbutolol concentrations detected by physico-chemical methods. It is concluded that the time course of antagonism against β-adrenoceptor-mediated effects after a single oral dose of penbutolol in man can readily be explained by its biphasic elimination kinetics from human plasma together with the properties of the β-adrenoceptor interaction detectable in vitro.

Notes: Summary Bupranolol is a non-selective beta-adrenoceptor antagonist with a Ki-value of 6–15 nmol/l (equivalent to 1.5–4 ng/ml in plasma) at beta1- (rat salivary gland) and beta2-adrenoceptors (rat reticulocytes) in receptor binding studies with3H-CGP 12177 in the presence of human plasma. After oral administration of 200 mg bupranolol to healthy volunteers, the maximal plasma concentration was observed within 1.2 h but it only reached a level close to the Ki-value. Elimination from plasma was rapid (t1/2=2.0 h). Administration of 30 mg bupranolol in a transdermal delivery system (TTS) every 24 h to 6 healthy volunteers for 72 h yielded steady state plasma concentrations 4- to 5-times above the Ki-value as shown by in vitro inhibition of beta-adrenoceptor binding by plasma samples. The pharmacodynamic effect, measured as the reduction in exercise tachycardia, showed a stable inhibitory effect; antagonism of a bolus injection of isoprenaline indicated a 10- to 15-fold right shift of the dose-response curve during the observation period of 72 h. It is concluded that steady-state plasma concentrations and effect of the elsewise rapidly eliminated beta-blocker bupranolol can be achieved by a transdermal delivery system applied each day.

Notes: Summary The pharmacokinetics of penbutolol 40 mg, its reduction in exercise-induced tachycardia, and the in vitro inhibition of radioligand binding to beta-adrenoceptors by plasma have been investigated in 7 healthy volunteers. The peak penbutolol concentration of 285 ng/ml was observed 1.2 h after administration, and the maximum of 4′-OH-penbutolol of 4.76 ng/ml was found after 1.64 h. Penbutolol was detected for up to 48 h, and 4′-OH-penbutolol dropped below the limit of detection after about 10 h. The terminal plasma concentration of penbutolol declined with an average half-life of 19 h. The maximum reduction in exercise-induced tachycardia was 33 beats/min 2.6 h after taking penbutolol. There was still a significant reduction of about 7 beats/min after 48 h. This effect could be adequately explained by the concentration-time course of penbutolol in combination with Clark's model of the concentration-effect relationship. Antagonist activity in plasma caused 91% inhibition of radioligand binding in vitro to beta2-adrenoceptors on rat reticulocyte membranes 1.6 h after intake of penbutolol. By 48 h after intake, radioligand binding was still significantly inhibited (23%). The in vitro inhibition of radioligand binding by plasma showed a linear correlation with the reduction in exercise-induced tachycardia for all phases of the workload. The time course of the reduction in heart rate was completely explained by the in vitro inhibition of radioligand binding. However, it was not possible to explain the in vitro inhibition of radioligand binding by the concentration-time course of penbutolol using a simple competition model, although both variables were based on the same sampling site. When the in vitro inhibition of radioligand binding was plotted against the penbutolol concentration at the same sampling times (with both variables transformed to multiples of the apparent inhibition constant) the discrepancy became even more apparent as time-related counterclockwise hysteresis. None of the known metabolites of penbutolol can explain the discrepancy between the penbutolol concentration and the inhibition of radioligand binding in vitro. It appears that an other active metabolite is formed, which contributes to the effect in vitro and in vivo and so can explain the observed discrepancy.

Notes: Summary A radioreceptor assay (RRA) for the assay of beta-adrenoceptor antagonists in native human plasma is described. The hydrophilic antagonist3H-CGP 12177 was used as the radioligand. In contrast to the hydrophobic radioligand3H-dihydroalprenolol, which was investigated in parallel, the beta-adrenoceptor binding of3H-CGP 12177 by rat reticulocyte membranes was found not to be affected by inclusion of increasing proportions (0–66% of incubation volume) of human plasma in the assay. Thus, solvent extraction of drug and/or active metabolites was not necessary to avoid binding of the radioligand tracer to plasma added in the RRA. The assay of unprocessed samples was possible. Drug concentrations in plasma after oral administration of propranolol (240 mg) or carteolol (30 mg) to 6 healthy volunteers were measured by the RRA and in parallel by a chemical method. The results from both methods agreed when the plasma concentration kinetics of propranolol were investigated (elimination half-life: 3.9 h). In contrast, plasma concentrations of carteolol were consistently higher according to the RRA after oral administration of the drug. Identical concentrations, however, were found by the RRA and chemical method using plasma samples spiked with carteolol. Plasma concentrations of carteolol detected by the chemical method decline monoexponentially (elimination half-life: 5.4 h). A similar half-life of elimination for parent drug was found by the RRA (5.9 h), but an additional term describing the appearance of an active metabolite was necessary to account for the biphasic drug elimination (elimination half-life of metabolite: 17.3 h). The latter result is in agreement with the appearance of 8-hydroxy-carteolol as an active metabolite, which shows similar affinity for beta-adrenoceptors as the parent drug. The active metabolite, with a 3-fold longer elimination half-life than the parent drug, will prolong the duration of the clinical effects of orally administered carteolol. In conclusion, the RRA permits the determination of beta-adrenoceptor antagonistic activity in native human plasma at concentrations as low as 0.1-fold the IC50-value of the drug or an active metabolite.

Notes: Summary In a double-blind, placebo-controlled study in 6 healthy volunteers, the correlation between beta-adrenoceptor binding, the time course of the effect and plasma concentration kinetics was investigated from 0 to 48 h after a single oral dose of propranolol 240 mg. First, the in vitro beta-adrenoceptor interaction of propranolol was investigated. Propranolol inhibited beta-adrenoceptor binding to rat parotid (beta1) and reticulocyte (beta2) membranes in the presence of pooled human plasma with a Ki of about 8 ng/ml plasma. After oral administration of 240 mg propranolol, concentration kinetics in plasma could be described by a Bateman function with a fictive concentration at time 0 of 275 ng/ml plasma, and a mean elimination half-life of 3.5 h. Using the concentration kinetics of propranolol in plasma together with its in vitro beta-adrenoceptor binding characteristics in the presence of placebo plasma from each individual, the time course of antagonism against beta-adrenoceptor mediated effects was predicted. The latter was in agreement with the time course of propranolol-induced inhibition of tachycardia due to orthostasis. After bicycle ergometry, however, the time course of inhibition of tachycardia was shorter than was predicted. Plasma sampled at various times after propranolol administration inhibited beta-adrenoceptor binding of the radioligand 3H-CGP 12177 to rat reticulocyte membranes in a fashion reflecting the time course of inhibition of exercise tachycardia observed in the volunteers. A direct, linear relation was shown between the in vitro inhibition of beta-adrenoceptor binding by the plasma samples withdrawn after propranolol administration and the inhibition of exercise tachycardia observed in parallel. The results show that the concentrations of antagonist present in plasma are representative of the concentrations in the effect compartment. Deep compartments of drug distribution appear irrelevant to the effects of the drugs. The relation between the plasma concentration of propranolol and the reduction in heart rate at various levels of physical effort shows no significant inhibition at rest and increasing IC50-values from orthostasis to 2 min and to 4 min of ergometry. IC50-values after orthostasis are in the range of the Ki-values from in vitro receptor binding studies, whereas the IC50-values after exercise are shifted 2-to 3-fold to the right relative to the Ki-values. This finding is in agreement with increased beta-adrenoceptor stimulation with increasing effort (release of endogenous noradrenaline), which shifts the antagonist concentration-effect curve to the right. Furthermore, the rightward shift can explain why with increasing effort the time course of the inhibitory effect of propranolol becomes shorter. Release of propranolol from presynaptic stores during exercise is irrelevant, since this would result in opposed effects on the concentration-effect relationship (leftward shift) and the time course of antagonism (longer effect) with increasing work load. It is concluded that the receptor interaction of propranolol together with its plasma concentration kinetics can fully explain the time course of effects after a single oral dose, and so receptor interaction will be the missing link in the correlation between concentration kinetics and effect kinetics of propranolol in man. In general, this mode of correlation should be expandable to any drug exerting its effects according to the law of mass action via receptors in the extracellular space. This approach provides a rational basis for the comparison of different drugs from one group irrespective of their receptor affinity and concentration kinetics.

Notes: Summary A randomized, double-blind, placebo-controlled study was performed in 8 white and 8 black volunteers matched for sex, age and mass. The effect of 3 intravenous doses of a new, cardiose-lective beta-adrenergic blocker, bisoprolol, on the heart rate increase after standardized exercise was compared to that of 3 doses of propranolol. As described previously for propranolol, black volunteers showed less response than whites to beta-blockade assessed in terms of the reduction in exercise-induced tachycardia. The effects of the two beta-blockers were similar and the apparent ethnic difference was seen with both drugs. It has previously been shown that black volunteers have a higher intrinsic heart rate (i.e. heart rate after parasympathetic and beta-adrenergic blockade of the heart) than whites, but their resting heart rates are similar because of greater parasympathetic tone in blacks. When exercise-load was calculated as increase in heart rate above that after atropinization, no ethnic differences were seen. It is suggested that in populations that are heterogenous in terms of the heart rate increase after atropine, work load should be standardized in terms of the increase in heart rate over the atropine heart rate rather than on absolute heart rate. The apparent ethnic difference represents a flaw in methodology as applied to a heterogenous volunteer population.

Notes: Schlußfolgerungen Die quantitative Charakterisierung von β-Adrenoreceptoren ausschließlich mit Antagonistliganden durchgeführt, ist daher unzureichend; eindeutige funktioneile Rückschlüsse lassen sich nur auf Grund von zusätzlichen Untersuchungen mit Agonistliganden ziehen. Die Untersuchungen wurden mit Unterstützung der Deutschen Forschungsgemeinschaft durchgeführt.