BETA(1)-ADRENOCEPTOR SUBTYPE-SELECTIVE ANTAGONISM OF ESMOLOL AND ITS MAJOR METABOLITE IN-VITRO AND IN MAN - INVESTIGATIONS USING TRICRESYLPHOSPHATE AS RED-BLOOD-CELL CARBOXYLESTERASE INHIBITOR

Esmolol (GAS 103598-03-4, ASL-8052, Brevibloc(R)) is an ultrashort act ing beta-adrenoceptor antagonist which is rapidly hydrolyzed by a red blood cell esterase. In order to investigate the affinity profile of e smolol and its acid metabolite at beta-adrenoceptor subtypes in plasma and in whole blood in radioligand binding studies a potent esterase i nhibitor was needed to prevent hydrolysis of esmolol and - in contrast to sodium fluoride - without influencing binding of the drugs at the receptor site. Tricresylphosphate was found to be a potent inhibitor o f hydrolysis of esmolol which did not affect the parameters of radioli gand binding. During an incubation time of 15 min at 25 degrees C in w hole blood no significant metabolism of esmolol took place whereas, wi thout addition of inhibitor about 20% were inactivated. Thus, for the first time exact determinations of plasma concentrations of esmolol by ligand binding studies could be carried out. In vitro in radioligand binding studies esmolol had a 34fold higher affinity for beta(1)-adren oceptors than for beta(2)-adrenoceptors. Its acid metabolite had a ver y low and nonselective affinity for both adrenoceptor subtypes: Compar ed with esmolol it was 400fold less potent at beta(1)-adrenoceptors. W hen plasma concentrations of esmolol and its metabolite after i.v. adm inistration of esmolol to healthy volunteers (3000 mu g/kg as a bolus and consecutively 500 mu g/ kg/min for 70 min) were determined in para llel by a beta(1)-selective radioreceptor assay and an HPLC-method the following conclusions could be drawn from the direct correlation betw een the respective results. 1. Up to the nose mentioned above esmolol is still PI-selective with only insignificant occupancy of beta(2)-adr enoceptors. 2. The acid metabolite occurring in concentrations more th an 40fold higher than the parent compound does not interfere with the beta(1)-adrenoceptor occupation by esmolol nor does it - due to its lo w affinity - occupy these receptive sites. From the comparative in vit ro and in vivo studies it becomes evident that in vitro radioligand bi nding studies carried out under appropriate conditions are of high pre dictive value for in vivo studies concerning subtype selectivity of be ta-adrenoceptor antagonists and interference of possibly occurring act ive metabolites.