ISSN:
0899-0042
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The stereoselectivity of carvedilol, a novel β-adrenoceptor antagonist and vasodilator with one asymmetric carbon atom, was examined at α1- and β1-adrenoceptors in vitro and in vivo. (-)-(S)-Carvedilol is a potent, competitive antagonist of the β1-adrenoceptor-mediated positive chronotropic response to isoproterenol in guinea pig atrium, with a dissociation constant (KB) of 0.4 nM. (+)-(R)-Carvedilol was more than 100-fold less potent than the (-)-S-enantiomer as an antagonist of β;1-andrenoceptors, having a KB of approximately 45 nM. Consistent with these findings (-)-(S)-carvedilol (0.1 mg/kg, i.v.) produced a 25-fold rightward shift in the β1-adrenoceptor-mediated positive chronotropic response to isoproterenol in pithed rats, whereas the (+)-R-enantiomer had no β1-adrenoceptor blocking activity in vivo at this dose. In contrast to the marked degree of stereoselectivity observed at βl1-adrenoceptors, both (-)-(S)- and (+)-(R)-carvedilol produced equal antagonism of the α1----adrenoceptor-mediated vasoconstrictor response to norepinephrine in rabbit aorta, with KB values of 14 and 16 nM, respectively. Furthermore, in the pithed rat, the α1-adrenoceptor-mediated pressor dose-response curve to cirazoline was shifted approximately 6-fold to the right by both the (+)-R- and (-)-S-enantiomers of carvedilol at a dose of 1 mg/kg, i.v. In anesthetized spontaneously hypertensive rats, (-)-(S)-carvedilol was 6-fold more potent as an antihypertensive than (+)-(R)-carvedilol. The vasodilator and acute antihypertensive activity of carvedilol results from α1-andrenoceptor blockade produced by both enantiomers, and the concomitant β1-adrenoceptor blockade produced by the (-)-S-enantiomer, which prevents reflex tachycardia that can offset the antihypertensive response, leading to greater overall antihypertensive potency of (-)-(S)-carvedilol relative to the (+)-R-enantiomer. These data also suggest that distinct regions of the carvedilol molecule are responsible for blocking α1- and β1-adrenoceptors, with β1-adrenoceptor blockade resulting from an area of the molecule containing the asymmetric carbon atom, specifically the carbazolyloxy propanolamine moiety, and α1-adrenoceptor blockade resulting from a part of the molecule that does not contain the asymmetric carbon atom, most likely the phenoxyethylamine moiety.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/chir.530010404
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