Stereoselective metabolism of carvedilol in the rat. Use of enantiomerically radiolabeled pseudoracemates.

14C-Labeled R(+)- and S(-)-carvedilol enantiomers were prepared by direct resolution of 14C-labeled racemic carvedilol on a chiral HPLC column. Two enantiomerically radiolabeled carvedilol pseudoracemates, 14C-labeled R(+)/unlabeled S(-)-carvedilol and 14C-labeled S(-)/unlabeled R(+)-carvedilol, were reconstituted and administered orally and iv to separate groups of bile duct-cannulated rats to determine the biliary excretion of carvedilol enantiomers and the stereochemical composition of metabolites excreted into the bile. The respective biliary excretions of the radioactivity derived from the radiolabeled R(+)- and S(-)-enantiomers accounted for 41.4 +/- 0.9 and 41.5 +/- 1.9% of the oral racemic dose, and 43.7 +/- 2.4 and 40.0 +/- 0.9% of the iv dose. Oral administration of these pseudoracemates produced no enantiomeric difference in the biliary excretion of the radioactivity derived from the enantiomers, whereas iv administration did result in an enantiomeric difference: the biliary excretion rate of the radioactivity derived from R(+)-enantiomer was higher than that from S(-)-enantiomer. After administration by the two routes, two carbazole ring-hydroxylated glucuronides, 1-hydroxycarvedilol O-glucuronide (1-OHCG) and 8-hydroxycarvedilol O-glucuronide (8-OHCG), were detected as the major metabolites in the bile. The S/R enantiomer ratios of 1-OHCG were 0.59 for oral dosing and 0.43 for iv dosing, suggesting that the formation of 1-OHCG is selective for R(+)-enantiomer, while the S/R ratios of 8-OHCG showed values of 3.29 and 2.63 for oral and iv administrations, respectively, favoring S(-)-enantiomer. Since corresponding hydroxylated metabolites are rapidly biotransformed to glucuronides that are excreted predominantly in the bile, the stereoselectivity of these glucuronides presumably reflects that of carbazole ring hydroxylation.