Abstract
The objective of this study was to elucidate stereoselective mechanisms of propranolol metabolism in man. Five normal subjects were given single 80 mg oral doses of deuterium-labeled pseudoracemates of propranolol, and the stereochemical composition of propranolol and its major metabolites in urine was determined by GC/MS. The (-)/(+)-enantiomer ratios for unchanged propranolol, 1.50 +/- 0.10 (mean +/- s.e. mean), and propranolol glucuronide, 1.76 +/- 0.10, were similar to previous findings in plasma. All products of side-chain oxidation also consisted mainly of the (-)-enantiomer, with an overall (-)/(+) ratio of 1.61 +/- 0.11. A (-)/(+) ratio of 1.04 +/- 0.17 for 4-hydroxypropranolol did not indicate stereoselectivity in ring oxidation. However, the ratio for its glucuronic acid conjugate of 1.78 +/- 0.19 and for its sulphate conjugate of 0.27 +/- 0.03 suggested stereoselectivity in either the glucuronidation or sulphation of 4-hydroxypropranolol, or both. When the stereoselectivity in these secondary pathways was taken into consideration, the overall ring oxidation strongly favoured (+)-propranolol with a (-)/(+)-enantiomer ratio of 0.59 +/- 0.09. The composite observations of the stereochemistry of propranolol metabolism in man are consistent with stereoselective ring oxidation of (+)-propranolol, leading to a greater bioavailability of the pharmacologically more active (-)-propranolol and subsequent preferential side-chain oxidation and glucuronidation of this enantiomer.
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Selected References
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