[Studies on the analysis of drugs by use of isotopes. IV. Reverse dilution analysis of the metabolites of 14C-ephedrine]

Studies on drug metabolism by use of isotopes. XXIX--Studies of the differences in biological fates of ephedrine isomers by use of a pseudo-racemic mixture

The differences in metabolic fates of the optical isomers of ephedrine were investigated by use of a pseudo-racemic mixture technique. In rats, the (-)-isomer was more easily p-hydroxylated than the (+)-isomer. A stereoselective reaction in the formation of the glucuronides of ephedrine, norephedrine and p-hydroxyephedrine was observed. The (-)-isomers were more easily subjected to glucuronide formation than the (+)-isomers. In human subjects, N-demethylation proved to be a stereoselective reaction. It is also suspected that oxidative dealkylation of norephedrine is a stereoselective reaction.

The metabolism of (-)-ephedrine in man

The metabolic fate of orally administered (-)-[14C]-ephedrine has been studied in 3 human subjects and the urinary excretion of metabolites determined quantitatively by solvent extraction, paper chromatography and reverse isotope dilution procedures. Following an oral dose of the drug (0.35 mg/kg, 1.6 muCi), 97% of the dose was excreted in the urine within 48 h, 88% in the first 24 h. Unchanged drug was the major urinary excretory product (53-74%), with N-demethylation occurring to a variable extent (8-20%) although there was little interindividual variation in urine pH. Oxidative deamination was also variable (4-13%); the main identified products of this were benzoic acid (free and conjugated) and 1,2-dihydroxy-1-phenylpropane (free and conjugated). No phenolic metabolites could be detected, and thus it would appear that these compounds cannot be implicated in the acquisition of tolerance to ephedrine which can occur on repeated dosage.

Species differences in the metabolism of norephedrine in man, rabbit and rat

1. (+/-)-2-Amino-1-phenyl[1-(14)C]propan-1-ol ([(14)C]norephedrine) was administered orally to man, rat and rabbit and the metabolites excreted in the urine were identified and measured. Pronounced species differences in the metabolism of the drug were found. 2. Three male human subjects, receiving 25mg each of [(14)C]norephedrine hydrochloride, excreted over 90% of the (14)C in the first day. The main metabolite was the unchanged drug (86% of the dose) and minor metabolites were hippuric acid and 4-hydroxynorephedrine. 3. In rats given 12mg of the drug/kg almost 80% of the (14)C administered was excreted in the first day. The major metabolites in the urine were the unchanged drug (48% of the dose), 4-hydroxynorephedrine (28%) and trace amounts of side-chain degradation products. 4. Rabbits given 12mg of the drug/kg excreted 85-95% of the dose of (14)C in the urine in the first 24h after dosing. The major metabolites in the urine were conjugates of 1,2-dihydroxy-1-phenylpropane (31% of the dose) and of 1-hydroxy-1-phenylpropan-2-one (27%) and hippuric acid (20%). The unchanged drug was excreted in relatively small amounts (8%).