Urinary metabolites of 4-isobutylphenylacetic acid studied by combined gas chromatography mass spectrometry.

After ingestion of 4-isobutylphenylacetic acid three metabolites were identified in the urine, viz. 4-(2-carboxypropyl)phenylacetic acid, 4-(2-hydroxy-2-methylpropyl)phenylacetic acid and 4-(1-hydroxy-2-methylpropyl)phenylacetic acid. These metabolites were identified by gas chromatography and mass spectrometry. In the in vivo formation of the metabolites the isobutyl sidechain of the drug is attacked by omega1-hydroxylation, followed by further oxidation of the primary alcohol to a carboxyl group, by omega2-hydroxylation and by omega3-hydroxylation, respectively. It has been shown previously that two other drugs with an isobutyl sidechain, viz. 2-p-methoxybenzenesulphonamido-5-isobutyl-1,3,4-thiadiazole and 2,4'-isobutylphenylpropionic acid, are metabolized in an analogous way.