Determination of paeonol metabolites in man by the use of stable isotopes.

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Abstract
The metabolism of paeonol (2-hydroxy-4-methoxyacetophenone: I, a component of Botan-pi having antipyretic, sedative, antiinflammatory and antibacterial activities) in man was studied by a stable isotope tracer technique. For the qualitative analysis of metabolites, an aliquot of 24 h urine from a man who had received orally an equimolar mixture of I and I[methoxy-d3, acetyl-13C2] was investigated by mass chromatography. Paeonol, 2,5-dihydroxy-4-methoxyacetophenone (II) and resacetophenone (III) were identified as free, .beta.-glucuronide, sulfate and enzyme-resistant conjugate form metabolites in the urine. By comparing the ion intensities of fragment ions of II and II-13C2d3 and those of III and III-13C2, an isotope effect in the process of metabolism was found. This isotope effect was explained in terms of the metabolic switching phenomenon. By dilution analysis using I[methoxy-d3], II[methoxy-d3] and III[acetyl-13C2] as internal standards, the amounts of metabolites in the urine were determined. The excretion of this drug was very rapid; the total amount of metabolites excreted in the urine within 24 h reached 78.8% of the dose (II 50.2%, III 21.0%, I 7.6%). Predominant metabolites were the .beta.-glucuronide of II (28.8% of dose) and an enzyme-resistant conjugate form of II (23.9%).

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