Identification of nifedipine metabolites and their determination by gas chromatography.

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The urinary metabolites of nifedipine in a dog were isolated and identified by chromatographic and spectrometric techniques. A new major metabolite, 2, 6-dimethyl-4-(2-nitrophenyl)-5-methoxycarbonylpyridine-3-carboxylic acid (M-I), was found in both dog and human urine. A highly sensitive and specific method is described for the determination of nifedipine, M-I, and a previously reported metabolite. The method for analysis of nifedipine is based on the oxidation of nifedipine to its pyridine analog, benzene extraction, and determination by electron-capture gas chromatography. The method for analysis of the metabolites is based on ethyl acetate extraction of the acidified specimen, derivatization of M-I to its methyl ester, and determination by electron-capture gas chromatography. Quantitative determination was possible down to 5 ng/ml of nifedipine using 1 ml samples of biological specimens. This method was used to measure plasma levels of nifedipine and its metabolites in dogs, and urinary excretion by humans after oral administration of a nifedipine preparation. It was found that nifedipine was absorbed, metabolized, and excreted rapidly, and that the main metabolite was M-I.