Biotransformation of amlodipine. Identification and synthesis of metabolites found in rat, dog and human urine/confirmation of structures by gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry

Metabolism of the dihydropyridine calcium antagonist (R,S)-2-[(2-aminoethoxy)methyl]-4-(2-chlorophenyl)-3-ethoxycarbony l-5- methoxycarbonyl- 6 -methyl- 1,4-dihydropyridine (amlodipine) has been studied in animals and man using 14C-labelled drug. The metabolite patterns are complex; 18 metabolites have been isolated from rat, dog and human urine. Based on chromatographic and mass-spectral evidence, structures have been proposed for the main metabolites and confirmed by synthesis of unambiguous reference compounds. Comparison of all reference compounds and isolated metabolites was made by gas chromatography-mass spectrometry pressure liquid chromatography on-line thermospray-mass spectrometry of underivatised compounds directly in urine. The metabolites are largely pyridine derivatives. The methods used in structure designation are presented, along with the proposed route of metabolism, which indicates that the metabolic pattern for amlodipine in man has features in common with those of both rat and dog.