Abstract

The metabolism of [¹⁴C]pioglitazone was studied in vitro in incubations with freshly isolated human, rat, and monkey hepatocytes. Radioactivity detection high-performance liquid chromatography analysis of incubation extracts showed the detection of thirteen metabolites (M1-M13) formed in incubations with human hepatocytes. An identical set of metabolites (M1-M13) was also detected in monkey hepatocytes. However, in rat hepatocytes, M1-M3, M5-M7, M9-M11, and M13 were also detected, but M4, M8, and M12 were not detected. The structures of the metabolites were elucidated by liquid chromatography/tandem mass spectrometry using electrospray ionization. Novel metabolites of pioglitazone detected using these methods included thiazolidinedione ring-opened methyl sulfoxide amide (M1), thiazolidinedione ring-opened N-glucuronide (M2), thiazolidinedione ring-opened methyl sulfone amide (M3), thiazolidinedione ring N-glucuronide (M7), thiazolidinedione ring-opened methylmercapto amide (M8), and thiazolidinedione ring-opened methylmercapto carboxylic acid (M11). In summary, based on the results from these studies, two novel metabolic pathways for pioglitazone in hepatocytes are proposed to be: 1) N-glucuronidation of the thiazolidinedione ring of pioglitazone to form M7 followed by hydrolysis to M2, and methylation of the mercapto group of the thiazolidinedione ring-opened mercapto carboxylic acid to form M11; and 2) methylation of the mercapto group of the thiazolidinedione ring-opened mercapto amide to form M8, oxidation of M8 to form M1, and oxidation of M1 to form M3.