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The in vitro metabolism of M445526 (ZD6126 phenol) was investigated by incubating [14C]-M445526 at a concentration of 10 µg ml-1 with human hepatic microsomes (4 mg ml-1) or human hepatocytes (2 × 106 cells ml-1) for up to 180 min. Following incubation with microsomes and hepatocytes, up to 78% and 40% of [14C]-M445526 was metabolized after 180 and 120 min, respectively. High-performance liquid chromatography (HPLC) with radiochemical detection confirmed extensive metabolism of [14C]-M445526 by microsomes and hepatocytes. Mass spectrometry and 1H-NMR spectroscopy enabled structural identification of up to eight metabolites. Human liver microsomes formed one major (O-desmethyl) and three minor (a further O-desmethyl and two different hydroxylated) phase I metabolites. Human hepatocytes produced one major metabolite, a sulphate conjugate of the major O-desmethyl metabolite formed by microsomes. Four minor metabolites were also formed, primarily by O-demethylation with subsequent glucuronidation. Taken collectively, [14C]-M445526 underwent extensive in vitro metabolism by human liver fractions. These data were confirmed by subsequent human in vivo studies. [ABSTRACT FROM AUTHOR]