The metabolism of [14C]-zibotentan (ZD4054) in rat, dog and human, the loss of the radiolabel and the identification of an anomalous peak, derived from the animal feed

This paper presents an overview of a cross-species investigation of the metabolic fate of [14C]-zibotentan (ZD4054), with particular focus on the main analytical challenges encountered during the study. A combination of detection methods were used including HPLC coupled to UV, RAD and/or MS(MS), and 1H NMR spectroscopy. The objective was to characterise and identify the major metabolites found in the circulation and excreta of rat and dog for comparison with those produced in human. Initial investigations in rat, using [14C]-labelled zibotentan positioned on the oxadiazole ring and HPLC–UV–RAD analysis, revealed seven labelled resolved metabolite peaks. Parallel analysis by HPLC–UV–MS (with in-source fragmentation) uncovered two additional metabolites, indicating loss of the radiolabel during biotransformation. Hence, in subsequent studies in rat, dog and human, dual-radiolabelled zibotentan was employed with the 14C-label positioned on the pyridine ring, which was shown to be less prone to metabolism. A total of 12 metabolites were found in the excreta and plasma in all species. One of these metabolites was found in the circulation in humans, which warranted further investigations. Characterisation of the isolated human circulating metabolite by 1H NMR was complicated by the co-extraction of a matrix component with a similar UV-chromophore to zibotentan, which was identified as daidzein, an isoflavone derived from the animal feed.