Characterization of conjugated metabolites of a new angiotensin II receptor antagonist, candesartan cilexetil, in rats by liquid chromatography/electrospray tandem mass spectrometry following chemical derivatization.

Combined liquid chromatography and electrospray mass spectrometry (LC/ESI-MS) and tandem mass spectrometry (MS/MS) were used for the characterization of the conjugated metabolites (glucuronides) of a new angiotensin II receptor antagonist, candesartan cilexetil (TCV-116; (+/-)-1-(cyclohexyloxycarbonyloxy)ethyl 2-ethoxy-1-¿[2'-(1H-tetrazol-5-yl)biphenyl- 4-yl]methyl¿-1H-benzimidazole-7-carboxylate) in the plasma and bile of rats given the drug. The glucuronides of the active component, M-I (candesartan), in rat plasma and bile were positional isomers with respect to the binding site of glucuronic acid. The site of glucuronidation in M-I was not directly identified by mass spectrometry. However, the structure of the isomers could be elucidated by the MS/MS analysis of dimethylated glucuronides prepared by the reaction of glucuronide isomers with diazomethane: N-glucuronide of M-I (M-I-NG) in the plasma and acyl glucuronide (M-I-AG) in the bile. The results obtained in this study indicated that LC/ESI-MS/MS analysis provides the detailed structure of conjugated metabolite by simple chemical derivatization.