Characterization of the phase I and phase II metabolic profile of tolvaptan by in vitro studies and liquid chromatography-mass spectrometry profiling: Relevance to doping control analysis

Our data show that tolvaptan is metabolized to at least 20 phase I metabolites, the biotransformation reactions being catalyzed mainly by CYP3A4 and CYP3A5 isoforms. The phase-I reactions include hydroxylation (in different positions), carboxylation, oxidation, hydrogenation, dealkylation, isomerization and a combination of the above. Most of the phase I metabolites undergo glucuronidation, carried out mostly by UGT2B7 and UGT2B17 isoforms. Dealkylated, mono-hydroxylated and carboxylated metabolites both in the free and in the glucuronidated form appear to be the most suitable urinary diagnostic markers for the detection of tolvaptan intake in doping control. Concerning the effects of physiological and non-physiological factors on the metabolic profile of tolvaptan, our results show that (i) no significant gender differences were detected; (ii) significant differences were registered in the presence of different CYP3A5 allelic variants, and finally (iii) a marked reduction of the detected metabolites was registered in the presence of antifungals, and, to a lesser extent, of steroidal progestins.