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Metabolism of ketotifen by human liver microsomes. In vitro characterization of a tertiary amine glucuronidation.

Biotransformation of ketotifen was investigated in vitro using human liver microsomes. Three of the four metabolic pathways observed in vivo in man were exhibited under the conditions of incubation, namely demethylation, N-oxidation, and N-glucuronidation, the absent route being the ketoreduction, which probably has a cytosolic localization. The kinetic parameters of the N-glucuronidation (KM for ketotifen and UDPGA and Vmax) were determined with native and detergent-treated microsomes. Treatment by Triton X-100 increased by about 3-fold the conjugation reaction. No sex difference was observed and N-glucuronidation did not seem to be inhibited either by bilirubin or by 4-nitrophenol. Thus, human liver microsomes are a useful and suitable in vitro model for studying metabolic routes, specific for man, as in the case of ketotifen. Obviously, the results obtained can only reflect partially the multiplicity of in vivo events and interpretation has to be complemented by investigations with other models.