Different Roles of Human Cytochrome P450 2C9 and 3A Enzymes in Diclofenac 4′- and 5-Hydroxylations Mediated by Metabolically Inactivated Human Hepatocytes in Previously Transplanted Chimeric Mice

To investigate the respective roles of cytochromes P450 2C9 and 3A in drug oxidation in human livers, the in vivo pharmacokinetics of S-warfarin and diclofenac were analyzed after intravenous administrations in chimeric mice that had been transplanted with human hepatocytes. P450 2C9 was metabolically inactivated in the humanized mice by orally pretreating them with tienilic acid. After intravenous administration of S-warfarin, a significant difference in the concentration-time profiles of the primary metabolite 7-hydroxywarfarin between untreated mice and mice treated with tienilic acid was observed. In contrast, there were no apparent differences in the profiles for S-warfarin between the treated and untreated groups. The mean values of the maximum concentrations (C-max) and the areas under the plasma concentration versus time curves (AUC(infinity)) for 7-hydroxywarfarin were significantly lower (22 and 16% of the untreated values, respectively) in the treated group. This presumably resulted from suppressed P450 2C9 activity in the primary oxidative metabolism in vivo in the treated group. After diclofenac administration, plasma levels of diclofenac, 5-hydroxydidofenac, and diclofenac acylglucuronide were roughly similar in pretreated and untreated mice. However, the mean C-max and AUC(infinity) values for 4'-hydroxydiclofenac were significantly lower (38 and 53% of the untreated group, respectively) in the treated group. The reported value of similar to 0.8 for the fraction of S-warfarin metabolized to 7-hydroxywarfarin mediated by P450 2C9 in in vitro systems was similar to the value implied by the present humanized-liver mouse model pretreated with tienilic acid in which the AUC of 7-hydroxywarfarin was reduced by 84%. In contrast, the fractions of diclofenac metabolized to 4'-hydroxydiclofenac in in vitro and in vivo experiments were inconsistent. These results suggested that humanized-liver mice orally treated with tienilic acid might constitute an in vivo model for metabolically inactivated P450 2C9 in human hepatocytes transplanted into chimeric mice. Moreover, diclofenac, a typical in vitro P450 2C9 probe substrate, was cleared differently in vitro and in humanized-liver mice in vivo.