Characterization of the cytochrome P450 enzymes involved in the in vitro metabolism of rosiglitazone

Aims To identify the human cytochrome P450 enzyme(s) involved in the in vit ro metabolism of rosiglitazone, a potential oral antidiabetic agent for the treatment of type 2 diabetes-mellitus. Method The specific P450 enzymes involved in the metabolism of rosiglitazon e were determined by a combination of three approaches; multiple regression analysis of the rates of metabolism of rosiglitazone in human liver micros omes against selective P450 substrates, the effect of selective chemical in hibitors on rosiglitazone metabolism and the capability of expressed P450 e nzymes to mediate the major metabolic routes of rosiglitazone metabolism. Result The major products of metabolism following incubation of rosiglitazo ne with human liver microsomes were para-hydroxy and N-desmethyl rosiglitaz one. The rate of formation varied over 38-fold in the 47 human livers inves tigated and correlated with paclitaxel 6 alpha-hydroxylation (P < 0.001). F ormation of these metabolites was inhibited significantly (>50%) by 13-cis retinoic acid, a CYP2C8 inhibitor, bur. not by furafylline, quinidine or ke toconazole. In addition, both metabolites were produced by microsomes deriv ed from a cell line transfected with human CYP2C8 cDNA. There was some evid ence for CYP2C9 playing a minor role in the metabolism of rosiglitazone. Su lphaphenazole caused limited inhibition (<30%) of both pathways in human li ver microsomes and microsomes from cells transfected with CYP2C9 cDNA were able to mediate the metabolism of rosiglitazone, in particular the N-demeth ylation pathway, albeit at a much slower rate than CYP2C8. Rosiglitazone ca used moderate inhibition of paclitaxel 6 alpha-hydroxylase activity (CYP2C8 ; IC50 = 18 mu M), weak inhibition of tolbutamide hydroxylase activity (CYP 2C9; IC50 = 50 mu M), but caused Ilo marked inhibition of the other cytochr ome P450 activities investigated (CYP1A2, 2A6, 2C9, 2C19, 2D6, 2E1, 3A and 4A). Conclusion CYP2C8 is primarily responsible for the hydroxylation and N-deme thylation of rosiglitazone ill human liver; with minor contributions from C YP2C9.