HUMAN LIVER CYTOCHROME P4503A BIOTRANSFORMATION OF THE CYCLOSPORINE DERIVATIVE SDZ IMM-125

In humans, cytochrome P4503A (CYP3A) is the major cytochrome P450 gene family that metabolizes SDZ IMM 125 (IMM) to its primary metabolites, Human liver microsomes could be used for this study, because the meta bolite profile matched that found in human blood. The apparent affinit y (K-M) of IMM for the cytochrome P450 proteins (5.1 +/- 1.8 mu M) is similar to that of cyclosporin A (CSA). CSA competitively inhibited th e metabolism of IMM, increasing the K-M 2- and 4.6-fold in the presenc e of 4 and 10 mu M CSA, respectively (K-i 3.8 +/- 1.1 mu M). Ketoconaz ole exhibited competitive inhibition kinetics toward IMM biotransforma tion, increasing the K-M of IMM 1.8-fold at 0.5 mu M ketoconazole and 3.5-fold at 1 mu M ketoconazole, with no effect on V-max (K-i of 0.5 /- 0.4 mu M). These results indicate that both CSA and ketoconazole wo uld cause drug interactions, interfering with the biotransformation of IMM. The metabolism of IMM was also greatly inhibited (similar to 80% ) by the CYP3A suicide substrate triacetyloleandomycin and a CYP3A inh ibitory antibody, indicating the involvement of CYP3A proteins in the biotransformation of IMM. Confirmation of CYP3A4 involvement in the fo rmation of the three primary IMM metabolites was demonstrated with rec ombinant cells expressing human CYP3A4. Therefore, compounds interacti ng with CYP3A proteins are expected to cause drug-drug interactions (i .e. the antimycotics ketoconazole and clotrimazole, the steroids ethin ylestradiol and testosterone, the ergots, the calcium channel blocker nifedipine, and the immunosuppressants FK-506 and rapamycin). In summa ry, this study has demonstrated that the biotransformation of the CSA derivative IMM to its three primary metabolites, which are analogous t o those of CSA, is CPY3A-dependent in humans, of which the CYP3A4 enzy me is primarily responsible.