PREDICTION OF DRUG-DRUG INTERACTIONS OF ZONISAMIDE METABOLISM IN HUMANS FROM IN-VITRO DATA

Objective: The purposes of this study were to identify the P450 enzyme (CYP) responsible for zonisamide metabolism in humans by using expres sed human CYPs and to predict drug interaction of zonisamide in vivo f rom in vitro data. Methods: Ten expressed human CYPs and human liver m icrosomes were used in the experiments for the identification of enzym es responsible for zonisamide metabolism and for the prediction of dru g-drug interactions of zonisamide metabolism in humans from in vitro d ata, respectively. Two-sulfamoylacetyl phenol, a reductive metabolite of zonisamide, was measured by the HPLC method. Results: From the expe riments using ten expressed human CYPs, CYP2C19, CYP3A4 and CYP3A5 wer e shown to be capable of catalyzing zonisamide reduction. However, an intrinsic clearance, V-max/k(M): of CYP3A4 was much higher than those of CYP2C19 and CYP3A5. From the point of view of enzyme amount in huma n liver CYPs isoform and their intrinsic clearance, it was suggested t hat CYP3A4 is mainly responsible for zonisamide metabolism in human CY Ps. Zonisamide metabolism in human liver microsomes was markedly inhib ited by cyclosporin A, dihydroergotamine, ketoconazole, itraconazole, miconazole and triazolam. We estimated the possibility and degree of c hange of zonisamide clearance in vivo in clinical dose range from in v itro inhibition constant of other drugs against zonisamide metabolism (Ki) and unbound inhibitor concentration in blood (Iu) in clinical usa ge. Clearance of zonisamide was maximally estimated to decrease by 31% , 23% and 17% of the clearance without inhibitors i.e. ketoconazole, c yclospolin A and miconazole, respectively. Fluconazole and carbamazepi ne are estimated to decrease by 5-6% of the clearance of zonisamide. O n the other hand, there may be lack of interaction of zonisamide metab olism by dihydroergotamine, itraconazole and triazolam in clinical dos e range. Conclusion: We demonstrated that: (1) zonisamide is metaboliz ed by recombinant CYP3A4, CYP2C19 and CYP3A5, (2) the metabolism is in hibited to a variable extent by known CYP3A4/5 substrates and/or inhib itors in human liver microsomes, and (3) in vitro-in vivo predictive c alculations suggest that several compounds demonstrating CYP3A4-affini ty might cause in vivo drug-drug interactions with zonisamide.