Objective: The purposes of this study were to identify the P450 enzyme (CYP) responsible for zonisamide metabolism in humans by using expressed human CYPs and to predict drug interaction of zonisamide in vivo from in vitro data.
Methods: Ten expressed human CYPs and human liver microsomes were used in the experiments for the identification of enzymes responsible for zonisamide metabolism and for the prediction of drug-drug interactions of zonisamide metabolism in humans from in vitro data, respectively. Two-sulfamoylacetyl phenol, a reductive metabolite of zonisamide, was measured by the HPLC method.
Results: From the experiments using ten expressed human CYPs, CYP2C19, CYP3A4 and CYP3A5 were shown to be capable of catalyzing zonisamide reduction. However, an intrinsic clearance, Vmax/kM, of CYP3A4 was much higher than those of CYP2C19 and CYP3A5. From the point of view of enzyme amount in human liver CYPs isoform and their intrinsic clearance, it was suggested that CYP3A4 is mainly responsible for zonisamide metabolism in human CYPs. Zonisamide metabolism in human liver microsomes was markedly inhibited by cyclosporin A, dihydroergotamine, ketoconazole, itraconazole, miconazole and triazolam. We estimated the possibility and degree of change of zonisamide clearance in vivo in clinical dose range from in vitro inhibition constant of other drugs against zonisamide metabolism (Ki) and unbound inhibitor concentration in blood (Iu) in clinical usage. Clearance of zonisamide was maximally estimated to decrease by 31%, 23% and 17% of the clearance without inhibitors i.e. ketoconazole, cyclospolin A and miconazole, respectively. Fluconazole and carbamazepine are estimated to decrease by 5-6% of the clearance of zonisamide. On the other hand, there may be lack of interaction of zonisamide metabolism by dihydroergotamine, itraconazole and triazolam in clinical dose range.
Conclusion: We demonstrated that: (1) zonisamide is metabolized by recombinant CYP3A4, CYP2C19 and CYP3A5, (2) the metabolism is inhibited to a variable extent by known CYP3A4/5 substrates and/or inhibitors in human liver microsomes, and (3) in vitro-in vivo predictive calculations suggest that several compounds demonstrating CYP3A4-affinity might cause in vivo drug-drug interactions with zonisamide.