CHARACTERIZATION OF HUMAN LIVER MICROSOMAL CYTOCHROME-P450 INVOLVED IN THE REDUCTIVE METABOLISM OF ZONISAMIDE

Zonisamide (1,2-benzisoxazole-3-methanesulfonamide) was metabolized to 2-sulfamoylacetylphenol (SMAP) in human liver microsomes under anaero bic conditions. The formation of SMAP was remarkably inhibited by cime tidine, n-octylamine, ketoconazole, and carbon monoxide, indicating th at a cytochrome P450 is involved in the metabolism of zonisamide to SM AP in human liver microsomes. The SMAP-producing activity did not corr elate with the spectrally determined amount of cytochrome P450. In con trast, the SMAP-producing activity from zonisamide correlated closely with the activity of testosterone 6beta-hydroxylase (r2 = 0.96) and co rrelated slightly but significantly with the activity of imipramine 2- hydroxylase (r2 = 0.28), but not with those of aniline hydroxylase (r2 = 0.09) or benzphetamine N-demethylase (r2= 0.20). In addition, immuno quantitation of cytochrome P450 enzymes in 21 human liver microsomal s amples revealed that SMAP formation correlated closely with the amount of P450 3A enzyme and correlated moderately well with that of P450 2D 6 but not with that of P450 2C enzyme in human liver microsomes. P450 3A4 exhibited SMAP-producing activity in a reconstituted monooxygenase system. The metabolism of zonisamide to SMAP was almost completely in hibited by anti-P450 3A4 antibody but not by anti-P450 2C9 or anti-P45 0 2D6 antibodies, suggesting that the amount of P450 3A enzyme may be a major factor influencing the level of metabolism of zonisamide to SM AP in human liver microsomes.