Abstract

Using selective cytochrome P450 (CYP) inhibitors and clinical concentrations (4 μM) of dapsone (DDS), we found a major contribution of CYP2C9 and little or no contribution (≤10%) of CYP3A4 and CYP2E1 to dapsone N-hydroxylation (DDS-NHY) in human liver microsomes. Sulfaphenazole (2.16 μM) and tolbutamide (500 μM), selective inhibitors of CYP2C9 (or 2C8/9), inhibited DDS-NHY by 48 ± 14 and 41 ± 15%, respectively. The apparent Michaelis-MentenK_m values for DDS-NHY by cloned CYP2C8, CYP2C9, CYP2C18, and CYP2C19 were 75 μM, 31 μM, 25 μM, and greater than 1 mM, respectively. CYP3A4 and CYP2E1 were incapable of DDS-NHY at 4 μM DDS. S-mephenytoin (360 μM) activated DDS-NHY by human liver microsomes and by CYP2C8 by 43 ± 36 and 193 ± 16%, respectively. This activation was cytochromeb₅-dependent. In contrast,S-mephenytoin inhibited DDS-NHY by CYP2C9, CYP2C18, and CYP2C19 by 27 ± 2, 49 ± 1, and 32 ± 4%, respectively. Because CYP2C18 and CYP19 are expressed at low concentrations in the human liver, these observations indicate that at clinical DDS concentrations, CYP2C9 is a major and CYP2C8 is a likely minor contributor to DDS-NHY in human liver microsomes.