TY - JOUR T1 - IDENTIFICATION OF CYTOCHROME P450 AND ARYLAMINE <em>N-</em>ACETYLTRANSFERASE ISOFORMS INVOLVED IN SULFADIAZINE METABOLISM JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 969 LP - 976 DO - 10.1124/dmd.104.002998 VL - 33 IS - 7 AU - Helen R. Winter AU - Jashvant D. Unadkat Y1 - 2005/07/01 UR - http://dmd.aspetjournals.org/content/33/7/969.abstract N2 - Sulfadiazine hydroxylamine has been postulated to be the mediator of the greatly increased rates of adverse reactions to sulfadiazine experienced by people with human immunodeficiency virus infection. Therefore, we investigated the in vitro human cytochrome P450 (P450) and N-arylamine acetyltransferase (detoxification) metabolism of sulfadiazine. Formation of both the hydroxylamine and 4-hydroxy sulfadiazine was NADPH-dependent in human liver microsomes (HLM). The average Km (±S.D.) and Vmax in HLM (n = 3) for hydroxylamine formation was 5.7 ± 2.2 mM and 185 ± 142 pmol/min/mg, respectively. Significant (p &lt; 0.05) inhibition by selective P450 isoform inhibitor sulfaphenazole (2.1 μM; CYP2C9) indicated a role for CYP2C9 in the formation of the hydroxylamine. Hydroxylamine formation correlated strongly with tolbutamide 4-hydroxylation (CYP2C8/9) in HLM (r = 0.76, p ≤ 0.004, n = 12). Fluconazole (CYP2C9/19 and CYP3A4 inhibitor at clinical concentrations) inhibited hydroxylamine formation, with one-enzyme model Ki estimates ranging from 9 to 40 μM. Acetylation of sulfadiazine in human liver cytosol (HLC) correlated strongly with NAT2 activity as measured by sulfamethazine N-acetylation (r = 0.92, p &lt; 0.001, n = 12). The average Km (±S.D.) and Vmax in HLC (n = 3) was 3.1 ± 1.7 mM and 221.8 ± 132.3 pmol/min/mg, respectively. The polymorphic acetylation of sulfadiazine may predispose slow acetylator patients to adverse reactions to sulfadiazine. On the basis of our Ki estimates, clinical fluconazole concentrations of 25 μM would produce decreases of 40 to 70% in hepatic-mediated hydroxylamine production. Therefore, we predict that fluconazole may prove useful in the clinic as an in vivo inhibitor of sulfadiazine hydroxylamine formation to suppress adverse reactions to this drug. The American Society for Pharmacology and Experimental Therapeutics ER -