PT - JOURNAL ARTICLE AU - Helen R. Winter AU - Jashvant D. Unadkat TI - IDENTIFICATION OF CYTOCHROME P450 AND ARYLAMINE <em>N-</em>ACETYLTRANSFERASE ISOFORMS INVOLVED IN SULFADIAZINE METABOLISM AID - 10.1124/dmd.104.002998 DP - 2005 Jul 01 TA - Drug Metabolism and Disposition PG - 969--976 VI - 33 IP - 7 4099 - http://dmd.aspetjournals.org/content/33/7/969.short 4100 - http://dmd.aspetjournals.org/content/33/7/969.full SO - Drug Metab Dispos2005 Jul 01; 33 AB - 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