DMD #16295 CHARACTERIZATION OF DIURON N-DEMETHYLATION BY MAMMALIAN HEPATIC MICROSOMES AND cDNA-EXPRESSED HUMAN CYTOCHROME P450 ENZYMES

in toxicokinetics; AK UF , uncertainty factor for human variability in toxicokinetics; HPLC, high-performance liquid chromatograph. ABSTRACT Diuron, a widely used herbicide and antifouling biocide, has been shown to persist in the environment and contaminate the drinking water. It has been characterized as a " known/likely " human carcinogen. While the environmental transformation and toxicity have been extensively examined, metabolic characteristics in mammalian livers have not been published. This study was designed to investigate diuron biotransformation and disposition because metabolic routes, metabolizing enzymes, interactions, interspecies differences, and interindividual variability are important for risk assessment purposes. The only metabolic pathway detected by LC–MS in human liver homogenates and seven mammalian liver microsomes including human was demethylation at the terminal nitrogen atom. No other phase I or phase II metabolites were observed. The rank order of N-demethyldiuron formation in liver microsomes based on intrinsic clearance (V max /K m) was dog > monkey > rabbit > mouse > human > minipig > rat. All tested recombinant human CYPs catalyzed diuron N-demethylation and the highest activities were possessed by CYP1A1, CYP1A2, CYP2C19 and CYP2D6. Relative contributions of human CYP1A2, CYP2C19 and CYP3A4 to hepatic diuron N-demethylation, based on average abundances of CYP enzymes in human liver microsomes, were about 60, 14 and 13%, respectively. Diuron inhibited relatively potently only CYP1A1/2 (IC 50 value 4 µM). Using the human-derived and quantitative chemical-specific data, the uncertainty factor for animal to human differences (AK UF) and for human variability (HK UF) in toxicokinetics were within range of the toxicokinetics default uncertainty/safety factors for chemical risk assessment.