TY - JOUR T1 - Species Differences in the in Vitro Metabolism of Deltamethrin and Esfenvalerate: Differential Oxidative and Hydrolytic Metabolism by Humans and Rats JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1764 LP - 1771 DO - 10.1124/dmd.106.010058 VL - 34 IS - 10 AU - Stephen J. Godin AU - Edward J. Scollon AU - Michael F. Hughes AU - Philip M. Potter AU - Michael J. DeVito AU - Matthew K. Ross Y1 - 2006/10/01 UR - http://dmd.aspetjournals.org/content/34/10/1764.abstract N2 - Pyrethroids are neurotoxic pesticides whose pharmacokinetic behavior plays a role in their potency. This study examined the elimination of esfenvalerate and deltamethrin from rat and human liver microsomes. A parent depletion approach in the presence and absence of NADPH was used to assess species differences in biotransformation pathways, rates of elimination, and intrinsic hepatic clearance. Esfenvalerate was eliminated primarily via NADPH-dependent oxidative metabolism in both rat and human liver microsomes. The intrinsic hepatic clearance (CLINT) of esfenvalerate was estimated to be 3-fold greater in rodents than in humans on a per kilogram body weight basis. Deltamethrin was also eliminated primarily via NADPH-dependent oxidative metabolism in rat liver microsomes; however, in human liver microsomes, deltamethrin was eliminated almost entirely via NADPH-independent hydrolytic metabolism. The CLINT for deltamethrin was estimated to be 2-fold more rapid in humans than in rats on a per kilogram body weight basis. Metabolism by purified rat and human carboxylesterases (CEs) were used to further examine the species differences in hydrolysis of deltamethrin and esfenvalerate. Results of CE metabolism revealed that human carboxylesterase 1 (hCE-1) was markedly more active toward deltamethrin than the class 1 rat CEs hydrolase A and B and the class 2 human CE (hCE-2); however, hydrolase A metabolized esfenvalerate 2-fold faster than hCE-1, whereas hydrolase B and hCE-1 hydrolyzed esfenvalerate at equal rates. These studies demonstrate a significant species difference in the in vitro pathways of biotransformation of deltamethrin in rat and human liver microsomes, which is due in part to differences in the intrinsic activities of rat and human carboxylestersases. The American Society for Pharmacology and Experimental Therapeutics ER -