An evaluation of the cytochrome P450 inhibition potential of selected pesticides in human hepatic microsomes

The goal of this work was to study the ability of 18 pesticides to inhibit selective model activities for all major xenobiotic-metabolizing enzymes, namely CYP1A1/2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 and 3A4. Generally organophosphorus insecticides were the most potent and extensive inhibitors, especially towards CYP1A1/2 (IC50 values of chlorpyrifos, fenitrothion and profenofos similar to 3 mu M), CYP2B6 (IC50 values of chlorpyrifos and fenitrothion 2.5 mu M), CYP2C8 (fenitrothion 4.3 mu M), CYP2C9 (fenitrothion and malathion 4.8 and 2.5 mu M, respectively), CYP2D6 (chlorpyrifos and phenthoate similar to 3 mu M) and CYP3A4 (chlorpyrifos, fenitrothion and phenthoate 3-4 mu M). Otherwise there were quite considerable differences in potency and extent of inhibition between different organophosphates. Pyrethroids were in general very weak or inactive. Deltamethrin and fenvalerate were potent inhibitors of CYP2D6 (IC50 values of similar to 3 mu M) while lambda-cyhalothrin potently inhibited both CYP2D6 and CYP3A4-mediated activities (IC50's about 3-4 mu M). Some pesticides caused relatively potent inhibitions sporadically (carbendazim, CYP2D6, IC50 = 12 mu M; atrazine, CYP3A4, IC50 = 2.8 mu M; glyphosate, CYP2C9, IC50 = 3.7 mu M; hexaflumuron, IC50 = 6.0 mu M). With the exceptions of alpha-cypermethrin, cypermethrin, isoproturon, carbaryl and abamectin, most pesticides inhibited relatively potently at least one CYP-selective activity, which may have relevance for potential interactions in occupational exposures and for further studies on the CYP-associated metabolism of respective pesticides.