The inhibition of major human hepatic cytochrome P450 enzymes by 18 pesticides: comparison of the N-in-one and single substrate approaches.

In the present study on human hepatic microsomes, the N-in-one assay with ten probe substrates for nine cytochrome-P450 enzymes (CYPs) was compared with the single substrate assays to investigate pesticides-CYP interactions. CYP inhibition was measured by liquid chromatography-tandem mass spectrometry (LC/MS-MS). As illustrated by the initial screening at 100 μM concentration of 18 pesticides, CYPs are more sensitive to organophosphates (OPs) than to other pesticide groups. Chlorpyrifos and fenitrothion were most effective in inhibiting CYP1A1/2, and CYP2B6. Profenofos was also inhibitory towards multiple CYPs. Pyrethroids, e.g. deltamethrin, fenvalerate and lambda-cyhalothrin, potently inhibited CYP2D6. CYP3A4 activity was moderately inhibited by fenvalerate and potently by alpha-cypermethrin. The correlations between IC50 values obtained from the N-in-one and single substrate approaches were highly significant for CYP2Cs (r(2)=0.94), CYP3A4, omeprazole-sulfoxidation, (r(2)=0.89), followed by CYP1A2 and CYP2B6 (r(2)=0.82), and CYP2D6 (r(2)=0.80). In contrast no correlation was observed with CYP2E1 and CYP3A4 (midazolam-1'-hydroxylation). The N-in-one screening assay seems useful and reliable for most CYP activities when a comprehensive and quick evaluation of potential interactions with CYPs is needed. However, at the present moment, it does not enable discrimination on the basis of mechanism of inhibition. A strict comparison between single and N-in-one assays is a prerequisite for more extensive routine use.