Insights into Free and Conjugated Forms of Neonicotinoid Insecticides in Human Serum and Their Association with Oxidative Stress

Following exposure, neonicotinoid insecticides (NEOs) can be metabolized by both Phase I and Phase II reactions catalyzed by human cytochrome P450 enzymes. However, toxicities of parent NEOs and their metabolites are still unclear, and little is known about biotransformation rates and pathways of NEOs in humans. In this study, 98 serum samples collected in China were analyzed for free, conjugated and total forms of six parent NEOs (i.e., acetamiprid (ACE), imidacloprid (IMI), clothianidin (CLO), thiacloprid (THD), thiamethoxam (THM), and dinotefuran (DIN)) and four metabolites (i.e., N-desmethyl-acetamiprid (N-dm-ACE), 1-methyl-3-(tetrahydro-3-furylmethyl) (DIN-U), 5-hydroxy-imidacloprid (5-OH-IMI), olefin-imidacloprid (Of-IMI)). NEOs and their metabolites were detected in all serum samples, and the total median concentrations of free, conjugated, and total forms of 10 NEOs were 2.04, 2.01, and 5.12 ng/mL, respectively. Conjugated forms of NEOs accounted for only half (53%) of the total forms of NEOs. Based on the profiles of Phase I and Phase II metabolites of NEOs in serum, it was found that age is a determinant in Phase I metabolism of DIN and Phase II metabolism of IMI. The Phase II metabolites of NEOs are associated with oxidative DNA damage, and the conjugated forms of IMI, DIN, and 5-OH-IMI in serum were significantly positively correlated with oxidative stress. Overall, the amount of NEOs present in conjugated forms in human serum was determined to document the existence of a considerable proportion of free forms of these insecticides.