Discovery of an unconventional metabolic pathway within neonicotinoids based on a characteristic fragment-dependent...

Neonicotinoid insecticides (NNIs), as a class of neuro-active compounds, have been detected in the environment and biological matrices. The metabolic fate of NNIs in biological media is of great significance for tracing their exposure levels and risks for humans. In this study, we investigate the metabolism of a suite of NNIs and identify potential metabolites of different NNIs by primary mouse hepatocytes (PMH). A comprehensive non-targeted screening strategy with characteristic fragment-dependent and Cl-specific screening approaches was used to identify potential unknown NNI metabolites (m-NNIs). In addition to known oxidation and reduction reactions (e.g., O-dealkylation, hydroxylation, and nitro reduction), we firstly report that NNIs undergo an unknown metabolic pathway to form metabolites with higher molecule weights as compared to parent NNIs. By use of high-resolution mass spectrometry, these self-coupled metabolites were identified as imidaclothiz (IMID)-U6, nitenpyram (NIT)-U6, NIT-U8, thiamethoxam (THIA)-U1, clothianidin (CLO)-U2, and thiacloprid (THI)-U4, respectively. Using computer prediction software (i.e., EPIWEB 4.1, ECOSAR v2.2 and ADMETlab 3.0), we estimated octanol-water partition coefficients (Log Pow), water solubility, LC₅₀ values for aquatic organisms, and probability of multiple biotoxicity for these m-NNIs. The results implied that m-NNIs with higher molecule weights could be more bioacummulative, and more toxic as compared to parent NNIs. Based on these findings, the identified m-NNIs reveal a previously uncharacterized metabolic disruption pathway for NNI exposure in organisms. Overall, this study provides a novel insight into the fate and persistence of NNIs in organisms.