Evaluation of the Safety of Fenbuconazole Monomers via Enantioselective Toxicokinetics, Molecular Docking and Enantiomer Conversion Analyses

Fenbuconazole, a chiral triazole fungicide, is produced and used as a racemate. Previous toxicological research on fenbuconazole in nontarget organisms primarily used the racemate, necessitating an investigation into each enantiomer's distribution and elimination for safety assessment. In this study, the absolute configurations of fenbuconazole enantiomers were first confirmed by ECD, designating them as S-(+)-fenbuconazole and R-(-)-fenbuconazole based on their optical activity. The UHPLC-QQQ/MS method was selected to systematically study the toxicokinetics and enantiomer conversion of fenbuconazole enantiomers in mice. The results revealed significant enantioselectivity, with S-(+)-fenbuconazole exhibiting 15.11 times higher AUC0-∞ than R-(-)-fenbuconazole, indicating greater blood absorption. In the distribution experiment involving the 14 examined tissues, S-(+)-fenbuconazole consistently exceeded R-(-)-fenbuconazole levels, except in the stomach. Notably, S-(+)-fenbuconazole concentration in the liver was second only to the stomach and was 4.35 times higher than R-(-)-fenbuconazole, suggesting a greater propensity for hepatic accumulation. Molecular docking studies further demonstrated a stronger interaction between S-(+)-fenbuconazole and the CYP2B enzyme in the liver, implying higher hepatotoxic potential. Both enantiomers were rarely excreted in urine or feces, with a cumulative excretion rate below 2.5‰. Enantiomer conversion occurred unidirectionally (R → S) in mice, and the rates were generally low in most tissue. Thus, enantiomeric conversion was not the primary factor driving the enantioselectivity. In summary, R-(-)-fenbuconazole exhibited poor absorption, limited distribution, and a weak interaction with the CYP2B enzyme, which may be considered a low-risk product that could guide monomer development and promote the reduction of pesticide usage.