Enantioselective in vitro metabolism of the herbicide diclofop-methyl: Prediction of toxicokinetic parameters and reaction phenotyping

Human exposure to contaminants of emerging concern, like pesticides, has increased in the past decades. Diclofop-methyl (DFM) is a chiral herbicide that is employed as a racemic mixture (rac-DFM) in soybean and other crops against wild oats. Studies have shown that DFM has enantioselective action (higher for R-DFM), degradation (faster for S-DFM), and metabolism, producing diclofop (DF) which is also a pesticide. Although toxic effects have been reported for DFM, information regarding how DFM affects humans is lacking, especially when its chirality is concerned. In this study, the in vitro metabolism of rac-DFM and its isolated enantiomers was assessed by using a human model based on human liver microsomes. The kinetic model and parameters were obtained, and the hepatic clearance (CL_H) and hepatic extraction ratio (E_H) were estimated. Enzyme phenotyping was carried out by employing carboxylesterase isoforms (CES 1 and CES 2). DFM was metabolized through positive homotropic cooperativity with slight preference for (-)-DFM metabolism to (-)-DF. CL_H and E_H were above 19.60 mL min^-1 kg^-1 and 98 % for all the monitored reactions, respectively, and CES 1 was the main enzyme underlying the metabolism. These findings point out that liver contributes to DFM metabolism, which is fast, resulting in nearly complete conversion to DF after exposition to DFM.