Stereoselective metabolism of fenoxaprop-ethyl and its chiral metabolite fenoxaprop in rabbits.

The stereoselective metabolism of the enantiomers of fenoxaprop-ethyl (FE) and its primary chiral metabolite fenoxaprop (FA) in rabbits in vivo and in vitro was studied based on a validated chiral high-performance liquid chromatography method. The information of in vivo metabolism was obtained by intravenous administration of racemic FE, racemic FA, and optically pure (-)-(S)-FE and (+)-(R)-FE separately. The results showed that FE degraded very fast to the metabolite FA, which was then metabolized in a stereoselective way in vivo: (-)-(S)-FA degraded faster in plasma, heart, lung, liver, kidney, and bile than its antipode. Moreover, a conversion of (-)-(S)-FA to (+)-(R)-FA in plasma was found after injection of optically pure (-)-(S)- and (+)-(R)-FE separately. Either enantiomers were not detected in brain, spleen, muscle, and fat. Plasma concentration-time curves were best described by an open three-compartment model, and the toxicokinetic parameters of the two enantiomers were significantly different. Different metabolism behaviors were observed in the degradations of FE and FA in the plasma and liver microsomes in vitro, which were helpful for understanding the stereoselective mechanism. This work suggested the stereoselective behaviors of chiral pollutants, and their chiral metabolites in environment should be taken into account for an accurate risk assessment.