In vitro-in vivo correlation of the chiral pesticide prothioconazole after interaction with human CYP450 enzymes.

Growing human demand for food has culminated in increased use of pesticides worldwide. Prothioconazole (PTC), a profungicide, is bioactivated by metabolic PTC oxidation to prothioconazole-desthio (D-PTC). Here, the in vitro phase I metabolism of PTC to D-PTC in human liver microsomes and human CYP450 forms was studied. The kinetic parameters for the formation of (+)-D-PTC (KM = 1.2 µmol L-1, VMAX = 1.7 pmol min-1 mg-1), (-)-D-PTC (KM = 7 µmol L-1, VMAX = 5.1 pmol min-1 mg-1), and both D-PTC enantiomers (KM = 9 µmol L-1, VMAX = 7 pmol min-1 mg-1) from rac-PTC indicated an enantioselective behavior. Formation of the enantiomer (+)-D-PTC was twice more extensive than the formation of the enantiomer (-)-D-PTC. Furthermore, CLH prediction revealed the same enantioselective behavior. The phenotyping study indicated that CYP2C19 was the sole CYP450 form accounting for the metabolism of PTC. The estimated apparent distribution volume of PTC was predicted as 2 L kg-1. This study showed that D-PTC may be formed in the human organism due to hepatic metabolism of PTC, mediated by CYP2C19 and that the enantiomer (+)-D-PTC is preferentially formed. However, it was not extensively formed (~1%). Considering a risk assessment point of view, this study provided positive evidence of PTC safety.

Enantioselective inhibition of human CYP2C19 by the chiral pesticide ethofumesate: Prediction of pesticide-drug interactions in humans.

Ethofumesate is a chiral herbicide that may display enantioselective behavior in humans. For this reason, the enantioselective potential of ethofumesate and its main metabolite ethofumesate-2-hydroxy to cause pesticide-drug interactions on cytochrome P450 forms (CYPs) has been evaluated by using human liver microsomes. Among the evaluated CYPs, CYP2C19 had its activity decreased by the ethofumesate racemic mixture (rac-ETO), (+)-ethofumesate ((+)-ETO), and (-)-ethofumesate ((-)-ETO). CYP2C19 inhibition was not time-dependent, but a strong inhibition potential was observed for rac-ETO (IC50 = 5 ± 1 µmol L-1), (+)-ETO (IC50 = 1.6 ± 0.4 µmol L-1), and (-)-ETO (IC50 = 1.8 ± 0.4 µmol L-1). The reversible inhibition mechanism was competitive, and the inhibition constant (Ki) values for rac-ETO (2.6 ± 0.4 µmol L-1), (+)-ETO (1.5 ± 0.2 µmol L-1), and (-)-ETO (0.7 ± 0.1 µmol L-1) were comparable to the Ki values of strong CYP2C19 inhibitors. Inhibition of CYP2C19 by ethofumesate was enantioselective, being almost twice higher for (-)-ETO than for (+)-ETO, which indicates that this enantiomer may be a more potent inhibitor of this CYP form. For an in vitro-in vivo correlation, the Food and Drug Administration's (FDA) guideline on the assessment of drug-drug interactions used in the early stages of drug development was used. The FDA's R1 values were estimated on the basis of the obtained ethofumesate Ki and distribution volume, metabolism, unbound plasma fraction, gastrointestinal and dermal absorption data available in the literature. The correlation revealed that ethofumesate probably inhibits CYP2C19 in vivo for both chronic (oral) and occupational (dermal) exposure scenarios.