Myclobutanil enantioselective risk assessment in humans through in vitro CYP450 reactions: Metabolism and inhibition studies.

Myclobutanil is a chiral triazole fungicide that is employed worldwide. Although enantiomers have the same physical-chemical properties, they may differ in terms of activity, metabolism, and toxicity. This investigation consisted of in vitro enantioselective metabolism studies that employed a human model to assess the risks of myclobutanil in humans. A LC-MS/MS enantioselective method was developed and validated. The enzymatic kinetic parameters (VMAX, KMapp, and CLINT) determined for in vitro rac-myclobutanil and S-(+)-myclobutanil metabolism revealed enantioselective differences. Furthermore, human CYP450 enzymes did not metabolize R-(-)-myclobutanil. The predicted in vivo toxicokinetic parameters indicated that S-(+)-myclobutanil may be preferentially eliminated by the liver and suffer the first-pass metabolism effect. However, because CYP450 did not metabolize R-(-)-myclobutanil, this enantiomer could reach the systemic circulation and stay longer in the human body, potentially causing toxic effects. The CYP450 isoforms CYP2C19 and CYP3A4 were involved in rac-myclobutanil and S-(+)-myclobutanil metabolism. Although there were differences in the metabolism of the myclobutanil enantiomers, in vitro inhibition studies did not show significant enantioselective differences. Overall, the present investigation suggested that myclobutanil moderately inhibits CYP2D6 and CYP2C9 in vitro and strongly inhibits CYP3A and CYP2C19 in vitro. These results provide useful scientific information for myclobutanil risk assessment in humans.