Gender and species differences in triadimefon metabolism by rodent hepatic microsomes

Understanding the potential differences in metabolic capacity and kinetics between various common laboratory species as well as between genders is an important facet of chemical risk assessment that is often overlooked, particularly for chemicals which undergo non-cytochrome P450 mediated metabolism. The use of physiologically based pharmacokinetic (PBPK) models to better describe chemical exposure is made more powerful by incorporation of high quality in vitro kinetic data. To this end, metabolism of the conazole fungicide triadimefon was studied in hepatic microsomes of both genders of SD rats and CD-1 mice. Triadimefon depletion and triadimenol formation were measured in each type of microsomes. Michaelis-Menten regressions were applied to metabolic data and VMAX and the Michaelis constant (KM) values calculated. Male SD rats metabolized triadimefon more rapidly than female SD rats or either gender of CD-1 mouse. KM values were in the micromolar range, indicating the possibility of competitive inhibition with endogenous substrates. Intrinsic clearances derived from kinetic parameters indicate that triadimefon metabolism is blood-flow limited in all organisms studied with the possible exception of female rat. The in vitro half-life method was investigated as a less resource intensive method for the derivation of intrinsic clearance, and was found to be useful as a complement to the traditional Michaelis-Menten approach.