A physiologically based pharmacokinetic (PBPK) model to align dosimetry of the isobutyl metabolic series in rats and humans.

We developed a physiologically based pharmacokinetic (PBPK) model in rats and humans for the isobutyl metabolic series, which includes isobutyl acetate, isobutanol, isobutyraldehyde, and isobutyric acid. Given chemical similarities, we used a previously developed PBPK model for the propyl metabolic series as a framework to create the isobutyl PBPK model. To support model development, we measured in vitro metabolism of isobutyl acetate in rat and human blood and liver S9 fractions. Our findings indicated that humans exhibited faster isobutyl acetate hydrolysis in liver S9 fractions compared to rats, while hydrolysis rates in blood were similar between the two species. Experiments involving closed chamber exposures of rats to isobutyl acetate or isobutanol revealed higher isobutanol concentrations in blood compared to other isobutyl compounds. Using these data to parameterize the model, the PBPK model accurately simulated available time-course concentrations of isobutyl compounds in blood of rats and humans. The isobutyl PBPK model enables comparisons of internal dose metrics across various isobutyl compound exposures and species and allows for calculation of equivalent external exposures that result in the same dose metric. Regulators can employ this PBPK model to predict and align internal dose metrics of isobutyl compounds for risk assessment purposes.