Updating physiologically based pharmacokinetic models for manganese by incorporating rapid association/dissociation processes in tissues.

Previously, we developed a series of physiologically based pharmacokinetic (PBPK) models for manganese (Mn) in which saturable tissue binding and dose-dependent increases in biliary excretion captured key aspects of Mn homeostasis biology. These models reproduced the non-linear behavior of Mn kinetics in different tissues, accounting for dose-dependent changes in Mn kinetics. The original model construct had relatively slow association and dissociation rate constants for Mn binding in tissues. In this updated model, both rates of entry into tissue and the interaction of Mn with binding sites are rapid, and the step limiting Mn accumulation is the saturation of tissue binding sites. This binding reflects general cellular requirements for Mn with high affinity but rapid exchange between bound and free forms, which we captured using a dissociation constant (KD) of ~ 0.5 μM across tissues while maintaining different maximum binding capacities in each tissue. Variability in the binding capacities accounted for different background levels of Mn in particular tissues. This alternative structure successfully described Mn kinetics in tissues in adult rats exposed to Mn either in their diet or by inhalation, indicating that both the original and the present models capture the dose-dependent and tissue-specific kinetic behavior of Mn in adult rats. Although the published models that emphasize the role of smaller tissue binding rate constants in non-linear behaviors capture all relevant dose-dependent kinetic behaviors of this metal, increasing biological relevance of the model structure and parameters should provide greater confidence in applying the Mn PBPK models to risk assessment.