Pharmacokinetic modeling of manganese. III. Physiological approaches accounting for background and tracer kinetics.

Manganese (Mn), an essential metal nutrient, produces neurotoxicity in workers exposed chronically to high concentrations of Mn-containing dusts. Our long-term goal was to develop a physiologically based pharmacokinetic (PBPK) model to support health risk assessments for Mn. A PK model that accounts for Mn-tracer kinetics and steady-state tissue Mn in rats on normal diets (about 45 ppm Mn) is described. The focus on normal dietary intakes avoids inclusion of dose-dependent processes that maintain Mn homeostasis at higher dose rates. Data used for model development were obtained from published literature. The model represents six tissues: brain, respiratory tract, liver, kidneys, bone, and muscle. Each of these has a shallow tissue pool in rapid equilibration with blood and a deep tissue store, connected to the shallow pool by transfer rate constants. Intraperitoneal (i.p.) tracer Mn is absorbed into systemic blood and equilibrated with the shallow and deep pools of tissue Mn. The model was calibrated to match steady-state tissue concentrations and radiotracer kinetics following an i.p. dose of 54Mn. Successful simulations showed uptake of 0.8% of dietary Mn, and estimated tissue partition coefficients and transfer rate constants in the tissues. Inhalation tracer 54Mn studies could only be adequately modeled by assuming that deposited Mn was absorbed into deep tissue stores in the lung before becoming available to move via blood to other tissues. In summary, this present effort provides the basic structure of a multiroute PBPK model for Mn that should now be easily extended to include homeostatic control and inhalation exposures in order to support risk assessment calculations for Mn.