Analysis of Manganese Tracer Kinetics and Target Tissue Dosimetry in Monkeys and Humans with Multi-Route Physiologically Based Pharmacokinetic Models

Manganese (Mn) is an essential nutrient with the capacity for toxicity from excessive exposure. Accumulation of Mn in the striatum, globus pallidus, and other midbrain regions is associated with neurotoxicity following high-dose Mn inhalation. Physiologically based pharmacokinetic (PBPK) models for ingested and inhaled Mn in rats and nonhuman primates were previously developed. The models contained saturable Mn tissue-binding capacities, preferential fluxes of Mn in specific tissues, and homeostatic control processes such as inducible biliary excretion of Mn. In this study, a nonhuman primate model was scaled to humans and was further extended to include iv, ip, and sc exposure routes so that past studies regarding radiolabeled carrier-free (54)MnCl(2) tracer kinetics could be evaluated. Simulation results accurately recapitulated the biphasic elimination behavior for all exposure routes. The PBPK models also provided consistent cross-species descriptions of Mn tracer kinetics across multiple exposure routes. These results indicate that PBPK models can accurately simulate the overall kinetic behavior of Mn and predict conditions where exposures will increase free Mn in various tissues throughout the body. Simulations with the human model indicate that globus pallidus Mn concentrations are unaffected by air concentrations < 10 mu g/m(3) Mn. The use of this human Mn PBPK model can become a key component of future human health risk assessment of Mn, allowing the consideration of various exposure routes, natural tissue background levels, and homeostatic controls to explore exposure conditions that lead to increased target tissue levels resulting from Mn overexposure.