Application of a multi-route physiologically based pharmacokinetic model for manganese to evaluate dose-dependent neurological effects in monkeys.

Manganese (Mn) is an essential element that is neurotoxic under certain exposure conditions. Monkeys and humans exposed to Mn develop similar neurological effects; thus, an improved understanding of the dose-response relationship seen in nonhuman primates could inform the human health risk assessment for this essential metal. A previous analysis of this dose-response relationship in experimental animals (Gwiazda, R., Lucchini, R., and Smith, D., 2007, Adequacy and consistency of animal studies to evaluate the neurotoxicity of chronic low-level manganese exposure in humans, J. Toxicol. Environ. Health Part A 70, 594-605.) relied on estimates of cumulative intake of Mn as the sole measure for comparison across studies with different doses, durations, and exposure routes. In this study, a physiologically based pharmacokinetic model that accurately accounts for the dose dependencies of Mn distribution was used to estimate increases in brain Mn concentrations in monkeys following Mn exposure. Experimental studies evaluated in the analysis included exposures by inhalation, oral, iv, ip, and sc dose routes, and spanned durations ranging from several weeks to over 2 years. This analysis confirms that the dose-response relationship for the neurotoxic effects of Mn in monkeys is independent of exposure route and supports the use of target tissue Mn concentration or cumulative target tissue Mn as the appropriate dose metric for these comparisons. These results also provide strong evidence of a dose-dependent transition in the mode of action for the neurological effects of Mn that needs to be considered in risk assessments for this essential metal.