Report of the Technical Advisory Panel Meeting for Manganese PBPK Models

The Hamner Institute for Health Sciences Research Triangle Park, NC October 31, 2013
Objectives
The Technical Advisory Panel (TAP) for manganese PBPK models developed over a period of time by the Hamner Institute for Health Sciences has provided scientific review and comment during the course of the model development process. The TAP empaneled to evaluate the latest PBPK models include : Drs. Da niel Krewski (chair), Jeffery Fisher, and Lynne Haber.

The current TAP meetin g was convened to evaluate PBPK models developed by Schroeter et al. (2011) for adult monkeys and humans, and the human models incorporating fetal and neonatal exposure in humans developed by Yoon et al. (2011). The TAP was asked specifically to comment on the extent to which the models have been validated, as well as their suitability for application in human health risk
assessment. The TAP was also asked to comment on the completeness of the documentation available for these two models.
Three supple mentary presentations on ongoing work were also considered, specifically: 1) a planned experiment to evaluate the effect of route of exposure (dietary/drinking water ad libitum/gavage) on manganese excretion and tissue concentrations; 2) an advanced uncert ainty analysis of Mn PBPK models for rodents and humans; and 3) a reparametrized PBPK model for Mn involving faster dissociation and association constants.

TAP Evaluation
Completed PBPK Models
The completion of the two models described by Schroeter et al. (2011) and Yoon et al. (2011) represents the successful completion of many years of work, which involved the initial development, refinement, and validation of these complex PBPK models.
The TAP considered that the PBPK model developed by Schroeter et al . (2011) to provide a plausible description of the pharmacokinetics of manganese in adult human and non

- human primates. Model simulations against measured Mn elimination curves served to validate the model quite well. Based on its general biological plau sibility and ability to describe experimental data the TAP considered the model suitable for risk assessment purposes.
The models provide considerable insight into the pharmacokinetics of manganese, including the identification of kinetic process that are dose

- dependent. For example, the saturation of binding processes and the induction of biliary elimination at high plasma concentrations resulting in increased elimination of manganese at high doses is well described within the model. The model also cle arlydescribes the relative importance of different routes of exposure to Mn, and the subsequent

concentration of manganese at key target tissues as a function of different exposure patterns.
An attractive feature of the model is its ability to predict Mn concentration in the following four different brain areas: globus pallidus, pituitary, cerebellum, and olfactory bulb. Although the model includes an omnibus compartment labelled ‘rest of body’, which includes a mix of heterogeneous tissues, the limite d mass of manganese in this compartment results in this not being an oversimplification that would appreciably distort model predictions.
The model is well documented, with the code having been successfully used by several organizations that the model has been provided to. The TAP did, however, recommend additional transparency in documentation, specifically: 1) the model diagram could be enhanced by colour coding those parameters that are dose

- dependent (Fdietup and biliary excretion); 2) figure legen ds for the model diagrams should explicitly note that the association/dissociation constants (k a and k d ) are tissue specific, and that k in and k out vary for each brain region; and 3) an explanation of how dietary or stable manganese and radiolabeled mangan ese were modelled together should be provided. As the research team moves forward to risk assessment applications, using the simplified structure of key elements of the model described by Meek et al. (2013) would be useful in communicating the model.

Th e model by Yoon et al. (2011) represents perhaps the most complex PBPK model developed to date, incorporating detailed descriptions of fetal and neonatal exposure through the addition of separate compartments for these two critical life stages. Despite th eir complexity, the models were clearly described. Although the models involve an unusually large number of parameters because of the inclusion of additional compartments, the authors have found sufficient data sources to permit the estimation of all of t he model parameters; independent verification of the model also appears to have been demonstrated using data not involved in model development, although the details of this verification might be made more transparent.
These models also provide important insights into exposure at different life stages: based on the results presented, foetal and neonatal tissue doses were comparable to, or less than, adult doses.
Both models use default respiratory parameters from MPPT deposition modeling (i.e., under res ting conditions), which differ from defaults used by EPA for risk assessment purposes (i.e., under light activity). The deposition modeling was correctly done in the verification process where the human subjects were at rest; however, if this model were t o be used in risk assessment, it would be more appropriate to use the respiratory parameters used by EPA. [The EPA defaults are based on 16 breaths per minute and 813 mL tidal volume (RIVM, 2002).]
Ongoing Work
The TAP considered the three ongoing activit ies described by the investigators to be of importance in further evaluating, and possibly refining, the two models discuss above. The proposed drinking water study comparing dietary, drinking water, and gavage dosing will be of great value in determining if these three routes of exposure result in different tissue concentrations of Mn; should this be the case, thePBPK models might be extended to distinguish between these three exposure routes. [On a technical

note regarding the design of this study, the TAP suggests that consideration be given to increasing the number of animals in the drinking water group slightly, to allow for the possibility that actual manganese doses for some animals in this group might be notably less than intended, because of mani pulation of the drinking water supply by the experimental animals during the course of the study.]
The planned uncertainty/variability analysis using sequential Bayesian methods and MCMC techniques is
state

- of

- the

- art, and will provide valuable information on those model parameters contributing most to uncertainty and variability in predicted tissue doses.
The alternative model based on higher dissociation/association constants not only appears to be more biologically plausible, but also appears to provide a slightly better fit to the available data. While this alternative model

– currently available only for the adult rat

– represents a refinement of the current model, the current model (which is available for human and non

- human primates) is still suffici ently accurate to be useful in risk assessment applications.

Conclusion
The work described above represents a significant achievement on the part of the investigators in developing and validating a series of PBPK models for manganese to the point where they are now suitable for risk assessment applications. These models

– currently available for the adult rat, monkey, and human, along with life

- stage specific components for the rat and human models

– have been well

validated according to current standar ds in this field, and have been subjected to sensitivity analyses that demonstrate reasonable model robustness. Although ongoing work can be expected to result in future model refinements, the current suite of models are sufficiently mature for use risk a ssessment.
An important aspect of this work has been the ongoing interaction between EPA, which requested that this work be conducted by industry, and the investigators at the Hamner Institutes . This interaction has resulted in much useful dialogue that h as been conducive to the achievement of a suite of models that have t he potential to lead to improvements in the manner in which potential Mn health risks are characterized and evaluated.

References
Meek, M.E., Barton, H.A., Bessems, J.G., Lipscomb, J.C. & Krishnan, K. (2013). Case study illustrating the WHO IPCS guidance on characterization and application of physiologically based models in risk assessment. Regulatory Toxicology and Pharmacology 66, 116

- 129.
RIVM (2002). From concentration to dose: Fa ctors influencing airborne particulate matter deposition in humans and rats. RIVM report 650010031/2002. Winter

- Sorkina and Cassee.

Schroeter , J.D., Nong, A., Yoon, M., Taylor, M.D., Dorman, D.C., Andersen, M.A. & Clewell, H.J. III (2011). Analysis of ma nganese tracer kinetics and target tissue dosimetry in monkeys and humans with multi

route physiologically based pharmacokinetic models. Toxicological Sciences 120, 481

- 498.Yoon , M., Schroeter, J. D. , Nong, A., Taylor, M.D., Dorman, D.C., Andersen, M.E . & Clewell, H.J. III (2011).

Physiologically based pharmacokinetic modeling of fetal and neonatal manganese exposure in humans: describing manganese homeostasis during development. Toxicological Sciences 122, 297

- 316.