Citation:

POWER, FRED W., J. N. BLANCATO, M. V. EVANS, A. TSANG, AND J. C. CALDWELL. PHYSIOLOGICALLY-BASED PHARMACOKINETIC ( PBPK ) MODEL FOR METHYL TERTIARY BUTYL ETHER ( MTBE ): A REVIEW OF EXISTING MODELS. Presented at Society of Toxicology, San Diego, CA, March 05 - 09, 2006.

Impact/Purpose:

Model Development

The overall goal of this work is to develop a modeling system that will enable risk assessors to apply PBPK/PD models to research and regulatory problems. The specific aim is to achieve Agency recognition of PBPK/PD modeling systems, such as ERDEM, as computational tools for risk characterization, research design, and diagnosis of resource allocation.

The key aspects of this research are to:

1. Add a multi-run graphical-user-interface (GUI) in the ERDEM "Front End" to assist the user in defining model input and to improve multi-run capability so that suites of parameter values can be run to speed up the parameter fitting process. Add intra-cellular and uncertainty analysis capability to the graphical-user-interface (GUI) in the ERDEM "Front End." Add the interface for additional compartments and subsystems as needed.(Sub-Task: Model Graphical User Interface and Development GUI)

2. Develop a repository in the ERDEM Front End database to store exposure time histories for processing ERDEM model runs. This Exposure/Time History Repository/Bio-monitoring Interface is expected to help users input various exposure model parameters into a generic PBPK model to process ADME functions simultaneously with a pharmacodynamic (PD) component to determine target tissue dose and effects, e.g., acetyl cholinesterase (AChE) inhibition (Sub-Task: Model Exposure/Time History Repository).

3. Generate Quantitative Structure Activity Relationship (QSAR) databases for chemicals of interest to test ADME and PD mechanisms and make predictions about activity for chemicals where data is lacking. These QSARs may be used to probe the in silico biological layers in ERDEM to examine ADME and PD mechanisms at the organism (e.g., body burden and lethality), tissue and organ, and cellular and sub-cellular levels (Sub-Task: QSAR and Intracellular modeling).

4. Provide exposure and risk assessment specialist's computational modeling tools to establish commonality among dermal exposure and dose related algorithms used in risk assessment. Recognition of the need for a "harmonization" of approaches arose through publication of international reports on dermal absorption (OECD, 2004a, 2004b, 2004c and WHO, 2005) and national colloquiums EPA, 2005 and AIHA, 2005) on dermal exposure methods comparisons (Sub-Task: Dermal Exposure to Dose Harmonization).

5. Develop symbolic solutions to PBPK models for application to risk assessment.

Description:

MTBE is a volatile organic compound used as an oxygenate additive to gasoline, added to comply with the 1990 Clean Air Act. Previous PBPK models for MTBE were reviewed and incorporated into the Exposure Related Dose Estimating Model (ERDEM) software. This model also included an explicit pulmonary compartment. Organs (compartments) included in the model were: liver (for metabolism), fat, kidney, brain, and slowly and rapidly perfused compartments. Both inhalation and oral dosing were included as routes of exposure. Numerous simulations were performed to compare performance between present and previous MTBE PBPK models with available data : (400 and 8000 ppm inhalation exposure for 6 hours, and oral dosing of 40 and 400 mg/kg to a 215g rat). Physiological and metabolic parameters were changed to human values and model performance was re-examined. Using available human data (inhalation, dermal and oral exposures), model comparisons were matched to MTBE peak values, and peak concentration for the metabolite TBA. An uncertainty analysis (representing errors due to parameters or model structure) and variability analysis (attempting to include assumed changes due to differences in the general population) was performed for rat. The simulation results showed model fit for peak MTBE levels to be similar for rat and human.

This work was reviewed by EPA and approved for publication but does not necessarily reflect official Agency policy.

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