Development of In Vitro-To-In Vivo (IVIVE) Extrapolation Methods for Chemical Disposition

Harvey J. Clewell, III, Miyoung Yoon, and Jerry L. Campbell, Jr.  The Hamner Institutes for Health Sciences.

The adsorption, distribution, metabolism, and excretion (ADME) properties of a chemical can have a significant impact on the toxicity of a chemical. In experimental animal models, the ADME properties of specific chemicals can be characterized using in vivo experiments. In humans, characterizing the ADME properties for most chemicals requires the use of in vitro assays or in silico estimation using quantitative structure activity relationships (QSAR). However, the current battery of in vitro ADME models has significant limitations.

In a previous study at the Hamner (see MTH0710), in vitro-to-in vivo extrapolation (IVIVE) was performed for 240 of the ToxCast Phase I chemicals. During this study it was recognized that one of the key uncertainties in IVIVE was the impact of transport limitations on hepatic clearance. The IVIVE modeling was performed using both restrictive and nonrestrictive clearance, but no method was available for experimentally measuring the extent of transport limitation. Also in previous studies, a life-stage physiologically-based pharmacokinetic (PBPK) model was developed to predict the relationship of the external exposures to internal dose throughout the life span of a human. This model incorporated physiological as well as metabolic changes that impact the pharmacokinetics of a chemical.

In 2012, the research will be focused in the following four different areas:

  1. IVIVE methods will be developed for estimating early-life exposures. We will collaborate with Barbara Wetmore of the Hamner to use the high-throughput recombinant cytochrome P450 data that is being collected on approximately 15 ToxCast chemicals to predict the relationship of the external exposures in children of different ages that would result in the same internal exposures as adults (see MTH1002-03). The IVIVE will be performed using SimCyp. The predicted exposures will then be compared with available exposure data.

  2. Metabolism data, including recombinant cytochrome P450 data, will be collected on key environmental chemicals of concern whose toxicity results from a metabolite rather than the parent chemical (e.g., DEHP), or that are subject to significant prehepatic clearance (e.g., parabens). These data will be used to support IVIVE with the life-stage model to predict the relationship of the external exposures in children of different ages that would result in the same internal exposures to the toxic metabolite as adults. The predicted exposures will then be compared with available exposure data.

  3. In collaboration with Ed LeCluyse of the Hamner, an experimental system together with the computational modeling methods will be developed to measure the impact of transport limitations on in vivo hepatic clearance. This research effort will address one of the key uncertainties in IVIVE identified during the ToxCast dosimetry project: restrictive vs. nonrestrictive clearance. The experimental methods are anticipated to include the development of a liver bioreactor.

  4. Experimental methods will be developed for simultaneously measuring intestinal absorption and pre-hepatic metabolism. Currently available assays for intestinal absorption do not provide quantitative data on pre-hepatic clearance during intestinal absorption due to the presence of cytochrome P450s, glucuronyl transferases and esterases in the enterocytes.

In 2013, the testing and validation of the experimental system for measuring the impact of transport limitations on in vivo hepatic clearance and the experimental system for simultaneously measuring intestinal absorption and pre-hepatic metabolism will continue. In addition, the development of a rapid approach to characterize complex metabolism will begin. This research effort will make use of a liver bioreactor to produce metabolites in sufficient quantities to permit identification and kinetic characterization. QSAR methods will be used to predict candidate metabolites that can then be confirmed with the bioreactor. The goal is to reduce the time necessary to characterize the complex metabolism of a compound from months to days.

In 2014, the development of the liver bioreactor for characterizing complex metabolism will continue and validation of the system will be performed. Initial validation of bioreactor will be done with acetaminophen, all-trans retinoic acid, and DEHP.

Implications

There is increasing need for improved in vitro-based methods by regulatory agencies in relation to pursuing the National Research Council’s “Toxicity Testing in the 21st Century” recommendations as well as to adapt to new regulatory requirements associated with European Unioin’s REACH (Registration, Evaluation, Authorisation and Restriction of Chemical substances) legislation. One key piece is linking the bioactivity or pathway-perturbations observed in the in vitro models at specific nominal concentrations to concentrations of the chemical in the target tissue or blood in vivo. The proposed studies aim to apply PBPK modeling in conjunction with targeted in vitro biokinetic studies to quantitatively translate in vitro observations into equivalent in vivo dose metrics. The outcome of this research will provide a more in-depth characterization of pharmacokinetics for high-priority chemicals identified in ToxCast, a better understanding of the limitations and utility of high-throughput pharmacokinetic approaches, and methods that can be applied for product selection in internal industry labs.        

Keywords

in vitro-to-in vivo extrapolation (IVIVE), physiologically-based pharmacokinetic (PBPK) modeling, linkage of exposure to effect, quantitative interpretation

Project Start and End Dates

January 2010 – December 2014

Project ID

MTH1002-04

Peer-reviewed Publication(s)

None to date.

Other Publication(s)

The Hamner Institutes for Health Sciences. (2011). The Hamner Course on PBPK Modeling and Its Applications Course book. March 28 – April 1, 2011.

Yoon, M., Campbell, J., and Clewell, H. Value of information analysis on utilizing in vitro metabolism data and modeling for conducting IVIVE from in vitro toxicity assays. Toxicological Sciences. (In preparation).

Abstract Revision Date

January 2012

*This abstract was prepared by the principal investigator for the project.

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