Assessing the Exposure-Dose-Toxicity Relationship Within the EPA’s ToxCast™ Program—Project Extension

Barbara Wetmore and Russell Thomas. The Hamner Institutes for Health Sciences.

Over the past five years, there has been a far-reaching discussion on the future direction of toxicology and how chemical testing is performed. The U.S. Environmental Protection Agency (EPA)'s National Center for Computational Toxicology (NCCT) has implemented a research program called ToxCast to assess whether the use of high-throughput in vitro screening assays that interrogate responses across hundreds of cellular pathways can be used to prioritize compounds for conventional in vivo testing. Most of the initial effort in the ToxCast program has focused on identifying potential high-throughput screening assays and collecting data on hundreds of agricultural and industrial chemicals in the selected assays. Less attention has been paid to determining the relationship between concentrations of the chemical active in vitro and expected concentrations in human populations. Pharmacokinetic properties and human exposure characteristics are equally important as the biological activity in determining a chemical's risk to human health.

The Hamner Institutes for Health Sciences has been collaborating with NCCT to incorporate dosimetry and exposure information into the ToxCast program. This collaborative effort evolved from a previously conducted pilot study evaluation of 35 ToxCast Phase I chemicals and the evaluation of the full set of 309 Phase I chemicals (see MTH0710 abstract). The current extended effort can be broken into the following three parts: 1) measurement of the metabolic clearance and plasma protein binding on a subset of the 700 ToxCast Phase II chemicals using primary human hepatocytes and human plasma, 2) measurement of the metabolic clearance and plasma protein binding on a subset of 59 Phase I chemicals using primary rat hepatocytes and rat plasma, and 3) measurement of the metabolic clearance of selected Phase I chemicals by individual recombinantly-expressed human hepatic enzymes.

In 2011, the metabolic clearance and plasma protein binding were experimentally measured for 200 chemicals from the ToxCast Phase II chemical library and exposure estimates were obtained for 7% of the compounds. Computational in vitro-to-in vivo extrapolation (IVIVE) methods and reverse dosimetry are currently being used to estimate human oral doses required to produce steady state in vivo concentrations equivalent to in vitro AC50 values. In the second part, measurement of the metabolic clearance and plasma protein binding of 59 Phase I chemicals using primary rat hepatocytes and rat plasma was completed. IVIVE modeling and reverse dosimetry were performed to calculate the oral equivalent doses which were then compared against doses and toxicities observed in in vivo studies compiled within the EPA ToxRef database. The manuscript describing this effort is in the final stages of preparation. Given the limited availability of exposure information for the Phase II chemicals, the third part of the project was added. The metabolic clearance of 10-15 ToxCast Phase I chemicals will be measured using a battery of individual hepatic enzymes. Different human subpopulations express these hepatic enzymes in different amounts. The metabolic clearance data will be incorporated into IVIVE modeling and reverse dosimetry to determine oral equivalent doses for different subpopulations (e.g., children, elderly). The experimental metabolic clearance assays for the individual enzymes are currently underway.

Implications

The reliance on in vitro assays without dosimetry and exposure information within the ToxCast program is likely to rapidly identify many hazardous, but possibly irrelevant, properties of chemicals. The extension of the incorporation of dosimetry and exposure into Phase II of the ToxCast program will continue to aid in interpretation of the high-throughput in vitro testing results and provide the relevant context for identifying appropriate priorities for follow-up testing and for risk evaluation exercises. The incorporation of the metabolism of the individual hepatic enzymes will also allow the identification of potential subpopulations that may be at higher risk of adverse health effects.

Keywords

reverse dosimetry, rat, ToxCast, high throughput screening, in vitro to in vivo extrapolation

Project Start and End Dates

January 2010 – December 2014

Project ID

MTH1002-03

Peer-reviewed Publication(s)

None to date.

Abstract Revision Date

January 2012

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

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