Non-animal Methods for Toxicity Testing

Tox21: Putting a Lens on the Vision of Toxicity Testing in the 21st Century

Last Updated: August 19, 2008
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By Christopher Austin, National Human Genome Research Institute, US National Istitutes of Health
     Robert Kavlock, US Environmental Protection Agency
     Raymond Tice, US National Institute of Environmental Health Sciences

©2008 NIH Chemical Genomics Center

In response to the release of the NRC report on "Toxicity Testing in the 21st Century, a Vision and Strategy" (NRC, 2007), two NIH institutes and EPA formed a collaboration (Tox21) to (1) identify mechanisms of chemically induced biological activity, (2) prioritize chemicals for more extensive toxicological evaluation, and (3) develop more predictive models of in vivo biological response. Success in achieving these goals is expected to result in methods for toxicity testing that are more scientific and cost-effective, and models for risk assessment that are more mechanistically based. As a consequence, a reduction or replacement of animals in regulatory testing is anticipated to occur in parallel with an increased ability to evaluate the large numbers of chemicals that currently lack adequate toxicological evaluation. Ultimately, Tox21 is expected to deliver biological activity profiles that are predictive of in vivo toxicities for the thousands of under-studied substances of concern to regulatory authorities in the United States, as well as in many other countries.

©2008 EPA/NCGC

The Tox21 collaboration is being coordinated through a five-year Memorandum of Understanding (MOU), which leverages the strengths of each organization. The MOU builds on the experimental toxicology expertise at the National Toxicology Program (NTP), headquartered at the NIH National Institute of Environmental Health Sciences (NIEHS); the high throughput screening (HTS) technology of the NIH Chemical Genomics Center (NCGC), managed by the National Human Genome Research Institute (NHGRI); and the computational toxicology capabilities of the EPA's National Center for Computational Toxicology (NCCT). Each party brings complementary expertise to bear on the application of novel methodologies to evaluate large numbers of chemicals for their potential to interact with the myriad of biological processes relevant to toxicity. A central aspect of Tox21 is the unique capabilities of the NCGC's high-speed, automated screening robots to simultaneously test thousands of potentially toxic compounds in biochemical- and cell-based HTS assays, and our ability to target this resource toward environmental health issues.

In addition to the testing activities, the MOU promotes coordination and sponsorship of workshops, symposia, and seminars to educate the various stakeholder groups including regulatory scientists and the public.

Focus Groups

To date, four focus groups have been established to support the Tox21 goals. Each focus group is co-chaired by a representative of each of the three organizations.

Chemical Selection: This group will coordinate the selection of chemicals for the Tox21 compound library to test at the NCGC. In the near term, this library is expected to include ~2800 compounds selected by the NTP, ~2800 compounds selected by the EPA, and ~2800 clinically approved drugs selected by the NCGC. A subset of compounds will be included multiple times in the library to evaluate within-assay reproducibility. Compound selection is based largely on the compound having a defined chemical structure and known purity, and on the extent of its solubility and stability in dimethyl sulfoxide (DMSO), the preferred solvent for HTS assays conducted at the NCGC. In addition to test compounds that are relatively insoluble in DMSO, a second, but smaller, library of compounds that are preferentially soluble in aqueous solutions is being identified. Implementing quality control procedures for identify and purity of all compounds in the library is an important charge to this group.

Biological Pathways and Assays: This group will identify and prioritize reliable HTS assays for use at the NCGC. Assays currently used include ones to assess (1) cytotoxicity and activation of caspases in a number of human and rodent cell types, (2) up-regulation of p53, (3) agonist/antagonist activity for a number of nuclear receptors, and (4) differential cytotoxicity in several cell lines associated with an inability to repair various classes of DNA damage. Others under consideration include assays for a variety of physiologically important molecular pathways (e.g., cellular stress responses) and methods for integrating human and rodent hepatic metabolic activation into reporter gene assays.

Informatics: This group will address methods for the analysis, interpretation, and storage of Tox21 HTS data, its public availability, and how to prioritize compounds for follow-up targeted testing. Approaches to making the data publicly accessible include uplinks to various existing databases: NCBI's PubChem, EPA's ACToR (Aggregated Computational Toxicology Resource), and NIEHS' CEBS (Chemical Effects in Biological Systems).

Targeted Testing: This group will develop strategies and capabilities that address follow-up to HTS assays using targeted testing in assays that involve higher order testing systems (e.g., roundworms [Caenorhabditis elegans], zebrafish embryos, rodents).


A Governance Board composed of the Director of the NCGC, the Director of the NCCT, and the Chief of the NTP Biomolecular Screening Branch meets biweekly by teleconference and provides leadership to the collaboration. In addition to the Governance Board, a leadership group consisting of co-chairs of the four focus group and members of the Governance Board meets monthly to review progress and identify goals. Every three months, all of the scientists participating in the Tox21 initiative meet to review progress and discuss policies and future activities. The respective Board of Scientific Counselor for each participating organization will review that organization's progress toward Tox21 goals.

Public Participation

Persons interested in following the progress of Tox21 are invited to join the Chemical Prioritization Community of Practice, which meets by teleconference at 11:00 am ET on the fourth Thursday of every month.

Disclaimer: Although the research described in this report has been funded by one or more of the participating federal agencies, the report does not necessarily reflect the views of the respective organizations.


Author Biography

©2008 Christopher Austin, Robert Kavlock, and Raymond Tice