Strengths: Endocrine disruptors are chemicals that interfere with the hormone system. Disruption in regulatory hormonal systems can cause devastating long-term effects such as problems in brain, metabolism, and disease states such as diabetes and autoimmune dysfunction. It may be of interest to test a group of new chemicals for potential endocrine disrupting activity before they are released for public use. Assays in this tier allow for the opportunity to further explore findings from computer based assessment by means of targeted nuclear receptor assays.
Target Cell assays allow: a) direct testing for possible disruption of biological signaling pathways by new compounds, b) to refine the findings from computer based toxicity predictions and pinpoint the endocrine systems that may be affected by the new compounds, and c) more efficiently target in vivo testing in the Cell Processes tier .
Additionally, one can also test a large number of chemicals at once using rapid, robotic tools. These “high-throughput screens” (HTS) give relatively fast information and, because they can simultaneously test such a large number of chemicals, can be relatively cost-efficient. HTS are now available using cell-based and cell-free methods. The two primary examples in development in the U.S. are ToxCast at the U.S. E.P.A.21; and Tox21, a joint effort U.S. E.P.A., National Institutes of Environmental Health Sciences/National Toxicology Program, National Institutes of Health and the Food and Drug Administration22.
Weaknesses:
The current design of the Targeted Cell battery of assays will allow one to obtain information about the ability of chemicals to interact with any one of 48 different nuclear receptors. However, some chemicals can interfere with hormone action in the body by mechanisms that do not target these receptors. For example, some chemicals can activate enzyme systems in the body that can block hormone synthesis or metabolism. In addition, chemicals may be modified in the body to form compounds that directly interact with receptors despite no such activity in the parent compounds. Evaluating metabolism is important, but is not present in current assays in this tier tests. Future iterations of targeted cell tier tests will include metabolism as a component, but this is not a trivial issue and will require development.
Additionally, the technologies employed in this tier make use of recombinant nuclear receptors that, in some cases, are fragments of the receptor. These technologies have the advantage of being relative inexpensive and rapid, but they can produce a higher level of false positive and false negative results. This weakness is compensated for to some extent by having some redundancy in the battery of assays. However, false positives would also be identified in Cell Processes assays as well as higher level assays.
As an overall concern, the high-throughput screens (HTS) in common use were originally developed for use in drug discovery. They work well at detecting pharmacologically-active compounds with strong effects. However, HTS were not initially designed (or performed by most labs) to detect weak activities, and it is precisely these weaker signals that may be biologically relevant and indicative of EDC activity. Hence care must be exercised in HTS use and interpretation for detecting EDC activity.