To determine whether or not its guidelines for reproductive and developmental toxicity testing are adequate for endocrine active chemicals, the U.S. Environmental Protection Agency (EPA) asked the National Toxicology Program (NTP)/National Institute of Environmental Health Sciences (NIEHS) to organize and conduct a scientific peer review to evaluate reported low-dose effects and dose-response relationships for endocrine disrupting chemicals. For this meeting, "low-dose effects" referred to biological changes that occur in the range of human exposures or at doses that are lower than those typically used in the U.S. EPA's standard testing paradigm for evaluating reproductive and developmental toxicity. The peer review panel was divided into five subpanels: Bisphenol A, Estradiol and Other Estrogens, Androgens and Antiandrogens, Biological Factors and Study Design, and Statistics and Dose-Response Modeling. The Panel examined data from major, selected studies (excluding studies on dioxin and dioxin-like compounds) supporting the presence or absence of low-dose effects in laboratory animals that could be relevant for human health assessments. The Panel was also asked to evaluate the shape of the dose-response curve for endocrine active substances in the low-dose region. The Panel exceeded the NTP's expectations of high quality and rigorous review of the available scientific evidence for low-dose effects at this three-day meeting. This peer review defined the knowledge base and uncertainties within this field and, as such, helps to strengthen the link between science and the regulatory decision-making process.

This peer review created a unique and novel model to resolve a controversial but very important environmental health issue. For this review, principal investigators of the primary research groups active in this field willingly provided their research data on selected parameters for independent statistical re-analysis. The Statistics and Dose-Response Modeling Subpanel analyzed the data for 38 studies prior to the meeting and provided its analyses to the other subpanels. In addition, investigators from the primary research groups were available at the meeting to give formal presentations of their findings and to have informal discussions with individual subpanels. This unique scientific peer review provided an open, transparent, and objective review and evaluation of the scientific evidence showing the presence or absence of low-dose effects of endocrine disrupting agents and an opportunity for participation by all stakeholders.

Preliminary Conclusions

Based upon presentations by the individual subpanels and general discussions during the plenary session, preliminary conclusions from this peer review include the following:

Low-dose effects have been clearly demonstrated for estradiol and some estrogenic compounds. For example, low-dose findings for nonylphenol and the phytoestrogen genistein include effects on the immune system and on neurological structure. For other compounds, results from different laboratories indicate the presence or absence of low-dose effects (e.g., the effect of bisphenol A on prostate weight). Although attempts were made to replicate early results, the Panel concluded that in most cases both the positive and negative study findings are credible and sound within the context of the experimental design. Study design differences (e.g., species/strain, animal husbandry, diet, chemical purity, dosing regime, endpoints evaluated, genetic factors, etc.) were identified among laboratories that might account for discrepancies in experimental outcomes. With regard to the observed low-dose responses, there is a need for further research to better understand the overall and long-term health consequences of those effects (i.e., whether or not these effects represent risk factors for later developing diseases).

Effects of antiandrogenic compounds have been demonstrated for some endpoints and the dose-response curve appears linear to the lowest dose tested; however, it was noted that the available studies were not designed to evaluate low-dose effects as defined for this review. The Panel concluded that low dose effects of androgenic compounds have not been studied.

The Panel identified areas for additional research that would clarify uncertainties about the occurrence of low-dose effects and better characterize those observed effects. These include (but are not limited to) using pharmacological and genetic approaches to determine mechanisms of action and to characterize dose-response relationships, characterizing response longevity from gestation through adulthood, evaluating long-term health outcomes, investigating the basis for immune system effects, and determining the impact of variations in endogenous hormone levels. The Panel recommended the use of new technologies (e.g., cDNA microarrays and proteomics) and additional or more sensitive biomarkers of response for evaluating low-dose effects.

The Panel recommended that special consideration be given to study design and biological and environmental factors that might affect experimental outcomes (e.g., diet, animal model, animal husbandry, dose, route, timing and duration of exposure, etc.). In selecting animal models for study, the Panel advocated the use of species and strains that are highly responsive to endocrine active compounds. The Panel also recommended that laboratories demonstrate effects with positive controls before initiating new studies and supported collaborations among laboratories in standardizing study protocols for the purpose of replicating findings.

The Panel concluded that the shape of the dose-response curve for endocrine disrupting agents would depend on numerous factors including the endpoint being evaluated, the chemical being studied, the dosing regime, and the biological characteristics of the target tissue. This conclusion is consistent with other receptor-mediated responses, where the shape of the dose-response curve may be non-monotonic (e.g., "inverted-u"), linear, or threshold-appearing.

Final Report

The individual subpanels' presentations and the subsequent plenary discussion represent the first step toward the NTP preparing its peer review report. Following the meeting, each subpanel will prepare a report of its conclusions noting areas of consensus and disagreement. In addition, the subpanels are asked to identify areas for which additional research is needed to help resolve ambiguities. The NTP will receive the individual subpanel reports and synthesize this information into a single peer review report. The NTP will solicit public comments on this report and incorporate them, as appropriate, into its final transmission to the U.S. EPA and other Federal agencies. The NTP will transmit its report with an appendix containing the subpanel reports and public comments to the U.S. EPA in spring 2001. The information in this document should provide a strong scientific foundation upon which the U.S. EPA and other national and international agencies can base decisions regarding the selection of dose, endpoints, animal models, and testing regimes when designing reproductive and developmental studies of endocrine active agents.

The NTP report will be published in a scientific journal and posted on its web site (http://ntp-server.niehs.nih.gov). Requests for hard copies of the NTP report and inquiries about the Endocrine Disruptors Low-Dose Peer Review can be made to the NTP Liaison and Scientific Review Office (NIEHS, P.O. Box 12233, Research Triangle Park, NC 27709; t: 919-541-0530; f: 919-541-0295; liaison@starbase.niehs.nih.gov).

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