The enhancement of the subacute repeat dose toxicity test OECD TG 407 for the detection of endocrine active chemicals: comparison with toxicity tests of longer duration.

The OECD conventional 28-day repeat dose toxicity test (OECD TG 407) is widely employed in the initial hazard identification and characterization for commercial chemicals. The OECD has recently undertaken an international effort to "enhance" the conventional 28-day repeat dose toxicity test (OECD TG 407) in order to ensure that chemicals acting through (anti)estrogenic, (anti)androgenic, and (anti)thyroid mechanisms are identified. The enhancements include additional parameters based on the respective target organs from the male and female reproductive tracts, the thyroid, and circulating hormone levels. Ten chemicals with known endocrine modes of action and different potencies were administered using the "enhanced TG 407" test protocol to investigate the performance of this procedure. In the present evaluation, these "enhanced TG 407" protocol results, drawn from a report of the OECD validation studies, are compared to studies of the same or similar chemicals with longer and/or in utero exposures in order to evaluate the capability of the this "enhanced TG 407" in identifying the chemicals' mode of action. The major conclusions that can be drawn from these comparisons are: 1. The "enhanced TG 407" will reliably identify chemicals with a strong to moderate potential to act through endocrine modes of action on the gonads and the thyroid. In addition, this test method gives a first indication for the dose-related potency. 2. Substances with a low potency for an endocrine mode of action, i.e., having only marginal effects in the most comprehensive in vivo studies such as multi-generation studies, may not elicit clear endocrine-related effects in the "enhanced TG 407". In these cases, the primary or principal effects observed will be driven by other toxic actions of the test materials in the "enhanced TG 407". 3. It may be concluded from the present database that prolongation of exposure from 28 days up to 90 days is unlikely to improve the chance of detecting an endocrine-mediated effect 4. A number of higher tier studies with in utero and pubertal exposure show that prenatally exposed rats may be more sensitive to exposures to compounds with very low estrogenic or antiandrogenic potential in some cases than young adult rats as used in the "enhanced TG 407". 5. Overall, these comparisons support the use of the "enhanced TG407" for the detection of endocrine active chemicals. It is therefore recommended to fully accept the enhancements and include them in the test method for toxicological and regulatory use.

Critical review and evaluation of the uterotrophic bioassay for the identification of possible estrogen agonists and antagonists: in support of the validation of the OECD uterotrophic protocols for the laboratory rodent. Organisation for Economic Co-operation and Development.

A current issue for regulatory agencies is endocrine-related modes of action such as those mediated by the estrogen, androgen, and thyroid nuclear receptors. At the national and international levels, the consensus recommendation for the assessment of such modes of action is a tiered series of in vitro and in vivo protocols. The tiered framework begins with screens for structural alerts and then moves to rapid, mechanistic in vitro screening assays, and then to in vivo screening bioassays. The objective of these screens is to identify substances that may warrant testing for endocrine-mediated adverse effects. The final framework tier as needed is to test these substances in long-term bioassays for adverse endocrine-mediated reproductive and/or developmental effects. The subject of this review, the rodent uterotrophic bioassay, is intended to be a rapid in vivo screening bioassay for possible estrogen agonists and based on the response of the estrogen-sensitive uterus. The central metric of bioassay is a statistically significant increase in the weight of the uterus after 3 consecutive days of test substance administration. The extensive background literature is summarized in this review on the mode of action underlying the bioassay and the uterine response to estrogens. The review includes the bioassay's history of development and how its employment has changed and evolved over time. The review describes two major uterotrophic bioassay versions, the intact, immature female and the mature, ovariectomized female, and the protocol factors likely to influence relevance, reproducibility, and reliability of bioassay. The emphasis of the review is the ability of the uterotrophic bioassay to identify the substances of current interest: weak estrogen agonists with binding affinities relative to the natural 17beta-estradiol in the log 0 to log -3 range. Using selected model substances having RBAs in this target range, the bioassay's performance in a hierarchical, tiered approach is evaluated, including the predictive capability of the uterotrophic bioassay based on available reproductive and developmental testing data. The review concludes that the uterotrophic bioassay is reliable and can identify substances that may act via an estrogen-mode of action, supporting the validity of the uterotrophic bioassay and its regulatory use as an in vivo mechanistic screening bioassay for estrogen agonists and antagonists.

Chemical toxicity indicators for human health: case study for classification of chronic noncancer chemical hazards in life-cycle assessment.

The classification scheme for human health indicators in life-cycle assessment (LCA) and other toxicity scores is proposed based on identifiable target organ or system toxicity. This scheme represents a compromise between a subjective single overall score and highly detailed toxic mechanisms of action. A case study was used to evaluate the proposed classification scheme. The data on 117 high production volume organic chemicals were extracted from the U.S. Environmental Protection Agency (U.S. EPA) integrated risk information system (IRIS) database. Based on the critical effect listed in the database, almost all chemicals were easily and clearly classified into the proposed categories. However, there were significant variations in test species, protocol, duration, toxicological parameter measured, etc., among different toxicity studies. Importantly, the toxicological critical effects observed, even for the same target organ or system, differed from chemical to chemical and were not equivalent. Using hepatotoxicity as an example, critical effects included changes in organ weight, a variety of different histopathological changes, and changes in circulating hepatic enzyme levels. Therefore, no universal, common basis was identified for biological equivalency in order to compare or to aggregate chemicals into an overall toxicity score. A recent proposal to develop a characterization system based on disability-adjusted life years (DALYs) was also evaluated. No means were found to extrapolate animal critical effects data to predict any diminution in human lifetimes or the appearance and intensity of human disability. Still, LCA presents an ideal starting point for toxicological analysis by conducting a system-wide analysis that inventories and maps the sources of potentially important human pollutants. The LCA results could then facilitate initial toxicological and risk evaluations for those sources and chemicals and set the stage for a more detailed and informed analysis by professional toxicologists.