As global concerns over the possible human health and environmental effects of endocrine-disrupting chemicals (EDCs) has grown, so have concerted efforts by the European Union, United States, other regulatory authorities and global organizations (e.g., OECD) to develop adequate tests and assessment frameworks to evaluate substances for endocrine disruption (ED) potential.
Development of these frameworks depends on the specific regulatory goals. Both the United States Environmental Protection Agency (USEPA) and EU have introduced specific legislation with regards to evaluating the ED potential of chemicals. In the United States, screening pesticides and drinking water contaminants is just one element in the overall objective of ensuring food and drinking water quality. Here, a risk-based approach takes into account both adverse effects and the likelihood of exposure. In the EU, the legislative aim leads to the eventual phasing out of industrial chemicals, plant protection products and biocides with ED properties. A purely hazard-based regulatory approach that only considers effects is being implemented.
With both approaches, testing must be sufficiently robust to 1) identify an adverse effect and 2) determine if the adverse effect is due to an endocrine mode of action. For risk-based regulation, testing must also be adequate for performing a risk assessment (e.g., dose response information).
The Endocrine Disruptor Screening Program (EDSP) developed by the USEPA currently fulfills these criteria with its two-tiered framework focusing on estrogen, androgen and thyroid activity. The Tier 1 screening battery required for all chemicals consists of 11 in vitro and in vivo screens which are designed to minimize false negatives and to assess the potential of a chemical to interact with the endocrine system. Tier 2 tests include four in vivo reproduction tests in a variety of species, including three wildlife species. One or more of these tests may be required following a weight of evidence evaluation of the Tier 1 results and any additional data. These dose-response tests in turn assess whether effects are adverse and produce endpoints suitable for use in risk assessment.
Conservation of hormone receptors across vertebrate species enable the results of one screen to inform another – therefore, the results of ecotoxicity and toxicity testing in Tier 1 and 2 (in mammalian, fish and amphibian systems) are evaluated as a complete battery. In general, adequate guidance is given for interpretation of results from individual screening assays. However, more detail on the weight of evidence methodology and/or the creation of decision trees that provide greater transparency on how the results of the Tier 1 battery are evaluated as a whole and lead to the requirement of Tier 2 tests would be a useful addition to the program.
Though the EDSP testing is adequate in that it achieves the overall goals, the program is time- consuming, costly and requires a large number of animals at a time when the Tox21 initiative is attempting to minimize animal use. The USEPA has invested considerable time and resources into the development of high throughput screening assays and computational models through its Tox21/EDSP21 programs, with a view to eventually providing alternatives for some or all of the current tests in the Tier 1 screening battery.
The regulatory framework in the European Union is in an earlier stage of development to that in the United States. Currently, the criteria for identification of EDCs is still being discussed, as is the way in which they will ultimately be regulated. But for now, a hazard-based assessment is being considered for plant protection products and biocides; that is, a compound deemed to be an ED that causes adverse effects in humans or non-target organisms will be excluded from registration unless the exposure is negligible (yet to be fully defined in the context of environmental assessment). At this time, neither exposure levels, nor potency of a compound will be taken into account and since the criteria are not finalized, no testing scheme is yet available. It remains unclear, however, how closely the EU testing strategy will align with that of the evolving EDSP.
The OECD has proposed a conceptual framework and an associated guidance document (OECD 150)2 for testing and assessment of EDCs. The framework provides an exhaustive list of the available OECD Test Guidelines – standardized tests and methods developed on a wide range of species that might be used to evaluate chemicals for ED properties. The framework also divides the tests into five levels. Level 1 takes into account existing physicochemical and (eco)toxicity data as well as information gleaned from predictive computational models. Level 2 provides information on specific endocrine mechanisms using in vitro assays while Level 3 examines selected mechanisms in vivo. Levels 4 and 5 include in vivo tests designed to provide data on adverse effects on endocrine relevant endpoints. Tests in Level 4 focus on effects on growth and reproduction, while Level 5 includes full lifecycle studies. Although the framework is not intended to be a testing strategy for identifying and linking ED activity and adverse effects, it is a valuable tool for use in a weight of evidence approach and should be taken into consideration by the EU, though it remains to be seen to what extent it is utilized.
Clearly, these tests alone will not be sufficient to determine adverse effects in a whole organism. But whether or not full lifecycle tests are required to identify an ED in light of the hazard-based cut-off criteria proposed in the EU, or to enable risk assessment in the U.S., might have to be evaluated on a case by case basis. Distinguishing adverse effects caused by an endocrine mode of action from adverse effects elicited via a toxic mode of action is a further challenge. Complex lifecycle studies use apical endpoints such as reproduction to assess adverse effects. The stakes are high for substances deemed to be endocrine disruptors, so close attention should be paid to ruling out toxicity as the cause of an adverse effect and linking the effect to a specific endocrine mode of action.
The continuing controversy over the definition of EDCs, understanding how best to incorporate predictive computational models and HTP testing, the interpretation of adverse effects and necessity of using a weight of evidence approach to determine the ED potential of a chemical makes the challenge of designing testing strategies that achieve regulatory goals extremely complex. As these strategies continue to evolve in both the U.S. and EU, transparency and involvement of all stakeholders is a necessity – both to improve logistics for carrying out testing programs and to minimize disagreements over the interpretation of test results and regulatory decisions that follow from them.
Tessa Scown, Ph.D., is global regulatory ecotoxicologist at DuPont Crop Protection in Newark, Del., USA.