Scope of the Problem 21
metabolism processes for EDCs is MC, or bis[p-methoxydichlorophenyl] trichloro- ethane. This chemical is considerably less estrogenic, toxic, and persistent, with a shorter half life, than its parent compound, DDT.144 This reduced toxicity of MC is attributed to its conversion of polar metabolites.
1.5.8 QUANTITATIVE STRUCTURE–ACTIVITY RELATIONSHIP
(QSAR) STUDIES
This type of analysis correlates a chemical’s structural characteristics with its bio- logical activity, and the models derived from these relationships can be used to predict the activity of untested environmental contaminants. Models are based on the biological activity of small groups of compounds with similar activity and structural features. Such models could yield information that would be useful in developing assay batteries for toxicity testing and possibly replace the high-through- put screening for chemical priority setting.
These studies have been used to establish some three-dimensional and physic- ochemical characteristics (biophore and toxicophore). These parameters can be used to identify characteristics of molecules that are responsible for ED activity. Cun- ningham et al.145 used an artificial intelligence system to identify biophores associated with the activity of several estrogens and antiestrogens. It was found that chemicals possessed a common 6 Å biophore which modulates their activities. However, this biophore is not widespread among estrogens.146 The precise role this biophore plays in receptor binding and its relationship between estrogenicity and carcinogenicity has yet to be evaluated.
Several chemicals containing the biophore, and thus suspected EDCs, have been identified and are illustrated in Figure 1.10. It is worthwhile noting that 2,5,-dichloro- 4-hydroxylbiphenyl (Figure 1.7) also has the same biophore, analogous to 2-chloro- 4-hydroxybiphenyl.
Such studies have revealed that almost all chemical estrogens contain one or more phenolic groups, which are required for binding to the ER. Many natural and synthetic estrogens have a phenolic OH group on a small lipophilic molecule of about 200 to 300 Daltons.89 This chemical moiety is found in the body arising from the metabolism of endogenous and xenobiotic chemicals. For example, tamoxifen does not have the 6 Å biophore, but its 4-hydroxy derivative does (Figure 1.10).
Metabolites of compounds are also suspected to be EDCs based on the presence of this 6 Å biophore. An example is 2,2-bis (4-hydroxyphenyl)-1,1,1, trichloroethane, which is a metabolite of MC, and 2-chloro-4 hydroxy biphenyl, which is a metabolite of chlorobiphenyl.89
22 Endocrine Disrupters in Wastewater and Sludge Treatment Processes
chemicals for “effects in humans similar to that produced by a naturally occurring estrogen or other endocrine disrupting effects.” With this in mind, the EPA estab- lished the Endocrine Disrupter Screening and Testing Advisory Committee (EDSTAC) in 1996 to advise on the screening and testing of pesticides and other chemicals for their potential to act as EDCs. The aim of this program was to provide advice on the following:147
1. Developing a strategy for selection and prioritization of chemicals for screening and analysis
2. Defining a process for identifying new and existing screening assays 3. Assembling a set of available screens for early application
4. Determination of what definitive tests should be used and when, beyond screening
5. Identifying mechanisms for standardization and validation of screens and tests
The final report75 contains over 70 recommendations, many being incorporated by the EPA into its proposed screening program. The program prioritizes chemicals to be screened based on existing information followed by a screening battery (Tier 1) and a testing battery (Tier 2). Tier 1 is designed to detect the potential for a substance to be endocrine disrupting and consists of a variety of in vivo and in vitro FIGURE 1.10 Biophore identification on estradiol and some potential EDCs. (Adapted from Combes R.D., Endocrine disruptors: a critical review of in vitro and in vivo testing strategies for assessing their toxic hazard to humans, ATLA, 28, 81, 2000. With permission.)
OH 6Å
HO
HO
Cl
6Å
OH 6Å
6Å
HO HO
Cl Cl
Cl C H C C
C H3C
CH2 CH2 N
H2C
CH3 CH3
17β Estradiol (E2) 2-chloro-4-hydroxybiphenyl
4-hydroxytamoxifen
Methoxychlor metabolite (2,2-bis (4-hydroxyphenyl)
-1,1,1,trichloroethane O
Scope of the Problem 23
assays that are thought to include all known mechanisms of endocrine disruption in the three hormone systems: estrogen, androgen, and thyroid. Tier 2 is designed to characterize the endocrine effects of chemicals from Tier 1 testing that have shown to be endocrine disrupting. The initial number of chemicals identified for screening was around 87,000.75
Within the European Union, there are two Directorates General (DG) that are involved with the EDC issue: DG XII and DG XXIV. DG XII, Science, Research, and Development, has produced a number of research and monitoring recommen- dations covering modeling, methodology, human health, and wildlife studies.76 DGXXIV, Consumer Policy and Consumer Health Protection, considers the impact of chemical and biological compounds, including EDCs, on human health and the environment.
In the United Kingdom, the Department of the Environment, Food and Rural Affairs (DEFRA, formerly the DETR), in conjunction with the Ministry of Agricul- ture, Fisheries and Food (MAFF), the UK Environment Agency (UKEA), Scotland and Northern Ireland Forum for Environmental Research (SNIFFER), and the Euro- pean Chemical Industry Council, initiated the EDMAR program in 1998 (Endocrine Disruption in the Marine Environment). This program was to investigate whether there is evidence of changes associated with endocrine disruption in marine life and the possible causes and potential impacts. EDMAR has six main objectives:148
1. To develop biomarkers for detecting androgenic activity in marine fish and invertebrates and estrogenic activity in invertebrates
2. To conduct surveys of estrogenic and androgenic activity in key indicator species in the estuarine, coastal, and offshore environments
3. To observe the impact of this activity on a fish and invertebrate species 4. To conduct confirmatory experiments with suspect effluents and sub-
stances in laboratory test systems
5. To model the possible effects at the population level
6. To isolate the substances causing this marine endocrine disruption and identify the main sources of marine contamination
Much of this research has already been published66,149–156 as have two interim reports148,157 on the progress of this program.
Other work involving the U.K. Environment Agency has investigated the effects of EDCs on the fish population of U.K. rivers,47–50,58,158 the estrogenicity of STW effluents,45,52,61 and current work is focusing on the development of predicted-no- effect-concentrations (PNECs) for steroid estrogens159 and other EDCs. Since 1993, there have been over 140 U.K. government research-related projects concerning endocrine disrupters, ranging from methodology to monitoring and risk assessment.
In 1998, the Japanese Environment Agency produced a Strategic Program on Environmental Endocrine Disrupters (SPEED).81 Prior to this in 1997, they had set up an exogenous endocrine disrupting chemical task force. The 67 substances listed in the SPEED document as “chemicals suspected of causing endocrinological dis- ruption,” have not been conclusively proven to have, or not have, such a disruptive effect. They have only been identified as high-priority targets for future research,
24 Endocrine Disrupters in Wastewater and Sludge Treatment Processes
leading to frequent misunderstanding about “environmental hormones.” The program is designed to promote research to first assign priorities to these suspected substances, and then clarify the mechanisms, existence, and strength of endocrine disrupters,81 as well as promoting the strengthening of international networks.
The Organization for Economic Co-operation and Development (OECD) is an intergovernmental organization in which representatives of 30 industrialized coun- tries in North America, Europe, and the Pacific, as well as the European Commission, meet to coordinate and harmonize policies, discuss issues of mutual concern, and work together to respond to international problems. The OECD endocrine disrupter activity program was initiated in 1996, at the request of OECD member countries and industry to ensure that testing and assessment approaches would not differ substantially among countries.160 A task force on Endocrine Disrupter Testing and Assessment (EDTA) was set up to oversee this program. This has resulted in an initial framework for the testing of EDCs and consists of three tiers: initial assess- ment, screening, and testing.160 The development of this framework was necessary for consolidating the existing work in the United States, Europe, and Japan, and to provide a platform for the discussion of testing needs and future work.