Prediction of the health effects of polychlorinated biphenyls (PCBs) and their metabolites using quantitative structure-activity relationship (QSAR).

Polychlorinated biphenyls (PCBs) are a group of 209 persistent environmental contaminants that are slightly different but structurally related. PCBs are known to induce a variety of health effects and often have been toxicologically tested as complex commercial mixtures (Aroclors) but environmental exposure occurs separately to a small number of specific congeners. Recently, the Third National Report on Human Exposures to Environmental Chemicals, an assessment of exposure data of the National Health and Nutrition Examination Survey (NHANES), identified 35 individual PCB congeners in the U.S. population. These types of findings necessitate the toxicity evaluation of individual congeners but adequate toxicity data for most individual PCB congeners are not available. Due to this, a quantitative structure-activity relationship (QSAR) approach was used to assess the potential mutagenesis and carcinogenesis of individual congeners and their possible metabolites. The predictions were analyzed to define the underlying generalizations between the parent PCBs, their metabolites, and some important toxicological endpoints. This analysis reveals that (1) mono and di-chlorinated PCBs and their metabolites can be potential mutagens; (2) PCB benzoquinone metabolites could be carcinogenic but the weight of evidence is poor. These results support the hypothesis that environmental exposure to some PCBs and/or their metabolites could produce mutagenicity and/or carcinogenicity. Hence, these data should be considered as priority toxicological testing data needs. As with all computational toxicology analytical findings, these conclusions must yield to empirical data as they become available.

Toxicity assessment of unintentional exposure to multiple chemicals.

Typically exposure to environmental chemicals is unintentional, and often the exposure is to chemical mixtures, either simultaneously or sequentially. When exposure occurs, in public health practice, it is prudent to ascertain if thresholds for harmful health effects are exceeded, whether by individual chemicals or by chemicals in combination. Three alternative approaches are available for assessing the toxicity of chemical mixtures. Each approach, however, has shortcomings. As the procedures of each approach are described in this paper, at various steps research needs are identified. Recently, reliance has increased on computational toxicology methods for predicting toxicological effects when data are limited. Advances in molecular biology, identification of biomarkers, and availability of accurate and sensitive methods allow us to more precisely define the relationships between multiple chemical exposures and health effects, both qualitatively and quantitatively. Key research needs are best fulfilled through collaborative research. It is through such collaborations that resources are most effectively leveraged to further develop and apply toxicity assessment methods that advance public health practices in vulnerable communities.

Implications of chemical mixtures in public health practice.

The Agency for Toxic Substances and Disease Registry (ATSDR) is a federal public health agency that investigates and strives to prevent human health problems produced by exposure to toxic chemicals and their mixtures in the environment. Most human exposures involving toxic chemicals or mixtures are thought to originate from environmental and occupational sources; however, concurrent exposures are also likely from other sources, such as prescription and nonprescription drugs, indoor air pollutants, alcohol, and tobacco smoke. Thus, in evaluating the potential hazard following exposure to environmental mixtures, ATSDR not only considers the inherent joint toxicity of the mixture but also the influence of environmental, demographic, occupational, and lifestyle factors. To foster these goals, ATSDR has pursued a Mixtures Research and Assessment Program that consists of three component efforts: trend analysis, joint toxicity assessment, and experimental testing. Through trend analysis, ATSDR sets priorities for environmental mixtures of concern for which joint toxicity assessments are conducted as needed. If data are not available to conduct appropriate assessments, a research agenda is pursued through established extramural mechanisms. Ultimately, the data generated are used to support ATSDR's work at sites involving exposure to chemical mixtures. This pragmatic approach allows testable hypotheses or research needs to be identified and resolved and enhances our understanding of the mechanisms of joint toxicity. Several collaborative and cooperative efforts with national and international organizations such as the Toxicology and Nutrition Office, the Netherlands, and the Department of Energy are being pursued as part of these activities. ATSDR also develops guidance manuals to consistently and accurately apply current methodologies for the joint toxicity assessment of chemicals. Further, expert panels often are assembled to resolve outstanding scientific issues or obtain expert advice on pertinent issues. Recently, the need for studies on chemical mixtures has been proposed as one of the six priority areas the agency identified in its agenda for public health environmental research. This has been reinforced through the agency's close work with communities whose leaders have spoken passionately about their concern for information on exposures to chemical mixtures. The five other priority research areas the agency identified are exposure, susceptible populations, communities and tribal involvement, evaluation/surveillance of health effects, and health promotion/prevention.

Gene induction studies and toxicity of chemical mixtures.

As part of its mixtures program, the Agency for Toxic Substances and Disease Registry (ATSDR) supports in vitro and limited in vivo toxicity testing to further our understanding of the toxicity and health effects of chemical mixtures. There are increasing concerns that environmental chemicals adversely affect the health of humans and wildlife. These concerns have been augmented by the realization that exposure to chemicals often occurs to mixtures of these chemicals that may exhibit complex synergistic or antagonistic interactions. To address such concerns, we have conducted two studies with techniques that are being used increasingly in experimental toxicology. In the first study, six organochlorine pesticides (4,4 -DDT, 4,4 -DDD, 4,4 -DDE, aldrin, dieldrin, or endrin) were selected from the ATSDR Comprehensive Environmental Response, Compensation and Liability Act of 1980 (or Superfund) priority list and tested for their ability to modulate transcriptional activation of an estrogen-responsive reporter gene in transfected HeLa cells. In these assays, HeLa cells cotransfected with an expression vector encoding estrogen receptor and an estrogen-responsive chloramphenicol acetyltransferase (CAT) reporter plasmid were dosed with and without selected environmental chemicals either individually or in defined combinations. Estradiol consistently elicited 10- to 23-fold dose-dependent inductions in this assay. By contrast, all six of the organochlorine pesticides showed no detectable dose-related response when tested either individually or in binary combinations. Thus, these chemicals as binary mixtures do not exhibit any additional estrogenicity at the levels tested in these assays. In the second study, arsenic [As(V)], cadmium [Cd(II)], chromium [Cr(III, VI)], and lead [Pb(II)] were tested in a commercially developed assay system, CAT-Tox (L), to identify metal-responsive promoters and to determine whether the pattern of gene expression changed with a mixture of these metals. This assay employs a battery of recombinant HepG2 cell lines to test the transcriptional activation capacity of xenobiotics in any of 13 different signal-transduction pathways. Singly, As(V), Cd(II), Cr(III, VI), and Pb(II) produced complex induction profiles in these assays. However, no evidence of synergistic activity was detected with a mixture of Cd(II), Cr(III), and Pb(II). These results have shown metal activation of gene expression through several previously unreported signal-transduction pathways and thus suggest new directions for future studies into their biochemical mechanisms of toxicity. In conclusion, the (italic)in vitro(/italic) methods used in these studies provide insights into complex interactions that occur in cellular systems and could be used to identify biomarkers of exposure to other environmental chemical mixtures.

Public health assessment of hexachlorobenzene.

Recently, hexachlorobenzene (HCB) was proposed for inclusion in the system of toxicity equivalency factors (TEFs) currently used for dioxin-like compounds. In this paper, we explore the practical implications of the proposition to the Agency for Toxic Substances and Disease Registry (ATSDR) programs by comparing respective health guidance values for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and HCB (expressed as total toxicity equivalents [TEQs]), reviewing possible interactions between HCB and dioxin-like chemicals, and by providing information on actual co-existence of HCB and dioxin-like chemicals at hazardous waste sites. We found a good correlation between the TEF-adjusted oral exposure guidance values for HCB and guidance values for TCDD. The combination of HCB and other dioxin-like compounds was not found in soil, air, or water media at hazardous waste sites. Based on this fact, it is not necessary to include HCB in the total TEQ count at hazardous waste sites at this time.

Neurodevelopmental effects: making the case for biologic plausibility.

It has been suggested that the most critical missing link between science and policy is causality; that is, the establishment of a definite cause-effect relationship between exposure and adverse health effects. As has been clearly demonstrated by the decades-long tobacco debate, causality is extremely difficult to establish with absolute certainty, particularly in the minds of scientists. Because of this, it has been suggested that a "weight of evidence" approach based on biologic plausibility should be used as a surrogate for causality when translating science into policy and public health practice. In the case of neurodevelopmental effects, the case for biologic plausibility is supported by scientific findings from three broad areas consisting of wildlife biology, toxicology, and epidemiology. A striking example of this is provided by research findings from the Great Lakes Basin, an area which has been the focus of significant scientific research for the last thirty years in these three broad areas. In this paper, we examine relevant findings from the Great Lakes Basin and elsewhere as they relate to establishing and supporting the biologic plausibility of neurodevelopmental effects associated with environmental exposures to persistent toxic substances.

Animals as sentinels of human health hazards of environmental chemicals.

A workshop titled "Using Sentinel Species Data to Address the Potential Human Health Effects of Chemicals in the Environment," sponsored by the U.S. Army Center for Environmental Health Research, the National Center for Environmental Assessment of the EPA, and the Agency for Toxic Substances and Disease Registry, was held to consider the use of sentinel and surrogate animal species data for evaluating the potential human health effects of chemicals in the environment. The workshop took a broad view of the sentinel species concept, and included mammalian and nonmammalian species, companion animals, food animals, fish, amphibians, and other wildlife. Sentinel species data included observations of wild animals in field situations as well as experimental animal data. Workshop participants identified potential applications for sentinel species data derived from monitoring programs or serendipitous observations and explored the potential use of such information in human health hazard and risk assessments and for evaluating causes or mechanisms of effect. Although it is unlikely that sentinel species data will be used as the sole determinative factor in evaluating human health concerns, such data can be useful as for additional weight of evidence in a risk assessment, for providing early warning of situations requiring further study, or for monitoring the course of remedial activities. Attention was given to the factors impeding the application of sentinel species approaches and their acceptance in the scientific and regulatory communities. Workshop participants identified a number of critical research needs and opportunities for interagency collaboration that could help advance the use of sentinel species approaches.

Key environmental human health issues in the Great Lakes and St. Lawrence River basins.

In May 1997, Health Conference '97-Great Lakes/St. Lawrence, an international conference on the effects of the environment on human health in the Great Lakes and St. Lawrence River basins, was held in Montreal, Québec, Canada. This was the third international conference on this topic sponsored by agencies in the United States and Canada. More than 120 platform and poster presentations were given by scientists of different disciplines from the Great Lakes region and elsewhere. The presentations represented the most current research findings on the effects of the Great Lakes environment on human health. The reports covered environmental contaminant levels of persistent toxic substances (PTSs), routes and pathways of exposure, exposure assessment and human tissue levels of PTSs, human health outcomes, risk communication and assessment, and approaches to scientific collaboration. Reports indicate that levels of contaminants in the Great Lakes and St. Lawrence River basins have generally declined since the 1970s, although certain contaminants have plateaued or slightly increased. The findings include elevated body burden levels of contaminants in persons who consume large amounts of some Great Lakes sport fish, developmental deficits and neurologic problems in children of some fish-consuming parents, nervous system dysfunction in adults, and disturbances in reproductive parameters. The findings underscore the need for better public health intervention strategies.

Research management in the Great Lakes and St. Lawrence River basins: challenges and opportunities.

Research management in the Great Lakes and St. Lawrence River basins is both challenging and filled with opportunities. From the perspective of public health practice, research management is more than just research managers managing discrete programs; it requires everyone involved in the process to become active participants, including researchers, communities, potential interest groups, policymakers, and other stakeholders. Agencies, organizations, and individuals responsible for managing research and resources in the Great Lakes and St. Lawrence River basins are facing problems of decreased research funding, data gaps, and research quality. Managers of research and resources in the basins face many challenges as they address these problems. They are challenged with strengthening the link between research and management in the face of decreasing resources and increasing expectations of results and findings while extending those results and findings to public health practice. A number of actions and activities have been proposed that can lead to better management of constrained programs, pooled resources, partnerships, targeted priorities, and improved effectiveness. With guidance and assistance from the International Joint Commission (IJC), research managers in the Great Lakes and St. Lawrence River basins who have initiated and maintained traditional research programs based on sound science are now adopting different and innovative management strategies. The research community must be proactive in articulating the role of science in bridging the gaps in knowledge between public health practice and regulatory programs. Supported by a firm foundation of credible science, critical assessment, and public service, basin research managers are recognizing the need to move outside the comfort zone and extend to areas previously unwelcomed or uncomfortable.

Reducing uncertainty in the derivation and application of health guidance values in public health practice. Dioxin as a case study.

We were requested by the U.S. Environmental Protection Agency (EPA) to clarify the relationships among the minimal risk level (MRL), action level, and environmental media evaluation guide (EMEG) for dioxin established by the Agency for Toxic Substances and Disease Registry (ATSDR). In response we developed a document entitled "Dioxin and Dioxin-Like Compounds in Soil, Part I: ATSDR Interim Policy Guideline"; and a supporting document entitled "Dioxin and Dioxin-Like Compounds in Soil, Part II: Technical Support Document". In these documents, we evaluated the key assumptions underlying the development and use of the ATSDR action level, MRL, and EMEG for dioxin. We described the chronology of events outlining these different health guidance values for dioxin and identified the areas of uncertainty surrounding these values. Four scientific assumptions were found to have had a great impact on this process; these were: (1) the specific uncertainty factors used, (2) the toxicity equivalent (TEQ) approach, (3) the fractional exposure from different pathways, and (4) the use of body burdens in the absence of exposure data. This information was subsequently used to develop a framework for reducing the uncertainties in public health risk assessment associated with exposure to other chemical contaminants in the environment. Within this framework are a number of future directions for reducing uncertainty, including physiologically based pharmacokinetic modeling (PBPK), benchmark dose modeling (BMD), functional toxicology, and the assessment of chemical mixture interactions.

Public health challenges posed by chemical mixtures.

Approximately 40 million people live within a 4-mile radius of waste sites that the Agency for Toxic Substances and Disease Registry (ATSDR) has assessed to date. Human populations living in the vicinity of such sites are often subjected to complex chemical exposures that may contribute to the total body burden of oxogenous chemicals. Apart from the contaminants found at waste sites, exposure may also include environmental, occupational, and personal agents. Concurrent exposure to chemicals such as welding fumes, indoor air pollutants, tobacco smoke, alcohol, and prescription and nonprescription drugs makes the health assessment of exposure to waste site chemicals a more complex task. Voluntary exposures such as these frequently entail exposures to relatively high chemical concentrations and can usually be well defined and quantified. Conversely, involuntary exposures from waste sites may be at low concentrations and hence difficult to characterize and quantify. Of the approximately 1450 waste sites evaluated by the ATSDR, 530 (37%) had either completed or potentially completed exposure pathways. Results of public health assessments conducted at 167 sites during 1993 to 1995 show that about 1.5 million people have been exposed to site-specific contaminants. At 10% or more of the sites that had either completed or potentially completed exposure pathways, 56 substances were identified. Of these, 19 are either known or anticipated human carcinogens, and 9 are associated with reproductive or endocrine-disrupting effects. In this paper we present important concerns regarding hazardous waste sites including the impact on human health, ecology, and quality of life. To address such human-health related issues, the ATSDR has established a mixtures program that consists of three components: trend analysis to identify combinations of chemicals of concern, experimental studies to identify data that would be useful in the development and implementation of predictive decision support methodologies, and development of assessment methodologies and guidance to provide health assessors with the tools to incorporate the evaluation of multiple-chemical exposure into site assessments.

Estimation of toxicity of chemical mixtures through modeling of chemical interactions.

The Agency for Toxic Substances and Disease Registry (ATSDR), in collaboration with the Dutch Organization for Applied Scientific Research (TNO) Nutrition and Food Research Institute, is conducting studies to evaluate the role of chemical interactions in the expression of toxicity from low-level exposure to combinations of chemicals. The goal of this collaborative effort is to use a weight-of-evidence (WOE) approach to estimate joint toxicity of some simple chemical mixtures and to compare the estimations with test results from animal toxicity studies. The WOE approach uses individual chemical dose-response assessments and algorithms that incorporate various assumptions regarding potential chemical interactions. Qualitative evaluations were prepared for binary combinations of chemicals for the effect of butyl hydroxyanisole on di(2-ethylhexyl)phthalate, the effect of stannous chloride on Cd chloride (CdCl2), and the effect of CdCl2 on loperamide. Analyses of these evaluations and their comparison with the conclusions of laboratory animal experiments indicate that the WOE approach can be used to estimate qualitatively the joint toxicity of such simple mixtures. To further test the utility of the WOE approach, qualitative and semiquantitative evaluations were prepared for two chemical mixtures--one with similarly acting halogenated aliphatics (trichloroethylene, tetrachloroethylene, hexachloro-1,3-butadiene[HCBD], and 1,1,2-trichloro-3,3,3-trifluoropropene [TCTFP]) and the other with dissimilarly acting nephrotoxic components (mercuric chloride, lysinolalanine, D-limonene, and HCBD). These two sets of data were used to estimate the overall toxicities of the mixtures using the WOE algorithm for the mixture. The comparison of the results of the estimated toxicity with experimentally determined toxicity of the mixture of similarly acting nephrotoxicants demonstrated that the WOE approach correctly adjusted for the observed interactions in experimental animal studies. However, this was not true for the mixture of dissimilarly acting nephrotoxicants. This could be attributed to the fact that WOE evaluations are based on dose additivity that postulates that all chemicals in a given mixture act in the same way--by the same mechanism--and differ only in their potencies. In these cases the WOE approach evaluations, based on consideration of common mechanisms for simple chemical mixtures, can lead to better estimates of joint toxicity of chemical mixtures than the default assumption of dose additivity. The results also show that the WOE evaluations should be target-organ specific because none of the models tested could approximate the observed responses in organs other than the target organs in the laboratory animal studies.

Public health implications of environmental exposures.

The Agency for Toxic Substances and Disease Registry (ATSDR) is a public health agency with responsibility for assessing the public health implications associated with uncontrolled releases of hazardous substances into the environment. The biological effects of low-level exposures are a primary concern in these assessments. One of the tools used by the agency for this purpose is the risk assessment paradigm originally outlined and described by the National Academy of Science in 1983. Because of its design and inherent concepts, risk assessment has been variously employed by a number of environmental and public health agencies and programs as a means to organize information, as a decision support tool, and as a working hypothesis for biologically based inference and extrapolation. Risk assessment has also been the subject of significant critical review. The ATSDR recognizes the utility of both the qualitative and quantitative conclusions provided by traditional risk assessment, but the agency uses such estimates only in the broader context of professional judgment, internal and external peer review, and extensive public review and comment. This multifaceted approach is consistent with the Council on Environmental Quality's description and use of risk analysis as an organizing construct based on sound biomedical and other scientific judgment in concert with risk assessment to define plausible exposure ranges of concern rather than a single numerical estimate that may convey an artificial sense of precision. In this approach biomedical opinion, host factors, mechanistic interpretation, molecular epidemiology, and actual exposure conditions are all critically important in evaluating the significance of environmental exposure to hazardous substances. As such, the ATSDR risk analysis approach is a multidimensional endeavor encompassing not only the components of risk assessment but also the principles of biomedical judgment, risk management, and risk communication. Within this framework of risk analysis, the ATSDR may rely on one or more of a number of interrelated principles and approaches to screen, organize information, set priorities, make decisions, and define future research needs and directions.