Application of the Key Characteristics of Carcinogens to Bisphenol A

The ten key characteristics (KCs) of carcinogens are based on characteristics of known human carcinogens and encompass many types of endpoints. We propose that an objective review of the large amount of cancer mechanistic evidence for the chemical bisphenol A (BPA) can be achieved through use of these KCs. A search on metabolic and mechanistic data relevant to the carcinogenicity of BPA was conducted and web-based software tools were used to screen and organize the results. We applied the KCs to systematically identify, organize, and summarize mechanistic information for BPA, and to bring relevant carcinogenic mechanisms into focus. For some KCs with very large data sets, we utilized reviews focused on specific endpoints. Over 3000 studies for BPA from various data streams (exposed humans, animals, in vitro and cell-free systems) were identified. Mechanistic data relevant to each of the ten KCs were identified, with receptor-mediated effects, epigenetic alterations, oxidative stress, and cell proliferation being especially data rich. Reactive and bioactive metabolites are also associated with a number of KCs. This review demonstrates how the KCs can be applied to evaluate mechanistic data, especially for data-rich chemicals. While individual entities may have different approaches for the incorporation of mechanistic data in cancer hazard identification, the KCs provide a practical framework for conducting an objective examination of the available mechanistic data without a priori assumptions on mode of action. This analysis of the mechanistic data available for BPA suggests multiple and inter-connected mechanisms through which this chemical can act.

Key Characteristics of Cardiovascular Toxicants

BACKGROUND

The concept of chemical agents having properties that confer potential hazard called key characteristics (KCs) was first developed to identify carcinogenic hazards. Identification of KCs of cardiovascular (CV) toxicants could facilitate the systematic assessment of CV hazards and understanding of assay and data gaps associated with current approaches.

OBJECTIVES

We sought to develop a consensus-based synthesis of scientific evidence on the KCs of chemical and nonchemical agents known to cause CV toxicity along with methods to measure them.

METHODS

An expert working group was convened to discuss mechanisms associated with CV toxicity.

RESULTS

The group identified 12 KCs of CV toxicants, defined as exogenous agents that adversely interfere with function of the CV system. The KCs were organized into those primarily affecting cardiac tissue (numbers 1-4 below), the vascular system (5-7), or both (8-12), as follows: 1) impairs regulation of cardiac excitability, 2) impairs cardiac contractility and relaxation, 3) induces cardiomyocyte injury and death, 4) induces proliferation of valve stroma, 5) impacts endothelial and vascular function, 6) alters hemostasis, 7) causes dyslipidemia, 8) impairs mitochondrial function, 9) modifies autonomic nervous system activity, 10) induces oxidative stress, 11) causes inflammation, and 12) alters hormone signaling.

DISCUSSION

These 12 KCs can be used to help identify pharmaceuticals and environmental pollutants as CV toxicants, as well as to better understand the mechanistic underpinnings of their toxicity. For example, evidence exists that fine particulate matter [PM ≤2.5μm in aerodynamic diameter (PM2.5)] air pollution, arsenic, anthracycline drugs, and other exogenous chemicals possess one or more of the described KCs. In conclusion, the KCs could be used to identify potential CV toxicants and to define a set of test methods to evaluate CV toxicity in a more comprehensive and standardized manner than current approaches. https://doi.org/10.1289/EHP9321.

Consensus on the key characteristics of endocrine-disrupting chemicals as a basis for hazard identification

Endocrine-disrupting chemicals (EDCs) are exogenous chemicals that interfere with hormone action, thereby increasing the risk of adverse health outcomes, including cancer, reproductive impairment, cognitive deficits and obesity. A complex literature of mechanistic studies provides evidence on the hazards of EDC exposure, yet there is no widely accepted systematic method to integrate these data to help identify EDC hazards. Inspired by work to improve hazard identification of carcinogens using key characteristics (KCs), we have developed ten KCs of EDCs based on our knowledge of hormone actions and EDC effects. In this Expert Consensus Statement, we describe the logic by which these KCs are identified and the assays that could be used to assess several of these KCs. We reflect on how these ten KCs can be used to identify, organize and utilize mechanistic data when evaluating chemicals as EDCs, and we use diethylstilbestrol, bisphenol A and perchlorate as examples to illustrate this approach.

Key characteristics of 86 agents known to cause cancer in humans

Since the inception of the International Agency for Research on Cancer (IARC) in the early 1970s, the IARC Monographs Programme has evaluated more than 1000 agents with respect to carcinogenic hazard; of these, up to and including Volume 119 of the IARC Monographs, 120 agents met the criteria for classification as carcinogenic to humans (Group 1). Volume 100 of the IARC Monographs provided a review and update of Group 1 carcinogens. These agents were divided into six broad categories: (I) pharmaceuticals; (II) biological agents; (III) arsenic, metals, fibers, and dusts; (IV) radiation; (V) personal habits and indoor combustions; and (VI) chemical agents and related occupations. Data on biological mechanisms of action (MOA) were extracted from the Monographs to assemble a database on the basis of ten key characteristics attributed to human carcinogens. After some grouping of similar agents, the characteristic profiles were examined for 86 Group 1 agents for which mechanistic information was available in the IARC Monographs up to and including Volume 106, based upon data derived from human in vivo, human in vitro, animal in vivo, and animal in vitro studies. The most prevalent key characteristic was "is genotoxic", followed by "alters cell proliferation, cell death, or nutrient supply" and "induces oxidative stress". Most agents exhibited several of the ten key characteristics, with an average of four characteristics per agent, a finding consistent with the notion that cancer development in humans involves multiple pathways. Information on the key characteristics was often available from multiple sources, with many agents demonstrating concordance between human and animal sources, particularly with respect to genotoxicity. Although a detailed comparison of the characteristics of different types of agents was not attempted here, the overall characteristic profiles for pharmaceutical agents and for chemical agents and related occupations appeared similar. Further in-depth analyses of this rich database of characteristics of human carcinogens are expected to provide additional insights into the MOA of human cancer development.

Concordance between sites of tumor development in humans and in experimental animals for 111 agents that are carcinogenic to humans

Since the inception of the IARC Monographs Programme in the early 1970s, this Programme has developed 119 Monograph Volumes on more than 1000 agents for which there exists some evidence of cancer risk to humans. Of these, 120 agents were found to meet the criteria for classification as carcinogenic to humans (Group 1). Volume 100 of the IARC Monographs, compiled in 2008-2009 and published in 2012, provided a review and update of the 107 Group 1 agents identified as of 2009. These agents were divided into six broad categories: (I) pharmaceuticals; (II) biological agents; (III) arsenic, metals, fibers and dusts; (IV) radiation; (V) personal habits and indoor combustions; and (VI) chemical agents and related occupations. The Group I agents reviewed in Volume 100, as well as five additional Group 1 agents defined in subsequent Volumes of the Monographs, were used to assess the degree of concordance between sites where tumors originate in humans and experimental animals including mice, rats, hamsters, dogs, and non-human primates using an anatomically based tumor nomenclature system, representing 39 tumor sites and 14 organ and tissue systems. This evaluation identified 91 Group 1 agents with sufficient evidence (82 agents) or limited evidence (9 agents) of carcinogenicity in animals. The most common tumors observed in both humans and animals were those of the respiratory system including larynx, lung, and lower respiratory tract. In humans, respiratory system tumors were noted for 31 of the 111 distinct Group 1 carcinogens identified up to and including Volume 109 of the IARC Monographs, comprising predominantly 14 chemical agents and related occupations in category VI; seven arsenic, metals, fibers, and dusts in category III, and five personal habits and indoor combustions in category V. Subsequent to respiratory system tumors, those in lymphoid and hematopoietic tissues (26 agents), the urothelium (18 agents), and the upper aerodigestive tract (16 agents) were most often seen in humans, while tumors in digestive organs (19 agents), skin (18 agents), and connective tissues (17 agents) were frequently seen in animals. Exposures to radiation, particularly X- and γ-radiation, and tobacco smoke were associated with tumors at multiple sites in humans. Although the IARC Monographs did not emphasize tumor site concordance between animals and humans, substantial concordance was detected for several organ and tissue systems, even under the stringent criteria for sufficient evidence of carcinogenicity used by IARC. Of the 60 agents for which at least one tumor site was identified in both humans and animals, 52 (87%) exhibited tumors in at least one of the same organ and tissue systems in humans and animals. It should be noted that some caution is needed in interpreting concordance at sites where sample size is particularly small. Although perfect (100%) concordance was noted for agents that induce tumors of the mesothelium, only two Group 1 agents that met the criteria for inclusion in the concordance analysis caused tumors at this site. Although the present analysis demonstrates good concordance between animals and humans for many, but not all, tumor sites, limitations of available data may result in underestimation of concordance.

Key Characteristics of Carcinogens as a Basis for Organizing Data on Mechanisms of Carcinogenesis

BACKGROUND

A recent review by the International Agency for Research on Cancer (IARC) updated the assessments of the > 100 agents classified as Group 1, carcinogenic to humans (IARC Monographs Volume 100, parts A-F). This exercise was complicated by the absence of a broadly accepted, systematic method for evaluating mechanistic data to support conclusions regarding human hazard from exposure to carcinogens.

OBJECTIVES AND METHODS

IARC therefore convened two workshops in which an international Working Group of experts identified 10 key characteristics, one or more of which are commonly exhibited by established human carcinogens.

DISCUSSION

These characteristics provide the basis for an objective approach to identifying and organizing results from pertinent mechanistic studies. The 10 characteristics are the abilities of an agent to 1) act as an electrophile either directly or after metabolic activation; 2) be genotoxic; 3) alter DNA repair or cause genomic instability; 4) induce epigenetic alterations; 5) induce oxidative stress; 6) induce chronic inflammation; 7) be immunosuppressive; 8) modulate receptor-mediated effects; 9) cause immortalization; and 10) alter cell proliferation, cell death, or nutrient supply.

CONCLUSION

We describe the use of the 10 key characteristics to conduct a systematic literature search focused on relevant end points and construct a graphical representation of the identified mechanistic information. Next, we use benzene and polychlorinated biphenyls as examples to illustrate how this approach may work in practice. The approach described is similar in many respects to those currently being implemented by the U.S. EPA's Integrated Risk Information System Program and the U.S. National Toxicology Program.

CITATION

Smith MT, Guyton KZ, Gibbons CF, Fritz JM, Portier CJ, Rusyn I, DeMarini DM, Caldwell JC, Kavlock RJ, Lambert P, Hecht SS, Bucher JR, Stewart BW, Baan R, Cogliano VJ, Straif K. 2016. Key characteristics of carcinogens as a basis for organizing data on mechanisms of carcinogenesis. Environ Health Perspect 124:713-721; http://dx.doi.org/10.1289/ehp.1509912.

Recommendations and proposed guidelines for assessing the cumulative evidence on joint effects of genes and environments on cancer occurrence in humans

We propose guidelines to evaluate the cumulative evidence of gene-environment (G × E) interactions in the causation of human cancer. Our approach has its roots in the HuGENet and IARC Monographs evaluation processes for genetic and environmental risk factors, respectively, and can be applied to common chronic diseases other than cancer. We first review issues of definitions of G × E interactions, discovery and modelling methods for G × E interactions, and issues in systematic reviews of evidence for G × E interactions, since these form the foundation for appraising the credibility of evidence in this contentious field. We then propose guidelines that include four steps: (i) score the strength of the evidence for main effects of the (a) environmental exposure and (b) genetic variant; (ii) establish a prior score category and decide on the pattern of interaction to be expected; (iii) score the strength of the evidence for interaction between the environmental exposure and the genetic variant; and (iv) examine the overall plausibility of interaction by combining the prior score and the strength of the evidence and interpret results. We finally apply the scheme to the interaction between NAT2 polymorphism and tobacco smoking in determining bladder cancer risk.

Research recommendations for selected IARC-classified agents

OBJECTIVES

There are some common occupational agents and exposure circumstances for which evidence of carcinogenicity is substantial but not yet conclusive for humans. Our objectives were to identify research gaps and needs for 20 agents prioritized for review based on evidence of widespread human exposures and potential carcinogenicity in animals or humans.

DATA SOURCES

For each chemical agent (or category of agents), a systematic review was conducted of new data published since the most recent pertinent International Agency for Research on Cancer (IARC) Monograph meeting on that agent.

DATA EXTRACTION

Reviewers were charged with identifying data gaps and general and specific approaches to address them, focusing on research that would be important in resolving classification uncertainties. An expert meeting brought reviewers together to discuss each agent and the identified data gaps and approaches.

DATA SYNTHESIS

Several overarching issues were identified that pertained to multiple agents; these included the importance of recognizing that carcinogenic agents can act through multiple toxicity pathways and mechanisms, including epigenetic mechanisms, oxidative stress, and immuno- and hormonal modulation.

CONCLUSIONS

Studies in occupational populations provide important opportunities to understand the mechanisms through which exogenous agents cause cancer and intervene to prevent human exposure and/or prevent or detect cancer among those already exposed. Scientific developments are likely to increase the challenges and complexities of carcinogen testing and evaluation in the future, and epidemiologic studies will be particularly critical to inform carcinogen classification and risk assessment processes.

Improving prediction of chemical carcinogenicity by considering multiple mechanisms and applying toxicogenomic approaches

While scientific knowledge of the potential health significance of chemical exposures has grown, experimental methods for predicting the carcinogenicity of environmental agents have not been substantially updated in the last two decades. Current methodologies focus first on identifying genotoxicants under the premise that agents capable of directly damaging DNA are most likely to be carcinogenic to humans. Emphasis on the distinction between genotoxic and non-genotoxic carcinogens is also motivated by assumed implications for the dose-response curve; it is purported that genotoxicants would lack a threshold in the low dose region, in contrast to non-genotoxic agents. However, for the vast majority of carcinogens, little if any empirical data exist to clarify the nature of the cancer dose-response relationship at low doses in the exposed human population. Recent advances in scientific understanding of cancer biology-and increased appreciation of the multiple impacts of carcinogens on this disease process-support the view that environmental chemicals can act through multiple toxicity pathways, modes and/or mechanisms of action to induce cancer and other adverse health outcomes. Moreover, the relationship between dose and a particular outcome in an individual could take multiple forms depending on genetic background, target tissue, internal dose and other factors besides mechanisms or modes of action; inter-individual variability and susceptibility in response are, in turn, key determinants of the population dose-response curve. New bioanalytical approaches (e.g., transcriptomics, proteomics, and metabolomics) applied in human, animal and in vitro studies could better characterize a wider array of hazard traits and improve the ability to predict the potential carcinogenicity of chemicals.

Trichloroethylene and cancer: epidemiologic evidence

adopt a traditional review of the mutagenicity data on TCE and its metabolites but instead raise several issues regarding the interpretation of mutagenicity and genetic toxicity tests in shedding light on whether these processes are key events in tumor initiation. As discussed in the U.S. EPA proposed cancer guidelines, a salient question is whether TCE or its metabolites interacts directly with and mutates DNA to bring about changes in gene expression or whether DNA mutation is achieved through some other process. The Moore and Harrington-Brock article examines this question. Bull ((italic)10(/italic)), Lash et al. ((italic)11(/italic)), and Green ((italic)12(/italic)) present the experimental support for several modes of action for tumor development in rodents. These articles discuss a number of hypotheses including the influence on tumor development from mutagenesis, cytotoxicity, cell proliferation, *(subscript)2u(/subscript)-globin, peroxisome proliferation, oxidative stress, receptor binding, and perturbation of cell-signaling pathways. Quantitative dose-response issues important to the statistical modeling of both noncarcinogenic and carcinogenic effects are discussed in articles by Fisher ((italic)5(/italic)), Bois ((italic)13(/italic)), Clewell et al. ((italic)14(/italic)), Bois ((italic)15(/italic)), Boyes et al. ((italic)16(/italic)), Barton and Clewell ((italic)17(/italic)), Chen ((italic)18(/italic)), and Rhomberg ((italic)19(/italic)). Because pharmacokinetic data are available for the TCE assessment, dose metrics other than applied dose may be evaluated in benchmark and other dose-response analyses. Fisher's article ((italic)5(/italic)) describes modeling liver concentration of TCE and its oxidative metabolites, while Clewell et al. ((italic)14(/italic)) model plasma concentrations of the oxidative metabolites and flux through the kidney for metabolites of the glutathione-(italic)S(/italic)-transferase pathway. Both models are scaled from mice or rats to humans and provide estimates of human equivalent doses simulating inhalation and oral exposure routes. Parameters from these models have been further subjected to an uncertainty analysis in the articles by Bois ((italic)13,15(/italic)). The application of Baysian statistical methods is increasingly used for updating estimates of pharmacokinetic model parameters. Moreover, these analyses can provide an additional set of dosimetric estimates that in some instances are very different from those obtained with the original model. These findings make the risk assessor's job more complex. Boyes and others ((italic)16(/italic)) test whether Haber's Law or a dose metric that integrates time and concentration best describes neurologic effects with high-level TCE exposure. Barton and Clewell ((italic)17(/italic)) examine both experimental and pharmacokinetically derived dose metrics in their analysis of neurologic and systemic organ toxicity seen in the rodent studies. The article further applies benchmark dose methodology to the quantitative analysis of these effects. The U.S. EPA proposed cancer guidelines recommend that dose-response modeling be carried out in two parts: analysis of the curve shape within the range of the data and extrapolation below the observable data. Application of a biologically based model is preferred for evaluating the dose-response relationship for carcinogenic effects. Such an approach is described in the article by Chen ((italic)18(/italic)), which explores the relationship between TCE and two of its oxidative metabolites, dichloroacetic acid and trichloroacetic acid, under the hypothesis that these chemicals induce liver tumors in mice through promotion of preexisting initiated cells. Unfortunately, the data ne

Estimating cancer risk from outdoor concentrations of hazardous air pollutants in 1990

A public health concern regarding hazardous air pollutants (HAPs) is their potential to cause cancer. It has been difficult to assess potential cancer risks from HAPs, due primarily to lack of ambient concentration data for the general population. The Environmental Protection Agency's Cumulative Exposure Project modeled 1990 outdoor concentrations of HAPs across the United States, which were combined with inhalation unit risk estimates to estimate the potential increase in excess cancer risk for individual carcinogenic HAPs. These were summed to provide an estimate of cancer risk from multiple HAPs. The analysis estimates a median excess cancer risk of 18 lifetime cancer cases per 100,000 people for all HAP concentrations. About 75% of estimated cancer risk was attributable to exposure to polycyclic organic matter, 1,3-butadiene, formaldehyde, benzene, and chromium. Consideration of some specific uncertainties, including underestimation of ambient concentrations, combining upper 95% confidence bound potency estimates, and changes to potency estimates, found that cancer risk may be underestimated by 15% or overestimated by 40-50%. Other unanalyzed uncertainties could make these under- or overestimates larger. This analysis used 1990 estimates of concentrations and can be used to track progress toward reducing cancer risk to the general population.

Assessing the cancer risk from environmental PCBs

A new approach to assessing the cancer risk from environmental polychlorinated biphenyls (PCBs) considers both toxicity and environmental processes to make distinctions among environmental mixtures. New toxicity information from a 1996 cancer study of four commercial mixtures strengthens the case that all PCB mixtures can cause cancer, although different mixtures have different potencies. Environmental processes alter PCB mixtures through partitioning, chemical transformation, and preferential bioaccumulation; these processes can increase or decrease toxicity considerably. Bioaccumulated PCBs are of greatest concern because they appear to be more toxic than commercial PCBs and more persistent in the body. The new approach uses toxicity studies of commercial mixtures to develop a range of cancer potency estimates and then considers the effect of environmental processes to choose appropriate values for representative classes of environmental mixtures. Guidance is given for assessing risks from different exposure pathways, less-than-lifetime and early-life exposures, and mixtures containing dioxinlike compounds.

Trichloroethylene health risk assessment: a new and improved process

Trichloroethylene (TCE), an environmental contaminant of National concern, is the focus of a new health risk assessment process incorporating the Proposed Cancer Risk Assessment Guidelines. This paper describes not only how TCE became an environmental problem for the Air Force, but also details the new Risk Assessment process envisioned by the Environmental Protection Agency's (EPA) National Center for Environmental Assessment (NCEA). Insights on epidemiological evaluations, both past and future, and their impact on the cancer classification of TCE are discussed. Examples of how physiologically based pharmacokinetics and dose-response characterization described in the new Cancer Guidelines are applied to TCE are provided. In addition, a variety of modeling techniques are discussed for the development of reference doses (oral exposure) and reference concentrations (inhalation exposures) for TCE. Finally, the role of risk communication is included. This new process provides an example of how interagency (EPA, Department of Defense. Department of Energy) and extramural (industry, academia) partnerships can provide greater gains to the nation, as a whole, than any of the parts on their own.

Plausible upper bounds: are their sums plausible?

Quantitative cancer risk assessments are typically expressed as plausible upper bounds rather than estimates of central tendency. In analyses involving several carcinogens, these upper bounds are often summed to estimate overall risk. This begs the question of whether a sum of upper bounds is itself a plausible estimate of overall risk. This question can be asked in two ways: whether the sum yields an improbable estimate of overall risk (that is, is it only remotely possible for the true sum of risks to match the sum of upper bounds), or whether the sum gives a misleading estimate (that is, is the true sum of risks likely to be very different from the sum of upper bounds). Analysis of four case studies shows that as the number of risk estimates increases, their sum becomes increasingly improbable, but not misleading. Though the overall risk depends on the independence, additivity, and number of risk estimates, as well as the shapes of the underlying risk distributions, sums of upper bounds provide useful information about the overall risk and can be adjusted downward to give a more plausible [perhaps probable] upper bound, or even a central estimate of overall risk.

Quantitative cancer assessment for vinyl chloride: indications of early-life sensitivity

Complementary sources of information are analyzed to characterize the early-life cancer risk from inhaling vinyl chloride. A study of partial-lifetime exposures suggests that the lifetime cancer risk depends on age at exposure, with higher lifetime risks attributable to exposures at younger ages. Studies of newborn animal exposures further demonstrate that a brief exposure in newborns can, by the end of life, induce a higher incidence of tumors compared to long-term exposure occurring later in life, including tumor types not induced by exposure later in life. An empirical, quantitative approach is used to model early-life sensitivity to inhaled vinyl chloride, supplementing conventional approaches for estimating the increased cancer risk from lifetime exposure. A single estimate is not presumed to apply to the entire population; instead, the new approach makes distinctions about the cancer risks for different population segments. This assessment shows one way such information might be analyzed, presented, and used to assess actual exposure situations.

A case study of cancer data set combinations for PCBs

Results of several animal bioassays have demonstrated the carcinogenic potential of polychlorinated biphenyl (PCB) mixtures. Although PCBs are no longer manufactured, cancer risk assessment for PCBs remains an important issue because of continued potential human exposure from many sources. The existing cancer risk estimate for PCBs used by the U.S. EPA is based on liver tumors observed in female Sprague-Dawley rats in a lifetime bioassay. Liver cancer has been observed in other long-term bioassays as well. In this case study, experimental designs and biological characteristics of the data from these studies were evaluated to determine whether a combination of the data sets is scientifically reasonable. A statistical analysis of the data sets based on likelihood ratio theory was used to assess the compatibility of individual data sets to a common multistage dose-response model. The results from these biological and statistical assessments suggest that at least two data sets could be combined to derive a quantitative risk estimate for PCBs. Increased confidence in the quantitative estimate would result from such combination because more data are being used to assess the dose-response relationship.