Lorenz R. Rhomberg

An Updated Weight of the Evidence Evaluation of Reproductive and Developmental Effects of Low Doses of Bisphenol A

There is controversy over whether low doses of bisphenol A (BPA, CAS no. 80-05-7) cause reproductive and developmental effects in humans. We update the 2004 weight-of-evidence assessment of an expert panel convened by Harvards Center for Risk Analysis by critically evaluating over 50 additional studies published between April 2002 and February 2006 that examine in vivo reproductive and developmental toxicity in mammals at doses ≤5 mg/kg-d. Our findings are consistent with the Harvard study: some statistically significant findings in rats and mice exist but they are generally countered by more numerous studies showing no effect for similar endpoints. No effect is marked or consistent across species, doses, and time points. Some mouse studies report morphological changes in testes and sperm and some non-oral mouse studies report morphological changes in female reproductive organs. Owing to lack of first-pass metabolism, results from non-oral studies are of limited relevance to oral human exposure. Human biomonitoring indicates exposures lower than the “low” doses in the reviewed animal studies. Reports of human health impact are very limited and inconsistent. Taken together, the weight of evidence does not support the hypothesis that low oral doses of BPA adversely affect human reproductive and developmental health.

Mode of action framework analysis for receptor-mediated toxicity: The peroxisome proliferator-activated receptor alpha (PPARα) as a case study

Abstract Several therapeutic agents and industrial chemicals induce liver tumors in rodents through the activation of the peroxisome proliferator-activated receptor alpha (PPARα). The cellular and molecular events by which PPARα activators induce rodent hepatocarcinogenesis has been extensively studied and elucidated. This review summarizes the weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis and identifies gaps in our knowledge of this MOA. Chemical-specific and mechanistic data support concordance of temporal and dose–response relationships for the key events associated with many PPARα activators including a phthalate ester plasticizer di(2-ethylhexyl) phthalate (DEHP) and the drug gemfibrozil. While biologically plausible in humans, the hypothesized key events in the rodent MOA, for PPARα activators, are unlikely to induce liver tumors in humans because of toxicodynamic and biological differences in responses. This conclusion is based on minimal or no effects observed on growth pathways, hepatocellular proliferation and liver tumors in humans and/or species (including hamsters, guinea pigs and cynomolgous monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Overall, the panel concluded that significant quantitative differences in PPARα activator-induced effects related to liver cancer formation exist between rodents and humans. On the basis of these quantitative differences, most of the workgroup felt that the rodent MOA is “not relevant to humans” with the remaining members concluding that the MOA is “unlikely to be relevant to humans”. The two groups differed in their level of confidence based on perceived limitations of the quantitative and mechanistic knowledge of the species differences, which for some panel members strongly supports but cannot preclude the absence of effects under unlikely exposure scenarios.

Weight-of-Evidence Evaluation of Reproductive and Developmental Effects of Low Doses of Bisphenol A

Recent public concern has focused on potential reproductive and developmental effects from exposure to low levels of bisphenol A (BPA, CAS number 80-05-7). Two previous published reviews (; ) conducted weight-of-evidence evaluations of in vivo reproductive/developmental toxicity from BPA exposure ≤ 5 mg/kg-d based on studies published through February 2006. Here, an update of those analyses presents additional relevant studies that were published through July 25, 2008, and a weight-of-evidence analysis of the studies evaluated in all three reviews. As with the earlier literature, positive findings: (1) are countered by null findings in more numerous studies; (2) have not been replicated; (3) do not exhibit coherence and plausibility; (4) do not show consistency across species, doses, and time points; and/or (5) were from studies using non-oral exposure routes. Owing to the lack of first-pass metabolism, results from non-oral studies are of limited relevance to human exposure. Exposure levels in most of the low-dose oral and non-oral animal studies are generally much higher than those experienced by even the most exposed people in the general population. The weight of evidence does not support the hypothesis that low oral doses of BPA adversely affect human reproductive and developmental health.

Linear low-dose extrapolation for noncancer health effects is the exception, not the rule

The nature of the exposure-response relationship has a profound influence on risk analyses. Several arguments have been proffered as to why all exposure-response relationships for both cancer and noncarcinogenic endpoints should be assumed to be linear at low doses. We focused on three arguments that have been put forth for noncarcinogens. First, the general “additivity-to-background” argument proposes that if an agent enhances an already existing disease-causing process, then even small exposures increase disease incidence in a linear manner. This only holds if it is related to a specific mode of action that has nonuniversal properties—properties that would not be expected for most noncancer effects. Second, the “heterogeneity in the population” argument states that variations in sensitivity among members of the target population tend to “flatten out and linearize” the exposure-response curve, but this actually only tends to broaden, not linearize, the dose-response relationship. Third, it has been argued that a review of epidemiological evidence shows linear or no-threshold effects at low exposures in humans, despite nonlinear exposure-response in the experimental dose range in animal testing for similar endpoints. It is more likely that this is attributable to exposure measurement error rather than a true nonthreshold association. Assuming that every chemical is toxic at high exposures and linear at low exposures does not comport to modern-day scientific knowledge of biology. There is no compelling evidence-based justification for a general low-exposure linearity; rather, case-specific mechanistic arguments are needed.

Weight of the Evidence Evaluation of Low-Dose Reproductive and Developmental Effects of Bisphenol A

ABSTRACT A panel convened by the Harvard Center for Risk Analysis (HCRA) evaluated the weight of evidence for potential developmental and reproductive toxicity of bisphenol A (BPA, CASRN 80-05-7) in animals at doses well below the Lowest Observed Adverse Effect Level (LOAEL) of 50 mg/kg-day previously identified by the U.S. Environmental Protection Agency (US EPA) and even US EPAs reference dose (RfD) of 0.05 mg/kg-day. The effects are hypothesized to occur through an endocrine-modulating mode of action, specifically through estrogen receptors. The panel focused on potential male reproductive effects but also examined other endpoints possibly associated with hormone-like effects. The review considered studies published through April 2002. A formal deliberation framework focused on consistency, generalizability, and biological plausibility. The panel found no consistent affirmative evidence of low-dose BPA effects for any endpoint. Inconsistent responses across rodent species and strains made generalizability of low-dose BPA effects questionable. Lack of adverse effects in two multiple-generation reproductive and developmental studies casts doubt on suggestions of significant physiological or functional impairment. The panel was concerned about generalization of non-oral administration results to oral exposures. Differences in the pattern of BPA responses compared to estradiol or diethylstilbestrol (DES) cast doubt on estrogenicity as a low-dose mechanism of action for BPA. Finally, there is indirect evidence that humans may be less sensitive to possible estrogenic effects from BPA exposure due to pharmacodynamic factors. The panel recommended replication of existing studies under carefully controlled conditions and further study of BPAs pharmacokinetics and pharmacodynamics. The study was funded by a grant from the American Plastics Council.

A COMPREHENSIVE EVALUATION OF THE POTENTIAL HEALTH RISKS ASSOCIATED WITH OCCUPATIONAL AND ENVIRONMENTAL EXPOSURE TO STYRENE

HEALTH RISKS ASSOCIATED WITH OCCUPATIONAL AND ENVIRONMENTAL EXPOSURE TO STYRENE Joshua T. Cohen Harvard Center for Risk Analysis, Harvard School of Public Health, Boston, Massachusetts, USA Gary Carlson School of Health Sciences, Purdue University, West Lafayette, Indiana, USA Gail Charnley Health Risk Strategies, Washington, DC, USA David Coggon MRC Environmental Epidemiology Unit, University of Southampton, Southampton, United Kingdom Elizabeth Delzell Department of Epidemiology and International Health, University of Alabama at Birmingham, Birmingham, Alabama John D. Graham Harvard Center for Risk Analysis, Harvard School of Public Health, Boston, Massachusetts, USA Helmut Greim GSF-Institute of Toxicology, Neuerberg, Germany

A survey of frameworks for best practices in weight-of-evidence analyses

Abstract The National Academy of Sciences (NAS) Review of the Environmental Protection Agency’s Draft IRIS Assessment of Formaldehyde proposed a “roadmap” for reform and improvement of the Agency’s risk assessment process. Specifically, it called for development of a transparent and defensible methodology for weight-of-evidence (WoE) assessments. To facilitate development of an improved process, we developed a white paper that reviewed approximately 50 existing WoE frameworks, seeking insights from their variations and nominating best practices for WoE analyses of causation of chemical risks. Four phases of WoE analysis were identified and evaluated in each framework: (1) defining the causal question and developing criteria for study selection, (2) developing and applying criteria for review of individual studies, (3) evaluating and integrating evidence and (4) drawing conclusions based on inferences. We circulated the draft white paper to stakeholders and then held a facilitated, multi-disciplinary invited stakeholder workshop to broaden and deepen the discussion on methods, rationales, utility and limitations among the surveyed WoE frameworks. The workshop developed recommendations for improving the conduct of WoE evaluations. Based on the analysis of the 50 frameworks and discussions at the workshop, best practices in conducting WoE analyses were identified for each of the four phases. Many of these best practices noted from the analysis and workshop could be implemented immediately, while others may require additional refinement as part of the ongoing discussions for improving the scientific basis of chemical risk assessments.

Issues in the design and interpretation of chronic toxicity and carcinogenicity studies in rodents: approaches to dose selection.

For more than three decades chronic studies in rodents have been the benchmark for assessing the potential long-term toxicity, and particularly the carcinogenicity, of chemicals. With doses typically administered for about 2 years (18 months to lifetime), the rodent bioassay has been an integral component of testing protocols for food additives, pesticides, pharmaceuticals, industrial chemicals, and all manner of byproducts and environmental contaminants. Over time, the data from these studies have been used to address an increasing diversity of questions related to the assessment of human health risks, adding complexity to study design and interpretation. An earlier ILSI RSI working group developed a set of principles for the selection of doses for chronic rodent studies (). The present report builds on that work, examining some of the issues that arise and offering new perspectives and approaches for putting the principles into practice. Dose selection is considered both from the prospective viewpoint of the choosing of dose levels for a study and from the retrospective interpretation of study results in light of the doses used. A main theme of this report is that the purposes and objectives of chronic rodent studies vary and should be clearly defined in advance. Dose placement, then, should be optimized to achieve study objectives. For practical reasons, most chronic studies today must be designed to address multiple objectives, often requiring trade-offs and innovative approaches in study design. A systematic approach to dose selection should begin with recognition that the design of chronic studies occurs in the context of a careful assessment of the accumulated scientific information on the test substance, the relevant risk management questions, priorities and mandates, and the practical limitations and constraints on available resources. A stepwise process is described. The aim is to increase insofar as possible the utility of an expensive and time-consuming experiment. The kinds of data that are most commonly needed for dose selection and for understanding the dose-related results of chronic rodent studies, particularly carcinogenicity studies, are discussed as “design/interpretation factors.” They comprise both the inherent characteristics of the test substance and indicators of biological damage, perturbation or stress among the experimental animals. They may be primary toxicity endpoints, predictors or indicators of appropriate dose selection, or indicators of conditions to be avoided in dose selection. The application and interpretation of design/interpretation factors is conditioned by the study objectives–what is considered desirable will depend on the strategy for choice of doses that is being followed. The challenge is to select doses that accommodate all of the issues raised by the relevant design/interpretation factors. Three case studies are presented here that illustrate the interplay between study objectives and the design and selection of doses for chronic rodent studies. These examples also highlight issues associated with multiple plausible modes of action, multiple pathways for biotransformation of the chemical, extraneous high-dose effects, the use of modeling in dose selection, and the implications of human exposure levels. Finally, looking to the future, the report explores seven potential paradigm shifts for risk assessment that will significantly impact the design and interpretation of toxicity and carcinogenicity studies.

Hypothesis-based weight of evidence: a tool for evaluating and communicating uncertainties and inconsistencies in the large body of evidence in proposing a carcinogenic mode of action--naphthalene as an example.

Human health risk assessment consists of bringing to bear a large body of in vitro, animal, and epidemiologic studies on the question of whether environmental exposures to a substance are a potential risk to humans. The body of scientific information is typically less than definitive and often contains apparent contradictions. Often various possible conclusions about potential human risks may be drawn from the data and these may vary from very strong to tenuous. The task, therefore, is to communicate the uncertainties in the inferences from the data effectively, giving proper consideration to contrary data and alternative scientifically plausible interpretations. We propose an approach, Hypothesis-Based Weight of Evidence (HBWoE), to organize, evaluate, and communicate the large body of available relevant data on a given chemical, using naphthalene as an example. The goal for our use of the term “weight of evidence” (WoE) is broad in that we express the relative degrees of credence that should be placed in alternative possible interpretations of the naphthalene data and hypothesized carcinogenic modes of action (MoAs), expressed in a way that shows how such credence is tied to specific scientific interpretations, considering consistencies, inconsistencies, and contradictions within the data set.

Reproductive and Developmental Risks from Ethylene Oxide: A Probabilistic Characterization of Possible Regulatory Thresholds

Ethylene oxide is a gas produced in large quantities in the United States that is used primarily as a chemical intermediate in the production of ethylene glycol, propylene glycol, non-ionic surfactants, ethanolamines, glycol ethers, and other chemicals. It has been well established that ethylene oxide can induce cancer, genetic, reproductive and developmental, and acute health effects in animals. The U.S. Environmental Protection Agency is currently developing both a cancer potency factor and a reference concentration (RfC) for ethylene oxide. This study used the rich database on the reproductive and developmental effects of ethylene oxide to develop a probabilistic characterization of possible regulatory thresholds for ethylene oxide. This analysis was based on the standard regulatory approach for noncancer risk assessment, but involved several innovative elements, such as: (1) the use of advanced statistical methods to account for correlations in developmental outcomes among littermates and allow for simultaneous control of covariates (such as litter size); (2) the application of a probabilistic approach for characterizing the uncertainty in extrapolating the animal results to humans; and (3) the use of a quantitative approach to account for the variation in heterogeneity among the human population. This article presents several classes of results, including: (1) probabilistic characterizations of ED10s for two quantal reproductive outcomes-resorption and fetal death, (2) probabilistic characterizations of one developmental outcome-the dose expected to yield a 5% reduction in fetal (or pup) weight, (3) estimates of the RfCs that would result from using these values in the standard regulatory approach for noncancer risk assessment, and (4) a probabilistic characterization of the level of ethylene oxide exposure that would be expected to yield a 1/1,000 increase in the risk of reproductive or developmental outcomes in exposed human populations.