In honor of the Teratology Society's 50th anniversary: The role of Teratology Society members in the development and evolution of in vivo developmental toxicity test guidelines

Guidelines on Developmental Toxicity Tests: Brief Insights

Developmental toxicology is a constantly evolving research field which needs to attend to a complex underlying regulatory network. In order to ensure human health and environmental safety, new substances have to be tested for toxic effects on reproduction and development, before being commercialized. Traditional in vivo mammalian models represent the intricacy of human development and provide more adequately an assessment of the interaction of chemical compounds with the reproductive system. However, in the last years, the directives are to reduce the use of vertebrate animals, promoting their use only as a last resort. Consequently, the interest on the development and validation of alternative tests, able to cover the various aspects of the reproductive cycle, has significantly increased. Reproductive toxicity is probably the most difficult endpoint to be replaced by alternative assays, since it should provide information on mechanism interactions essential for female and male fertility and also knowledge on the animal development during the first phases of its life cycle. This complexity explains the slow progress in implementing alternative models for reproductive toxicity safety assays. Alternative test models may be based on in vitro systems and nonmammalian animal models. Many biological processes have been successfully addressed using in vitro models, opening the possibility to study the interference of teratogenic compounds. Their validation and implementation have lagged behind, in part because of difficulties in establishing their predictability. Nevertheless, the advance toward the process of validation is crucial to replace and reduce the use of living animals. Based on the present state of the art, it is not probable that such testing strategies will completely replace the need to assess reproductive toxicity in vivo in the near future, but they will contribute to reduce animal tests and will provide important information. In this chapter, the approved guidelines for standard methods and alternative methods, according to their regulatory and scientific status, are enumerated and briefly described.

Toxicity testing and endocrine disrupting chemicals

Regulatory agencies around the world depend on standardized testing approaches to evaluate environmental chemicals for endocrine disrupting properties. The US Environmental Protection Agency (EPA) has developed a two-tiered testing approach within its Endocrine Disruptor Screening Program (EDSP). The eleven Tier 1 and three Tier 2 EDSP assays can be used to identify chemicals that act as agonists or antagonists of estrogen receptor, androgen receptor, or thyroid hormone receptor, or chemicals that interfere with steroidogenesis. Additional assays have been developed in the context of Tox21, and others have been validated by the OECD. In spite of the availability of validated toxicity tests, problems have been identified with the approaches and methods used to identify endocrine disrupting chemicals (EDCs). This chapter will provide an overview of several of these issues including: (1) The way an EDC is defined by an agency impacts whether a specific test can be used to determine if a chemical is an EDC. This is especially important when considering which assays examine outcomes that are considered "adverse effects." (2) Some assumptions about the validated studies used to identify EDCs may not be true (e.g., their reproducibility has been questioned). (3) Many of the validated assays are less sensitive than other methods that have not yet been validated. Ultimately, these and other problems contribute to the current landscape, where testing approaches have failed to protect the public from known EDCs. The chapter concludes with a review of approaches that have been taken to improve current guideline studies.

The Use and Misuse of Historical Controls in Regulatory Toxicology: Lessons from the CLARITY-BPA Study

For many endocrine-disrupting chemicals (EDCs) including Bisphenol A (BPA), animal studies show that environmentally relevant exposures cause harm; human studies are consistent with these findings. Yet, regulatory agencies charged with protecting public health continue to conclude that human exposures to these EDCs pose no risk. One reason for the disconnect between the scientific consensus on EDCs in the endocrinology community and the failure to act in the regulatory community is the dependence of the latter on so-called "guideline studies" to evaluate hazards, and the inability to incorporate independent scientific studies in risk assessment. The Consortium Linking Academic and Regulatory Insights on Toxicity (CLARITY) study was intended to bridge this gap, combining a "guideline" study with independent hypothesis-driven studies designed to be more appropriate to evaluate EDCs. Here we examined an aspect of "guideline" studies, the use of so-called "historical controls," which are essentially control data borrowed from prior studies to aid in the interpretation of current findings. The US Food and Drug Administration authors used historical controls to question the plausibility of statistically significant BPA-related effects in the CLARITY study. We examined the use of historical controls on 5 outcomes in the CLARITY "guideline" study: mammary neoplasms, pituitary neoplasms, kidney nephropathy, prostate inflammation and adenomas, and body weight. Using US Food and Drug Administration-proposed historical control data, our evaluation revealed that endpoints used in "guideline" studies are not as reproducible as previously held. Combined with other data comparing the effects of ethinyl estradiol in 2 "guideline" studies including CLARITY-BPA, we conclude that near-exclusive reliance on "guideline" studies can result in scientifically invalid conclusions.

Endocrine disruptors and the future of toxicology testing - lessons from CLARITY-BPA

Five years ago, an ambitious collaboration, the Consortium Linking Academic and Regulatory Insights on Toxicity of BPA (CLARITY-BPA; henceforth CLARITY), was launched by three US agencies. The goal was to provide a definitive evaluation of bisphenol A (BPA) and explain disparities between traditional regulatory studies and findings from independent investigators. BPA or vehicle-treated rats from an FDA facility were used in a guideline study and animals and/or tissues were provided to academic researchers for analysis. An interim summary released in February 2018 by the FDA concluded that currently authorized uses of BPA continue to be safe. We disagree. In this Perspectives, we summarize the goals, design and problems of CLARITY. We conclude that, despite its flaws, CLARITY provides important insight and, taken together, the data provide compelling evidence that low-dose BPA exposure induces marked adverse effects. Indeed, the greatest number of effects were observed at doses 20,000 times lower than the current 'safe' dose of BPA for humans.

Assessment of Gross Fetal Malformations: The Modernized Wilson Technique and Skeletal Staining

Teratology is the study of anatomical and physiological abnormalities, commonly known as birth defects. If an embryo is exposed to a harmful substance, or teratogen, during the critical period of development, an ensuing malformation may occur. These malformations and their associated mechanisms are studied and analyzed in laboratory animals in order to prevent them from occurring in humans. Rodents such as rats and mice have commonly been used in such studies because of their similarity to humans. In 1959, James G. Wilson designed, developed, and tested a protocol on how to observe and analyze structural malformations in rodent fetuses, which included: external examination, skeletal evaluation, soft tissue analysis, and data collection/analysis. For standardization purposes, i.e., to normalize findings from one lab to another, it is important that this protocol be followed with precision. Although many years have passed since Wilson initially created this protocol, it is still widely used to this day, and only minor changes have been made to his instructions such as the chemical reagents used in the experiments and methods of analysis of the experimental data. Such testing has resulted in major advances in the dissemination of teratology information, including the identification of an increasing number of teratogens and the understanding of the pathogenesis of birth defects. While mechanistically birth defect prevention will include the understanding of individual genomes and pharmacogenomics, overall, morphological assessment will still be required as an integral part of birth defects research. As the interaction between teratogenic and genetic factors is better understood, it is anticipated that the incidence of most types of defects will substantially be reduced.

Teratology Study Guidelines: An Overview

Developmental toxicology is a constantly evolving research field which needs to attend to a complex underlying regulatory network. Before entering the market new substances have to be tested for toxic effects on reproduction and development in order to ensure human health and environmental safety. Traditional in vivo mammalian models represent more adequately the intricacy of human development and provide an assessment of the interaction of chemicals on the reproductive system. However, in the last years, the main goal is to reduce the use of vertebrate animals, using those only as last resort. Consequently, the interest in the development and validation of a battery of alternative tests able to cover the various aspects of the reproductive cycle has increased. Reproductive toxicity is probably the most difficult endpoint to be replaced by alternative assays, since it should provide information on mechanisms interactions essential for female and male fertility, and also knowledge on the development of a new human being during its prenatal life. This complexity explains the slow progress in implementing alternatives for reproductive toxicity safety assessments. Alternative test methods may be based on in vitro systems and non-mammalian animal models. Many biological processes have been successfully implemented using in vitro models, opening the possibility to study the interference of teratogenic compounds using these models. Their validation and implementation have lagged behind, in part because of difficulties in establishing their predictability. Nevertheless, the advance toward the process of validation is crucial for a strategy aiming to replace and reduce the use of living animals. Based on the present state of the art, it is not probable that such testing strategies will completely replace the need to assess reproductive toxicity in vivo in the near future, but they contribute to reduce the animal testing and provide important information. In this chapter the approved guidelines for standard methods and alternative methods according to their regulatory and scientific status are enumerated and described.

Using systematic reviews for hazard and risk assessment of endocrine disrupting chemicals

The possibility that endocrine disrupting chemicals (EDCs) in our environment contribute to hormonally related effects and diseases observed in human and wildlife populations has caused concern among decision makers and researchers alike. EDCs challenge principles traditionally applied in chemical risk assessment and the identification and assessment of these compounds has been a much debated topic during the last decade. State of the science reports and risk assessments of potential EDCs have been criticized for not using systematic and transparent approaches in the evaluation of evidence. In the fields of medicine and health care, systematic review methodologies have been developed and used to enable objectivity and transparency in the evaluation of scientific evidence for decision making. Lately, such approaches have also been promoted for use in the environmental health sciences and risk assessment of chemicals. Systematic review approaches could provide a tool for improving the evaluation of evidence for decision making regarding EDCs, e.g. by enabling systematic and transparent use of academic research data in this process. In this review we discuss the advantages and challenges of applying systematic review methodology in the identification and assessment of EDCs.

Beyond a means of exposure: a new view of the mother in toxicology research

Toxicological studies generally view pregnant animals as a conduit through which gestational exposure of offspring to chemicals can be achieved, allowing for the study of developmental toxicity.

A path forward in the debate over health impacts of endocrine disrupting chemicals

Several recent publications reflect debate on the issue of "endocrine disrupting chemicals" (EDCs), indicating that two seemingly mutually exclusive perspectives are being articulated separately and independently. Considering this, a group of scientists with expertise in basic science, medicine and risk assessment reviewed the various aspects of the debate to identify the most significant areas of dispute and to propose a path forward. We identified four areas of debate. The first is about the definitions for terms such as "endocrine disrupting chemical", "adverse effects", and "endocrine system". The second is focused on elements of hormone action including "potency", "endpoints", "timing", "dose" and "thresholds". The third addresses the information needed to establish sufficient evidence of harm. Finally, the fourth focuses on the need to develop and the characteristics of transparent, systematic methods to review the EDC literature. Herein we identify areas of general consensus and propose resolutions for these four areas that would allow the field to move beyond the current and, in our opinion, ineffective debate.

Regulatory decisions on endocrine disrupting chemicals should be based on the principles of endocrinology

For years, scientists from various disciplines have studied the effects of endocrine disrupting chemicals (EDCs) on the health and wellbeing of humans and wildlife. Some studies have specifically focused on the effects of low doses, i.e. those in the range that are thought to be safe for humans and/or animals. Others have focused on the existence of non-monotonic dose-response curves. These concepts challenge the way that chemical risk assessment is performed for EDCs. Continued discussions have clarified exactly what controversies and challenges remain. We address several of these issues, including why the study and regulation of EDCs should incorporate endocrine principles; what level of consensus there is for low dose effects; challenges to our understanding of non-monotonicity; and whether EDCs have been demonstrated to produce adverse effects. This discussion should result in a better understanding of these issues, and allow for additional dialog on their impact on risk assessment.

Assessment of gross fetal malformations: the modernized Wilson technique and skeletal staining

Teratology is the study of anatomical and physiological abnormalities, commonly known as birth defects. If an embryo is exposed to a harmful substance, or teratogen, during the critical period of development, an ensuing malformation may occur. These malformations and their associated mechanisms are studied and analyzed in laboratory animals in order to prevent them from occurring in humans. Rodents, such as rabbits, rats, and mice, have commonly been used in such studies because of their similarity to humans. In 1959, James G. Wilson designed, developed, and tested a protocol on how to observe and analyze structural malformations in rodent fetuses, which included external examination, skeletal evaluation, soft tissue analysis, and data collection/analysis. Although many years have passed since Wilson created this protocol, it is still widely used to this day, and only minor changes have been made to his instructions such as the chemicals used in the experiments and also the analysis of the experimental data. While only minor modifications have been made to this protocol since its beginning, major advances have been made in the dissemination of teratology information to the public such that information is now available through the Internet--information including the identification of an increasing number of teratogens and the understanding of the pathogenesis as it relates to the etiology of birth defects. Despite these advances, however, there has been little decrease in the overall incidence of major birth defects, although significantly improved reporting and ascertainment of birth defects must be factored into the equation in determining birth defect rates. Future birth defect prevention may be based on the understanding of individual genomes and pharmacogenomics, and as the interaction between teratogenic and genetic factors is better understood--with the hope that the incidence of both chemically induced and genetic defects will one day be substantially reduced.

Hormones and endocrine-disrupting chemicals: low-dose effects and nonmonotonic dose responses

For decades, studies of endocrine-disrupting chemicals (EDCs) have challenged traditional concepts in toxicology, in particular the dogma of "the dose makes the poison," because EDCs can have effects at low doses that are not predicted by effects at higher doses. Here, we review two major concepts in EDC studies: low dose and nonmonotonicity. Low-dose effects were defined by the National Toxicology Program as those that occur in the range of human exposures or effects observed at doses below those used for traditional toxicological studies. We review the mechanistic data for low-dose effects and use a weight-of-evidence approach to analyze five examples from the EDC literature. Additionally, we explore nonmonotonic dose-response curves, defined as a nonlinear relationship between dose and effect where the slope of the curve changes sign somewhere within the range of doses examined. We provide a detailed discussion of the mechanisms responsible for generating these phenomena, plus hundreds of examples from the cell culture, animal, and epidemiology literature. We illustrate that nonmonotonic responses and low-dose effects are remarkably common in studies of natural hormones and EDCs. Whether low doses of EDCs influence certain human disorders is no longer conjecture, because epidemiological studies show that environmental exposures to EDCs are associated with human diseases and disabilities. We conclude that when nonmonotonic dose-response curves occur, the effects of low doses cannot be predicted by the effects observed at high doses. Thus, fundamental changes in chemical testing and safety determination are needed to protect human health.