Society for Birth Defects Research and Prevention 2022-2027 strategic plan

BACKGROUND

The sixth Strategic Planning Session of the Society for Birth Defects Research and Prevention (BDRP) was held on April 24-25, 2022, in Alexandria, VA.

METHODS

This effort built upon previous strategic planning sessions, conducted every 5 years.

RESULTS

The overall process was designed to identify BDRP's vision, purpose, culture, and potential, as well as to communicate the value that BDRP brings to its members, volunteers, partners, and the greater community.

CONCLUSIONS

The BDRP 2022-2027 Strategic Plan provides the BDRP leadership, members, and staff with a clearly articulated framework and direction to support long-term sustainability and growth of the society.

25th anniversary of the Berlin workshop on developmental toxicology: DevTox database update, challenges in risk assessment of developmental neurotoxicity and alternative methodologies in bone development and growth

25 years after the first Berlin Workshop on Developmental Toxicity this 10th Berlin Workshop aimed to bring together international experts from authorities, academia and industry to consider scientific, methodologic and regulatory aspects in risk assessment of developmental toxicity and to debate alternative strategies in testing developmental effects in the future. Proposals for improvement of the categorization of developmental effects were discussed as well as the update of the DevTox database as valuable tool for harmonization. The development of adverse outcome pathways relevant to developmental neurotoxicity (DNT) was debated as a fundamental improvement to guide the screening and testing for DNT using alternatives to animal methods. A further focus was the implementation of an in vitro mechanism-based battery, which can support various regulatory applications associated with the assessment of chemicals and mixtures. More interdisciplinary and translation research should be initiated to accelerate the development of new technologies to test developmental toxicity. Technologies in the pipeline are (i) high throughput imaging techniques, (ii) models for DNT screening tests, (iii) use of computer tomography for assessment of thoracolumbar supernumerary ribs in animal models, and (iv) 3D biofabrication of bone development and regeneration tissue models. In addition, increased collaboration with the medical community was suggested to improve the relevance of test results to humans and identify more clinically relevant endpoints. Finally, the participants agreed that this conference facilitated better understanding innovative approaches that can be useful for the identification of developmental health risks due to exposure to chemical substances.

Hazards of diethyl phthalate (DEP) exposure: A systematic review of animal toxicology studies

BACKGROUND

Diethyl phthalate (DEP) is widely used in many commercially available products including plastics and personal care products. DEP has generally not been found to share the antiandrogenic mode of action that is common among other types of phthalates, but there is emerging evidence that DEP may be associated with other types of health effects.

OBJECTIVE

To inform chemical risk assessment, we performed a systematic review to identify and characterize outcomes within six broad hazard categories (male reproductive, female reproductive, developmental, liver, kidney, and cancer) following exposure of nonhuman mammalian animals to DEP or its primary metabolite, monoethyl phthalate (MEP).

METHODS

A literature search was conducted in online scientific databases (PubMed, Web of Science, Toxline, Toxcenter) and Toxic Substances Control Act Submissions, augmented by review of online regulatory sources as well as forward and backward searches. Studies were selected for inclusion using PECO (Population, Exposure, Comparator, Outcome) criteria. Studies were evaluated using criteria defined a priori for reporting quality, risk of bias, and sensitivity using a domain-based approach. Evidence was synthesized by outcome and life stage of exposure, and strength of evidence was summarized into categories of robust, moderate, slight, indeterminate, or compelling evidence of no effect, using a structured framework.

RESULTS

Thirty-four experimental studies in animals were included in this analysis. Although no effects on androgen-dependent male reproductive development were observed following gestational exposure to DEP, there was evidence including effects on sperm following peripubertal and adult exposures, and the overall evidence for male reproductive effects was considered moderate. There was moderate evidence that DEP exposure can lead to developmental effects, with the major effect being reduced postnatal growth following gestational or early postnatal exposure; this generally occurred at doses associated with maternal effects, consistent with the observation that DEP is not a potent developmental toxicant. The evidence for liver effects was considered moderate based on consistent changes in relative liver weight at higher dose levels; histopathological and biochemical changes indicative of hepatic effects were also observed, but primarily in studies that had significant concerns for risk of bias and sensitivity. The evidence for female reproductive effects was considered slight based on few reports of statistically significant effects on maternal body weight gain, organ weight changes, and pregnancy outcomes. Evidence for cancer and effects on kidney were judged to be indeterminate based on limited evidence (i.e., a single two-year cancer bioassay) and inconsistent findings, respectively.

CONCLUSIONS

These results suggest that DEP exposure may induce androgen-independent male reproductive toxicity (i.e., sperm effects) as well as developmental toxicity and hepatic effects, with some evidence of female reproductive toxicity. More research is warranted to fully evaluate these outcomes and strengthen confidence in this database.

Potential frameworks to support evaluation of mechanistic data for developmental neurotoxicity outcomes: A symposium report

A key challenge in systematically incorporating mechanistic data into human health assessments is that, compared to studies of apical health endpoints, these data are both more abundant (mechanistic studies routinely outnumber other studies by several orders of magnitude) and more heterogeneous (e.g. different species, test system, tissue, cell type, exposure paradigm, or specific assays performed). A structured decision-making process for organizing, integrating, and weighing mechanistic DNT data for use in human health risk assessments will improve the consistency and efficiency of such evaluations. At the Developmental Neurotoxicology Society (DNTS) 2016 annual meeting, a symposium was held to address the application of existing organizing principles and frameworks for evaluation of mechanistic data relevant to interpreting neurotoxicology data. Speakers identified considerations with potential to advance the use of mechanistic DNT data in risk assessment, including considering the context of each exposure, since epigenetics, tissue type, sex, stress, nutrition and other factors can modify toxicity responses in organisms. It was also suggested that, because behavior is a manifestation of complex nervous system function, the presence and absence of behavioral change itself could be used to organize the interpretation of multiple complex simultaneous mechanistic changes. Several challenges were identified with frameworks and their implementation, and ongoing research to develop these approaches represents an early step toward full evaluation of mechanistic DNT data for assessments.

Issues in assessing the health risks of n-butanol

A literature review and health effects evaluation were conducted for n-butanol, a chemical that occurs naturally in some foods, which is an intermediate in the production of butyl esters and can be used as a gasoline additive or blend. Studies evaluating n-butyl acetate were included in the review as n-butyl acetate is rapidly converted to n-butanol following multiple routes of exposure. The primary n-butanol health effects identified were developmental and nervous system endpoints. In conducting the literature review and evaluating study findings, the following observations were made: (1) developmental findings were consistently identified; (2) neurodevelopmental findings were inconsistent; (3) evidence for nervous system effects was weak; (4) comparing internal doses from oral and inhalation exposures using physiologically based pharmacokinetic models introduces uncertainties; and (5) a lack of mechanistic information for n-butanol resulted in the reliance on mechanistic data for ethanol, which may or may not be applicable to n-butanol. This paper presents findings from a literature review on the health effects of n-butanol and proposes research to help reduce uncertainty that exists due to database limitations.

Practical considerations for developmental thyroid toxicity assessments: What's working, what's not, and how can we do better?

Thyroid hormones (THs; T3 and T4) play a role in development of cardiovascular, reproductive, immune and nervous systems. Thus, interpretation of TH changes from rodent studies (during pregnancy, in fetuses, neonates, and adults) is critical in hazard characterization and risk assessment. A roundtable session at the 2017 Society of Toxicology (SOT) meeting brought together academic, industry and government scientists to share knowledge and different perspectives on technical and data interpretation issues. Data from a limited group of laboratories were compiled for technical discussions on TH measurements, including good practices for reliable serum TH data. Inter-laboratory historical control data, derived from immunoassays or mass spectrometry methods, revealed: 1) assay sensitivities vary within and across methodologies; 2) TH variability is similar across animal ages; 3) laboratories generally achieve sufficiently sensitive TH quantitation levels, although issues remain for lower levels of serum TH and TSH in fetuses and postnatal day 4 pups; thus, assay sensitivity is critical at these life stages. Best practices require detailed validation of rat serum TH measurements across ages to establish assay sensitivity and precision, and identify potential matrix effects. Finally, issues related to data interpretation for biological understanding and risk assessment were discussed, but their resolution remains elusive.

Hazards of diisobutyl phthalate (DIBP) exposure: A systematic review of animal toxicology studies

BACKGROUND

Biomonitoring studies indicate a trend towards increased human exposure to diisobutyl phthalate (DIBP), a replacement for dibutyl phthalate (DBP). Recent reviews have found DIBP to be a male reproductive toxicant, but have not evaluated other hazards of DIBP exposure.

OBJECTIVE

To inform chemical risk assessment, we performed a systematic review to identify and characterize outcomes within six broad hazard categories (male reproductive, female reproductive, developmental, liver, kidney, and cancer) following exposure of nonhuman mammalian animals to DIBP or the primary metabolite, monoisobutyl phthalate (MIBP).

METHODS

A literature search was conducted in four online scientific databases [PubMed, Web of Science, Toxline, and Toxic Substances Control Act Test Submissions 2.0 (TSCATS2)], and augmented by review of regulatory sources as well as forward and backward searches. Studies were identified for inclusion based on defined PECO (Population, Exposure, Comparator, Outcome) criteria. Studies were evaluated using criteria defined a priori for reporting quality, risk of bias, and sensitivity using a domain-based approach. Evidence was synthesized by outcome and life stage of exposure, and strength of evidence was summarized into categories of robust, moderate, slight, indeterminate, or compelling evidence of no effect, using a structured framework.

RESULTS

Nineteen toxicological studies in rats or mice met the inclusion criteria. There was robust evidence that DIBP causes male reproductive toxicity. Male rats and mice exposed to DIBP during gestation had decreased testosterone and adverse effects on sperm or testicular histology, with additional phthalate syndrome effects observed in male rats. There was also evidence of androgen-dependent and -independent male reproductive effects in rats and mice following peripubertal or young adult exposure to DIBP or MIBP, but confidence was reduced because of concerns over risk of bias and sensitivity in the available studies. There was also robust evidence that DIBP causes developmental toxicity; specifically, increased post-implantation loss and decreased pre- and postnatal growth. For other hazards, evidence was limited by the small number of studies, experimental designs that were suboptimal for evaluating outcomes, and study evaluation concerns such as incomplete reporting of methods and results. There was slight evidence for female reproductive toxicity and effects on liver, and indeterminate evidence for effects on kidney and cancer.

CONCLUSION

Results support DIBP as a children's health concern and indicate that male reproductive and developmental toxicities are hazards of DIBP exposure, with some evidence for female reproductive and liver toxicity. Data gaps include the need for more studies on male reproductive effects following postnatal and adult exposure, and studies to characterize potential hormonal mechanisms in females.

Rethinking developmental toxicity testing: Evolution or revolution?

BACKGROUND

Current developmental toxicity testing adheres largely to protocols suggested in 1966 involving the administration of test compound to pregnant laboratory animals. After more than 50 years of embryo-fetal development testing, are we ready to consider a different approach to human developmental toxicity testing?

METHODS

A workshop was held under the auspices of the Developmental and Reproductive Toxicology Technical Committee of the ILSI Health and Environmental Sciences Institute to consider how we might design developmental toxicity testing if we started over with 21st century knowledge and techniques (revolution). We first consider what changes to the current protocols might be recommended to make them more predictive for human risk (evolution).

RESULTS

The evolutionary approach includes modifications of existing protocols and can include humanized models, disease models, more accurate assessment and testing of metabolites, and informed approaches to dose selection. The revolution could start with hypothesis-driven testing where we take what we know about a compound or close analog and answer specific questions using targeted experimental techniques rather than a one-protocol-fits-all approach. Central to the idea of hypothesis-driven testing is the concept that testing can be done at the level of mode of action. It might be feasible to identify a small number of key events at a molecular or cellular level that predict an adverse outcome and for which testing could be performed in vitro or in silico or, rarely, using limited in vivo models. Techniques for evaluating these key events exist today or are in development.

DISCUSSION

Opportunities exist for refining and then replacing current developmental toxicity testing protocols using techniques that have already been developed or are within reach.

Recommendations for harmonization of data collection and analysis of developmental neurotoxicity endpoints in regulatory guideline studies: Proceedings of workshops presented at Society of Toxicology and joint Teratology Society and Neurobehavioral Teratology Society meetings

The potential for developmental neurotoxicity (DNT) of environmental chemicals may be evaluated using specific test guidelines from the US Environmental Protection Agency or the Organisation for Economic Cooperation and Development (OECD). These guidelines generate neurobehavioral, neuropathological, and morphometric data that are evaluated by regulatory agencies globally. Data from these DNT guideline studies, or the more recent OECD extended one-generation reproductive toxicity guideline, play a pivotal role in children's health risk assessment in different world areas. Data from the same study may be interpreted differently by regulatory authorities in different countries resulting in inconsistent evaluations that may lead to inconsistencies in risk assessment decisions internationally, resulting in regional differences in public health protection or in commercial trade barriers. These issues of data interpretation and reporting are also relevant to juvenile and pre-postnatal studies conducted more routinely for pharmaceuticals and veterinary medicines. There is a need for development of recommendations geared toward the operational needs of the regulatory scientific reviewers who apply these studies in risk assessments, as well as the scientists who generate DNT data sets. The workshops summarized here draw upon the experience of the authors representing government, industry, contract research organizations, and academia to discuss the scientific issues that have emerged from diverse regulatory evaluations. Although various regulatory bodies have different risk management decisions and labeling requirements that are difficult to harmonize, the workshops provided an opportunity to work toward more harmonized scientific approaches for evaluating DNT data within the context of different regulatory frameworks. Five speakers and their coauthors with neurotoxicology, neuropathology, and regulatory toxicology expertise discussed issues of variability, data reporting and analysis, and expectations in DNT data that are encountered by regulatory authorities. In addition, principles for harmonized evaluation of data were suggested using guideline DNT data as case studies.

Comparing rat and rabbit embryo-fetal developmental toxicity data for 379 pharmaceuticals: on systemic dose and developmental effects

A database of embryo-fetal developmental toxicity (EFDT) studies of 379 pharmaceutical compounds in rat and rabbit was analyzed for species differences based on toxicokinetic parameters of area under the curve (AUC) and maximum concentration (C_max) at the developmental lowest adverse effect level (dLOAEL). For the vast majority of cases (83% based on AUC of n = 283), dLOAELs in rats and rabbits were within the same order of magnitude (less than 10-fold different) when compared based on available data on AUC and C_max exposures. For 13.5% of the compounds the rabbit was more sensitive and for 3.5% of compounds the rat was more sensitive when compared based on AUC exposures. For 12% of the compounds the rabbit was more sensitive and for 1.3% of compounds the rat was more sensitive based on C_max exposures. When evaluated based on human equivalent dose (HED) conversion using standard factors, the rat and rabbit were equally sensitive. The relative extent of embryo-fetal toxicity in the presence of maternal toxicity was not different between species. Overall effect severity incidences were distributed similarly in rat and rabbit studies. Individual rat and rabbit strains did not show a different general distribution of systemic exposure LOAELs as compared to all strains combined for each species. There were no apparent species differences in the occurrence of embryo-fetal variations. Based on power of detection and given differences in the nature of developmental effects between rat and rabbit study outcomes for individual compounds, EFDT studies in two species have added value over single studies.

Comparison of rat and rabbit embryo-fetal developmental toxicity data for 379 pharmaceuticals: on the nature and severity of developmental effects

Regulatory non-clinical safety testing of human pharmaceuticals typically requires embryo–fetal developmental toxicity (EFDT) testing in two species (one rodent and one non-rodent). The question has been raised whether under some conditions EFDT testing could be limited to one species, or whether the testing in a second species could be decided on a case-by-case basis. As part of a consortium initiative, we built and queried a database of 379 compounds with EFDT studies (in both rat and rabbit animal models) conducted for marketed and non-marketed pharmaceuticals for their potential for adverse developmental and maternal outcomes, including EFDT incidence and the nature and severity of adverse findings. Manifestation of EFDT in either one or both species was demonstrated for 282 compounds (74%). EFDT was detected in only one species (rat or rabbit) in almost a third (31%, 118 compounds), with 58% (68 compounds) of rat studies and 42% (50 compounds) of rabbit studies identifying an EFDT signal. For 24 compounds (6%), fetal malformations were observed in one species (rat or rabbit) in the absence of any EFDT in the second species. In general, growth retardation, fetal variations, and malformations were more prominent in the rat, whereas embryo–fetal death was observed more often in the rabbit. Discordance across species may be attributed to factors such as maternal toxicity, study design differences, pharmacokinetic differences, and pharmacologic relevance of species. The current analysis suggests that in general both species are equally sensitive on the basis of an overall EFDT LOAEL comparison, but selective EFDT toxicity in one species is not uncommon. Also, there appear to be species differences in the prevalence of various EFDT manifestations (i.e. embryo–fetal death, growth retardation, and dysmorphogenesis) between rat and rabbit, suggesting that the use of both species has a higher probability of detecting developmental toxicants than either one alone.

A systematic evaluation of the potential effects of trichloroethylene exposure on cardiac development

The 2011 EPA trichloroethylene (TCE) IRIS assessment, used developmental cardiac defects from a controversial drinking water study in rats (Johnson et al. [51]), along with several other studies/endpoints to derive reference values. An updated literature search of TCE-related developmental cardiac defects was conducted. Study quality, strengths, and limitations were assessed. A putative adverse outcome pathway (AOP) construct was developed to explore key events for the most commonly observed cardiac dysmorphologies, particularly those involved with epithelial-mesenchymal transition (EMT) of endothelial origin (EndMT); several candidate pathways were identified. A hypothesis-driven weight-of-evidence analysis of epidemiological, toxicological, in vitro, in ovo, and mechanistic/AOP data concluded that TCE has the potential to cause cardiac defects in humans when exposure occurs at sufficient doses during a sensitive window of fetal development. The study by Johnson et al. [51] was reaffirmed as suitable for hazard characterization and reference value derivation, though acknowledging study limitations and uncertainties.

Micro-CT imaging: Developing criteria for examining fetal skeletons in regulatory developmental toxicology studies - A workshop report

During the past two decades the use and refinements of imaging modalities have markedly increased making it possible to image embryos and fetuses used in pivotal nonclinical studies submitted to regulatory agencies. Implementing these technologies into the Good Laboratory Practice environment requires rigorous testing, validation, and documentation to ensure the reproducibility of data. A workshop on current practices and regulatory requirements was held with the goal of defining minimal criteria for the proper implementation of these technologies and subsequent submission to regulatory agencies. Micro-computed tomography (micro-CT) is especially well suited for high-throughput evaluations, and is gaining popularity to evaluate fetal skeletons to assess the potential developmental toxicity of test agents. This workshop was convened to help scientists in the developmental toxicology field understand and apply micro-CT technology to nonclinical toxicology studies and facilitate the regulatory acceptance of imaging data. Presentations and workshop discussions covered: (1) principles of micro-CT fetal imaging; (2) concordance of findings with conventional skeletal evaluations; and (3) regulatory requirements for validating the system. Establishing these requirements for micro-CT examination can provide a path forward for laboratories considering implementing this technology and provide regulatory agencies with a basis to consider the acceptability of data generated via this technology.

Assessment of learning, memory, and attention in developmental neurotoxicity regulatory studies: synthesis, commentary, and recommendations

Cognitive tests of learning and memory (L&M) have been required by U.S. Environmental Protection Agency (EPA) developmental neurotoxicity test (DNT) guidelines for more than two decades. To evaluate the utility of these guidelines, the EPA reviewed 69 pesticide DNT studies. This review found that the DNT provided or could provide the point-of-departure for risk assessment by showing the Lowest Observable Adverse Effect Level (LOAEL) in 28 of these studies in relation to other reported end points. Among the behavioral tests, locomotor activity and auditory/acoustic startle provided the most LOAELs, and tests of cognitive function and the Functional Observational Battery (FOB) the fewest. Two issues arose from the review: (1) what is the relative utility of cognitive tests versus tests of unconditioned behavior, and (2) how might cognitive tests be improved? The EPA sponsored a symposium to address this. Bushnell reviewed studies in which both screening (locomotor activity, FOB, reflex ontogeny, etc.) and complex tests (those requiring training) were used within the same study; he found relatively little evidence that complex tests provided a LOAEL lower than screening tests (with exceptions). Levin reviewed reasons for including cognitive tests in regulatory studies and methods and evidence for the radial arm maze and its place in developmental neurotoxicity assessments. Driscoll and Strupp reviewed the value of serial reaction time operant methods for assessing executive function in developmental neurotoxicity studies. Vorhees and Williams reviewed the value of allocentric (spatial) and egocentric cognitive tests and presented methods for using the Morris water maze for spatial and the Cincinnati water maze for egocentric cognitive assessment. They also reviewed the possible use of water radial mazes. The relatively lower impact of cognitive tests in previous DNT studies in the face of the frequency of human complaints of chemical-induced cognitive dysfunction indicates that animal cognitive tests need improvement. The contributors to this symposium suggest that if the guidelines are updated, they be made more specific by recommending preferred tests and providing greater detail on key characteristics of such tests. Additionally, it is recommended that guidance be developed to address important issues with cognitive tests and to provide the information needed to improve the design, conduct, and interpretation of tests of higher function within a regulatory context. These steps will maximize the value of cognitive tests for use in hazard evaluation and risk assessment.

Evaluation of the ToxRTool's ability to rate the reliability of toxicological data for human health hazard assessments

Regulatory agencies often utilize results from peer reviewed publications for hazard assessments. A problem in doing so is the lack of well-accepted tools to objectively, efficiently and systematically assess the quality of published toxicological studies. Herein, we evaluated the publicly available software-based ToxRTool (Toxicological data Reliability assessment Tool) for use in human health hazard assessments. The ToxRTool was developed by the European Commission's Joint Research Center in 2009. It builds on Klimisch categories, a rating system established in 1997, by providing additional criteria and guidance for assessing the reliability of toxicological studies. It also transparently documents the study-selection process. Eight scientists used the ToxRTool to rate the same 20 journal articles on thyroid toxicants. Results were then compared using the Finn coefficient and "AC1" to determine inter-rater consistency. Ratings were most consistent for high-quality journal articles, but less consistent as study quality decreased. Primary reasons for inconsistencies were that some criteria were subjective and some were not clearly described. It was concluded, however, that the ToxRTool has potential and, with refinement, could provide a more objective approach for screening published toxicology studies for use in health risk evaluations, although the ToxRTool ratings are primarily based on study reporting quality.

Improving Concordance in Environmental Epidemiology: A Three-Part Proposal

In observational research, evidence is usually derived from multiple studies, and any single result is rarely considered sufficient for public health decision making. Despite more than five decades of research and thousands of studies published, the ability to draw robust conclusions regarding the presence or absence of causal links between specific environmental exposures and human health remains limited. To develop policies that are protective of public health and can withstand scrutiny, agencies need to rely on investigations of satisfactory quality that follow sufficiently concordant protocols in terms of exposure assessment, outcome ascertainment, data analysis, and reporting of results. Absent such concordance, the ability of environmental epidemiology studies to inform decision making is greatly diminished. Systems and tools are proposed here to improve concordance among environmental epidemiology studies. Specifically, working systems in place in other fields of research are critically examined and used as guidelines to develop analogous policies and procedures for environmental epidemiology. A three-part path forward toward more concordant, transparent, and readily accessible environmental epidemiology evidence that parallels ongoing efforts in medical research is proposed. The three parts address methods for improving quality and accessibility of systematic reviews, access to information on ongoing and completed studies, and principles for reporting. The goals are to increase the value of epidemiological research in public health decision making and to stimulate discussions around solutions proposed herein.

Human health effects of tetrachloroethylene: key findings and scientific issues

BACKGROUND

The U.S. Environmental Protection Agency (EPA) completed a toxicological review of tetrachloroethylene (perchloroethylene, PCE) in February 2012 in support of the Integrated Risk Information System (IRIS).

OBJECTIVES

We reviewed key findings and scientific issues regarding the human health effects of PCE described in the U.S. EPA's Toxicological Review of Tetrachloroethylene (Perchloroethylene).

METHODS

The updated assessment of PCE synthesized and characterized a substantial database of epidemiological, experimental animal, and mechanistic studies. Key scientific issues were addressed through modeling of PCE toxicokinetics, synthesis of evidence from neurological studies, and analyses of toxicokinetic, mechanistic, and other factors (tumor latency, severity, and background rate) in interpreting experimental animal cancer findings. Considerations in evaluating epidemiological studies included the quality (e.g., specificity) of the exposure assessment methods and other essential design features, and the potential for alternative explanations for observed associations (e.g., bias or confounding).

DISCUSSION

Toxicokinetic modeling aided in characterizing the complex metabolism and multiple metabolites that contribute to PCE toxicity. The exposure assessment approach-a key evaluation factor for epidemiological studies of bladder cancer, non-Hodgkin lymphoma, and multiple myeloma-provided suggestive evidence of carcinogenicity. Bioassay data provided conclusive evidence of carcinogenicity in experimental animals. Neurotoxicity was identified as a sensitive noncancer health effect, occurring at low exposures: a conclusion supported by multiple studies. Evidence was integrated from human, experimental animal, and mechanistic data sets in assessing adverse health effects of PCE.

CONCLUSIONS

PCE is likely to be carcinogenic to humans. Neurotoxicity is a sensitive adverse health effect of PCE.

A systematic approach for identifying and presenting mechanistic evidence in human health assessments

Clear documentation of literature search and presentation methodologies can improve transparency in chemical hazard assessments. We sought to improve clarity for the scientific support for cancer mechanisms of action using a systematic approach to literature retrieval, selection, and presentation of studies. The general question was "What are the mechanisms by which a chemical may cause carcinogenicity in the target tissue?". Di(2-ethylhexyl)phthalate was used as a case study chemical with a complex database of >3000 publications. Relevant mechanistic events were identified from published reviews. The PubMed search strategy included relevant synonyms and wildcards for DEHP and its metabolites, mechanistic events, and species of interest. Tiered exclusion/inclusion criteria for study pertinence were defined, and applied to the retrieved literature. Manual curation was conducted for mechanistic events with large literature databases. Literature trees documented identification and selection of the literature evidence. The selected studies were summarized in evidence tables accompanied by succinct narratives. Primary publications were deposited into the Health and Environmental Research Online (http://hero.epa.gov/) database and identified by pertinence criteria and key terms to permit organized retrieval. This approach contributes to human health assessment by effectively managing a large volume of literature, improving transparency, and facilitating subsequent synthesis of information across studies.

Developmental neurotoxicity of engineered nanomaterials: identifying research needs to support human health risk assessment

Increasing use of engineered nanomaterials (ENM) in consumer products and commercial applications has helped drive a rise in research related to the environmental health and safety (EHS) of these materials. Within the cacophony of information on ENM EHS to date are data indicating that these materials may be neurotoxic in adult animals. Evidence of elevated inflammatory responses, increased oxidative stress levels, alterations in neuronal function, and changes in cell morphology in adult animals suggests that ENM exposure during development could elicit developmental neurotoxicity (DNT), especially considering the greater vulnerability of the developing brain to some toxic insults. In this review, we examine current findings related to developmental neurotoxic effects of ENM in the context of identifying research gaps for future risk assessments. The basic risk assessment paradigm is presented, with an emphasis on problem formulation and assessments of exposure, hazard, and dose response for DNT. Limited evidence suggests that in utero and postpartum exposures are possible, while fewer than 10 animal studies have evaluated DNT, with results indicating changes in synaptic plasticity, gene expression, and neurobehavior. Based on the available information, we use current testing guidelines to highlight research gaps that may inform ENM research efforts to develop data for higher throughput methods and future risk assessments for DNT. Although the available evidence is not strong enough to reach conclusions about DNT risk from ENM exposure, the data indicate that consideration of ENM developmental neurotoxic potential is warranted.

Human health effects of trichloroethylene: key findings and scientific issues

BACKGROUND

In support of the Integrated Risk Information System (IRIS), the U.S. Environmental Protection Agency (EPA) completed a toxicological review of trichloroethylene (TCE) in September 2011, which was the result of an effort spanning > 20 years.

OBJECTIVES

We summarized the key findings and scientific issues regarding the human health effects of TCE in the U.S. EPA's toxicological review.

METHODS

In this assessment we synthesized and characterized thousands of epidemiologic, experimental animal, and mechanistic studies, and addressed several key scientific issues through modeling of TCE toxicokinetics, meta-analyses of epidemiologic studies, and analyses of mechanistic data.

DISCUSSION

Toxicokinetic modeling aided in characterizing the toxicological role of the complex metabolism and multiple metabolites of TCE. Meta-analyses of the epidemiologic data strongly supported the conclusions that TCE causes kidney cancer in humans and that TCE may also cause liver cancer and non-Hodgkin lymphoma. Mechanistic analyses support a key role for mutagenicity in TCE-induced kidney carcinogenicity. Recent evidence from studies in both humans and experimental animals point to the involvement of TCE exposure in autoimmune disease and hypersensitivity. Recent avian and in vitro mechanistic studies provided biological plausibility that TCE plays a role in developmental cardiac toxicity, the subject of substantial debate due to mixed results from epidemiologic and rodent studies.

CONCLUSIONS

TCE is carcinogenic to humans by all routes of exposure and poses a potential human health hazard for noncancer toxicity to the central nervous system, kidney, liver, immune system, male reproductive system, and the developing embryo/fetus.

Current and future needs for developmental toxicity testing

A review is presented of the use of developmental toxicity testing in the United States and international regulatory assessment of human health risks associated with exposures to pharmaceuticals (human and veterinary), chemicals (agricultural, industrial, and environmental), food additives, cosmetics, and consumer products. Developmental toxicology data are used for prioritization and screening of pharmaceuticals and chemicals, for evaluating and labeling of pharmaceuticals, and for characterizing hazards and risk of exposures to industrial and environmental chemicals. The in vivo study designs utilized in hazard characterization and dose-response assessment for developmental outcomes have not changed substantially over the past 30 years and have served the process well. Now there are opportunities to incorporate new technologies and approaches to testing into the existing assessment paradigm, or to apply innovative approaches to various aspects of risk assessment. Developmental toxicology testing can be enhanced by the refinement or replacement of traditional in vivo protocols, including through the use of in vitro assays, studies conducted in alternative nonmammalian species, the application of new technologies, and the use of in silico models. Potential benefits to the current regulatory process include the ability to screen large numbers of chemicals quickly, with the commitment of fewer resources than traditional toxicology studies, and to refine the risk assessment process through an enhanced understanding of the mechanisms of developmental toxicity and their relevance to potential human risk. As the testing paradigm evolves, the ability to use developmental toxicology data to meet diverse critical regulatory needs must be retained.

Current assessment of the effects of environmental chemicals on the mammary gland in guideline rodent studies by the U.S. Environmental Protection Agency (U.S. EPA), Organisation for Economic Co-operation and Development (OECD), and National Toxicology Program (NTP)

BACKGROUND

Evaluation of the structural and/or functional integrity of the mammary gland (MG) across life stages is integral to the assessment of developmental, reproductive, and carcinogenic risk for environmental chemicals.

OBJECTIVES

In this commentary I characterize MG assessment recommended in U.S. Environmental Protection Agency, Organisation for Economic Co-operation and Development, and National Toxicology Program guideline toxicology study protocols and identify any information gaps for the evaluation of MG development, structure, and function.

DISCUSSION

Several data gaps, issues, and challenges were identified. Current guidelines that include a lactation phase do not provide specific recommendations to record observations on maternal or offspring lactation or nursing behavior. In guideline studies, the assessment of MG toxicity often relies upon indirect, nonspecific, or surrogate end points, and information that could be useful in the interpretation of these data (e.g., mode of action or toxicokinetics) is often unavailable. Most guideline studies designed to assess general organ toxicity do not expose test animals during sensitive stages of MG development; histopathological evaluation of the developing MG is not routinely conducted; and evaluation of MG tissue for both sexes is inconsistently recommended.

CONCLUSIONS

I propose the following general recommendations to enhance MG assessment in guideline toxicology studies: a) inclusion of more specific criteria for the evaluation of MG end points in guideline language, b) inclusion of histopathological evaluation of MG development (using whole-mount techniques) in existing or new guideline protocols that include offspring with perinatal and/or pubertal treatment, c) incorporation of perinatal exposures into rodent subchronic and carcinogenicity assays, and d) expansion of the histopathological evaluation of male MG tissue.

Environmental exposures and mammary gland development: state of the science, public health implications, and research recommendations

OBJECTIVES

Perturbations in mammary gland (MG) development may increase risk for later adverse effects, including lactation impairment, gynecomastia (in males), and breast cancer. Animal studies indicate that exposure to hormonally active agents leads to this type of developmental effect and related later life susceptibilities. In this review we describe current science, public health issues, and research recommendations for evaluating MG development.

DATA SOURCES

The Mammary Gland Evaluation and Risk Assessment Workshop was convened in Oakland, California, USA, 16-17 November 2009, to integrate the expertise and perspectives of scientists, risk assessors, and public health advocates. Interviews were conducted with 18 experts, and seven laboratories conducted an MG slide evaluation exercise. Workshop participants discussed effects of gestational and early life exposures to hormonally active agents on MG development, the relationship of these developmental effects to lactation and cancer, the relative sensitivity of MG and other developmental end points, the relevance of animal models to humans, and methods for evaluating MG effects.

SYNTHESIS

Normal MG development and MG carcinogenesis demonstrate temporal, morphological, and mechanistic similarities among test animal species and humans. Diverse chemicals, including many not considered primarily estrogenic, alter MG development in rodents. Inconsistent reporting methods hinder comparison across studies, and relationships between altered development and effects on lactation or carcinogenesis are still being defined. In some studies, altered MG development is the most sensitive endocrine end point.

CONCLUSIONS

Early life environmental exposures can alter MG development, disrupt lactation, and increase susceptibility to breast cancer. Assessment of MG development should be incorporated in chemical test guidelines and risk assessment.

A proposal to facilitate weight-of-evidence assessments: Harmonization of Neurodevelopmental Environmental Epidemiology Studies (HONEES)

The ability to conduct weight-of-evidence assessments to inform the evaluation of potential environmental neurotoxicants is limited by lack of comparability of study methods, data analysis, and reporting. There is a need to establish consensus guidelines for conducting, analyzing, and reporting neurodevelopmental environmental epidemiologic studies, while recognizing that consistency is likewise needed for epidemiology studies examining other health outcomes. This paper proposes a set of considerations to be used by the scientific community at-large as a tool for systematically evaluating the quality of proposed and/or published studies in terms of their value for weight-of-evidence assessments. Particular emphasis is placed on evaluating factors influencing the risk of incorrect conclusions at the level of study findings. The proposed considerations are the first step in what must be a larger consensus-based process and can serve to catalyze such a discussion. Achieving consensus in these types of endeavors is difficult; however, opportunities exist for further interdisciplinary discussion, collaboration, and research that will help realize this goal. Broad acceptance and application of such an approach can facilitate the expanded use of environmental epidemiology studies of potential neurodevelopmental toxicants in the protection of public health, and specifically children's health.

Continuing education course #3: current practices and future trends in neuropathology assessment for developmental neurotoxicity testing

The continuing education course on Developmental Neurotoxicity Testing (DNT) was designed to communicate current practices for DNT neuropathology, describe promising innovations in quantitative analysis and noninvasive imaging, and facilitate a discussion among experienced neuropathologists and regulatory scientists regarding suitable DNT practices. Conventional DNT neuropathology endpoints are qualitative histopathology and morphometric endpoints of particularly vulnerable sites (e.g., cerebral, cerebellar, or hippocampal thickness). Novel imaging and stereology measurements hold promise for automated analysis of factors that cannot be effectively examined in routinely processed specimens (e.g., cell numbers, fiber tract integrity). The panel recommended that dedicated DNT neuropathology data sets be acquired on a minimum of 8 sections (for qualitative assessment) or 3 sections (for quantitative linear and stereological analyses) using a small battery of stains to examine neurons and myelin. Where guidelines permit discretion, immersion fixation is acceptable for younger animals (postnatal day 22 or earlier), and peripheral nerves may be embedded in paraffin. Frequent concerns regarding DNT data sets include false-negative outcomes due to processing difficulties (e.g., lack of concordance among sections from different animals) and insensitive analytical endpoints (e.g., qualitative evaluation) as well as false-positive results arising from overinterpretation or misreading by inexperienced pathologists.

Use of genomic data in risk assessment case study: I. Evaluation of the dibutyl phthalate male reproductive development toxicity data set

A case study was conducted, using dibutyl phthalate (DBP), to explore an approach to using toxicogenomic data in risk assessment. The toxicity and toxicogenomic data sets relative to DBP-related male reproductive developmental outcomes were considered conjointly to derive information about mode and mechanism of action. In this manuscript, we describe the case study evaluation of the toxicological database for DBP, focusing on identifying the full spectrum of male reproductive developmental effects. The data were assessed to 1) evaluate low dose and low incidence findings and 2) identify male reproductive toxicity endpoints without well-established modes of action (MOAs). These efforts led to the characterization of data gaps and research needs for the toxicity and toxicogenomic studies in a risk assessment context. Further, the identification of endpoints with unexplained MOAs in the toxicity data set was useful in the subsequent evaluation of the mechanistic information that the toxicogenomic data set evaluation could provide. The extensive analysis of the toxicology data set within the MOA context provided a resource of information for DBP in attempts to hypothesize MOAs (for endpoints without a well-established MOA) and to phenotypically anchor toxicogenomic and other mechanistic data both to toxicity endpoints and to available toxicogenomic data. This case study serves as an example of the steps that can be taken to develop a toxicological data source for a risk assessment, both in general and especially for risk assessments that include toxicogenomic data.

The use of developmental neurotoxicity data in pesticide risk assessments

Following the passage of the Food Quality Protection Act, which mandated an increased focus on evaluating the potential toxicity of pesticides to children, the number of guideline developmental neurotoxicity (DNT) studies (OPPTS 870.6300) submitted to the U.S. Environmental Protection Agency (EPA) Office of Pesticide Programs (OPP) was greatly increased. To evaluate the impact of available DNT studies on individual chemical risk assessments, the ways in which data from these studies are being used in pesticide risk assessment were investigated. In addition, the neurobehavioral and neuropathological parameters affected at the lowest observed adverse effect level (LOAEL) for each study were evaluated to ascertain whether some types of endpoints were consistently more sensitive than others. As of December 2008, final OPP reviews of DNT studies for 72 pesticide chemicals were available; elimination of studies with major deficiencies resulted in a total of 69 that were included in this analysis. Of those studies, 15 had been used to determine the point of departure for one or more risk assessment scenarios, and an additional 13 were determined to have the potential for use as a point of departure for future risk assessments (selection is dependent upon review of the entire database available at the time of reassessment). Analysis of parameters affected at the study LOAELs indicated that no single parameter was consistently more sensitive than another. Early assessment time points (e.g., postnatal day (PND) 11/21) tended to be more sensitive than later time points (e.g., PND 60). These results demonstrate that data generated using the current guideline DNT study protocol are useful in providing points of departure for risk assessments. The results of these studies also affirm the importance of evaluating a spectrum of behavioral and neuropathological endpoints, in both young and adult animals, to improve the detection of the potential for a chemical to cause developmental neurotoxicity.

Terminology of developmental abnormalities in common laboratory mammals (version 2)

This update (Version 2) of the Terminology of Developmental Abnormalities in Common Laboratory Mammals (Version 1) incorporates improvements and enhancements to both content and organization of the terminology to enable greater flexibility in its application, while maintaining a consistent approach to the description of findings. The revisions are the result of an international collaboration among interested organizations, advised by individual experts and the outcomes of several workshops. The terminology remains organized into tables under the broad categories of external, visceral, and skeletal observations, following the manner in which data are typically collected and recorded in developmental toxicity studies. This arrangement of the tables, as well as other information provided in appendices, is intended to facilitate the process of specimen evaluation at the laboratory bench level. Only the commonly used laboratory mammals (i.e. rats, mice, rabbits) are addressed in the current terminology tables. The inclusion of other species that are used in developmental toxicity testing, such as primates, is considered outside the scope of the present update. Similarly, categorization of findings as, for example, 'malformation' or 'variation' remains unaddressed, in accordance with the overall principle that the focus of this document is descriptive terminology and not diagnosis or interpretation. The skeletal terms have been augmented to accommodate cartilage findings.

Recent progress and diverse effects in developmental immunotoxicology: overview of a symposium at the 46th Annual SOT Meeting, Charlotte, NC

It has long been known that the developing immune system is more sensitive and susceptible than the adult immune system to some drugs and environmental contaminants. However, notable advances have been made in the database of studies supporting developmental immunotoxicity (DIT) over the past 5 years. There is considerable evidence that responses of the immune system can be quantitatively or qualitatively different from normal adult responses when xenobiotic exposure occurs during critical periods of immune system development. Qualitative differences of DIT relative to adult exposures include examples of more persistent effects, a latency of effects, and immune dysfunction that is fundamentally different than effects observed when adults are exposed. A symposium was presented at the Society of Toxicology annual meeting to provide an update on advances in the maturing field of developmental immunotoxicology and to facilitate discussion on the range of DIT and later life effects following developmental exposure. In particular, presentations focused on implications of neuroendocrine cross-talk for DIT, the association between developmental air pollutant exposure and asthma, and recent evidence that developmental exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin may increase the risk of autoimmune responses. Several important concepts relative to DIT assessment were illustrated, i.e., (1) Screening for immunosuppression alone is not sufficient to identify all potential immunotoxic effects; (2) DIT cannot be reliably predicted from studies that only utilize adult exposures; (3) Functional testing protocols are preferred in the assessment of DIT; (4) Gender-related differences should be routinely assessed; (5) Latency (i.e., later-life adverse outcomes resulting from developmental exposures) is an important consideration that cannot be detected in adult exposure studies; and, (6) There is increasing support for DIT testing protocols with continuous exposure throughout development until the immune assay is performed.

Evidence of autoimmune-related effects of trichloroethylene exposure from studies in mice and humans

OBJECTIVE

Our objective was to examine experimental and epidemiologic studies pertaining to immune-related, and specifically autoimmune-related, effects of trichloroethylene (TCE).

DATA SOURCES AND EXTRACTION

We performed a literature search of PubMed and reviewed bibliographies in identified articles. We then systematically reviewed immune-related data, focusing on clinical and immunologic features and mechanistic studies.

DATA SYNTHESIS

Studies conducted in MRL+/+ lupus mice report an accelerated autoimmune response in relation to exposure to TCE or some metabolites. Effects have been reported after 4 weeks of exposure to TCE at doses as low as 0.1 mg/kg/day in drinking water and have included increased antinuclear antibodies and interferon-gamma (IFN-gamma) and decreased secretion of interleukin-4 (IL-4), consistent with an inflammatory response. Autoimmune hepatitis, inflammatory skin lesions, and alopecia have been found after exposures of 32-48 weeks. Recent mechanistic experiments in mice examined oxidative stress and, specifically, effects on lipid-peroxidation-derived aldehydes in TCE-induced autoimmune disease. Two studies in humans reported an increase in IL-2 or IFN-gamma and a decrease in IL-4 in relation to occupational or environmental TCE exposure. Occupational exposure to TCE has also been associated with a severe, generalized hypersensitivity skin disorder accompanied by systemic effects, including hepatitis. In three case-control studies of scleroderma with a measure of occupational TCE exposure, the combined odds ratio was 2.5 [95% confidence interval (CI), 1.1-5.4] in men and 1.2 (95% CI, 0.58-2.6) in women.

CONCLUSION

The consistency among the studies and the concordance between the studies in mice and humans support an etiologic role of TCE in autoimmune disease. Multisite collaborations and studies of preclinical immune markers are needed to further develop this field of research.

A lifestage-specific approach to hazard and dose-response characterization for children's health risk assessment

In 2006, the U.S. EPA published a report entitled A Framework for Assessing Health Risks of Environmental Exposures to Children (hereafter referred to as the "Framework") describing a lifestage approach to risk assessment that includes the evaluation of existing data from a temporal perspective (i.e., the timing of both the exposure and the outcome). This article summarizes the lifestage-specific issues discussed in the Framework related to the qualitative and the quantitative hazard and dose-response characterization. Lifestage-specific hazard characterization includes an evaluation of relevant human and experimental animal studies, focusing on the identification of critical windows of development (i.e., exposure intervals of maximum susceptibility) for observed outcomes, evaluation of differential exposure at individual lifestages, the relevance and impact of lifestage-specific toxicokinetic and toxicodynamic data, mode of action information, variability and latency of effects from early lifestage exposure, and describing uncertainties. The interpretation of the hazard data to determine the strength of association between early life exposures and the timing and type of outcomes depends upon the overall weight of evidence. Lifestage-specific dose-response characterization relies on the identification of susceptible lifestages in order to quantify health risk, information on the point of departure, key default assumptions, and descriptions of uncertainty, sensitivity, and variability. Discussion of the strength and limitations of the hazard and dose-response data provides a basis for confidence in risk determinations. Applying a lifestage approach to hazard and dose-response characterization is likely to improve children's health risk assessment by identifying data gaps and providing a better understanding of sources of uncertainty.

A retrospective performance assessment of the developmental neurotoxicity study in support of OECD test guideline 426

OBJECTIVE

We conducted a review of the history and performance of developmental neurotoxicity (DNT) testing in support of the finalization and implementation of Organisation of Economic Co-operation and Development (OECD) DNT test guideline 426 (TG 426).

INFORMATION SOURCES AND ANALYSIS

In this review we summarize extensive scientific efforts that form the foundation for this testing paradigm, including basic neurotoxicology research, interlaboratory collaborative studies, expert workshops, and validation studies, and we address the relevance, applicability, and use of the DNT study in risk assessment.

CONCLUSIONS

The OECD DNT guideline represents the best available science for assessing the potential for DNT in human health risk assessment, and data generated with this protocol are relevant and reliable for the assessment of these end points. The test methods used have been subjected to an extensive history of international validation, peer review, and evaluation, which is contained in the public record. The reproducibility, reliability, and sensitivity of these methods have been demonstrated, using a wide variety of test substances, in accordance with OECD guidance on the validation and international acceptance of new or updated test methods for hazard characterization. Multiple independent, expert scientific peer reviews affirm these conclusions.

A Tiered Approach to Life Stages Testing for Agricultural Chemical Safety Assessment

A proposal has been developed by the Agricultural Chemical Safety Assessment (ACSA) Technical Committee of the ILSI Health and Environmental Sciences Institute (HESI) for an improved approach to assessing the safety of crop protection chemicals. The goal is to ensure that studies are scientifically appropriate and necessary without being redundant, and that tests emphasize toxicological endpoints and exposure durations that are relevant for risk assessment. The ACSA Life Stages Task Force proposes a tiered approach to toxicity testing that assesses a compound's potential to cause adverse effects on reproduction, and that assesses the nature and severity of effects during development and adolescence, with consideration of the sensitivity of the elderly. While incorporating many features from current guideline studies, the proposed approach includes a novel rat reproduction and developmental study with enhanced endpoints and a rabbit development study. All available data, including toxicokinetics, ADME data, and systemic toxicity information, are considered in the design and interpretation of studies. Compared to existing testing strategies, the proposed approach uses fewer animals, provides information on the young animal, and includes an estimation of human exposure potential for making decisions about the extent of testing required.

What’s So Special about the Developing Immune System?

The evolution of the subdiscipline of developmental immunotoxicology (DIT) as it exists today has been shaped by significant regulatory pressures as well as key scientific advances. This review considers the role played by legislation to protect children’s health, and on the emergence of immunotoxcity and developmental immunotoxicity guidelines, as well as providing some context to the need for special attention on DIT by considering the evidence that the developing immune system may have unique susceptibilities when compared to the adult immune system. Understanding the full extent of this potential has been complicated by a paucity of data detailing the development of the immune system during critical life stages as well as by the complexities of comparisons across species. Notably, there are differences between humans and nonhuman species used in toxicity testing that include specific differences relative to the timing of the development of the immune system as well as more general anatomic differences, and these differences must be factored into the interpretation of DIT studies. Likewise, understanding how the timing of the immune development impacts on various immune parameters is critical to the design of DIT studies, parameters most extensively characterized to date in young adult animals. Other factors important to DIT, which are considered in this review, are the recognition that effects other than suppression (e.g., allergy and autoimmunity) are important; the need to improve our understanding of how to assess the potential for DIT in humans; and the role that pathology has played in DIT studies in test animals. The latter point receives special emphasis in this review because pathology evaluations have been a major component of standard nonclinical toxicology studies, and could serve an important role in studies to evaluate DIT. This possibility is very consistent with recommendations to incorporate a DIT evaluation into standard developmental and reproductive toxicology (DART) protocols. The overall objective of this review is to provide a ‘snapshot’ of the current state-of-the-science of DIT. Despite significant progress, DIT is still evolving and it is our hope that this review will advance the science.

Developmental Toxicology Evaluations—Issues with Including Neurotoxicology and Immunotoxicology Assessments in Reproductive Toxicology Studies

Developmental and reproductive toxicology (DART) has routinely been a part of safety assessment. Attention is now focused on the effects of chemicals on the developing nervous and immune systems. This focus on developmental neurotoxicology (DNT) and developmental immunotoxicology (DIT) is based on the premise that children differ from adults in some aspects of their biology and, thus, may also differ in their responses to chemicals. This session's objective was to discuss issues common to DNT and DIT as they relate to DART protocols, including high dose selection and maternal toxicity, adequacy of pup exposure during lactation, use of a different dosing paradigm for DART versus DNT or DIT studies, and whether DIT and DNT endpoints can be incorporated into a single DART study for hazard identification purposes. Consensus was achieved on all topics except the adequacy for risk assessment purposes of the use of a limited number of endpoints for DIT and DNT, with the DNT endpoints being the primary focus of disagreement. Panelists indicated that a combination study design for hazard identification was feasible, though flexibility to meet the scientific needs of the project was emphasized. The adequacy of existing triggers for additional developmental studies was also questioned. Panelists iterated the importance of understanding pup exposure during the various life stages and the use of toxicokinetic data in designing these studies. The group agreed to consider the HESI ACSA Life Stages Task Force recommendations as a next step to address some of the issues and challenges raised during this session.

A Proposed Testing Framework for Developmental Immunotoxicology (DIT)

A group of thirty immunotoxicology experts from the U.S. and E.U. representing government, industry, and academia met in May 2003, in Washington, D.C., to reach consensus regarding the most appropriate methods to assess developmental immunotoxicology (DIT) for hazard identification, including under what conditions such testing might be required. The following points represent the major conclusions from this roundtable discussion: (1) the rat is the preferred model; (2) any DIT protocol should be based on immune assays already validated; (3) DIT methods should be incorporated into standard developmental and reproductive toxicity protocols to the extent possible rather than a "stand-alone" protocol; (4) the approach to address DIT potential should be similar for chemicals and drugs, but the experimental design should be flexible and should reflect the specific questions to be answered; (5) it is possible to utilize a study design that assesses all critical windows in one protocol, with the results leading to further study of specific effects, as warranted; (6) animals should be exposed throughout the treatment protocol; (7) the triggers for DIT may include structure-activity-relationships, results from other toxicity studies, the intended use of a drug/chemical and/or its anticipated exposure of neonates and/or juveniles.

A qualitative retrospective analysis of positive control data in developmental neurotoxicity studies

Testing for neurodevelopmental effects commonly involves both functional and neuropathological assessments in offspring during and following maternal exposure. The use of positive controls in neurotoxicity screening has been advocated by numerous expert groups. Evaluation of positive control data allows evaluation of laboratory proficiency in detecting changes in the structure and function of the developing nervous system and comparison of the sensitivity of assessments in different studies and laboratories. This project surveyed approaches taken in contract and industrial laboratories in generating and providing these data. Positive control data submitted in support of 34 developmental neurotoxicity (DNT) studies from 16 different laboratories were summarized by test method for information on the following: age relevance of test subjects, the presence of a dose-response relationship, gender, group size, statistics, report quality, quality assurance, and the year the study was conducted. Endpoints included the following: developmental landmarks, clinical observations (CO), motor activity, startle response, learning and memory, qualitative neuropathology, and quantitative brain morphometry (linear measurements of selected brain regions). Results ranged from no positive control data for three laboratories, to one laboratory that submitted 17 separate positive control reports. The qualitative range was similarly broad, from excellent to poor. Various problems were identified, including the following: inappropriate report structure (e.g., copies of poster presentations), lack of individual data, inadequate methodological details, submission of very old data (>10 years) or data from completely different laboratories, use of inappropriate positive control chemicals or doses that were without effect, lack of statistical analysis, use of only one sex, and use of incompatibly aged animals. Analyses revealed that there were only 3 out of 16 laboratories that had submitted positive control data adequate for proficiency purposes for all of the major endpoints in the DNT study. Adequate positive control data are very useful in a weight-of-evidence approach to help determine the biological significance of results, and also to increase the confidence in negative results from DNT studies.

Recent developments in regulatory requirements for developmental toxicology

A number of legislative and regulatory changes have occurred over the past 5 years to prompt the re-evaluation of the regulatory requirements for developmental toxicity testing and use of the data for risk assessment. In particular, passage of the 1996 Food Quality Protection Act (FQPA) in the United States required the USEPA to evaluate children's health risks in a more rigorous fashion, and to apply an additional 10-fold safety factor if data were inadequate or children appeared to be more sensitive than adults. A review of the testing protocols required by USEPA led to extension of the dosing period to term in the prenatal developmental toxicity study and the addition of endpoints to the 2-generation reproduction study protocol as indicators of possible neurologic, reproductive, or immune alterations. Revised testing guidelines for pesticides and toxic substances were published by USEPA in 1998, including a developmental neurotoxicity testing protocol. Further review for FQPA implementation resulted in the proposal for a core set of required toxicology studies, including routine developmental neurotoxicity, adult neurotoxicity, and adult immunotoxicity studies. In addition, development of new testing guidelines in several areas was recommended, these guidelines to be used in conjunction with or as follow-up when indicated from standard testing: developmental immunotoxicity, carcinogenesis, specialized neurotoxicity studies, endocrine disruptor studies, pharmacokinetics, and direct dosing of neonates. The impact of these efforts on the policies for toxicity testing of pesticides are discussed, and these issues are currently being reviewed on a broader scale, in particular, by evaluating the adequacy of the methods used for reference values (e.g. chronic RfD, RfC). Three major areas of focus for this review include life stages evaluated, endpoints assessed, and the duration of exposure used in various studies. A major focus of these efforts is to ensure that children's health risks are being adequately addressed in the risk assessment process.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. III: pharmacokinetic and pharmacodynamic considerations

We review pharmacokinetic and pharmacodynamic factors that should be considered in the design and interpretation of developmental neurotoxicity studies. Toxicologic effects on the developing nervous system depend on the delivered dose, exposure duration, and developmental stage at which exposure occurred. Several pharmacokinetic processes (absorption, distribution, metabolism, and excretion) govern chemical disposition within the dam and the nervous system of the offspring. In addition, unique physical features such as the presence or absence of a placental barrier and the gradual development of the blood--brain barrier influence chemical disposition and thus modulate developmental neurotoxicity. Neonatal exposure may depend on maternal pharmacokinetic processes and transfer of the xenobiotic through the milk, although direct exposure may occur through other routes (e.g., inhalation). Measurement of the xenobiotic in milk and evaluation of biomarkers of exposure or effect following exposure can confirm or characterize neonatal exposure. Physiologically based pharmacokinetic and pharmacodynamic models that incorporate these and other determinants can estimate tissue dose and biologic response following in utero or neonatal exposure. These models can characterize dose--response relationships and improve extrapolation of results from animal studies to humans. In addition, pharmacologic data allow an experimenter to determine whether exposure to the test chemical is adequate, whether exposure occurs during critical periods of nervous system development, whether route and duration of exposure are appropriate, and whether developmental neurotoxicity can be differentiated from direct actions of the xenobiotic.