Overview of a workshop on quantitative models for developmental toxicity risk assessment

An Overview of the U.S. EPA Proposed Amendments to the Guidelines for the Health Assessment of Suspect Developmental Toxicants

The U.S. Environmental Protection Agency has recently proposed amendments to the Guidelines for the Health Assessment of Suspect Developmental Toxicants. These amendments expand and clarify points made in the original guidelines, and add new information based on advances in the field. For example, the original risk assessment guidance was developed around several basic assumptions that were implicit in the earlier document, but that are clearly stated in the proposed amendments. Also, several consensus workshops were held following the completion of the 1986 guidelines, and the conclusions of these workshops have been incorporated. These include workshops dealing with the relationship of maternal and developmental toxicity, and with the development of an approach for a weight-of-evidence classification. In addition, a reference dose for developmental toxicity (RfDT) is proposed, based on short-term exposure, to distinguish it from the Rf D for chronic exposure. Other proposed changes include the expansion of the functional developmental toxicity section to reflect the Agency's testing guidelines for developmental neurotoxicity, and the human studies section which now gives more guidance on the use of human data in risk assessment. A number of other minor proposed changes are discussed. The final amended guidelines are currently undergoing Agency review and should be completed within the next year.

A biologically-based dose-response model for developmental toxicology

The methods currently used to evaluate the risk of developmental defects in humans from exposure to potential toxic agents do not reflect biological processes in extrapolating estimated risks to low doses and from test species to humans. We develop a mathematical model to describe aspects of the dynamic process of organogenesis, based on branching process models of cell kinetics. The biological information that can be incorporated into the model includes timing and rates of dynamic cell processes such as differentiation, migration, growth, and replication. The dose-response models produced can explain patterns of malformation rates as a function of both dose and time of exposure, resulting in improvements in risk assessment and understanding of the underlying mechanistic processes. To illustrate the use of the model, we apply it to the prediction of the effects of methylmercury on brain development in rats.

Developmental toxicity risk assessment: consensus building, hypothesis formulation, and focused research

Risk assessment for developmental toxicity has become more defined over the last decade and the most recent EPA guidelines for developmental toxicity risk assessment were published in 1991. Development of approaches for risk assessment in this area have relied on building of consensus opinion among experts about the interpretation of developmental toxicity data. These discussions have aided in strengthening the scientific basis for risk assessment for developmental toxicity, including the default assumptions that must be used when more complete information on mechanisms of action and pharmacokinetics are unavailable. Such discussions continue on both a national and international basis. The EPA risk assessment guidelines outlined several major areas of research needed to strengthen risk assessment for developmental toxicity and led to the formulation of hypotheses to be tested in focused research projects. Several major research efforts have focused on dose-related mechanisms and biologically based modeling of specific agents, physiologically based pharmacokinetic models of pregnancy that can be scaled across species, and the influence of dose and duration of exposure on developmental outcomes, as well as the delineation of specific biomarkers of adverse developmental effects. Although such research initiatives will require a long-term effort, it is important that attention be focused now on those approaches that can improve the risk assessment process to avoid continued reliance on default approaches that have been used for the past 30 years or more. The impact of the EPA Guidelines for Developmental Toxicity Risk Assessment has been widespread. One of the main purposes for developing risk assessment guidelines by EPA was to communicate procedures to risk assessors inside the agency in an effort to promote consistency. In addition, the guidelines were to communicate to those outside the agency in private industry and other regulatory agencies how EPA would evaluate and interpret data. The developmental toxicity guidelines have standardized terminology used in developmental toxicity risk assessments. They also have influenced the revision of testing guidelines and the writing of new guidelines, such as the developmental neurotoxicity testing guidelines. More extensive use of developmental toxicity data within the RfD/RfC process was influenced by the guidelines, as well. More recently, the risk assessment guidelines have had a major impact on the harmonization of international testing and risk assessment guidelines through cooperative efforts with the World Health Organization under the auspices of the International Program on Chemical Safety (IPCS) and the Organization for Economic Cooperation and Development (OECD). We foresee these types of interagency and international cooperative efforts continuing as we attempt to make more efficient use of the limited resources available for testing, risk assessment, and research to better understand and prevent developmental toxicity.

Reproductive Health Hazards in the Workplace: Guidelines for Policy Development and Implementation

To provide occupational health professionals and employers with guidelines for reproductive health policy making, the authors-both of whom are obstetrician-gynecologists and occupational medicine physicians-review the history of and legal basis for U.S. policy on work and reproduction, delineate the goals and important elements of a reproductive health policy (RHP), and emphasize the need to incorporate an RHP into a comprehensive occupational injury and illness prevention program. After suggesting practical ways to achieve the goals of an RHP (maximal health protection, compliance with legal mandates, and reduction of liability), the authors propose a team-based model for RHP implementation with shared responsibility of management and workers in designing, enforcing, and evaluating the policy.

Approaches to evaluating reproductive hazards and risks

Development of approaches to risk assessment for reproductive toxicity has aided in the critical evaluation of the scientific basis for interpretation of data and the description of assumptions underlying the process. In addition, it has helped to standardize, to the extent possible, the use of qualitative and quantitative data in the hazard identification and dose-response processes and to identify research needed to fill gaps in the available database. The standard study protocols for evaluating reproductive and developmental hazards include developmental toxicity studies and both short-term and longer-term reproductive studies. These study protocols have been in use for several decades, but development of risk assessment approaches has prompted the recommendation of additional end point measures to these protocols. These include evaluation of specific neurologic and behavioral function of offspring following prenatal and postnatal exposure, evaluation of sperm production and quality, reproductive organ weights, and more in-depth testicular histopathology in males, as well as measures of age at vaginal opening, vaginal cytology, oocyte toxicity, time to mating, gestation length, and reproductive organ weights in females. Current approaches to risk assessment in reproductive toxicity involve the determination of a no-observed-adverse-effect level (NOAEL) and the application of uncertainty factors (UFs) to account for differences between the experimental animal species and humans, variability in sensitivity within the human population, and other factors as necessary to derive the reference dose (RfD), or a specified RfD for developmental toxicity to account for the short period of exposure required.(ABSTRACT TRUNCATED AT 250 WORDS)

Sites of female reproductive vulnerability: implications for testing and risk assessment

All reproductive toxicity testing, whether in vivo or in vitro, should be conducted with consideration of the ultimate use of the data for reproductive risk assessment and the protection of human reproductive health. In this review selected sites of vulnerability in the female reproductive system are identified. These sites of female reproductive vulnerability may be utilized for in vitro toxicity testing assays, and the data from assays may be employed in the hazard identification and hazard characterization steps necessary for reproductive risk assessment. Using biomarkers of female reproductive function derived from in vitro toxicity testing it is possible to define functions that characterize female fecundity. These characterizations of female fecundity may be used to quantitate reproductive risk in human populations.

Comparing alternative approaches to establishing regulatory levels for reproductive toxicants: DBCP as a case study

This paper compares four alternative approaches for deriving regulatory levels for reproductive toxicants by applying them to the available data on the human spermatotoxicant 1,2-dibromo-3-chloropropane (DBCP). The alternatives examined include the Proposition 65 approach (application of a mandatory 1000-fold uncertainty factor to a no-observed-adverse-effect level [NOAEL]), the Environmental Protection Agency (EPA) approach (application of flexible uncertainty factors to a NOAEL), the Benchmark Dose approach (application of flexible uncertainty factors to a dose associated with a known level of change in a reproductive parameter), and the Quantitative Risk Estimation approach (using low-dose linear extrapolation and a model of the relationship between sperm count and infertility). Applied to DBCP, these approaches do not produce substantially different estimates of allowable exposure levels. However, the approaches do have different data requirements and provide different amounts of information on reproductive hazards to risk managers and the public. Neither the Proposition 65 nor the EPA approach provides information about the extent of health risk remaining at a regulatory level. In contrast, the Benchmark Dose approach can provide estimates of the magnitude of sperm count reduction at a regulatory level, and the Quantitative Risk Estimation approach can provide estimates of exposure-induced infertility.

Characterization of a developmental toxicity dose-response model

The Rai and Van Ryzin dose-response model proposed for teratology experiments has been characterized for its appropriateness and applicability in modeling the dichotomous response data from developmental toxicity studies. Modifications were made in the initial probability statements to reflect more accurately biological events underlying developmental toxicity. Data sets used for the evaluation were obtained from the National Toxicology Program and U.S. EPA laboratories. The studies included developmental evaluations of ethylene glycol, diethylhexyl phthalate, di- and triethylene glycol dimethyl ethers, and nitrofen in rats, mice, or rabbits. Graphic examination and statistical evaluation demonstrate that this model is sensitive to the data when compared to directly measured experimental outcomes. The model was used to interpolate to low-risk dose levels, and comparisons were made between the values obtained and the no-observed-adverse-effect levels (NOAELs) divided by an uncertainty factor. Our investigation suggests that the Rai and Van Ryzin model is sensitive to the developmental toxicity end points, prenatal deaths, and malformations, and appears to model closely their relationship to dose.