Gary L. Kimmel

Overview: Using mode of action and life stage information to evaluate the human relevance of animal toxicity data

A complete mode of action human relevance analysis—as distinct from mode of action (MOA) analysis alone—depends on robust information on the animal MOA, as well as systematic comparison of the animal data with corresponding information from humans. In November 2003, the International Life Sciences Institutes Risk Science Institute (ILSI RSI) published a 2-year study using animal and human MOA information to generate a four-part Human Relevance Framework (HRF) for systematic and transparent analysis of MOA data and information. Based mainly on non-DNA-reactive carcinogens, the HRF features a “concordance” analysis of MOA information from both animal and human sources, with a focus on determining the appropriate role for each MOA data set in human risk assessment. With MOA information increasingly available for risk assessment purposes, this article illustrates the further applicability of the HRF for reproductive, developmental, neurologic, and renal endpoints, as well as cancer. Based on qualitative and quantitative MOA considerations, the MOA/human relevance analysis also contributes to identifying data needs and issues essential for the dose-response and exposure assessment steps in the overall risk assessment.

Workshop on perinatal exposure to dioxin-like compounds. II. Reproductive effects.

This summary report focuses on current studies on reproductive effects reported at the workshop on Perinatal Exposure to Dioxin-like Compounds and supporting data noted in the discussion. Recent laboratory studies have suggested that altered development (e.g., low birth weight, spontaneous abortion, congenital malformation) and reproductive health (e.g., fertility, sex organ development, reproductive behavior) may be among the most sensitive end points when examining the effects of dioxinlike compounds. Thus, future research should target the reproductive health of both males and females exposed postnatally and prenatally. Studies in humans are needed and are on-going. In animal models, postnatal exposure to dioxin or dioxinlike compounds has been associated with abnormal spermatogenesis and abnormal testicular morphology and size in males and with reduced fertility and endometriosis in females. In utero exposure may also produce profound reproductive consequences in both males and females including delays in sexual maturation, abnormalities in development of sexual organs, and abnormal sexual behavior. The mechanism by which dioxin-like compounds cause reproductive effects is not well delineated.

Heat shock during rat embryo development in vitro results in decreased mitosis and abundant cell death.

Epidemiologic studies strongly suggest that in utero exposure to hyperthermia results in developmental defects in humans. Rats, mice, guinea pigs, and other species exposed to hyperthermia also exhibit a variety of developmental defects. Studies in our laboratory have focused on exposure to hyperthermia on Gestation Day (GD) 10 of rats in vivo or in vitro. Within 24 h after in vivo or in vitro exposure, delayed or abnormal CNS, optic cup, somite, and limb development can be observed. At birth, only rib and vertebral malformations are seen after hyperthermia on GD 10, and these have been shown to be due to alterations in somite segmentation. Unsegmented somites have been thought to result from a cell-cycle block in the presomitic mesoderm, from which somites emerge individually during normal development. In the present study, DNA fragmentation (terminal deoxynucleotidyl transferase (TdT) catalyzed fluorescein-12-dUTP DNA end-labelling), indicative of apoptotic cell death, and changes in cell proliferation were examined in vitro in 37 degrees C control and heat treated (42 degrees C for 15 min) GD 10 CD rat embryos. Embryos were returned to 37 degrees C culture following exposure and evaluated 5, 8, or 18 h later. A temperature-related increase in TdT labelled cells was observed in the CNS, optic vesicle, neural tube, and somites. Increased cell death in the presomitic mesoderm also was evident. Changes in cell proliferation were examined using the cell-specific abundance of proliferating cell nuclear antigen (PCNA) and the quantification of mitotic figures. In neuroectodermal cells in the region of the optic cup, a change in the abundance of PCNA was not apparent, but a marked decrease in mitotic figures was observed. A significant change in cell proliferation in somites was not detected by either method. These results suggest that acute hyperthermia disrupts embryonic development through a combination of inappropriate cell death and/or altered cell proliferation in discrete regions of the developing rat embryo. Furthermore, postnatal vertebral and rib defects following disrupted somite development may be due, in part, to abundant cell death occurring in the presomitic mesoderm.

Proceedings of the Workshop on One-vs Two-Generation Reproductive Effects Studies

The Workshop on One-vs Two-Generation Reproductive Effects Studies was held on October 21-22, 1987. It was organized by the USEPAs Office of Pesticides and Toxic Substances and Office of Research and Development and was supported by the Agencys Risk Assessment Forum. The purpose of the workshop was to address the central question: Is a single-generation reproductive effects study sufficient to assess the reproductive toxicity potential of chemicals that do not bioaccumulate? In response to this question, the panel of nine participants concluded that, by itself, a one-generation reproductive effects study is insufficient to identify all potential reproductive toxicants and that a two-generation study is needed for an adequate assessment. The panel did not support placing much importance on bioaccumulation as related to reproductive toxicity testing mainly because it is not the only critical factor that may account for effects in a second generation but not in the first. In studying broader issues, the panel accomplished several goals that hopefully may serve to direct future development in reproductive toxicity testing: (1) the purpose of a reproductive effects study was defined, (2) a minimal set of end points necessary for adequate evaluation of reproductive toxicity was determined, (3) some alternative reproductive effects test methods were recommended, (4) greater flexibility in choosing a test protocol on a case by case basis was encouraged, and (5) areas that need further research were identified.

Exposure-Duration Relationships: The Risk Assessment Process for Health Effects Other Than Cancer

AbstractThe development of the risk assessment process over the past 10 years not only has provided us with a framework to carry out an important regulatory function, but also has aided in the identification of major gaps in our knowledge and the scientific database. One of those gaps is the influence of the duration of exposure on the toxicity of an agent. Currently, approaches to estimating short-duration exposure limits are only generally defined, assuming a constant relationship between the level of an exposure and its duration with respect to the response that can be expected. Within developmental toxicology, even this type of exposure duration adjustment is generally not carried out, the risk assessment being based on the overall daily exposure, regardless of the actual timing, duration, or frequency of exposure or the physicochemical properties of the agent. This article summarizes several current approaches to estimating short-duration exposure limits and outlines preliminary model development tha...

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 Agencys 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.

Framework for Human Health Risk Assessment

The U.S. Environmental Protection Agency has recognized the need to develop a framework for human health risk assessment that puts a perspective on the approaches in practice throughout the Agency. In response, the Agencys Risk Assessment Forum has begun the long-term process of developing a framework for human health risk assessment. The framework will be a communication piece that will lay out the scientific basis, principles, and policy choices underlying past and current risk assessment approaches and will provide recommendations for integrating/harmonizing risk assessment methodologies for all human health endpoints.

On the Subject of Incorrect Terminology

To the Editor: A recent article was devoted to the subject of “incorrect terminology” as the basis for problems in developmental toxicology (Christian, 1993). As perceived by Christian, poor terminology has misguided the discipline over the past 25 years. Christian purports that calling a Food and Drug Administration (FDA) Segment I1 study a “teratology” study has led to an “exclusion of the study of the three other potential outcomes of embryonic exposure: death, growth retardation, and functional alteration.” On the basis of an anecdotal reference, Christian lays the blame for this catastrophe at the feet of Dr. James G. Wilson because of his failure to remove the term from the blackboard at a workshop where the experimental design for this study was apparently introduced. There are at least three major flaws in this premise. First, the experimental study design really is a “teratology” study. An FDA Segment I1 study specifies that “effort is concentrated on determining whether a drug has a potential for embryotoxicity and/or teratogenic effects” (emphasis added) (FDA, 1966). Exposure is limited to the period of prenatal development, characteristic for each species, when the organs of the embryo are in the formative stage (i.e., the period of organogenesis). This period was chosen because previous work had clearly indicated that it is during this time that congenital malformations can most readily be induced. Thus, contrary to the denunciation by Christian (1993), we do not believe that it is incorrect to identify an FDA Segment I1 study as a “teratology” study, although the term developmental toxicity, as well as many other designations, are also appropriate. Second, and far more egregious, is Christian’s conclusion that this terminology resulted in an exclusion of the study of other forms of embryotoxicity. The FDA guidelines, which according to Christian arose from the workshop in question, clearly specify the need to record the number and placement of dead fetuses and resorption sites and indicate that each fetus needs to be weighed individually. The fourth manifestation of prenatal injury, altered postnatal function, was not a clearly recognized potential outcome at the time of the workshop, but in subsequent years, Dr. Wilson was quite convinced that this manifestation, along with the other three, needed to be searched for in any estimation of adverse effects (Wilson, 1973, 1975). When the Segment I1 study is conducted in combination with the other two segments (I and 111), as intended, this series of studies does indeed evaluate many aspects of both reproductive and developmental toxicity. Third, if calling the FDA Segment I1 study a “teratology” study was a scientific misnomer of great impact, why did it take almost 2 decades before the issue was raised? The assessment of potential risk from exposure of pregnant women to drugs or environmental chemicals is not an exact science, but it was through the process of evaluating approaches to data interpretation and risk assessment (e.g., Kimmel et al., 1986; U.S. Environmental Protection Agency, 1986, 1991), that the inappropriate elevation of malformations as more important than other end points

Stress Proteins as Biomarkers of Toxicity

The development of more sensitive and predictive test methods to characterize the safety of drugs and chemicals is an important part of the missions of the U.S. Food and Drug Administration and the U.S. Environmental Protection Agency. One approach is to develop methodologies which would define biomarkers of exposure and toxicity. Biomarkers have been proposed to be used to: 1) identify potential hazards, 2) establish dose-response relationships, 3) estimate risk at low-dose exposures, 4) serve as short-term in vitro toxicity tests, and 5) improve risk assessment and risk management capabilities (Committee on Biological Markers, 1987).