Statistical methods for analyzing developmental toxicity data

Analysis of incidence data in developmental toxicity studies: Statistical tests to account for litter effects in fetal defect data

BACKGROUND

When analyzing fetal defect incidence in laboratory animal studies, correlation in responses within litters (i.e., litter effects) can lead to increased false-positive rates if litter effects are not incorporated into the analysis. Studies of fetal defects require analysis methods that are robust across a broad range of defect types, including those with zero or near-zero incidence rates in control groups.

METHODS

A simulation study compared power and false-positive rates for six approaches across a range of background defect rates and litter size distributions. Statistical methods evaluated included ignoring the litter effect as well as parametric and nonparametric approaches based on litter proportions, generalized linear mixed models (GLMMs), the Rao-Scott Cochran-Armitage (RSCA) trend test, and a modification to the RSCA (mRSCA) introduced here to improve estimation at low background rates. These methods were also applied to a common and a rare defect from two prenatal developmental toxicology studies conducted by the National Toxicology Program (NTP).

RESULTS

At background defect rates of 1%, the mRSCA and parametric litter proportion methods provided gains in power over the nonparametric litter proportion method, the GLMM method, and the RSCA method. Simulations involving litter loss in high-dose groups showed loss of power for both litter proportion methods.

CONCLUSIONS

The mRSCA test developed here compares favorably with other litter-based approaches and is robust across a range of background defect rates and litter size distributions, making it a practical choice for prenatal developmental toxicology studies involving both common and rare fetal defects.

Unnecessary use of additional animals for determination of sexual maturation in the EOGRTS

The Extended-One-Generation Study [EOGRTS, OECD 443] is a study in which the toxic effects of test substances on reproduction (Cohort 1), neurodevelopment (Cohort 2), and development of the immune system (Cohort 3) in rats are evaluated. The latter two Cohorts are not always required according to the European Chemicals Agency (ECHA) based on data from previously performed toxicity studies. Although the Cohorts for developmental neurotoxicity (DNT) and developmental immunotoxicity (DIT) are often omitted, the F₁-animals normally required for these Cohorts are still maintained for evaluation of sexual maturation since three F₁-animals/sex/litter/group are required according to OECD Guidance Document (GD) No. 151. This review investigates whether two F₁-animals/sex/litter/group would suffice for this endpoint by investigating the rationale provided by the GD and by comparing results of eighteen EOGRTSs in which three versus two F₁-animals/sex/litter/group were evaluated. After a comprehensive literature research, we concluded that the rationale in the GD does not substantiate the decision to use three F₁-animals/sex/litter/group. The scientific papers provided as rationale focused on male observations and the observations discussed do not match the observations for sexual maturation mentioned by the guidelines. The investigation using data from eighteen EOGRTSs showed that the toxicological conclusions, whether the test substance affected sexual maturation or not, matched when comparing data of two F₁-animals/sex/litter/group to three F₁-animals/sex/litter/group. To conclude, two F₁-animals/sex/litter/group would suffice for the evaluation of sexual maturation, which negates the requirement for a so called "Cohort 1 C" (i.e. 160 animals (80 males and 80 females)) per EOGRTS, as well as the number of regulated procedures that need to be performed.

A new regression model for overdispersed binomial data accounting for outliers and an excess of zeros

Binary outcomes are extremely common in biomedical research. Despite its popularity, binomial regression often fails to model this kind of data accurately due to the overdispersion problem. Many alternatives can be found in the literature, the beta-binomial (BB) regression model being one of the most popular. The additional parameter of this model enables a better fit to overdispersed data. It also exhibits an attractive interpretation in terms of the intraclass correlation coefficient. Nonetheless, in many real data applications, a single additional parameter cannot handle the entire excess of variability. In this study, we propose a new finite mixture distribution with BB components, namely, the flexible beta-binomial (FBB), which is characterized by a richer parameterization. This allows us to enhance the variance structure to account for multiple causes of overdispersion while also preserving the intraclass correlation interpretation. The novel regression model, based on the FBB distribution, exploits the flexibility and large variety of the distribution's possible shapes (which includes bimodality and various tail behaviors). Thus, it succeeds in accounting for several (possibly concomitant) sources of overdispersion stemming from the presence of latent groups in the population, outliers, and excessive zero observations. Adopting a Bayesian approach to inference, we perform an intensive simulation study that shows the superiority of the new regression model over that of the existing ones. Its better performance is also confirmed by three applications to real datasets extensively studied in the biomedical literature, namely, bacteria data, atomic bomb radiation data, and control mice data.

Genotoxicity as a toxicologically relevant endpoint to inform risk assessment: A case study with ethylene oxide

The purpose of the present investigation is to analyze the in vivo genotoxicity dose-response data of ethylene oxide (EO) and the applicability of the derived point-of-departure (PoD) values when estimating permitted daily exposure (PDE) values. A total of 40 data sets were identified from the literature, and benchmark dose analyses were conducted using PROAST software to identify a PoD value. Studies employing the inhalation route of exposure and assessing gene or chromosomal mutations and chromosomal damage in various tissues were considered the most relevant for assessing risk from EO, since these effects are likely to contribute to adverse health consequences in exposed individuals. The PoD estimates were screened for precision and the values were divided by data-derived adjustment factors. For gene mutations, the lowest PDE was 285 parts per trillion (ppt) based on the induction of lacI mutations in the testes of mice following 48 weeks of exposure to EO. The corresponding lowest PDE value for chromosomal mutations was 1,175 ppt for heritable translocations in mice following 8.5 weeks of EO exposure. The lowest PDE for chromosomal aberrations was 238 ppt in the mouse peripheral blood lymphocytes following 48 weeks of inhalation exposure. The diverse dose-response data for EO-induced genotoxicity enabled the derivation of PoDs for various endpoints, tissues, and species and identified 238 ppt as the lowest PDE in this retrospective analysis.

A Reproductive and Developmental Toxicity Study in CD Rats of LY275585, [Lys(B28),Pro(B29)]-Human Insulin

LY275585, [Lys(B28),Pro(B29)]-human insulin, was administered daily by subcutaneous injection at doses of 0, 1, 5, or 20 U/kg. Male rats were treated with LY275585 beginning 2 weeks prior to cohabitation and throughout the mating period. Females assigned to the teratology component of the study were treated for 2 weeks prior to cohabitation with the males and through gestation day (GD) 19. These dams were killed on GD 20 and uterine and fetal examinations were performed. Female rats assigned to the delivery component were treated for 2 weeks prior to cohabitation through postpartum day (PD) 20. Dams were allowed to deliver and maintain their progeny through a 21-day lactation period. After weaning, 1 pup/sex/litter was assigned to the F1 generation, and these animals received no treatment. Survival, growth, behavior, and reproductive performance were evaluated, and reproductive organs were collected for histological evaluation. Treatment of F0 male and female rats with LY275585 resulted in isolated incidences of severe hypoglycemia at 5 and 20 U/kg/day and some modest changes in food consumption and body weight measures at all treatment levels. These changes were anticipated and attributed to the pharmacology of this compound. Mating and fertility of the F0 animals were unaffected by treatment. While slight decreases in fetal body weights and increased fetal runts/litter were observed in the 20-U/kg/day group, PD 1 progeny weights were not affected in the delivery component, and there was no indication of teratogenicity in this study. There were no remarkable treatment-related effects on offspring growth patterns, survival, or reproductive performance, but the F1 animals from the 20-U/kg/day treatment-derived group were more reactive than controls in the startle habituation test. Thus, F0 parental toxicity, related to the hypoglycemic action of LY275585, was found at all doses. A dose of 5 U/kg/day was considered a no-adverse-effect level for developmental toxicity. There were no remarkable effects of LY275585 treatment on F0 or F1 generation reproductive performance at 20 U/kg/day, the highest dose tested in this study.

Effects of n-butylparaben on steroidogenesis and spermatogenesis through changed E₂ levels in male rat offspring

Parabens are widely used as antibacterial agents, which are concerned recently in the relationship between the use of parabens and reproductive toxicity. So that reassessment of the risk of parabens is needed. In this study, one of parabens, n-butylparaben (n-BP) was orally administered to pregnant Wistar rats (0, 64, 160, 400 and 1000 mg/kg/day) from gestation day (GD) 7 through postnatal day (PND) 21. Reduced anogenital distance (AGD) and delayed preputial separation (PPS) were observed in the male offspring. The weights of the testes were significantly reduced at PND 21-90. The weights of the epididymides were significantly reduced at all monitoring points, except PND 35. Seminal vesicle weights were significantly reduced on PND 21. Serum testosterone (T) was significantly decreased, especially on PND 49. The levels of 17β-estradiol (E2) showed an increase at each of the tested points except on PND 180. Serum luteinising hormone (LH) and follicle-stimulating hormone (FSH) levels in the n-BP treated groups were lower on PND 21, 35 and 49 but elevated on PND 90 compared to control levels. n-BP reduced epididymal cauda sperm counts and daily sperm production in a dose-dependent manner; this difference was statistically significant at exposure groups of 400 and 1000 mg/kg/day. The present study strongly suggests that exposure to n-BP in utero and during lactation has adverse effects on the reproductive system in male offspring, with a no observed adverse effect level (NOAEL) of 160 mg/kg/day. To our knowledge, this is the first study that reports increased E2 levels of male rats following n-BP exposure; we suggest that E2 levels may be considered as biomarkers for some endocrine disrupting chemicals (EDCs).

Cerebral gene expression and neurobehavioural development after perinatal exposure to an environmentally relevant polybrominated diphenylether (BDE47)

Nutrients in seafood are known to be beneficial for brain development. Effects of maternal exposure to 2,2',4,4' tetrabromo diphenylether (BDE47) was investigated, alongside the potential ameliorating impact of seafood nutrients, through assessment of neurobehaviour and gene expression in brain and liver. Developing mice were exposed during gestation and lactation via dams dosed through casein- or salmon-based feed, spiked with BDE47. Two concentrations were used: a low level (6 μg/kg feed) representing an environmentally realistic concentration and a high level (1,900 μg/kg feed) representing a BDE47 intake much higher than expected from frequents consumption of contaminated seafood. Experimental groups were similar with respect to reproductive success, growth and physical development. Minor, transient changes in neurobehavioural metrics were observed in groups given the highest dose of BDE47. No significant differences in behaviour or development were seen on postnatal day 18 among maternally exposed offspring. Cerebral gene expression investigated by microarray analyses and validated by RT-qPCR showed low fold changes for all genes, despite dose-dependent accumulation of BDE47 in brain tissue. The gene for glutamate ammonia ligase was upregulated compared to control in the casein-based high BDE47diet, suggesting potential impacts on downstream synaptic transmission. The study supported a previously observed regulation of Igfbp2 in brain with BDE47 exposure. Genes for hepatic metabolic enzymes were not influenced by BDE47. Potential neurotoxic effects and neurobehavioural aberrations after perinatal exposure to high levels of BDE47 were not readily observed in mice pups with the present experimental exposure regimes and methods of analysis.

Fish consumption reduces transfer of BDE47 from dam to murine offspring

Fish and seafood are important contributions to a healthy diet, but also contain persistent organic pollutants (POPs) like polybrominated diphenylethers (PBDEs) and polychlorinated biphenyls (PCBs). Discrepancies have been found between intake and accumulated levels of POPs, where fish consumers have had similar levels of POPs to the general population. Similarly fish oil consumption has been found to reduce accumulation of POPs. This study examined the accumulation of BDE47 or PCB153 in mice fed diets with different nutritional composition, using female mice with pre-weanling pups exposed through gestation and lactation. A fish-based diet was compared to a standard casein-based rodent diet. All diets had low background levels of environmental contaminants and were spiked with BDE47 or PCB153 to levels representing a realistic (∼0.004 μmol kg bw(-1) d) or a high dietary exposure (∼1.3 μmol kg bw(-1) d). Accumulation of BDE47 or PCB153 in offspring tissues after 18d lactation reflected the maternal exposure levels. However, the pups of dams fed a fish-based diet had consistently lower BDE47 accumulation in liver, fat and stomach than pups from casein-fed dams. Similarly the pups of dams fed a high dose of PCB153 in a fish diet also accumulated less PCB153 than pups of the dams fed a casein diet, although not significant. In conclusion, the fish based diets seemed to reduce transfer of BDE47 and PCB153 from dams to pups. The study highlights that in-depth knowledge about nutritional impact on toxicokinetics is of great interest to vulnerable consumers.

Long-term effects of environmentally relevant doses of 2,2',4,4',5,5' hexachlorobiphenyl (PCB153) on neurobehavioural development, health and spontaneous behaviour in maternally exposed mice

Background

Polychlorinated biphenyls (PCBs) are widespread in the environment, human food and breast milk. Seafood is known to contain nutrients beneficial for the normal development and function of the brain, but also contaminants such as PCBs which are neurotoxic. Exposure to non-coplanar PCBs during brain development can disrupt spontaneous behaviour in mice and lead to hyperactive behaviour. Humans are chronically exposed to the highest relative levels of organochlorines in early childhood during brain development, though usually at doses which do not give clinical symptoms of toxicity. This study aimed to elucidate the developmental and behavioural effects of 2,2',4,4',5,5' hexachlorobiphenyl (PCB153) in mice, mimicking human exposure during gestation and lactation.

Methods

Environmentally relevant doses of PCB153 were added to the experimental diets. Feed concentrations were approximately 0.5, 6.5, and 1500 μg PCB153/kg feed, representing a realistic and a worst case scenario of frequent consumption of contaminated fish. The study also investigated the effects of maternal nutrition, i.e. a standard rodent diet versus a high inclusion of salmon. Mice pups were examined for physical- and reflex development, sensorimotor function and spontaneous behaviour from five days after birth until weaning. A selection of pups were followed until 16 weeks of age and tested for open field behaviour and the acoustic startle response (ASR) with prepulse inhibition (PPI). Blood thyroid hormones and liver enzymes, blood lipids and PCB153 content in fat were examined at 16 weeks. Statistical analyses modelled the three way interactions of diet, PCB exposure and litter size on behaviour, using generalized linear models (GLM) and linear mixed effect models (LME). The litter was used as a random variable. Non-parametric tests were used for pair wise comparisons of biochemical analyses.

Results

Litter size consistently influenced pup development and behaviour. Few lasting PCB153 related changes were observed, but results indicated effects on synchronization of physical development. Perinatal PCB153 exposure appeared to reduce habituation and cause aggression in males, though not statistically significant.

Conclusions

Litter size and maternal diet influenced physical development and function more than PCB153 in perinatally exposed mouse pups and supports the developmental importance of maternal care and the social environment.

Perinatal/postnatal study of D-003, a mixture of long-chain fatty acids, in rats

D-003 is a mixture of long-chain fatty acids isolated and purified from sugar cane wax with cholesterol-lowering and antiplatelet effects. In order to further characterize the developmental toxicity during the treatment period from late gestation up to weaning of the offspring, pregnant females received 0 (control), 500, and 1,000 mg/kg/day D-003 daily by oral gavage beginning at day 15 of pregnancy and through gestation until day 21 postpartum. Maternal clinical signs, body weight, and food intake were measured at regular intervals during gestation and lactation. Live pups were weighed, sexed, and examined for developmental signs. One female and male of each litter were randomly selected to evaluate the reproductive potential. There were no spontaneous or dose-related maternal deaths during the course of this study. The general health and behavioral condition of offspring was good in all groups. No significant differences among groups were found in comparisons of litter size, survival through the weaning period, sex ratio, and male and female weights. This peri- and postnatal study conducted with D-003 in rats indicated that treatment of the dam during late gestation and lactation did not show adversely effects on reproductive performance or fetal development over two generations.

Evaluation of the reproductive and developmental toxicity of the D-003, a mixture of long-chain fatty acids, in rats and rabbits

D-003 is a mixture of long-chain fatty acids isolated and purified from sugar cane wax with cholesterol-lowering properties. D-003 given orally (500 and 1000 mg/kg/day) to female rats for 15 days prior to mating, through mating and gestation to day 21 of lactation and male rats for 4 weeks prior and during mating did not induce toxic effects on reproduction. There were no significant reductions in the number of animals that conceived, in the numbers of pups born to those that did conceive, in the numbers of pups that survived until weaning, and in their body weights at weaning. Drug-treated and control groups' offspring were comparable in growth, physical and behavioral development, spontaneous activity and reproductive performance. Pregnant New Zealand rabbits were given D-003 as oral doses of 500 and 1000 mg/kg/day on days 6 through 18 of gestation without any evidence of embryotoxicity or teratogenicity. The no-observed-effect dose in these two experimental studies was 1000 mg/kg/day. After assessment of the potential of high doses of D-003 to act on developing embryo and reproduction process, no evidence supports the conclusion that D-003 is a reproductive and developmental toxicant/teratogen.

Sexual maturation data for Crl Sprague-Dawley rats: criteria and confounding factors

BACKGROUND

Considerable concern exists in the scientific community regarding potential effects of endocrine disruptive or modulating environmental agents on male and female reproductive development and capacity. Existing data show that in utero and postnatal exposure of rodents to endocrine modulating chemicals can influence the timing and progression of sexual differentiation and/or maturation (e.g., balanopreputial separation and vaginal opening).

METHODS

Sexual maturation data from various types of littering studies using International Gold Standard (IGS) Crl Sprague-Dawley rats were evaluated for consistency with both historical observations and published values from other laboratories. In addition, litters from two developmental neurotoxicology studies were statistically analyzed to identify whether increasing the numbers of pups per litter evaluated affected the interpretation of sexual maturation data sets.

RESULTS

Control values for preputial separation and vaginal opening ages ranged from PD 45.0 to 48.0 and from PD 32.0 to 34.0, respectively, regardless of the number of pups evaluated per litter. However, statistically significant delays in sexual maturation were present when three rats/sex/litter were evaluated that were not present when only one randomly selected rat/sex/litter was evaluated.

CONCLUSIONS

Standardized procedures and criteria are required to provide consistent intra-laboratory values and reduce inter-laboratory differences in sexual maturation observations. When such criteria are used, these endpoints provide sensitive measures for detecting alterations in sexual maturation. However, our analyses demonstrate that the ability to detect statistically significant and biologically important differences in these endpoints is sometimes impaired by the currently common practice of evaluating only one randomly selected rat/sex/litter. Evaluation of three rats/sex/litter improved the sensitivity of the statistical analysis in detection of treatment-related effects and reduced the probability of identifying a false negative result.

Recommendations for statistical designs of in vivo mutagenicity tests with regard to subsequent statistical analysis

A workshop was held on September 13 and 14, 1993, at the GSF, Neuherberg, Germany, to start a discussion of experimental design and statistical analysis issues for three in vivo mutagenicity test systems, the micronucleus test in mouse bone marrow/peripheral blood, the chromosomal aberration tests in mouse bone marrow/differentiating spermatogonia, and the mouse dominant lethal test. The discussion has now come to conclusions which we would like to make generally known. Rather than dwell upon specific statistical tests which could be used for data analysis, serious consideration was given to test design. However, the test design, its power of detecting a given increase of adverse effects and the test statistics are interrelated. Detailed analyses of historical negative control data led to important recommendations for each test system. Concerning the statistical sensitivity parameters, a type I error of 0.05 (one tailed), a type II error of 0.20 and a dose related increase of twice the background (negative control) frequencies were generally adopted. It was recommended that sufficient observations (cells, implants) be planned for each analysis unit (animal) so that at least one adverse outcome (micronucleus, aberrant cell, dead implant) would likely be observed. The treated animal was the smallest unit of analysis allowed. On the basis of these general consideration the sample size was determined for each of the three assays. A minimum of 2000 immature erythrocytes/animal should be scored for micronuclei from each of at least 4 animals in each comparison group in the micronucleus assays. A minimum of 200 cells should be scored for chromosomal aberrations from each of at least 5 animals in each comparison group in the aberration assays. In the dominant lethal test, a minimum of 400 implants (40-50 pregnant females) are required per dose group for each mating period. The analysis unit for the dominant lethal test would be the treated male unless the background frequency of dead implants (DI) is so low that multiple males would need to be integrated to meet the minimum observation of one adverse outcome (DI) per analysis unit. A three-step strategy of data analysis was proposed for the cytogenetic assays. Use of negative historical controls was allowed in certain circumstances for interpretation of results from micronucleus tests and chromosomal aberration tests.

The selective estrogen receptor modulator, raloxifene: segment II studies in rats and rabbits

Raloxifene is a nonsteroidal, selective estrogen receptor modulator developed by Eli Lilly and Company primarily as a therapeutic agent for postmenopausal osteoporosis. Two Segment II studies were conducted that examined maternal reproductive parameters and fetal outcome following gestational exposure to raloxifene. Pregnant CD rats (25/group) and New Zealand white rabbits (20/group) were dosed once daily by oral gavage with 0, 0.1, 1, or 10 mg/kg on Gestation Days (GD) 6 through 17 and 7 through 19, respectively. Maternal body weight and food consumption were monitored throughout pregnancy. Caesarean sections were performed on GD 20 and GD 28 for rats and rabbits, respectively, to evaluate fetal viability, weight, and morphology. In rats, maternal body weight, body weight gain, and food consumption were reduced in all raloxifene treatment groups. Fetal viability was depressed in the 10-mg/kg group and was often associated with signs of hemorrhaging from the vagina. Fetal growth retardation was indicated in the 1- and/or 10-mg/kg groups by increased incidences of fetal runts and the developmental deviations, wavy ribs and kidney cavitation. There was no evidence of treatment-related malformations in rat fetuses. In rabbits, depressions in body weight gain and food consumption occurred in the 10-mg/kg group, and a single abortion occurred in the 1-mg/kg group. Fetal viability and weights were not affected in any of the raloxifene treatment groups. The overall proportions of fetuses with malformations, deviations, or variations were not affected by treatment with raloxifene; however, one fetus each from the 0.1-, 1-, and 10-mg/kg groups had incomplete closure of the interventricular septum. Therefore, maternal and fetal no-effect levels were not obtained in this study of raloxifene.

The selective estrogen receptor modulator, raloxifene: reproductive assessments following preimplantation exposure in mated female rats

Raloxifene is a nonsteroidal, selective estrogen receptor modulator being developed for postmenopausal osteoporosis. As part of an integrated reproductive toxicity assessment, two studies were conducted in which raloxifene was administered orally to CD rats during Gestation Days (GD) 0 through 5. In each study, animals received daily raloxifene doses of 0, 0.1, 1, or 10 mg/kg. In Study 1, GD 20 evaluations of maternal reproductive parameters identified dose-related increases in pre- and postimplantation loss, reductions in the numbers of corpora lutea and live conceptuses, and reduced fetal weight. The low fetal weights were consistent with an extent of morphologic development that corresponded to developmental ages up to 8 d younger than GD 20. Study 2 characterized the potential impact of this disrupted and apparently delayed implantation on gestation length, parturition, and progeny viability. Dams were allowed to deliver and rear their offspring through Postpartum Day 21. Gestation lengths were extended up to 1 week, and litter sizes were reduced in a dose-dependent manner. Nevertheless, parturition occurred normally and pup morphology, survival, and physical and behavioral development were unaffected.

The toxicological effects following the ingestion of chinook salmon from the Great Lakes by Sprague-Dawley rats during a two-generation feeding-reproduction study

A two-generation reproduction-feeding study was undertaken with Sprague-Dawley rats to ascertain the effects of ingesting chinook salmon fillets caught in the Credit River, which empties into Lake Ontario (LO), or in the Owen Sound region of Lake Huron (LH). Rats (30/sex/group) were randomly assigned to groups whose dietary protein consisted of casein and/or lyophilized salmon [Group 1: 20% casein (controls); Group 2: 15% casein + 5% LO salmon (LO-5%); Group 3: 20% LO salmon (LO-20%); Group 4: 15% casein + 5% LH salmon (LH-5%); Group 5: 20% LH salmon (LH-20%)]. After 70 days on test, the males and females were mated on a 1:1 basis within diet groups. Approximately 70 days postweaning, one F1 male and one F1 female from 24 litters were mated within diet groups, avoiding sibling matings. At weaning, the F0 and F1 adults and the F1 and F2 neonates not randomly selected for further testing were necropsied. Evaluated parameters included growth, feed consumption, organ weights, reproduction indices, serum chemistry, hematology, and coagulation times. The only statistically significant effects which were present in both generations were increased relative liver and kidney weights of both sexes in the LO-20% and LH-20% groups; the LH-20% females had lower alanine transaminase activity than the controls; the controls had lower creatinine levels than the fish groups and the LO-20% females; the LH-20% and LO-20% males had a lower blood urea nitrogen than the controls; and the LH-20% females had a heavier terminal body weight than the controls and a lower number of red blood cells, hematocrit, hemoglobin values, and mean platelet volume. There was a tendency for the fish-fed groups to grow faster, eat more feed, and have larger litters with heavier pups. Overall, there was little to suggest that the myriad of contaminants in chinook salmon from the Great Lakes presented an appreciable toxicological risk to Sprague-Dawley growth and reproduction.

Design of developmental toxicity studies for assessing joint effects of dose and duration

In the assessment of developmental and reproductive effects, the timing and duration of exposures to chemical compounds or other environmental contaminants are of particular interest, as the gestational cycle is known to have periods of increased sensitivity. The goal of this research is to identify optimal experimental designs for conducting developmental toxicity studies when the effects of both exposure level and duration of exposure are of interest. The elements of the study design considered in this evaluation are the allocation of animals to dose-duration exposure groups and the determination of the most efficient intermediate exposure levels. The optimality of various designs is assessed via the accuracy of the estimated excess risk as well as testing criteria. Simulation studies are conducted to compare these criteria and determine optimal design strategies under various underlying dose-response patterns. Asymptotic results are also derived to lend support to the simulation studies.

Estimation and hypothesis testing of treatment effects in animal reproductive toxicology studies

Healy (1) and Dempster et al. (8) proposed statistical methods to evaluate the treatment effects in animal reproductive toxicology research. Both methods assume homogeneous variance for the dams and the pups, respectively, in all the treatment groups. In this paper, via mixed effect modeling, we propose a method to estimate the treatment effects allowing heterogeneous variances for the dams and the pups, respectively, in different treatment groups. Covariates will also be included in the model. A procedure to test the fixed effects is also discussed. An example from an animal reproductive toxicological study is used to illustrate the procedures.

A comparison of methods for testing homogeneity of proportions in teratologic studies

We consider teratologic studies in which the aim is to compare the survival rate of animals in a treatment group to the corresponding rate in a control group. The design of such studies often involves the allocation of intact litters of animals to treatment, invalidating the application of standard statistical methods. We review the strengths and weaknesses of several approaches for dealing with this problem including methodology recently developed for the analysis of clustered binary data. A simulation study is conducted in which litter sizes are generated from a distribution having specified mean and degree of imbalance. It is recommended on the basis of this study and on theoretical considerations that the choice of method should depend on whether the comparison of interest is experimental or observational. For experimental comparisons, involving the random assignment of litters to different treatment groups, methods based on the adjustment of standard chi-square statistics are recommended unless the number of litters in each group is very large.

Assessing overdispersion and dose-response in the male dominant lethal assay

In dominant lethal studies the primary variables of interest are typically expressed as discrete counts or proportions (e.g., live implants, resorptions, percent pregnant). Simple statistical sampling models for discrete data such as binomial or Poisson generally do not fit this type of data because of extra-binomial or extra-Poisson departures from variability predicted under these simple models. Extra-variability in the fetal response may originate from parental contributions. These can lead to over- or under-dispersion seen as, e.g., extra-binomial variability in the proportion response. Utilizing a large control database, we investigated the relative impact of extra-variability from male or female contributions on the endpoints of interest. Male-related effects did not seem to contribute to overdispersion in our database; female-related effects were, however, evidenced. Various statistical methods were considered to test for significant treatment differences under these forms of sampling variability. Computer simulations were used to evaluate these methods and to determine which are most appropriate for practical use in the evaluation of dominant lethal data. Our results suggest that distribution-free statistical methods such as a nonparametric permutation test or rank-based tests for trend can be recommended for use.

Developmental response of zygotes exposed to similar mutagens

Exposure of mouse zygotes to ethylene oxide (EtO) or ethyl methanesulfonate (EMS) led to high incidences of fetal death and of certain classes of fetal malformations (Generoso et al., 1987, 1988; Rutledge and Generoso, 1989). These effects were not associated with induced chromosomal aberrations (Katoh et al., 1989) nor are they likely to be caused by gene mutations (Generoso et al., 1990). Nevertheless, the anomalies observed in these studies resemble the large class of stillbirths and sporadic defects in humans that are of unknown etiology, such as cleft palate, omphalocoel, clubfoot, hydrops and stillbirths (Czeizel, 1985; Oakley, 1986). Therefore, we continue to study the possible mechanisms relating to induction of these types of zygote-derived anomalies in mice. Effects of zygote exposure to the compounds methyl methanesulfonate (MMS), dimethyl sulfate (DMS), and diethyl sulfate (DES), which have similar DNA-binding properties as EtO and EMS, were studied. DMS and DES, but not MMS, induced effects that are similar to those induced by EtO and EMS. Thus, no site-specific alkylation product was identifiable as the critical target for these zygote-derived anomalies. We speculate that the developmental anomalies arose as a result of altered programming of gene expression during embryogenesis.