Approaches to defining the relationship of maternal and developmental toxicity

Metabolic regulation of preimplantation embryo development in vivo and in vitro: Molecular mechanisms and insights

Understanding of embryonic development has led to the clinical application of Assisted Reproductive technologies (ART), with the resulting birth of millions of children. Recent developments in metabolomics, proteomics, and transcriptomics have brought to light new insights into embryonic growth dynamics, with implications spanning reproductive medicine, stem cell research, and regenerative medicine. The review explores the key metabolic processes and molecular pathways active during preimplantation embryo development, including PI3K-Akt, mTOR, AMPK, Wnt/β-catenin, TGF-β, Notch and Jak-Stat signaling pathways. We focused on analyzing the differences occurring in vitro as opposed to in vivo development and we discussed significant physiological and clinical implications.

Developmental Toxicity: Extrapolation Across Species

There is a pervasive lack of understanding about the definition of a developmental toxicant and the experimental design necessary to differentiate between maternal toxicants and those uniquely, i.e., selectively, hazardous to the conceptus. Thus, the acceptance of the adult-to-developmental (A/D) ratio and its ability to identify those compounds uniquely hazardous to the conceptus has been compromised, as has the extrapolation of hazard across species. The failure to establish a shared understanding is discussed, together with the role of an in vitro screen for determining the A/D ratio. The addition of an environmental exposure factor permitting the extrapolation of hazard assessment across species and the estimation of potential human developmental toxicity is detailed.

Influence of maternal toxicity on the outcome of developmental toxicity studies

The relevance of fetal abnormalities noted at maternally toxic doses is a long-standing issue regarding the interpretation of findings of segment II studies. A number of diseases and conditions during pregnancy are known to adversely affect embryo/fetal development, and along this line many scientists believe that any marked disturbance of maternal homeostasis produced by chemical exposure may eventually produce a teratogenic effect. Although there is little doubt that developmental toxicity may be maternally mediated, the notion that, in principle, any maternal toxicity leads to birth defects is disputed. When embryotoxicity is noted only within the maternally toxic dose range, it is not possible to ascertain whether it is in fact maternally mediated or not (i.e., embryo development may have been impaired by a direct action of the chemical at doses that also adversely affect the mother; in these circumstances it would still be a selective developmental toxicant). However, currently, a chemical is not regarded as a "developmental toxicant" (or "teratogenic agent") if embryotoxicity is apparent only at doses that are also toxic to the mother. In the European Union, developmental hazard identification exerts a strong influence on the classification and labeling of chemicals. In Brazil, registration of any pesticide that proved to be teratogenic in animal studies is strictly forbidden by law (Pesticide Law, Federal Law 7.802, 1989). Therefore, interpretation of findings from developmental toxicity studies in light of maternal toxicity is particularly relevant to regulatory agencies, and becomes even more important when labeling or cutoff decision-making criteria are adopted regarding teratogenicity.

Combined Treatment Potentiates the Developmental Toxicity of Ibuprofen and Acetazolamide in Rats

Aspirin (ASA), an irreversible cyclooxygenase (COX) inhibitor, induces ventricular septal defect (VSD) and diaphragmatic hernia (DH) in rat fetuses when administered on gestation days (GDs) 9-10, a critical period for cardiovascular (CV) and midline development. Evaluation of a spectrum of nonsteroidal antiinflammatory drugs (NSAIDs; reversible COX inhibitors) showed that while some NSAIDs induced VSD in rats, none of the NSAIDs evaluated produced DH. In addition to inhibiting COX, ASA also inhibits carbonic anhydrase. The purpose of this study was to determine whether concurrent inhibition of COX and carbonic anhydrase would produce a teratogenic profile that includes both VSD and DH. To inhibit both COX and carbonic anhydrase, ibuprofen (COX inhibitor) and acetazolamide (carbonic anhydrase inhibitor) were coadministered on GDs 9-10. Groups of 20 female Crl:CD(SD)IGS BR rats were given either 300 mg kg(-1) day(-1) ibuprofen, 1000 mg kg(-1) day(-1) acetazolamide, or both (combination of ibuprofen and acetazolamide). Fetuses were evaluated on GD 21 for external and visceral development. Ibuprofen induced VSD in 3.7% of fetuses per litter; no defects in appendicular skeletal development were noted. Acetazolamide induced VSD in 5.9% of the fetuses per litter and appendicular defects in 41% of the fetuses per litter. Coadministration of ibuprofen and acetazolamide produced VSD in 18.7% of the fetuses per litter and appendicular defects in 77% of the fetuses per litter; however, there were no DH. Therefore, while concurrent inhibition of COX and carbonic anhydrase did not produce DH, potentiation was noted for the induction of VSD and appendicular anomalies.

Effects of deoxynivalenol (DON, vomitoxin) on in utero development in rats

Deoxynivalenol (DON, vomitoxin), is one of the most common contaminants of cereal grains world-wide. The effects of DON on fetal development were assessed in Charles River Sprague-Dawley rats. Pregnant female rats were gavaged once daily with DON at doses of 0, 0.5, 1, 2.5, or 5 mg/kg body weight on gestation days (GD) 6-19. At cesarean section on GD 20, reproductive and developmental parameters were measured. All females survived to cesarean section. DON caused a dose-related increase in excessive salivation by the pregnant females, a reaction probably linked to the lack of emetic reflex in rats. At 5 mg/kg, feed consumption and mean body weight gain were significantly decreased throughout gestation, mean weight gain (carcass weight), and gravid uterine weight were significantly reduced, 52% of litters (12/23) were totally resorbed, the average number of early and late deaths per litter was significantly increased, average fetal body weight and crown-rump length were significantly decreased, the incidence of runts was significantly increased, and the ossification of fetal sternebrae, centra, dorsal arches, vertebrae, metatarsals, and metacarpals was significantly decreased. At 2.5 mg/kg, DON significantly decreased average fetal body weight, crown-rump length, and vertebral ossification. These effects may be secondary to maternal toxicity and the reduced size of the fetuses. The incidence of misaligned and fused sternebrae was significantly increased at 5.0 mg/kg. No adverse developmental effects were observed at 0.5 and 1.0 mg/kg. Dose-related increases in maternal liver weight-to-body weight ratios were observed in all treated groups (significant at 1, 2.5, and 5 mg/kg). The weight changes were correlated with dose-related cytoplasmic alterations of hepatocytes. The NOEL for maternal toxicity for this study is 0.5 mg/kg based on the dose-related increase in liver-body weight ratio at 1 mg/kg. The NOEL for fetal toxicity is 1 mg/kg based on the general reduction in fetal development at 2.5 and 5 mg/kg. DON is considered a teratogen at 5 mg/kg day in Sprague-Dawley rats based on the anomalous development of the sternebrae.

Inhibiting matrix metalloproteinases with prinomastat produces abnormalities in fetal growth and development in rats

BACKGROUND

Matrix metalloproteinases (MMPs) play key roles in remodeling of the extracellular matrix during embryogenesis and fetal development. The objective of this study was to determine the effects of prinomastat, a potent selective MMP inhibitor, on fetal growth and development.

METHODS

Prinomastat (25, 100, 250 mg/kg/day, p.o.) was administered to pregnant female Sprague-Dawley rats on gestational days (GD) 6-17. A Cesarian section was carried out on GD 20 and the fetuses were evaluated for viability and skeletal and soft tissue abnormalities.

RESULTS

Prinomastat treatment at the 250 mg/kg/day dose produced a decrease in body weight and food consumption in the dams. A dose-dependent increase in post-implantation loss was observed in the 100 and 250 mg/kg/day-dose groups, resulting in only 22% of the dams having viable litters for evaluation at the 250 mg/kg/day dose. Fetal skeletal tissue variations and malformations were present in all prinomastat treated groups and their frequency increased with dose. Variations and malformation in fetal soft tissue were also increased at the 100 and 250 mg/kg/day doses. Prinomastat also interfered with fetal growth of rat embryo cultures in vitro.

CONCLUSIONS

These data confirm that MMP inhibition has a profound effect on fetal growth and development in vivo and in vitro.

The effects of feed restriction during organogenesis on embryo-fetal development in the rat

BACKGROUND

Given the role of nutrition and body weight gain in normal development, pharmaceuticals intended to reduce appetite and promote weight loss will generate safety data that may be challenging to interpret. To aid with this, the effects of feed restriction and subsequent body weight reductions on embryo-fetal development were investigated in the rat.

METHODS

Groups of 20 timed pregnant female Sprague-Dawley rats were offered Certified Rodent Diet 5002 either ad libitum or in restricted amounts of 20, 15, 10, and 7.5 g/day from Gestation Day (GD) 6-17. Clinical signs, body weights, and food consumption were recorded. Cesarean sections were performed on GD 21 and fetuses were sexed, weighed, and examined for external, visceral, and skeletal development.

RESULTS

Mean maternal body weights at the end of the feed restriction period, GD 18, were reduced 0.87 x, 0.80 x, 0.69 x, and 0.63 x control mean in the 20, 15, 10, and 7.5 g/day groups, respectively. Mean body weight gains for the restriction period inclusive, GD 6-18, were 0.49 x and 0.24 x control at 10 and 7.5 g/day, respectively, and a mean body weight loss occurred at 10 and 7.5 g/day (0.95 x and 0.85 x mean GD 6 body weight, respectively). Fetal body weights were reduced 0.95 x, 0.93 x, 0.90 x, and 0.76 x control at 20, 15, 10, and 7.5 g/day, respectively. This resulted in a reduction in gravid uterine weight at 10 and 7.5 g/day. There were no external, visceral, or skeletal malformations attributed to feed restriction. There was an increase in the skeletal variation of wavy ribs and a decrease in ossification at 7.5 g/day.

CONCLUSIONS

These data demonstrate that feed restriction-induced reductions in maternal gestational body weight gain of approximately 50% compared to ab lib fed rats only caused a reduction in fetal body weight. Even up to a 15% maternal gestational body weight loss had no effect on embryo viability in rats, but retarded fetal growth significantly enough to induce minor changes in skeletal development. There were no external, visceral, or skeletal malformations associated with any of the levels of maternal body weight reduction or loss.

Characterization of maternal influence on teratogenicity: an assessment of developmental toxicity studies for the herbicide cyanazine

The contribution of maternal toxicity to the teratogenic effects of the herbicide cyanazine has been assessed to determine whether it may be a hazard to development. Eye defects such as anophthalmia and microphthalmia were observed in rat fetuses and pups. Maternal toxicity was determined from body weight data and clinical signs. Two approaches were used. First, the timing of maternal toxicity was correlated with the specific period of gestation during which the observed fetal defect was most likely to have occurred. Second, individual dams, as well as mean values for each group, were evaluated. The data at the individual level, i.e., in dams with affected litters, did not support conclusions based on the group means. Instead, it is suggested that the developmental effects were not a direct result of maternal toxicity of cyanazine. Data from a rabbit developmental toxicity study supported the findings from the Fischer 344 rat studies. The strategy employed may thus enable direct toxicity to the fetus to be distinguished from developmental toxicity arising as a secondary consequence of maternal toxicity.

Spontaneous and induced alterations in the cardiac membranous ventricular septum of fetal, weanling, and adult rats

Alterations of the cardiac membranous ventricular septum were studied using macrodissection, scanning electron and light microscopy of fetal, weanling, and adult Sprague-Dawley rats. Membranous ventricular septal defects (VSDs) were observed in 2.0% of fetuses on day 21 postcoitus (pc) but not in weanling or adult rats. The most common observation was a nonpatent depression in the membranous septum with an incidence of 38.1, 10.5, 4.3% for fetuses on days 17, 19, or 21 pc, respectively, 11.8% for weanlings, and 9.1% for adults. VSDs were characterized by a split in the endocardial cushion cells in the interventricular component of the membranous septum. Nonpatent depressions were characterized by a split in the endocardial cushion cells in the atrioventricular component of the septum, and they persisted postnatally as a blind-ended diverticulum directed above the tricuspid valve. The cardiovascular teratogens, trimethadione and trypan blue, produced in fetuses nonpatent depressions and VSDs morphologically similar to untreated fetuses. Maternal diet restriction (25% of controls) lowered fetal (day 21 pc) body weight by 47% but did not affect the incidence of ventricular septal alterations, suggesting that intrauterine growth retardation is not necessarily associated with alterations in the development of the ventricular septum. We conclude that neither VSDs nor nonpatent depressions in Sprague-Dawley rats affect postnatal survival and that VSDs close spontaneously during neonatal life.

Developmental toxicity study in rats treated with the anticonvulsant, ralitoline

The developmental toxicity of the anticonvulsant compound, ralitoline, was investigated in Sprague-Dawley rats administered oral doses of 0, 15, 60, 120, 180, or 240 mg/kg on days 6 through 15 of gestation. An untreated control group and a vehicle control group pair-fed to the high dose group were included. Maternal and fetal parameters were evaluated on day 21 of gestation. Fetuses were examined for external, visceral, and skeletal malformations and variations. Maternal death occurred at 180 and 240 mg/kg. Dose-dependent decreases in body weight, food consumption, and water consumption were observed at 60 mg/kg and above. Body weight gain during treatment was similar in the pair-fed and 240 mg/kg groups. Dose-related CNS signs (hypoactivity, ataxia, prostration, and/or convulsions) were observed at 60 mg/kg and above. Decreased numbers of live fetuses and increased postimplantation loss were observed in a dose-related manner at 120, 180, and 240 mg/kg while no changes occurred in pair-fed controls. Fetal body weights and placental weights were decreased in pair-fed controls and in the 120, 180, and 240 mg/kg groups. The percent fetuses per litter, and the percent litters with external/visceral malformations, were significantly increased at 120, 180, and 240 mg/kg compared with vehicle and pair-fed controls. Dose-related increases in cardiovascular malformations, specifically of the aortic arch (interrupted, stenotic, extra vessel), were apparent at 120 mg/kg and above. The incidence of skeletal variations was increased at 120 mg/kg and above.(ABSTRACT TRUNCATED AT 250 WORDS)

Housing of pregnant rats in metabolism cages: maternal and developmental effects

The influence of the caging conditions on maternal and gestational variables was assessed for pregnant rats housed individually in two cage types. Plug-positive Sprague-Dawley females were caged either in Makrolon or in metabolism (Tecniplast) cages, and were not disturbed throughout all the gestational period. Cesarean sections were performed on gestation day 20. All live fetuses were examined for external, internal, and skeletal malformations and variations. Pregnant rats were affected by the housing system, as evidenced by a significant weight loss and reduced food consumption in the animals housed in metabolism cages. A moderate increase in the number of total skeletal defects was also observed in the fetuses of dams housed in metabolism cages. An important implication of these results would be that in maternal and developmental toxicity studies of xenobiotics, pregnant animals should not be housed in metabolism cages.

Maternal toxicity and the identification of inorganic arsenic as a developmental toxicant

Assessment of the potential developmental toxicity of arsenic in humans must be based entirely on the extensive animal literature; no appropriate human data are available. Hazard identification of developmental toxicity of arsenic in animal studies is complicated by the co-occurrence of maternal and developmental toxicity when the pregnant dam is exposed to the toxicant. Current regulatory guidance requires that, when maternal and developmental toxicity occur at the same or similar doses, detailed consideration needs to be given to whether developmental toxicity is secondary to maternal toxicity or whether it represents a distinct hazard. In this review, these principles were applied to the relatively large database of animal studies available for hazard identification of inorganic arsenic as a developmental toxicant. It is concluded that maternal and developmental toxicity occur in the same dose range for this potent cytotoxicant, although differential no observed adverse effect levels can be identified depending on the endpoints used. Various evidence from the basic science literature indicates that developmental toxicity is not secondary to maternal toxicity. Current regulatory guidance falls short of defining effective approaches to resolving the difficulties posed by co-occurrence of maternal and developmental toxicity.

Developmental toxicity of orally administered thiabendazole in Sprague-Dawley rats and New Zealand white rabbits

The developmental toxicity of thiabendazole (TBZ) was assessed in studies in New Zealand white rabbits and Sprague-Dawley rats. The doses of TBZ for these studies were based on decreases in maternal weight gain in dose range-finding studies in pregnant females of each species. In rabbits, TBZ was administered orally at doses ranging from 24 to 600 mg/kg body weight/day in two separate studies. In rats, TBZ was administered at doses of 10, 40 or 80 mg/kg body weight/day. In all studies, TBZ was administered daily by gavage in aqueous suspension on gestation days 6 to 17. In the initial rabbit study, weight loss occurred in the 600 mg/kg/day group and weight gain decreased in the 120 mg/kg/day group. In addition, there were four complete litter resorptions and four abortions in the 120 and 600 mg/kg/day groups, respectively. To determine whether these changes (known to occur spontaneously in rabbits) were reproducible, the study was repeated. In this study, decreased maternal weight gain and decreased foetal weights were found at 600 mg TBZ/kg/day, but there was no evidence of selective developmental toxicity. Similarly, in rats TBZ produced decreased maternal weight gain (12 to 26%) associated with slight (5 to 7%) decreases in foetal weights at doses of 30 and 80 mg/kg/day. No changes were found at 10 mg TBZ/kg/day and no evidence of selective developmental toxicity or teratogenicity was found at any dose. On the basis of these results, it is concluded that TBZ is not teratogenic or selectivity foetotoxic in rats or rabbits.

Embryotoxicity induced by alkylating agents: 7. Low dose prenatal-toxic risk estimation based on NOAEL risk factor approach, dose-response relationships, and DNA adducts using methylnitrosourea as a model compound

Prenatal-toxic risk estimation for the alkylating model compound methylnitrosourea (MNU) was performed using different procedures. Risk of low doses was estimated using linear extrapolation to zero (estimated ED0.1%: 0.1 mg/kg body wt MNU) as well as extrapolation by probit analysis based on a dose-response study (estimated ED0.1%: 1.6 mg/kg body wt). Furthermore, a "virtually safe dose" was established by means of the NOAEL risk factor approach (e.g., factor 30:0.03 mg MNU per kg body wt). In previous studies in murine embryos using MNU, we combined dose-response data and DNA adduct rate measurements and deduced that O6-methylguanine is a suitable variable for molecular dosimetry. In a tentative approach, we estimated the teratogenic risk of low doses based on the adduct rates of O6-methylguanine in the DNA of the embryos. It is concluded that in the case of steep dose-response relationships, which are typical for the majority of teratogenic effects, the NOAEL risk factor approach is more conservative than extrapolation based on probit analysis. Risk estimation using dosimetry with this model compound yields estimated incidences similar to linear extrapolation.

Teratological study of stobadin after single and repeated administration in rats

The toxic developmental potential of the anti-arrhythmic drug stobadin was assessed after single intravenous or repeated oral doses to pregnant rats. Stobadin was studied in the form of dihydrochloride (DH 1011) at doses of 2 and 6 mg/kg, given in single intravenous injections on days 3, 6, 9, or 12 of gestation. Immediately after injection of the 6-mg/kg dose of DH 1011 to pregnant rats, saccade abdominal respiration, tremor of hindlimbs, and sedative behaviour were observed on each day of medication. No deaths of females occurred in either the control or experimental groups. Slight foetal toxicity was manifested by significantly decreased foetal weight only after treatment on day 3 of gestation at 6 mg/kg and by significantly increased incidence of delayed ossification of the parietal and supraoccipital bone also at 6 mg/kg DH 1011 given on day 12 of gestation. The effect of repeated oral treatment in the form of dipalmitate salt (DP 1031) was studied in doses of 5, 15, and 45 mg/kg from days 2-15 of gestation. Oral exposure to 45 mg/kg DP 1031 resulted in significant reduction of maternal body weight gain and in embryofoetal toxicity, namely, increased preimplantation foetal loss, anomalies of sternebrae, and, after 15 and 45 mg/kg DP 1031, significantly decreased foetal weight and smaller litter size. The relevance of the two routes of stobadin administration for risk extrapolation is discussed.

Assessing developmental hazard: the reliability of the A/D ratio

The quantitative relationship between developmentally toxic exposure levels and adult toxic exposure levels has been used as an index of developmental hazard and has figured prominently in discussions of legal regulation of developmentally toxic agents. Perhaps the most frequently cited index of developmental hazard is the A/D ratio. This index, a ratio of marginally toxic adult and developmental dose levels (e.g., NOAELs or LOAELs), is attractive because it is easily calculated from published toxicity assays and because it has been argued that A/D is relatively constant across species for a given agent. We explored some quantitative aspects of the A/D ratio and of the concept of developmental hazard by simulating 661,500 mammalian developmental toxicity assays on 441 hypothetical compounds. In our simulations, A/D often varied substantially among replicate assays: the median ratio of the upper and lower limits of the distribution of A/D values that include about 95% of the observed A/D values is 16. In addition, A/D did a poor job of predicting the relative developmental and adult responses at dosages lower than those used to calculate the index: among simulated compounds with A/Ds of about 1.0, the developmental response at 1/100th of the NOAEL ranged from about 0.1% to 20,000% of the adult response. Finally, we measured the concordance between pairs of four different indices of developmental hazard, including A/D. Concordance was greatest when the indices were based on the same portion of the dose response, and was much weaker between indices that examined different portions of the dose response. Therefore, it seems likely that no single index can quantify "developmental hazard," as defined by relative adult and developmental susceptibility, and more effort needs to be expended in refining the concept if it is to be useful for hazard assessment.

Developmental toxicity and pharmacokinetics of oral and intravenous phenytoin in the rat

Correlations between oral and intravenous (i.v.) doses of phenytoin, maternal plasma levels, and subsequent developmental toxicity were examined in the Sprague-Dawley rat. Oral administration of 150 to 1500 mg/kg and i.v. administration of 25 to 100 mg/kg phenytoin from gestational days (GD) 8 to 17 resulted in a dose-dependent increase in maternal death and toxicity [impaired motor function, decreased maternal weight gain (oral dose only)], embryolethality, and intrauterine growth retardation, in addition to significant increases in craniofacial (1125 mg/kg oral; 75 mg/kg i.v.) and urogenital (1125 mg/kg oral) malformations. Pharmacokinetic sampling in oral and i.v. groups on GD 8-9 and 16-17 revealed significant increases in maternal drug exposure over the treatment period, as evidenced by 2- to 3-fold increases in total plasma phenytoin (bound + free) half-life, area under the concentration curve, peak concentration (oral dose only), and decreases in clearance. These findings emphasize the importance of pharmacokinetics in the evaluation of phenytoin-induced developmental toxicity.

Screening for neurotoxicity: complementarity of functional and morphologic techniques

Our philosophy is that screening tests should be applicable across species and emphasize complementarity to neuropathology. Within this context, electrophysiological tests comparable to those in human clinical neurology are powerful screening tools. For example, while histopathologic evaluation of the cochlea for ototoxicity is difficult, evoked potential audiometry is fast and easy. In this instance, one might routinely screen for deficits in auditory function, and reserve morphologic techniques for a characterization role rather than one of discovery. Lesions of neurons, axons and myelin are, however, readily assessed by light microscopy. A suitable combination of functional and morphologic screening tests, therefore, enhances the ability to discover neurotoxicity, and these data often are ideal for generation of refined hypotheses for subsequent characterization studies.

An overview of maternal toxicity and prenatal development: considerations for developmental toxicity hazard assessments

The objective of testing xenobiotics for potential developmental toxicity is to extrapolate laboratory animal information to the human species, thereby deriving biologically rational regulatory policies. One of the problems that significantly contributes to the difficulty of this task is the possibility that general effects on the maternal organism could affect the developing conceptus. Published data have indicated that factors intrinsic to the maternal organism affect developmental outcome. This overview examines factors which may bear directly or indirectly upon developmental outcome, with emphasis on those of greatest relevance to the hazard assessment process. Standard teratology testing protocols often call for dose levels that induce overt maternal toxicity, and the developmental effects of this toxicity (both alone, and with concurrent embryo/fetal insult) continue to present regulators with considerable interpretive difficulties. In response to these problems there have been both research and literature review efforts dealing with the relationship of maternal and developmental toxicity. Relevant studies are reviewed here, and suggestions for avenues of future research are offered including the identification of any syndromes of developmental effects occurring at maternally toxic levels irrespective of the causative agent, and experimental approaches for the characterization of maternal toxicity.

Important recent advances in the practice of health risk assessment: implications for the 1990s

Health risk assessments have been so widely adopted in the United States that their conclusions are a major factor in many environmental decisions. The procedure by which these assessments are conducted is one which has evolved over the past 10-15 years and a number of short-comings have been widely recognized. Unfortunately, improvements in the process have often occurred more slowly than advancements in technology or scientific knowledge. Recent significant advances for more accurately estimating the risks posed by environmental chemicals are likely to have a dramatic effect on the regulation of many substances. Each of the four portions of risk assessment (hazard identification, dose-response assessment, exposure assessment, and risk characterization) has undergone significant refinement since 1985. This paper reviews some of the specific changes and explains the likely benefits as well as the implications. Emphasis is placed on the improved techniques for (a) identifying those chemicals which may pose a human cancer or developmental hazard, (b) using statistical approaches which account for the distribution of interindividual biological differences, (c) using lognormal statistics when interpreting environmental data, (d) using physiologically based pharmacokinetic models for estimating delivered dose and for scaling up rodent data, (e) using biologically based cancer models to account for the seven or more apparently different mechanisms of chemical carcinogenesis, (f) describing the severity of the public health risks by considering those portions of the population exposed to various concentrations of a contaminant, and (g) reviewing how criteria for acceptable risk have been influenced by the number of exposed persons. The net benefit of these improvements should be a reduction in the uncertainty inherent in current estimates of the health risks posed by low level exposure to carcinogens and developmental toxicants.

Retinoic acid and 2,3,7,8-tetrachlorodibenzo-p-dioxin selectively enhance teratogenesis in C57BL/6N mice

TCDD is one of the most toxic man-made compounds and an extremely potent teratogen in mice. Many of its toxic symptoms resemble those seen during vitamin A deficiency. Vitamin A and its derivatives, such as alltrans-retinoic acid (RA), are also teratogenic in mice, as well as many other species. Both TCDD and RA produce cleft palate in susceptible strains of mice. However, while TCDD produces hydronephrosis, RA does not, and TCDD does not produce limb bud defects while RA does. To determine whether TCDD and RA would enhance or antagonize the teratogenic effects of the other compound, C57BL/6N dams were treated po on Gestation Day (gd) 10 or 12 with 10 ml corn oil/kg containing TCDD (0-18 micrograms/kg), RA (0-200 mg/kg), or combinations of the two chemicals. Dams were killed on gd 18 and toxicity and teratogenicity assessed. Coadministration of TCDD and RA had no effect on maternal or fetal toxicity beyond what would be expected by either compound alone. Cleft palate was induced by RA at lower doses on gd 10 than on gd 12, but by TCDD at lower doses on gd 12 than on gd 10. Sensitivity to TCDD-induced hydronephrosis was similar on both gd 10 and 12. The limb bud defects were only observed when RA was administered on gd 10, not when given on gd 12. No other soft tissue or skeletal malformations were related to administration of TCDD or RA. No effect of TCDD was observed on the incidence or severity of limb bud defects induced by RA, nor did RA influence the incidence or severity of hydronephrosis induced by TCDD. However, the incidence of cleft palate was dramatically enhanced by coadministration of the xenobiotic and vitamin. On both gd 10 and 12, the dose-response curves for cleft palate induction were parallel, suggesting some similarities in mechanism between the two compounds. However, combination treatment resulted in a synergistic response that varied with the stage of development and was tissue specific.

Prenatal ethanol and stress in mice: 1. Pup behavioral development and maternal physiology

On days 12 to 17 of pregnancy, B6D2F1 mice were pair-fed liquid diets containing either 25% ethanol-derived calories or an isocaloric amount of maltose-dextrin. During this period, half the mice in each dietary condition also underwent two daily one-hour periods of restraint stress. A fifth group, given lab chow and water ad lib, was left undisturbed throughout gestation. Neither treatment affected offspring body weight on days 22 or 32 postconception, but undernutrition produced by the pair feeding procedure reduced day 32 body weight in all groups relative to the ad lib-fed group. Both prenatal ethanol and pair feeding led to delayed neurobehavioral development on day 32, while prenatal stress significantly reduced the degree of developmental delay caused by these factors. In a second study, restraint stress significantly reduced blood alcohol concentrations in pregnant dams on day 15 of gestation while elevating plasma corticosterone concentrations, and this elevation was consistent regardless of the dietary condition of the dam. The pair feeding procedure also produced corticosterone elevations but the effect of ethanol was not significant. These results suggest that prenatal stress in the presence of other physiological insults may act to counter the actions of those insults.

Potential human developmental toxicants and the role of animal testing in their identification and characterization

Some 50 chemicals have been identified from environmental, occupational, or therapeutic exposure data as being potential developmental toxicants in humans. The toxicity pattern of these chemicals in humans has been characterized and correlated with developmental toxicity end points in laboratory animal models in order to determine the relevance and predictiveness of the results of testing in animals in extrapolation to human data. In general, animal developmental toxicity data closely paralleled human outcomes, and while humans in most cases were more sensitive than animals, the data support the concept that, imperfections aside, studies in animals serve a vital role in the hazard identification process.

The mouse teratogen dinocap has lower A/D ratios and is not teratogenic in the rat and hamster

The fungicide dinocap is currently used in the control of powdery mildew. We have reported that dinocap is teratogenic in the CD-1 mouse, causing cleft palate, otolith defects, and fetal weight deficits well below maternotoxic dose levels. In this study the maternal and fetal toxicity of dinocap was determined in the Sprague-Dawley rat and Syrian golden hamster, and adult-to-developmental (A/D) toxicity ratios were calculated and compared with the previously established A/D ratio of dinocap in the mouse. Dinocap in corn oil was administered by gavage to pregnant rats on gestation days 7-20 (0, 100, 150, 200 mg/kg/day) and to hamsters on gestation days 7-14 (0, 12.5, 25, 50, 75, 100, 200 mg/kg/day). Dams were killed on day 21 (rat) or day 15 (hamster), and litters were removed, counted, and weighed; half of each litter was necropsied for soft tissue defects, and the remaining half was processed for skeletal examination. In the rat, maternal extrauterine weight gain was significantly affected at 150 and 200 mg/kg/day, relative liver weight was elevated at 100 mg/kg/day and above, and fetal weight was lower at 150 and 200 mg/kg/day. In the hamster, maternal extrauterine weight was lower at 12.5 mg/kg/day and above; fetal weight was reduced, and the incidence of dilated renal pelvis was higher, at 25 mg/kg/day and above. Thus the A/D ratios for dinocap in the rat and hamster are similar, approximately 1.(ABSTRACT TRUNCATED AT 250 WORDS)