Drug-induced damage to the embryo or fetus (molecular and multilateral approach to prenatal toxicology)

Receptor- and reactive intermediate-mediated mechanisms of teratogenesis

Drugs and environmental chemicals can adversely alter the development of the fetus at critical periods during pregnancy, resulting in death, or in structural and functional birth defects in the surviving offspring. This process of teratogenesis may not be evident until a decade or more after birth. Postnatal functional abnormalities include deficits in brain function, a variety of metabolic diseases, and cancer. Due to the high degree of fetal cellular division and differentiation, and to differences from the adult in many biochemical pathways, the fetus is highly susceptible to teratogens, typically at low exposure levels that do not harm the mother. Insights into the mechanisms of teratogenesis come primarily from animal models and in vitro systems, and involve either receptor-mediated or reactive intermediate-mediated processes. Receptor-mediated mechanisms involving the reversible binding of xenobiotic substrates to a specific receptor are exemplified herein by the interaction of the environmental chemical 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or "dioxin") with the cytosolic aryl hydrocarbon receptor (AHR), which translocates to the nucleus and, in association with other proteins, binds to AH-responsive elements (AHREs) in numerous genes, initiating changes in gene transcription that can perturb development. Alternatively, many xenobiotics are bioactivated by fetal enzymes like the cytochromes P450 (CYPs) and prostaglandin H synthases (PHSs) to highly unstable electrophilic or free radical reactive intermediates. Electrophilic reactive intermediates can covalently (irreversibly) bind to and alter the function of essential cellular macromolecules (proteins, DNA), causing developmental anomalies. Free radical reactive intermediates can enhance the formation of reactive oxygen species (ROS), resulting in oxidative damage to cellular macromolecules and/or altered signal transduction. The teratogenicity of reactive intermediates is determined to a large extent by the balance among embryonic and fetal pathways of xenobiotic bioactivation, detoxification of the xenobiotic reactive intermediate, detoxification of ROS, and repair of oxidative macromolecular damage.

Embryotoxicity induced by alkylating agents: 10. Analysis of the combined teratogenic effects of methylnitrosourea and ethylmethanesulfonate in mice

In previous studies the direct-acting alkylating model compounds methylnitrosourea (MNU) and ethylmethanesulfonate (EMS) were investigated with regard to dose-response of teratogenicity as well as DNA adduct formation in mice. In this study the teratogenic effects induced by combined treatment with these substances were analyzed using doses which, following single treatment with either substance, were around the threshold level, i.e., no adverse effect level (NOAEL) and lowest observed adverse effect level (LAOEL). Combined treatment of LAOELs resulted in a threshold-like response, while the combination of the NOAEL of one substance with the LAOEL of the other increased the response rate dramatically to nearly 100%. This phenomenon was further evaluated using biometrical methods. The dose-response surface as well as isobolograms were calculated in order to describe the combination effect. In addition, a dose-response model was fitted to the data. In conclusion, the initially surprising high combination effect revealed to be not so extraordinary when considering the steepness of the dose-response relationships of the single substances.

Whole embryo culture: interpretation of abnormal development in vitro

An overview will be given of a number of problems that arise when we attempt an interpretation of data obtained with the whole embryo culture method (WEC). The following aspects will be considered: 1) Types of deviation from normal development in the whole-embryo culture system: (a) problem of "artifacts" (for example, due to preparation mistakes and suboptimal culture conditions), (b) types and frequency of abnormal development in controls, retardation as an abnormal outcome, and the difficulty of assessing its significance, (c) gross-structural defects and, specifically, attempts to answer the following questions: (i) What are the limitations for an assessment of abnormal development with the WEC? (ii) Is a specific abnormality pattern recognizable under specific pathologic conditions? (iii) Is the rate of induced abnormal development concentration-dependent? (2) Problems encountered when interpreting concentration-effect relationships (for example, statistical and pharmacokinetic aspects). (3) Necessity of verifying in vitro effects with in vivo studies.

Cross-species extrapolations and the biologic basis for safety factor determinations in developmental toxicology

Designations of agents as teratogenic or nonteratogenic often are inaccurate, as adverse effects are more a reflection of the timing and severity of treatment during pregnancy than agent nature. Careful consideration of both the similarities and the differences between developmental effects in animals and humans and the extent and nature of the data available are essential for protection of the human conceptus. Animal surrogates prove reliable predictors of human developmental effect levels. When the data are evaluated consistent with contemporary concepts of developmental toxicity, for example, where the effect in the embryo is only seen at maternally toxic doses and exposure is below the adult toxic level, relatively modest safety factors are sufficient for safe cross-species extrapolation. Developmental toxicity safety factor magnitude is predicated on data quality and the fact that thresholds of effect exist in mammalian pregnancy. Safety of human concepti is achieved by considering both the developmental hazard index of the chemicals in question and the severity of exposure.

Assessment of the developmental risks resulting from occupational exposure to select glycol ethers within the semiconductor industry

This risk assessment evaluates the potential human hazards of adverse developmental effects posed by exposure to 2-ethoxyethanol (2-EE), 2-ethoxyethanol acetate (2-EEA), 2-methoxyethanol (2-ME), and 2-methoxyethanol acetate (2-MEA) as they are currently used in semiconductor manufacturing. These glycol ethers are contained in positive photoresists used in the wafer fabrication process. The available data on the developmental toxicology of these glycol ethers indicates that each can selectively affect the offspring of pregnant animals that have been exposed to relatively low vapor concentrations. For these chemicals, the ratio of the lowest dose which adversely affected the pregnant animals (A) and the lowest dose which produced developmental effects in offspring (D), e.g., A/D ranged from 1-5. Approximately 400 workplace air samples of 4-8 h duration, both personal and area, from seven different companies were used to assess the degree of inhalation exposure during the manufacture of wafers. The geometric mean results obtained during personal sampling of workplace air for 2-EE, 2-EEA, 2-ME, and 2-MEA were 0.36, 0.02, 0.10, and 0.01 ppm, respectively. These levels are 14- to 500-fold lower than the applicable threshold limit value (TLV) currently recommended by the American Conference of Governmental Industrial Hygienists (ACGIH). Specifically, the margins of safety between the typical occupational exposure and the TLV for 2-ME, 2-EE, 2-MEA, and 2-EEA are 50, 14, 500, and 250, respectively. The TLVs for these chemicals were set at levels considered sufficiently low to protect workers and their offspring from adverse effects and are about 2- to 10-fold lower than the various no-observed-effect levels (NOELs) obtained in animal tests. Based on more recent data, lower TLVs are indicated. The safety-factor approach, rather than mathematical models developed for estimating cancer risks, was used in this analysis. Historical data have shown that the application of safety factors of 10-100 to the NOEL, as determined in Segment II developmental toxicology tests in animals, should be adequate to protect humans. In its risk assessment guidelines, the U.S. Environmental Protection Agency (EPA) selected the uncertainty-factor approach as the most reasonable one for evaluating the hazards of developmental toxicants. This assessment indicates that the airborne concentrations of these glycol ethers in the semiconductor industry are, in general, sufficiently low to protect employees against their adverse developmental and reproductive effects as well as any other toxic effects as long as dermal exposure is minimal.

Preferential alcoholic embryopathy among contiguous siblings of Long-Evans rats

Ethanol exposure alters sex steroidogenesis and sexually dimorphic behaviors in rodent offspring. Contiguity to siblings of the same or opposite sex in utero also affects steroidogenesis and sexual dimorphism in rodents. The present study with Long-Evans rats shows that maternal exposure to ethanol during the critical period of rodent organogenesis preferentially affects body weights and increases malformations in offspring dependent on their in utero contiguity to siblings of the opposite sex.

Studies on the embryopathic effects of ethanolamine in Long-Evans rats: preferential embryopathy in pups contiguous with male siblings in utero

The embryopathic effects of high doses of ethanolamine were evaluated in pregnant Long-Evans rats during the "critical period" or organogenesis. Ethanolamine was given by gavage at levels of 0, 500, 300, or 50 mg/kg/day (24%, 14.4%, or 2.4% of the LD50 value). Ethanolamine caused dose-dependent increases in intrauterine deaths, malformations, and intrauterine growth retardation. Embryolethality caused by 500 mg/kg of ethanolamine was not random: male pups contiguous to two male siblings (designated mMm) were almost quantitatively replaced by resorptions that were contiguous to two live male pups (designated mRm) (mMm pups constituted 6.7% of control implants and decreased to only 0.9% of group II implants while mRm resorptions increased from 0.3% in controls to 5.6% in group II dams). Intrauterine growth retardation and increases in gross structural anomalies (considered indicative of depressed fetal growth) more severely affected male than female offspring at all dose levels. Pups of either sex who were contiguous to male siblings were more adversely affected than those offspring contiguous to one or more female siblings. As ethanolamine was given prior to the period of greatest fetal growth and fetal sex steroidogenesis, it is suggested that intrauterine levels of female sex steroids (estradiol) enhance fetal repair of cellular damage while testosterone inhibits fetal repair or exacerbates previous embryonic damage by some unknown mechanism. Such interaction furthers the concept that intrauterine position affects the endpoints of developmental toxicity, as expressed at parturition.

Effects of ethanol on reproduction and arterial hypertension in spontaneously hypertensive and normotensive rats: a preliminary communication

Ethanol consumption and spontaneous (essential) hypertension are important fetal and maternal risk factors. Alone, they contribute to embryopathy (fetal alcohol syndrome) or maternal organ pathology and fetal loss in hypertensive pregnancies. Combined, the effects of ethanol consumption on the progress of a hypertensive pregnancy have not been adequately investigated. In the present study, groups of O-A strain genetic hypertensive (SHR: groups 1 and 2) and Wistar-Kyoto normotensive (WKY: groups 3 and 4) pregnant rats were given 20 ml/kg of distilled water by gavage to serve as controls [groups 1 (SHR) and 3 (WKY)] or 3.2 g/kg of ethanol [groups 2 (SHR) and 4 (WKY)] from days 6 to 15 of gestation. During acclimation, hypertension developed in SHR rats (WKY pressures were 105 to 114 mm Hg; SHR pressures were 137 to 148 mm Hg). From day 6 to 15 of gestation, ethanol-consuming rats (groups 2 and 4) had higher arterial pressures than controls (groups 1 and 3). Pregnant SHR rats given ethanol did not experience a prebirthing hypotension. On gestation day 20, most offspring (84%, group 2; 86%, group 4) of alcoholic dams were dead or malformed. Intrauterine growth retardation occurred in group 4. Hydrocephalus, microphthalmia, and mild hydronephrosis and hydroureter were common in live offspring of group 2 dams. Hydronephrosis and hydroureter were increased in group 4 pups. Variant cranial ossification was noted in group 2 and 4 pups. These preliminary data suggest an altered hypertensive response during pregnancy in alcohol-consuming rats and confirm the embryopathic effects of relatively high levels of ethanol consumed during the critical period of organogenesis in two additional strains of rats.

Correlations between the degree and type of forebrain malformations and the simultaneous neuro-oncogenic properties of ethylnitrosourea after diaplacental exposure in rats, alone and in combination with X-irradiation

Single and combined treatments were performed in rats on day 13 of gestation with either ENU or ENU subsequent to various X-irradiation doses between 0.5 and 1.5 Gy. At this time of gestation, developmental anomalies of the brain are still inducible by any of these treatments, in addition to neurocarcinogenic effects after ENU alone or in combination with X-irradiation. We looked for correlations between the degree of brain malformations still detectable in the adult animals and the simultaneous occurrence of brain tumors. These evaluations were based on a histopathological analysis regarding the type and degree of malformation residues, as well as the type and distribution pattern of the tumors (especially regarding gliomas) within the forebrain. Both after ENU and X-irradiation plus ENU-treatment, the occurrence of glioma in the offspring was positively correlated with the degree of brain dysplasia. This effect was not only restricted to the total glioma incidence but also confirmed by the higher glioma multiplicity in major dysplastic brains. Additionally, gliomas were preferentially located within the subependymal layer, which simultaneously was most severely affected by the teratogenic effects after prenatal treatment. Although forebrain dysplasia generally presents a significant predisposition for glioma inducibility, this oncogenic event is apparently strictly inversely related to a certain type of forebrain malformation, namely the occurrence of heterotopic neuronal nodules within the telencephalic roof. They emerge from "rosettes," which are typical radiation lesions occurring only after doses above 1.0 Gy. In none of the forebrains which still revealed rosette-residues in later life could a simultaneous occurrence of gliomas be observed. This explains not only the substantial decrease of glioma incidence after combined treatment with 1.0 and 1.5 Gy X-irradiation doses, but also the consistent glioma multiplicity despite the decrease of frequency.

Effects of various exposure levels of 2-phenylethanol on fetal development and survival in Long-Evans rats

Phenylethanol was given at different levels (432, 43, or 4.3 mg/kg) by gavage to pregnant Long-Evans rats during the "critical period" of organogenesis. Examination of offspring revealed adverse reproductive and teratogenic effects in a dose-related manner. Intrauterine growth retardation occurred at levels of 432 and 4.3 mg/kg. Embryolethality was 18% at 43 mg/kg and 10% at 4.3 mg/kg. Malformations occurred in the following sequence: 100% at 432 mg/kg; 93% at 43 mg/kg, and 50% at 4.3 mg/kg. Noteworthy dose-related teratogenic effects of phenylethanol in offspring manifested themselves in increased incidences of malformed eyes, neural-tube defects, hydronephrosis, and limb defects.

The relative teratogenic index and teratogenic potency: proposed components of developmental toxicity risk assessment

Teratogenicity tests should provide answers to three questions: (1) Can the agent induce developmental defects? ("teratogenic potential"); (2) What are the effective doses? ("teratogenic potency"); and (3) Are effective doses below adult toxic doses? ("teratogenic hazard"). The answers to (2) and (3) should be quantitative in nature, but there are no accepted parameters to express these properties. In this paper we propose parameters for the description of teratogenic potency and hazard in quantitative terms. Derivation and calculation of the parameters are illustrated by the analysis of adult lethality and teratogenicity data of eight structurally related anhydrides and imides, following testing in the CD-1 mouse. Teratogenicity was evaluated following treatment on Days 8-10 of gestation, using an average of four dose groups per compound and at least 10 dams per group. Adult lethality was estimated following a similar 3-day dosage schedule with an average of 6 dose groups per compound and at least 8 animals per group. Dose-response relationships of teratogenicity were fitted to a probit model from which tD50 (median effective dose), and other effective doses were computed. It is proposed that tD05, as a minimum teratogenic dose, best represents teratogenic potency. In this study, potency ranged from 0.17 mmol/kg/day for phenytoin to 5.2 mmol/kg/day for ethosuximide. In order to measure teratogenic hazard a ratio between adult toxic (lethality was chosen as the most appropriate measure) and teratogenic responses was made. Since the dose-response slopes of lethality and teratogenicity were different, a simple ratio between median effective doses could not be used. It is shown that a ratio of LD01 to tD05 provides a "Relative Teratogenic Index" (RTI) which reflects the teratogenic hazard of a test agent. The following RTI values (LD01/tD05) were computed in this study: phthalic anhydride, 0.9; phensuximide, 1.0; succinic anhydride, 1.0; ethosuximide, 1.2; phenytoin, 1.6; phenacemide, 3.3; trimethadione, 4.0; and sodium valproate, 4.1. For these data, tD05 and RTI clearly represent the differing teratogenic potencies and hazards of the tested compounds. it is suggested that these parameters may be useful in comparative teratogenicity studies and may be valuable components of developmental toxicity risk assessment.

Significance of organ culture techniques for evaluation of prenatal toxicity

A discussion of the applicability of in vitro techniques now available for research in prenatal toxicology is presented. Advantages and disadvantages of the various in vitro methods (such as cultivation of preimplantation embryos, whole embryo culture, and organ culture) as applied to various problems of experimental research are described. As a typical example, the experience gained in our laboratory with the organ culture of mammalian limb buds is detailed. Various aspects of the research with this type of organ culture - e.g., different techniques of culturing, extent of differentiation achieved in culture, induction of abnormalities in culture, supplementing the system with drug-metabolizing capacities and means for quantification of the data - are discussed. It is concluded that certain in vitro techniques using mammalian embryonic tissues are very suitable tools for elucidating the mode of action of teratogenic agents, and that they may serve as a "model" for several basic processes also for the situation probably existing in humans. Such organ culture, and other in vitro methods, provide little, if any advantage over in vivo experiments if a "mass screening" of a possible teratogenic potential of chemicals (hazards for the human population) is attempted.