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

The Xenopus laevis teratogenesis assay for developmental toxicity of phthalate plasticizers and alternatives

Contamination of phthalate ester plasticizers threatens the wildlife as well as human health. To evaluate the developmental toxicity of commonly used phthalate esters and emerging alternatives, the frog embryo teratogenesis assay-Xenopus (FETAX) was conducted for dibutyl-phthalate (DBP), benzyl-butyl-phthalate (BBP), dioctyl-terephthalate (DOTP), di(2-propylheptyl)-phthalate (DPHP), diisononyl-phthalate (DINP), diisodecyl-phthalate (DIDP), diethyl hexyl cyclohexane (DEHCH), and diisononyl-cyclohexane-1,2-dicarboxylate (DINCH). The 96-hrs LC₅₀ for DBP, BBP, DOTP, DIDP, DINCH, DINP, DPHP, and DEHCH were 18.3, 20.1, 588.7, 718.0, 837.5, 859.3, 899.0, and 899.0 mg/L, respectively. The 96-hrs EC₅₀ of developmental abnormality of DBP, BBP, DPHP, DOTP, DINP, DEHCH, DINCH, and DIDP were 7.5, 18.2, 645.1, 653.6, 664.4, 745.6, 813.7, and 944.5 mg/L, respectively. The lowest observed effective concentration for embryonic survival, malformation, and growth was DINP, DBP, BBP, DIDP, DPHP, DINCH, DEHCH, and DOTP in increasing order. In tadpoles, DBP, BBP, DEHCH, DINP, and DIDP caused inositol-requiring enzyme 1 or protein kinase R-like endoplasmic reticulum kinase pathway endoplasmic reticulum stress (ERS) in order, and BBP, DBP, DOTP, DPHP, DINP, and DIDP caused long term ERS-related apoptosis or mitochondrial apoptosis in order. Together, in Xenopus embryos, the developmental toxicity and the cellular stress-inducing potential of tested plasticizers were DEHCH, DINCH, DPHP, DIDP, DINP, DOTP, BBP, and DBP in increasing order. In consideration of public as well as environmental health this information would be helpful for industrial choice of phthalate ester plasticizers and their alternatives.

Alternative Tests for Teratogenicity

Summary

The major features of the tests surveyed are shown in Table I. In a tier system of tests for teratogenicity, the Chernoff test is at a different level than the other assays described here. It is not appropriate for screening large numbers of chemicals, but may be useful for studies of smaller groups of agents, for example to confirm data from a prescreen. Although the test is certainly easier, cheaper and uses less than half the animals of a Segment II test, it is still much more expensive and time-consuming than most alternative tests.

Of the remaining alternatives, whole embryos or organs in culture encompass the widest range of mammalian developmental events and are invaluable in the study of teratogenic mechanisms. They are, however, also inappropriate for screening large numbers of chemicals. The methods are technically demanding, relatively expensive and use reasonably large numbers of pregnant mammals. To screen a group of, say, 20 chemicals involves a considerable investment of time and, in fact, no study of this size has been reported. In certain specific circumstances, they may be a useful adjunt to testing; for example, if treated human serum samples are freely available, if a drug has a unique action on rodent dams which confounds evaluation of the standard in vivo tests, or if human metabolism is important and can be mimicked in vitro.

Sub-mammalian and sub-vertebrate species offer considerable advantages; reduced cost, relative rapidity and no requirement for laboratory animals. FETAX provides some indication of teratogenicity in relation to embryotoxicity, while CHEST and the planarian and Drosophila assays measure only teratogenic potential, or more strictly speaking, embryotoxic potential, although it should be possible to derive some assessment of hazard with each of the latter systems. The Hydra system is cheap, quick and easy and is commercially available. It is the only assay specifically designed to estimate teratogenic hazard and may offer considerable advantages as an alternative screen.

The metabolic cooperation assay has not generated sufficient data to enable evaluation. The neural crest cell assay is not well developed as a routine screen, and objective endpoints which are not measures of general cytotoxicity must be devised. The viral morphogenesis and Drosophila embryo cell assays have both produced encouraging validation data. With further assessment, the viral system may be shown to be useful, but it is a relatively complex assay and its relevance to teratogenesis is obscure. The Drosophila system is easier, has been used with more chemicals and is developmentally relevant. However, it has not produced dose-response data to evaluate potency or hazard, and must be improved so that it can more clearly distinguish cytotoxicity. The measurement of endpoints in the neuroblastoma cell line assay requires further refinement, and contributions of growth inhibition or stimulation to effects on differentiation must be examined. In combination, tumour cell attachment and HEPM may prove valuable. Alone, HEPM appears to be an assay for cellular toxicity, not teratogenicity, and the attachment assay suffers from a high rate of false negatives because it measures only one cell phenomenon. Although micromass cultures use mammalian tissue, are not the cheapest assays and require some skill for full evaluation of the results obtained, they show considerable promise. Validation data are encouraging, the assay includes several developmental processes and the use of multiple endpoints permits specific developmental toxicities to be evaluated.

Assessing Teratogenicity from the Clustering of Abnormal Phenotypes in Individual Zebrafish Larvae

In previous publications, we described the population incidence of abnormalities in zebrafish larvae exposed to toxicants. Here, we examine the phenomenon of clustering or co-occurrence of abnormalities in individual larva. Our aim is to see how this clustering can be used to assess the specificity and severity of teratogenic effect. A total of 11,214 surviving larvae, exposed continuously from 1 day postfertilization (dpf) to one of 60 toxicants, were scored at 5 dpf for the presence of eight different abnormal phenotypes. These were as follows: pericardial edema, yolk sac edema, dispersed melanocytes, bent tail, bent trunk, hypoplasia of Meckel's cartilage, hypoplasia of branchial arches, and uninflated swim bladder. For 43/60 compounds tested, there was a concentration-dependent increase in the severity score (number of different abnormalities per larva). Statistical analysis showed that abnormalities tended to cluster (i.e., to occur in the same larva) more often than expected by chance alone. Yolk sac edema and dispersed melanocytes show a relatively strong association with one another and were typically the first abnormalities to appear in single larvae as the concentration of compound was increased. By contrast, hypoplastic branchial arches and hypoplastic Meckel's cartilage were only frequently observed in the most severely affected larvae. We developed a metric of teratogenicity (TC_3/8), which represents the concentration of a compound that produces, on average, 3/8 abnormalities per larva. On this basis, the most teratogenic compounds tested here are amitriptyline, chlorpromazine hydrochloride, and sodium dodecyl sulfate; the least teratogenic is ethanol. We find a strong correlation between TC_3/8 and LC₅₀ of the 43 compounds that showed teratogenic effects. When we examined the ratio of TC_3/8 to LC₅₀, benserazide hydrochloride, copper (II) nitrate trihydrate, and nicotine had the highest specific teratogenicity, while aconitine, hesperidin, and ouabain octahydrate had the lowest. We conclude that analyzing the clustering of abnormalities per larva can provide an enriched teratogenic dataset compared with simple measurement of the population frequency of abnormalities.

Developmental Toxicity of Cumene Vapor in Cd Rats and New Zealand White Rabbits

The potential developmental toxicity of cumene vapor (99.9% pure) was assessed in pregnant CD (Sprague-Dawley) rats and New Zealand White rabbits exposed for 6 h per day by inhalation, the most relevant route of potential human exposure. Groups of 25 rats were exposed on gestational days (GD) 6–15 to concentrations of 0 (filtered air), 100, 500, or 1200 ppm, and groups of 15 rabbits were exposed on GD 6–18 to 0, 500, 1200, and 2300 ppm cumene vapor. In rats, reduced maternal body weight gain and increased relative liver weight was observed at 1200 ppm cumene. In rats and rabbits, reduced food consumption was observed at concentrations of 500 and 1200 ppm. A t 2300 ppm, 2 rabbits died, body weight gain and food consumption were reduced during the exposure period, and relative liver weights were increased. None of the gestational parameters, including numbers of viable implantations per litter, sex ratio, and fetal body weights, were affected at any exposure level in rats or rabbits. There were no treatment-related increases in incidences of external, visceral, or skeletal malformations or in the incidences of variations at any level. Thus, in rats, the no observable adverse effect level (NO A EL) for maternal toxicity was 100 ppm and the NO A EL for developmental toxicity was 1200 ppm, the highest concentration of cumene vapor tested. In rabbits, there was no NO A EL for maternal toxicity, but the NO A EL for developmental toxicity was 2300 ppm for cumene, the highest concentration tested. Therefore, even at exposure levels associated with maternal toxicity, cumene was not a developmental toxicant by inhalation exposure in either rats or rabbits.

Comparative teratogenicity analysis of valnoctamide, risperidone, and olanzapine in mice

OBJECTIVES

Based on the recent findings from animal studies, it has been proposed that the therapeutic use of valnoctamide, an anxiolytic drug developed in the early 1960s, be extended to treat other neurological disorders such as epilepsy and bipolar disease. Given the scarcity of adequate data on its prenatal toxicity, a comparative teratogenicity study of valnoctamide and two of the most commonly used drugs to treat bipolar disorder, risperidone and olanzapine, was carried out in a mouse model system.

METHODS

Pregnant dams were treated with the aforementioned three drugs at the dose levels calculated as an equal proportion of the respective LD50 values of these drugs. The main reproductive indices examined included the numbers of implantations and resorptions, viable and dead fetuses, and fetal gross, visceral and skeletal abnormalities.

RESULTS

The outcomes of the present study indicated that olanzapine was the most teratogenic of the three drugs, inducing maternal-, embryo-, and fetotoxicity. Risperidone also exerted a significant prenatal toxicity, but its adverse effect was less pronounced than that induced by olanzapine. Valnoctamide did not show any teratogenic effect, even when used in relatively higher dosages than olanzapine and risperidone. The observed increased skeletal abnormalities in one of the valnoctamide treatment groups were nonspecific and, as such, signaled a modest developmental delay rather than an indication that the compound could induce structural malformations.

CONCLUSIONS

Under our experimental conditions, valnoctamide demonstrated the lowest prenatal toxicity of the three tested drugs.

Comparison of rat and chick limb bud micromass cultures for developmental toxicity screening

This study compares the responses of rat and chick limb bud micromass cultures to chemical treatment. Eight chemicals, of diverse structure, potency and mechanism, were tested, using two endpoints: extractable alcian blue stain as a measure of differentiation to chondrocytes, and extractable neutral red stain as an index of proliferation. Each chemical reduced differentiation and proliferation in a concentration-related manner. IC(50)s, concentrations that reduced staining by 50%, ranged from 10 nm (colchicine) to 4 mm (acetazolamide). Rat and chick responses to acetazolamide, colchicine and diazepam were indistinguishable. For diphenhydramine and sulphisoxazole, concentration-response curves were very similar, but rat IC(50)s were half that of chick. For two chemicals, concentration-response slopes were markedly steeper for chick; in the case of beta-aminopropionitrile, IC(50)s were similar, but rat cultures were three-fold more sensitive than chick to cytosine arabinoside. 6-Aminonicotinamide gave a U-shaped response curve, for both endpoints and both species, so IC(50)s may be misleading, but the IC(50) for proliferation was lower for chick (0.6 mum) than rat (4 mum). In vivo and in vitro parameters for validation of developmental toxicity screens are contentious. Diphenhydramine apart, these chemicals can be teratogenic in vivo, although their 'hazard' can be debated. An IC(50)-proliferation/IC(50)-differentiation ratio > 2 has been suggested to predict specific developmental toxicity. Only sulphisoxazole and 6-aminonicotinamide had significantly different IC(50)s for proliferation and differentiation, with ratios of 4.4 (both species), and 10.4 for rat and 1.9 for chick, respectively. All other ratios were close to 1. The general consistency of this ratio, and the concentration-responses, in the two species suggests that the chick is a viable alternative to laboratory mammals, but the predictive ability of micromass remains to be determined.

Drug target-gene signatures that predict teratogenicity are enriched for developmentally related genes

Drugs prescribed during pregnancy affect two populations simultaneously: fetuses and their mothers. Drug-induced fetal injury (teratogenicity) has a significant impact on current and future public health. Teratogenic risk designation of many drugs relies on associating rare fetal events with rare environmental exposures. Therefore we aim to develop preclinical predictive models of clinical teratogenicity. We collated public databases for drug-target-gene relationships for 619 drugs spanning the 5 pregnancy risk classes. Genes targeted by high risk but not low risk drugs demonstrated 79% accuracy (p < 0.0001 vs. random) for predicting high vs. low fetal risk on cross validation. Functional enrichment analysis revealed that target genes of drugs known to be safe in pregnancy contained no developmentally related terms, while target genes of known teratogens contained 85 developmentally related terms. Drug target gene signatures that are enriched for known developmental genes may provide valuable preclinical predictive information regarding drug pregnancy risk.

A baseline inhalation toxicity model for narcosis in mammals

This paper presents the results of an analysis of the rodent inhalation literature and the development of a quantitative structure-activity relationships (QSAR) model for 4-hour LC50 as baseline toxicity to complement the baseline toxicity model for aquatic animals. We used the same literature review criteria developed for the ECOTOX database which selects only primary references with explicit experimental methods to form a high-quality database. Our literature review focused on the primary references reporting a 4-hour exposure for a single species of rodent in which the chemical had been clearly tested as a vapour and for which the exposure concentrations were not ambiguous. An expert system was used to remove reactive chemicals, receptor-mediated toxicants, and any test that produced symptoms inconsistent with non-polar narcosis. The QSAR model derived for narcosis in rodents was log LC50 = 0.69 x log VP + 1.54 which had an r(2) of 0.91, which is significantly better than the baseline toxicity model for aquatic animals. This simple model suggests that there is no intrinsic barrier to estimating baseline toxicity for in vivo endpoints in mammalian or terrestrial toxicology.

Ethosuximide: from bench to bedside

Ethosuximide, 2-ethyl-2-methylsuccinimide, has been used extensively for "petit mal" seizures and it is a valuable agent in studies of absence epilepsy. In the treatment of epilepsy, ethosuximide has a narrow therapeutic profile. It is the drug of choice in the monotherapy or combination therapy of children with generalized absence (petit mal) epilepsy. Commonly observed side effects of ethosuximide are dose dependent and involve the gastrointestinal tract and central nervous system. Ethosuximide has been associated with a wide variety of idiosyncratic reactions and with hematopoietic adverse effects. Typical absence seizures are generated as a result of complex interactions between the thalamus and the cerebral cortex. This thalamocortical circuitry is under the control of several specific inhibitory and excitatory systems arising from the forebrain and brainstem. Corticothalamic rhythms are believed to be involved in the generation of spike-and-wave discharges that are the characteristic electroencephalographic signs of absence seizures. The spontaneous pacemaker oscillatory activity of thalamocortical circuitry involves low threshold T-type Ca2+ currents in the thalamus, and ethosuximide is presumed to reduce these low threshold T-type Ca2+ currents in thalamic neurons. Ethosuximide also decreases the persistent Na+ and Ca2+ -activated K+ currents in thalamic and layer V cortical pyramidal neurons. In addition, there is evidence that in a genetic absence epilepsy rat model ethosuximide reduces cortical gamma-aminobutyric acid (GABA) levels. Also, elevated glutamate levels in the primary motor cortex of rats with absence epilepsy (but not in normal animals) are reduced by ethosuximide.

A toxicity and hazard assessment of fourteen pharmaceuticals to Xenopus laevis larvae

The toxicity of fourteen widely used human pharmaceuticals was determined using the Frog Embryo Teratogenesis Assay-Xenopus (FETAX). Stage 9 Xenopus blastulae were exposed for 96 h to single concentrations of commonly prescribed selective serotonin reuptake inhibitors (SSRIs), statin blood lipid regulators, non-steroidal anti-inflammatories, antibiotics, a stimulant, and an anti-epileptic. Toxicity, teratogenicity, minimum concentration to inhibit growth, and types and severity of associated malformations were determined. EC(10)s ranged from 3.0 mg/l to >100 mg/l and LC(10)s ranged from 3.6 mg/l to >100 mg/l. Toxicity varied between and within compound class of pharmaceutical. The fluoroquinolones, stimulants, anti-epileptics, and antibiotics tested were determined to be nontoxic and non-teratogenic at singular, water-soluble concentrations. The hazard quotients (HQ) for the pharmaceuticals ranged from 6.10 x 10(-7 )to 1.6 x 10(-4), all of which are orders of magnitude below EPA's levels for concern for harm to aquatic animals. Thus, based on the data from the present study, concentrations of individual pharmaceuticals currently detected in surface water are far below concentrations of effective and lethal concentrations.

Effects of anticonvulsant drugs on fetal mouse palates cultured in vitro

Maxillary regions of day-12.5 ICR mouse fetuses were dissected and cultured in a chemically defined serumless medium, and the effects of anticonvulsant drugs on in vitro palatogenesis were studied. The explants were treated for 72 h in vitro with 50 to 200 micrograms/mL diphenylhydantoin (DPH), 200 to 800 micrograms/mL sodium phenobarbital (PB), 12.5 to 400 micrograms/mL sodium valproate (VPA), and 3 to 100 micrograms/mL diazepam (DAZ). During the culture, the secondary palatal shelves of control explants elevated, grew medially, and fused after 72-h culture in a manner similar to the palatogenetic process in vivo. The fusion of palatal shelves was inhibited dose-dependently by treatments with DPH, VPA, and DAZ. PB showed no significant inhibitory effects on palatal fusion at concentrations up to 800 micrograms/mL. The in vitro toxicity of the anticonvulsants tested appeared to correlate with the relative in vivo teratogenic potential of the drugs. The present study demonstrated that the in vitro organ culture system should be useful for screening teratogenic agents, especially those causing cleft palate, and for exploring the mechanisms of cleft palate formation.

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.

A critical review of the toxicology of glutaraldehyde

Glutaraldehyde, a low molecular weight aldehyde, has been investigated for toxicity in humans and animals. Examination of this dialdehyde was indicated from previous studies with other aldehydes in which carcinogenicity of formaldehyde and toxicity of acetaldehyde and malonaldehyde have been disclosed. Information gaps concerning the actions of glutaraldehyde have been identified in this review and recommendations are suggested for additional short- and long-term studies. In particular, information regarding irritation of the respiratory tract, potential neurotoxicity, and developmental effects would assist in a complete hazard evaluation of glutaraldehyde. Further study related to disposition, metabolism, and reactions of glutaraldehyde may elucidate the mechanism of action.

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.

Parental epilepsy, anticonvulsant drugs, and reproductive outcome: epidemiologic and experimental findings spanning three decades; 1: Animal studies

In conclusion, it is clear that the experimental animal literature has been extremely beneficial in validating the teratogenicity of selected anticonvulsant drugs such as phenytoin and valproic acid, and in providing much needed information on pharmacokinetic parameters that are involved in altering normal embryogenesis. Continued efforts are needed to further elucidate the mechanism of teratogenic action for these drugs. It is clear from the work on phenytoin that reactive intermediates are important, and care must be taken to either avoid drug therapies that promote the formation of or inhibit the rapid degradation of toxic oxidative metabolites. For valproic acid and carbamazepine the pathogenesis of congenital defects remains much less defined. Until adequate information is ascertained on just how antiepileptic drugs disrupt normal development, it will be difficult, if not impossible, to develop either alternative medications or treatment strategies that maximize clinical effectiveness without the risk of an adverse pregnancy outcome. Such information emanating from animal studies shall, hopefully, be available in the not-too-distant future.

Use of fetal mouse salivary glands in culture to detect embryotoxicity: evaluation of eight additional chemicals

Several developmental processes interact to convert an epithelial bud into a gland with many lobes. For each chemical tested, 20 glands were placed into a control and each of 3 concentrations of the chemical. From dose-response curves, the concentration that reduced gland growth by 50% was determined and divided into the LD50 for mice. These ratios were used to compare the toxicity of the chemicals. The ratios of cyclamate, diphenhydramine, allopurinol, nitrofen and urethane would indicate that embryotoxicity would not be expected without maternal toxicity. Promethazine, diethyldithiocarbamate, and 5-fluorouracil would be expected to show embryotoxicity without maternal toxicity.

Susceptibility to cyclophosphamide and thalidomide of fetal rat limb buds grafted in athymic (nude) mice

Limb buds of day-14 rat fetuses were cut into pieces and transplanted into athymic (nude) mice. On the 7th and 9th days after grafting, the host nude mice were given cyclophosphamide intraperitoneally (10-120 mg/kg) or thalidomide orally (30-240 mg/kg). On the 20th day, the grafted tissue was examined macroscopically and histiologically. The grafts maintained in vehicle-treated nude mice showed considerable growth and tissue differentiation similar to in vivo. Growth and histogenesis of the grafts were significantly inhibited by treatment with cyclophosphamide (greater than or equal to 20 mg/kg). There was no indication that treatment with thalidomide (less than or equal to 240 mg/kg) adversely affects the development of grafted limbs. Thus, the susceptibility of transplanted rat limb buds to these two human teratogens was identical to the susceptibility of living rat fetuses. The heterotransplantation method of embryonic tissues may be of potential use for the study of teratogenic mechanisms and for the screening of human teratogens.

Teratogenic drugs inhibit the differentiation of fetal rat limb buds grafted in athymic (nude) mice

Forelimb buds of day-14 rat fetuses were cut into pieces and transplanted subcutaneously into athymic (nude) mice. On the 7th, 9th, and 11th days after grafting, the nude mice were treated with various drugs including rat teratogens. On the 20th day, the grafted tissue was examined macroscopically and histologically. While control grafts showed substantial growth and tissue differentiation similar to that observed in vivo, the differentiation of grafts was significantly inhibited by the treatment with 5-fluorouracil, cyclophosphamide, hydroxyurea, cycloheximide, mitomycin C, caffeine, aspirin, retinol palmitate, all-trans-retinoic acid, and ascorbic acid. Hydrocortisone, tetracycline, and thalidomide did not adversely affect the differentiation of grafts. Thus, the susceptibility of transplanted rat limb buds was generally close to the teratologic sensitivity of rat fetuses in vivo. The heterotransplantation method of embryonic tissues may be useful as a new experimental system in developmental toxicology.

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.

Criteria for identifying and listing substances known to cause developmental toxicity under California's Proposition 65

Because of the automatic restrictions and warning requirements imposed on substances identified by the state as "known to cause developmental toxicity," the Expert Committee recommends the use of criteria that emphasize human relevancy, biological plausibility, and evidence in support of a selective, adverse developmental effect at non-maternally-toxic doses. In many instances, data for substances of public concern will be insufficient at present to meet these criteria. The fact that a substance is not listed as "known to cause developmental toxicity" does not create a presumption that the substance is safe. The Expert Committee, therefore, urges that these substances be recommended for further testing and that high priority be given to conducting the necessary tests. The Expert Committee reiterates its concern that substances listed by the SAP be identified according to the toxic endpoints (cancer, male reproductive toxicity, female reproductive toxicity, and/or developmental toxicity) that led to listing. Further, the Expert Committee recommends that the state Health and Welfare Agency institute education programs emphasizing appropriate courses of action for citizens informed of exposures to substances known to the state to cause cancer, birth defects, or reproductive toxicity.

Teratogenic activity of lithium carbonate: an experimental update

The teratogenic potential of lithium carbonate was re-examined using mouse embryos as an experimental model. Externally it was found that, like many other psychoactive drugs, it exhibits a teratogenic activity that is confined to the development of the central nervous system. When administered intraperitoneally to pregnant females in doses ranging from 330 to 340 mg/kg at the very beginning of the ninth gestation day, i.e., during the critical period of the neural tube closure, it is embryotoxic, causes retardation of development, and induces the "neurotropic syndrome of malformations" comprising exencephaly, craniorachischisis, rachischisis, kinking of the spinal cord, and dilation of the fourth brain ventricle. Depending on the exact time of gestation when lithium carbonate is administered, the frequencies of exencephaly and spinal kinking after treatment 1 hour into the ninth day are compared with those after treatment 2 or 3 hours into the ninth day, incidence of exencephaly is increased at the later time and the incidence of spinal kinking is decreased.

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.

Embryotoxicity induced by alkylating agents: 5. Dose-response relationships of teratogenic effects of methylnitrosourea in mice

The teratogenic potency of the directly acting alkylating agent methylnitrosourea (MNU) was analysed in mice. Skeletal abnormalities were evaluated after treatment on either day 11 or 12 of pregnancy. Ectrodactyly was the predominant effect after treatment on day 11. Treatment on day 12 triggered especially double-sided microdactyly (method of analysis: measuring digit lengths). Litter variabilities were analysed using a new biometrical procedure. Using probit analysis, dose-response curves were computed from the experimental data obtained and the effective doses were calculated and compared with maternal toxicity. Low dose extrapolation was performed by use of various mathematical models which yielded very similar ED1/100 and ED1/1000 values.

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.

Estimating chemical developmental hazard in a chicken embryo limb bud micromass system

A chicken limb bud micromass system measuring the production of cartilage proteoglycans and the incorporation of radiolabelled thymidine and leucine was used to calculate potential developmental hazard. This hazard index was based upon the 50% inhibitory concentration (IC50) for these parameters and enabled classification of substances according to their intrinsic ability to inhibit differentiation at concentrations which were not cytotoxic. All-trans-retinoic acid, a potent teratogen, inhibited cartilage proteoglycan synthesis at a concentration which was more than 100 times lower than those affecting the incorporation of 14C-thymidine and 14C-leucine. On the other hand, sodium valproate, salicylate and isoniazid inhibited cartilage proteoglycan synthesis only near concentrations which were also cytotoxic. The results from testing with chicken limb bud micromass compared favorably, both with respect to effective concentrations and ratios describing developmental hazard to those from rodent micromass systems.

Teratogen update: valproic acid

Valproic acid use during pregnancy results in an absolute risk for spina bifida of 1-2%. This increased risk is comparable to the recurrence risk for neural tube defects and warrants informed counselling and access to prenatal diagnosis. There is no substantial evidence that valproic acid use increases the risk for other specific major malformations above the increased risk due to maternal epilepsy. Valproic acid may cause a characteristic pattern of minor facial malformations. Further definition and confirmation are required, and the magnitude of the risk needs to be determined. There are inadequate data to assess the magnitude, if any, of the risks for postnatal growth abnormalities and developmental disabilities associated with the use of valproic acid during pregnancy. Birth-defect monitoring programs and international collaboration among the staffs of monitoring programs played a major role in determining that valproic acid is a human teratogen.

An evaluation system for ranking chemicals with teratogenic potential

An evaluation system has been developed which permits comparison of the relative toxicity of chemicals based on their potential to alter embryonal and early-stage fetal development. It was considered necessary, from a regulatory standpoint, to develop a system for ranking chemicals which possess teratogenic potential. A detailed discussion of the various parameters selected and their relative importance is provided.

Approaches to defining the relationship of maternal and developmental toxicity

Maternal and developmental toxicities reported in the literature were examined in an attempt to define more clearly their relationship. Relationships were difficult to ascertain because maternal toxicity end points are not clearly defined, or even assessed, in every study. However, maternal toxicity accompanied by developmental toxicity is the most common outcome of in vivo testing. Approaches to define these associations have included assessment of acute maternal toxicity and teratogenicity; evaluation of maternal toxicity and its association to developmental toxicity in general, and to malformations, specifically; and examination of developmental variations, embryolethality, and altered growth. None has demonstrated an unequivocal relationship between specific maternal and developmental toxicities: Developmental disruption appears not to result unconditionally from maternal toxicity. Maternal "stress" appears to have some impact on development but resists further definition at this time. Variations in association may be due to the extent to which maternal homeostasis has been compromised. Several quantitative approaches to relating maternal toxicity and developmental toxicity in animal systems (ie, relative teratogenic index, adult/developmental toxicity ratio) may provide the most satisfactory means of evaluating developmental toxicity testing for assessment of hazard.

Comparison of maternal and fetal toxic dose responses in mammals

The role of maternal toxicity in adverse developmental outcome and the importance of maternal toxicity as a factor in developmental risk assessment have received increasing attention in the recent literature. This paper reviews these concepts and some of the experimental approaches that have been used to assess their importance. The often non-parallel nature of maternal and fetal toxic dose response curves makes specific comparisons of maternal and fetal toxic doses quite difficult. The use of specific ratios of maternal to fetal toxic doses, such as the A/D ratio and the Relative Teratogenic Index is discussed, including some of the difficulties encountered in assigning such ratios, and a compilation of A/D ratios for compounds for which this ratio could be estimated in more than one mammalian species is included. Results of our studies on the fungicide, dinocap, for which the A/D ratio does not seem to be consistent across species, are briefly reviewed. Maternal toxicity is usually evaluated only on the basis of significant mortality, dose-related weight loss, or obvious external observations. Some examples of other more specific indications of maternal toxicity that may be involved in teratogenesis are presented.

Dose-response relationship of cadmium embryotoxicity in cultured mouse embryos

Mouse embryos were exposed in vitro to 1.2 to 2.2 microM cadmium, and effects on embryotoxicity were examined after 39 h of culture. Teratogenic responses similar to in vivo were obtained at 1.2 to 2.2 microM with concomitant reduction in embryonic protein, while embryo deaths were increased from 13.8 to 93.3% at 2.0 to 2.2 microM. The response data of both teratogenicity and growth parameters, including embryonic protein, head length, crown-rump length, somite number, and protein and diameter of yolk sac, were acceptably fitted to a linear log-probit regression. These results suggest that (a) In chronic exposure conditions, the concentration of cadmium is a critical parameter in the manifestation of teratogenic potential, (b) as an estimation of interference in the growth of embryos, embryonic protein is one of the most sensitive endpoints while somite number is an insensitive criterion, and (c) a linear log-probit regression is applicable to the analyses of embryotoxicity data, including growth parameters in whole-embryo culture systems.

Weak acids may act as teratogens by accumulating in the basic milieu of the early mammalian embryo

Among the eleven drugs or chemicals which are well-documented human teratogens, eight (or their main metabolites) are weak acids whereas none is a weak base. Moreover, 23 out of 32 acids tested have been found to be teratogenic in at least one animal species. The acidic property of drugs may therefore be an important determinant of teratogenicity. We demonstrate here that the intracellular pH (pHi) of the mouse and rat embryo is higher than that of maternal plasma, as determined by the relative accumulation of dimethadione. The antiepileptic drug valproic acid and its pharmacologically active unsaturated metabolite accumulate in embryonic tissue to higher concentrations than in maternal plasma, whereas the essentially neutral amide of valproic acid (valpromide) or ethosuximide do not accumulate in the embryo; we further demonstrate in the rat that the pHi of the embryo decreases with advancing gestation; in general agreement with the pH partition hypothesis, the exposure of the embryo to valproic acid also decreases significantly during that period. Furthermore, the amides of two weak acid teratogens, valpromide and methoxyacetamide, and the imide ethosuximide, are much less teratogenic than their acid counterparts. Our results suggest that weakly acidic drugs, by virtue of their physico-chemical nature, accumulate in the early embryo with its relatively high pHi.

A methodology for assessing developmental and reproductive hazards of chemicals

A method to evaluate developmental and reproductive toxicity for a wide variety of chemicals has been developed. The assessment includes all effects on male and female reproductive function as well as effects in the developing embryo or fetus. Effects are evaluated using primary literature. Studies are classified as adequate, supportive or inadequate. Only adequate studies are used in further assessment. Reproductive and developmental toxicity of each chemical is assessed using both qualitative and quantitative information. Three elements are used to describe developmental toxicity of chemicals: Lowest Observed Effect Level (LOEL), weight-of-evidence classification and a Risk Ratio (RR). For reproductive toxicity only two elements are used, LOEL and weight-of-evidence. LOEL is defined as the lowest dose at which statistically significant effects are found. A risk ratio is calculated using the levels of the chemical producing acute maternal lethality (LD50 or LC50) to the levels producing developmental toxicity in the same animal species and route of administration. The risk ratio is used to indicate the extent to which the developing embryo or fetus is more susceptible than the dam to the toxic effects of a chemical. A toxicity score (A through E) is assigned to each chemical from a two- or three-dimensional matrix based on the elements of assessment. The score reflects the degree of hazard associated with each chemical. To date, 110 chemicals have been evaluated using this system. From the selected databases, 188 articles were reviewed. Of these, 103 (55%) were judged to be adequate, 23 (12%) provide supportive information, and 62 (33%) were inadequate and excluded from the assessment methodology. Scores for 47 chemical with some adequate data available show that high hazard substances (28 A and B) outnumber those associated with lower hazards (a total of 19 C, D and E). This may reflect a selection process for testing which favors substances suspected of causing adverse reproductive and developmental effects.

Post-implantation whole embryo culture and the study of teratogenesis

The two major applications of whole post-implantation rat embryo culture (WEC) are as a test for teratogenicity in safety evaluation studies and as a tool in the investigation of mechanisms of teratogenesis. As a test system, WEC possesses many of the characteristics necessary for an in vitro screen. However, its use on a large scale is disqualified for reasons of cost and the demand for technical expertise to perform cultures and interpret findings, compared with other in vitro teratogenicity screens, although its use might be preferred when only a few compounds require testing. The major value of WEC lies in its use for studying mechanisms of teratogenesis. The opportunity to study the embryo in isolation and the versatility afforded by the technique offer considerable advantages in this context.

Developmental phase-specific and dose-related teratogenic effects of ethylene glycol monomethyl ether in CD-1 mice

Several animal species have shown a teratogenic response to inhaled or ingested ethylene glycol monomethyl ether (EGME). The present study examined the developmental phase specificity and dose-response characteristics of EGME-induced embryotoxicity. Pregnant CD-1 mice (vaginal plug positive day = gestation Day [gd]0) received multiple or single doses of EGME by gavage between gd 7 and 14. Fetuses were examined on gd 18 for external and skeletal malformations. EGME was not maternally toxic after multiple doses of 250 mg/kg or a single administration of up to 500 mg/kg. EGME induced embryotoxicity as manifested by reduced gd 18 fetal weights and increased resorptions. The observed malformations were specifically related to the developmental stage at the time of exposure. Exencephaly resulted after EGME exposure between gd 7 to 10 whereas paw anomalies (syndactyly, oligodactyly, and stunted digit No. 1) predominated during later stages of development. Paw anomalies were maximal after administration on gd 11, and forepaws exhibited greater susceptibility than hindpaws. The no observed effect dose for the induction of digit malformations after a single administration of EGME on gd 11 was 100 mg/kg. At 175 mg EGME/kg digit anomalies were induced without any concurrent reduction in fetal body weight while at 250 mg/kg and above, digit anomalies occurred concurrently with reduced fetal body weight.

Teratogenic potency of TCDD, TCDF and TCDD-TCDF combinations in C57BL/6N mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDF) cause the same spectrum of fetal anomalies in C57BL/6N mice. Pregnant dams were treated with TCDD, TCDF and combinations of the 2 compounds on gestation day 10, and examined for maternal and fetal effects on day 18. The fetal kidneys were the most sensitive target for teratogenicity. The dose response for cleft palate induction fit the probit model for both compounds, suggesting that TCDD was approximately 30 times more potent than TCDF. The interaction between these 2 compounds was consistent with a model for additive toxicity.

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.

Mouse limb bud development in submerged culture: quantitative assessment of the effects of in vivo exposure to retinoic acid

Retinoic acid, suspended in cottonseed oil, was administered via gavage to pregnant mice (ICR strain) on day 11 (E 11) of gestation at doses of either 20, 40, or 80 mg/kg. Fetuses were examined for external malformations on day 17 (E 17). Retinoic acid treatment induced micromelia (with the elimination of several long bones at higher doses) and digital defects (ectrodactyly and syndactyly) in a dose-dependent manner in fetuses examined on day 17. Hindlimbs were affected more than forelimbs. In another group of experiments, limbs exposed to retinoic acid treatment in utero on E 11 were cultured on E 12 and maintained for 3 days in submerged culture. Cultured limbs were examined qualitatively for digital and long bone defects, and image analysis of the area and form of bone anlagen of cultured limbs was used to quantitatively evaluate the teratogenic potential of retinoic acid. The qualitative evaluation indicated that the retinoic acid-induced effects obtained in vivo and with pretreated, cultured limbs were essentially the same, except that the severity of regional effects changed as a result of culture. The incidence of ectrodactyly was higher with cultured limbs than with E 17 fetal limbs, but fewer cultured limbs were missing long bones. These results suggest that culturing limbs, after they have been pretreated in utero, modifies their response to a teratogen and demonstrates that the paw skeleton is extremely sensitive to teratogen treatment under these experimental conditions. Therefore, care must be exercised when attempting to compare in vivo and in vitro teratogenic data. This study also clearly demonstrates the power and usefulness of image analysis for quantitative evaluation of both the area and form of a cultured specimen such as the developing limb bud. Quantitative, image analysis of cultured limbs showed a dose-dependent decrease in area of both fore- and hindlimbs. The effect was most severe in hindlimbs. In the forelimb, the paw was affected more than the long bones; as the dose increased, this disparity of effect also increased. With the hindlimb, a greater effect on the paw occurred only at 80 mg/kg. Computing the soft tissue/bone ratio illustrated that retinoic acid had a greater effect on chondrogenic tissue than on soft tissue.

Acute embryopathic effects of ethanol in the Long-Evans rat

Thirty-two pregnant Long-Evans rats were divided into 10 groups of 3 or 4 pregnant rats, and each rat was given a single dose of 4 ml ethanol/kg (20 ml/kg of a 20% solution) between d 6 and 15 of gestation. An 11th group of 50 pregnant rats received distilled water and served as controls. Offspring body weights were decreased in groups of rats given ethanol as compared to controls (3.0-3.6 g, versus 3.9 g for controls). Total litter weight was decreased in dams given ethanol on d 6. Skeletal variants were seen in 13-78% of the offspring given ethanol, compared to 0.6% of the controls. Variations may be considered as additional signs of embryotoxicity. Malformations such as hydronephrosis, pelvic kidney, microcephalus, cranioschisis, and microphthalmia occurred in 72-100% of the ethanol treated offspring, as compared to 12% of controls. Hydronephrosis was most frequent on d 9 or 14, pelvic kidney on d 8 and 11, and microphthalmia from d 10-12. Cranioschisis was maximal on d 7, 11, and 15, and microcephalic offspring were most frequently born to dams given ethanol on d 7 or 14. Skeletal defects were usually single entities, while soft-tissue anomalies occurred in a consistent pattern. These results suggest that ethanol is a stage-specific teratogen in the rat at comparable exposure levels attained by many humans.

Embryotoxicity induced by alkylating agents. Teratogenicity of acetoxymethyl-methylnitrosamine: dose-response relationship, application route dependency and phase specificity

Acetoxymethyl-methylnitrosamine, the acetate ester of the presumed reactive metabolite of dimethylnitrosamine, is an effective teratogen in NMRI mice. An unusual phenomenon of application route specificity and a pronounced phase specificity of the teratogenic effects induced are demonstrated. Dose-response relationships are established. Some further arguments are given to support the basic hypothesis that the teratogenicity of alkylating agents is closely correlated to the DNA alkylation rate of embryonic cells.

A selection of candidate compounds for in vitro teratogenesis test validation

The Consensus Workshop on In Vitro Teratogenesis Testing recommended that test validation be facilitated by a listing of agents with defined teratogenicity; subsequently, a panel was convened to review and select such agents. This communication established a list of 47 compounds or conditions which demonstrate a wide range of teratogenicity in vivo. The agents were chosen primarily on the strength of the literature base denoting their in vivo effects. The tables note a number of general biological and toxicological characteristics for each agent, and the details of representative in vivo teratology studies are summarized and referenced. This list is intended to serve as a base for in vitro teratogenesis test validation and should prove useful in developing and identifying those systems which will contribute to a more effective testing program.