Developmental toxicity defined

Developmental toxicity, defined in its widest sense to include any adverse effect on normal development either before or after birth, has become of increasing concern in recent years. Developmental toxicity can result from exposure of either parent prior to conception, from exposure of the embryo or fetus in utero or from exposure of the progeny after birth. Adverse developmental effects may be detected at any point in the life span of the organism. In addition to stmcmral abnormalities, examples of manifestations of developmental toxicity include fetal loss, altered growth, functional defects, latent onset of adult disease, early reproductive senescence, and shortened life span (WHO/IPCS 2001b). 

Zebrafish embryo assay results were compared to the ToxCast in vitro assay features from the predictive model of developmental toxicity (50). A majority of the features were significant between the zebrafish data and predictive models, despite the fact that the zebrafish assay did not correlate with global developmental toxicity defined by species-specific ToxRefDB data. The top 15 chemicals predicted to be developmental toxicants and bottom 15 chemicals predicted not to be developmental toxicants varied in their endpoint responses and logP values. Padilla et al. (35) noted that chemical-physical characteristics could limit the amount of chemical seen by the embryo due to poor solubility or poor uptake. This may be the reason that a majority of the bottom 15 chemicals with no zebrafish embryo activity had logP values less than 1.0. The bottom 15 chemicals with zebrafish embryo activity could almost exclusively be characterized by the negative predictors of the species-specific developmental toxicity models, which may be indicating that these predictors have differing roles between mammalian and zebrafish development. 

The proposed US EPA weight-of-evidence (WOE) scheme for suspect developmental toxicants defines three levels of confidence for data used to identify developmental hazards and to assess the risk of human developmental toxicity (1) definitive evidence for human developmental toxicity or for no apparent human developmental toxicity, (2) adequate evidence for potential human developmental toxicity or no apparent potential human developmental toxicity, and (3) inadequate evidence for determining potential human developmental toxicity. The scheme may require scientific judgment based on experience to weigh the implications of study design, statistical analyses, and biological significance of the data. 

Even if there is little evidence that maternal toxicity (defined as reductions in maternal body weight) is consistently associated with major malformations, there is clear evidence that substantial reduction in maternal weight is linked with other manifestations of developmental toxicity. These manifestations include decreased fetal weights, and skeletal anomalies (e.g., wavy ribs) in rats and decreased fetal weights, post implantation loss, abortions, and skeletal defects in rabbits (e.g., unossified sternebrae, metatarsals, metacarpals, or caudal vertebrae). 

Profiling environmental chemicals by a defined set of biochemical and cellular assays raises important concerns about the means by which this information can be used to predict in vivo developmental toxicity and in broader terms the use of this HTS information for problem formulation steps in a lifestage-specific risk assessment. 

Since most chemicals caused different phenotypic outcomes between the rats and rabbits, species-specific models were analyzed, with 251 chemicals evaluated in the rat model and 234 in the rabbit (Fig. 2). Cross-validation balanced accuracies in the resulting classification models were 71% for the rat model (12 features), and 74% for the rabbit model (7 features). Each model contained positive predictors or assay features generally affected by the developmental toxicants (as defined above) and negative predictors or assay features that were generally affected by the nondevelopmen-tal toxicants (as defined above). 

It is in this context, that in 2009, the DART committee formed a Steering Team to work on a project titled Consensus List of Developmental Toxicants. The Steering Team published a report of their deliberations and defined developmental toxicant in terms of its concentration in vitro (27). Daston et al. (27) based the definitions of positive and negative developmental toxicants according to their exposure conditions. That is, compounds on the list could have an exposure concentration that is unequivocally positive and a concentration that is unequivocally negative. In addition, only permanent effects that alter fetal organization, particularly structural malformations, were considered developmental toxicity. For example, fetal weight decreases (which are commonly used endpoints in risk assessment) are not considered developmental toxicity for the purposes of this list. 

The second objective of the hazard assessment concerns characterization of the identified hazards of a particular substance. Under REACH this means that the registrant must define so-called derived no-effect levels., abbreviated DNELs. With respect to human health, these values constitute exposure levels above which humans should not be exposed and below which risks for humans are considered controlled. The DNEL derivation is a complex process which comprises several conversion steps and the application of different assessment factors. In the case of reproductive toxicity, the registrant derives separate DNELs with respect to developmental toxicity on the one hand and to impairment of sexual function and fertility on the other hand. 

The risk assessor should be sensitive to certain dose-response patterns that are often encountered in studies on developmental toxicity. For example, the lowest effective doses in adults and young are often similar or may be the same, but the type of effects may be very different as well, the effects on the developing child may be permanent (or lead to latent effects), whereas the effects on the adult may be transient. Also, the end-points used in evaluating alterations in children s health may vary considerably. The difference between the maternal toxic dose and the developmental toxic dose may at times be related to the relative thoroughness with which end-points are evaluated. Also, the variability and level of severity within a particular end-point need to be defined, since end-point variability and level of severity can have a significant effect on the power of the study and the ability to establish an effect level. Approaches to carrying out dose-response assessments are described below. 

Reproductive and developmental toxicity data from animal experiments and human studies should be assessed based on defined criteria. One of the following judgments can be made either the toxicity data are sufficient (or insufficient) to ascribe an adverse effect to a specific agent under specified conditions, or the data are sufficient (or insufficient) to conclude that there is no adverse effect. To be characterized as sufficient, the database must include information on the full range of potential adverse male and female reproductive effects and developmental effects, and the actual range of conditions of exposure must be known in sufficient detail to determine whether the dose, duration,... 

The dose-response evaluation defines the range of doses that produce reproductive and developmental toxicity, the routes of exposure, the timing and duration of exposure, the species specificity of effects, and any pharmacokinetic or other considerations that might influence comparison with human exposure. Much of the focus is on identification of the adverse effect observed at the LOAEL and the NOAEL for the study. 

The low dose should be a no-observed-adverse-ef-fect level (NOAEL) for both dams and conceptuses. The NOAEL is defined as the highest dose (or exposure concentration) at which no statistically significant and/or biologically relevant adverse effects are observed in any adequate developmental toxicity study . The middle dose may or may not result in maternal and/or developmental toxicity and should be a lowest-observed-adverse-effect level (LOAEL), defined as the lowest dose or exposure concentration at which a statistically significant and/or biologically relevant adverse effect is observed in any adequate developmental toxicity study . The characteristics of the NOAEL (or the LOAEL) are that (1) it is obviously experimentally derived and therefore dependent on the statistical power of the study (which is in turn dependent on the number of animals employed) (2) it is dependent on the number and sensitivity of the parameters examined and (3) its presence implies a... 

Once a NOAEL (or LOAEL) is provided by the experimental data, the proposed next step by risk assessors is to define a reference dose for developmental toxicity (RfDoT) according to the following equation ... 

A second use for NOAELs (or LOAELs) is in the calculation of a proposed margin of exposure (MOE) for developmental toxicity to be used in risk characterization. The MOE is defined as the ratio of the NOAEL from the most sensitive or appropriate species to the estimated human exposure level from all potential sources. If the MOE is very high relative to the estimated human exposure level, then risk to the human population would be considered low. 

The list of alternative tests for reproductive toxicity, at variable stage of development, is fairly long given the complexity of the reproductive cycle and the multiple cell types and functions involved. An official source of information on alternative test development is the website of the European Centre for the Validation of Alternative Methods (ECVAM) (http //ecvam-dbalm.jrc.ec. europa.eu/ updated to 15 June 2013). Forty methods are listed for the area of reproductive toxicity. They are split into four categories effects on female fertility (n = 8), effects on male fertility n= 10), developmental toxicity (n = 21), and genotoxicity-mutagenicity (n= 1). Only eight of these methods have been developed up to fully defined protocols that can be downloaded from the same website ... 

Developmental Toxicity. Evidence indicates that NDMA is fetotoxic to rats and mice, but NOAELs have not been defined. Well-conducted developmental studies using several exposure levels and environmentally relevant routes of exposure could provide the dose-response information necessary to determine the threshold for fetotoxicity and to determine the possible relevance and risk for humans. Additional studies also could determine if NDMA is a transplacental carcinogen. 

Developmental Toxicity. Data regarding the developmental toxicity of hydrazines in humans are not available. Data regarding the developmental effects of hydrazines in animals are limited to a study which reported increased fetal and neonatal mortality following exposure to hydrazine by the parenteral route (Lee and Aleyassine 1970). No apparent developmental effects were seen after oral exposure of pregnant hamsters to 1,2-dimethylhydrazine dihydrochloride (Schiller et al. 1979). Studies that investigate the developmental effects of 1,1-dimethylhydrazine for any exposure route, as well as studies that better define the dose-response relationship for the developmental effects of hydrazine and... 

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