Developmental toxicity embryo models

Liu, P. et al., Developmental toxicity research of ginsenoside Rbl using a whole mouse embryo culture model, Birth Defects Res B Dev Reprod Toxicol, 74, 207, 2005. 

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 methods presented here provide the basis for conducting zebrafish developmental toxicity research. Some fundamental procedures for the care and maintenance of a zebrafish colony for the purpose of collecting embryos has been provided, but greater detail and other procedures have been published elsewhere (12-14). While there are many very large zebrafish facilities and several very technically advanced zebrafish labs, one of the most attractive elements of the zebrafish (Fig. 1) as a model of embryonic development is how readily and inexpensively experiments can be performed. 

The FETAX has been in use as a screening test in our laboratory since 1999 and is based on the standard guide of the American Society for Testing and Materials (1). FETAX is conducted under the approval of our local ethical committee using Xenofus laevis embryos and constitutes an efficient developmental toxicity screening test when performed early in drug safety development. Its possible use as a mechanistic model is not discussed herein. 

Regarding the positive compounds, six out of eight compounds showed a developmental toxicity both in mammalian embryo-fetal studies and in FETAX. Concerning the negative compounds, four out of five chemicals were found negative in both models. There were two false negative compounds, acetylsalicylic acid and dexame-... 

These studies are designed to test chemicals for developmental toxicity. The program is also developing several techniques for evaluating potential toxic effects of chemical exposure on the reproductive system of humans and rodent models and on the developing embryos of rodents. Research efforts include increasing current knowledge about the site and mechanism of action for reproductive and developmental toxicity. 

As described elsewhere in this book, SARs of increasing sophistication have been used for some time to make toxicity predictions for a variety of organ systems, including the developing embryo/fetus.20 In silico or computer-based analyses are of two primary kinds knowledge or "rule-based" systems and correlative or "statistically based" systems. The details of the different prediction systems have been reviewed in detail elsewhere,21-22 but for the purposes of developmental toxicity prediction two models are discussed briefly here. 

The ECVAM models were designed to facilitate the blinded categorization of a broad array of structurally unrelated agents into three categories of embryotox-icity. Thus, the correct embryotoxicity prediction is critical. We do not use the tests in that way instead, we use them to rank-order the relative developmental toxicity risk within or across chemical series, to identify the agent with the least inherent risk- irrespective of the ECVAM-predicted embryotoxicity class. We are not alone in this approach, as the chick embryo retinal culture is also used for ranking (G. Daston (daston.gp pg.com), personal communication). 

Figure 9-5 The sensitivity of the conceptus to a theoretical teratogen during rat gestation (modified from 161). The most susceptible window is organogenesis with low levels of vulnerability at the time of implantation and the period of functional maturation. Superimposed are the approximations of when the developmental landmarks that are represented in the four in vitro tests occur. The chick embryo neural retina model (CENR) represents events around GD 10-13. The mouse embryonic stem cell test (EST) corresponds roughly to the period of GD 6-10 in the rat, near the peak of sensitivity. Whole embryo culture (WEC) recapitulates the window at the peak of sensitivity, between GD 9-11 or GD 10-12 depending upon the window within which the culture is conducted. Rabbit cultures are also done between GD 10-12. Represented by the single ( ) and double asterisk ( ), respectively, are the initiation and termination of the dosing period in regulatory compliant preclinical embryo/fetal toxicity studies. Thus, the zebrafish is the only model that permits exposure to test article during this important period.

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