Developmental toxicity animal tests

The combined repeated dose toxicity study with the reproduction/developmental toxicity screening test (OECD TG 422, US-EPA OPPTS 870.3650) comprises a basic repeated dose toxicity study and a fertility/developmental toxicity screening test and, therefore, can be used to provide initial information on possible effects on a limited number of reproductive performance parameters. The test does not provide complete information on all aspects of reproduction, has a relatively short period of exposure, and does not provide evidence for dehnite claims of no reproductive effects, while positive results are useful for initial hazard assessment. Furthermore, results regarding repeated dose toxicity are influenced by the pregnant state of the female animals (see also Sections 4.7.3.1 and 4.7.5.2.2). 

The reproductive/developmental toxicity screening test can provide initial information on possible effects on reproduction and/or development and may make it possible to identify a substance as being toxic to reproduction, i.e., the test gives a clear positive result. However, this test offers only limited means of detecting postnatal manifestations of prenatal exposure or effects that may be induced during postnatal exposure. In addition, because of the study design (e.g., relatively small numbers of animals per dose level, relatively short smdy duration), the test will not provide evidence for definite claims of no effects. 

OECD TG 421 and TG 422 Reproduction/ Developmental Toxicity Screening Test (with or without the Combined Repeat Dose Toxicity Study) From 2 weeks prior to mating until at least day 4 postnatally 3 dose levels plus control n = 8-10 pregnant females Fertility Pregnancy length and birth Fetal and pup growth and survival until day 3 + short-term test - limited exposure period - limited number of end-points - limited sensitivity due to number of animals... 

In vitro developmental toxicity systems have clearly been usefid for studies of mechanisms of developmental effects (e.g., Datson et al., 1989) — use (3) in the list above. It is unclear, though, whether in vitro developmental toxicity tests will provide useful information about developmental toxicity that is not derived from whole animal studies [use (4) from the list]. As is true for a possible use as a prescreen, the interpretation of a positive finding in an in vitro test will depend on knowing the exposure level in vivo. When this is known, the in vitro information could be helpful. The results of in vivo studies, though, would still likely be considered definitive for that species. 

Although chemical sensitization is a more complex process than immunosuppression, by far most of the efforts on developing in vitro assays are in this field. An important reason for this is that from the various fields of immunotoxicity, most of the animals are used for sensitization testing. In fact the number of animals required for sensitization is second only after developmental toxicity testing. 

When maternal toxicity and developmental toxicity occur together in the same animals, it is ofren difficult to determine whether the observed fetal effects are secondary to maternal toxicity or due to selective developmental toxicity. The FDA Redbook states the following In situations where developmental effects are observed only at doses where there is a substantial amount of maternal toxicity, then the possible relationship between maternal toxicity and the developmental effects should be evaluated in order to make a proper assessment regarding the toxicity of a test substance. No guidance is given on how this should be done. A more useful consensus document on this subject has recently been published (9), which is discussed in detail in Chapter 24. 

In Europe, the developmental toxicity testing (including teratogenicity) of new cosmetic ingredients is performed according to the Cosmetics Directive 76/768/EEC only alternatives leading to full replacement of animal experiments should be used. This chapter presents the three scientifically validated animal alternative methods for the assessment of embryotoxicity the embryonic stem cell test (EST), the micromass (MM) assay, and the whole embryo culture (WEC) assay. 

Reproductive and developmental toxicity studies in laboratory animals are conducted under regulatory testing guidelines (e.g., (1-3) as part of the process of evaluating the risk of pharmaceuticals and... 

Therefore, a validation exercise with a variety of compounds with unknown mechanisms of developmental toxicity has only limited value if only used to derive an overall predictability rate of a single assay. It is more useful to elucidate the applicability domain of the assay in terms of the mechanisms of development covered, and to validate that aspect by testing compounds that do or do not affect the applicability domain. For single end point assays such as specific receptor activation assays, this exercise is relatively straightforward. For more complex assays such as those involving embryonic cell differentiation, the understanding of the applicability domain is more complex, as extensive research with the embryonic stem cell test has learned (27, 47). Whole embryo cultures are probably more straightforward in terms of applicability domain as they involve the entire embryo in a limited window of development, but such assays are complex and not animal free. 

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