A review of the contribution of whole embryo culture to the determination of hazard and risk in teratogenicity testing.

Whole embryo culture appears to be an excellent method to screen chemicals for teratogenic hazard. Compared to in vivo testing it is cheap and rapid and does not involve experimentation on live adult animals. Also in the important area of risk estimation whole embryo culture offers distinct advantages over in vivo teratogenicity testing. Adverse embryonic outcomes (malformations or embryotoxicity) are directly related to the serum concentration of the compound being tested and can be compared to the serum concentration in the human. A similar comparison is not possible after in vivo testing because for most compounds there are major pharmacokinetic differences between humans and experimental animals. In vivo testing is also limited by the possibility that metabolites that occur in the human do not occur in the test animal. This problem can be overcome in the in vitro system by adding the metabolite directly at the desired concentration either with or without the parent compound. There is only one major disadvantage to in vitro testing and that is the limited period of embryogenesis that is undertaken in the commonly used culture system. This restricts the range of malformations that can be induced and may render the testing system unsuitable for compounds that are likely to exert their major toxicological effect late in gestation. Any evaluation of whole embryo culture for hazard and risk assessment in teratology must take into account the limited value of currently used in vivo methods. Over 2000 chemicals have been reported to be teratogenic in experimental animals exposed in vivo (Shepard, Catalog of Teratogenic Agents, 1989). In comparison only about 20 chemicals are known to cause birth defects in the human. This large number of in vivo false-positive cannot easily be distinguished from true-positives. In this respect in vivo testing is severely deficient. The embryo culture testing system would also be expected to produce many false-positives; but by comparing effective drug concentrations with human therapeutic concentrations they can be differentiated from true-positives. The most serious deficiency for an in vivo or in vitro teratogenicity testing system would be false-negatives. This has not been a problem in the validation of in vitro testing so far (except perhaps procarbazine), but difficult drugs such as thalidomide were not included. Thalidomide remains an important index chemical because it is not teratogenic in rats or mice but is teratogenic in the rabbit and human. It is likely that these species differences are due to metabolic differences between species and it is possible that if the proximate teratogen/s of thalidomide were identified they would be teratogenic in rat embryo culture. Whole embryo culture remains a very powerful technique that should continue to contribute to the determination of the safety of drugs and other chemicals during pregnancy.