Abstract:The rat whole embryo culture was compared to an in vivo experiment with regard to embryotoxicity as well as exposure characteristics, using phenytoin as a model compound. Intra-embryonic concentrations and their embryotoxic effects were determined on gestation day 11 after in vitro administration of 50-150 mg/ml or in vivo gavage of 500-1500 mg/kg body-weight on gestation day 10. In addition, exposure kinetics were studied in vivo after a single oral dose on gestation day 10, and developmental defects on gestation day 21 were scored. The embryotoxic effects observed on gestation day 11 were more pronounced after in vitro exposure in comparison to in vivo exposure at similar intra-embryonic concentrations. Exposure of phenytoin on gestation day 10 in vitro via the culture medium resulted in general embryotoxicity on gestation day 11, whereas in vivo effects as determined on gestation day 11 were minimal. Plasma concentrations of phenytoin increased and plateaued around 35 mg/ml during the 48 hr monitoring period. Plasma concentration curves and pharmacokinetic parameters did not show remarkable differences between the dose groups, indicating that absorption is the limiting factor at the dose range used. Although the developmental effects were minimal as observed in vivo on gestation day 11, specific malformations (defects encompassing the urogenital, craniofacial and skeletal systems) were observed on gestation day 21. These findings show that with similar intra-embryonic concentrations of phenytoin the embryotoxicity in rat whole embryo culture was not comparable with the in vivo embryotoxicity as determined on gestation day 11. This discrepancy may at least partly be explained by differences in exposure characteristics.In embryotoxicity testing the in vitro test with closest resemblance to in vivo complexity available is the rodent whole embryo culture (New 1978). This system can be used to detect developmental effects of compounds on the isolated intact embryo during a developmentally critical 48 hr period (Schmid 1985; Bechter & Schö n 1988;Piersma et al. 1995Piersma et al. & 1996. For the interpretation of results of in vitro embryotoxicity testing, the nature of the effects observed and the lowest effective compound concentration are essential. As in vitro systems necessarily represent a small part of the living animal, extrapolation of these findings to the in vivo situation is not straightforward.The present study was carried out to compare an embryo culture experiment with an in vivo experiment in the rat with regard to exposure and effect characteristics, using phenytoin as a model compound. In man, phenytoin causes growth retardation, microcephaly, mental deficiency and craniofacial anomalies, known as the foetal hydantoin syndrome (Hanson & Smith 1975). Growth retardation and craniofacial defects have been produced in mice and rats in vivo (Lorente et al. 1981;Rowland et al. 1990; Finnell & Dansky 1991) and in embryo culture (Bruckner et al. 1983;Lindhout et al. 1987;Ozolins et al. 1995). T...