Distribution of phenobarbital and phenytoin in pregnant rats and their fetuses.

Studies were conducted to determine the extent of fetal exposure to phenobarbital (PB) and phenytoin (PHT) in pregnant rats. Two groups of rats determined to be in the third week of pregnancy received 30 mg/kg as a single intraperitoneal (i.p.) dose of either PB or PHT. At various times (from 15 min to 48 h for PB and from 20 min to 16 h for PHT), the rats were killed and whole blood, plasma, maternal brain, fetal brain and liver, whole fetuses, and placentas were rapidly collected. Concentrations of PB, PHT, and its major metabolite, p-hydroxyphenytoin (HPPH), were measured by high-performance liquid chromatography (HPLC). PB and PHT appeared rapidly in plasma and all tissues; the maximum HPPH concentrations appeared much later. The highest PB concentrations were detected in plasma, followed by placenta, whole fetus, maternal brain, fetal liver, and fetal brain. In contrast, PHT concentrations were higher in fetal brain and liver than in whole fetuses. HPPH concentrations were much lower than PHT and were not detected in maternal brain. These results show that PB and PHT readily distribute to the rat fetus, that drug distribution within the fetus is regional, and that changes in maternal plasma concentrations of these antiepileptic drugs (AEDs) are accompanied by parallel changes in fetal tissue.