Numerous life-threatening diseases, such as epilepsy, necessitate long-term treatment of patients even during pregnancy and lactation, since discontinuation can cause serious harm to the mother, and subsequently her offspring. However, exposing the highly sensitive developing brain to some drugs, including antiseizure medications valproate and lamotrigine, may result in permanent congenital defects or neurobehavioral issues in later life. The developing brain is protected by its own brain barrier mechanisms (i.e. blood-brain and blood-CSF barriers) but also by the placental interface between maternal and fetal circulations. Factors determining transfer across these barriers include cellular drug transfer mechanisms such as transporters and drug metabolising enzymes but also the extent of drug binding to circulating plasma protein.
GAERS (Genetic Absence Epilepsy Rat from Strasbourg) at embryonic day (E) 19, postnatal day (P) 0, 4 and 21, and young adults were administered valproate (30 or 100 mg/kg) or lamotrigine (6 mg/kg) either alone or in combination with respective [3H]-tracers. In chronic experiments, females commenced on a valproate-containing diet 2 weeks prior to mating and throughout pregnancy. The offspring were studied. Thirty minutes following drug injection, blood, CSF and brain samples were collected from terminally anaesthetised animals (urethane, 2g/kg). Radioactivity was measured and transfer expressed as ratios of radioactivity (Mena±SD%) in fetal over maternal plasma (placental barrier) or in CSF or cortex over fetal plasma (fetal brain barriers). Drug binding to plasma protein was established by ultrafiltration. Transcriptomic analysis of brain cortex and choroid plexus was used to determine expression of cellular drug transfer mechanisms at E19, P5 and adults
Brain entry of valproate was higher in fetal than postnatal animals (76±12%, n=6 E19 and 21±3%, n=4 adults), which may be partially attributed to reduced valproate binding to plasma protein as its free fraction decreased from 85±2% in E19 to 65±10% in adults. Brain entry of lamotrigine was not age-dependent. In acute experiments, combination therapy had no effects on brain and CSF entry of valproate but enhanced entry of lamotrigine into the adult brain (60±9% to 124±20%). Following long-term valproate exposure using a formulated diet, its entry into adult brain decreased. However, maternal exposure to valproate during pregnancy led to increased brain and CSF entry in E19 fetuses and postnatal pups. Placental transfer of valproate at from maternal into fetal circulation decreased substantially from 64±21% to 36±15% (n=8) when co-administered with lamotrigine. Developmental differences in brain entry of valproate may be correlated with expression of some transporters, including Slc22a8, and drug metabolising enzymes.
Present results suggest that combination therapy may mitigate risk of toxicity as placental transfer of valproate decreased when co-administered with lamotrigine. However, in fetuses chronically exposed to valproate throughout gestation, lamotrigine co-administration resulted in increased brain entry of valproate but decreased lamotrigine entry. Overall, findings from this study demonstrated the need to understand mechanisms that influence drug entry into the brain and across the placenta in order to provide an evidence base for devising modulatory strategies to achieve desired clinical outcomes such as reducing collateral exposure of developing brain to maternally administered drugs.