Abstracts

Rationale: Women with epilepsy typically require continuation of pharmacotherapy throughout pregnancy. Changes in antiepileptic drug (AED) dosage during gestation must be approached with caution in order to minimize harmful effects on the fetus. Dramatic changes in maternal physiology occurring during pregnancy can significantly alter AED concentrations in the maternal brain, evidenced by studies in humans where altered efficacy of AED during gestation has been observed. While increases in metabolic enzyme activity can contribute to decreased brain concentrations of AEDs, increased activity of xenobiotic efflux transporters at the blood-brain barrier (BBB) is also suspected to be a factor leading to diminished drug levels in the central nervous system. Two abundant efflux transporters at the BBB are P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP). In humans, some AEDs that are P-gp substrates exhibit decreased efficacy, requiring increased doses during pregnancy.Methods: We tested whether changes in membrane localization of P-gp and BCRP at the maternal BBB during gestation would be associated with altered uptake and efficacy of AEDs. Beginning with a model P-gp substrate, methadone, we found the antinociceptive response in non-pregnant (NP) CD1 female mice highly variable throughout the estrus cycle. Significant differences between NP females and gestational day 8 (gd8) females were observed when NP females were tested during estrus (AUEC NP= 7374 +/- 373; gd8 = 2767 +/- 690, n= 8; p < 0.01). This decrease in antinociceptive activity was correlated with a shift in membrane localization of P-gp and BCRP. Estradiol and progesterone levels, and their cognate receptors were also quantitated in these groups. Prior to conducting pharmacokinetic and pharmacodynamic analyses of AEDs, we determined thresholds for minimal and maximal seizures in 6 week old CD1 female mice. Corneal stimulation was administered using an ECT Unit (Ugo Basile, Italy), stimulus parameters 90Hz, 0.9ms pulse width, 0.2s duration. The staircase estimation of median seizure threshold was done using 33 mice, n>3 for each current setting (10-26mA). Behavioral endpoints for minimal seizures included jaw and forelimb clonus, ventral neck flexion, and loss of posture; maximal seizure was tonic hindlimb extension (180 degrees). Results: An intensity response curve was generated and the convulsive threshold was evaluated as CS50 (behavioral endpoint seen in 50% of mice tested). The CS50 for minimal seizures was 16mA; for maximal seizures was 23mA. Phenytoin (10mg/kg i.p.) and lamotrigine (10mg/kg i.p.) blocked minimal seizures in non-pregnant mice when administered 30 min prior to stimulation. Gestational effects of seizure suppression using phenytoin (a P-gp substrate), lamotrigine (a weak P-gp substrate) and carbamazepine (not a P-gp substrate) were examined, along with the pharmacokinetic characteristics in non-pregnant, gd8 and gd18 mice. Conclusions: Our data suggest that changes transporter location underlies gestational changes in AED efficacy.