In vitro concentration dependent transport of phenytoin and phenobarbital, but not ethosuximide, by human P-glycoprotein.

AIMS: One possible mechanism for epilepsy drug resistance is overexpression of P-glycoprotein in the blood-brain barrier, but whether (or which) antiepileptic drugs (AEDs) are transported by P-gp remains unclear. We evaluated AEDs as P-gp substrates using cell monolayers. MAIN
METHODS: Bi-directional transport assays and concentration equilibrium transport assays (CETAs) were performed for phenytoin (PHT), phenobarbital (PB), and ethosuximide (ESM) using wildtype Madin-Darby Canine Kidney II cell line MDCKII and porcine renal endothelial cell line LLC-PK1 cells and these cells transfected with human MDR1 cDNA to express P-gp. KEY
FINDINGS: Wildtype cells demonstrated no efflux transport of PHT, PB, or ESM. In CETAs, both MDR1-transfected cell lines transported PHT from basolateral to apical when PHT loading concentrations were 5 or 10, but not 20microg/ml. MDCK-MDR1 cells transported PB when initial concentrations were 10 or 20, but not 5microg/ml. LLC-MDR1 did not transport PB. P-gp inhibitor verapamil blocked efflux transport. MDR1-transfected cells did not transport ESM at 5.6 or 56microg/ml. Bi-directional transport assays demonstrated weak transport for PHT but not PB or ESM. SIGNIFICANCE: Human P-gp transports PHT and PB, but not ESM, in a concentration dependent manner. CETA may be more sensitive than bi-directional assays to detect transport of drugs with high passive diffusion. Potential P-gp substrates should be tested at clinically relevant concentration ranges.