title: In vitro concentration dependent transport of phenytoin and phenobarbital, but not ethosuximide, by human P-glycoprotein.
abstract: OBJECTIVE
  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.
  
  
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: School of Pharmacy, Faculty of Medicine, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China.
authors.forename: Chunbo
authors.initials: C
authors.lastname: Zhang
authors.forename: Patrick
authors.initials: P
authors.lastname: Kwan
authors.forename: Zhong
authors.initials: Z
authors.lastname: Zuo
authors.forename: Larry
authors.initials: L
authors.lastname: Baum
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000494:pharmacology*; D000927:Anticonvulsants / Q000493:pharmacokinetics*; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D001812:Blood-Brain Barrier / Q000187:drug effects* / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D005013:Ethosuximide / Q000493:pharmacokinetics*; D006801:Humans; D066298:In Vitro Techniques; D010634:Phenobarbital / Q000493:pharmacokinetics*; D010672:Phenytoin / Q000493:pharmacokinetics*; D020112:Rhodamine 123 / Q000493:pharmacokinetics; D014700:Verapamil / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000927:Anticonvulsants; D002121:Calcium Channel Blockers; D020112:Rhodamine 123; D005013:Ethosuximide; D010672:Phenytoin; D014700:Verapamil; D010634:Phenobarbital