title: pH-dependent bidirectional transport of weakly basic drugs across Caco-2 monolayers: implications for drug-drug interactions.
abstract: OBJECTIVE
  The purpose of this study was to investigate the pH-dependent passive and active transport of weakly basic drugs across the human intestinal epithelium.
  
  
  METHODS
  The bidirectional pH-dependent transport of weak bases was studied in Caco-2 cell monolayers in the physiologic pH range of the gastrointestinal tract.
  
  
  RESULTS
  A net secretion of atenolol and metoprolol was observed when a pH gradient was applied. However, the bidirectional transport of both compounds was equal in the nongradient system. Hence, at lower apical than basolateral pH a change in passive transport caused by an imbalance in the concentration of the uncharged drug species resulted in a "false" asymmetry (efflux ratio). Furthermore, a mixture of pH-dependent passive and active efflux was found for the P-glycoprotein (P-gp, MDR1, ABCB1) substrates, talinolol and quinidine, but not for the neutral drug, digoxin. However, the clinically important digoxin-quinidine interaction depended on the presence of a pH gradient. Hence, the degree of interaction depends on the amount of quinidine available at the binding site of the P-gp.
  
  
  CONCLUSIONS
  Active efflux of weak bases can only be accounted for when the fraction of unionized drug species is equal in all compartments because the transport is biased by a pH-dependent passive component. However, this component may take part in vivo and contribute to drug-drug interactions involving P-gp.
authors.affiliation: AstraZeneca, DMPK & Bioanalytical Chemistry, SE-431 83 Mölndal, Sweden.
authors.forename: Sibylle
authors.initials: S
authors.lastname: Neuhoff
authors.forename: Anna-Lena
authors.initials: AL
authors.lastname: Ungell
authors.forename: Ismael
authors.initials: I
authors.lastname: Zamora
authors.forename: Per
authors.initials: P
authors.lastname: Artursson
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism; D001692:Biological Transport; D018938:Caco-2 Cells; D002463:Cell Membrane Permeability / Q000187:drug effects*; D004077:Digoxin / Q000493:pharmacokinetics; D004347:Drug Interactions; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007413:Intestinal Mucosa / Q000166:cytology / Q000378:metabolism; D004364:Pharmaceutical Preparations / Q000378:metabolism*; D010599:Pharmacokinetics
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D004364:Pharmaceutical Preparations; D004077:Digoxin