title: Atorvastatin transport in the Caco-2 cell model: contributions of P-glycoprotein and the proton-monocarboxylic acid co-transporter.
abstract: OBJECTIVE
  The purpose of this study was to elucidate the mechanisms by which an HMG-CoA reductase inhibitor, atorvastatin (an organic acid with a pKa of 4.46), was transported in the secretory and absorptive directions across Caco-2 cell monolayers.
  
  
  METHODS
  Caco-2 cells were grown on polycarbonate membrane inserts in 6-well Snapwell plates (Costar). The permeability of radiolabeled compounds across Caco-2 cell monolayers was determined using a side-by-side diffusion apparatus (NaviCyte) and an automated liquid handler (Hamilton Microlab 2200). The apical uptake of 14C-atorvastatin was also determined in Caco-2 cells. Cyclosporin A (20 microM) was present in the uptake media to block potential P-glycoprotein-mediated atorvastatin efflux.
  
  
  RESULTS
  Polarized permeation of atorvastatin was observed with the basolateral-to-apical (B-to-A) permeability being 7-fold greater than the A-to-B permeability (35.6 x 10(-6) and 4.9 x 10(-6) cm/s, respectively). The secretion of atorvastatin was a saturable process with an apparent Km of 115 microM. The B-to-A permeability of atorvastatin was significantly reduced by cyclosporin A (10 microM), verapamil (100 microM), and a P-glycoprotein specific monoclonal antibody, UIC2(10 microg/ml) (43%, 25%, and 13%, respectively). Furthermore, both CsA and verapamil significantly increased the A-to-B permeability of atorvastatin by 60%; however, UIC2 did not affect the A-to-B permeability of atorvastatin. CsA uncompetitively inhibited the B-to-A flux of atorvastatin with a Ki of 5 microM. In addition, atorvastatin (100 microM) significantly inhibited the B-to-A permeability of vinblastine by 61%. The apical uptake of atorvastatin increased 10.5-fold when the apical pH decreased from pH 7.4 to pH 5.5 while the pH in the basolateral side was fixed at pH 7.4. A proton ionophore, carbonylcyanide p-trifluoro-methoxyphenylhydrazone (FCCP) significantly decreased atorvastatin uptake. In addition, atorvastatin uptake was significantly inhibited by benzoic acid, nicotinic acid, and acetic acid each at 20 mM (65%, 14%, and 40%, respectively). Benzoic acid competitively inhibited atorvastatin uptake with a Ki of 14 mM. Similarly, benzoic acid, nicotinic acid, and acetic acid significantly, inhibited the A-to-B permeability of atorvastatin by 71%, 21%, and 66%, respectively.
  
  
  CONCLUSIONS
  This study demonstrated that atorvastatin was secreted across the apical surface of Caco-2 cell monolayers via P-glycoprotein-mediated efflux and transported across the apical membrane in the absorptive direction via a H(+)-monocarboxylic acid cotransporter (MCT). In addition, this study provided the first evidence that negatively charged compounds, such as atorvastatin, can be a substrate for P-glycoprotein.
authors.affiliation: Department of Pharmacokinetics, Dynamics, & Metabolism, Parke-Davis Pharmaceutical Research, Warner-Lambert Company, Ann Arbor, Michigan, USA.
authors.forename: X
authors.initials: X
authors.lastname: Wu
authors.forename: L R
authors.initials: LR
authors.lastname: Whitfield
authors.forename: B H
authors.initials: BH
authors.lastname: Stewart
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000494:pharmacology*; D019342:Acetic Acid / Q000494:pharmacology; D027321:Anion Transport Proteins; D000972:Antineoplastic Agents, Phytogenic / Q000493:pharmacokinetics; D000069059:Atorvastatin; D019817:Benzoic Acid / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D018938:Caco-2 Cells / Q000737:chemistry / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002250:Carbon Radioisotopes; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D002264:Carboxylic Acids / Q000493:pharmacokinetics; D002352:Carrier Proteins / Q000378:metabolism*; D016764:Cell Polarity / Q000187:drug effects; D016572:Cyclosporine / Q000494:pharmacology; D004347:Drug Interactions; D006538:Heptanoic Acids / Q000737:chemistry / Q000493:pharmacokinetics*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors / Q000737:chemistry / Q000493:pharmacokinetics*; D007166:Immunosuppressive Agents / Q000494:pharmacology; D007202:Indicators and Reagents / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D009525:Niacin / Q000494:pharmacology; D011522:Protons; D011758:Pyrroles / Q000737:chemistry / Q000493:pharmacokinetics*; D014700:Verapamil / Q000494:pharmacology; D014747:Vinblastine / Q000493:pharmacokinetics
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D027321:Anion Transport Proteins; D000972:Antineoplastic Agents, Phytogenic; D002121:Calcium Channel Blockers; D002250:Carbon Radioisotopes; D002264:Carboxylic Acids; D002352:Carrier Proteins; D006538:Heptanoic Acids; D019161:Hydroxymethylglutaryl-CoA Reductase Inhibitors; D007166:Immunosuppressive Agents; D007202:Indicators and Reagents; D007476:Ionophores; D011522:Protons; D011758:Pyrroles; D009525:Niacin; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D014747:Vinblastine; D016572:Cyclosporine; D019817:Benzoic Acid; D000069059:Atorvastatin; D014700:Verapamil; D019342:Acetic Acid