title: Impact of P-glycoprotein on blood-retinal barrier permeability: comparison of blood-aqueous humor and blood-brain barrier using mdr1a knockout rats.
abstract: OBJECTIVE
  The purpose of this study was to clarify the impact of P-glycoprotein (P-gp) on blood-retinal barrier (BRB) and blood-aqueous humor barrier (BAB) permeability, in contrast to blood-brain barrier (BBB) permeability.
  
  
  METHODS
  Permeabilities of six compounds, including P-gp substrates (quinidine, digoxin, and verapamil), were investigated in wild-type and mdr1a knockout rats using retinal, aqueous humor, and brain uptake index (RUI, AHUI, and BUI, respectively) methods and integration plot analysis.
  
  
  RESULTS
  In both rat strains, quinidine, digoxin, and verapamil were transported by P-gp across each barrier; however, the impact of P-gp on retinal uptake of quinidine and verapamil was less pronounced than that on brain uptake. The apparent influx permeability clearance (Kin) values of verapamil in retina obtained from wild-type and knockout rats were similar (0.824 ± 0.201 and 0.849 ± 0.980 mL/min·g retina, respectively; mean ± SD; n = 3 rats). The Kin in aqueous humor and brain obtained from knockout rats was, respectively, 3-fold and 12-fold higher than that of wild-type (P < 0.05). In P-gp-deficient conditions, the RUI and AHUI of quinidine, digoxin, and verapamil, as well as the BUI of quinidine and digoxin, were decreased by P-gp inhibitors. However, the BUI of verapamil was not changed by P-gp inhibitors. Results suggest that carrier-mediated influx transporters exist in the blood-ocular barriers and that the function of verapamil influx transporters is markedly different between the retina and brain.
  
  
  CONCLUSIONS
  In both rat strains, P-gp operates in the blood-ocular barriers, and the impact of P-gp on BRB permeability to quinidine and verapamil is lower than that on BBB permeability.
authors.affiliation: Nara Research and Development Center, Santen Pharmaceutical Co., Ltd., Nara, Japan Department of Pharmaceutics, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Toyama, Japan.
authors.forename: Shinobu
authors.initials: S
authors.lastname: Fujii
authors.affiliation: Nara Research and Development Center, Santen Pharmaceutical Co., Ltd., Nara, Japan.
authors.forename: Chikako
authors.initials: C
authors.lastname: Setoguchi
authors.affiliation: Nara Research and Development Center, Santen Pharmaceutical Co., Ltd., Nara, Japan.
authors.forename: Kouichi
authors.initials: K
authors.lastname: Kawazu
authors.affiliation: Department of Pharmaceutics, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Toyama, Japan.
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Hosoya
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000037:antagonists & inhibitors / Q000502:physiology*; D000818:Animals; D001082:Aqueous Humor / Q000378:metabolism*; D001693:Biological Transport, Active / Q000187:drug effects; D018916:Blood-Aqueous Barrier / Q000502:physiology*; D001812:Blood-Brain Barrier / Q000502:physiology*; D001813:Blood-Retinal Barrier / Q000502:physiology*; D001921:Brain / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002199:Capillary Permeability; D004077:Digoxin / Q000378:metabolism / Q000494:pharmacology; D055786:Gene Knockout Techniques; D008297:Male; D011802:Quinidine / Q000378:metabolism / Q000494:pharmacology; D051381:Rats; D011922:Rats, Mutant Strains; D017207:Rats, Sprague-Dawley; D060888:Real-Time Polymerase Chain Reaction; D012160:Retina / Q000378:metabolism*; D014700:Verapamil / Q000378:metabolism / Q000494:pharmacology; D061567:Voltage-Gated Sodium Channel Blockers / Q000494:pharmacology
chemical_list: D018435:ATP Binding Cassette Transporter, Subfamily B; D002121:Calcium Channel Blockers; D061567:Voltage-Gated Sodium Channel Blockers; D004077:Digoxin; C095122:multidrug resistance protein 3; D014700:Verapamil; D011802:Quinidine
keywords: P-glycoprotein; blood–ocular barrier; blood–retinal barrier; mdr1a knockout rat