title: Effect of photodynamic therapy on the function of the outer blood-retinal barrier in an in vitro model.
abstract: BACKGROUND
  Photodynamic therapy (PDT) is a well established clinical treatment for age-related macular degeneration (AMD), and comprises intravenous injection of verteporfin and subsequent application of a non-thermal laser beam to the area of AMD to induce selective vascular occlusion. Since there is evidence that PDT may cause outer blood-retinal barrier (BRB) breakdown and possibly RPE cell alteration, we investigated the effect of PDT on the BRB function of the RPE in an in vitro model.
  
  
  METHODS
  Twenty-one monolayers of human RPE cells were cultured on semipermeable membranes until a stable barrier function was achieved as determined by transepithelial electrical resistance (TER) and sodium fluorescein permeability. To test the effect of PDT on the outer BRB function, non-thermal laser (692 nm), verteporfin or a combination of both were applied. TER assessment prior to and after PDT was utilized to identify changes in barrier function of the RPE in this in vitro model. Finally, monolayers of RPE cells were evaluated by transmission electron microscopy (TEM).
  
  
  RESULTS
  No significant TER decrease was observed after application of non-thermal laser alone or after administration of verteporfin in therapeutic concentrations, but combination of these modalities resulted in significantly decreased TER within 4 h. Except for intercellular blisters, no damage to the RPE was evident in TEM. Verteporfin added at concentrations higher than therapeutic doses (2 mg/ml) resulted in an immediate decrease in TER and damage to the RPE cells.
  
  
  CONCLUSIONS
  The combination of a therapeutic concentration of verteporfin and application of non-thermal laser resulted in a morphologically and functionally detectable breakdown of the outer BRB function of the RPE without any damage to the RPE cells themselves in vitro. However, increasing the concentration of verteporfin can result in RPE cell damage.
authors.affiliation: Department of Ophthalmology, Philipps-University Marburg, Robert-Koch-Str. 4, 35037, Marburg, Germany. stefan.mennel@lycos.com
authors.forename: Stefan
authors.initials: S
authors.lastname: Mennel
authors.forename: Silvia
authors.initials: S
authors.lastname: Peter
authors.forename: Carsten H
authors.initials: CH
authors.lastname: Meyer
authors.forename: Gabriele
authors.initials: G
authors.lastname: Thumann
mesh_terms: D001813:Blood-Retinal Barrier / Q000502:physiology*; D002199:Capillary Permeability / Q000187:drug effects; D002470:Cell Survival; D002478:Cells, Cultured; D017097:Electric Impedance; D019793:Fluorescein / Q000378:metabolism; D006801:Humans; D007365:Intercellular Junctions / Q000502:physiology; D010778:Photochemotherapy; D017319:Photosensitizing Agents / Q000494:pharmacology*; D010857:Pigment Epithelium of Eye / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D011166:Porphyrins / Q000494:pharmacology*; D000077362:Verteporfin
chemical_list: D017319:Photosensitizing Agents; D011166:Porphyrins; D000077362:Verteporfin; D019793:Fluorescein