In vitro model of the outer blood–retina barrier

The outer blood–retina barrier (BRB) is formed by the retinal pigment epithelium (rpe) and functions similarly to the blood–brain barrier (BBB). In contrast to the BBB, which is composed of a myriad of capillaries, the rpe can in principle be prepared as an intact planar tissue sheet without disruption of its barrier and carrier functions. Both a rapid and gentle procedure to isolate porcine rpe and a method to implement the harvested rpe in drug penetration testing are presented. Enucleated eyes were flat-mounted and the RPE/choroid tissue sheets with or without the retina were isolated. Fluorescence microscopy based on double-labeling with propidium iodide/calcein and scanning electron microscopy revealed well-preserved cell and tissue architecture. For drug evaluation, specimens were immobilized as the interface between test compartments in a dual-chamber device. Ten different test agents were added to one chamber at defined concentrations. After an incubation time of 30 min at 37 °C permeated drug levels in both compartments were quantified by HPLC-tandem mass spectrometry or HPLC with fluorescence detection. Sodium fluorescein used as a barrier marker indicated that the rpe model had excellent seal integrity. The use of a representative subset of pharmaceuticals with known BBB permeability characteristics demonstrated that the rpe model had a large permeability dynamic range (factor >350). These findings showed that the model represents a valuable tool for the investigation of the blood barrier penetration of test compounds.