IOVS Am. J. Pathology
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(Investigative Ophthalmology and Visual Science. 2005;46:641-646.)
© 2005 by The Association for Research in Vision and Ophthalmology, Inc.
DOI:  10.1167/iovs.04-1051
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Permeability of Retinal Pigment Epithelium: Effects of Permeant Molecular Weight and Lipophilicity

Leena Pitkänen, Veli-Pekka Ranta, Hanna Moilanen, and Arto Urtti

From the Department of Pharmaceutics, University of Kuopio, Kuopio, Finland.

PURPOSE. To determine the effects of solute molecular weight and lipophilicity on the permeability of a retinal pigment epithelium (RPE)-choroid preparation.

METHODS. Fresh RPE-choroid specimens from bovine eyes were placed in diffusion chambers for permeability experiments with carboxyfluorescein, fluorescein isothiocyanate (FITC)-labeled dextrans with molecular masses from 4 to 80 kDa, and ß-blockers exhibiting a wide range of lipophilicity (atenolol, nadolol, pindolol, timolol, metoprolol, and betaxolol). Permeability experiments were performed both in the choroid-to-retina (inward) direction and in the retina-to-choroid (outward) direction. Carboxyfluorescein and FITC-dextrans were determined by fluorometry, and ß-blockers by HPLC. The transepithelial electrical resistance and potential difference were monitored during the experiments.

RESULTS. Permeability of the fluorescent FITC-dextran probes through RPE-choroid decreased significantly with the increasing size of the probe. RPE-choroid was 35 times more permeable to carboxyfluorescein (376 Da) than to FITC-dextran 80 kDa. The permeabilities of lipophilic ß-blockers were up to 8 and 20 times higher than that of hydrophilic atenolol and carboxyfluorescein, respectively. The lag time of solute flux across the RPE-choroid increased with the molecular weight and lipophilicity. Compared with published data on isolated sclera, bovine RPE-choroid was 10 to 100 times less permeable to hydrophilic compounds and macromolecules. The permeability of lipophilic molecules in RPE-choroid was in the same range as in the sclera.

CONCLUSIONS. RPE is a major barrier and may be the rate-limiting factor in the retinal delivery of hydrophilic drugs and macromolecules through the transscleral route. For lipophilic molecules, RPE-choroid, and sclera are approximately equal barriers.




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