Transscleral-RPE permeability of PEDF and ovalbumin proteins: implications for subconjunctival protein delivery

To investigate the in vitro, ex vivo, and in vivo transscleral-retinal pigment epithelium (RPE) permeability of PEDF and structurally related proteins for the exploration of novel routes of protein delivery to the retina.

Monkey RPE cells were cultured on permeable supports to separate apical and basal compartments. Porcine scleral tissue was placed in Ussing chambers to separate uveal and orbital compartments. Transepithelial resistance and voltage were measured by an electrical resistance system, and paracellular tracer flux was evaluated with trypan blue. Subconjunctival administration in rat eyes was by injections of soluble protein or by implantation of polyvinyl alcohol devices containing protein. Fluorescein-conjugated (Fl-) PEDF and ovalbumin were determined by spectrofluorometry, laser scanning, immunoblotting, epifluorescence, and confocal microscopy. Permeability was assessed by fluoresceinated-protein flux.

Transepithelial resistance, impermeability to trypan blue, and confocal microscopy confirmed functional and structural tight junction formation of RPE cells cultured on permeable supports. Full-length Fl-PEDF and Fl-ovalbumin proteins diffused through RPE cell monolayers from either the apical or basal side. Fl-ovalbumin diffused through scleral tissues at constant rates. Subconjunctival Fl-PEDF or Fl-ovalbumin administration in vivo revealed movement of full-length protein into the choroid and retina as early as 1 hour.

The sclera and RPE were permeable in vitro, ex vivo, and in vivo to PEDF and ovalbumin proteins. These large proteins can traverse through the sclera-RPE to reach the retina. In addition, these data prompt the proposal that subconjunctival protein delivery may represent a feasible and minimally invasive route for PEDF administration in the clinic.