Characterization of paracellular and aqueous penetration routes in cornea, conjunctiva, and sclera.

PURPOSE To characterize quantitatively the paracellular permeation routes in rabbit cornea, conjunctiva, and sclera using polyethylene glycol (PEG) oligomers. METHODS Corneal, conjunctival, and scleral tissues from New Zealand white rabbits were tested individually in a modified two-chamber Ussing apparatus with the mixture of PEGs with mean molecular weights 200, 400, 600, and 1000 in glutathione bicarbonated Ringer's solution buffer on the donor side of the chamber. The samples and standards were analyzed with high-performance liquid chromatography-thermospray mass spectrometry method. The pore sizes and the pore densities of the corneal and conjunctival epithelia were calculated using an effusion-like approach. RESULTS The conjunctival and scleral tissues were 15 to 25 times more permeable than the cornea and the molecular size affected the conjunctival permeability less than that of the cornea. The palpebral and bulbar conjunctivas had equal permeabilities. The scleral permeability was approximately half of that in the conjunctiva and approximately 10 times more than in the cornea. The conjunctival epithelia had 2 times larger pores and 16 times higher pore density than the cornea. The total paracellular space in the conjunctiva was estimated to be 230 times greater than that in the cornea. CONCLUSIONS The conjunctival epithelium, due to its higher membrane permeability and larger absorptive and intercellular space surface areas, is the most viable route for ocular delivery of peptides and oligonucleotides.