Protein transport across biological membranes depends on a number of physical properties, among which the size and polarity of the compound are most significant. From the changes in permeability coefficients under different experimental conditions, the major pathways of protein permeation can be hypothesized. The kinetic approach provides indirect evidence of the permeability characteristics and can be a powerful tool in deducing pathways.
Electrophysiological measurements and fluorescent confocal microscopy have been used to understand the mechanism of corneal drug permea tion (22,23). The latter method provides a sensitive approach to monitoring flux across the corneal tissue and characterizing tissue damage. Confocal microscopy utilizes a laser computer system that permits visualization of a tissue specimen under light microscopy.
Insulin, polylysine, and thyrotropin-releasing hormone have been studied for their corneal permeability behavior using confocal microscopy (24). Based on the results obtained, the movement of these peptides across the cornea appears to be governed by the following factors:
1. A paracellular route is favored irrespective of the charge or size of the peptide.
2. The outermost two cell layers of the corneal epithelium offer the maximum resistance; the intercellular spaces then widen, making the transport across the corneal layer easier.
3. Charge plays an important role in the transport of the small- to medium-sized peptides. The cornea offers greater resistance to negatively charged compounds than it does to positively charged ones.
Minor inflammation of the cornea and calcium chelation may cause a widening of the intercellular spaces, thereby allowing a substantial flux of the peptide compounds. If polypeptide absorption in therapeutic amounts across the cornea is to be achieved, it will be necessary to maintain prolonged contact of the peptide with the corneal surface and also to employ a proper penetration enhancer to potentiate flux across the intercellular spaces.
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