(578c) Mechanisms of Binding and Transport of Molecules in Cornea

Understanding mechanisms of binding and transport of molecules across cornea is critical to developing optimal ocular formulations for delivery of drugs and for understanding mechanisms that cause ocular toxicity. Cornea is a three layer membrane consisting of a 60 micron thick lipophillic epithelium with about 6 layers of cells with tight junctions, 400 micron thick water like stroma and a single layer of about 10 micron thick endothelium with a structure similar to epithelium. This talk will focus on mechanisms of transport of hydrophobic and hydrophilic molecules across an excised rabbit cornea. The mechanisms were explored by exposing either the front (tear side) or the back (aqueous humor side) of an excised rabbit cornea to fluorescent dyes (hydrophobic dye Rhodamine B and hydrophilic dye Fluorescein) and then measuring the temporal evolution of the spatially resolved fluorescence profiles. The profiles were then fitted to multi-scale transport models to determine the transport parameters for the dyes. The transport model for Rhodamine B includes diffusive transport through each layer, along with a cell-layer model that accounts for the slow partitioning of the dye from the lipid bilayers of the epithelial cells into the internal hydrophobic organelles. A similar model is developed for the hydrophilic dye fluorescein. The models fit the experimental data well and sensitivity analysis shows that the model is sensitive to all epithelium and stroma transport parameters. The sensitivity to endothelium is minimal due to its small thickness which results in a relatively small amount of data from this layer. The key findings of the study are that fluorescein transport through the endothelium occurs primarily through the transcellular route even though it is hydrophilic and that RhB exhibit a slow partitioning into the cells due to the low solubility of the dye in the cytoplasm which results in a slow transport of the dye from the bilayers to the internal organelles. The fitted values of the parameters match values obtained through scaling analysis or to the reported values.

For some molecules the most critical step in interaction with the epithelium involves binding and transport in the outermost layers of the epithelium. Such molecules include surfactants that are commonly found in most ocular formulations. In this talk we will also focus on transport on binding and interaction of ionic surfactants with cornea. These interactions were explored by developing liposomes that have lipid bilayers that mimic the compositions of the corneal epithelium. The liposomes were loaded with calcein dye that self quenches at high concentrations. After removal of the external dye, the liposomes were exposed to surfactant solutions and fluorescence was measured to determine the leakage rate of the dye across the lipid bilayers after exposure to the surfactants.