(331b) Transcorneal Penetration of Rhodamine B Across the Rabbit Cornea Mounted in Vitro

A cornea is three-layer membrane comprising of the outermost epithelium, the middle water-like stroma and then a single layer of endothelium. The mechanisms of transport of drugs across the corneal membrane play an important role in determining the bioavailability of the ophthalmic drugs. This talk will focus on the mechanisms of transport across the corneal membrane of Rhodamine B (RhB), a fluorescent lipophilic molecule.

To experimentally measure the transport of RhB through the rabbit cornea, rabbit corneas were mounted in vitro and perfused with HCO3- Ringers at the anterior and posterior surfaces. About 30 min after mounting, the epithelial surface was exposed to RhB (10 µM) dissolved in the Ringers. Corneal thickness and concentration profile of RhB across the cornea were obtained periodically (every 2-3 min) using a custom-built scanning microfluorometer at a high depth resolution (at least 8 µm using a 40x water immersion objective with 0.75 NA). A multiscale cornea transport model was developed and the transient concentration profiles are fitted to the model to obtain all the parameters. Sensitivity analysis was conducted to determine the reliability of the fitted parameters.

After topical administration, RhB distribution across the corneal membrane showed discontinuities at the cellular boundaries. Specifically, RhB fluorescence was elevated in the lipophilic cellular layers relative to the hydrophilic stromal layer. A mathematical model consisting of three partial differential equations governing diffusion of RhB across the cornea was developed. Additionally slow transport of RhB from the bilayers to hydrophobic sites in the interior of the cells in the epithelium and endothelium was included. Also a transport barrier was included at the interface between the epithelium and the stroma. The experimental data was fitted to the model with reliable parameters except the endothelium parameters, which show less sensitivity due to a limited amount of data from endothelium because of its small thickness.

The mathematical model developed here accurately characterizes the transient solute transport through the corneal membrane. The conventional approaches using a lumped permeability cannot adequately describe the mechanistic of transcorneal penetration of lipophilic drugs particular for delivery by eye drops because the residence time of drugs in eyes is much small than the time needed to establish a pseudo-steady profile, and thus the lumped permeability of the membrane changes with time.