A novel in-situ model for continuous observation of transient drug concentration gradients across buccal epithelium at the microscopical level

Buccal drug delivery is hindered by the low permeability of the buccal epithelium. The use of a time- and space-resolved visualization technique is inevitable for finding the location of the rate-limiting regions in the epithelium. In this study an in-situ method is presented with which permeation of fluorescent compounds in and across buccal epithelium can be visualized by means of confocal laser scanning microscopy (CLSM) and their topographical distribution quantified. This direct approach to visualize the penetration of fluorescently labelled drugs in thick epithelia avoids artifacts due to fixation, dehydration and embedding and can be executed on a single piece of tissue. Using this method, i.e. visualizing and quantifying the transepithelial drug distribution as a function of time, a rate-limiting zone in porcine buccal epithelium was observed at approximately 150 mu m from the surface, in the epithelium. The region where diffusion of fluorescein isothiocyanate (FITC), a hydrophilic marker, was slowed down corresponds with the intermediate strata, the cell layers where membrane coating granules extrude their content in the intercellular space.