Nanomedicine in Drug Delivery

The physiological characteristics of the eye show unique structural and functional properties. The structure of the eye is composed of the retinal-lined fibrovascular sphere enclosing the aqueous humor, the lens, and the vitreous body (McCaa 1982). The eye is a transparent tissue, which is necessary for normal vision. To attain a clear and normal vision, the communication between the anterior and posterior parts of the eye should be kept in a highly controlled manner. The tight cellular barrier restricts particles to traverse inward/outward of the eye segments, in which various static barriers (different layers of the cornea, sclera, and the retina including blood–aqueous barrier [BAB] and blood–retinal barrier [BRB]), dynamic barriers (choroidal and conjunctival blood flow, lymphatic clearance, and tear dilution), and efflux pumps in conjunction control such transportation. As a result, the unique barrier functions in the eye pose a significant challenge for delivery of a drug alone or in a dosage form, especially to the posterior segment (Gaudana et al. 2010). Figure 11.1 is a schematic illustration of the structure of the eye and the presence of major functional barriers, namely the corneal–epithelial barrier, BAB, and BRB. Despite architectural commonalities among biological barriers, permeability of various substances across BRB and BAB compared with the blood–brain barrier (BBB) in rats has demonstrated significant differences. Lipophilic substances were shown to have high permeabilities across BBB and BRB, in which their retinal uptake index (RUI) values were also markedly higher than the brain uptake index (BUI) values. It seems there is a linear relationship between permeability and log D7.4, while efflux transport machineries play important roles in the absorption of ocular drugs from the circulating blood after systemic administration (Toda et al. 2011).