Figure 1.

Biological barriers of the eye. The tear film barrier comprises three layers (lipid, aqueous, and mucin) and is continuously removed from the eye surface by lacrimal fluid secretion, resulting in rapid drug removal. The corneal barrier serves as a mechanical and chemical barrier, limiting the access of exogenous substances into the eye. Tight junctions on the corneal epithelium surface prevent the diffusion of macromolecular and hydrophilic molecules. Intravitreal administration offers a direct path to the vitreous and retina but may impede the diffusion of larger, positively charged drugs across the retinal pigment epithelium (RPE) barrier to the choroid. The blood-ocular barrier (BOB) poses a significant challenge to systemic and topical drug delivery in the anterior and posterior chambers of the eye. It comprises the blood-aqueous barrier (BAB) and the blood-retinal barrier (BRB). The BAB, associated with the anterior chamber, consists of endothelial cells, iris, ciliary muscle, and pigmented and non-pigmented epithelium cells with tight junctions that restrict drug molecule entry.