OCULAR BIOAVAILABILITY

The ocular bioavailability of topically applied drugs is determined by their absorption through the anterior parts of the eye. The barriers in the anterior parts of the eye restrict and regulate the intraocular drug absorption. Topically applied drugs can reach the intraocular tissues by corneal and non-corneal (conjunctival–scleral) pathways. Small and lipophilic molecules (most clinically used drugs) are absorbed via the cornea. However, the attention to drug perme­ability through the conjunctiva and sclera has been increasing in recent years because this pathway can be of high importance to the intraocular delivery of hydrophilic and large molecules, i.e., new biotech drugs, such as proteins, peptides and DNA- or RNA-based therapeutics. The physicochemical properties of each ophthalmic formulation, including the incorporated drug (e.g., particle or mole­cule size, charge, lipophilicity, solubility in the tear fluid, and meta­bolic stability); the drug carrier (size, surface charge, morph­ology, and drug release properties); and the formulation charac­teristics (e.g., pH, refractive index, surface tension, tonicity, and viscosity), can significantly influence drug bioavailability. The topical ophth­almic formulation is applied directly on the ocular surface, i.e., onto the absorptive membrane from which the drug permeates to the site of action. The action sites of the topically applied drugs are typically the various layers of the cornea, conjun­ctiva, sclera, and the other tissues of the anterior segment, such as the iris and ciliary body.