Abstract
Topical administration is the most common route of ocular drug delivery. Despite its apparent easy accessibility, the eye is well protected from foreign materials and drugs by a number of very efficient mechanisms such as blinking, induced lacrimation, tear turnover, nasolacrimal drainage, which cause rapid removal of substances from the eye surface and by the cornea, which forms the physical-biological barrier (Figure (Figure1).1). Consequently, these protective mechanisms and structural obstacle may cause subtherapeutic drug levels at the tissue target, particularly at retinal level. Delivery of drugs to the posterior eye is challenging, and there is an increasing need for managing rapidly progressing for posterior eye diseases, such as diabetic retinopathy, age-related macular degeneration, and optic neuropathy (Bucolo et al., 2012a,b; Musumeci et al., 2012). Currently, the intravitreal route is widely used to deliver therapeutic molecules to the retina. However, frequent administration of drugs via this route can lead to retinal detachment, endophthalmitis and increased intraocular pressure. For this reason ophthalmic drug delivery, particularly targeted to posterior segment, is one of the most challenging endeavors facing the ocular pharmacologists. Topical route represents a safer administration, therefore a major challenge to the scientists is to overcome the ocular barriers and reach the tissue target. Systemic route (e.g., oral, parenteral) is also used to reach the eye, though the drug transport across the ocular barriers (blood-aqueous barrier and blood-retinal barrier) is quite difficult.
Highlights
Under normal condition the human eye can hold about 25–30 μl of an ophthalmic solution; after a single blink the volume is reduced to 7–10 μl through nasolacrimal drainage which cause the drug to be systemically absorbed across the nasal mucosa or the gastrointestinal tract
The lipophilic corneal epithelium contains 5–7 layers of cells each connected by tight junctions and represents the rate-limiting barrier for transcorneal diffusion of most hydrophilic drugs; on the contrary the stroma, which is mainly composed of hydrated collagen, exerts a diffusional barrier to highly lipophilic drugs
The endothelium is not a significant barrier to the transcorneal diffusion, its permeability depends on molecular weight rather than characteristic of compound
Summary
Under normal condition the human eye can hold about 25–30 μl of an ophthalmic solution; after a single blink the volume is reduced to 7–10 μl through nasolacrimal drainage which cause the drug to be systemically absorbed across the nasal mucosa or the gastrointestinal tract. A significant systemic loss from topically applied drugs occurs from conjunctival absorption into the local circulation (Figure 1). The limited permeability of cornea contributes to the low absorption of ocular drugs.
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