Ocular Anatomy And Physiology Relevant To The Noncorneal Route

The ocular anatomy and physiology relevant to ocular drug delivery was reviewed by Robinson (95). Some of the main points relevant to noncorneal drug delivery are summarized below.

A. Noncorneal Drug Penetration and Tissue Distribution

The shape of the eye in humans and rabbits, the primary animal model used in ocular research, approximates that of a globe, as shown in Figure 1. From the perspective of ocular drug delivery, the eye can be regarded as consisting of two parts: an anterior and a posterior segment. The tear fluid, the cornea, anterior chamber filled with aqueous humor, iris-ciliary body, and the lens comprise the anterior segment. The posterior segment of the eye consists of

Figure 1 Anatomy of the eye. (Adapted from Ref. 154.)

four structures—the conjunctiva, sclera, choroid, and retina—surrounding the vitreous cavity that contains the vitreous humor. Taking into consideration the geometry of the eyeball and the diffusional pathways, drug penetrating via the corneal route has direct access to the anterior segment tissues, providing high levels in the cornea, aqueous humor, and the iris-ciliary body. In contrast, drug entering via the noncorneal route traverses the conjunctiva and sclera entering the choroid, retina, and eventually the vitreous humor. This selective tissue distribution of drugs entering the eye via the noncorneal route may be promising for drug delivery targeting the posterior eye (23,96).

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