The external eye is readily accessible for drug administration. As a consequence of its function as the visual apparatus, mechanisms are strongly developed for the clearance of foreign materials from the cornea to preserve visual acuity. This presents problems in the development of formulations for ophthalmic therapy.
Topical administration is direct, but conventional preparations of ophthalmic drugs, such as ointments, suspensions, or solutions, are relatively inefficient as therapeutic systems. Following administration, a large proportion of the topically applied drug is immediately diluted in the tear film and excess fluid spills over the lid margin and the remainder is rapidly drained into the nasolacrimal duct. A proportion of the drug is not available for immediate therapeutic action since it binds to the surrounding extraorbital tissues. In view of these losses, frequent topical administration is necessary to maintain adequate drug levels. Systemic administration of a drug to treat ocular disease would require a high concentrations of circulating drug in the plasma to achieve therapeutic quantities in the aqueous humor, with the increased risk of side effects.
Three important factors have to be considered when attempting drug delivery to the eye-
1. how the blood-eye barrier (systemic to ocular) or cornea (external to ocular) is crossed to reach the site of action,
2. how to localize the pharmacodynamic action at the eye and minimise drug action on other tissues
3. how to prolong the duration of drug action such that the frequency of drug administration can be reduced.
Central Retinal Artery and Vein
Figure 11.1 Vertical section through the human eye
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