Ophthalmic Drug Delivery Systems

OPHTHALMIC DRUG DELIVERY SYSTEMS

Absorption of ophthalmic drugs across the corneal membrane is a diffusion process and depends to a large extent on:

1. Physicochemical properties of the permeating molecule, and

2. Drainage and output of tears.

Most drugs for ophthalmic use like pilocarpine, epinephrine, local anesthetics, atropine, etc. are weak bases which are generally formulated at acidic pH to enhance stability. But due to their highly ionized form, ocular diffusion is poor. This, coupled with tear drainage, further reduces the rate and extent of absorption. Moreover, if the drug has short half-life, the problems become more complicated. Frequent dosing of large doses of such drugs becomes necessary to achieve the therapeutic objective which often results in corresponding increase in local (e.g. irritation) and systemic side effects. One of the approaches to improve drug effectiveness is to prolong its contact with corneal surface. Highly viscous preparations like suspensions and ointments are intended to achieve this purpose but do not offer the amount of control desired. Continuous delivery of drugs in a controlled manner can overcome most of these problems. A number of ocular drug delivery systems have been developed for providing zero-order input. The best known system is ocular insert or ocusert developed to deliver pilocarpine in the treatment of glaucoma. Available in two release forms—20 and 40 mcg/hour, the system provides relief for 7 days (following insertion in the cul-de-sac, just below the cornea) in contrast to eyedrops which are required to be instilled 3 to 4 times daily. The system is basically a thin, flexible wafer, composed of a drug reservoir core surrounded on either side by rate-controlling membranes of ethylene-vinyl acetate copolymer (Fig. 14.20).

Fig. 14.20 Schematic representation of ocusert