A REVIEW ON TOPICAL OPHTHALMIC DRUG DELIVERY SYSTEMS

For many drugs K loss (First order elimination rate) is approxi­mately 0.5-0.7/min and K absorption (First order absorp­tion rate) is about 0.001/min. The sum of these two rate constants control the fraction of the applied dose absorbed into the eye3. So the ocular bioavailability can be increased by decreasing K Ioss or by increasing K absorption. The former can be achieved by modifying the ocular dosage forms and the latter by formulating ocular dosage forms containing lipophilic prodrugs or by adding penetration enhancers. Therefore to optimize topical ocular drug deliv­ery system prolonged contact time with the corneal surface and better penetration through cornea is necessary4. A considerable amount of effort has been made in ophthalmic drug delivery since 1970's. The two main ap­proqches attempted are improvement in bioavailability and controlled release drug delivery. ENHANCEMENT IN BIOAVAILABILITY   Topical bioavailability can be improved by maximizing precorneal drug absorption and minimizing precorneal drug loss. Viscosity improver: In order to prolong precorneal residence time and to improve bioavailability attempts were made to increase the viscosity of the formulation. The viscosity enhancers used were hydrophilic polymers such as cellulose, polyalcohol and polyacrylic acid. sodium carboxy methyl cellulose is one of the most important mucoadhesion polymers having mono adhosive strength5. The effects of polyacrylic acid and polyacrylamide based hydrogels are tested on miotic re­sponse of pilocarpine. Carbomer were used in liquid and semisolid formulations as suspending or viscosity increas­ing agents.

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