Carbopol/chitosan based pH triggered in situ gelling system for ocular delivery of timolol maleate

The poor bioavailability and therapeutic response exhibited by conventional ophthalmic preparations due to rapid precorneal elimination, dilution and nasolacrimal drainage of the drug may be vanquished by the use of in situ gelling systems that are instilled as drops in to the eye and undergo a sol-gel transition in the cul-de-sac. Timolol eye drops may cause systemic side effects in glaucoma patients due to absorption of the drug into systemic circulation. In situ gelling system of this drug can provide localized effect with reduced contraindications, improved patient compliance and better therapeutic index. The present work describes the formulation and evaluation of an ophthalmic delivery system of an antiglaucoma drug, timolol maleate (TM) based on the concept of pH-triggered in situ gelation. Polyacrylic acid (carbopol) was used as the gelling agent in combination with chitosan (amine polysaccharide), which was acted as a viscosity-enhancing agent. Formulations were evaluated for pH, viscosity, gelling capacity and drug content. The 0.4% w/v carbopol/0.5% w/v chitosan based in situ gelling system was in liquid state at room temperature and at the pH formulated (pH 6.0) and underwent rapid transition into the viscous gel phase at the pH of the tear fluid (lacrimal fluid) (pH 7.4). The in vitro drug release and in vivo effects of the developed in situ gelling system were compared with that of Glucomol (a 0.25% TM ophthalmic solution), 0.4% w/v carbopol solution as well as liposomal formulation. The results clearly demonstrated that developed carbopol-chitosan based formulation was therapeutically efficacious and showed a fickian (diffusion controlled) type of release behaviour over 24 h periods. The developed system is thus a viable alternative to conventional eye drops and can also prevent the rapid drainage as in case of liposomes.