Rheological evaluation of Gelrite in situ gels for ophthalmic use

One of the reasons for the relatively low bioavailability of conventional eye drops is their short precorneal contact times. In situ gels are promising ocular drug delivery systems since they are conveniently dropped into the eye as a liquid whereafter they undergo a transition into a gel. Due to their elastic properties hydrogels resist ocular drainage leading to longer contact times. In this paper the rheology of Gelrite in situ gels was studied. A complementary in vivo study for determining precorneal contact times in humans and in rabbits was performed. The elastic moduli of the gels increased with increasing concentration of electrolytes. At physiological concentration of the electrolytes, the elasticity of the gels was independent of Gelrite concentration. The human contact times increased up to 20 h with decreasing osmolality of the formulations. The results indicate that a high rate of the sol/gel transition results in long contact times.