Phase transition water-in-oil microemulsions as ocular drug delivery systems: in vitro and in vivo evaluation.

Microemuslion (ME)-based phase transition systems were evaluated for ocular delivery of pilocarpine hydrochloride (model hydrophilic drug). These used two non-ionic surfactants, sorbitan mono laurate and polyoxyethylene sorbitan mono-oleate with ethyl oleate (oil component) and water. These systems undergo phase change from ME to liquid crystalline (LC) and to coarse emulsion (EM) with a change in viscosity depending on water content. This study selected five formulations containing aqueous phase at 5% (w/w) (ME 5%), 10% (w/w) (ME 10%), 26% (w/w) (LC), 85% (w/w) (O/W EM) and 100% (solution) with the model drug at 1% (w/w). Incorporation of pilocarpine hydrochloride did not affect the phase behaviour. The viscosity was increased initially with dilution from ME 5% to ME 10% then LC, indicating structuring of the system, before being reduced in the EM formulation. Drug release depended on the viscosity with lower release rates obtained from formulations with high viscosity. The miotic response and duration of action were greatest in case of ME and LC formulations indicating high ocular bioavailability. Thus, phase transition ME is promising for ocular drug delivery as it provides the fluidity with its viscosity being increased after application increasing ocular retention while retaining the therapeutic efficiency.