Emulsions and microemulsions for ocular drug delivery

Ocular drug delivery through aqueous eye drops is the most common approach for administering ophthalmic drugs, in spite of the low residence time of a few minutes, which limits the bioavailability to less than 5 %. A number of drug delivery approaches have been explored to overcome the drawbacks of the aqueous eye drops including addition of colloidal particles to the eye drops. Emulsions and microemulsions are particularly appealing for delivering hydrophobic drugs to the cornea because of the possibility of loading the drugs in the oil particles. There are some other potential advantages such as the possibility of increasing the residence time through binding between the surfactant covered oil drops and the cornea, or through designing systems that undergo phase transition to a liquid crystalline system with high viscosity. Additionally, some researchers have dispersed microemulsions in contact lenses to further increase the residence time. The benefits of the emulsions and microemulsions listed above are at least partially offset by potential toxicity issues, and also formulation issues such as shelf life. Furthermore, issues related to the ocular physiology including tear biochemistry, and rapid tear turnover also sometimes limit the potential benefits of using microemulsions and emulsions. This review paper focuses on summarizing recent research on ophthalmic delivery through emulsions and microemulsions, with a particular focus on mechanistic and formulation issues.