Microemulsions: A Novel Approach to Enhanced Drug Delivery

Microemulsions are isotropic, thermodynamically stable transparent (or translucent) systems of oil, water and surfactant, frequently in combination with a cosurfactant with a droplet size usually in the range of 20-200 nm. They can be classified as oil-in-water (o/w), water-in-oil (w/o) or bicontinuous systems depending on their structure and are characterized by ultra low interfacial tension between oil and water phases. These versatile systems are currently of great technological and scientific interest to the researchers because of their potential to incorporate a wide range of drug molecules (hydrophilic and hydrophobic) due to the presence of both lipophilic and hydrophilic domains. These adaptable delivery systems provide protection against oxidation, enzymatic hydrolysis and improve the solubilization of lipophilic drugs and hence enhance their bioavailability. In addition to oral and intravenous delivery, they are amenable for sustained and targeted delivery through ophthalmic, dental, pulmonary, vaginal and topical routes. Microemulsions are experiencing a very active development as reflected by the numerous publications and patents being granted on these systems. They have been used to improve the oral bioavailability of various poorly soluble drugs including cyclosporine and paclitaxel as professed by Hauer et al., US patent 7235248, and Gao et al., US patent 7115565, respectively. Furthermore, they can be employed for challenging tasks such as carrying chemotherapeutic agents to neoplastic cells and oral delivery of insulin as diligently described by Maranhao, US patent 5578583 and Burnside et al., US patent 5824638 respectively. The recent commercial success of Sandimmune Neoral (Cyclosporine A), Fortovase (Saquinavir), Norvir (Ritonavir), etc. also reflects the tremendous potential of these newer drug therapeutic systems. A critical evaluation of recent patents claiming different approaches to improve the drug delivery is the focus of the current review.