Microemulsions for oral administration and their therapeutic applications

The microemulsion concept was introduced in 1943 by Hoar and Schulman. Self-microemulsifying drug delivery systems (S(M)EDDS) are much more recent and can be described as isotropic solutions of oils and surfactants that form oil-in-water O/W microemulsions when they are poured into an aqueous medium. When they are presented as soft capsules for oral delivery, S(M)EDDS have the ability to considerably improve the intestinal absorption of agents that are incorporated into the S(M)EDDS. Forty percent of newly discovered drug candidates have little or no water solubility and therefore have low and/or variable bioavailability profiles. Many of these drugs are good candidates for formulation into S(M)EDDS.

This paper describes the preparation and assessment of these formulations and their current applications. The characterisation of this type of formulation has improved, and in vitro models (Caco-2 cell cultures, Ussing chambers, the everted sac technique, etc.) can be used for screening different formulations. It describes also marketed formulations (i.e., cyclosporin and saquinavir S(M)EDDS) and some other formulations.

Actual applications of S(M)EDDS remain rare. The first drug marketed as a S(M)EDDS was cyclosporin, and it had significantly improved bioavailability compared with the conventional solution. In the last decade, several S(M)EDDS loaded with antiviral drugs (e.g., ritonavir, saquinavir) were tested for treatment of HIV infection, but the relative improvement in clinical benefit was not significant. The S(M)EDDS formulation of Norvir® (soft capsules) has been withdrawn in some countries.