Model membrane systems as drug delivery vehicles

Model membrane systems have been developed primarily to provide simplified versions of biological membranes, which allow the properties of individual components to be studied in detail. Model systems have provided a great deal of information on the conformation of lipids in membranes, the rates and type of motions undergone by individual lipid molecules, and the types of polymorphic phases preferred by lipids in isolation and in mixtures. This has led to many insights concerning the roles of lipids in membranes, such as the relationship between non-bilayer phases and membrane fusion (Cullis and De Kruijff, 1979; Bloom et al., 1991; Siegel, 1993). However, model membranes and the techniques used to generate them have considerable utility in their own right for drug delivery applications. This utility is straightforward to illustrate. An ideal drug delivery system will exhibit several characteristics: (i) a small size (<100 nm diameter) combined with reasonably long circulation lifetimes to be able to access disease sites such as tumours, (ii) efficient loading with a biologically active agent, and (iii) fusogenic properties that allow the contents of the carrier to be delivered into target cells. All of these properties can be satisfied by utilizing our knowledge of model membrane systems.