Rapid and efficient method for the size separation of homogeneous fluorescein-encapsulating liposomes

Liposomes are colloidal structures formed by the self-assembly of lipid molecules in solution into spherical, self-closed structures through their amphiphilic properties. All liposome preparation protocols reported consist of several steps of preparation, homogenization, and purification, which are labor-intensive, arduous, and lengthy to execute. In this work, a new procedure has been developed to reduce the time of the postrehydration sizing of liposomes from multilamellar vesicles, while improving the uniformity of the resulting liposomes produced and achieving high encapsulation efficiencies. For the homogenization step, the typically used method of filter extrusion was substituted by centrifugation. Purification of liposomes to eliminate nonencapsulated molecules and lipids is routinely carried out via gel permeation chromatography, an extremely lengthy procedure, and in the method we report, this lengthy step was replaced by the use of molecular-weight cut-off filters. Using this novel method, large unilamellar vesicles were produced and the time required, postrehydration, was dramatically reduced from almost 48 to less than 2 hours, with a highly uniformly sized population of liposomes being produced-the homogeneity of the liposome population achieved using our method was 99%, as compared to 88% attained by using the traditional method of production. We have used this approach to encapsulate fluorescein isothiocyanate (FITC), and 160,000 FITC molecules were encapsulated and the liposomes were demonstrated to be stable for at least 10 weeks at 4°C.