Comparison of particle size and encapsulation parameters of three liposomal preparations

Liposomes can be produced by using various techniques such as mechanical agitation, ultrasonication, ether injection, detergent dialysis, reversed-phase evaporation, extrusion through polycarbonate filters of appropriate pore size, etc. Each procedure provides liposomes with their own perculiar features so the choice of a given one to obtain suitable vesicles for use as drug delivery systems should be based on careful experimentation. In this work we tested three liposome preparation methods that provide unilamellar structures with high yields. In fact, unilamellar vesicles of small diameters (SUV) were prepared by ultrasonication and large unilamellar vesicles (LUV) were obtained by (a) reversed-phase evaporation (REV) and (b) extrusion through polycarbonate filters--with or without preliminary freezing-thawing (VETI and VETII, respectively). According to the results obtained, subsequent filtration of the SUV and REV liposomal preparations results in dramatically decreased average diameters and polydispersity indices, as well as in marked changes in their encapsulation parameters and in lipid losses. Among extruded liposomes, those obtained with preliminary freezing-thawing (VETII) resulted in a higher lipid recovery, even though their average hydrodynamic diameter and polydispersity index as measured by quasi-elastic light-scattering spectroscopy (QELSS) were quite similar to those of the VETI counterparts.