A novel method for the preparation of liposomes: freeze drying of monophase solutions

A novel method is described for the preparation of sterile and pyrogen-free submicron liposomes of narrow size distribution. The method is based on the formation of a homogeneous dispersion of lipids in water-soluble carrier materials. To obtain the lipid-containing solid dispersion, liposome-forming lipids and water-soluble carrier materials are dissolved in tert-butyl alcohol/water cosolvent systems to form an isotropic monophase solution, and then the resulting solution is lyophilized after sterilization by filtration through 0.2 microm pores. On addition of water, the lyophilized product spontaneously forms homogeneous liposome preparation. After investigation of the various parameters associated with this method it is found that the lipid/carrier ratio is the key factor affecting the size and the polydispersity of liposome preparation. Based on the data from DSC, X-ray diffraction, and size measurements, a possible liposome formation mechanism is proposed. In addition, the application of this new method to the passive loading and active loading of drugs into liposomes is discussed in detail.