Encapsulation of doxorubicin in neutral liposomes by passive methods: evidence of drug-lipid interaction at neutral pH.

Doxorubicin, an antineoplastic agent, was encapsulated in liposomes of dipalmitoylphosphatidylcholine with or without cholesterol, by the extrusion procedure. Doxorubicin was added to the lipid before drying, or was present in the rehydration buffer, and the influence of the method of encapsulation on size and polydispersity was determined by photon correlation spectroscopy. Results showed an important interaction between doxorubicin and liposomes, although cholesterol-containing vesicles were those that underwent the strongest insertion of the drug. One important parameter, which determined the extension of such interaction, was the curvature of the vesicle bilayer. So, liposomes extruded through a 50 nm membrane filter suffered the highest relative size variation in comparison with empty liposomes. Doxorubicin also produced an increase in polydispersity of vesicle population; therefore its presence resulted in some fusion and/or aggregation processes. The stability of liposomes was dependent on lipid content, on the method of drug trapping and on the presence or absence of such drug. Encapsulation efficiency seemed to be inversely related to liposome stability. Maximal values, which never exceed 0.015 +/- 0.005 mumol of drug per mumol of lipid, were obtained when the drug was dried together with the lipids.