Encapsulation of doxorubicin in thermosensitive small unilamellar vesicle liposomes

The optimisation of the formulation of thermosensitive, doxorubicin-containing small unilamellar liposomes is described. The liposomes were first strictly defined in terms of size distribution and size stability and a quality level was defined. The suspension contained more than 95% vesicles with a maximal diameter of 50 nm and kept this level for a minimum of 24 hours. Several lipid mixtures were tested in defined thermal conditions usable for in vitro experiments: 43°C in fetal calf serum-containing medium. The mixture yielding the best differential thermal stability (DTS) defined as the difference of release between 37°C and 43°C exposures was found to be a dipalmitoylphosphatidylcholine/distearoylphosphatidyl-choline/cholesterol mixture in 5:4:2 molar ratio yielding 72% DTS. These thermosensitive liposomes were evaluated betwen pH 6.00 and 8.00 since hyperthermia-induced lethality was reported to be enhanced by pH variations. Their release capacity was not altered by any pH variations. Incorporation of doxorubicin within these liposomes was then performed. The release kinetics at 37° and 43°C were determined. It is proposed to use this formulation in in vitro experiments on tumour cells, although a decrease of DTS was evident.