Liposomal l-asparaginase: in vitro evaluation

The purpose of this work was the development of liposomal formulations of l-asparaginase (l-ASNase) with the following characteristics: preservation of active enzyme, high entrapment efficiency, prolonged serum half-life and reduced toxicity compared with the free enzyme. Several liposome formulations were developed using simplified dehydration-rehydration vesicles (sDRV) or extruded vesicles (VET). The effect of lipid composition, vesicle size, ionic strength and osmolarity on enzyme encapsulation was investigated. Using a simplified dehydration-rehydration method (sDRV) we were able to achieve encapsulation efficiencies of up to 100% with full preservation (99%) of the specific activity of the encapsulated enzyme. The protein to lipid ratios of the liposomal formulations ranged from 5 to 27 μg/μmol, depending on the lipid composition. Extruded vesicles ranging from 85 to 250 nm in diameter were also tested. The encapsulation efficiency of extruded vesicles was lower than that of large vesicles and the range of preservation of specific activity was dependent on the lipid composition. Lipid combinations of phosphatidylcholine and cholesterol and either stearylamine, phosphatidylinositol or monosialoganglioside resulted in a high encapsulation efficiency (40 and 98% in VET and sDRV, respectively), high stability in saline and human serum (65-90% after 48 h) and considerable preservation of enzymatic activity (74-98%). The liposomal formulations were significantly less toxic than the free enzyme against normal CHO cells. In vivo toxicity, pharmacokinetics, biodistribution and antitumour activity studies are planned with the best formulations described in this paper.