Surface modification of continuously extruded contrast-carrying liposomes: Effect on their physical properties

Surface-modified, contrast-carrying liposomes were generated by incorporation of amphipathic polymers into the membranes of continuously extruded vesicles. Besides the well described distearoylphosphatidylethanolamine monomethoxypolyethyleneglycol (DSPE-PEG), a new substance, cholesterylhemisuccinate monomethoxypolyethyleneglycol (CholHS-PEG) was tested for the first time. Using the water-soluble radiographic contrast agent iopromide as well as the nuclear magnetic contrast agent Gd-DTPA, the impact of surface modification (SM) on liposome properties like Vesicle size distribution, encapsulation efficiency, zeta potential and storage as well as plasma stability was investigated. In the course of the studies, the molar amount of amphipathic polymer employed as well as the time point of SM during the production process were varied. Incorporation of both, DSPE-PEG and CholHS-PEG into the lipid films formed before continuous extrusion resulted in a concentration-dependent decline of encapsulation efficiencies. When SM was carried out after vesicle formation, the observed effect diminished and even disappeared, as soon as PEG-coating was carried out after the last extrusion step. However, when using the latter procedure with DSPE-PEC, mean vesicle diameters showed a strong increase in the course of the pegylation process. The extent of bilayer modification was studied by zeta potential measurements of liposomes containing the negatively charged phospholipid SPG. In the presence of PEG-derivatives the high zeta potentials of unmodified vesicles were significantly reduced, irrespective of whether SM was carried out before, during or after extrusion. This result indicated a successful association of the PEG-derivatives with liposomal bilayers for all procedures. For CholHS-PEG complete incorporation into liposomes after extrusion could be demonstrated using gel filtration. Stability testing revealed an unchanged macroscopic appearance, encapsulation efficiency and vesicle size distribution of unmodified and CholHS-bearing liposomes after 4 months' storage at 2-8 degrees C. In contrast to this, DSBE-PEG-containing vesicles displayed a pronounced size increase when SM was carried out during extrusion. Another important effect of DSPE-PEG incorporation was found during plasma stability experiments. Whereas CholHS-PEG-carrying and unmodified liposomes had similar leakage rates in human plasma, DSPE-PEG caused a concentration-dependent decrease in plasma stability, but only when SM had been carried out before extrusion. Altogether, from a merely technological point of view, CholHS-PEG revealed superior properties over DSPE-PEG for SM of continuously extruded contrast-carrying liposomes.