SURFACE MODIFICATION OF CONTINUOUSLY EXTRUDED CONTRAST-CARRYING LIPOSOMES - EFFECT ON THEIR PHYSICAL-PROPERTIES

Surface-modified, contrast-carrying liposomes were generated by incorp oration of amphipathic polymers into the membranes of continuously ext ruded vesicles. Besides the well described distearoylphosphatidylethan olamine monomethoxypolyethyleneglycol (DSPE-PEG), a new substance, cho lesterylhemisuccinate monomethoxypolyethyleneglycol (CholHS-PEG) was t ested for the first time. Using the water-soluble radiographic contras t agent iopromide as well as the nuclear magnetic contrast agent Gd-DT PA, the impact of surface modification (SM) on liposome properties lik e Vesicle size distribution, encapsulation efficiency, zeta potential and storage as well as plasma stability was investigated. In the cours e 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-depende nt decline of encapsulation efficiencies. When SM was carried out afte r vesicle formation, the observed effect diminished and even disappear ed, as soon as PEG-coating was carried out after the last extrusion st ep. However, when using the latter procedure with DSPE-PEC, mean vesic le diameters showed a strong increase in the course of the pegylation process. The extent of bilayer modification was studied by zeta potent ial measurements of liposomes containing the negatively charged phosph olipid SPG. In the presence of PEG-derivatives the high zeta potential s of unmodified vesicles were significantly reduced, irrespective of w hether SM was carried out before, during or after extrusion. This resu lt indicated a successful association of the PEG-derivatives with lipo somal bilayers for all procedures. For CholHS-PEG complete incorporati on into liposomes after extrusion could be demonstrated using gel filt ration. 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 pronou nced size increase when SM was carried out during extrusion. Another i mportant effect of DSPE-PEG incorporation was found during plasma stab ility experiments. Whereas CholHS-PEG-carrying and unmodified liposome s had similar leakage rates in human plasma, DSPE-PEG caused a concent ration-dependent decrease in plasma stability, but only when SM had be en carried out before extrusion. Altogether, from a merely technologic al point of view, CholHS-PEG revealed superior properties over DSPE-PE G for SM of continuously extruded contrast-carrying liposomes.