Influence of osmotic stress on liposome size and morphology

Abstract

Liposomes are currently being investigated as potential parenterally used drug carriers. The main factor that influences the in vivo behavior of such liposomes is their vesicle size. A detailed and reliable knowledge of vesicle size is therefore necessary in order to interpret results of physical and biological investigations in a correct manner. It has earlier been discovered that it is feasible to determine the size distribution of vesicle dispersions in a reliable manner and it appears especially useful to employ the combination of SEC fractionation, PCS and the enzymatic PC quantitation. A drawback discovered was for certain liposome dispersions that in some of the dispersions SEC fractionation showed incomplete recovery of the vesicles. This indicated a demand for a different fractionation method which does not have the limitations that the SEC method described above has. To this end flow field-flow fractionation was chosen. In a previous study the influence of some key factors such as ionic strength of the eluent as well as pore size of the semi-permeable membrane on liposome fractionation behavior has been evaluated. Neutral liposomes were found very dependent of the ionic strength when it comes to elution time. In this study, the intention was to find out if the retention behavior and calculated geometric radius of liposomes obtained by flow field-flow fractionation in combination with multi-angle light scattering is affected by the osmotic pressure of the medium used for diluting the liposomes and/or running the AF4. In order to exclude ionic-strength effects the salt concentration was kept constant while the osmotic pressure was varied by using mono- and disaccharides. In conclusion, this project has demonstrated that a change in osmotic pressure, with constant ionic strength, affects both elution time and calculated size of liposomes that were prepared by high pressure filter extrusion. But, osmotic stress was found to affect liposomes of different sizes in a different manner; liposomes that were smaller than the pore size of the filter used for extrusion were found to shrink in hyperosmotic medium but stay quite constant in size in hypo-osmotic medium. In contrast, liposomes that were larger than the pore size of the filter were found to shrink in hyperosmotic medium and swell in hypo-osmotic medium. A hypothesis is presented to explain this behavior.