Radiopaque liposomes: effect of formulation conditions on encapsulation efficiency

Liposomes containing sodium ioxitalamate were prepared by sonication. Suitable amounts of purified soybean phosphatidylcholine and cholesterol were used at various molar ratios. Stearylamine or dicetylphosphate were added to this lipid composition when charged liposomes were required. After sonication and removal of unencapsulated solute, this manufacturing process yielded small multilamellar vesicles as confirmed by electron microscopy. These liposomes did not exhibit a narrow range of size distribution; the mean particle size varied from 135 to 145 nm. With respect to the efficiency of encapsulation, two parameters were distinguishable: the volume of encapsulated aqueous space per unit of lipid weight and the percentage of the contrast agent added that became encapsulated in the liposomes. Investigation of the preparative parameters revealed that increased molar ratios of cholesterol yielded higher aqueous volume and iodine contents in the liposomes, which were attributed to a reduction of the liposome permeability to the contrast agent. However, the inclusion of cholesterol into the bilayer liposomal membrane was limited, probably by solubility restrictions. Negatively and positively charged liposomes had higher rates of encapsulation than did neutral liposomes. This result was expected since efficient encapsulation of polar compounds requires formation of large aqueous spaces within the vesicles per mole of lipids. Increase of the lipid fractions at a constant, reduced the aqueous volume entrapped per millimole of lipid and, consequently, the iodine content in the liposomes. However, an increase in the initial sodium ioxitalamate concentration diminished the aqueous volume entrapped in the liposomes but increased the iodine content.