Liposomal composition and storage temperature affect loading efficiency of heparin

Abstract

Two of the major factors affecting the encapsulation of any drug in liposomes is liposomal composition and storage temperature. However, this project evaluated the influence of the combination of both these factors on loading efficiency of heparin. Liposomal composition is mainly described by lipid charge, lipid transition temperature and liposomal density. Different charged molecules were used to construct three kinds of liposomal carriers, lecithin:cholesterol, lecithin:cholesterol:stearylamine and lecithin: cholesterol:phosphatidic acid. A mixture of fluorescent and normal heparin was used for encapsulation. Three different storage temperatures (4°C, RT, and 37°C) were used after entrapment. Fluorescent absorbance was measured in order to determine heparin concentrations in liposomal preparations, after liposome lysis by Triton-X 100. Results showed an increased entrapment of heparin by positively charged liposomes, containing stearylamine. The least encapsulation was recorded by liposomes containing phosphatidic acid. Differences in loading efficiency were interpreted mainly in terms of lipid charge - drug charge interactions, with the negatively charged heparin being more readily entrapped in positively charged liposomes. However, unusual high entrapment of heparin was recorded, giving rise to "heparin-coated" theories. In the case of storage temperature, the effect on drug leakage was the focal point of interest. Heparin fluorescence was measured at one week of storage. No significant differences were recorded (p>0.5) among the same species of liposomes stored in different temperatures. However, the difference between fluorescence recorded by stearylamine-containing liposomes and phosphatidic acid-containing ones, both stored at 4°C, was significantly higher (p<0.5) than the difference between fluorescence recorded by the above type of liposomes when stored at 37°C.