A method to determine the incorporation capacity of camptothecin in liposomes

The purpose of this study was to establish a new experimental approach to determine the maximum amount of camptothecin (CPT) that can be incorporated in liposomes, and to use this method to compare the CPT-incorporation capacity of various liposome formulations. Small, CPT-saturated liposomes were prepared by dispersing freeze-dried blends of lipids and drug in phosphate buffer, and subsequent probe-sonication. Excess precipitated CPT could be separated from the liposomes by ultracentrifugation. The small and homogeneous liposome size obtained gave a good and reproducible recovery of liposomes in the supernatant (>80%), whereas the acidic pH (pH 6.0) kept CPT in its hydrophobic lactone form, which is poorly soluble in the buffer. The maximum CPT-incorporation capacity of 12 different liposome formulations was investigated, using the described method, and was found to vary widely. With liposomes made of neutral and anionic phospholipids, the solubility of CPT in the buffer was improved by approximately a factor of 10 (from 2.7 to 15-50 microg/mL) as compared with buffer. With cationic liposomes containing 1,2-dioleoyl-3-trimethyl-ammonium-propane (DOTAP), a maximum CPT-solubilization of 100-fold, the buffer solubility was reached, probably owing to an electrostatic interaction between the cationic lipids and the carboxylate-CPT isomer. Increasing DOTAP fractions within egg-phosphatidylcholine (EPC)/DOTAP liposomes reached a CPT-incorporation plateau at 20 mol% DOTAP. The presented approach appears suitable to study the incorporation capacity of any drug component within small vesicles as long as the liposome incorporation is high relative to the intrinsic water solubility of the drug.