PREPARATION AND CHARACTERIZATION OF ECHOGENIC LIPOSOME AS AN ULTRASOUND CONTRAST AGENT - SIZE-DEPENDENCY AND STABILIZING EFFECT OF CHOLESTEROL ON THE ECHOGENICITY OF GAS-ENTRAPPING LIPOSOME

The liposome entrapping CO2 gas inside the vesicle, which is called th e echogenic liposome, has been made and characterized in vitro as an u ltrasound contrast agent. The small unilamellar vesicle (SUV), large u nilamellar vesicle (LUV) and multilamellar vesicle (MLV) as echogenic liposomes were compared in their echogenic efficiency and stability an d the effect of size and acoustic property was tested. The acoustic re flectivity increased with the increase in size of the vesicle, largest for the gas filled MLV among the three liposome suspensions. The acou stic reflectivity obtained with the echogenic MLV was larger than that of the gas bubbles enclosed within a surfactant mixture. A half-lifet ime of 39 min was observed for the MLV prepared from egg-yolk phosphat idylcholine liposomes, The duration of reflectivity was prolonged dras tically to a half-lifetime of 866 min by incorporating cholesterol int o the MLV, although the echogenicity was decreased by such incorporati on, The stabilizing effect of cholesterol for the ordinary liposomal m embrane was thus ascertained in the present case of the gas-entrapping liposome, Our findings encourage the future development of improved g as-entrapping liposomes for the clinical trials of ultrasound contrast agents.