PREPARATION OF LIPOSOMES USING A MINI-LAB-8.30-H HIGH-PRESSURE HOMOGENIZER

The range of liposome preparation methods has recently been extended by a number of techniques which are based on the use of homogenizers. However, only a few suitable types of apparatus have been proposed so far. This report describes work with a continously working Mini-Lab 8.30 H high-pressure homogenizer of APV Rannie. Homogenization of dispersions of preformed vesicles which were multilamellar and heterogeneous in size resulted in uniform, small unilamellar vesicles (SUVs). The previously described one-step method (Brandl et al., Drug Dev. Ind Pharm., 16 (1990) 2167-2191) for liposome preparation could also be carried out using the Mini-Lab homogenizer; the resulting vesicles were again very small and homogeneous after one cycle only. Higher homogenization pressures and repeated recirculation led to further reduction in vesicle diameter and heterogeneity. Finally, after about 15 cycles size distributions with peaks of about 30 nm and upper limits below 100 nm could be achieved. With higher lipid concentrations (maximum of 100 mg/ml) size reduction by homogenization was less effective. In the case of cholesterol-containing vesicles, maximum size reduction during repeated homogenization was observed after five to ten cycles. Further recirculation caused vesicle re-growth. The encapsulation efficiency of one-step vesicles for carboxyfluorescein at lipid concentrations (100 mg/ml) was nearly 12%, the entrapped volume being about 0.90 l/mol lipid. Repeated homogenization (up to 30 cycles) reduced these values by about 50%. The erosion of the most stressed part of the apparatus (i.e., the valve) was low, as monitored microscopically. During homogenization a slight leakage from the product compartment into the cooling water compartment was observed. Leakage of cooling water into the product compartment could be seen only during standby periods.