Large-Scale Production of Liposomes of Defined size by a New Continuous High Pressure Extrusion Device

Abstract

A new continuous high pressure extrusion apparatus for the generation of liposomes was designed and tested in the present study. The extruder, which basically consists of an open supply vessel and a high pressure filter holder mounted on a gas-driven pump exhibited superior abilities compared to currently available, discontinuous extrusion devices. Due to continuous flows up to 500 ml/min (filter diameter 47 mm), large batches on a liter scale could be extruded in one step. The maximum pressure of 10.5 MPa employed here, enabled the rapid passage of liposomal preparations with various lipid compositions at lipid concentrations as high as 400 mg/g through polycarbonate membranes without any clogging of the system. Employing final pore sizes between 5.0 and 0.03 μm, the mean vesicle diameter of liposomal preparations could be varied from 400 to 60 nm. Surprisingly high encapsulation efficiencies of two water-soluble contrast agents, iopromide and Gd-DTPA, which we used as model substances, were obtained for continuously extruded liposomes. Values of over 50 % for iopromide and more than 60 % for Gd-DTPA, which were achieved in combination with a freeze-thaw protocol, to our knowledge exceed all results reported so far for passive entrapment of water-soluble contrast agents. The generation of 6 batches of iopromide-carrying liposomes under identical conditions revealed good reproducibility of the new method. Physico-chemical characterization of void as well as contrast-carrying liposomes after storage at 2–8°C for 6 months showed satisfactory long-term stability of continuously extruded liposomes.

The new structural design of the high pressure extrusion device permits, for the first time, reproducible, continuous, large-scale production of stable liposomes of defined size. Scaling up the widely used extrusion method to an industrial level seems feasible now.