LARGE-SCALE PRODUCTION OF LIPOSOMES OF DEFINED SIZE BY A NEW CONTINUOUS HIGH-PRESSURE EXTRUSION DEVICE

A new continuous high pressure extrusion apparatus for the generation of liposomes was designed and tested in the present study. The extrude r, which basically consists of an open supply vessel and a high pressu re filter holder mounted on a gas-driven pump exhibited superior abili ties compared to currently available, discontinuous extrusion devices. Due to continuous flows up to 500 ml/min (filter diameter 47 mm), lar ge 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 lipo somal preparations with various lipid compositions at lipid concentrat ions as high as 400 mg/g through polycarbonate membranes without any c logging of the system. Employing final pore sizes between 5.0 and 0.03 mu m, the mean vesicle diameter of liposomal preparations could be va ried from 400 to 60 nm. Surprisingly high encapsulation efficiencies o f two water-soluble contrast agents, iopromide and Gd-DTPA, which we u sed as model substances, were obtained for continuously extruded lipos omes. Values of over 50 % for iopromide and more than 60 % for Gd-DTPA , which were achieved in combination with a freeze-thaw protocol, to o ur knowledge exceed all results reported so far for passive entrapment of water-soluble contrast agents. The generation of 6 batches of iopr omide-carrying liposomes under identical conditions revealed good repr oducibility of the new method. Physico-chemical characterization of vo id as well as contrast-carrying liposomes after storage at 2-8 degrees C for 6 months showed satisfactory long-term stability of continuousl y extruded liposomes. The new structural design of the high pressure e xtrusion device permits, for the first time, reproducible, continuous, large-scale production of stable liposomes of defined size. Scaling u p the widely used extrusion method to an industrial level seems feasib le now.