GENERATION OF CONTRAST-CARRYING LIPOSOMES OF DEFINED SIZE WITH A NEW CONTINUOUS HIGH-PRESSURE EXTRUSION METHOD

A novel continuous high pressure extrusion method was evaluated for th e generation of radiopaque and paramagnetic liposomes. The magnetic re sonance contrast agent gadolinium diethylenetriaminepentaacetic acid ( Gd-DTPA) as well as the X-ray contrast agent iopromide were used as wa ter-soluble model substances for liposomal encapsulation. The continuo us process, which is introduced here, allows the fast and efficient ex trusion of large batches of liposomal preparations with maximum flow r ates of 500 ml/min. Applying high pressures up to 10.5 MPa, MLV prepar ed by the film method were sequentially extruded through polycarbonate membranes of decreasing pore size. Encapsulation efficiency was found to be dependent on lipid composition and concentration, amount of con trast agent in the preparation as well as choice of final pore size fo r extrusion, Application of freeze-thaw cycles markedly improved the e ntrapment of iopromide, whereas for Gd-DTPA freeze-thaw surprisingly t urned out to have only minor effects. Entrapment values rose with incr easing lipid concentration and fell sharply with increasing solute con centration. Mean liposome diameters could be varied using polycarbonat e membranes of differing pore sizes. Smaller final pore sizes led to v esicle populations with smaller mean diameters and lower encapsulation efficiencies. Excellent maximum encapsulation efficiencies of more th an 50% for iopromide and over 60% for Gd-DTPA were obtained for vesicl es with mean diameters of around 100 nm, as determined by photon corre lation spectroscopy (PCS) and confirmed by negative-staining electron microscopy. Employing medium contrast (100 mg/g iodine and 180 mu mol/ g Gd) and lipid concentrations (150 mg/g), entrapment values as high a s 40% for the X-ray and 50% for the paramagnetic contrast agent could still be achieved. Best results were obtained using a lipid mixture of soy phosphatidylcholine (SPC), cholesterol (Chol) and soy phosphatidy lglycerol (SPG) in a molar ratio of 6:3:1 for iopromide and SPC, Chol 7:3 for Gd-DTPA. Liposomal preparations remained stable upon storage a t 2-8 degrees C for 6 months. The new continuous high pressure extrusi on method proved to be suitable for the generation of large volumes of stable, contrast-carrying liposomes with outstanding encapsulation ef ficiencies.