Liposomes in Diagnostic Imaging – Comparison of Modalities – In-vivo Visualization of Liposomes -

Abstract

Liposomes provide advantages in the areas of X-ray and magnetic resonance imaging, ultrasound and nuclear medicine over other forms of tissue-selective contrast media due to their high capacity loading system, their good tolerability and their tissue selectivity. Target organs are primarily the liver and spleen. However, the lymphatic system may also be considered as a target tissue. We have compiled the available data on the use of liposomes in the imaging area and will review their advantages and disadvantages compared to “standard diagnostics''. At present liposomes are most advanced in nuclear medicine where several preparations are in clinical trials, followed by X-ray imaging. In MRI and, especially, in ultrasound development is less advanced. Furthermore, we have investigated by combined incidental light/fluoroscopic microscopy using continuous 35-mm cine film and video recordings the flow of iopromide-carrying liposomes through the microcapillary bed and the active uptake into macrophages after intravenous injection in the intact rat. Target tissues were the microcirculatory pathways, the liver and the lymphatic system. Using this technique, the uptake of liposomes by macrophages was visualized in the rat liver. Two mechanisms of phagocytosis could be demonstrated, passage through the macrophage membrane followed by degradation (endocytosis) and fusion of the liposome and macrophage membranes. Penetration through fenestrated endothelium could also be shown. In the lymph node, passage through vessels could be visualized. In-vivo microscopy seems to be a suitable method for the investigation of liposomal fate in living animals.