Liposomes as carriers of imaging agents.

This review discusses the utilization of liposomes as imaging agents or as vehicles for contrast materials. The initial approach was the use of radiolabeled liposomes for scintigraphy. To this end liposomes were either labeled in the lipid membrane or aqueous radiotracers were incorporated inside the lipid vesicles. The lipid labeling provides a more stable association of the radioactive tracer and the lipid vesicles, while the use of water-soluble radiotracers provides a wider selection of compounds. Early attempts at selective tumor imaging using radiolabeled liposomes were unsuccessful. The use of monoclonal antibodies attached to liposomes offers new hopes. Several strategies have been proposed in this respect and several others can be envisioned. The use of liposomes permits the use of several administration routes for imaging agents. Of particular interest is the subcutaneous administration for lymph node visualization. Liposomes offer clear advantages over most radiocontrast agents for prolonged hepatosplenic contrast enhancement. This is particularly relevant in the diagnostic evaluation of the abdomen with computed tomography. Important research efforts are being conducted in this area. Two different approaches have been advanced: the incorporation of contrast agents into liposomes and the preparation of radiopaque liposomes from radiodense lipids. Nuclear magnetic resonance imaging can also benefit from contrast agents. Several centers are investigating this exciting field using liposomes loaded with paramagnetic elements.