Liposomes for scintigraphic imaging: Optimization of in vivo behavior

Liposomes, microscopic lipid vesicles consisting of concentric phospholipid bilayers enclosing discrete aqueous spaces, have been investigated extensi vely as carriers for drugs in attempts to achieve selective deposition and/ or reduced toxicity. Liposomes radiolabeled with gamma emitters (Ga-67, In- 111 and (99)mTc) have been used for imaging purposes. Liposomes as formulat ed in the past, are rapidly taken up by cells of the mononuclear phagocyte system, primarily those located in Liver and spleen. However, it has been s hown during the last two decades that the in vivo behavior of liposomes can be modulated by modifying their formulation. The size and the lipid compos ition have a major influence on the blood clearance rate, hepatic uptake an d splenic uptake of liposomes. The development of long circulating liposome s, in particular coating of the bilayer with polyethyleneglycol (PEG) resul ted in Liposomes that oppose recognition by the MPS, thus displaying even l onger circulatory half-lives. By carefully adjusting the liposomal formulat ion, the in vivo characteristics of liposomes can be tailored such that the y become suitable vehicles for imaging various pathological processes in vi vo. Liposomes have been proposed for tumor imaging, for infection imaging a nd as blood pool markers. Here, the factors that determine the in vivo beha vior of liposomes and the current status of liposome-based radiopharmaceuti cals are reviewed.