Phospholipid and Liposome
for Pharmaceutical

As an attempt to achieve active targeting using high affinity binding of antibody to the target, immunoliposome, a liposome with antibody attached to its surface, was developed. Ordinary liposome conjugated by antibody insufficiently avoids the reticuloendothelial system, so a PEG-modified liposome is necessary. Two types of approach were considered; the antibody is conjugated directly to phospholipid, or else to an end of the PEG chain. Experimental results indicated that binding to an end of the PEG chain is essential to preserve antigen recognition capacity. Antibody-conjugated liposome is also called pendant-type immunoliposome because of its shape. Pendant-type immunoliposome is expected to play an important role in active targeting, since it has long retention due to PEG and antigen recognition capacity thanks to antibody conjugation. But in the case of the IgG antibody, macrophages recognize it and uptake in the liver increases, for macrophages have Fc receptors. In order to solve this, an immunoliposome using Fab' fragment that lacks Fc region was prepared, and it was demonstrated by Maruyama et al. to have longer retention after intravenous administration than IgG-PEG-liposome. The pattern of Fab'-PEG-liposome disappearance in blood was the same as PEG-liposome, and it had two stages of disappearance, namely an initial, fast disappearance due to phagocytosis by macrophages, etc. and a late, slow disappearance. Since longer retention was achieved by Fab'-PEG-liposome, it was shown to avoid the initial uptake due to phagocytosis. From all these results, Fab' modification on the liposomal surface is considered to be effective in targeting using immunoliposome.