Effect of serum protein binding on real-time trafficking of liposomes with different charges analyzed by positron emission tomography

Liposomes have been used as carriers of various materials and as tools for gene transfer: for the latter purpose, positively charged liposomes are usually used. To evaluate the stability in the presence of serum and the in vivo behavior of such liposomes as well as those aspects of neutral and negatively charged liposomes, we investigated liposomal agglutinability in the presence of serum, serum protein binding to these liposomes, and real-time liposomal trafficking by a non-invasive method using positron emission tomography (PET). Liposomes composed of dipalmitoylphosphatidylcholine, cholesterol without or with charged lipid were prepared in the presence of mannitol, and the turbidity change in the presence of serum was determined. Turbidity increase was not observed for so-called long-circulating liposomes, i.e., liposomes modified with glucuronic acid or with poly(ethylene glycol), or for negatively charged liposomes containing dicetyl phosphate (DCP), phosphatidylglycerol, or phosphatidylserine. On the contrary, a significant turbidity increase was observed when positively charged liposomes modified with stearylamine, stearyltrimethylammmonium chloride or 1,2-dimyristyloxypropyl-3-dimethylhydroxyethyl bromide (DMRIE), which is known as a component of liposomes for gene transfer, were used. These liposomes were found to have bound a high amount of serum proteins after separation of unbound serum proteins by use of a spin column. The liposomal trafficking in vivo was determined for three kinds of liposomes, i.e., liposomes with DMRIE, those with DCP, and those without charged lipids. These liposomes were prepared in the presence of 2-[F-18]fluoro-2-deoxy-D-glucose ([2-F-18]FDG), and the [2-F-18]FDG-labeled liposomes were administered to mice to perform PET scans. Positively charged liposomes containing DMRIE showed high accumulation in the liver compared with neutral and negatively charged liposomes. Since DMRIE-liposomes tended to aggregate in the presence of serum, and to be associated with serum protein, these characters may lead to the high uptake of DMRIE-liposomes by the liver.