Temperature-dependent associating property of liposomes modified with a thermosensitive polymer

Novel temperature-sensitive liposomes having surface properties that can be controlled by temperature were designed as liposomes coated with poly(N-isopropylacrylamide), which exhibits a hydrated coil to dehydrated globule transition at ca. 32 degrees C. To obtain the polymer with anchoring groups to the liposome, a copolymer of N-isopropylacrylamide and octadecyl acrylate (99:1, mol/mol) was synthesized by radical copolymerization. The copolymer revealed the transition near 30 degrees C. Liposomes made from various phospholipids were prepared by sonication and coated with the copolymer. When dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine were used as liposome lipids, remarkable aggregation and fusion of the copolymer-modified liposomes took place between the transition temperature of the copolymer and the gel-liquid-crystalline transition temperature (Tc) of the lipid membranes. However, above the Tc, association between the liposomes was much less significant, although the copolymer is still hydrophobic. In the case of the copolymer-modified dilauroylphosphatidylcholine liposome, the membrane of which takes on a liquid-crystalline state under the experimental conditions, association between the liposomes also hardly occurred even when the copolymer became hydrophobic. On the other hand, below the transition temperature of the copolymer, the copolymer-modified liposomes were very stable and aggregation of the liposomes was not observed, irrespective of membrane lipid. Results obtained in this study demonstrate that the copolymer chains fixed on the surface of the liposome with a solid membrane promote aggregation and fusion of the liposomes by hydrophobic interactions between the copolymer chains and/or between the copolymer chains and the liposome membranes above the transition temperature of the copolymer.