Preparation and Characterization of Reconstituted Lipid-Synthetic Polymer Discoidal Particles

Discoidal high-density lipoproteins generated by the apolipoprotein-mediated solubilization of membrane lipids in vivo can be reconstituted with phospholipids and apolipoproteins in vitro. Recently, it has been reported that such particles can be prepared using the hydrolyzed acid form of styrene-maleic anhydride copolymer (SMAaf) instead of apolipoproteins, but characterization of its physicochemical properties has remained less elucidated. In the present study, with the aim of applying SMAaf-based lipid nanoparticles as novel delivery vehicles of drugs and/or imaging agents, we investigated the preparation conditions and evaluated the physicochemical properties of lipid-SMAaf complexes. SMAaf induced spontaneous turbidity clearance of dimyristoylphosphatidylcholine (DMPC) vesicles accompanied by the formation of smaller particles not only at the phase transition temperature of DMPC but also above it. Such reductions in the turbidity were not observed with some other amphiphilic synthetic polymers tested under the same experimental conditions. Size exclusion chromatography analyses showed that homogeneously sized particles were prepared at lipid to SMAaf weight ratios of less than 1/1.5. Dynamic light scattering and transmission electron microscopy revealed that gel-filtered DMPC-SMAaf complexes were approximately 8-10 nm in diameter and discoidal in shape. The DMPC-SMAaf complexes were relatively stable even after lyophilization but were sensitive to pH changes. Fluorescence techniques demonstrated that the gel to liquid-crystalline phase transition temperature of DMPC in the discoidal complexes broadened significantly relative to that of liposomes, despite their common bilayer structure, which is a typical feature of discoidal lipid nanoparticles. These results provide fundamental insights into discoidal SMAaf-based lipid nanoparticles for the development of novel delivery vehicles.