Adsorption of plasmid DNA onto lipid/polymer particle assemblies

In this study, the adsorption of plasmid DNA onto preformed lipid/polymer particle assemblies was investigated. These assemblies, called LipoParticles, with a great potential in the biomedical field, were previously studied and thoroughly characterized. They consist of submicrometer anionic spherical polymer particles composed of either poly(styrene) or poly(lactic acid) which are coated with lipid layers formed by mixtures of zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cationic 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP) in tunable proportions. The plasmid adsorption onto the LipoParticle surface was clearly established by the inversion of the surface charge of the assemblies, fluorescence microscopy observations, and agarose gel electrophoresis. From a quantitative standpoint, adsorption isotherms were found to be independent on the polymer core nature but rather strongly dependent on the adsorbed lipid shell composition. Furthermore, the data were fitted with the Langmuir model providing interesting information about the plasmid adsorption process such as the affinity constants between species, as well as the saturation levels of adsorbed plasmids onto assemblies. Finally, the compaction state of the adsorbed DNA was examined thanks to displacement experiments of a DNA-intercalated fluorescent dye, and found to be tightly related to the proportion of the cationic lipid in the adsorbed lipid layer. DPTAP-rich lipid formulations indeed displayed an important plasmid compaction that could be linked to a high affinity constant and a high amount adsorbed.