Encapsulation of plasmid DNA in PLGA-stearylamine microspheres: a comparison of solvent evaporation and spray-drying methods

Stearylamine, a positively charged hydrophobic molecule, was tested as a formulation agent for the encapsulation of a model plasmid in PLGA microspheres. The primary objective was to compare the spray-drying and double emulsion solvent evaporation methods and evaluate their suitability for fabricating PLGA-stearylamine plasmid-entrapped microspheres. A luciferase reporter gene plasmid (pGL3-Con) was formulated into microspheres using a 64 kDa PLGA 50:50 polymer blended with stearylamine (SA) at a range of concentrations up to 15%(m)/(m), by the solvent evaporation and spray-drying methods. The microspheres were characterized regarding their size distributions, zeta potentials and morphology by laser diffraction, electrophoretic mobility and scanning electron microscopy (SEM), respectively. Formulated plasmid extracts were assessed for physical damage by agarose gel electrophoresis, and the in vitro biological activity was determined by tranfection of a human embryo kidney epithelial (293) cell line. Size distribution analysis showed that SA reduced the median diameters of spray-dried particles from 8.32 to 3.64 microns, with a corresponding reduction in the spread of the distribution, but solvent evaporation microspheres showed an increased median diameter on addition of SA. Concentrations of SA above 10%(m)/(m) resulted in disruption of the smooth morphology of the solvent evaporation particles. There was a SA concentration-dependent tendency in the increase of surface positive charge and resistance to serum nuclease assault and in vitro expression of luciferase protein. These results show that SA and possibly other charged hydrophobic molecules may be useful agents in the formulation of particulate DNA vaccines by both methods.