Preparation of DNA dry powder for non-viral gene delivery by spray-freeze drying: effect of protective agents (polyethyleneimine and sugars) on the stability of DNA.

This study investigates the feasibility of using the process of spray-freeze drying (SFD) to produce DNA dry powders for non-viral gene delivery. The effect of protective agents was assessed on the stability of DNA dry powders after SFD. The process of SFD had adverse effects on the tertiary structure of DNA with the protective agents of sucrose, trehalose and mannitol. With the protection of these sugars, a band corresponding to the linear form of DNA was observed during gel electrophoresis between the supercoiled form (SC) and the open circular (OC) form. On the contrary, excess cationic condensing polyethyleneimine (PEI), in conjunction with the above sugars, had the ability to provide protection for DNA from degradation after SFD. This is indicated by the reservation in SC and OC forms of DNA during agarose gel electrophoresis. The electrostatic forces between PEI polymer and DNA are critical for providing protection against various stresses generated by the process of SFD. Furthermore, on rehydration, the particle size and zeta potential of PEI/DNA complexes at weight ratios 3:1 of SFD dry powders were well maintained. Also, no transfection activity loss of PEI/DNA complexes at weight ratios 3:1 on NIH/3T3 cells was observed for reconstituted powders as compared with untreated control solutions. These results give a better understanding of preparing stable DNA dry powders by the process of SFD.