Optimization of Formulations and Conditions for the Aerosol Delivery of Functional Cationic Lipid:DNA Complexes

Abstract

We have examined several variables inherent in aerosolizing cationic lipid:DNA complexes using a jet nebulizer and thereby have optimized the delivery of functional complexes. Maximal aerosol transfer efficiency of cationic lipid:pDNA complexes was quantitated and shown to require the presence of at least 25 mM NaCl as an excipient. This is possibly related to effects on the measured zeta potentials of the complex, which indicate that the complexes are more highly charged in solutions of physiological ionic strength than in solutions of low ionic strength. Inclusion of saline also resulted in retention of the starting lipid to plasmid DNA (pDNA) ratio following nebulization. These data were used to design in vitro aerosolization experiments with tissue culture cells that resulted in the identification of a cationic lipid:pDNA ratio of 0.75:1 (mol:mol) as being optimal for aerosolization. This formulation largely protected pDNA from shear degradation during nebulization and produced a respirable aerosol droplet size (1–3 μm). It was tested further in a mouse model and shown to result in the dose-dependent transfection of mouse lungs, generating the equivalent of several picograms of reporter gene activity per mouse lung. The results of these experiments have provided a set of optimal conditions for nebulizing cationic lipid:pDNA complexes that can be used as a starting point for the further evaluation of aerosol delivery of these nonviral gene delivery vectors in vivo. Conditions for nebulizing cationic lipid:DNA complexes were evaluated and optimized for efficient transfer of DNA, maintenance of the lipid:DNA ratio, and protection of pDNA from degradation during nebulization. These studies indicated that the suspending medium must have an ionic component to obtain maximal transfer of the DNA and to maintain the lipid:DNA ratio in the aerosolized material. Maximal DNA transfer was found to occur at cationic lipid:DNA ratios where some of the DNA remained uncomplexed. However, free plasmid DNA (pDNA) was also shown to be rapidly degraded during aerosolization, whereas complexed pDNA was largely protected. In vitro transfection results from cells exposed to cationic lipid:pDNA aerosols indicated that complexes prepared at a cationic lipid (GL-53):pDNA ratio of 0.75:1 were optimal for transfection. Exposure of BALB/c mice to an aerosol of GL-67:pCF1-CAT prepared at this ratio resulted in the dose-dependent expression of chloramphenicol acetyltransferase (CAT) in the mouse lungs.