Cationic lipids are essential for gene delivery mediated by intravenous administration of lipoplexes

It was recently suggested that intravenously administered lipoplexes serve as a depot for the extracellular release of naked DNA and it is the naked DNA that mediates gene delivery in the lung. If this is the mechanism responsible for gene expression, we reasoned that continuous infusion of plasmid DNA should also result in significant lung expression in the absence of lipoplexes. Moreover, the infusion of non-coding plasmid DNA should inhibit gene delivery by lipoplexes. Infusion of plasmid DNA at a rate of 80 microg/min into the tail vein of a mouse resulted in a DNA serum concentration of 800 microg/ml. This was equivalent to a transcriptionally active DNA concentration of 120 microg/ml plasma as determined by an in vitro transfection assay. In spite of this high level of transcriptionally active DNA, there was no significant gene expression in the lung or any other organ tested. In addition, when lipoplex containing a reporter gene was injected, followed by an infusion of non-coding plasmid DNA as a potential competing molecule for DNA released from the lipoplex there was no effect on gene expression. These experiments indicate that the cationic lipid component of the lipoplex functions in an active capacity beyond that of a simple passive release matrix for plasmid DNA.