Enhancing nucleic acid delivery, insights from the cationic phospholipid carriers

The development of nucleic acid-based drugs has attracted considerable interest in the past two decades as a new category of biologics. A key challenge in successfully achieving the full potential of nucleotide therapeutics is their efficient delivery. Synthetic cationic lipids are currently the most extensively used non-viral nucleotide carriers because of their ability to form complexes with the nucleic acids. Here we examine the properties of oligonucleotide lipoplexes with a particularly noteworthy cationic lipid class, the cationic phosphatidylcholines (PCs) which exhibit low toxicity and good nucleotide delivery efficacy. Studies on a set of cationic PCs reveal the existence of a strong, systematic dependence of their carrier efficiency on the lipid hydrocarbon chain structure. Their activity rises with the increase in chain unsaturation and declines with the increase in chain length. Maximum transfection is detected for ethyl-PC (ePC) with monounsaturated 14:1 chains. The same lipid exhibits maximum activity also in intracellular delivery of siRNA. As the lipid phase behavior is known to depend substantially on the hydrocarbon chain structure, the above relationships validate a view that cationic PC phase properties are an important factor for their activity. Indeed, time-resolved X-ray diffraction studies showed that the rate of the nucleotide release from the lipoplexes, as well as their transfection activity, correlate with the non-lamellar phase progressions detected in mixtures of cationic PCs with biomembrane lipids. These findings emphasize the role of the non-lamellar lipid mesophases in the nucleic acid transport across the cellular membranes and their intracellular release.