(55a) Rational Design of Polyelectrolyte Complexes for Nucleic Acid Delivery

Therapeutic nucleic acids (genetic DNA, mRNA, siRNA, miRNA) hold immense potential for treating 'undruggable' disease targets and are ideally suited to personalized medicine, but their utility has been severely hampered by difficulties in delivering them to their intracellular sites of action. Present clinical efforts are dominated by lipid nanoparticle formulations, viral vectors, and ligand-conjugated, chemically-modified nucleic acids, all of which have significant limitations in biodistribution, toxicity, and off-target effects. Nanoparticle vectors assembled by polyelectrolyte complexation with hydrophilic polycations offer an attractive alternative. In particular, cationic peptides readily complex nucleic acids, with entropy gain from counterion release driving phase separation into liquid coacervates or solid precipitates. If a neutral hydrophilic polymer (e.g. PEG) is grafted to either polyelectrolyte, phase separation occurs on the microscale, producing micelles with the polyelectrolyte complex in the core and the neutral polymer in the corona. This protects the nucleic acid from nucleolytic degradation, reduces immunogenicity, and allows incorporation of targeting moieties on the outside of the corona block. A number of promising results have been obtained with this method in vitro and in animal models, but we currently lack the ability to predict polyelectrolyte complex and micelle properties from the characteristics of the constituent polymers, an essential requirement for translating laboratory successes to clinical application. We recently showed that the phase of nucleic acid complexes is controlled by the hybridization state of the nucleic acid, with single-stranded nucleic acids forming liquid coacervates and double-stranded nucleic acids forming solid precipitates. I will discuss these results, the underlying polymer physics, and the effect of polyelectrolyte length and structure on the characteristics of both bulk complexes and micelles. Developing a set of design rules for constructing micelles of designed shape and size should enable polyelectrolyte complexes to realize their potential as an improved delivery technology for therapeutic nucleic acids.