Recent Advances in Nonviral Vectors for Gene Delivery

Gene therapy has long been regarded a promising treatment for many diseases, whether acquired (such as AIDS or cancer) or inherited through a genetic disorder. A drug based on a nucleic acid, however, must be delivered to the interior of the target cell while surviving an array of biological defenses honed by evolution. Successful gene therapy is thus dependent on the development of an efficient delivery vector. Researchers have pursued two major vehicles for gene delivery: viral and nonviral (synthetic) vectors. Although viral vectors currently offer greater efficiency, nonviral vectors, which are typically based on cationic lipids or polymers, are preferred because of safety concerns with viral vectors. So far, nonviral vectors can readily transfect cells in culture, but efficient nanomedicines remain far removed from the clinic. Overcoming the obstacles associated with nonviral vectors to improve the delivery efficiency and therapeutic effect of nucleic acids is thus an active area of current research. The difficulties are manifold, including the strong interaction of cationic delivery vehicles with blood components, uptake by the reticuloendothelial system (RES), toxicity, and managing the targeting ability of the carriers with respect to the cells of interest. Modifying the surface with poly(ethylene glycol), that is, PEGylation, is the predominant method used to reduce the binding of plasma proteins to nonviral vectors and minimize clearance by the RES after intravenous administration. Nanoparticles that are not rapidly cleared from the circulation accumulate in the tumors because of the enhanced permeability and retention effect, and the targeting ligands attached to the distal end of the PEGylated components allow binding to the receptors on the target cell surface. Neutral and anionic liposomes have been also developed for systemic delivery of nucleic acids in experimental animal models. Other approaches include (i) designing and synthesizing novel cationic lipids and polymers, (ii) chemically coupling the nucleic acid to peptides, targeting ligands, polymers, or environmentally sensitive moieties, and (iii) utilizing inorganic nanoparticles in nucleic acid delivery. Recently, the different classes of nonviral vectors appear to be converging, and the ability to combine features of different classes of nonviral vectors in a single strategy has emerged. With the strengths of several approaches working in concert, more hurdles associated with efficient nucleic acid delivery might therefore be overcome. In this Account, we focus on these novel nonviral vectors, which are classified as multifunctional hybrid nucleic acid vectors, novel membrane/core nanoparticles for nucleic acid delivery, and ultrasound-responsive nucleic acid vectors. We highlight systemic delivery studies and consider the future prospects for nucleic acid delivery. A better understanding of the fate of the nanoparticles inside the cell and of the interactions between the parts of hybrid particles should lead to a delivery system suitable for clinical use. We also underscore the value of sustained release of a nucleic acid in this endeavor; making vectors targeted to cells with sustained release in vivo should provide an interesting research challenge.