Barriers to gene delivery using synthetic vectors.

Progress has been made in the development of different types of nucleic acids such as DNA and siRNA with the potential to form the basis of new treatments for genetic and acquired disorders. The lack of suitable vectors for the delivery of nucleic acids, however, represents a major hurdle to their continued development and therapeutic application. Synthetic vectors based on polycations are promising vectors for gene delivery as they are relatively safe and can be modified by the incorporation of ligands for targeting to specific cell types. However, the levels of gene expression mediated by synthetic vectors are low compared to viral vectors. The aim of this chapter is to give an overview of the main barriers that have been identified as limiting gene transfer using polycation-based synthetic vectors. The chapter is divided into two sections to focus on both extracellular and intracellular barriers. We describe novel strategies that are being used to develop increasingly sophisticated vectors in an attempt to overcome these barriers. For instance, we describe approaches to prolong the plasma circulation of polyplexes by the incorporation onto their surface of hydrophilic polymers such as polyethylene glycol (PEG) and poly[N-(2-hydroxypropyl)methacrylamide] (pHPMA). In addition, strategies to improve transfer of nucleic acids from the outside of the cell to the nucleus are described to overcome barriers such as escape from endocytic vesicles and translocation across the nuclear membrane. Furthermore, we highlight new types of vectors based on reducible polycations that are triggered by the intracellular environment to facilitate efficient cytoplasmic release of nucleic acids.