Nucleic acid delivery: the missing pieces of the puzzle?

The delivery of genes or RNA interference (RNAi) agents can increase or decrease the expression of virtually any protein in a cell, and this process opens the path for cures to most diseases that afflict humans. However, the high molecular weight, anionic nature, and instability of nucleic acids in the presence of enzymes pose major obstacles to their delivery and frustrates their use as human therapies. This Account describes current ideas about the mechanisms in nonviral nucleic acid delivery and how lipidic and polymeric carriers can overcome some of the critical barriers to delivery. Over the last 20 years, researchers have developed a multitude of polymeric and lipidic vectors, but only a small fraction of these have progressed into clinical trials. None of these vectors has received FDA approval, which indicates that the current vectors do not yet have suitable properties for effective in vivo nucleic acid delivery. Nucleic acid delivery is a multistep process and inefficiencies at any stage result in a dramatic decrease in gene delivery or gene silencing. However, the majority of studies investigating synthetic vectors focus solely on optimization of endosomal escape. A small number of studies address how to improve uptake via targeted delivery, and an even smaller fraction examine the intracellular fate of the delivery systems and nucleic acid cargo. The internalization of genes into the cell nucleus remains an inefficient and mysterious process. In the case of DNA delivery, strategies are needed to increase and accelerate the migration of DNA through the cytoplasm and transport it through the nuclear membrane. siRNA delivery involves fewer barriers. siRNA is more readily released from the carrier and more resistant to enzymatic degradation, and its target is in the cytoplasm; hence, siRNA delivery systems are becoming a clinical reality. With regard to siRNA therapy, the exact cytoplasmic location of RNA-induced silencing complex (RISC) formation and activity is unknown, which makes specific targeting of the RISC for more efficient delivery difficult. Furthermore, we would like to identify the factors that favor the binding of siRNA to Ago-2. If we could understand how the half-life of siRNA and Ago-2/siRNA complex in the cytoplasm can be modulated without interfering with RISC functions that are essential for normal cell activity, we could increase siRNA delivery efficiency. In this Account, we review the current synthetic vectors and propose alternative strategies in a few cases. We also suggest how certain cellular mechanisms might be exploited to improve gene transfection and silencing. Finally, we discuss whether some carriers that deliver the siRNA to cells could also repackage the siRNA into exosomes. The exosomes would then transport the siRNA into a subsequent population of cells that manifest the siRNA effect. This piggy-back mechanism may be responsible for reported deep tissue siRNA effects using certain carriers.