siRNA delivery and targeting.

RNA interference (RNAi) is one of the most dramatic findings over the past decade, and the number of publications related to RNAi increased exponentially from 5 in 1998 to more than 2400 in 2008. Generally, RNAi can be achieved by three strategies: chemical synthesized small interfering RNA (siRNA), long double-stranded RNA (dsRNA), and DNA-based (plasmid or viral vector) short hairpin RNA (shRNA). Remarkable progress has been made in different aspects of the RNAi technology, and attempts are being made to use siRNA for therapeutic applications. In this issue, we will focus on the delivery and targeting of chemically synthesized siRNA. Currently, there are 12 ongoing clinical trials using siRNA for numerous diseases. Eight of these clinical trials employ naked siRNA for local treatment of ocular and respiratory diseases. Inefficient delivery to target cells and off-target effect are the two major barriers to turning siRNA into therapeutics. Off-target effect is not the focus of this issue, since significant progress has already been made in overcoming this problem. This special issue contains several research and review articles, which highlight the recent advances in siRNA delivery and targeting. First, a thorough understanding of different biological barriers to siRNA therapeutics is essential for developing successful siRNA delivery system. Juliano and associates describe the major biological barriers between the initial administration of siRNAs and their final targets within cells, as well as possible strategies to overcome these barriers. The authors particularly point out the differences in barriers encountered by monomeric siRNA and siRNA encapsulated in nanocarriers. Nanocarriers efficiently deliver siRNA to tumor cells and other normal tissues with fenestrated endothelia, while molecular conjugates of siRNA with targeting ligands have advantages in targeting siRNA to specific cells or tissues after systemic administration. Generally, an efficient siRNA delivery system includes a cationic group for ionic interaction with siRNA, poly(ethylene glycol) (PEG) for steric hindrance, an endosomolytic group for endosomal disruption, and a targeting ligand for site-specific delivery. The article by Huang and colleagues reviews the current delivery strategies for siRNA, especially the targeted, self-assembled nanoparticles which have shown a great potential for cancer therapy. Major components of these nanoparticles are cationic lipids and protamine which interact with the negatively charged siRNA. The surface of nanoparticles is modified with PEG, and the targeting ligand is attached to the distal end of PEG to achieve targeted delivery. In another paper, the same group describes a novel cationic lipid which can deliver siRNA and enhance its therapeutic effect in lung cancer cells. Lipid-polycation-DNA (LPD) nanoparticles were prepared with a non-glycerol based cationic lipid (DSGLA) which contains a lysine and guanidinium as the cationic headgroup. Compared to nanoparticles containing 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), nanoparticles prepared using DSGLA showed significant increase in tumor regression. Since all these conventional siRNA lipoplexes are formed electrostatically, it is possible that other charged molecules could disrupt the complexes before they reach target cells. Wagner and coworkers describe a strategy to conjugate siRNA directly to the poly(L-lysine) (PLL) backbone via a bioreducible disulfide bond which is stable in the presence of naturally occurring polyanions like heparin, but will be cleaved in the reducing intracellular environment. The other end of PLL is conjugated with an endosomolytic peptide which promotes the endosomal release of siRNA. The most successful story for systemic delivery of siRNA is the self-assembling, cyclodextrin polymer-based nanoparticles developed by Mark Davis and his colleagues. Till now, this is the first targeted delivery system of siRNA which was approved for clinical trial in April 2008. This nanoparticle formulation contains a cyclodextrin-containing polycation, PEG, and human transferrin as a ligand for transferrin receptors on cancer cells. All these three components and siRNA self-assemble into nanoparticles which can be administered intravenously to patients. This article summarizes the discovery and translation of this approach all the way from concept to the clinic. It provides us new insights on how to design carriers based on the special characteristics of siRNA, and move the system into the clinic. siRNA delivery to specific cell types is a key challenge for turning siRNA into therapeutics. A number of cell specific ligands including polysaccharide, antibody, antibody fragment, and peptide have been employed to achieve targeted delivery of siRNA. Lu and associates describe a siRNA nanocarrier modified with bombesin or RGD peptide via a PEG spacer. Systemic administration of this siRNA formulation showed a significant tumor growth inhibition compared to nontargeted siRNA nanocarrier or naked siRNAs. Although most targeted siRNA delivery systems are formed by surface modification of the siRNA complex with targeting ligands, siRNA can also be directly attached to a targeting ligand. Pardridge and colleagues attached siRNA to a monoclonal antibody against the human insulin receptor via a stable avidin-biotin linkage. This siRNA conjugate showed comparable silencing effect compared to siRNA delivered with cationic polymers. editorial