Effective stabilization and delivery of siRNA: reversible siRNA-phospholipid conjugate in nanosized mixed polymeric micelles.

siRNA is a powerful tool to control cellular processes at the post-transcriptional level. However, its therapeutic potential is limited because of low stability in biological fluids and the lack of simple and efficient delivery systems. Chemical modification of siRNA could be used to increase its intracellular delivery, but may affect its specific activity. To overcome these obstacles, we suggest a simple and effective system capable of stabilization, delivery, and subsequent release of free active siRNA within cells. With this in mind, we reversibly modified the double-stranded GFP-siRNA with a phosphothioethanol (PE) portion via the reducible disulfide bond and incorporated the resulting siRNA-S-S-PE conjugate in nanosized PEG-PE micelles. In the mixed siRNA-S-S-PE/PEG-PE micelles obtained, siRNA was well-protected against degradation by nucleases for at least 24 h, and was released easily from these nanoparticles in free form in the presence of glutathione (GSH) at a concentration mimicking the intracellular levels. In GFP-C166 endothelial cells, mixed GFP-siRNA-S-S-PE/PEG-PE micelles down-regulate the GFP production 50-fold more effectively than free siRNA. In addition, siRNA-containing micelles showed none of the cytotoxic side effects typical for siRNA delivery systems that are based on electrostatic association of siRNA with cationic carriers. Thus, a reversible siRNA-phospholipid conjugate formulated into mixed micelles with PEG-PE can be an effective, nontoxic system for stabilization and delivery of siRNA.