Phosphorothioate-stimulated uptake of siRNA by mammalian cells: A novel route for delivery

The efficient delivery of biologically functional short interfering RNA (siRNA) in vivo remains a widely unresolved technical problem in therapeutic drug development. The repertoire of concepts for the cellular uptake of oligonucleotide-based tools and drugs has been extended by the mechanistically novel finding that phosphorothioate (PS)-modified single-stranded oligodeoxyribonucleotides (ON) promote the intracellular accumulation of naked extra-cellular siRNA in a variety of cell types. This mode of delivery gives rise to substantial intracellular amounts of siRNA, up to 10⁴ siRNA molecules per cell. Conversely, the moderate biological effectiveness strongly indicates that intracellular release of siRNA from sub-cellular compartments where it seems to be trapped is a necessary step towards efficient target suppression. Here, we summarize key characteristics of the PS-stimulated cellular uptake of siRNA and describe concepts for the increase of intracellular delivery of biologically functional siRNA.