Strategies for siRNA navigation to desired cells

Whilst small interfering (si) RNAs have emerged as a promising therapeutic modality for treating a diversity of human diseases, delivery constitutes the most serious obstacle to siRNA drug development. As the most used delivery agents can enter all cell types, specificity must be built into the delivery agents or directly attached to the siRNA molecules. The use of antibodies, peptides, Peptide-Fc fusions, aptamers, and other targeting ligands has now enabled efficient gene silencing in the desired cell populations/tissues in vitro and in vivo. The present review summarizes these current innovations, which are important for the design of safe therapeutic siRNAs.