Conjugating Phosphospermines to siRNAs for Improved Stability in Serum, Intracellular Delivery and RNAi-Mediated Gene Silencing

siRNAs are usually formulated with cationic polymers or lipids to form supramolecular particles capable of binding and crossing the negatively charged cell membrane. However, particles hardly diffuse through tissues when administered in vivo. We therefore are developing cationic siRNAs, composed of an antisense sequence annealed to an oligophosphospermine-conjugated sense strand. Cationic siRNAs have been previously shown to display gene silencing activity in human cell line (Nothisen et al. J. Am. Chem. Soc. 2009). We have improved the synthesis, purification and characterization of oligospermine-oligoribonucleotide conjugates which provide cationic siRNAs with enhanced biological activity. We show data supporting their carrier-free intracellular delivery in a molecular, soluble state. Additional results on the relationship between global charge, uptake and silencing activity confirm the requirement for an overall positive charge of the conjugated siRNA in order to enter cells. Importantly, conjugated siRNAs made of natural phosphodiester nucleotides are protected from nuclease degradation by the oligophosphospermine moiety, operate through the RNAi mechanism and mediate specific gene silencing at submicromolar concentration in the presence of serum.