Rigorous control of vesicle-forming lipid pK a by fluorine-conjugated bioisosteres for gene-silencing with siRNA.

While the influence of pK _a provided by amine-containing materials in siRNA delivery vectors for use in gene-silencing has been widely studied, there are little reports in which amine pK _a is controlled rigorously by using bioisosteres and its effect on gene-silencing. Here, we report that amine pK _a could be rigorously controlled by replacement of hydrogen atom(s) with fluorine atom(s). A series of mono- and di-amine lipids with a different number of fluorine atoms were synthesized. The pK _a of the polyamine lipids was shifted to a lower value with an increase in the number of fluorine atoms. The optimal pK _a for high gene-silencing efficiency varied according to the number of amine residues in the polyamine lipid. Whereas the endosomal escape ability of mono-amine lipid-containing lipid vesicles (LVs) depended on the pK _a , that of all tested di-amine lipid-containing LVs showed equal membrane-destabilizing activity. LVs showing moderately weak interactions with siRNA facilitated cytoplasmic release of siRNA, resulting in strong gene-silencing. These findings indicate that appropriate amine pK _a engineering depending on the number of amines is important for the induction of effective RNA interference.
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