SiRNAs with Neutral Phosphate Triester Hydrocarbon Tails Exhibit Carrier-Free Gene-Silencing Activity

Short interfering RNAs (siRNAs) show promise as gene-silencing therapeutics, but their cellular uptake remains a challenge. We have recently shown the synthesis of siRNAs bearing a single neutral phenylethyl phosphotriester linkage within the sense strand. Here, we report the synthesis of siRNAs bearing three different hydrophobic phosphate triester linkages at key positions within the sense strand and assess their gene silencing in the absence of a transfection carrier. The best siRNAs bearing hydrophobic phosphate triester tails were not aromatic and exhibited effective gene silencing (IC₅₀ ≈ 56-141 nM), whereas the aromatic derivative with three hydrophobic tails did not exhibit carrier-free gene silencing.