Oligonucleotides, including antisense oligonucleotides and siRNA, are promising therapeutic agents against a variety of diseases. Effective delivery of these molecules is critical in view of their clinical application. Therefore, cation-based nanoplexes have been developed to improve the stability as well as the intracellular penetration of these short fragments of nucleic acids. However, this approach is clearly limited by the strong interaction with proteins after administration and by the inherent toxicity of these positively charged transfection materials. Neutral lipid-oligonucleotide conjugates have become a subject of considerable interest to improve the safe delivery of oligonucleotides. These molecules have been chemically conjugated to hydrophobic moieties such as cholesterol, squalene, or fatty acids to enhance their pharmacokinetic behavior and trans-membrane delivery. The present review gives an account of the main synthetic methods available to conjugate lipids to oligonucleotides and will discuss the pharmacological efficacy of this approach.

We report the conjugation of the natural lipid squalene (SQ) with a small interfering RNA (siRNA), against the junction oncogene RET/PTC1, usually found in papillary thyroid carcinoma (PTC). The acyclic isoprenoid chain of squalene has been covalently coupled with siRNA RET/PTC1 at the 3'-terminus of the sense strand via maleimide-sulfhydryl chemistry. Remarkably, the linkage of siRNA RET/PTC1 to squalene led to an amphiphilic molecule that self-organized in H(2)O as siRNA-SQ RET/PTC1 nanoparticles (NPs). The siRNA-SQ RET/PTC1 NPs, stable in H(2)O, were used for biological studies. In vitro, they did not show any cytotoxicity. Interestingly, in vivo, on a mice xenografted RET/PTC1 experimental model, RET/PTC1-SQ NPs were found to inhibit tumor growth and RET/PTC1 oncogene and oncoprotein expression after 2.5 mg/kg cumulative dose intravenous injections. In conclusion, these results showed that the "squalenoylation" offers a new noncationic plate-form for the siRNA delivery.