## 5 Conclusions  
To conclude, we have obtained lipid conjugates of oligodeoxynucleotides and siRNAs carrying one, two or four hydrophobic ((4-dodecylphenyl)sulfonyl)phosphoramide or (hexadecylsulfonyl)phosphoramide groups at either 5\({}^{\prime}\)- or 3\({}^{\prime}\)-internucleotide positions by a modified protocol of the phosphoramidite synthesis replacing usual aqueous iodine oxidation by the Staudinger reaction between a solid-supported phosphtite triseter and the corresponding long-chain sulfonyl azide.  
The oligodeoxynucleotide conjugates were characterized by low cytotoxicity in either murine or human macrophages and improved cell uptake into the same in the absence of a transfection agent. Spontaneous formation of nanoparticles was observed in aqueous solutions of doubly-conjugated oligonucleotides having either of the two lipophilic modifications at the 5\({}^{\prime}\)-end, which may be responsible for the improved cellular uptake of the respective conjugates. Notably, the size of the nanoparticles decreased with the increase in the length of the hydrocarbon chain from dodecylphenyl to hexadecyl. Flow cytometry revealed the superior uptake of hexadecyl conjugates over the dodecyl ones into human macrophages. Intracellular localization of the conjugates according to confocal microscopy was predominantly cytosolic with the majority of the fluorescence within endosomes.  
Finally, two adjacent lipophilic dodecylphenyl or hexadecyl groups placed within the 3\({}^{\prime}\)-terminal overhang and at the 3\({}^{\prime}\)-terminal phosphate of the passenger strand did not significantly impair the gene-s Silencing ability of siRNAs targeting repulsive guidance molecule (RGMa) mRNA in PK-59 cell culture, whereas the siRNAs carrying four of the same at the 3\({}^{\prime}\)-ends of both passenger and antisense strands were considerably less active. Thus, lipid-siRNA conjugates with optimized number and position of the lipid chains could be a prospective candidate for future experiments in an in vivo model of multiple sclerosis and, potentially, other diseases when RNA targeting is applicable.  
The following are available online at [[https://www.mdpi.com/2076-3](https://www.mdpi.com/2076-3)]([https://www.mdpi.com/2076-3](https://www.mdpi.com/2076-3)) 417/11/3/1174/s1, Figures S1-S3: IR, \({}^{1}\)H and \({}^{13}\)C spectra of 1-hexadecanesulfonyl azide; reverse-phased HPLC profiles and ESI mass spectra of modified oligonucleotides, Figure S4: Intracellular accumulation of \(\eta\)-\(F\) conjugate in KB-8-5 cells, Figure S5: Intracellular accumulation of \(\delta\)-\(F\) and \(\eta\)-\(F\) conjugates in RAW264.7 cells and THP-1-induced macrophages.  
The first three authors ([PERSON], [PERSON] and [PERSON]) made equal contributions to the work. Conceptualization, D.S.; methodology, M.Z., M.F., D.S. and T.Z.; software, O.M.; validation, D.S., M.F. and M.Z.; formal analysis, O.M. and Y.S.; investigation, A.D. and Y.S.; resources, M.Z. and T.Z.; data curation, O.M., A.F. and Y.S.; writing--original draft preparation, O.M. and D.S.; writing--review and editing, O.M., M.Z. and D.S.; visualization, O.M. and A.D.; supervision, O.M. and A.F.; project administration, M.Z., M.F. and D.S.; funding acquisition, M.Z., D.S. and M.F. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Russian Science Foundation, grant No. 19-74-30011 (cytotoxicity, confocal microscopy, flow cytometry, dynamic light scattering), Russian Foundation for Basic Research, grant No. 18-515-57006 (siRNA synthesis), the Ministry of Science and Higher Education of the Russian Federation, project No. FSUS-2020-0035 to Novosibirsk State University (oligodeoxynucleotide synthesis), and Japan Medical Research Foundation, grant No. 2018 JP002 (biological activity).  
Data sharing is not applicable to this article.  
The authors declare no conflict of interest.