Cholesterol-Modified Anti-Il6 siRNA Reduces the Severity of Acute Lung Injury in Mice.

Small interfering RNA (siRNA) holds significant therapeutic potential by silencing target genes through RNA interference. Current clinical applications of siRNA have been primarily limited to liver diseases, while achievements in delivery methods are expanding their applications to various organs, including the lungs. Cholesterol-conjugated siRNA emerges as a promising delivery approach due to its low toxicity and high efficiency. This study focuses on developing a cholesterol-conjugated anti-Il6 siRNA and the evaluation of its potency for the potential treatment of inflammatory diseases using the example of acute lung injury (ALI). The biological activities of different Il6-targeted siRNAs containing chemical modifications were evaluated in J774 cells in vitro. The lead cholesterol-conjugated anti-Il6 siRNA after intranasal instillation demonstrated dose-dependent therapeutic effects in a mouse model of ALI induced by lipopolysaccharide (LPS). The treatment significantly reduced Il6 mRNA levels, inflammatory cell infiltration, and the severity of lung inflammation. IL6 silencing by cholesterol-conjugated siRNA proves to be a promising strategy for treating inflammatory diseases, with potential applications beyond the lungs.

Cholesterol Conjugates of Small Interfering RNA: Linkers and Patterns of Modification.

Cholesterol siRNA conjugates attract attention because they allow the delivery of siRNA into cells without the use of transfection agents. In this study, we compared the efficacy and duration of silencing induced by cholesterol conjugates of selectively and totally modified siRNAs and their heteroduplexes of the same sequence and explored the impact of linker length between the 3' end of the sense strand of siRNA and cholesterol on the silencing activity of "light" and "heavy" modified siRNAs. All 3'-cholesterol conjugates were equally active under transfection, but the conjugate with a C3 linker was less active than those with longer linkers (C8 and C15) in a carrier-free mode. At the same time, they were significantly inferior in activity to the 5'-cholesterol conjugate. Shortening the sense strand carrying cholesterol by two nucleotides from the 3'-end did not have a significant effect on the activity of the conjugate. Replacing the antisense strand or both strands with fully modified ones had a significant effect on silencing as well as improving the duration in transfection-mediated and carrier-free modes. A significant 78% suppression of MDR1 gene expression in KB-8-5 xenograft tumors developed in mice promises an advantage from the use of fully modified siRNA cholesterol conjugates in combination chemotherapy.

Cholesterol-Conjugated Supramolecular Multimeric siRNAs: Effect of siRNA Length on Accumulation and Silencing In Vitro and In Vivo.

Conjugation of small interfering RNA (siRNA) with lipophilic molecules is one of the most promising approaches for delivering siRNA in vivo. The rate of molecular weight-dependent siRNA renal clearance is critical for the efficiency of this process. In this study, we prepared cholesterol-containing supramolecular complexes containing from three to eight antisense strands and examined their accumulation and silencing activity in vitro and in vivo. We have shown for the first time that such complexes with 2'F, 2'OMe, and LNA modifications exhibit interfering activity both in carrier-mediated and carrier-free modes. Silencing data from a xenograft tumor model show that 4 days after intravenous injection of cholesterol-containing monomers and supramolecular trimers, the levels of MDR1 mRNA in the tumor decreased by 85% and 68%, respectively. The in vivo accumulation data demonstrated that the formation of supramolecular structures with three or four antisense strands enhanced their accumulation in the liver. After addition of two PS modifications at the ends of antisense strands, 47% and 67% reductions of Ttr mRNA levels in the liver tissue were detected 7 days after administration of monomers and supramolecular trimers, respectively. Thus, we have obtained a new type of RNAi inducer that is convenient for synthesis and provides opportunities for modifications.

Soloxolone Methyl Reduces the Stimulatory Effect of Leptin on the Aggressive Phenotype of Murine Neuro2a Neuroblastoma Cells via the MAPK/ERK1/2 Pathway.

Despite the proven tumorigenic effect of leptin on epithelial-derived cancers, its impact on the aggressiveness of neural crest-derived cancers, notably neuroblastoma, remains largely unexplored. In our study, for the first time, transcriptome analysis of neuroblastoma tissue demonstrated that the level of leptin is elevated in neuroblastoma patients along with the severity of the disease and is inversely correlated with patient survival. The treatment of murine Neuro2a neuroblastoma cells with leptin significantly stimulated their proliferation and motility and reduced cell adhesion, thus rendering the phenotype of neuroblastoma cells more aggressive. Given the proven efficacy of cyanoenone-bearing semisynthetic triterpenoids in inhibiting the growth of neuroblastoma and preventing obesity in vivo, the effect of soloxolone methyl (SM) on leptin-stimulated Neuro2a cells was further investigated. We found that SM effectively abolished leptin-induced proliferation of Neuro2a cells by inducing G1/S cell cycle arrest and restored their adhesiveness to extracellular matrix (ECM) proteins to near control levels through the upregulation of vimentin, zonula occludens protein 1 (ZO-1), cell adhesion molecule L1 (L1cam), and neural cell adhesion molecule 1 (Ncam1). Moreover, SM significantly suppressed the leptin-associated phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and ribosomal protein S6 kinase A1 (p90RSK), which are key kinases that ensure the survival and proliferation of cancer cells. Further molecular modeling studies demonstrated that the inhibitory effect of SM on the mitogen-activated protein kinase (MAPK)/ERK1/2 signaling pathway can be mediated by its direct interaction with ERK2 and its upstream regulators, son of sevenless homolog 1 (SOS) and mitogen-activated protein kinase kinase 1 (MEK1). Taken together, our findings in murine Neuro2a cells provide novel evidence of the stimulatory effect of leptin on the aggressiveness of neuroblastoma, which requires further detailed studies in human neuroblastoma cells and relevant animal models. The obtained results indicate that SM can be considered a promising drug candidate capable of reducing the impact of adipokines on tumor progression.

Structural Modifications of siRNA Improve Its Performance In Vivo.

The use of small interfering RNA (siRNA) in the clinic gives a wide range of possibilities for the treatment of previously incurable diseases. However, the main limitation for biomedical applications is their delivery to target cells and organs. Currently, delivery of siRNA to liver cells is a solved problem due to the bioconjugation of siRNA with N-acetylgalactosamine; other organs remain challenging for siRNA delivery to them. Despite the important role of the ligand in the composition of the bioconjugate, the structure and molecular weight of siRNA also play an important role in the delivery of siRNA. The basic principle is that siRNAs with smaller molecular weights are more efficient at entering cells, whereas siRNAs with larger molecular weights have advantages at the organism level. Here we review the relationships between siRNA structure and its biodistribution and activity to find new strategies for improving siRNA performance.

siRNA-Mediated Timp1 Silencing Inhibited the Inflammatory Phenotype during Acute Lung Injury.

Acute lung injury is a complex cascade process that develops in response to various damaging factors, which can lead to acute respiratory distress syndrome. Within this study, based on bioinformatics reanalysis of available full-transcriptome data of acute lung injury induced in mice and humans by various factors, we selected a set of genes that could serve as good targets for suppressing inflammation in the lung tissue, evaluated their expression in the cells of different origins during LPS-induced inflammation, and chose the tissue inhibitor of metalloproteinase Timp1 as a promising target for suppressing inflammation. We designed an effective chemically modified anti-TIMP1 siRNA and showed that Timp1 silencing correlates with a decrease in the pro-inflammatory cytokine IL6 secretion in cultured macrophage cells and reduces the severity of LPS-induced acute lung injury in a mouse model.

A Novel 3-meta-Pyridine-1,2,4-oxadiazole Derivative of Glycyrrhetinic Acid as a Safe and Promising Candidate for Overcoming P-Glycoprotein-Mediated Multidrug Resistance in Tumor Cells.

Given the pharmacophore properties of the nitrogen-containing moiety in the molecular structure of P-glycoprotein (P-gp) inhibitors, we report the evaluation of the P-gp inhibitory and MDR reversal activities of 2g, a 3-meta-pyridin-1,2,4-oxadiazole derivative of 18ßH-glycyrrhetinic acid. Through molecular docking, we have shown that 2g has the potential to directly interact with the transmembrane domain of P-gp with a low free binding energy (-10.2 kcal/mol). Using KB-8-5 human cervical carcinoma cells and RLS40 murine lymphosarcoma cells, both of which exhibit a multidrug-resistant (MDR) phenotype mediated by P-gp activation, we have shown that 2g, at nontoxic concentrations, effectively increased the intracellular accumulation of fluorescent P-gp substrates (rhodamine 123 or doxorubicin (DOX)), leading to a marked sensitization of the model cells to the cytotoxic effect of DOX. Considering the comparable activity of 2g with verapamil, a known P-gp inhibitor, 2g can be considered as a promising candidate for the development of agents capable of overcoming P-gp-mediated MDR in tumor cells.

Symmetric lipophilic polyamines exhibiting antitumor activity.

Unsymmetric lipophilic polyamine derivatives are considered as potential antitumor agents. Here, a series of novel symmetric lipophilic polyamines (LPAs) based on norspermine and triethylenetetramine (TETA) backbones bearing alkyl substituents with different lengths (from decyl to octadecyl) at C(1) atom of glycerol were synthesized. Performed screening of the cytotoxicity of novel compounds on the panel of tumor cell lines (MCF-7, KB-3-1, B16) and non-malignant fibroblasts hFF3 in vitro revealed a correlation between the length of the aliphatic moieties in LPAs and their toxic effects - LPAs with the shortest decyl substituent were found to exhibit the highest cytotoxicity. Furthermore, norspermine-based LPAs displayed somewhat more pronounced cytotoxicity compared with their TETA-based counterparts. Further mechanistic studies demonstrated that hit LPAs containing the norspermine backbone and tetradecyl or decyl substituents efficiently induced apoptosis in KB-3-1 cells. Moreover, decyl-bearing LPA inhibited motility and enhanced adhesiveness of murine B16 melanoma cells in vitro, showing promising antimetastatic potential. Thus, developed novel symmetric norspermine-based LPAs can be considered as promising anticancer chemotherapeutic candidates.

Antisense oligonucleotide gapmers containing phosphoryl guanidine groups reverse MDR1-mediated multiple drug resistance of tumor cells.

Antisense gapmer oligonucleotides containing phosphoryl guanidine (PG) groups, e.g., 1,3-dimethylimidazolidin-2-imine, at three to five internucleotidic positions adjacent to the 3' and 5' ends were prepared via the Staudinger chemistry, which is compatible with conditions of standard automated solid-phase phosphoramidite synthesis for phosphodiester and, notably, phosphorothioate linkages, and allows one to design a variety of gapmeric structures with alternating linkages, and deoxyribose or 2'-O-methylribose backbone. PG modifications increased nuclease resistance in serum-containing medium for more than 21 days. Replacing two internucleotidic phosphates by PG groups in phosphorothioate-modified oligonucleotides did not decrease their cellular uptake in the absence of lipid carriers. Increasing the number of PG groups from two to seven per oligonucleotide reduced their ability to enter the cells in the carrier-free mode. Cationic liposomes provided similar delivery efficiency of both partially PG-modified and unmodified oligonucleotides. PG-gapmers were designed containing three to four PG groups at both wings and a central "window" of seven deoxynucleotides with either phosphodiester or phosphorothioate linkages targeted to MDR1 mRNA providing multiple drug resistance of tumor cells. Gapmers efficiently silenced MDR1 mRNA and restored the sensitivity of tumor cells to chemotherapeutics. Thus, PG-gapmers can be considered as novel, promising types of antisense oligonucleotides for targeting biologically relevant RNAs.

Folate-Equipped Cationic Liposomes Deliver Anti-MDR1-siRNA to the Tumor and Increase the Efficiency of Chemotherapy.

In this study, we examined the in vivo toxicity of the liposomes F consisting of 1,26-bis(cholest-5-en-3-yloxycarbonylamino)-7,11,16,20-tetraazahexacosan tetrahydrochloride, lipid-helper 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine and folate lipoconjugate (O-{2-[rac-2,3-di(tetradecyloxy)prop-1-yloxycarbonyl]aminoethyl}-O'-[2-(pteroyl-L-glutam-5-yl)aminoethyl]octadecaethyleneglycol) and investigated the antitumor effect of combined antitumor therapy consisting of MDR1-targeted siMDR/F complexes and conventional polychemotherapy using tumor xenograft initiated in immunodeficient mice. Detailed analysis of acute and chronic toxicity of this liposomal formulation in healthy C57BL/6J mice demonstrated that formulation F and parent formulation L (without folate lipoconjugate) have no acute and chronic toxicity in mice. The study of the biodistribution of siMDR/F lipoplexes in SCID mice with xenograft tumors formed by tumor cells differing in the expression level of folate receptors showed that the accumulation in various types of tumors strongly depends on the abandons of folate receptors in tumor cells and effective accumulation occurs only in tumors formed by cells with the highest FR levels. Investigating the effects of combined therapy including anti-MDR1 siRNA/F complexes and polychemotherapy on a multidrug-resistant KB-8-5 tumor xenograft in SCID mice demonstrated that siMDR/F increases the efficiency of polychemotherapy: the treatment leads to pronounced inhibition of tumor growth, reduced necrosis and inflammation, and stimulates apoptosis in KB-8-5 tumor tissue. At the same time, it does not induce liver toxicity in tumor-bearing mice. These data confirm that folate-containing liposome F mediated the extremely efficient delivery of siRNA in FR-expressing tumors in vivo and ensured the safety and effectiveness of its action.

Transport Oligonucleotides-A Novel System for Intracellular Delivery of Antisense Therapeutics.

Biological activity of antisense oligonucleotides (asON), especially those with a neutral backbone, is often attenuated by poor cellular accumulation. In the present proof-of-concept study, we propose a novel delivery system for asONs which implies the delivery of modified antisense oligonucleotides by so-called transport oligonucleotides (tON), which are oligodeoxyribonucleotides complementary to asON conjugated with hydrophobic dodecyl moieties. Two types of tONs, bearing at the 5'-end up to three dodecyl residues attached through non-nucleotide inserts (TD series) or anchored directly to internucleotidic phosphate (TP series), were synthesized. tONs with three dodecyl residues efficiently delivered asON to cells without any signs of cytotoxicity and provided a transfection efficacy comparable to that achieved using Lipofectamine 2000. We found that, in the case of tON with three dodecyl residues, some tON/asON duplexes were excreted from the cells within extracellular vesicles at late stages of transfection. We confirmed the high efficacy of the novel and demonstrated that MDR1 mRNA targeted asON delivered by tON with three dodecyl residues significantly reduced the level of P-glycoprotein and increased the sensitivity of KB-8-5 human carcinoma cells to vinblastine. The obtained results demonstrate the efficacy of lipophilic oligonucleotide carriers and shows they are potentially capable of intracellular delivery of any kind of antisense oligonucleotides.

1-Hydroxyanthraquinones Containing Aryl Substituents as Potent and Selective Anticancer Agents.

A series of 1,2-, 1,4-disubstituted or 1,2,4-trisubstituted anthraquinone-based compounds was designed, synthesized, characterized and biologically evaluated for anticancer efficacy. 2- or 4-arylated 1-hydroxy-9,10-antraquinones (anthracene-9,10-diones) were prepared by Suzuki-Miyaura cross-coupling reaction of 1-hydroxy-2-bromoanthraquinone, 1-hydroxy-4-iodoanthraquinone or 1-hydroxy-2,4-dibromoanthraquinone with arylboronic acids. The cross-coupling reaction of 2,4-dibromo-9,10-anthraquinone with arylboronic acids provide a convenient approach to 2,4-bis arylated 1-hydroxyanthraquinones with a variety of aryl substituent in the 2 and 4 position. The cytotoxicity of new anthraquinone derivatives was evaluated using the conventional MTT assays. The data revealed that six of the aryl substituted compounds among the entire series 3, 15, 16, 25, 27, 28 were comparable potent with the commercially available reference drug doxorubicin on the human glioblastoma cells SNB-19, prostate cancer DU-145 or breast cancer cells MDA-MB-231 and were relatively safe towards human telomerase (h-TERT)immortalized lung fibroblasts cells. The results suggested that the in vitro antitumor activity of synthesized 2-aryl, 4-aryl- and 2,4-diaryl substituted 1-hydroxyanthraquinones depends on the nature of the substituent within the cyclic backbone. Docking interaction of 2-, 4-substituted and 2,4-disubstituted 1-hydroxyanthraquinones indicates intercalative mode of binding of compounds with DNA topoisomerase. The interaction with the DNA of 4-aryl-13, 15, 16 and 4-(furan-3-yl)-23 1-hydroxyanthraquinones was experimentally confirmed through a change in electroforetic mobility. Further experiments with 1-hydroxy-4-phenyl-anthraquinone 13 demonstrated that the compound induced cell cycle arrest at sub-G1 phase in DU-145 cells in the concentration 1.1 µM, which is probably achieved by inducing apoptosis. 4-Arylsubstituted 1-hydroxyanthraquinones 13 and 16 induced the enhancement of DNA synthesis on SNB19 cell lines.

Trimeric Small Interfering RNAs and Their Cholesterol-Containing Conjugates Exhibit Improved Accumulation in Tumors, but Dramatically Reduced Silencing Activity.

Cholesterol derivatives of nuclease-resistant, anti-MDR1 small-interfering RNAs were designed to contain a 2'-OMe-modified 21-bp siRNA and a 63-bp TsiRNA in order to investigate their accumulation and silencing activity in vitro and in vivo. The results showed that increasing the length of the RNA duplex in such a conjugate increases its biological activity when delivered using a transfection agent. However, the efficiency of accumulation in human drug-resistant KB-8-5 cells during delivery in vitro in a carrier-free mode was reduced as well as efficiency of target gene silencing. TsiRNAs demonstrated a similar biodistribution in KB-8-5 xenograft tumor-bearing SCID mice with more efficient accumulation in organs and tumors than cholesterol-conjugated canonical siRNAs; however, this accumulation did not provide a silencing effect. The lack of correlation between the accumulation in the organ and the silencing activity of cholesterol conjugates of siRNAs of different lengths can be attributed to the fact that trimeric Ch-TsiRNA lags mainly in the intercellular space and does not penetrate sufficiently into the cytoplasm of the cell. Increased accumulation in the organs and in the tumor, by itself, shows that using siRNA with increased molecular weight is an effective approach to control biodistribution and delivery to the target organ.

Preparation, Determination of Activity, and Biodistribution of Cholesterol-Containing Nuclease-Resistant siRNAs In Vivo.

RNA interference (RNAi) is a powerful tool for suppressing gene expression associated with various diseases that are not amenable to treatment with low molecular weight drugs. Despite significant progress in this area, the potential for therapeutic use of RNAi in humans is limited due to the lack of efficient delivery systems. Bioconjugation is one of the most promising methods for delivering siRNA to cells and tissues, since conjugation of siRNA with molecules capable of penetrating cells through natural transport mechanisms can provide specificity of delivery without toxic effects and unwanted immunostimulation. Here we describe the design, preparation, and in vivo evaluation of cholesterol-containing siRNA conjugates able to accumulate in the tumor, penetrate into cells without a carrier, and suppress the expression of the target genes.

Current Development of siRNA Bioconjugates: From Research to the Clinic.

Small interfering RNAs (siRNAs) acting via RNA interference mechanisms are able to recognize a homologous mRNA sequence in the cell and induce its degradation. The main problems in the development of siRNA-based drugs for therapeutic use are the low efficiency of siRNA delivery to target cells and the degradation of siRNAs by nucleases in biological fluids. Various approaches have been proposed to solve the problem of siRNA delivery in vivo (e.g., viruses, cationic lipids, polymers, nanoparticles), but all have limitations for therapeutic use. One of the most promising approaches to solve the problem of siRNA delivery to target cells is bioconjugation; i.e., the covalent connection of siRNAs with biogenic molecules (lipophilic molecules, antibodies, aptamers, ligands, peptides, or polymers). Bioconjugates are "ideal nanoparticles" since they do not need a positive charge to form complexes, are less toxic, and are less effectively recognized by components of the immune system because of their small size. This review is focused on strategies and principles for constructing siRNA bioconjugates for in vivo use.

Fluorophore Labeling Affects the Cellular Accumulation and Gene Silencing Activity of Cholesterol-Modified siRNAs In Vitro.

The objective of this study was to analyze the effects of fluorophores on the intracellular accumulation and biological activity of small interfering RNA (siRNA) and its cholesterol conjugates. In this study, we used stem-loop real-time PCR and calibration curves to quantitate cellular siRNA accumulation. Attachment of fluorophores significantly affected both the accumulation and biological activity of siRNA conjugates. The severity of this effect depended significantly on the structure of the conjugate; fluorophores (Cy5.5 or Alexa-488) attached to siRNA, facing the side of the duplex opposite to cholesterol, enhanced the unproductive intracellular accumulation of the conjugate when delivered in carrier-free mode. Enhanced cellular accumulation of siRNA conjugates did not result in enhanced biological activity of the conjugate. Moreover, the attachment of a hydrophobic fluorophore, such as Cy5.5, to conventional siRNA also enhanced its apparent intracellular accumulation, but not its biological activity. Thus, the use of fluorescent labels for the study of the intracellular accumulation of siRNA and its conjugates formed with different molecules is possible only for a limited range of structures, and requires verification using alternative methods.

Autocrine-based selection of ligands for personalized CAR-T therapy of lymphoma.

We report the development of a novel platform to enhance the efficacy and safety of follicular lymphoma (FL) treatment. Since lymphoma is a clonal malignancy of a diversity system, every tumor has a different antibody on its cell surface. Combinatorial autocrine-based selection is used to rapidly identify specific ligands for these B cell receptors on the surface of FL tumor cells. The selected ligands are used in a chimeric antigen receptor T cell (CAR-T) format for redirection of human cytotoxic T lymphocytes. Essentially, the format is the inverse of the usual CAR-T protocol. Instead of being a guide molecule, the antibody itself is the target. Thus, these studies raise the possibility of personalized treatment of lymphomas using a private antibody binding ligand that can be obtained in a few weeks.

Nuclease-resistant 63-bp trimeric siRNAs simultaneously silence three different genes in tumor cells.

We designed a multimeric nuclease-resistant 63-bp trimeric small-interfering RNA (tsiRNA) comprising in one duplex the sequence of siRNAs targeting mRNAs of MDR1, LMP2, and LMP7 genes. We show that such tsiRNA is able to suppress the expression of all the target genes independently and with high efficiency, acting via a Dicer-dependent mechanism. tsiRNA is diced into 42- and 21-bp duplexes inside the cell. tsiRNA-induced gene silencing is characterized by kinetics similar to that of canonical siRNA, while the silencing efficiency is significantly higher than that of canonical siRNA with the same sequence.

Cholesterol-Containing Nuclease-Resistant siRNA Accumulates in Tumors in a Carrier-free Mode and Silences MDR1 Gene.

Chemical modifications are an effective way to improve the therapeutic properties of small interfering RNAs (siRNAs), making them more resistant to degradation in serum and ensuring their delivery to target cells and tissues. Here, we studied the carrier-free biodistribution and biological activity of a nuclease-resistant anti-MDR1 cholesterol-siRNA conjugate in healthy and tumor-bearing severe combined immune deficiency (SCID) mice. The attachment of cholesterol to siRNA provided its efficient accumulation in the liver and in tumors, and reduced its retention in the kidneys after intravenous and intraperitoneal injection. The major part of cholesterol-siRNA after intramuscular and subcutaneous injections remained in the injection place. Confocal microscopy data demonstrated that cholesterol-siRNA spread deep in the tissue and was present in the cytoplasm of almost all the liver and tumor cells. The reduction of P-glycoprotein level in human KB-8-5 xenograft overexpressing the MDR1 gene by 60% was observed at days 5-6 after injection. Then, its initial level recovered by the eighth day. The data showed that, regardless of the mode of administration (intravenous, intraperitoneal, or peritumoral), cholesterol-siMDR efficiently reduced the P-glycoprotein level in tumors. The designed anti-MDR1 conjugate has potential as an adjuvant therapeutic for the reversal of multiple drug resistance of cancer cells.

Carrier-free cellular uptake and the gene-silencing activity of the lipophilic siRNAs is strongly affected by the length of the linker between siRNA and lipophilic group.

The conjugation of siRNA to molecules, which can be internalized into the cell via natural transport mechanisms, can result in the enhancement of siRNA cellular uptake. Herein, the carrier-free cellular uptake of nuclease-resistant anti-MDR1 siRNA equipped with lipophilic residues (cholesterol, lithocholic acid, oleyl alcohol and litocholic acid oleylamide) attached to the 5'-end of the sense strand via oligomethylene linker of various length was investigated. A convenient combination of H-phosphonate and phosphoramidite methods was developed for the synthesis of 5'-lipophilic conjugates of siRNAs. It was found that lipophilic siRNA are able to effectively penetrate into HEK293, HepG2 and KB-8-5 cancer cells when used in a micromolar concentration range. The efficiency of the uptake is dependent upon the type of lipophilic moiety, the length of the linker between the moiety and the siRNA and cell type. Among all the conjugates tested, the cholesterol-conjugated siRNAs with linkers containing from 6 to 10 carbon atoms demonstrate the optimal uptake and gene silencing properties: the shortening of the linker reduces the efficiency of the cellular uptake of siRNA conjugates, whereas the lengthening of the linker facilitates the uptake but retards the gene silencing effect and decreases the efficiency of the silencing.