Biodistribution of phosphodiester and phosphorothioate siRNA

Targeted delivery systems of small interfering RNA by systemic administration

RNA interference (RNAi) is induced by 21-25 nucleotide, double-stranded small interfering RNA (siRNA), which is incorporated into the RNAi-induced silencing complex (RISC) and is a guide for cleavage of the complementary target mRNA in the cytoplasm. There are many obstacles to in vivo delivery of siRNAs, such as degradation by enzymes in blood, interaction with blood components and non-specific uptake by the cells, which govern biodistribution in the body. In order to achieve the knockdown by siRNAs in vivo, many delivery systems of siRNAs based on physical and pharmaceutical approaches have been proposed. In addition, the immune responses of siRNA must be taken into account when considering the application of siRNAs to in vivo therapy. This review focuses on recent reports about delivery systems and immune responses of siRNAs.

Applications of RNA interference: current state and prospects for siRNA-based strategies in vivo

Within the recent years, RNA interference (RNAi) has become an almost-standard method for in vitro knockdown of any target gene of interest. Now, one major focus is to further explore its potential in vivo, including the development of novel therapeutic strategies. From the mechanism, it becomes clear that small interfering RNAs (siRNAs) play a pivotal role in triggering RNAi. Thus, the efficient delivery of target gene-specific siRNAs is one major challenge in the establishment of therapeutic RNAi. Numerous studies, based on different modes of administration and various siRNA formulations and/or modifications, have already accumulated promising results. This applies to various animal models covering viral infections, cancer and multiple other diseases. Continuing efforts will lead to the development of efficient and “double-specific” drugs, comprising of siRNAs with high target gene specificity and of nanoparticles enhancing siRNA delivery and target organ specificity.

Hydrophobization and bioconjugation for enhanced siRNA delivery and targeting

RNA interference (RNAi) is an evolutionarily conserved process by which double-stranded small interfering RNA (siRNA) induces sequence-specific, post-transcriptional gene silencing. Unlike other mRNA targeting strategies, RNAi takes advantage of the physiological gene silencing machinery. The potential use of siRNA as therapeutic agents has attracted great attention as a novel approach for treating severe and chronic diseases. RNAi can be achieved by either delivery of chemically synthesized siRNAs or endogenous expression of small hairpin RNA, siRNA, and microRNA (miRNA). However, the relatively high dose of siRNA required for gene silencing limits its therapeutic applications. This review discusses several strategies to improve therapeutic efficacy as well as to abrogate off-target effects and immunostimulation caused by siRNAs. There is an in-depth discussion on various issues related to the (1) mechanisms of RNAi, (2) methods of siRNA production, (3) barriers to RNAi-based therapies, (4) biodistribution, (5) design of siRNA molecules, (6) chemical modification and bioconjugation, (7) complex formation with lipids and polymers, (8) encapsulation into lipid particles, and (9) target specificity for enhanced therapeutic effectiveness.

Therapeutic potential for microRNAs

MiRNAs are a conserved class of non-coding RNAs that negatively regulate gene expression post-transcriptionally. Although their biological roles are largely unknown, examples of their importance in cancer, metabolic disease, and viral infection are accumulating, suggesting that they represent a new class of drug targets in these and likely many other therapeutic areas. Antisense oligonucleotide approaches for inhibiting miRNA function and siRNA-like technologies for replacement of miRNAs are currently being explored as tools for uncovering miRNA biology and as potential therapeutic agents. The next few years should see significant progress in our understanding of miRNA biology and the advancement of the technology for therapeutic modulation of miRNA activity.

RNA learns from antisense

RNA interference provides powerful tools for controlling gene expression in cultured cells. Whether RNAi will provide similarly powerful drugs is unknown. Lessons from development of antisense oligonucleotide drugs may provide some clues.

Innovative nanotechnologies for the delivery of oligonucleotides and siRNA

One way to reach intracellular therapeutic targets in cells consists in the use of short nucleic acids which will bind specifically to on targets thanks to either Watson-Crick base pairing or protein nucleic acids recognition rules. Among these short nucleic acids an important class of therapeutic agents is antisense oligonucleotides and siRNAs. However, the major problem of nucleic acids is their poor stability in biological media. One method, among others, to solve the stability problem is the use of colloïdal carriers such as nanoparticles. Nanoparticles have already been applied with success to in vitro drug delivery to particular types of cells and in vivo in several experimental models. Many membrane and intracellular processes deal with nanosized structure (typically 100 nm) which are processed further through the recognition sites of receptors and enzymes. Thus non-viral nanoparticles are interesting candidates to present biochemical molecules such as nucleic acids and proteins to cells as well as to protect them in vivo during delivery. This review focuses on the recent developments in the design of nanotechnologies to improve the delivery of antisense oligonucleotides and siRNAs.

Ligand-Targeted Delivery of Therapeutic siRNA

RNA interference (RNAi) is a post-transcriptional gene-silencing phenomenon that is triggered by double-stranded RNA (dsRNA). Since many diseases are associated with the inappropriate production of specific proteins, attempts are being made to exploit RNAi in a clinical settings. However, before RNAi can be exploited as therapeutically, several obstacles must be overcome. For example, small interfering RNA (siRNA) is unstable in the blood stream so any effects of injected siRNA are only transient. Accordingly, methods must be developed to prolong its activity. Furthermore, the efficient and safe delivery of siRNA into target tissues and cells is critical for successful therapy. Any useful delivery method should be designed to target siRNA to specific cells and to promote gene-silencing activity once the siRNA is inside the cells. Recent chemical modifications of siRNA have overcome problems associated with the instability of siRNA, and various ligands, including glycosylated molecules, peptides, proteins, antibodies and engineered antibody fragments, appear to be very useful or have considerable potential for the targeted delivery of siRNA. The use of such ligands improves the efficiency, specificity and, as a consequence, the safety of the corresponding delivery systems.

Reconstituted influenza virus envelopes as an efficient carrier system for cellular delivery of small-interfering RNAs

Application of RNA interference for in vivo evaluation of gene function or for therapeutic interventions has been hampered by a lack of suitable delivery methods for small interfering RNA (siRNA). Here, we present reconstituted viral envelopes (virosomes) derived from influenza virus as suitable vehicles for in vitro as well as in vivo delivery of siRNAs. Virosomes are vesicles that bear in their membrane the influenza virus spike protein hemagglutinin (HA). This protein mediates binding of native virus to and fusion with cellular target membranes. Accordingly, virosomes with membrane-incorporated HA bind to cells, are taken up by receptor-mediated endocytosis, and fuse with the endosomal membrane to release their contents into the cytoplasm. When complexed to cationic lipids, siRNA was successfully encapsulated in virosomes. Virosomes with encapsulated siRNA fused with target membranes in a pH-dependent manner and delivered the encapsulated siRNA to several cell lines in vitro. Virosome-delivered siRNA markedly downregulated the synthesis of newly induced and constitutively expressed green fluorescent protein. Moreover, intraperitoneal injection of siRNA-loaded virosomes resulted in delivery of the nucleotides to cells in the peritoneal cavity. Our results indicate that virosomes are a promising delivery device for in vivo application, especially where topical administration of siRNA, for example, to the respiratory tract is envisaged.

Intravenously administered short interfering RNA accumulates in the kidney and selectively suppresses gene function in renal proximal tubules

Different gene-silencing methods, like antisense and short interfering RNA (siRNA), are widely used as experimental tools to inhibit gene expression. In the present study, the in vivo behavior of siRNA in rats and siRNA-mediated silencing of genes in the renal proximal tubule were investigated. To study the biodistribution of siRNA, rats were injected i.v. with radiolabeled siRNA or radiolabel alone (control), and scintigraphic images were acquired at different time intervals postinjection. The siRNA preferentially accumulated in the kidneys and was excreted in the urine. One hour after injection, the amount of siRNA present in both kidneys (1.7 +/- 0.3% of injected dose/g tissue) was on average 40 times higher than in other tissues (liver, brain, intestine, muscle, lung, spleen, and blood). Besides the biodistribution, the effect of siRNA on multidrug resistance protein isoform 2 (Mrp2/Abcc2, siRNAMrp2) in renal proximal tubules was investigated. Mrp2 function was assessed by measuring the excretion of its fluorescent substrate calcein in the isolated perfused rat kidney. Four days after administration, siRNAMrp2 reduced the urinary calcein excretion rate significantly (35% inhibition over the period 80-150 min of perfusion). This down-regulation was specific because another siRNA sequence directed against a different transporter in the proximal tubule, Mrp4 (Abcc4, siRNAMrp4), did not alter the Mrp2-mediated excretion of calcein. In conclusion, siRNA accumulates spontaneously in the kidney after i.v. injection, where it selectively suppresses gene function in the proximal tubules. Therefore, i.v. administered siRNA provides a novel experimental and potential therapeutic tool for gene silencing in the kidney.

Improving RNA interference in mammalian cells by 4'-thio-modified small interfering RNA (siRNA): effect on siRNA activity and nuclease stability when used in combination with 2'-O-alkyl modifications

A systematic structure-activity relationship study of 4'-thioribose containing small interfering RNAs (siRNAs) has led to the identification of highly potent and stable antisense constructs. To enable this optimization effort for both in vitro and in vivo applications, we have significantly improved the yields of 4'-thioribonucleosides by using a chirally pure (R)-sulfoxide precursor. siRNA duplexes containing strategically placed regions of 4'-thio-RNA were synthesized and evaluated for RNA interference activity and plasma stability. Stretches of 4'-thio-RNA were well tolerated in both the antisense and sense strands. However, optimization of both the number and placement of 4'-thioribonucleosides was necessary for maximal potency. These optimized siRNAs were generally equipotent or superior to native siRNAs and exhibited increased thermal and plasma stability. Furthermore, significant improvements in siRNA activity and plasma stability were achieved by judicious combination of 4'-thioribose with 2'-O-methyl and 2'-O-methoxyethyl modifications. These optimized 4'-thio-siRNAs may be valuable for developing stable siRNAs for therapeutic applications.

A novel siRNA-lipoplex technology for RNA interference in the mouse vascular endothelium

For the application of RNA interference (RNAi) in vivo the functional delivery of short interfering RNAs (siRNAs) is still the major obstacle. Therefore, delivery technologies need to be established for the systemic application of RNAi in vivo. Here we report uptake, biodistribution and in vivo efficacy of siRNA molecules formulated into siRNA-lipoplexes. The applied formulation is based on complex formation of positively charged liposomes, a mixture of cationic and fusogenic lipids complexed with the negatively charged siRNA. We determined by fluorescence microscopy the temporal and spatial distribution of fluorescently labeled siRNA-lipoplexes, the body clearance and endothelial cell type specific uptake after single intravenous injection. Furthermore, by using siRNA molecules for targeting endothelia-specifically expressed genes, such as CD31 and Tie2, we were able to demonstrate downregulation of the corresponding mRNA and protein in vivo. Taken together, we show the applicability of this non-viral delivery technology for inducing RNAi in the vasculature of mice after systemic application.

RNA interference as a potential tool in the treatment of leukaemia

Leukaemias are often characterised by nonrandom chromosomal translocations that, at the molecular level, induce the activation of specific oncogenes or create novel chimeric genes. They have frequently been regarded as optimal targets for gene silencing approaches, as these single abnormalities may directly initiate or maintain the malignant process. Since the ground-breaking discovery that double-stranded RNA molecules 21 - 23 nucleotides in length, named small interfering RNAs (siRNAs), are able to elicit gene-specific inhibition also in mammalian cells, the interest of the scientific community has rapidly been drawn to the potential of these siRNAs for targeting oncogenic fusion genes in leukaemic cells. There has been a flurry of reports describing overexpressed or mutated genes that may also serve as attractive targets for therapeutic intervention by RNA silencing methods. Although this approach seems to be relatively straightforward, many problems remain to be solved before siRNAs may become clinically implemented as 'leukaemia drugs'. Difficulties in delivering siRNAs into the leukaemic cell, inefficient target mRNA cleavage, prolonged protein half-life in cancer cells, nonspecific side effects caused by targeting other genes than those originally thought, immunological reactions of the host organism against the siRNAs, such as interferon responses, or even acquired resistance mechanisms, such as escape mutants, should be overcome. This paper reviews the current knowledge regarding the use of siRNAs, either chemically synthesised or intracellular-generated via specialised expression constructs, in order to suppress the falsely activated oncogenes in haematopoietic malignancies.

Progress towards in vivo use of siRNAs

RNA interference (RNAi) has become the method of choice to suppress gene expression in vitro. It is also emerging as a powerful tool for in vivo research with over 90 studies published using synthetic small interfering RNAs in mammals. These reports demonstrate the potential for use of synthetic small interfering RNAs (siRNAs) as therapeutic agents, especially in the areas of cancer and viral infection. The number of reports using siRNAs for functional genomics applications, for validation of targets for small-molecule drug development programs, and to address questions of basic biology will rapidly grow as methods and protocols for use in animals become more established. This review will first discuss aspects of RNAi biochemistry and biology that impact in vivo use, especially as relates to experimental design, and will then provide an overview of published work with a focus on methodology.

Short interfering RNA (siRNA): tool or therapeutic?

Gene silencing by siRNA (short interfering RNA) is a still developing field in biology and has evolved as a novel post-transcriptional gene silencing strategy with therapeutic potential. With siRNAs, virtually every gene in the human genome contributing to a disease becomes amenable to regulation, thus opening unprecedented opportunities for drug discovery. Besides the well-established role for siRNA as a tool for target screening and validation in vitro, recent progress of siRNA delivery in vivo raised expectations for siRNA drugs as the up-and-coming 'magic bullet'. Whether siRNA compounds will make it as novel chemical entities from 'bench to bedside' will probably depend largely on improving their pharmacokinetics in terms of plasma stability and cellular uptake. Whereas locally administered siRNAs have already entered the first clinical trials, strategies for successful systemic delivery of siRNA are still in a preclinical stage of development. Irrespective of its therapeutic potential, RNAi (RNA interference) has unambiguously become a valuable tool for basic research in biology and thereby it will continue to have a major impact on medical science. In this review, we will give a brief overview about the history and current understanding of RNAi and focus on potential applications, especially as a therapeutic option to treat human disease.

Development of siRNA for therapeutics: efficient synthesis of phosphorothioate RNA utilizing phenylacetyl disulfide (PADS)

Efficient synthesis of phosphorothioate RNA (PS-RNA) is demonstrated by using phenylacetyl disulfide (PADS) in a mixture of pyridine and acetonitrile (1:1, v/v) for 3 min. Sulfurization is achieved with >99.8% stepwise efficiency. This reagent also performs efficiently during synthesis of RNA containing PS:PO mixed backbone.

On the delivery of small interfering RNAs into mammalian cells

RNA interference is becoming the technique of choice for analysing gene function and drug target validation. In this process, sequence-specific gene inhibition is initiated by small RNA duplexes, known as small interfering RNAs (siRNAs). The possibility that exogenously delivered siRNAs or endogenously expressed hairpin siRNAs can cause the destruction of specific target mRNA in vitro and in animal models has been demonstrated. However, the key challenges for the development of siRNAs as human therapeutics is largely dependent on the development of suitable delivery agents and improved siRNA specificity. This review highlights recent advances in siRNA delivery, as well as challenging problems related to immune stimulation.

Sequence-specific knockdown of EWS-FLI1 by targeted, nonviral delivery of small interfering RNA inhibits tumor growth in a murine model of metastatic Ewing's sarcoma

The development of effective, systemic therapies for metastatic cancer is highly desired. We show here that the systemic delivery of sequence-specific small interfering RNA (siRNA) against the EWS-FLI1 gene product by a targeted, nonviral delivery system dramatically inhibits tumor growth in a murine model of metastatic Ewing's sarcoma. The nonviral delivery system uses a cyclodextrin-containing polycation to bind and protect siRNA and transferrin as a targeting ligand for delivery to transferrin receptor-expressing tumor cells. Removal of the targeting ligand or the use of a control siRNA sequence eliminates the antitumor effects. Additionally, no abnormalities in interleukin-12 and IFN-alpha, liver and kidney function tests, complete blood counts, or pathology of major organs are observed from long-term, low-pressure, low-volume tail-vein administrations. These data provide strong evidence for the safety and efficacy of this targeted, nonviral siRNA delivery system.

Transporting silence: Design of carriers for siRNA to angiogenic endothelium

The recently developed siRNA oligonucleotides are an attractive alternative to antisense as a therapeutic modality because of their robust, gene selective silencing of drug target protein expression. To achieve therapeutic success, however, several hurdles must be overcome including rapid clearance, nuclease degradation, and inefficient intracellular localization. In this presentation, we discuss design strategies for development of self-assembling nanoscale carriers for neovasculature targeted delivery of siRNA inhibiting tumor or ocular angiogenesis.

RNA interference: from gene silencing to gene-specific therapeutics

In the past 4 years, RNA interference (RNAi) has become widely used as an experimental tool to analyse the function of mammalian genes, both in vitro and in vivo. By harnessing an evolutionary conserved endogenous biological pathway, first identified in plants and lower organisms, double-stranded RNA (dsRNA) reagents are used to bind to and promote the degradation of target RNAs, resulting in knockdown of the expression of specific genes. RNAi can be induced in mammalian cells by the introduction of synthetic double-stranded small interfering RNAs (siRNAs) 21–23 base pairs (bp) in length or by plasmid and viral vector systems that express double-stranded short hairpin RNAs (shRNAs) that are subsequently processed to siRNAs by the cellular machinery. RNAi has been widely used in mammalian cells to define the functional roles of individual genes, particularly in disease. In addition, siRNA and shRNA libraries have been developed to allow the systematic analysis of genes required for disease processes such as cancer using high throughput RNAi screens. RNAi has been used for the knockdown of gene expression in experimental animals, with the development of shRNA systems that allow tissue-specific and inducible knockdown of genes promising to provide a quicker and cheaper way to generate transgenic animals than conventional approaches. Finally, because of the ability of RNAi to silence disease-associated genes in tissue culture and animal models, the development of RNAi-based reagents for clinical applications is gathering pace, as technological enhancements that improve siRNA stability and delivery in vivo, while minimising off-target and nonspecific effects, are developed.

Interfering with the brain: use of RNA interference for understanding the pathophysiology of psychiatric and neurological disorders

Psychiatric and neurological disorders are among the most complex, poorly understood, and debilitating diseases in medicine. The burgeoning advances in functional genomic technologies have led to the identification of a vast number of novel genes that are potentially implicated in the pathophysiology of such disorders. However, many of these candidate genes have not yet been functionalized and require validation in vivo. Traditionally, abrogating gene function is one of the primary means of examining the physiological significance of a given gene product. Several methods have been developed for gene ablation or knockdown, however, with limited levels of success. The recent discovery of RNA interference (RNAi), as a highly efficient method for gene knockdown, has been one of the major breakthroughs in molecular medicine. In vivo application of RNAi is further demonstrating the promise of this technology. Recent efforts have focused on applying RNAi-based knockdown to understand the genes implicated in neuropsychiatric disorders. However, the greatest challenge with this approach is translating the success of RNAi from mammalian cell cultures to the brain in animal models of disease and, subsequently, in patients. In this review, we describe the various methods that are being developed to deliver RNAi into the brain for down-regulating gene expression and subsequent phenotyping of genes in vivo. We illustrate the utility of various approaches with a few successful examples and also discuss the potential benefits and pitfalls associated with the use of each delivery approach. Appropriate tailoring of tools that deliver RNAi in the brain may not only aid our understanding of the complex pathophysiology of neuropsychiatric disorders, but may also serve as a valuable therapy for disorders, where there is an immense unmet medical need.

Challenges and successes in developing new therapies for hepatitis C

Hepatitis C virus (HCV) will continue to be a serious global health threat for many years to come because of the chronic nature of the infection, its high prevalence and the significant morbidity of the resulting disease. Recently, a small number of molecules have produced encouraging results in proof-of-concept clinical trials. At the same time, preclinical evidence is accumulating that development of resistance will eventually limit the efficacy of new drugs. Thus, combinations of multiple agents will be required to treat chronic HCV infection.

Positional effect of chemical modifications on short interference RNA activity in mammalian cells

A systematic study on the effect of 2'-sugar modifications (2'-F (2'-F-2'-deoxy-nucleoside residues), 2'-O-Me (2'-O-methyl-nucleoside residues), and 2'-O-MOE [2'-O-(2-methoxyethyl)]-nucleoside residues) in the antisense and sense strands of short interference RNA (siRNA) was performed in HeLa cells. The study of the antisense strand of siRNAs demonstrated that activity depends on the position of the modifications in the sequence. The siRNAs with modified ribonucleotides at the 5'-end of the antisense strand were less active relative to the 3'-modified ones. The 2'-F sugar was generally well-tolerated on the antisense strand, whereas the 2'-O-Me showed significant shift in activity depending on the position of modification. The 2'-O-MOE modification in the antisense strand resulted in less active siRNA constructs regardless of placement position in the construct. The incorporation of the modified residues, e.g., 2'-O-Me and 2'-O-MOE, in the sense strand of siRNA did not show a strong positional preference. These results may provide guidelines to design effective and stable siRNAs for RNA interference mediated therapeutic applications.

Rnai as an experimental and therapeutic tool to study and regulate physiological and disease processes

Over the past four years RNA interference (RNAi) has exploded onto the research scene as a new approach to manipulate gene expression in mammalian systems. More recently, RNAi has garnered much interest as a potential therapeutic strategy. In this review, we briefly summarize the current understanding of RNAi biology and examine how RNAi has been used to study the genetic basis of physiological and disease processes in mammalian systems. We also explore some of the new developments in the use of RNAi for disease therapy and highlight the key challenges that currently limit its application in the laboratory, as well as in the clinical setting.

The design and exogenous delivery of siRNA for post-transcriptional gene silencing

RNA interference (RNAi) is a natural cellular process that effects post-transcriptional gene silencing in eukaryotic systems. Small interfering RNA (siRNA) molecules are the key intermediaries in this process which when exogenously administered can inhibit or "silence" the expression of any given target gene. Thus, siRNA molecules hold great promise as biological tools and as potential therapeutic agents for targeted inhibition of disease-causing genes. However, key challenges to the effective and widespread use of these polyanionic, macromolecular duplexes of RNA are their appropriate design and efficient delivery to cells in vitro and in vivo. This review highlights the current strategies used in the design of effective siRNA molecules and also summarises the main strategies being considered for the exogenous delivery of siRNA for both in vitro and in vivo applications.

Fully 2'-modified oligonucleotide duplexes with improved in vitro potency and stability compared to unmodified small interfering RNA

We have identified a small interfering RNA (siRNA) motif, consisting entirely of 2'-O-methyl and 2'-fluoro nucleotides, that displays enhanced plasma stability and increased in vitro potency. At one site, this motif showed remarkable >500-fold improvement in potency over the unmodified siRNA. This marks the first report of such a potent fully modified motif, which may represent a useful design for therapeutic oligonucleotides.

Lactosylated poly(ethylene glycol)-siRNA conjugate through acid-labile beta-thiopropionate linkage to construct pH-sensitive polyion complex micelles achieving enhanced gene silencing in hepatoma cells

The remarkably enhanced gene silencing in hepatoma cells was achieved by assembling lactosylated-PEG-siRNA conjugates bearing acid-labile beta-thiopropionate linkages into polyion complex (PIC) micelles through the mixing with poly(l-lysine). The PIC micelles with clustered lactose moieties on the periphery were successfully transported into hepatoma cells in a receptor-mediated manner, releasing hundreds of active siRNA molecules into the cellular interior responding to the pH decrease in the endosomal compartment. Eventually, almost 100 times enhancement in gene silencing activity compared to that of the free conjugate was achieved for the micelle system, facilitating the practical utility of siRNA therapeutics.

Challenges for RNAi in vivo

Synthetic small interfering RNA (siRNA) has become a valuable tool for investigating gene function in cell culture. This success has led to high expectations for siRNA as a tool for in vivo investigation and as a platform for therapeutic development. siRNA in cell culture owes much of its success to years of development of traditional antisense oligonucleotides, and in vivo applications will also benefit from previous experience in this regard. However, the duplex nature of siRNA presents significant obstacles that will need to be overcome. Here, we discuss the current status of in vivo siRNA technology and describe some of the barriers to widespread application of RNAi-mediated gene silencing in mammals.

Antisense and siRNA as agonists of Toll-like receptors

About 25 years ago, researchers first demonstrated that a short synthetic oligodeoxynucleotide, referred to as antisense, can inhibit replication of Rous sarcoma virus through hybridization to viral RNA. Since then, several hybridization-based oligonucleotide approaches have been developed to elucidate the functions of genes and their potential as therapeutic agents. Short-interfering (si) RNA is the most recent example. To effectively inhibit gene expression, an antisense or siRNA must be resistant to nucleases, be taken up efficiently by cells, hybridize efficiently with the target mRNA and activate selective degradation of the target mRNA or block its translation without causing undesirable side effects. However, both antisense and siRNA agents have been shown to exert non-target-related biological effects including immune stimulation. Do antisense and siRNA agents work as ligands for Toll-like receptors (TLRs), a family of pathogen-associated, molecular pattern recognition receptors?