Emerging RNA interference (RNAi) therapies, which reversibly silence target genes through the use of short-strand RNAs, such as small interfering RNAs (siRNAs), have the potential to treat a variety of diseases.
RNAi-based therapeutics for gene silencing. (Mainini F, et al., 2020)
Mechanism of siRNA
siRNAs, also known as short interfering RNAs or silencing RNAs, are typically 20-24 base pairs in length and achieve gene therapy effects by silencing target genes. The specific mechanism of action of siRNA is that it has a short double-stranded structure. siRNA dissociates into single strands and binds to the target messenger RNA (mRNA) sequence, triggering a series of reactions that lead to cleavage of the target gene and degradation of the mRNA, thereby preventing translation and gene expression.
Mechanism of RNA interference and strategies for siRNA synthesis and delivery. (Dong Y, et al., 2019)
Challenges of siRNA Delivery
- Easily Degradable
Unmodified and unprotected siRNAs are easily degraded by RNases and are unstable in the bloodstream.
- Low Bioavailability
Due to the relatively large molecular weight (~13 kDa) and anionic charge of siRNA, it is difficult to diffuse across the cell membrane, resulting in low bioavailability.
- Short Half-life
siRNA injected into the bloodstream is easily cleared by the kidneys.
- Does not Have Endosomal Escape Strategies
Although some delivery systems enable efficient uptake of siRNA by target cells, if siRNA does not have an endosomal escape strategy, it will remain trapped in endosomes, causing degradation and making gene therapy impossible.
Considering these different challenges, there is a need to develop suitable delivery methods to achieve siRNA therapy. These approaches are able to protect siRNA from degradation and induce endosomal escape, while minimizing off-target effects and increasing bioavailability.
How to Meet the Challenges of siRNA Delivery
- Chemical Modification
Chemical modifications can be made on the sugar-phosphate backbone of the siRNA molecule as well as on the purine and pyrimidine bases.
- PS Modification
Replacing the highly charged, labile phosphodiester backbone with a phosphorothioate (PS) backbone is one of the first discovered and most commonly used chemical modification methods. This modification not only increases the resistance of the siRNA to nucleases and phosphodiesterase, but also increases the hydrophobicity, which promotes the binding of the siRNA to the carrier plasma protein and ultimately increases the drug circulation time.
- Sugar Modification
Because the existing 2'-OH group is not essential for the siRNA's mechanism of action, sugar modifications are most common at the 2' position. Endoribonuclease degradation can be prevented when siRNA is sugar modified.
- Development of siRNA Delivery System
Although chemical modification can increase siRNA stability and resistance to nucleases, it does not solve the problem of permeability through the lipid bilayer, so the delivery system is critical for siRNA drugs.
- LNP as siRNA Delivery System
As the most successful delivery strategy for siRNA, lipid nanoparticles (LNP) greatly improve the pharmacokinetics and bioavailability of siRNA drugs. Despite the proven clinical utility of LNPs, their use still suffers from a number of drawbacks, the main ones being the toxicity of their excipients, associated inflammation and the necessity to be administered by a medical professional by intravenous infusion.
- VLP as siRNA Delivery System
Other formulation-based strategies are currently under development, including cationic transfection agents, polymeric nanoparticles, micelles, dendrimers, and virus-like particles (VLPs) with nanostructures. Among them, VLPs produced by plants are widely used in plant biomedicine because they are easy to produce and have no possibility of human virus contamination. Furthermore, when VLPs were chemically modified and conjugated with folic acid (FA), the targeting of siRNA drugs was significantly improved.
Chemical modification and delivery system (LNP) of siRNA. (Zhang MM, et al., 2021)
How We can Help
As a global leader in the production and application of VLPs from VLPlantTM platform, CD BioSciences leverages its expertise to help our customers deliver siRNA for gene therapy. We are good at tailoring our services to the needs of our clients. Please contact us if you are interested.
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- Mainini F.; Eccles MR. Lipid and Polymer-Based Nanoparticle siRNA Delivery Systems for Cancer Therapy. Molecules. 2020, 25:1-18.
- Dong Y.; et al., Strategies, design, and chemistry in siRNA delivery systems. Adv Drug Deliv Rev. 2019, 144:133-147.
- Zhang MM.; et al., The growth of siRNA-based therapeutics: Updated clinical studies. Biochem Pharmacol. 2021, 189:1-29.