Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) first arose in Wuhan in 2019 and has since spread worldwide. Initially, many governments were forced to enact costly and restrictive measures to reduce the transmission of the disease, including mandatory face masks, social distancing restrictions and full lockdowns/stay-at-home orders. With mass vaccination programmes allowing many developed countries to dismantle these restrictions, disease cases have risen, but deaths have fallen significantly.
However, new treatments are still necessary for those at risk of severe disease or those in developing countries with low vaccination rates. In a review published in the International Journal of Molecular Sciences, researchers have been reviewing the use of siRNAs to target coronaviruses.
siRNAs begin as long double-stranded RNAs before Dicer-mediated cleavage cuts them down to strands around ~21 nucleotides long. These molecules can induce gene silencing in mammalian cells. This primarily occurs through either post-transcriptional gene silencing (PTGS) or transcriptional gene silencing (TGS).
PTGS occurs in the cytoplasm, where the guide strand of the siRNAs will bind to Argonaute-2 (Ago-2) protein of the RNA-inducing silencing complex (RISC), while the non-guide strand is cleaved and ejected by Ago-2, which then carries the guide strand to the target mRNA, resulting in the degradation of the binding site.
TGS is less well characterized, but it is known that siRNA will bind to the Ago-1 protein on the RNA-induced transcriptional silencing (RITS) complex, which will result in methylation and reduced acetylation of the target genes to promote heterochromatin production and epigenetic silencing.
Some siRNAs are known to be effective against different coronaviruses, including the three that cause the most severe disease in humans – severe acute respiratory syndrome coronavirus (SARS-CoV-1), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Middle East respiratory syndrome–related coronavirus (MERS-CoV).
The researchers identify several important factors to be considered for the efficacy of siRNAs in targeting these diseases, including the identification of highly conserved targets to prevent viral escape. SARS-CoV-2, in particular, has shown significant changes in the conformation and electrostatic charge of the spike protein, allowing evasion of both vaccine-induced and natural immunity. They also suggest targeting furin, the cell protease responsible for the cleavage of the spike protein (S) S1 and S2 subunits required for viral pathogenicity.
Several in silico experiments have been performed that identified possible candidates, with in vitro follow-ups confirming the efficacy of four siRNAs able to decrease the expression of the spike protein, four more that could reduce the expression of the nucleocapsid gene, and three that can decrease membrane protein gene expression.
Nanoparticle-mediated delivery, or NP delivery, is often the chosen method to carry siRNAs into the body for targeted therapy. The NPs form vesicle suspensions after encapsulating the siRNAs, allowing them to remain stable for longer. All four currently approved siRNA drugs require a delivery system, either using N-acetylgalactosamine or lipid NP pathways.
Lipid NPs (LNPs) have a phospholipid bilayer, with hydrophilic ends exposed to both the inside and outside of the vesicle and hydrophobic ends facing towards the hydrophobic ends of other phospholipid molecules. They have proven effective with multiple drugs in the past, and studies have shown that LNP siRNAs can target tumours as well as deliver siRNAs to the lungs – which could be particularly effective for anti-SARS-CoV-2 drugs. Other NPs can be formed of polymers, glycogen and non-organic particles such as gold and magnetic molecules.
Delivery of NP siRNAs can be difficult. It is well known that therapeutic siRNAs can be delivered to the liver via the circulatory system. All currently approved siRNA drugs are delivered to the liver, but other organs can be tougher to target. The long-term build-up in off-target tissues must be avoided, as well as damage to other organs.
The lung is one of the more obvious targets, as there is already significant research into where particles of certain sizes that are passed through the mouth or nose will eventually deposit. There are three potential methods by which NPs can be delivered to the respiratory tract – intratracheal, intranasal and orotracheal. Inhalation devices to target these are already present and ubiquitous in the medical field. Some are even currently used to treat SARS-CoV-2 by other methods, such as nebulizers. Inhalation is the most popular method, but more mechanical devices are sometimes necessary if a patient has trouble breathing.
The researchers highlight the need to optimize RNAi treatment strategies and appropriate carrier platforms for SARS-CoV-2 infection. While treatment with these strategies does look promising, it is still in the early stages of research. Other more promising pharmaceuticals have already been trialled and approved, and it seems more likely that these will continue to be the dominant choice of healthcare workers.