Antiviral applications of RNAi for coronavirus

siRNA-Based Novel Therapeutic Strategies to Improve Effectiveness of Antivirals: An Insight

Since the ground-breaking discovery of RNA interference (RNAi), scientists have made significant progress in the field of small interfering RNA (siRNA) treatments. Due to severe barriers to the therapeutic application of siRNA, nanoparticle technologies for siRNA delivery have been designed. For pathological circumstances such as viral infection, toxic RNA abnormalities, malignancies, and hereditary diseases, siRNAs are potential therapeutic agents. However, systemic administration of siRNAs in vivo remains a substantial issue due to a lack of "drug-likeness" (siRNA are relatively larger than drugs and have low hydrophobicity), physiological obstacles, and possible toxicities. This write-up covers important accomplishment in the field of clinical trials and patents specially based of siRNAs using targeting viruses. Furthermore, it offers deep insight of nanoparticle applied for siRNA delivery and strategies to improve the effectiveness of antivirals.

Involvement of epigenetics in affecting host immunity during SARS-CoV-2 infection

Coronavirus disease 19 (COVID-19) is caused by a highly contagious RNA virus Severe Acute Respiratory Syndrome coronavirus-2 (SARS-CoV-2), originated in December 2019 in Wuhan, China. Since then, it has become a global public health concern and leads the disease table with the highest mortality rate, highlighting the necessity for a thorough understanding of its biological properties. The intricate interaction between the virus and the host immune system gives rise to diverse implications of COVID-19. RNA viruses are known to hijack the host epigenetic mechanisms of immune cells to regulate antiviral defence. Epigenetics involves processes that alter gene expression without changing the DNA sequence, leading to heritable phenotypic changes. The epigenetic landscape consists of reversible modifications like chromatin remodelling, DNA/RNA methylation, and histone methylation/acetylation that regulates gene expression. The epigenetic machinery contributes to many aspects of SARS-CoV-2 pathogenesis, like global DNA methylation and receptor angiotensin-converting enzyme 2 (ACE2) methylation determines the viral entry inside the host, viral replication, and infection efficiency. Further, it is also reported to epigenetically regulate the expression of different host cytokines affecting antiviral response. The viral proteins of SARS-CoV-2 interact with various host epigenetic enzymes like histone deacetylases (HDACs) and bromodomain-containing proteins to antagonize cellular signalling. The central role of epigenetic factors in SARS-CoV-2 pathogenesis is now exploited as promising biomarkers and therapeutic targets against COVID-19. This review article highlights the ability of SARS-CoV-2 in regulating the host epigenetic landscape during infection leading to immune evasion. It also discusses the ongoing therapeutic approaches to curtail and control the viral outbreak.

Epigenetic perspectives of COVID-19: Virus infection to disease progression and therapeutic control

COVID-19 has caused numerous deaths as well as imposed social isolation and upheaval world-wide. Although, the genome and the composition of the virus, the entry process and replication mechanisms are well investigated from by several laboratories across the world, there are many unknown remaining questions. For example, what are the functions of membrane lipids during entry, packaging and exit of virus particles? Also, the metabolic aspects of the infected tissue cells are poorly understood. In the course of virus replication and formation of virus particles within the host cell, the enhanced metabolic activities of the host is directly proportional to viral loads. The epigenetic landscape of the host cells is also altered, particularly the expression/repression of genes associated with cellular metabolism as well as cellular processes that are antagonistic to the virus. Metabolic pathways are enzyme driven processes and the expression profile and mechanism of regulations of the respective genes encoding those enzymes during the course of pathogen invasion might be highly informative on the course of the disease. Recently, the metabolic profile of the patients' sera have been analysed from few patients. In view of this, and to gain further insights into the roles that epigenetic mechanisms might play in this scenario in regulation of metabolic pathways during the progression of COVID-19 are discussed and summarised in this contribution for ensuring best therapy.

SMRI: A New Method for siRNA Design for COVID-19 Therapy

First discovered in Wuhan, China, SARS-CoV-2 is a highly pathogenic novel coronavirus, which rapidly spread globally and became a pandemic with no vaccine and limited distinctive clinical drugs available till March 13th, 2020. Ribonucleic Acid interference (RNAi) technology, a gene-silencing technology that targets mRNA, can cause damage to RNA viruses effectively. Here, we report a new efficient small interfering RNA (siRNA) design method named Simple Multiple Rules Intelligent Method (SMRI) to propose a new solution of the treatment of COVID-19. To be specific, this study proposes a new model named Base Preference and Thermodynamic Characteristic model (BPTC model) indicating the siRNA silencing efficiency and a new index named siRNA Extended Rules index (SER index) based on the BPTC model to screen high-efficiency siRNAs and filter out the siRNAs that are difficult to take effect or synthesize as a part of the SMRI method, which is more robust and efficient than the traditional statistical indicators under the same circumstances. Besides, to silence the spike protein of SARS-CoV-2 to invade cells, this study further puts forward the SMRI method to search candidate high-efficiency siRNAs on SARS-CoV-2's S gene. This study is one of the early studies applying RNAi therapy to the COVID-19 treatment. According to the analysis, the average value of predicted interference efficiency of the candidate siRNAs designed by the SMRI method is comparable to that of the mainstream siRNA design algorithms. Moreover, the SMRI method ensures that the designed siRNAs have more than three base mismatches with human genes, thus avoiding silencing normal human genes. This is not considered by other mainstream methods, thereby the five candidate high-efficiency siRNAs which are easy to take effect or synthesize and much safer for human body are obtained by our SMRI method, which provide a new safer, small dosage and long efficacy solution for the treatment of COVID-19.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11390-021-0826-x.

Antisense technology as a potential strategy for the treatment of coronaviruses infection: With focus on COVID-19

After the outbreak of coronavirus disease 2019 (COVID-19) in December 2019 and the increasing number of SARS-CoV-2 infections all over the world, researchers are struggling to investigate effective therapeutic strategies for the treatment of this infection. Targeting viral small molecules that are involved in the process of infection is a promising strategy. Since many host factors are also used by SARS-CoV-2 during various stages of infection, down-regulating or silencing these factors can serve as an effective therapeutic tool. Several nucleic acid-based technologies including short interfering RNAs, antisense oligonucleotides, aptamers, DNAzymes, and ribozymes have been suggested for the control of SARS-CoV-2 as well as other respiratory viruses. The antisense technology also plays an indispensable role in the treatment of many other diseases including cancer, influenza, and acquired immunodeficiency syndrome. In this review, we summarised the potential applications of antisense technology for the treatment of coronaviruses and specifically COVID-19 infection.

Therapeutic perceptions in antisense RNA-mediated gene regulation for COVID-19

COVID-19 was first reported in Wuhan, China, in December 2019. It is widely accepted that the world will not return to its prepandemic normality until safe and effective vaccines are available and a global vaccination program has been successfully implemented. Antisense RNAs are single-stranded RNAs that occur naturally or are synthetic and enable hybridizing and protein-blocking translation. Therefore, the main objective of this study was to identify target markers in the RNA of the severe acute respiratory syndrome coronavirus, or SARS-CoV-2, with a length between 21 and 28 bases that could enable the development of vaccines and therapies based on antisense RNA. We used a search algorithm in C language to compare 3159 complete nucleotide sequences from SARS-CoV-2 downloaded from the repository of the National Center for Biotechnology Information. The objective was to verify whether any common sequences were present in all 3159 strains of SARS-CoV-2. In the first of three datasets (SARS-CoV-2), the algorithm found two sequences each of 21 nucleotides (Sequence 1: CTACTGAAGCCTTTGAAAAAA; Sequence 2: TGTGGTTATACCTACTAAAAA). In the second dataset (SARS-CoV) and third dataset (MERS-CoV), no sequences of size N between 21 and 28 were found. Sequence 1 and Sequence 2 were input into BLAST® ≫ blastn and recognized by the platform. The gene identified by the sequences found by the algorithm was the ORF1ab region of SARS-CoV-2. Considerable progress in antisense RNA research has been made in recent years, and great achievements in the application of antisense RNA have been observed. However, many mechanisms of antisense RNA are not yet understood. Thus, more time and money must be invested into the development of therapies for gene regulation mediated by antisense RNA to treat COVID-19 as no effective therapy for this disease has yet been found.

A SARS-CoV-2 targeted siRNA-nanoparticle therapy for COVID-19

Coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in humans. Despite several emerging vaccines, there remains no verifiable therapeutic targeted specifically to the virus. Here we present a highly effective small interfering RNA (siRNA) therapeutic against SARS-CoV-2 infection using a novel lipid nanoparticle (LNP) delivery system. Multiple siRNAs targeting highly conserved regions of the SARS-CoV-2 virus were screened, and three candidate siRNAs emerged that effectively inhibit the virus by greater than 90% either alone or in combination with one another. We simultaneously developed and screened two novel LNP formulations for the delivery of these candidate siRNA therapeutics to the lungs, an organ that incurs immense damage during SARS-CoV-2 infection. Encapsulation of siRNAs in these LNPs followed by in vivo injection demonstrated robust repression of virus in the lungs and a pronounced survival advantage to the treated mice. Our LNP-siRNA approaches are scalable and can be administered upon the first sign of SARS-CoV-2 infection in humans. We suggest that an siRNA-LNP therapeutic approach could prove highly useful in treating COVID-19 disease as an adjunctive therapy to current vaccine strategies.

A SARS-CoV-2 targeted siRNA-nanoparticle therapy for COVID-19

Coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in humans. Despite several emerging vaccines, there remains no verifiable therapeutic targeted specifically to the virus. Here we present a highly effective siRNA therapeutic against SARS-CoV-2 infection using a novel lipid nanoparticle delivery system. Multiple small-interfering RNAs (siRNAs) targeting highly conserved regions of the SARS-CoV-2 virus were screened and three candidate siRNAs emerged that effectively inhibit virus by greater than 90% either alone or in combination with one another. We simultaneously developed and screened two novel lipid nanoparticle formulations for the delivery of these candidate siRNA therapeutics to the lungs, an organ that incurs immense damage during SARS-CoV-2 infection. Encapsulation of siRNAs in these LNPs followed by in vivo injection demonstrated robust repression of virus in the lungs and a pronounced survival advantage to the treated mice. Our LNP-siRNA approaches are scalable and can be administered upon the first sign of SARS-CoV-2 infection in humans. We suggest that an siRNA-LNP therapeutic approach could prove highly useful in treating COVID-19 disease as an adjunctive therapy to current vaccine strategies.

The significance of bioengineered nanoplatforms against SARS-CoV-2: From detection to genome editing

COVID-19 outbreak can impose serious negative impacts on the infrastructures of societies including the healthcare systems. Despite the increasing research efforts, false positive or negative results that may be associated with serologic or even RT-PCR tests, inappropriate or variable immune response, and high rates of mutations in coronavirus may negatively affect virus detection process and effectiveness of the vaccines or drugs in development. Nanotechnology-based research attempts via developing state-of-the-art techniques such as nanomechatronics ones and advanced materials including the sensors for detecting the pathogen loads at very low concentrations or site-specific delivery of therapeutics, and real-time protections against the pandemic outbreaks by nanorobots can provide outstanding biomedical breakthroughs. Considering the unique characteristics of pathogens particularly the newly-emerged ones and avoiding the exaggerated optimism or simplistic views on the prophylactic and therapeutic approaches including the one-size-fits-all ones or presenting multiple medications that may be associated with synergistic toxicities rather than enhanced efficiencies might pave the way towards the development of more appropriate treatment strategies with reduced safety concerns. This paper highlights the significance of nanoplatforms against the viral disorders and their capabilities of genome editing that may facilitate taking more appropriate measures against SARS-CoV-2.

The Potential Therapeutic Effect of RNA Interference and Natural Products on COVID-19: A Review of the Coronaviruses Infection

The SARS-CoV-2 virus was reported for the first time in Wuhan, Hubei Province, China, and causes respiratory infection. This pandemic pneumonia killed about 1,437,835 people out of 61,308,161cases up to November 27, 2020. The disease's main clinical complications include fever, recurrent coughing, shortness of breath, acute respiratory syndrome, and failure of vital organs that could lead to death. It has been shown that natural compounds with antioxidant, anticancer, and antiviral activities and RNA interference agents could play an essential role in preventing or treating coronavirus infection by inhibiting the expression of crucial virus genes. This study aims to introduce a summary of coronavirus's genetic and morphological structure and determine the role of miRNAs, siRNAs, chemical drugs, and natural compounds in stimulating the immune system or inhibiting the virus's structural and non-structural genes that are essential for replication and infection of SARS-CoV-2.

Emerging Technologies for the Treatment of COVID-19

The new coronavirus, named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), turned into a pandemic affecting more than 200 countries. Due to the high rate of transmission and mortality, finding specific and effective treatment options for this infection is currently of urgent importance. Emerging technologies have created a promising platform for developing novel treatment options for various viral diseases such as the SARS-CoV-2 virus. Here, we have described potential novel therapeutic options based on the structure and pathophysiological mechanism of the SARS-CoV-2 virus, as well as the results of previous studies on similar viruses such as SARS and MERS. Many of these approaches can be used for controlling viral infection by reducing the viral damage or by increasing the potency of the host response. Owing to their high sensitivity, specificity, and reproducibility, siRNAs, aptamers, nanobodies, neutralizing antibodies, and different types of peptides can be used for interference with viral replication or for blocking internalization. Receptor agonists and interferon-inducing agents are also potential options to balance and enhance the innate immune response against SARS-CoV-2. Solid evidence on the efficacy and safety of such novel technologies is yet to be established although many well-designed clinical trials are underway to address these issues.

Nanotheranostics against COVID-19: From multivalent to immune-targeted materials

Destructive impacts of COVID-19 pandemic worldwide necessitates taking more appropriate measures for mitigating virus spread and development of the effective theranostic agents. In general, high heterogeneity of viruses is a major challenging issue towards the development of effective antiviral agents. Regarding the coronavirus, its high mutation rates can negatively affect virus detection process or the efficiency of drugs and vaccines in development or induce drug resistance. Bioengineered nanomaterials with suitable physicochemical characteristics for site-specific therapeutic delivery, highly-sensitive nanobiosensors for detection of very low virus concentration, and real-time protections using the nanorobots can provide roadmaps towards the imminent breakthroughs in theranostics of a variety of diseases including the COVID-19. Besides revolutionizing the classical disinfection procedures, state-of-the-art nanotechnology-based approaches enable providing the analytical tools for accelerated monitoring of coronavirus and associated biomarkers or drug delivery towards the pulmonary system or other affected organs. Multivalent nanomaterials capable of interaction with multivalent pathogens including the viruses could be suitable candidates for viral detection and prevention of further infections. Besides the inactivation or destruction of the virus, functionalized nanoparticles capable of modulating patient's immune response might be of great significance for attenuating the exaggerated inflammatory reactions or development of the effective nanovaccines and medications against the virus pandemics including the COVID-19.

The epigenetic implication in coronavirus infection and therapy

Epigenetics is a relatively new field of science that studies the genetic and non-genetic aspects related to heritable phenotypic changes, frequently caused by environmental and metabolic factors. In the host, the epigenetic machinery can regulate gene expression through a series of reversible epigenetic modifications, such as histone methylation and acetylation, DNA/RNA methylation, chromatin remodeling, and non-coding RNAs. The coronavirus disease 19 (COVID-19) is a highly transmittable and pathogenic viral infection. The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), which emerged in Wuhan, China, and spread worldwide, causes it. COVID-19 severity and consequences largely depend on patient age and health status. In this review, we will summarize and comparatively analyze how viruses regulate the host epigenome. Mainly, we will be focusing on highly pathogenic respiratory RNA virus infections such as coronaviruses. In this context, epigenetic alterations might play an essential role in the onset of coronavirus disease complications. Although many therapeutic approaches are under study, more research is urgently needed to identify effective vaccine or safer chemotherapeutic drugs, including epigenetic drugs, to cope with this viral outbreak and to develop pre- and post-exposure prophylaxis against COVID-19.

Potential use of noncoding RNAs and innovative therapeutic strategies to target the 5'UTR of SARS-CoV-2

After the increasing number of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections all over the world, researchers and clinicians are struggling to find a vaccine or innovative therapeutic strategies to treat this viral infection. The severe acute respiratory syndrome coronavirus infection that occurred in 2002, Middle East respiratory syndrome (MERS) and other more common infectious diseases such as hepatitis C virus, led to the discovery of many RNA-based drugs. Among them, siRNAs and antisense locked nucleic acids have been demonstrated to have effective antiviral effects both in animal models and humans. Owing to the high genomic homology of SARS-CoV-2 and severe acute respiratory syndrome coronavirus (80–82%) the use of these molecules could be employed successfully also to target this emerging coronavirus. Trying to translate this approach to treat COVID-19, we analyzed the common structural features of viral 5’UTR regions that can be targeted by noncoding RNAs and we also identified miRNAs binding sites suitable for designing RNA-based drugs to be employed successfully against SARS-CoV-2.

RNA interference as a promising treatment against SARS-CoV-2

Until now, there is no current vaccine or treatment against SARS-CoV-2. There are previous successful RNAi studies performed on SARS-CoV. Therefore, similar line of investigation against SARS-CoV-2 could be successful.

Prospects for RNAi Therapy of COVID-19

COVID-19 caused by the SARS-CoV-2 virus is a fast emerging disease with deadly consequences. The pulmonary system and lungs in particular are most prone to damage caused by the SARS-CoV-2 infection, which leaves a destructive footprint in the lung tissue, making it incapable of conducting its respiratory functions and resulting in severe acute respiratory disease and loss of life. There were no drug treatments or vaccines approved for SARS-CoV-2 at the onset of pandemic, necessitating an urgent need to develop effective therapeutics. To this end, the innate RNA interference (RNAi) mechanism can be employed to develop front line therapies against the virus. This approach allows specific binding and silencing of therapeutic targets by using short interfering RNA (siRNA) and short hairpin RNA (shRNA) molecules. In this review, we lay out the prospect of the RNAi technology for combatting the COVID-19. We first summarize current understanding of SARS-CoV-2 virology and the host response to viral entry and duplication, with the purpose of revealing effective RNAi targets. We then summarize the past experience with nucleic acid silencers for SARS-CoV, the predecessor for current SARS-CoV-2. Efforts targeting specific protein-coding regions within the viral genome and intragenomic targets are summarized. Emphasizing non-viral delivery approaches, molecular underpinnings of design of RNAi agents are summarized with comparative analysis of various systems used in the past. Promising viral targets as well as host factors are summarized, and the possibility of modulating the immune system are presented for more effective therapies. We place special emphasis on the limitations of past studies to propel the field faster by focusing on most relevant models to translate the promising agents to a clinical setting. Given the urgency to address lung failure in COVID-19, we summarize the feasibility of delivering promising therapies by the inhalational route, with the expectation that this route will provide the most effective intervention to halt viral spread. We conclude with the authors' perspectives on the future of RNAi therapeutics for combatting SARS-CoV-2. Since time is of the essence, a strong perspective for the path to most effective therapeutic approaches are clearly articulated by the authors.

Coronavirus Pandemic-Therapy and Vaccines

The current coronavirus COVID-19 pandemic, which originated in Wuhan, China, has raised significant social, psychological and economic concerns in addition to direct medical issues. The rapid spread of severe acute respiratory syndrome-coronavirus (SARS-CoV)-2 to almost every country on the globe and the failure to contain the infections have contributed to fear and panic worldwide. The lack of available and efficient antiviral drugs or vaccines has further worsened the situation. For these reasons, it cannot be overstated that an accelerated effort for the development of novel drugs and vaccines is needed. In this context, novel approaches in both gene therapy and vaccine development are essential. Previous experience from SARS- and MERS-coronavirus vaccine and drug development projects have targeted glycoprotein epitopes, monoclonal antibodies, angiotensin receptor blockers and gene silencing technologies, which may be useful for COVID-19 too. Moreover, existing antivirals used for other types of viral infections have been considered as urgent action is necessary. This review aims at providing a background of coronavirus genetics and biology, examples of therapeutic and vaccine strategies taken and potential innovative novel approaches in progress.

Role of siRNAs and miRNAs in the processes of RNA-mediated gene silencing during viral infections

Phenomenon of RNA-induced gene silencing is a highly conservative mechanism among eukaryotic organisms. Several classes of small RNAs (siRNAs and miRNAs) 21-25 nt in length, which play a significant role in the processes of development of an organism, occurred important components of antiviral defence in animals and plants. This review shortly describes the main stages of gene silencing mechanism, features of antiviral RNA silencing in plants, invertebrates, mammals, ways of suppression of RNA-interference by viruses, as well as possible approaches of utilization of abovementioned phenomenon for struggling against viral infections.

siRNA silencing of angiotensin-converting enzyme 2 reduced severe acute respiratory syndrome-associated coronavirus replications in Vero E6 cells

The outbreak of severe acute respiratory syndrome (SARS) in 2002–2003 has had a significant impact worldwide. No effective prophylaxis or treatment for SARS is available up to now. Angiotensin-converting enzyme 2 (ACE2) is the cellular receptor for SARS-associated coronavirus (SARS-CoV). By expressing a U6 promoter-driven small interfering RNA containing sequences homologous to part of ACE2 mRNA, we successfully silenced ACE2 expression in Vero E6 cells. By detecting negative strand SARS-CoV RNA and measuring RNA copy numbers of SARS-CoV by real-time reverse transcription polymerase chain reaction (RT-PCR), we demonstrated that SARS-CoV infection was reduced in the ACE2-silenced cell lines. These findings support the involvement of ACE2 in SARS-CoV infections and provide a basis for further studies on potential use of siRNA targeting ACE2 as a preventive or therapeutic strategy for SARS.

Oligonucleotide antiviral therapeutics: antisense and RNA interference for highly pathogenic RNA viruses

RNA viruses are a significant source of morbidity and mortality in humans every year. Additionally, the potential use of these viruses in acts of bioterrorism poses a threat to national security. Given the paucity of vaccines or postexposure therapeutics for many highly pathogenic RNA viruses, novel treatments are badly needed. Sequence-based drug design, under development for almost 20 years, is proving effective in animal models and has moved into clinical trials. Important advances in the field include the characterization of RNA interference in mammalian cells and chemical modifications that can dramatically increase the in vivo stability of therapeutic oligonucleotides. Antisense strategies utilize single-stranded DNA oligonucleotides that inhibit protein production by mediating the catalytic degradation of target mRNA, or by binding to sites on mRNA essential for translation. Double-stranded RNA oligonucleotides, known as short-interfering RNAs (siRNAs), also mediate the catalytic degradation of complementary mRNAs. As RNA virus infection is predicated on the delivery, replication, and translation of viral RNA, these pathogens present an obvious target for the rapidly advancing field of sequence-specific therapeutics. Antisense oligonucleotides or siRNAs can be designed to target the viral RNA genome or viral transcripts. This article reviews current knowledge on therapeutic applications of antisense and RNA interference for highly pathogenic RNA viral infections.

Therapeutic and prophylactic potential of small interfering RNAs against severe acute respiratory syndrome: progress to date

Severe acute respiratory syndrome (SARS), caused by the novel coronavirus SARS-CoV, produced a scare when it appeared in 2003 in China and later quickly spread to other countries around the world. Although it has since disappeared, its threat to human health remains. Therefore, studies on the prevention and treatment of SARS are important for dealing with epidemics of this and other infectious diseases. The most promising newly developed technology for intervention in SARS may be RNA interference, an endogenous cellular process for the inhibition of gene expression mediated by sequence-specific double-stranded RNAs. Numerous studies have reported the therapeutic potential of RNA interference for the treatment of various human diseases ranging from cancers to infectious diseases such as HIV and hepatitis. To date, most studies on inhibition of SARS-CoV replication using small interfering RNAs (siRNAs) have been conducted in cell lines in vitro. One study using siRNAs to inhibit SARS-CoV infection in Rhesus macaques demonstrated that siRNAs were effective both prophylactically and therapeutically with no adverse effects in the animals. Challenges remaining for the application of siRNA in vivo for SARS prevention and treatment include the specificity of the siRNAs and the efficiency of delivery. However, with improvements in siRNA design and delivery methods, RNA interference has the potential to become another major weapon for combating dangerous infections due to viruses such as SARS-CoV.

Prospects of RNA interference therapy in respiratory viral diseases: update 2006

Respiratory viruses, such as influenza, parainfluenza and respiratory syncytial virus (RSV), claim millions of lives annually. At present, there is no completely effective vaccine or drug against these highly mutable RNA viruses. Passive antibody therapies for RSV, despite their limited application and staggering cost, enjoy a virtual monopoly in a multibillion-dollar global market. Recently, however, pioneering discoveries have launched RNA interference as a novel, nucleic acid-based therapy against viral pathogens. Specifically, small interfering RNAs (siRNAs) offered protection against respiratory syncytial virus, parainfluenza and influenza. siRNA against RSV has entered Phase I clinical trials in humans, and preliminary reports are promising. If appropriately formulated for improved specificity, delivery and pharmacokinetics, siRNAs may indeed become effective antivirals in the clinics of the future. This paper provides an overview of the prospects and hurdles facing the antiviral siRNA drugs, with special emphasis on RSV.