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History of RNAi use in medicine The first instance of RNA silencing in animals was documented in 1996, when Guo and Kemphues observed that, by introducing sense and antisense RNA to par-1 mRNA in Caenorhabditis elegans caused degradation of the par-1 message. It was thought that this degradation was triggered by single stranded RNA (ssRNA), but two years later, in 1998, Fire and Mello discovered that this ability to silence the par-1 gene expression was actually triggered by double-stranded RNA (dsRNA). They would eventually share the Nobel Prize in Physiology or Medicine for this discovery. Just after Fire and Mello's ground-breaking discovery, Elbashir et al. discovered, by using synthetically made small interfering RNA (siRNA), it was possible to target the silencing of specific sequences in a gene, rather than silencing the entire gene. Only a year later, McCaffrey and colleagues demonstrated that this sequence specific silencing had therapeutic applications by targeting a sequence from the Hepatitis C virus in transgenic mice. Since then, multiple researchers have been attempting to expand the therapeutic applications of RNAi, specifically looking to target genes that cause various types of cancer. Finally, in 2004, this new gene silencing technology entered a Phase I clinical trial in humans for wet age-related macular degeneration. Six years later the first-in-human Phase I clinical trial was started, using a nanoparticle delivery system to target solid tumors. Although most research is currently looking into the applications of RNAi in cancer treatment, the list of possible applications is extensive. RNAi could potentially be used to treat viruses, bacterial diseases, parasites, maladaptive genetic mutations, control drug consumption, provide pain relief, and even modulate sleep.

Viral infection
Antiviral treatment is one of the earliest proposed RNAi-based medical applications, and two different types have been developed. The first type is to target viral RNAs. Many studies have shown that targeting viral RNAs can suppress the replication of numerous viruses, including HIV, HPV, hepatitis A, hepatitis B, Influenza virus, and Measles virus. The other strategy is to block the initial viral entries by targeting the host cell genes. For example, suppression of chemokine receptors (CXCR4 and CCR5)on host cells can prevent HIV viral entry.

Cancer
While traditional chemotherapy can effectively kill cancer cells, lack of specificity for discriminating normal cells and cancer cells in these treatments usually cause severe side effects. Numerous studies have demonstrated that RNAi can provide a more specific approach to inhibit tumor growth by targeting cancer-related genes (i.e., oncogene). It has also been proposed that RNAi can enhance the sensitivity of cancer cells to chemotherapeutic agents, providing a combinatorial therapeutic approach with chemotherapy. Another potential RNAi-based treatment is to inhibit cell invasion and migration.

Neurological diseases
RNAi strategies also show potential for treating neurodegenerative diseases. Studies in cells and in mouse have shown that specifically targeting Amyloid beta-producing genes (e.g. BACE1 and APP) by RNAi can significantly reduced the amount of Aβ peptide which is correlated with the cause of Alzheimer's disease. In addition, this silencing-based approaches also provide promising results in treatment of Parkinson's disease and Polyglutamine disease.

Difficulties in Therapeutic Application
To achieve the clinical potential of RNAi, siRNA must be efficiently transportated to the cells of target tissues. However, there are various barriers that must be fixed before it can be used clinically. For example, "Naked" siRNA is susceptible to several obstacles that reduce its therapeutic efficacy. Additionally, once siRNA has entered the bloodstream, naked RNA can be degraded by serum nucleases and can stimulate the innate immune system. Due to its size and highly polyanionic (containing negative charges at several sites) nature, unmodified siRNA molecules cannot readily enter the cells through the cell membrane. Therefore, artificial or nanoparticle encapsulated siRNA must be used. However, transporting siRNA across the cell membrane still has its own unique challenges. If siRNA is transferred across the cell membrane, unintended toxicities can occur if therapeutic doses are not optimized, and siRNAs can exhibit off-target effects (e.g. unintended downregulation of genes with partial sequence complementarity). Even after entering the cells, repeated dosing is required since their effects are diluted at each cell division.

Safety and Uses in Cancer treatment
Compared with chemotherapy or other anti-cancer drugs, there are a lot of advantages of siRNA drug. SiRNA acts on the post-translational stage of gene expression, so it doesn’t modify or change DNA in a deleterious effect. SiRNA can also be used to produced a specific response in a certain type of way, such as by downgrading suppression of gene expression. In a single cancer cell, siRNA can cause dramatic suppression of gene expression with just several copies. This happens by silencing cancer-promoting genes with RNAi, as well as targeting an mRNA sequence.

RNAi drugs treat cancer by silencing certain cancer promoting genes. This is done by complementing the cancer genes with the RNAi, such as keeping the mRNA sequences in accordance with the RNAi drug. Ideally, RNAi is should be injected and/or chemically modified so the RNAi can reach cancer cells more efficiently. RNAi uptake and regulation is monitored by the kidneys.

Stimulation of immune response
The human immune system is divided into two separate branches: the innate immune system and the adaptive immune system. The innate immune system is the first defense against infection and responds to pathogens in a generic fashion. On the other hand, the adaptive immune system, a system that was evolved later than the innate, is composed mainly of highly specialized B and T cells that are trained to react to specific portions of pathogenic molecules.

The challenge between old pathogens and new has helped create a system of guarded cells and particles that are called safe framework. This framework has given humans an army systems that search out and destroy invader particles, such as pathogens, microscopic organisms, parasites, and infections. The mammalian safe framework has developed to incorporate siRNA as a tool to indicate viral contamination, which has allowed siRNA is create  an intense innate immune response.

siRNA is controlled by the innate immune system, which can be divided into the acute inflammatory responses and antiviral responses. The inflammatory response is created with signals from small signaling molecules, or cytokines. These include  interleukin-1 (IL-1), interleukin-6 (IL-6), interleukin-12 (IL-12) and tumor necrosis factor α (TNF-α). The innate immune system generates inflammation and antiviral responses, which cause  the release pattern recognition receptors (PRRs). These receptors help in labeling which pathogens are viruses, fungi, or bacteria. Moreover, the importance of siRNA and the innate immune system is to include more PRRs to help recognize different  RNA structures. This makes it more likely for the siRNA to cause an immunostimulant response in the event of the pathogen.

Prospects as a Therapeutic Technique
Clinical Phase I and II studies of siRNA therapies conducted between 2015 and 2017 have demonstrated potent and durable gene knockdown in the liver, with some signs of clinical improvement and without unacceptable toxicity. Two Phase III studies are in progress to treat familial neurodegenerative and cardiac syndromes caused by mutations in transthyretin (TTR). Numerous publications have shown that in vivo delivery systems are very promising and are diverse in characteristics, allowing numerous applications. The nanoparticle delivery system shows the most promise yet this method presents additional challenges in the scale-up of the manufacturing process, such as the need for tightly controlled mixing processes to achieve consistent quality of the drug product. The table below shows different drugs using RNA interference and what their phases and status is in clinical trials.