Small interfering RNA (siRNA) represents a new class of therapeutics with the potential to treat diseases by directly intervening in the genetic process. This approach is rooted in the central dogma of molecular biology: Deoxyribonucleic acid (DNA) is transcribed into messenger RNA (mRNA), and that mRNA is then translated into protein. In many diseases, a faulty gene leads to the production of a toxic or malfunctioning protein. siRNA offers a way to stop this process at the RNA stage. These molecules are short, double-stranded pieces of RNA designed to specifically target the problematic mRNA. By eliminating the mRNA, siRNA effectively silences the gene’s expression, preventing the harmful protein from ever being made. This strategy allows researchers to address the fundamental genetic causes of illness rather than merely treating the symptoms.
Understanding RNA Interference
The therapeutic application of small interfering RNA is based on a natural cellular process called RNA interference (RNAi), which evolved as a defense mechanism against viruses. Small interfering RNA molecules are typically 20 to 24 base pairs in length. Once inside the cell, they are incorporated into a specialized structure known as the RNA-Induced Silencing Complex (RISC), a multiprotein complex that serves as the molecular machinery for gene silencing.
The RISC complex separates the double-stranded siRNA, keeping only one, the “guide strand,” to direct its activity. The RISC complex, armed with the guide strand, then scans the cell’s interior for complementary sequences of messenger RNA.
Once the RISC complex binds to the target messenger RNA with near-perfect complementarity, a protein component within the complex, Argonaute 2 (Ago2), activates. The Argonaute protein acts as a molecular scissor, cleaving the messenger RNA and initiating its rapid degradation. Destroying the messenger RNA prevents the cell’s ribosomes from translating the faulty genetic code into a disease-causing protein. The RISC complex is then released, ready to bind to another identical messenger RNA molecule and repeat the silencing process, which allows for sustained suppression of the target gene’s expression.
Delivering siRNA to Target Cells
A major challenge in transforming small interfering RNA into a viable medicine involves ensuring the molecule safely reaches its target inside the cell. Naked RNA is chemically unstable and would be quickly degraded by enzymes in the bloodstream before it could reach the intended tissue. Furthermore, the siRNA molecule is too large and negatively charged to easily cross the fatty, protective cell membrane on its own.
Pharmaceutical engineering has developed two primary strategies to overcome these obstacles and facilitate targeted delivery. One method involves encapsulating the siRNA within Lipid Nanoparticles (LNPs), which are tiny, chemically synthesized spheres. The LNPs protect the RNA payload from degradation and possess special lipids that help them fuse with the cell membrane, enabling the release of the siRNA into the cell’s cytoplasm. LNPs tend to accumulate naturally in the liver due to their affinity for a protein called apolipoprotein E (ApoE), making them highly effective for treating liver-based diseases.
A second successful strategy involves Chemical Conjugation, which attaches the siRNA to a targeting ligand. The most widely used example is the N-acetylgalactosamine (GalNAc) conjugate, a sugar molecule that acts as a key for a specific lock. GalNAc has a high affinity for the asialoglycoprotein receptor (ASGPR), which is abundantly expressed on the surface of liver cells, or hepatocytes. This targeted delivery system allows for the siRNA to be administered subcutaneously, resulting in highly specific uptake by the liver cells, which is ideal for silencing genes whose proteins are primarily produced in the liver.
Therapeutic Applications in Disease
The ability to silence specific genes has opened up new avenues for treating numerous diseases, with several siRNA therapeutics now approved by regulatory agencies. The first FDA-approved siRNA drug, patisiran (Onpattro), targets a hereditary disorder called hereditary transthyretin amyloidosis (hATTR). This drug uses Lipid Nanoparticles to deliver the siRNA to the liver, where it silences the mRNA for the transthyretin protein, stopping the production of the toxic, misfolded protein that causes nerve damage. Another GalNAc-conjugated drug, inclisiran, targets the liver to silence a protein involved in cholesterol regulation, offering a twice-yearly subcutaneous treatment for high cholesterol.
siRNA is also proving useful for localized diseases, particularly in ophthalmology, where the eye’s anatomy allows for direct administration. Delivering the therapy via an intravitreal injection allows the drug to bypass systemic circulation and achieve high concentrations at the site of disease, minimizing the risk of side effects in other parts of the body. This localized approach is currently being explored in clinical trials to treat conditions such as age-related macular degeneration and various forms of glaucoma.
Beyond approved treatments, the technology is being actively investigated for complex diseases like oncology and infectious diseases. Researchers are working on siRNA designs that can silence messenger RNA for genes that are overexpressed in cancer cells, such as those involved in tumor growth or resistance to chemotherapy. Additionally, the inherent antiviral mechanism of RNA interference is being harnessed to develop therapeutics that target the messenger RNA of viruses, preventing the virus from replicating within the host cell.