siRNA Drugs: Here to Stay

Main Text

RNA interference (RNAi) is a naturally occurring phenomenon that inhibits gene expression or translation via Watson-Crick base pairing by triggering the targeted degradation of mRNAs. It was first described in plants several decades ago and more recently in animals by Fire et al.¹ with the key observation that 21-base pair duplexes, termed small interfering RNAs (siRNAs), could drive sequence specific degradation of complementary mRNAs or viral RNAs (Figure 1, first published in Hannon and Rossi²).³ These siRNAs could be synthesized in the lab and delivered to target cells via lipid nanoparticles or other nonlipid carriers, opening up a universe of therapeutic possibilities as any known disease-related protein could be targeted.

Figure 1.

Long dsRNA and miRNA Precursors Are Processed to siRNA/miRNA Duplexes by the RNase-III-like Enzyme Dicer

These short dsRNAs are subsequently unwound and assembled into effector complexes, RNA-induced silencing complexes (RISCs), which can direct RNA cleavage, mediate translational repression, or induce chromatin modification. S. pombe, C. elegans, and mammals carry only one Dicer gene. In D. melanogaster and A. thaliana, specialized Dicer or DLC proteins preferentially process long dsRNA or miRNA precursors. 7mG, 7-methyl guanine; AAAA, poly-adenosine tail; Me, methyl group; P, 5′ phosphate

Once gene silencing was shown to take place in human cells by Elbashir et al.³ and Caplen et al.,⁴ several biotech companies were established to develop siRNA-based therapeutics; however, as macromolecules, many challenges had to be overcome before their promise could be realized. Alnylam Pharmaceuticals developed backbone chemistries for the RNAs, which stabilized them into almost indestructible agents that could be administered systemically in patients suffering from inherited or acquired diseases. Initially, most disease targets were liver specific because siRNA delivery formulations with lipid nanoparticles homed to the liver. Despite limitations in delivery to tissues outside of the liver, the first siRNA drug was approved by the US Food and Drug Administration (FDA) a mere decade after Fire and Mello shared the 2006 Nobel Prize in Physiology or Medicine.

Alnylam’s patisiran—branded as Onpattro—became the first siRNA drug to hit the market in 2018, following approval by the FDA for the treatment of polyneuropathy in patients with hereditary transthyretin-mediated amyloidosis, for which it has become a first-in-class treatment. Patisiran targets a mutated form of the TTR gene, which encodes a dysfunctional transthyretin protein. The latter is misfolded and degrades into peptides that form disease-causing myeloid deposits in nervous tissue, the heart, and other organs. The siRNA specifically targets the mutant mRNA in the liver, thereby preventing formation of the misfolded protein and subsequently the deposits. Indeed, the conceptual beauty of siRNA drugs derives from their specific destruction of a target mRNA owing to Watson-Crick base-pairing. Patisiran is delivered in a lipid-nanoparticle formulation that both protects it from degradation and facilitates its delivery to hepatocytes.

Givosiran—branded Givlaari—became both the second approved siRNA as well as Alnylum’s second marketed product following FDA approval in November 2019. Givosiran treats acute hepatic porphyria, a genetic disease resulting in the buildup of porphyrin molecules due to a mutant copy of aminolevulinic acid synthase (ALAS1), a liver enzyme involved in an early step of heme production. Givosiran reduces expression of the mRNA encoding ALAS1, which lowers blood levels of aminolevulinic acid and porphobilinogen, neurotoxic intermediates that are associated with symptoms in patients. A notable difference between this drug and patisiran is the introduction of GAlNac-conjugation, entirely eliminating the need for a lipid nanoparticle carrier since the GAlNac moiety binds to a liver cell surface receptor.⁵ This leads to internalization of the siRNA in addition to targeted uptake by hepatocytes. Furthermore, the elimination of a lipid-nanoparticle carrier enables subcutaneous injection, eliminating long treatment times and patient discomfort associated with intravenously administered therapies.

In November of 2020 Alnylam’s lumasiran—branded Oxlumo—became the third siRNA therapy following FDA approval for its use to treat primary hyperoxaluria type 1 (PH1). PH1 is caused by a mutant gene encoding serine-pyruvate aminotransferase, which results in insufficient detoxification of glyoxylate from the liver, resulting in renal and bladder stones and eventually damage to the kidneys. The drug works by reducing oxalate synthesis in the liver by targeting mRNA encoding glycolate oxidase, in turn reducing the amount of available glyoxylate, a substrate required for the production of oxalate. Like givosiran, lumasiran achieves efficient delivery to hepatocytes using GAlNac conjugation. This third rapid approval is a strong indication that siRNA drugs are here to stay, have almost unlimited potential to treat human diseases and metabolic disorders, and more approvals for both Alnylum and other siRNA companies can be expected in the near future.

The challenge for the field of siRNA therapeutics going forward is moving beyond the liver to open up the application of this powerful technology to a wider array of diseases affecting the heart, lungs, central nervous system, and beyond. Several strategies are emerging, such as delivering siRNA into epithelial tissues using coated microneedles.⁶ Short circulation times, another central problem to the technology, can be overcome with strategies that increase the size of administered siRNA macromolecules, thus decreasing susceptibility to renal filtration, such as conjugation to polyethylene glycol (PEG) or by achieving significant binding to plasma proteins. Another hurdle can be found in cellular uptake—specifically, escape from the intracellular endosome once the molecule has been internalized. Ultimately, a combination of methods to improve siRNA pharmacokinetics in plasma, increase endosomal escape, and enhance tissue-specific delivery will be required to target a broader array of diseases using siRNA technology, depending on the context of the target cell population.⁷

Despite lingering questions pertaining to delivery, the rapid regulatory approvals of Alnylam’s portfolio drugs validate the fundamental underlying science of RNAi and set the stage for other applications of small RNAs, such as small-activating RNAs that direct transcription complexes to targeted promoter regions. The core promise of siRNA drugs remains—only the primary nucleotide sequence of the target is required to design a new drug, allowing the possibility for rational drug design that avoids the complexity of high-throughput screening and can, theoretically, target any protein in the human organism.