In recent years, oligonucleotide drugs have become a highly regarded type of molecule in new drug development, seen as the third generation of therapeutic drugs after small molecule drugs and antibody drugs. Oligonucleotides are small molecules 8–50 nucleotides in length that bind via Watson-Crick base pairing to enhance or repress the expression of target RNA. Depending on their structure and mechanism of action, they are categorized into antisense oligonucleotides (ASO), small interfering RNA (siRNA), small activating RNA (saRNA), microRNA (miRNA), and aptamers.

siRNA drugs have gained significant attention due to their effectiveness and recent technological advances. These drugs consist of double-stranded RNA molecules (an antisense strand and a sense strand) that form an RNA-induced silencing complex (RISC) with enzymes in the body. This complex degrades target mRNA, reducing or inhibiting the expression of the target gene.

FDA Approved siRNA Drugs

To date, the FDA has approved six siRNA drugs. Notably, four of these are from Alnylam, one is co-developed by Alnylam and Novartis, and one is from Novo Nordisk. These drugs mainly target the endocrine and metabolic, neurological, urological, and cardiovascular systems. All six drugs are designed to target the liver and are chemically modified. Except for Onpattro, which uses lipid nanoparticles (LNP) and is administered intravenously, the other drugs use the GalNAc delivery system.

Table 1. FDA approved siRNA drugs

Delivery System of siRNA Drugs

Due to the physicochemical properties of siRNA, unmodified siRNA is quickly cleared from the body and carries a risk of off-target toxicity. Therefore, both chemical modifications and suitable delivery systems are needed to achieve therapeutic effects. All six approved siRNA drugs use chemical modifications and delivery systems.

First-generation chemical modifications involve altering the phosphate backbone, such as with phosphorothioate (PS). Second-generation modifications involve ribose modifications, with the most common types being  2′-O-methyl (2′-OMe), 2′-O-methoxyethyl (2′-MOE), 2′-O-aminopropyl (2′-O-AP), and 2′-fluoro. Third-generation modifications involve alterations to the ribose ring.

Chemical modifications increase the stability of siRNA but also have some drawbacks. For example, phosphorothioate (PS) modifications can reduce affinity for the target gene and may cause cytotoxicity due to nonspecific interactions with certain proteins.

Chemical modifications alone are often insufficient to deliver siRNA to the target site effectively. Therefore, an efficient delivery system is crucial for the success of oligonucleotide drugs and is a key competitive advantage for pharmaceutical companies. Among the six approved siRNA drugs, only Patisiran uses a lipid nanoparticle (LNP) delivery system, while the others use the GalNAc delivery system. (Figure 1)

Figure 1. LNP system and GalNAc system

LNPs are mainly composed of four parts: ionizable lipids, cholesterol, helper phospholipids, and PEG-lipids. Ionizable lipids are the most crucial component, exhibiting different charge characteristics at varying pH levels. They are positively charged under acidic conditions and nearly neutral at physiological pH, providing intelligent protection for siRNA to cross biological barriers.

The five GalNAc-siRNA conjugates all use L96 as the carrier, which consists of three parts: a triantennary GalNAc ligand, a linker, and the siRNA molecule. GalNAc is covalently linked to the 3’ end of the nucleic acid in a trivalent form, forming the GalNAc-siRNA conjugate. GalNAc is a ligand for the asialoglycoprotein receptor (ASGPR), which is highly specifically expressed on the surface of liver cells. The GalNAc-siRNA conjugate binds specifically to ASGPR, transporting the siRNA from the cell surface into the cell through endocytosis. Once inside, the GalNAc-siRNA conjugate dissociates from ASGPR, which returns to the cell surface, while the GalNAc-siRNA conjugate further breaks down. The released free siRNA then silences genes in the cytoplasm to exert its therapeutic effect.

Advantages and Disadvantages of siRNA Drugs

1. Fast Screening, Short Development Time, Low Risk

Compared to chemical drugs or antibodies, the primary advantage of siRNA drugs lies in their platform nature. By simply rearranging the sequence of the four nucleotide bases A, G, C, T (U), new drugs can be developed rapidly. The screening and development process is much quicker than that of chemical drugs and antibodies (see Table 1). Unlike treatments at the genomic DNA level, nucleic acid drugs do not carry the risk of genetic integration. They also offer flexibility in treatment timelines, allowing for cessation of treatment when not needed.

2. Targeting Undrugable Targets

Unlike conventional drugs that target downstream proteins, siRNA targets upstream mRNA. Therefore, it can target many drug targets that small molecules and monoclonal antibodies cannot reach. This includes targets such as AAT, APOC3, DMPK, HTT, and dozens of trinucleotide repeat disorders, significantly expanding the range of drug-targetable sites.

3. Low Bioavailability, Delivery Challenges

Compared to small molecule drugs, siRNA drugs suffer from low bioavailability and difficulties in delivery.

The Future of siRNA Drugs

Currently, the six approved siRNA drugs are primarily used in the treatment of neurological disorders, cardiovascular diseases, and endocrine/metabolic conditions such as amyloidosis, familial amyloid polyneuropathy, and hyperlipidemia.

With the introduction of new chemical modifications and delivery systems, the success rate of siRNA drug development is expected to improve. Development is expanding beyond rare diseases into chronic disease markets, with potential breakthroughs anticipated in oncology. This expansion significantly increases market opportunities, particularly in segments like lipid disorders and "functional cure" for hepatitis B.

Pharmaceutical companies are increasingly recognizing the market potential of siRNA drugs and are actively developing treatments for indications such as hepatitis B, ophthalmic diseases, and cancer.

Due to immature delivery technologies, current siRNA drugs primarily target liver tissues, especially in chronic conditions like lipid and blood pressure control. Indications are mostly focused on genetic and rare diseases, as well as specific chronic liver-related conditions, serving smaller patient populations. Consequently, the market is still in its early stages. However, once the liver targeting limitations are overcome, the widespread application of siRNA drugs is expected to create substantial market opportunities, particularly in cancer therapy.

Huateng Pharma provides large scale GMP manufacture of PEG lipids for LMP drug delivery syetem for your siRNA drug delivery.

References:

1. Kulkarni, J.A., Witzigmann, D., Thomson, S.B. et al. The current landscape of nucleic acid therapeutics. Nat. Nanotechnol. 16, 630–643 (2021). https://doi.org/10.1038/s41565-021-00898-0

2. Okamura K, Ishizuka A, Siomi H, et al. Distinct roles for Argonaute proteins in small RNA-directed RNA cleavage pathways. Genes Dev. 2004;18(14):1655–1666.

3. Ahmed Khaled Abosalha, Waqar Ahmad, Jacqueline Boyajian, Paromita Islam, Merry Ghebretatios, Sabrina Schaly, Rahul Thareja, Karan Arora and Satya Prakash, A comprehensive update of siRNA delivery design strategies for targeted and effective gene silencing in gene therapy and other applications, EXPERT OPINION ON DRUG DISCOVERY, 2023, VOL. 18, NO. 2, 149–161.