Small nucleic acid drugs, or oligonucleotide drugs, are short strands of nucleic acids composed of several dozen nucleotide tandems. By exploiting complementary base pairing, oligonucleotides can selectively bind to mRNA sequences, leading to degradation, inhibition of translation, or modulation of splicing, resulting in precise and targeted therapeutic effects. Depending on their mechanism of action and structural characteristics, oligonucleotide drugs are classified into several categories, including antisense oligonucleotides (ASO), small interfering RNA (siRNA), small activating RNA (saRNA), microRNA (miRNA), and aptamers.

To date, 23 nucleic acid drugs have been approved by the FDA and/or EMA, including 14 ASO drugs, 8 siRNA drugs, and 2 aptamers (with 3 products later withdrawn from the market). The main development difficulties for ASO drugs and siRNA based drugs lie in structural modifications to improve their stability, as well as selecting optimized drug delivery systems.

Table. Approved nucleic acid drugs

The sites of action for oligonucleotide drugs lay within the intracellular space thus need to overcome several biological barriers to reach their pharmacological targets in vivo. However, oligonucleotide drugs' large size, hydrophilicity, and negative charge largely prevent them from crossing cell membranes and promote their rapid clearance from circulation. Exogeneous nucleic acids are also readily degraded by cellular and serum nucleases and are potent activators of the innate immune system. Therefore, the development of efficient, safe, and precisely targeted delivery systems is critical for the success of oligonucleotide drugs. Currently, the main approved oligonucleotide drug delivery systems are lipid-based systems (like lipid nanoparticles (LNPs)) and ligand conjugation (especially GalNAc), etc.

Lipid Nanoparticle Delivery Systems

LNPs emerged as favourable vehicles for nucleic acid delivery (like mRNA and DNA), due to their biocompatibility, bioavailability, and versatility.

A typical LNP formulation consists of four key components:

• ● Ionizable lipids - designed to carry a positive charge at low pH, such as the mildly acidic environment found within endosomes, facilitating endosomal escape.

• ● Helper phospholipids - usually saturated neutral lipids that enhance the phase transition temperature and support the formation and stability of the bilayer structure.

• ● Cholesterol - enhances particle stability by regulating membrane integrity and rigidity.

• ● PEGylated lipids - enhance LNP colloidal stability in vitro and circulation time in vivo but may reduce uptake and inhibit endosomal release at the cellular level.

Compared with viral vectors, LNPs eliminate risks associated with infection, oncogenicity, and immunogenicity. They can be functionalized with surface ligands for cell-specific targeting, protect mRNA from enzymatic degradation, and facilitate endocytosis and endosomal escape, thereby enhancing antigen expression and therapeutic efficacy. In addition, adjuvants can be incorporated into LNP formulations to further promote immune activation and strengthen vaccine responses.

To date, Patisiran (Onpattro®) remains the only FDA-approved siRNA therapeutic utilizing a LNP delivery system, indicated for the treatment of hereditary transthyretin-mediated amyloidosis (hATTR). The therapeutic efficacy of Patisiran relies on liver-targeted delivery achieved through intravenous administration, as LNPs naturally tend to accumulate in the liver. This hepatic accumulation is primarily driven by opsonization followed by clearance via the mononuclear phagocyte system (MPS).

GalNAc Conjugate Delivery System

GalNAc, or N-acetylgalactosamine, is a sugar molecule that can recognize and bind to a cell surface protein, the asialoglycoprotein receptor (ASGPR), which is abundantly expressed on liver cells (hepatocytes). Upon binding to ASGPR, GalNAc–oligonucleotide conjugates are efficiently internalized through clathrin-mediated endocytosis, allowing the drug to enter hepatocytes and exert its therapeutic effect.

This conjugation strategy addresses three major challenges in nucleic acid drug development—limited tissue specificity, off-target effects, and insufficient in vivo stability—by achieving highly efficient and selective hepatic delivery. With its strong targeting capability, the GalNAc platform has become the preferred delivery system for liver-directed oligonucleotide therapeutics.

To date, nine GalNAc-based oligonucleotide drugs have received regulatory approval for indications such as cardiovascular and metabolic diseases.The first approved product utilizing this technology was Givosiran, a siRNA drug developed by Alnylam Pharmaceuticals, which has also established broad patent coverage and a leading position in GalNAc delivery innovation. Following Alnylam’s success, companies such as Ionis Pharmaceuticals and Arrowhead Pharmaceuticals have accelerated research efforts, driving the emergence of multiple next-generation GalNAc delivery platforms.

Despite its remarkable efficiency in both ASO and siRNA therapeutics, it is important to note that GalNAc conjugation remains primarily restricted to liver-targeted applications, leaving room for further innovation in extrahepatic delivery systems.

Other Delivery Technologies

Beyond the delivery systems already applied in approved oligonucleotide drugs, several emerging platforms—such as antibody, peptide, and exosome-based technologies—show promising potential for future development.

Antibody–Oligonucleotide Conjugates (AOCs)

Antibody–oligonucleotide conjugates (AOCs) are composed of an antibody, a linker, and an oligonucleotide. By leveraging the antibody’s tissue specificity to transport oligonucleotides directly to target cells, AOCs combine the precision of nucleic acid therapeutics with the selective targeting power of antibodies.

AOCs have the potential to overcome a major challenge encountered with many established RNA-based therapeutics: delivery to tissues outside the liver, expanding the therapeutic reach of oligonucleotide drugs to tissues such as muscle and the central nervous system.

Avidity Biosciences—recently the subject of a proposed acquisition by Novartis—has advanced two muscle-targeted AOC candidates into Phase III clinical trials. Other leading players exploring this field include Dyne Therapeutics, Tallac Therapeutics, and Denali Therapeutics. Together, these efforts are shaping a new generation of oligonucleotide delivery systems with broader and more precise therapeutic applications.

Cell-Penetrating Peptides (CPPs)

Cell-penetrating peptides (CPPs) are short peptides capable of transporting large molecular drugs across biological barriers and cellular membranes. They can be linked to oligonucleotides either covalently or non-covalently, providing a versatile tool for intracellular delivery. CPP-based strategies show particular promise for targeted delivery to the central nervous system, heart, and skeletal muscle, where conventional delivery systems face limitations.

Exosomes

Exosomes are naturally secreted nanoscale vesicles capable of carrying nucleic acids and proteins. They offer excellent biocompatibility and can efficiently transfer cargo to recipient cells, making them a promising platform for oligonucleotide delivery. However, challenges remain, including low nucleic acid loading capacity and potential restrictions on therapeutic activity. Ongoing research into engineered or biomimetic exosome-based carriers seeks to overcome these limitations and expand their potential in targeted drug delivery.

Outlook for Oligonucleotide Drug Delivery

Efficient delivery has long been a major barrier in oligonucleotide drug development. The GalNAc platform has successfully achieved liver-specific delivery, greatly enhancing the treatment of hepatic and liver-related diseases. Yet, precise targeting to extrahepatic tissues and organs remains a significant unmet challenge. To date, most approved or clinically investigated siRNA therapeutics are focused on liver indications, reflecting the pivotal role of effective hepatic delivery systems in their success.

Emerging strategies, including AOCs, CPPs, exosomes, and other innovative carriers, hold the potential to expand tissue-specific delivery beyond the liver. Developing efficient, safe, and controllable delivery platforms will be essential to extending the clinical reach of oligonucleotide therapeutics and unlocking new treatment possibilities.

References:

[1] Hammond SM, Aartsma-Rus A, Alves S, Borgos SE, Buijsen RAM, Collin RWJ, Covello G, Denti MA, Desviat LR, Echevarría L, Foged C, Gaina G, Garanto A, Goyenvalle AT, Guzowska M, Holodnuka I, Jones DR, Krause S, Lehto T, Montolio M, Van Roon-Mom W, Arechavala-Gomeza V. Delivery of oligonucleotide-based therapeutics: challenges and opportunities. EMBO Mol Med. 2021 Apr 9;13(4):e13243. doi: 10.15252/emmm.202013243. Epub 2021 Apr 6. PMID: 33821570; PMCID: PMC8033518.

[2] Roberts, T. C., Langer, R., & Wood, M. J. (2020). Advances in oligonucleotide drug delivery. Nature Reviews Drug Discovery, 19(10), 673-694. https://doi.org/10.1038/s41573-020-0075-7

[3] Gagliardi, M., & Ashizawa, A. T. (2021). The Challenges and Strategies of Antisense Oligonucleotide Drug Delivery. Biomedicines, 9(4). https://doi.org/10.3390/biomedicines9040433

[4] Malecova, B., Burke, R. S., Cochran, M., Hood, M. D., Johns, R., Kovach, P. R., Doppalapudi, V. R., Erdogan, G., Arias, J. D., Darimont, B., Miller, C. D., Huang, H., Geall, A., Younis, H. S., & Levin, A. A. (2023). Targeted tissue delivery of RNA therapeutics using antibody–oligonucleotide conjugates (AOCs). Nucleic Acids Research, 51(12), 5901-5910. https://doi.org/10.1093/nar/gkad415

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