To date, 15 oligonucleotide therapeutics have been approved by the FDA for several different indications , including nine ASO drugs, five siRNA drugs and one aptamer.

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Overview of Oligonucleotide Therapeutics &
Its Delivery
Oligonucleotide therapeutics are a rapidly growing class of therapeutics, which
consists of modified or unmodified short nucleic acid molecules, and
includes antisense oligonucleotides (ASOs), small interfering RNA
(siRNAs), microRNA (miRNAs), aptamers, and DNAzymes. The
mechanism of action of oligonucleotide therapeutics mainly relies on
Watson–Crick base pairing to targeted mRNAs, resulting in either gene
silencing, a steric block, or altered splicing patterns, with the exception of
aptamers, which recognize their targets by their three-dimensional structures.
Approved Oligonucleotide Therapeutics
To date, 15 oligonucleotide therapeutics have been approved by the U.S.
Food and Drug Administration.
(FDA) for several different indications , including nine ASO drugs, five siRNA
drugs and one aptamer. Besides, there are more than 100 in clinical studies
and many more in preclinical development.
Type Drug FDA Approval Company Indication
ASO
Fomivirse
n
1998 (Withdra
wn)
Ionis/Novartis
Cytomegalovirus
retinitis in
immunocompromis
ed patients
Mipomers
en
2013 (Withdra
wn)
Kastle
Homozygous famili
al hypercholesterol
emia
Nusinerse
n
2016 Ionis/Biogen
Spinal muscular
atrophy
Eteplirsen 2016 Sarepta
Duchenne
muscular dystrophy
Defibrotid
e
2016
Jazz Pharmaceuti
cals plc
Veno-occlusive
disease
Inotersen 2018 Ionis
Hereditary
transthyretin-media
ted amyloidosis
Golodirse
n
2019 Sarepta
Duchenne
muscular dystrophy
Viltolarsen 2020 Nippon Shinyaku Duchenne

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muscular dystrophy
Casimerse
n
2021 Sarepta
Duchenne
muscular dystrophy
siRNA
Patisiran 2018 Alnylam
Hereditary
transthyretin-media
ted amyloidosis
Givosiran 2019 Alnylam
Acute hepatic
porphyria
Lumasiran 2020 Alnylam
Primary
hyperoxaluria type
1
Inclisiran 2021 Novartis
Primary hyperchol
esterolemia
Vutrisiran 2022 Alnylam
Hereditary
transthyretin-media
ted amyloidosis
Aptam
er
Pegaptani
b
2004 (Withdra
wn)
Pfizer/Eyetech
Neovascular (Wet)
Age-Related
Macular
Degeneration
Table. Approved Oligonucleotide therapeutics
It has been 25 years since fomivirsen, the first small oligonucleotide
therapeutic, was introduced in 1998. However, the market for oligonucleotide
therapeutics is still immature, and there is still a lot of room for development. In
the past 5 years, the market for oligonucleotide therapeutics has
maintained a promising growth momentum, with the market volume
expanding from $2 billion in 2018 to nearly $4 billion today.
Mechanism of Action of Oligonucleotide Drugs
RNA interference (RNAi)
RNAi is a process whereby long double-stranded RNA is sheared into short
double-stranded RNA and then bound to a protein to form an RNA-induced
silencing complex (RISC). After the degradation of the sense strand of the
short-stranded RNA, RISC then binds to specific mRNAs to degrade the
mRNAs and ultimately silence the expression of the corresponding genes.
This is a highly conserved process.

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Figure. Mechanism of Action of RNAi [1]
Antisense Oligonucleotides (ASO)
ASO is a single-stranded DNA or RNA sequence complementary to the mRNA
of a target gene, usually consisting of a dozen to several dozen bases,
produced by chemical synthesis. After certain specific chemical modifications
of ASO, ASO drugs enter the cell by certain means and are able to specifically
regulate the expression of target genes. ASO targets various types of nucleic
acids (pre-mRNA, mRNA, non-coding RNA) in the cell. ASO inhibits protein
production primarily by stimulating RNAase H activity, which in turn leads to
the degradation of the target mRNA (ASO "Gapmers").
Figure. Mechanism of Action of ASO [1]
Aptamers

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Aptamers are single-stranded DNA or RNA molecules screened by large
oligonucleotide libraries (called SELEX) that bind specific targets with high
selectivity and specificity. Common targets include small metal ions and
organic molecules, proteins, viruses, bacteria, and whole cells. Target
recognition and binding involves three-dimensional, shape-dependent
interactions as well as hydrophobic interactions. The diagram below shows
aptamer Pegaptanib inhibiting the action of target protein VEGF-165 by
binding to its receptor VEGFR.
Figure. Mechanism of action of aptamers [1]
Advantages of Oligonucleotide Drugs
❖ High specificity: Oligonucleotide drugs are artificially designed based on
target RNA, so the target is clear and the target specificity is strong.
❖ Easy design and short R&D cycle: Preclinical development of
oligonucleotide drugs is firstly done by determining gene sequences and
making proper designs for disease genes to silence gene targets, so it can
avoid blind development and save R&D time to a great extent.
❖ Rich targets: Oligonucleotide drugs are treated from post-transcriptional
level, which can make a breakthrough for some special targets where protein
targets can be efficacious, and are expected to overcome genetic diseases for
which there is no drug yet.

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Figure. Oligonucleotide Drugs vs. small molecule drugs [2]
Delivery of Oligonucleotide Drugs
The key to the drug production of oligonucleotide drugs lies in chemical
modification and delivery system technology. Unmodified Oligonucleotide
Drugs are not only susceptible to degradation by nucleases in vivo, but also
susceptible to induce immune reactions; moreover, without the help of
targeted delivery systems, negatively charged oligonucleotide drugs are
difficult to enter cells for action.
Currently, the following delivery platform technologies are commonly used for
oligonucleotide therapy.

❖ Antisense Oligonucleotides (ASOs)


❖ GalNAc-siRNA conjugates


❖ Lipid Nanoparticles (LNPs)


❖ Adeno-Associated Viral Vectors (AAV vectors)


❖ Exosomes

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
Among them, the development of delivery carriers is more critical. At
present, GalNAc-siRNA conjugates and LNPs are the most studied and
discussed delivery systems in RNAi therapy owing to their practicality, stability,
and safety.
GalNac Conjugates
GalNAc conjugation is emerging as a dominant strategy for delivery of
therapeutic oligonucleotides. To date, four GalNAc-siRNA therapeutics,
Leqvio® (inclisiran), GIVLAARI™ (givosiran), Oxlumo™ (lumasiran) and
Amvuttra (vutrisiran), have been approved for commercial applications.
Figure. Delivery of GalNAc-siRNA conjugates into hepatocytes. [3]
GalNAc 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).
GalNac-siRNA conjugates can rapidly enter the liver through the circulatory
system after subcutaneous injection. It is then rapidly endocytosed by liver
cells mediated by ASPGR and accumulated in lysosomes, slowly released and
persistently loaded onto RISC, thus achieving long-acting inhibitory effects.
At present, Alnylam has more comprehensive patent coverage on GalNac,
covering GalNac-siRNA, including double-stranded nucleic acid structure,
limiting some site-specific modifications, etc. Dicema mainly avoids patents by
constructing single-stranded structures and end-loop formations, etc. In
addition to Alnylam, GalNAc-based technology platforms have emerged in
recent years, including Dicerna's GalXC, Arrowhead's TRiM, Ionis' LICA, etc.
Lipid Nanoparticle (LNP)

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Lipid nanoparticles (LNPs) are chemically synthesized multicomponent lipid
formulations (~100 nm in size) encapsulating siRNAs for delivery to the target
tissue. Since RNA is negatively charged, positively charged lipids can bind to it,
forming very small particles with a reverse interlaced structure, which are
wrapped by phospholipids.
LNPs typically consist of four components—ionizable lipid, phospholipid,
cholesterol, and PEGylated lipid. Among them, PEGylated lipid can improve
the hydrophilicity of the drug, avoid the rapid clearance of the drug by the
immune system, prevent the aggregation of particles, and increase the
stability.
Huateng Pharma, as a leading supplier of PEG derivatives, can provide a wide
range of PEG lipids to our clients worldwide.
Conclusion
Although the development of oligonucleotide therapies is still difficult, it is
believed that with the continuous development, improvement and progress of
related technologies, oligonucleotide therapies will surely set off a new wave in
the pharmaceutical industry.
References:
[1] Ageliki Laina, et al. RNA Therapeutics in Cardiovascular Precision Medicine. Front Physiol. 2018 Jul
25;9:953. doi: 10.3389/fphys.2018.00953. eCollection 2018.
[2] Phuc Tran, et al. Delivery of Oligonucleotides: Efficiency with Lipid Conjugation and Clinical
Outcome. Pharmaceutics. 2022 Feb 1;14(2):342. doi: 10.3390/pharmaceutics14020342.
[3]Aaron D. Springer and Steven F. Dowdy.GalNAc-siRNA Conjugates: Leading the Way for Delivery of
RNAi Therapeutics.Nucleic Acid Therapeutics.Jun 2018.109-118.http://doi.org/10.1089/nat.2018.0736