Overcoming the Delivery Barriers of RNA Therapeutics
Small interfering RNA (siRNA) and
antisense oligonucleotides (ASOs) represent a class of therapeutics with
exceptional mechanistic precision.
Through complementary base pairing, they can selectively modulate gene expression at the mRNA
level. In theory, virtually any pathogenic gene
could be silenced or corrected once an appropriate sequence is designed.
This therapeutic principle was initially verified in 1998 with the FDA approval of Fomivirsen,
the world's first ASO drug, for the treatment
of cytomegalovirus retinitis. However, despite the elegance of the mechanism, the clinical
development of RNA therapeutics progressed
far more slowly than anticipated. For more than two decades, the field was constrained by a
single dominant challenge: delivery.
Triple Biological Barriers
Oligonucleotide therapeutics face three major biological barriers in vivo: serum stability,
cellular uptake, and tissue-selective delivery.
Serum Stability
The first challenge is serum stability. Naked RNA molecules are rapidly degraded by nucleases in
circulation, with plasma half-lives measured in minutes.
To overcome this issue, extensive chemical modification strategies have been developed.
First-generation chemical modifications mainly focused on
phosphorothioate (PS) backbone substitutions, which significantly enhance the resistance of
small nucleic acids to nucleases. Second-generation
modifications introduced 2'-substituents such as 2'-O-methyl (2'-OMe), 2'-O-methoxyethyl
(2'-MOE), and 2'-fluoro (2'-F), which further enhance
molecular stability and prolong systemic half-life. Third-generation modifications evolved
toward engineered sugar-ring chemistries, including
locked nucleic acid (LNA) and morpholine nucleic acid (PMO), which significantly
improve thermal stability, target recognition, and nuclease resistance[1].
Collectively, these chemical modifications have laid the foundation for the modern development
of oligonucleotide therapeutics.
Figure 1. Common nucleic acid chemical modifications[1].
Cellular Uptake
The second challenge is cellular membrane permeability. Oligonucleotides possess relatively
large molecular weights (siRNA molecules are
approximately 13-14 kDa) and carry strong negative charges, preventing passive diffusion across
the negatively charged cellular membrane.
As a result, oligonucleotides require specialized delivery carriers to enter cells. Current
mainstream oligonucleotide delivery technologies
include lipid nanoparticle (LNP)-based encapsulation systems and ligand-mediated targeting
technologies such as
N-acetylgalactosamine (GalNac) conjugation[2].
Figure 2. The drug delivery carrier can reach the target cells through two different
mechanisms[2].
Tissue-Selective Delivery
The third challenge is tissue specificity. Even when serum stability and cellular uptake are
successfully addressed, ensuring selective delivery to specific organs rather than broad
systemic distribution remains a major obstacle.
Current strategies mainly rely on ligand-mediated targeting approaches. For example, GalNAc
conjugation enables efficient the liver targeting, whereas antibody-mediated delivery provides
opportunities for targeting extrahepatic tissues.
GalNAc and LNP: The Limitations of Liver-Centric Success
Over the past decade, the maturation of GalNAc and LNP delivery technologies has driven
significant breakthroughs in RNA therapy.
GalNAc conjugation utilizes the high-affinity interaction between N-acetylgalactosamine and the
asialoglycoprotein receptor (ASGPR)
expressed on hepatocytes, enabling efficient liver delivery of siRNA and ASO therapeutics. The
success of
Inclisiran, a lipid-lowering
siRNA therapy targeting
PCSK9 is based on this principle.
Inclisiran utilizes RNA interference mechanisms to suppress PCSK9 production in liver cells.
Liver-specific delivery is achieved through GalNAc
conjugation, while extensive chemical modifications enhance the stability and pharmacokinetic
properties of the molecule[3].
Figure 3. Structural information of inclisiran[3].
LNPs are the core delivery vehicles for mRNA vaccines and several siRNA therapeutics, delivering
nucleic acid cargos through endosomal
fusion mechanisms. Early clinical success in RNA therapeutics was largely driven by
liver-targeted delivery. In 2018, Patisiran became the
first FDA-approved siRNA therapeutic and the first non-viral RNAi therapy for hereditary
transthyretin amyloidosis (ATTR)[4].
Figure 4. Drug structure information of Patisiran[4].
However, despite their clinical success, both GalNAc and LNP technologies share a fundamental
limitation: they predominantly accumulate in the
liver. For extrahepatic tissues such as muscles, the central nervous system (CNS), and the
heart, delivery efficiency declines markedly due
to the lack of appropriate targeting receptors and cellular uptake mechanisms. As a result,
effective RNA delivery beyond the liver remains
a major challenge, limiting the development of RNA therapies for neuromuscular diseases,
hereditary cardiomyopathy, and other extrahepatic diseases.
Antibody: A Natural Targeted Delivery Tool
To overcome the challenge of extrahepatic delivery, researchers increasingly turned their
attention toward monoclonal antibodies.
Antibodies naturally possess two key properties: high-affinity recognition of specific cell
surface antigens and efficient receptor-mediated endocytosis. These characteristics align
closely with the delivery requirements of RNA therapy.
One representative example is TAC001, which employs an antibody targeting the B-cell surface
receptor CD22. Through CD22-mediated internalization,
the oligonucleotide payload is selectively delivered into B cells, thereby enhancing targeting
specificity and minimizing off-target effects.
The payload subsequently inhibits the
Toll-like receptor 9 (TLR9)
signaling pathway, regulating both innate and adaptive immune responses,
and potentially suppressing excessive inflammation and tumor immune evasion
[5].
Figure 5. Monoclonal antibody targeting CD22, which delivers the drug to B cells through
specific binding[5].
The core design logic of AOC is straightforward: antibodies act as molecular guidance systems,
directing oligonucleotides payloads to specific
cell types in tissues beyond the liver, including skeletal muscles, the heart, and other
extrahepatic organs.
Molecular Engineering of AOCs and Key Technical Challenges
The Three Elements of AOCs
A typical AOC consists of three essential components: an antibody, an oligonucleotide
payload, and a linker.
Antibody
The antibody is responsible for target recognition and cellular internalization. Its
selection largely determines the tissue specificity and
delivery efficiency of AOC. Factors such as target receptor expression levels, endocytosis
rates, and intracellular trafficking pathways can
all significantly affect the final delivery efficiency[6].
One of the most important targets is
transferrin receptor 1 (TfR1), which is highly
expressed in muscle tissue and undergoes rapid
receptor-mediated endocytosis, making it an attractive target for muscle directed AOC
delivery.
Oligonucleotide
The oligonucleotide serves as the functional payload and is responsible for gene regulation.
Common payload types include siRNA,
ASO and phosphorodiamidate morpholino oligomers (PMOs)[6].
siRNAs exert their effects through the RNA-induced silencing complex (RISC), leading to
degradation of target mRNA in the cytoplasm. Consequently,
siRNA payloads must successfully escape from endosomes and reach the cytoplasm to achieve
therapeutic activity.
ASOs function through various mechanisms. They can recruit RNase H to degrade target RNA,
block translation through spatial obstruction, or
regulate pre-mRNA splicing events such as exon skipping. Depending on the mechanism, ASOs
must ultimately reach either the cytoplasm or nucleus.
PMOs represent a specialized class of oligonucleotide analogs. Unlike conventional
oligonucleotides that possess negatively charged phosphodiester
backbones, PMOs contain a neutral phosphodiesteramine morpholino backbone. They primarily
regulate gene expression through steric blockade of
RNA processing events, such as exon skipping, and therefore must reach the nucleus to exert
their effects.
This neutral charge characteristic fundamentally differentiates PMO-AOCs from siRNA-AOCs in
terms of drug–antibody ratio (DAR) design and pharmacokinetic behavior.
Linker
The linker connects the antibody and the oligonucleotide payload and must remain stable
during systemic circulation while enabling effective
intracellular release under specific conditions, such as acidic endosomal environments or
intracellular reducing environments. The chemical
design of the linker directly affects the pharmacokinetic behavior and intracellular release
efficiency of AOC[6].
Figure 6. Schematic representation of the structure of an antibody-oligonucleotide
conjugate, consisting of an antibody, linker, and oligonucleotide[6].
AOCs and ADCs: Similar Architectures, Distinct Therapeutic Strategies
Although AOCs share a modular architecture similar to that of antibody-drug conjugates
(ADCs), the two modalities differ fundamentally in therapeutic mechanism, intracellular
trafficking requirements, and molecular engineering strategies.
From a payload perspective, ADCs employ small cytotoxic molecules, typically around 1 kDa in
size, whereas AOCs use substantially larger oligonucleotide payloads, including siRNAs
(~13-14 kDa) and PMOs (~7-10 kDa).
Their therapeutic objectives are also fundamentally different. ADCs are designed to
eliminate target cells through cytotoxic mechanisms, while
AOCs aim to regulate gene expression and restore normal cellular function. Consequently,
ADCs are mainly used in oncology, while AOCs are
being developmed mainly for genetic diseases, neuromuscular diseases, and cardiovascular
indications.
The most critical difference lies in payload release and intracellular trafficking. ADC
payloads can exert their effects following lysosomal release
and degradation. In contrast, oligonucleotide payloads must escape endosomal compartment and
reach either the cytoplasm or nucleus to function.
The DAR strategy also differs substantially between payload classes. Due to their size and
negative charge, siRNA-AOCs typically maintain a relatively low DAR (1-2). In contrast,
PMO-AOCs can tolerate significantly higher DAR values, often up to 8, because of the neutral
charge of the PMO backbone.
Immunogenicity considerations also differ. While the immunogenicity risk of small-molecule
ADC payloads is generally manageable, AOCs must account for potential anti-drug antibodies
(ADA) responses triggered by oligonucleotides and linkers.
Collectively, these differences create a unique set of engineering challenges for AOCs,
primarily centered on DAR optimization, endosomal escape, and immunogenicity management.
DAR Optimization: Why Not All AOCs Are Created Equal
In the ADC field, increasing the DAR is a common strategy for enhancing payload delivery.
However, this principle does not universally apply to AOCs.
For siRNA-AOCs, the large molecular size and dense negative charge of siRNA molecules create
significant constraints. Excessively high DAR values can
significantly alter the isoelectric point (pI) of the antibody, accelerate non-specific
clearance, and drastically shorten the half-life.
As a result, siRNA-AOCs generally maintain DAR values at a low level (1-2) to balance
targeted delivery efficiency with pharmacokinetic stability.
PMO-AOCs follow a different paradigm. Because PMOs possess a neutral backbone, higher DAR
values can be achieved without significantly
affecting antibody binding affinity or pharmacokinetic behavior. Several studies have
demonstrated that PMO-AOCs can tolerate DAR values
of up to 8 while maintaining favorable delivery characteristics. High DAR values increase
payload delivery volume per antibody unit, and
also improve manufacturing efficiency by reducing production complexity and cost.
Endosomal Escape: The Critical Bottleneck Limiting AOC Efficacy
Successful cellular uptake does not guarantee therapeutic activity. Even after efficient
receptor-mediated internalization, AOCs must overcome one of the most challenging barriers
in intracellular delivery: endosomal escape.
Following receptor-mediated endocytosis, AOCs are encapsulated within endosomes. As
endosomes mature into lysosomes, the internal pH value continuously decreases, and the
activity of hydrolases increases. During this process, a large amount of oligonucleotides
payloads are degraded before reaching their intracellular sites of action.
Current studies suggest that the endosomal escape efficiency of AOCs is generally below 1%,
which means that over 99% of internalized oligonucleotides fail to reach the cytoplasm or
nucleus. As a result, endosomal escape is widely regarded as the primary bottleneck limiting
the therapeutic efficacy of current AOC platforms.
To address this challenge, several strategies are being actively explored.
pH-Sensitive Linker Design
These linkers exploit the acidic environment of endosomes to trigger the release of
oligonucleotides, combined with membrane-disruptive materials to facilitate endosomal
membrane penetration.
Cationic Polymer Strategy
Polyarginine and other cationic copolymers can promote the endosomal membrane
destabilization through the "proton sponge effect", enhancing the escape efficiency.
Target Receptor Optimization
Different receptors utilize distinct endocytic pathways and intracellular trafficking
routes. Identifying receptors with favorable trafficking characteristics or prolonged
endosomal residence times may improve intracellular delivery efficiency.
Oligonucleotide Chemical Modification
Modifications such as 2'-F and 2'-OMe not only enhance the serum stability but also
influence intracellular release behavior. Phosphorothioate (PS) backbone modifications
further improve resistance to nuclease-mediated degradation following release.
Improving endosomal escape efficiency will be a key turning point in the evolution of AOC
technology, transforming intracellular delivery from merely effective to highly efficient.
Consequently, this area remains one of the most active fields of investigation across both
academia and industry.
Immunogenicity: An Underappreciated Development Challenge
As complex macromolecular conjugates, AOCs carry a risk of inducing ADAs, which may affect
pharmacokinetics, pharmacodynamics, and safety.
From a pharmacokinetic perspective, ADA formation can accelerate drug clearance and reduce
systemic exposure. From a pharmacodynamic perspective, neutralizing antibodies may interfere
with target binding and diminish therapeutic efficacy. In addition, ADA responses may
increase the risk of hypersensitivity reactions, infusion-related reactions and other safety
concerns.
Current clinical data show that the overall immunogenicity profile of AOCs remains
manageable. Nevertheless, ADA incidence may increase with higher DAR values, repeated
dosing, and prolonged treatment duration. Therefore, comprehensive immunogenicity assessment
should be incorporated early in the development process.
Clinical Translation and the Expanding AOC Landscape
Avidity Biosciences: The First Clinical Validation of Muscle-Targeted RNA Delivery
Avidity Biosciences currently represents the most advanced company in the AOC field from a
clinical development perspective. Its platform is built around monoclonal antibodies
targeting TfR1, enabling the selective delivery of siRNA, ASO, and PMO payloads into
skeletal and cardiac muscles.
Importantly, this platform provides one of the first systematical clinical demonstrations
that extrahepatic RNA delivery to muscle tissue can be achieved through antibody-mediated
targeting.
The company's leading pipeline currently addresses three severe neuromuscular disorders.
Del-zota is an AOC-delivered PMO designed to induce exon 44
skipping in the dystrophin gene, thereby restoring expression of a shortened but functional
dystrophin protein in patients with DMD.
Updated EXPLORE44 data (September 2025) demonstrated robust biological activity, with
dystrophin levels reaching approximately 25% of normal, exon skipping efficiency approaching
40% and creatine kinase (CK) levels decreasing by more than 80% from baseline. Functional
assessments, including NSAA and PUL scores, also showed favorable trends compared with
natural history controls. The therapy was generally well tolerated and received FDA
Breakthrough Therapy Designation in July 2025.
Del-desiran targets DMPK mRNA to reduce toxic RNA
accumulation, the underlying driver of DM1.
Results from the MARINA Phase 1/2 trial, published in The New England Journal of Medicine in
2026, showed an average reduction of approximately 40% in DMPK mRNA levels, accompanied by
improvements in multiple clinical endpoints, including splicing abnormalities, myotonia,
muscle strength, and patient-reported outcomes. The Phase 3 HARBOR trial has completed
enrollment, with primary endpoint data expected in the second half of 2026.
FSHD is caused by abnormal activation of the DUX4 gene.
Del-brax is designed to silence DUX4 mRNA and thereby suppress
disease-driving gene expression.
The FORTITUDE biomarker cohort and FORTITUDE-3 confirmatory studies are in progress.
Competitive Landscape: Beyond Avidity
The AOC field is expanding rapidly, with multiple companies advancing differentiated
technological approaches.
Dyne Therapeutics has adopted a different approach through its FORCE™ platform, which
utilizes TfR1-targeting Fab fragment rather than
full-length antibodies. The smaller molecular size is expected to improve tissue penetration
while potentially reducing immunogenicity.
Its core pipelines include
DYNE-251 for DMD (exon 51
skipping) and
DYNE-101 for DM1. Clinical data released in
2025 showed that DYNE-251
achieved significant increases in dystrophin expression, further validating the the
feasibility of antibody-mediated muscle delivery.
Tallac Therapeutics' TRAAC™ platform utilizes
CD22-targeted
oligonucleotide conjugates to modulate immune signaling in B-cell malignancies, while Gennao
Bio is developing antibody-free targeted delivery approaches for oncology indications.
Collectively, these efforts demonstrate that the AOC landscape is rapidly evolving beyond a
single platform or company, with multiple delivery strategies competing to define the next
generation of extrahepatic RNA therapeutics.
Novartis' $12 Billion Acquisition: A Vote of Confidence in AOCs
In October 2025, Novartis announced its acquisition of Avidity Biosciences, completing the
transaction officially in February 2026. Importantly, the deal was widely viewed as a
validation of Avidity's AOC platform rather than any single pipeline asset.
The acquisition highlights the growing confidence in extrahepatic RNA delivery and
underscores the commercial potential of AOCs beyond neuromuscular diseases. It also signals
increasing competition in the field as multiple companies advance differentiated delivery
platforms.
The Next Frontier: Beyond Neuromuscular Disease
While current clinical validation has largely focused on neuromuscular diseases, the
therapeutic scope of AOCs is expanding rapidly.
Cardiovascular Diseases
Avidity's cardiovascular spinout is advancing early-stage projects targeting PLN-related
cardiomyopathy and PRKAG2 syndrome, aiming to extend AOC-mediated delivery into hereditary
cardiac diseases. Successful cardiac delivery would represent a major expansion beyond
skeletal muscle applications.
Immuno-Oncology
Tallac Therapeutics' TAC-001 represents an alternative application of the AOC concept. By
combining CD22-targeted
delivery with
TLR9 pathway modulation, the platform seeks to
activate anti-tumor immune responses while maintaining cellular specificity. Early clinical
studies have shown preliminary pharmacodynamic activity.
Central Nervous System Disorders
The central nervous system remains one of the most challenging frontiers for RNA delivery.
Overcoming the blood–brain barrier will require identification of receptors capable of both
efficient endothelial uptake and transcytosis into brain tissue. Several groups are actively
exploring whether TfR1-mediated delivery can be adapted for CNS applications.
As receptor biology, delivery engineering, and intracellular trafficking technologies
continue to advance, the potential applications of AOCs are likely to extend far beyond
their current neuromuscular focus.
Summary
AOCs combine the targeting capability of antibodies with the gene-regulatory functions of
oligonucleotides, providing a clinically validated approach to extrahepatic RNA delivery. With
encouraging clinical results and growing industry investment, AOCs have rapidly evolved from a
delivery concept into an emerging therapeutic platform. Future advances in endosomal escape, DAR
optimization, and receptor biology will further shape the development of this field.
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Overcoming the Delivery Barriers of RNA Therapeutics
Molecular Engineering of AOCs and Key Technical
Challenges
Clinical Translation and the Expanding AOC Landscape
