The high specificity of monoclonal antibodies has been extensively leveraged in developing therapeutics with different mechanisms of action and addressing a range of pathologies, from infectious diseases to cancer, autoimmune diseases, and more. Through neutralizing, agonistic, and inhibitory mechanisms, antibodies have proven to be potent biologics. Moreover, beyond their intrinsic therapeutic activity, antibodies have emerged as powerful targeting vehicles that enhance the efficacy of otherwise promising therapeutics by enabling improved and more selective delivery to disease-relevant targets.
Among antibody-based next generation therapeutics, antibody drug conjugates (ADCs) are a well-validated and established modality with over seventeen FDA approvals since 2020. Following up on the clinical success of ADCs, the field is increasingly leveraging the targeting strengths of antibodies for the efficient delivery of oligonucleotides.
Antibody-oligonucleotide conjugates (AOC) open the door to a whole new therapeutic modality, one where antibodies can improve the efficiency of gene modulation beyond the liver. While AOC development remains an emerging therapeutic area, with no FDA-approved therapies to date and a relatively limited clinical pipeline, the technology’s potential to unlock the full promise of gene therapies based on small interfering RNA (siRNA) and antisense oligonucleotide (ASO) is fueling substantial growth and innovation in the field.
How are AOCs different from ADCs?
The rapid advancement of ADCs has established a robust framework for targeted therapeutic delivery, providing valuable lessons in antibody selection, conjugation chemistry, pharmacokinetics, and manufacturability. AOC developers are now leveraging many of these foundational principles. However, AOCs are not simply ADCs with a nucleic acid payload. Oligonucleotides introduce physicochemical and biological properties that fundamentally differ from those of small molecule payloads.
These distinctions extend beyond the payload itself and influence virtually every aspect of therapeutic design. In particular, the differences between AOCs and ADCs can be understood through three key dimensions: mechanism of action, payload biology, and the requirements for productive intracellular delivery.
Mechanism of Action
Although AOCs and ADCs share a similar antibody-linker-payload architecture, their therapeutic objectives are fundamentally different.
ADC payloads are highly potent; once internalized and released, these cytotoxic drugs diffuse and reach their intended targets, interfering with essential cellular processes such as microtubule assembly or DNA replication, ultimately triggering cell death. Therefore, selection of targets that ensure specific payload delivery to disease tissues and cells is prioritized in ADC development to reduce off-target toxic effects [1].
In contrast, oligonucleotide payloads are not leveraged for cytotoxicity but rather to modulate gene expression by inhibiting protein production, degrading RNA, or modifying splicing. Rather than killing cells, AOCs aim to selectively alter cellular function by regulating the expression of disease-associated genes [1].
Payload Biochemistry
The distinct mechanisms of action of ADCs and AOCs are closely linked to the fundamentally different biochemical properties of their payloads.
Unlike the typical small, membrane-permeable payloads in ADCs, oligonucleotides are large, highly charged macromolecules that cannot readily cross cell membranes. These fundamental biophysical differences reshape how antibodies, linkers, and conjugation strategies must be selected and optimized for AOC development. Moreover, oligonucleotide payloads must maintain structural integrity throughout systemic circulation, cellular uptake, and intracellular trafficking before reaching their RNA targets.
As a result, the considerations governing payload delivery, biodistribution, and intracellular fate differ substantially between AOCs and ADCs.
Cytotoxic Delivery vs. Productive Delivery
These payload-specific differences ultimately shift the primary challenge facing each modality.
For ADCs, therapeutic success largely depends on selectively delivering a highly potent cytotoxic payload to diseased cells while minimizing exposure to healthy tissues. Once intracellular release occurs, the payload can often exert its activity without overcoming the same biological barriers faced by nucleic acid therapeutics.
For AOCs, however, the primary challenge is often not avoiding payload toxicity but achieving efficient intracellular delivery of a biologically active oligonucleotide.
To exert their therapeutic effect, ASOs and siRNAs must remain intact throughout cellular uptake and successfully navigate multiple intracellular barriers before reaching their target RNA. Consequently, factors such as receptor-mediated uptake, intracellular trafficking, endosomal escape, and productive payload release are central determinants of AOC therapeutic performance and hence prioritized in target selection [1].
Key Development Challenges Distinguishing AOCs from ADCs
| Development Challenges | AOC | ADC |
|---|---|---|
| Target | The ideal target must not only be tissue-associated but also support efficient receptor-mediated uptake, intracellular trafficking, and productive delivery of the oligonucleotide cargo. Receptors that function as effective cellular “delivery gateways” may be prioritized even if they are not perfectly tissue-specific. | Target selection is heavily driven by differential antigen expression between diseased and healthy tissues to maximize efficacy while minimizing exposure to potent cytotoxic payloads. |
| Intracellular Delivery | Inefficient endosomal escape remains a major intracellular barrier and may limit therapeutic activity even after successful cellular uptake. Productive delivery, rather than internalization alone, is often the critical determinant of efficacy. | Internalization is important, but once the cytotoxic payload is released intracellularly, highly potent payloads can often achieve activity without overcoming the same trafficking barriers faced by oligonucleotides. |
| Safety | Off-target uptake may reduce efficacy and alter biodistribution, although oligonucleotide payloads are generally less intrinsically cytotoxic than ADC payloads. | Risk of off-target toxicity due to premature release of cytotoxic payloads, target expression on healthy tissues, or bystander killing in neighboring cells. |
| Payload | Oligonucleotide-to-antibody ratio (OAR) can influence binding, pharmacokinetics, tissue distribution, and overall conjugate stability. | High drug-antibody ratios (DAR) can negatively affect pharmacokinetics, increase aggregation, and accelerate systemic clearance. |
| Biophysical Properties | Large, highly charged oligonucleotide payloads may alter antibody surface charge, increase self-association, affect biodistribution, and complicate formulation development. | Hydrophobic payloads can increase aggregation, nonspecific interactions, and formulation challenges, particularly at elevated DAR values. |
| Analytical | The hybrid protein-polyanion nature of AOCs requires integrated characterization of antibody integrity, oligonucleotide integrity, conjugation level, linker stability, and functional potency. | Characterization focuses primarily on DAR distribution, free payload, linker stability, aggregation, and product heterogeneity. |
| Resistance Mechanisms | Therapeutic activity may be reduced by impaired uptake, altered intracellular trafficking, insufficient endosomal escape, or compensatory cellular responses that restore gene expression. | Resistance can arise through antigen loss, reduced internalization, altered lysosomal processing, or multidrug resistance mechanisms such as efflux pumps. |
| Manufacturing | Conjugation of large nucleic acid payloads introduces additional challenges related to stability, scalability, purification, and formulation. | Manufacturing workflows are comparatively mature, although linker stability, DAR control, and payload-related aggregation remain important challenges. |
| Clinical & Regulatory Maturity | Limited clinical experience and less established regulatory precedent compared with ADCs. | Well-established development, manufacturing, and regulatory pathways supported by multiple approved therapeutics. |
Table References: [1-4]
Antibody Properties That Determine AOC Performance: What Makes an Antibody Ideal for Oligonucleotide Delivery?
Antibody Format
Antibody format selection is a critical consideration in AOC design because the antibody must function not only as a targeting agent but also as a delivery vehicle for a large nucleic acid payload. Unlike the small-molecule drugs in ADCs, oligonucleotides are highly charged macromolecules with limited membrane permeability and complex intracellular delivery requirements. Consequently, the antibody format must support favorable pharmacokinetics, efficient receptor engagement, intracellular trafficking, and sufficient payload delivery while retaining manufacturability and stability following conjugation.
AOCs generally benefit from formats that preserve the long serum half-life and tissue exposure associated with full-length antibodies. Specifically, full-length IgG and Fc-containing formats benefit from FcRn-mediated recycling, which prolongs circulation time and increases opportunities for receptor-mediated uptake and productive oligonucleotide delivery [1,2]. Beyond their greater stability in circulation, full-length antibodies also provide more flexibility for conjugation site selection, including site-specific conjugation.
Smaller antibody formats such as VHHs, scFvs, and Fabs may offer enhanced tissue penetration compared with full-length IgGs [5]. However, their reduced size can limit conjugation options and may present greater developability challenges. This is because the steric bulk and high negative charge of oligonucleotide payloads can exert a proportionally larger influence on AOC biophysical properties and stability. In turn, this reduced stability can increase the risk of aggregation or degradation, potentially elevating immunogenicity risk and complicating clinical development [2].
Compact formats lacking Fc-fragments generally require additional engineering strategies, such as half-life extension technologies (e.g., PEGylation, albumin-binding, or Fc fusion), to achieve adequate pharmacokinetic performance [2].
Lastly, in addition to scaffold selection, antibody origin can significantly influence AOC developability and clinical value. Non-human antibody sequences may increase the likelihood of anti-drug immune responses which can result in drug clearance and neutralization of therapeutic effects. Although antibody humanization is commonly used to reduce this risk, the process requires extensive sequence engineering and may introduce liabilities affecting stability, manufacturability, or binding properties [2].
Fully human antibodies generated from transgenic platforms (e.g., Harbour Mice®) eliminate the need for subsequent humanization while providing therapeutically diverse and developable binders with potentially lower immunogenicity risk. These properties may be especially valuable for AOCs that require repeated dosing to sustain therapeutic activity.
Target Specificity
Many of the target-selection criteria established for ADCs remain relevant to AOCs, including antigen abundance, tissue-restricted expression, tissue accessibility, and internalization kinetics. However, the unique delivery needs of oligonucleotide payloads impose additional constraints. For AOCs, therapeutic activity depends on successful intracellular trafficking and access to RNA targets, making productive delivery a key criterion for target selection. Therefore, the most suitable AOC target may not be the most tissue-restricted antigen but rather the receptor that most effectively supports cellular uptake, recycling, endosomal escape, and delivery of the oligonucleotide cargo to the cytosol or nucleus [2].
For example, receptors that undergo recycling are attractive targets because they return to the cell surface after internalization and can engage additional AOC molecules. This repeated use of the same receptor enhances uptake capacity while preserving cell-surface receptor levels, supporting sustained delivery over multiple dosing cycles. TfR1 is a notable example of a receptor that continually undergoes endocytosis and recycling and has become the main target being leveraged for AOC therapeutics in clinical evaluation [5].
Beyond target selection, epitope specificity is also critical to AOC design because antibody binding must preserve the receptor’s ability to support intracellular delivery. Antibodies that interrupt native ligand binding, receptor recycling, or intracellular trafficking may compromise oligonucleotide transport, even when target engagement is maintained. Consequently, antibodies targeting epitopes that minimize disruption of normal receptor biological function are often preferred [3].
In this regard, single-domain antibodies, such as those derived from camelids, are well known for their ability to recognize recessed, sterically restricted, or otherwise challenging epitopes that may be less accessible to conventional IgGs. Similarly, fully human HCAbs (e.g., Harbour Mice®) may offer advantages by enabling access to a broader epitope landscape. In the context of AOCs, this expanded epitope diversity could facilitate targeting binding sites that preserve receptor function, support productive intracellular trafficking, and ultimately improve oligonucleotide delivery.
Antibody Affinity
Antibody affinity is a critical parameter that must be carefully optimized for both ADCs and AOCs; however, the underlying objective is different for each modality. For ADCs, antibody affinity tuning is required to ensure desired tissue selectivity and tumor penetration. Binders with too high affinity can more readily interact with targets in healthy tissues leading to toxicities and increase trapping near blood vessels reducing tumor penetration. Therefore, affinity optimization serves to ensure both ADC safety and efficacy.
In contrast, for AOCs, affinity optimization is required to ensure efficient antibody-receptor interactions and delivery. Although high-affinity binding can enhance receptor engagement, therapeutic efficacy depends on effective intracellular delivery rather than target occupancy alone. For example, studies of TfR1-targeting delivery systems have demonstrated that increasing antibody receptor affinity negatively impacts delivery by promoting peripheral clearance and target-mediated receptor degradation.
Smith et al. systematically showed that affinity, valency, and epitope collectively influence transcytosis, receptor degradation, pharmacokinetics, and safety, highlighting that optimal delivery is achieved through careful tuning rather than maximal receptor binding [6]. More recently, Sela and colleagues at Roche reported similar conclusions for BBB shuttle-antisense oligonucleotide conjugates, demonstrating that lower-affinity TfR1 engagement, particularly when combined with complementary receptor pathways such as CD98hc, can improve transport characteristics by reducing intracellular retention and lysosomal degradation, ultimately supporting more productive delivery of oligonucleotide cargo [7].
Together, these studies support the principle that optimal therapeutic activity depends on fine-tuned antibody-receptor interactions rather than maximizing binding affinity alone.
Fully human HCAb-derived VH domains offer a versatile engineering platform ideally suited to this challenge. Because VH domains function as autonomous antigen-binding units, they enable rapid affinity optimization and facilitate the generation of molecules spanning a range of binding strengths. Furthermore, the modular nature of HCAb platforms enables the straightforward construction of multivalent and multispecific architectures, providing additional opportunities to modulate tissue distribution, receptor trafficking, and intracellular delivery.
As AOC development increasingly shifts from target binding toward engineering productive delivery pathways, this flexibility may become a key advantage for designing next-generation oligonucleotide therapeutics.
How Nona Biosciences Supports Advanced Conjugated Therapeutics
Although AOCs introduce unique delivery challenges compared with ADCs, both modalities rely on antibodies that combine precise target engagement with favorable developability and manufacturability characteristics.
Through its Harbour Mice® platform, Nona Biosciences provides access to fully human IgGs, HCAbs, and single VH domains that can support the design of next-generation conjugated therapeutics. These diverse antibody formats offer opportunities for target-specific delivery, multispecific engineering, and the exploration of novel receptor biology across both ADC and AOC development programs.
As targeted delivery technologies continue to evolve, clinically validated, flexible antibody discovery platforms will play an increasingly important role in enabling innovative therapeutic designs.
Related Resources
To explore next-generation CAR design and delivery strategies in more detail:
- View our Infographic on Single Domain Binders for a condensed, visual overview of how fully human VH domains function as modular building blocks across advanced biotherapeutic modalities.
- Read our White Paper on Fully Human HCAbs and VH domains for a deeper discussion on why HCAb-derived VH formats serve as foundational building blocks for next-generation therapeutic modalities
