Key takeaways
- An AOC is not an ADC with a different payload – the oligonucleotide’s chemistry changes the analytical questions
- UV-Vis and SEC stay in the process, but IEX, CE, and HRMS become the analytical backbone for AOC characterization
- Oligonucleotide input quality sets the floor for every downstream impurity profile – characterize before you conjugate
- Conjugation yield alone doesn’t predict a viable conjugate and linker stability should be measured directly
- Defining CQAs early is what makes analytical work usable for regulatory submission later
On paper, an antibody-oligonucleotide conjugate (AOC) may look like antibody-drug conjugate (ADC) with a different payload – antibody, linker, cargo – but its chemistry sets it apart. AOCs combine antibody-mediated targeting with an oligonucleotide designed to modulate gene expression (1). The oligonucleotide carries charge, sequence-related impurities, secondary structure, enzymatic vulnerability, and conjugation-specific failure modes that simply don’t appear in a small-molecule warhead. So, the analytical question changes, too.
Why do AOCs need a dedicated analytical strategy?
AOCs need a dedicated analytical strategy because each of their three components – antibody, oligonucleotide, and linker – generates its own critical quality attributes (CQAs).
The antibody brings binding specificity, Fc considerations, and biologic manufacturability constraints. The oligonucleotide adds sequence specificity, backbone chemistry, charge, molecular weight, and a degradation risk profile. The linker sits between them and isn’t passive: it can shift binding, payload integrity, stability, and impurity formation.
Define those CQAs early and match each to a method that can resolve it specifically enough to drive construct selection, process development, and regulatory justification later. Late-stage method development is where small ambiguities become large delays.
How does oligonucleotide chemistry increase analytical complexity?
It adds two complications that don’t exist for ADCs, such as chemistry-induced product species and physicochemical incompatibility between the oligonucleotide and antibody. The oligonucleotide’s high negative charge density and propensity to form non-covalent complexes can alter charge variant behavior and intact mass profiles, meaning ion-exchange chromatography (IEX) and high-resolution mass spectrometry (HRMS) methods often need to be tailored rather than transferred directly from ADC workflows.
What do phosphorothioate modifications add to the impurity profile?
They add diastereomers. Phosphorothioate (PS) bonds resist nuclease cleavage and improve stability, but each PS linkage creates a chiral center, which generates diastereomers that can differ in biological activity, enzymatic resistance, and toxicity (2, 3). Confirming the sequence does not amount to confirming the molecule – two oligonucleotides with identical sequences and different diastereomer ratios can behave differently in vivo.
Why does secondary structure matter for analytics?
Because oligonucleotides fold, and folded oligonucleotides don’t always work as designed. Hairpins and intramolecular folds form under specific buffer conditions and can blunt activity even when the sequence is intact. Formulation, sequence design, and analytics need to be developed together, rather than these in series. A formulation that protects the antibody while allowing oligonucleotide self-interaction will still produce a conjugate that underperforms.
Are standard ADC analytical methods enough for AOCs?
No – they’re necessary but not sufficient. UV-Vis spectroscopy and size-exclusion chromatography (SEC) are useful for concentration estimates, aggregation, and broad profiling, but neither resolves the oligonucleotide-specific questions that decide whether an AOC is well characterized.
Which advanced analytical technologies are needed?
Three methods are valuable here:
- IEX resolves charge-based heterogeneity
- Capillary electrophoresis (CE) gives high-resolution separation by size and charge
- HRMS, especially when coupled to IEX or SEC under native MS conditions, confirms identity, assigns impurities, and helps assess full AOC assembly, including drug-to-oligonucleotide species and non-covalent complex formation.
Used together, they pull apart charge variants, sequence impurities, conjugation heterogeneity, and product-related species that any single method could miss. SEC still has a place here, but as part of a panel, not as the decision-maker. A clean SEC profile says nothing about diastereomer control, co-eluting oligo impurities, or whether the conjugate carries the intended payload distribution. Single-method approaches are how programs end up rerunning their entire characterization process at IND.
How does impurity profiling guide AOC development?
Impurity profiling determines whether the oligonucleotide input, conjugation reaction, and purification process can produce a controllable product, and where each one fails.
Solid-phase oligonucleotide synthesis generates a predictable mess: truncated sequences, missed incorporations, incomplete deprotection products, diastereomers, and self-hybridized species. Some co-elute with the intended product and that co-elution sets the floor for everything downstream, and so if impurities move into the conjugation reaction, they move into the conjugate pool, and they make every subsequent purification harder.
What do orthogonal methods reveal that single assays hide?
Heterogeneity, mostly. When IEX, CE, and HRMS agree, confidence rises. When they disagree, that’s also a useful signal – disagreement reveals heterogeneity a single method would have hidden. This matters at the regulatory layer too. Regulatory agencies are increasingly scrutinizing oligonucleotide impurities that may affect safety or function, particularly diastereomeric and co-eluting species (4–6). Methods need to support both internal decisions and the regulatory file built on them.
What does conjugation analytics actually need to measure?
Whether the reaction produced the intended oligonucleotide attached at the target conjugation site, and whether it did so without compromising antibody binding, oligonucleotide integrity, or stability.
Why isn’t conjugation efficiency enough on its own?
Because a high yield can still produce a weak candidate. If the linker’s unstable, if aggregation rose, if the antibody’s binding profile shifted, or if the oligo lost functional integrity at the conjugation site, the yield number is misleading. Yield is necessary but yield alone isn’t sufficient.
The analytical process has to answer a connected set of questions in parallel: did the intended conjugate form, is the species distribution consistent, did aggregation increase, does the antibody still hit its target, and does the oligonucleotide remain intact?
How does linker choice change the analytical risk profile?
It changes what has to be measured and when(7). Cleavable linkers are designed to release the oligonucleotide under defined biological conditions, such as enzymatic cleavage after cellular internalization. Non-cleavable linkers are designed for greater plasma stability and rely on downstream intracellular processing, such as lysosomal degradation of the antibody, releasing the oligonucleotide with residual linker or amino-acid fragments attached. Linker length and chemistry govern stability and activity; the negatively charged oligonucleotide and bulky antibody add electrostatic and steric constraints that have to be measured rather than predicted.
What’s the issue with maleimide-thiol chemistry in AOCs?
It works – with caveats that need analytical support. Maleimide-thiol, the workhorse of antibody conjugation, has been applied to AOC formats. But thiosuccinimide linkages can undergo retro-Michael exchange or related deconjugation pathways depending on linker structure and conjugation site (8). Hydrolysis of the succinimide ring can also stabilize some maleimide-thiol conjugates, so the same chemistry can succeed or fail depending on construct context (9). Stability has to be measured, not inferred from conjugation yield. Alternative disulfide-rebridging approaches, such as ThioBridge™, are worth evaluating where a more controlled antibody conjugation strategy is needed, but the benefit should be tested for the specific AOC construct.
How should AOC purification and formulation decisions be made?
They should be made against product quality, material recovery, stability, and function – together.
What trade-offs come with additional purification steps?
Yield is the primary one. Oligonucleotide-related species behave differently from small-molecule payload impurities, so additional steps – including anion-exchange chromatography (AEX) – may be needed, and each step costs material. The question is whether the process produces the required quality at a scale compatible with manufacturing.
A method that works at small scale, but bleeds material is a manufacturing problem waiting to happen. A method that preserves yield but leaves impurity peaks unresolved will raise regulatory concerns. Purification development needs to track impurity clearance, aggregation, conjugate integrity, residual unconjugated components, and recovery at every step. These are trade-offs that cannot be ignored.
What should AOC stability studies track?
Degradation, aggregation, conjugate integrity, oligonucleotide structure, and biological activity over time. The formulation has to preserve antibody and oligonucleotide simultaneously – two molecules with different vulnerabilities. Oligonucleotides degrade under nuclease activity and require structural integrity through storage and delivery, while antibodies need to stay non-aggregated and target competent.
That means evaluating pH, ionic strength, excipients, stabilizing agents, storage conditions, and degradation risk. Formulations may include chelators such as EDTA, surfactants, cryoprotectants, buffers, and tonicity modifiers. Long-term studies confirm activity is retained across the intended shelf life. When stability data show degradation, aggregation, or activity loss, the cause is usually upstream, such as linker chemistry, conjugation conditions, purification, or oligonucleotide design. Formulation is a poor place to fix a chemistry problem and analytical data should point back to the source.
How do analytical datasets support regulatory readiness?
By connecting purity, stability, safety, manufacturability, and function across the AOC in a single coherent dataset.
Regulatory expectations for AOCs are still developing because they combine features of biologics, oligonucleotide therapeutics, and conjugated products. Recent oligonucleotide guidance offers reference points, but AOC-specific expectations still need to be justified through product-specific analytical, stability, safety, and manufacturing data (4–6).
Why define critical quality attributes be defined early?
Because doing so converts complex analytical findings into development decisions. Target product profiles (TPPs) organize those decisions (purity, stability, efficacy, safety, manufacturability) so that analytical strategy aligns with development goals from the start, so CQAs then become operational decision points.
Analytical and biological data must be read together. Cell-based assays, in vitro models, and in vivo studies generate safety-relevant signals – but those signals carry more weight when interpreted alongside impurity and stability profiles. Disconnecting them creates ambiguity that costs months later.
How does Abzena support analytics-led AOC development?
By characterizing oligonucleotide inputs supplied by clients or specialist oligonucleotide manufacturers, then connecting those data to conjugation, purification, formulation, stability, and regulatory decisions. The aim is that analytical findings inform process changes and decisions.
Our AOC capabilities include the advanced analytics this work demands: IEX, CE, and HRMS for input characterization and impurity assignment. We support conjugation process development through linker selection, conjugation chemistry, reaction conditions, conjugation efficiency, stability, and antibody binding assessment. Where needed, analytical findings are interpreted alongside bioassay, binding, immunogenicity, and safety data so that impurity and stability profiles link to biological relevance. Regulatory support covers requirements, documentation, and responses to agency queries.
What that produces, across a program, is a dataset that can carry construct selection, impurity control, stability, manufacturability, and regulatory decision-making – and reads consistently across the antibody, the oligonucleotide, and the chemistry that joins them. AOC development is complex and advanced analytics make it interpretable. That’s the difference between a program that moves forward and one that stalls.
FAQs
How are AOCs analytically different from ADCs?
The oligonucleotide payload introduces charge-based heterogeneity, sequence-related impurities (truncations, deletions, diastereomers), secondary-structure behavior, and enzymatic-degradation vulnerability – none of which appear with small-molecule payloads. Methods routine for ADCs (UV-Vis, SEC) stay useful for AOCs but are not sufficient on their own.
Which analytical methods are most useful for AOC characterization?
For oligonucleotide-specific characterization, the working trio is IEX (charge variants), CE (size and charge species), and HRMS (identity and impurity assignment). SEC, UV-Vis, and bioassays support the overall picture but rarely answer the decisive questions in isolation.
What’s the role of CQAs in AOC development?
CQAs convert analytical observations into go/no-go decisions. Defined early, alongside a TPP, they align analytics with construct selection, impurity control, stability strategy, and regulatory submission, rather than treating each as a separate workstream.
Abzena Featured Expert Contributors
-Nicolas Camper, VP of ADCs & Bioconjugates
-Fabio Rossi, Director of Analytics
References
- C.-H. Yu, S. Y. Y. Ha, Z. An, K. Tsuchikama, W. Li, Antibody-oligonucleotide conjugates: an emerging modality for precision RNA therapeutics. Antib. Ther., tbag022 (2026).
- S. T. Crooke, T. A. Vickers, X. Liang, Phosphorothioate modified oligonucleotide–protein interactions. Nucleic Acids Res. 48, 5235–5253 (2020).
- S. T. Crooke, P. P. Seth, T. A. Vickers, X. Liang, The Interaction of Phosphorothioate-Containing RNA Targeted Drugs with Proteins Is a Critical Determinant of the Therapeutic Effects of These Agents. J. Am. Chem. Soc. 142, 14754–14771 (2020).
- Development and manufacture of oligonucleotides, European Medicines Agency (EMA). https://www.ema.europa.eu/en/development-manufacture-oligonucleotides-scientific-guideline.
- Clinical Pharmacology Considerations for the Development of Oligonucleotide Therapeutics, Food and Drug Administration (FDA). https://www.fda.gov/regulatory-information/search-fda-guidance-documents/clinical-pharmacology-considerations-development-oligonucleotide-therapeutics.
- Nonclinical Safety Assessment of Oligonucleotide-Based Therapeutics, Food and Drug Administration (FDA). https://www.fda.gov/media/183496/download.
- M. Cochran, I. Marks, T. Albin, D. Arias, P. Kovach, B. Darimont, H. Huang, U. Etxaniz, H. W. Kwon, Y. Shi, M. Diaz, O. Tyaglo, A. Levin, V. R. Doppalapudi, Structure–Activity Relationship of Antibody–Oligonucleotide Conjugates: Evaluating Bioconjugation Strategies for Antibody–Phosphorodiamidate Morpholino Oligomer Conjugates for Drug Development. J. Med. Chem. 67, 14868–14884 (2024).
- Y. Wang, F. Xie, L. Liu, X. Xu, S. Fan, W. Zhong, X. Zhou, Development of applicable thiol-linked antibody–drug conjugates with improved stability and therapeutic index. Drug Delivery 29, 754–766 (2022).
- M. Lahnsteiner, A. Kastner, J. Mayr, A. Roller, B. K. Keppler, C. R. Kowol, Improving the Stability of Maleimide–Thiol Conjugation for Drug Targeting. Chem. A Eur. J. 26, 15867–15870 (2020).
