Are Peptide-Oligonucleotide Conjugates Regulated as Small Molecules or Biologics?

Introduction:

Peptide-oligonucleotide conjugate (POC) developers face a question that shapes every downstream decision — from IND strategy to exclusivity planning: does Peptide-Oligonucleotide Conjugates Regulation follow the small-molecule drug pathway or the biologic licensing pathway? The honest answer is that there is no universal classification. Most synthetic POCs are developed under the FDA’s drug framework, but classification ultimately depends on the molecule’s composition, manufacturing process, mechanism of action, intended use, and the specific regulatory framework of the target market. At ResolveMass Laboratories Inc., we support biotech and pharma teams with the analytical characterization data that regulatory and CMC teams need to build a defensible classification position, so this guide reflects the questions we see raised most often in early-phase POC programs.

Summary:

• There is no single universal classification for POCs — the FDA treats oligonucleotide therapeutics as a distinct modality, not a simple small-molecule-or-biologic binary.
• In the United States, synthetic oligonucleotide and peptide-oligonucleotide products are generally developed under the drug pathway (Section 505, FDCA), though certain conjugates can raise biologics-related considerations.
• In the European Union, EMA’s draft oligonucleotide guideline states that an oligonucleotide conjugated to a monoclonal antibody is a biological medicinal product — so the identity of the conjugated partner matters as much as the oligonucleotide itself.
• Classification depends on molecular composition, mechanism of action, manufacturing process, product heterogeneity, and immunogenicity risk — not molecular size alone.
• Full characterization of the complete conjugate — not the peptide or oligonucleotide in isolation — is essential for quality, safety, efficacy, and regulatory submissions.
• Early regulatory strategy matters because expectations can differ meaningfully between FDA, EMA, and Health Canada.
• ResolveMass Laboratories provides the analytical, structural, and sequence-confirmation data that support this regulatory determination.

1: What Are Peptide-Oligonucleotide Conjugates?

A peptide-oligonucleotide conjugate is a molecule in which a peptide is chemically linked to an oligonucleotide, combining nucleic-acid activity with peptide-mediated targeting or delivery. The oligonucleotide may be an antisense oligonucleotide (ASO), siRNA, or other chemically modified nucleic-acid sequence, while the peptide is often designed to improve cellular uptake, membrane penetration, tissue targeting, or intracellular delivery — including emerging applications in peptide-oligonucleotide conjugates for blood-brain barrier delivery, where the peptide’s targeting function is central to the therapeutic’s viability.

A typical conjugate can include:

• An antisense or RNA-targeting oligonucleotide
• A cell-penetrating or targeting peptide
• A chemical linker
• Modified nucleotides
• Modified amino acids or peptide residues
• Additional functional groups designed to improve stability or delivery

Conjugation creates a single molecular entity whose properties can differ significantly from those of its individual components — which is exactly why regulators expect developers to characterize the complete conjugated product rather than relying on separate data for the peptide and oligonucleotide alone.

2: Peptide-Oligonucleotide Conjugates Regulation: Small Molecule or Biologic?

There is no blanket rule. The safest regulatory conclusion is that peptide-oligonucleotide conjugates should be treated as a distinct, emerging modality requiring product-specific regulatory assessment, rather than being automatically labeled as either a small molecule or a biologic. “Synthetic” does not automatically mean “small molecule,” and the presence of a peptide does not automatically make a product a biologic.

United States: FDA Regulatory Perspective

In the U.S., oligonucleotide therapeutics are generally developed under the drug framework. FDA’s 2024 guidance on clinical pharmacology considerations for oligonucleotide therapeutics provides recommendations for products developed under Section 505 of the FDCA and 21 CFR Parts 312 and 314, and explicitly describes oligonucleotide therapeutics as a distinct modality rather than conventional small molecules or large biologics. FDA’s orphan-drug database already lists peptide-oligonucleotide conjugates in active development, including conjugates targeting specific mRNA sequences for rare diseases. For a closer look at how this plays out procedurally — pre-IND interactions, application type, and center jurisdiction — see our detailed breakdown of the FDA regulatory pathway for peptide-oligonucleotide conjugates.

Developers should not assume that every POC will automatically follow the same regulatory pathway as a prior oligonucleotide or peptide approval — each construct needs its own assessment.

European Union: EMA Perspective

The European framework introduces an additional layer of distinction based on the nature of the conjugated component. EMA’s draft guideline on the development and manufacture of oligonucleotides addresses conjugated oligonucleotides directly, stating that when an oligonucleotide is conjugated to a monoclonal antibody, the resulting product is considered a biological medicinal product, because monoclonal antibodies are manufactured using recombinant technologies and are themselves biological medicinal products.

This illustrates a key principle: the regulatory status of a conjugate can be influenced by the identity and regulatory nature of the molecule the oligonucleotide is attached to. A synthetic peptide-oligonucleotide conjugate should therefore not automatically be equated with an antibody-oligonucleotide conjugate — the two can sit in entirely different regulatory categories even though both are “conjugates.”

Health Canada and Other Jurisdictions

Health Canada also distinguishes biologic drugs within its own framework and requires evidence appropriate to the product and its risk profile. Its biologic-drug roadmap explains that biologic drugs are regulated under specific provisions and reviewed by the Biologic and Radiopharmaceutical Drugs Directorate — reinforcing that multinational POC programs need a jurisdiction-specific regulatory strategy rather than a single global assumption.

3: Why Is Peptide-Oligonucleotide Conjugates Regulation So Complex?

Regulation is complex because the final conjugate combines multiple chemically and biologically relevant attributes into a single therapeutic entity, and each attribute can independently influence regulatory expectations.

4: What Determines the Classification? Five Core Factors

Regulators typically evaluate the complete scientific and manufacturing profile of a POC rather than relying on molecular size alone.

1. Molecular composition — A conjugate built from a synthetic oligonucleotide and a synthetic peptide can be evaluated very differently from one containing a recombinant protein, antibody, or other biological component. The source and manufacturing method of each component is highly relevant to the classification outcome.

2. Mechanism of action — The oligonucleotide may bind complementary RNA, promote RNase H-mediated degradation, modulate splicing, or silence gene expression, while the peptide may function purely as a delivery vehicle or as an independently active pharmacological agent. Regulators expect the relative contribution of each component to be scientifically established, not assumed.

3. Manufacturing process — Manufacturing is one of the most important considerations because the final conjugate can contain multiple sources of process- and product-related impurities, including incomplete peptide coupling, incomplete oligonucleotide synthesis, deletion or truncated sequences, unconjugated peptide or oligonucleotide, incorrect conjugation products, linker-related impurities, oxidation products, stereochemical variants, and residual reagents. As synthesis scales move from research to GMP production, many sponsors are also evaluating automation in peptide-oligonucleotide conjugate synthesis to improve batch-to-batch consistency and reduce the very impurity classes regulators scrutinize most closely.

4. Product heterogeneity — Unlike a simple, well-defined small molecule, a conjugated nucleic-acid therapeutic can present multiple closely related molecular species. The analytical strategy must demonstrate that the manufacturing process consistently produces a product with an appropriate, reproducible quality profile.

5. Immunogenicity — Conjugation can meaningfully change the immune-response profile of an oligonucleotide therapeutic. FDA notes that chemical modifications and conjugation can influence immunogenicity considerations, with peptide sequence, peptide length, chemical modifications, conjugation chemistry, dose, route of administration, repeated dosing, and patient population all playing a role.

5: How Does the FDA Regulate Oligonucleotide Therapeutics?

The FDA has established specific clinical pharmacology considerations for oligonucleotide therapeutics rather than simply applying conventional small-molecule development principles. FDA’s final 2024 guidance covers QTc and proarrhythmic risk, immunogenicity risk assessment, hepatic impairment, renal impairment, and drug-drug interactions, reflecting the agency’s accumulated review experience with this modality.

This matters for POC developers specifically: the presence of a peptide does not eliminate the need to address the established clinical pharmacology considerations for the oligonucleotide component. At the same time, sponsors should independently assess whether the peptide introduces additional pharmacological, toxicological, immunological, or analytical considerations of its own.

6: FDA vs. EMA: A Side-by-Side Comparison

7: What Quality Attributes Must Be Characterized?

A comprehensive analytical control strategy must characterize both the oligonucleotide and peptide components, as well as the integrity of the final conjugate as a whole — this is the core subject of our detailed guide on the analytical characterization of peptide-oligonucleotide conjugates.

• Identity — confirmation of oligonucleotide sequence, peptide sequence, conjugation site, linker identity, and overall molecular identity. Rigorous peptide-oligonucleotide conjugate sequence confirmation strategies are foundational here, since sequence-level evidence underpins nearly every other quality claim in a regulatory submission.
• Purity — differentiating full-length conjugate from truncated sequences, deletion products, unconjugated peptide or oligonucleotide, process-related impurities, and degradation products.
• Molecular mass — mass spectrometry provides evidence for molecular weight confirmation, conjugation confirmation, sequence-related impurities, and structural characterization.
• Chromatographic profile — HPLC, UHPLC, LC-MS, ion-exchange chromatography, reverse-phase chromatography, and size-based separation techniques, depending on the product.
• Structural characterization — additional orthogonal techniques are often required to establish full molecular structure and integrity, including NMR-based structural elucidation for confirming linker connectivity, stereochemistry, and conjugation geometry that mass spectrometry alone cannot fully resolve.
• Potency — the potency assay must be scientifically linked to the intended mechanism of action, using validated biochemical, cellular, or other functional assays.

8: Key CMC Challenges for Peptide-Oligonucleotide Conjugates

The primary CMC challenge is demonstrating consistent control of a structurally complex conjugated product throughout manufacturing and storage:

• Sequence confirmation — both peptide and oligonucleotide sequences require appropriate, orthogonal characterization.
• Conjugation efficiency — the manufacturing process must consistently achieve the intended conjugation yield.
• Impurity control — unconjugated components and incomplete conjugation products require suitable analytical methods to detect and quantify.
• Linker stability — the linker must remain appropriate under manufacturing, storage, and physiological conditions; sponsors increasingly invest in optimizing linker hydrolytic stability early, since a linker that degrades prematurely can undermine both efficacy data and the impurity profile reviewers scrutinize.
• Batch-to-batch consistency — critical quality attributes must stay within scientifically justified specifications across production runs.
• Stability — the complete conjugate must be evaluated for chemical and physical degradation over its intended shelf life.
• Analytical method development — conventional methods may not adequately resolve all closely related molecular species, often requiring bespoke method development.

EMA’s oligonucleotide guidance specifically addresses conjugation-related manufacturing and analytical considerations, including documentation and batch/stability data requirements for conjugation components and linkers.

9: How Does Conjugation Affect Pharmacokinetics and Safety?

Conjugation can substantially alter the absorption, distribution, metabolism, elimination, and intracellular behavior of an oligonucleotide therapeutic. The peptide component may affect:

• Cellular uptake
• Tissue distribution
• Plasma protein binding
• Proteolytic degradation
• Renal clearance
• Endosomal trafficking
• Intracellular release
• Target tissue exposure

Pharmacokinetic and pharmacodynamic studies should therefore consider the behavior of the complete conjugate, as well as relevant released or metabolized species where scientifically justified. FDA emphasizes that oligonucleotide therapeutics can have PK/PD characteristics that differ meaningfully from conventional small-molecule drugs — another reason product-specific study design matters more than defaulting to a standard small-molecule protocol.

10: Building a Regulatory Strategy for Your POC Program

Developers should establish their regulatory strategy early and align analytical, CMC, pharmacology, toxicology, and clinical development plans with the expected pathway.

1. Define the product — document the complete molecular structure, peptide sequence, oligonucleotide sequence, linker, chemical modifications, conjugation site, and mechanism of action.
2. Establish a target product profile — define intended indication, route of administration, dose, target population, desired pharmacological effect, and critical quality attributes.
3. Build a CMC strategy — address raw materials, synthesis, conjugation, purification, process controls, analytical methods, specifications, stability, and packaging.
4. Establish analytical control — deploy a fit-for-purpose, orthogonal analytical platform: LC-MS, HPLC/UHPLC, mass spectrometry, spectroscopic techniques, peptide mapping, oligonucleotide sequence analysis, chromatographic impurity profiling, and functional potency assays.
5. Seek regulatory advice early — because POCs are an emerging modality, early interaction with the relevant authority can resolve open questions on classification, CMC expectations, nonclinical studies, clinical pharmacology, immunogenicity, potency, and comparability before they become late-stage obstacles.

11: How ResolveMass Laboratories Supports POC Characterization and Regulatory Readiness

Classification arguments are only as strong as the analytical data behind them. Reviewers expect sponsors to demonstrate — with orthogonal, validated methods — exactly what their conjugate is: confirmed sequence identity for both the peptide and oligonucleotide, conjugation site and efficiency, linker integrity, purity, and a well-documented manufacturing route.

ResolveMass Laboratories Inc. works with biotech and pharma sponsors to generate this evidence base, including:

• High-resolution LC-MS/MS characterization and peptide-oligonucleotide conjugate sequence confirmation strategies for both the peptide and oligonucleotide domains.
• Conjugation site and stoichiometry confirmation, supported by NMR-based structural elucidation where mass spectrometry alone cannot resolve linker geometry or stereochemistry.
• Linker stability assessment, including work on optimizing linker hydrolytic stability to support degradation and impurity control strategies.
• Purity, impurity, and degradation profiling to support CMC sections of IND and NDA submissions, including evaluation of manufacturing approaches such as automation in peptide-oligonucleotide conjugate synthesis.
• Method development and validation support for sponsors preparing pre-IND or Type B meeting briefing packages where classification is a discussion point — grounded in our broader work on the analytical characterization of peptide-oligonucleotide conjugates and the FDA regulatory pathway for peptide-oligonucleotide conjugates.

Our team has supported multiple oligonucleotide- and peptide-conjugate programs through exactly this kind of characterization work — including delivery-focused constructs such as those explored for blood-brain barrier delivery — which is why sponsors bring us in early, before the regulatory strategy is locked in, rather than after a classification challenge from the agency.

Conclusion:

Peptide-Oligonucleotide Conjugates Regulation cannot be reduced to a simple small-molecule-versus-biologic decision. The appropriate pathway depends on the complete molecular construct, manufacturing process, mechanism of action, biological characteristics, and jurisdiction — with the FDA treating oligonucleotide therapeutics as a distinct modality and EMA drawing a clear line around antibody-conjugated oligonucleotides as biological medicinal products. For developers, the priorities are consistent regardless of jurisdiction: define the complete molecular structure, understand the regulatory status of every component, build a robust CMC strategy, characterize the complete conjugate with orthogonal methods, control conjugation-related impurities, evaluate pharmacokinetics and immunogenicity, and obtain early regulatory feedback wherever classification is uncertain.

Frequently Asked Questions:

Reference

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• Turner JJ, Jones S, Fabani MM, Ivanova G, Arzumanov AA, Gait MJ. RNA targeting with peptide conjugates of oligonucleotides, siRNA and PNA. Blood Cells, Molecules, and Diseases. 2007 Jan 1;38(1):1-7.https://www.sciencedirect.com/science/article/pii/S1079979606002038
• Manoharan M. Oligonucleotide conjugates as potential antisense drugs with improved uptake, biodistribution, targeted delivery, and mechanism of action. Antisense and Nucleic Acid Drug Development. 2002 Apr 1;12(2):103-28.https://journals.sagepub.com/doi/abs/10.1089/108729002760070849
• Thakur S, Sinhari A, Jain P, Jadhav HR. A perspective on oligonucleotide therapy: Approaches to patient customization. Frontiers in pharmacology. 2022 Oct 19;13:1006304.https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2022.1006304/full

About the Author

Priyal Darji