Next-Generation ADC Payloads: Beyond MMAE and DM1

Antibody-drug conjugates (ADCs) have moved well beyond their first-generation payloads, and the choice of cytotoxic warhead now shapes every downstream decision from linker chemistry to clinical indication. This FAQ addresses the most common questions from R&D scientists and BD professionals evaluating next-generation ADC payload strategies, with a focus on what the science supports and where the field is heading.

What are ADC payloads, and why does payload selection matter so much?

An ADC payload is the cytotoxic small molecule delivered by the antibody to the target cell, and its mechanism of action, potency, and physicochemical properties are major drivers of the overall ADC therapeutic index. Payload selection is not an isolated chemistry decision: it directly affects linker design, conjugation site, drug-to-antibody ratio (DAR), bystander killing potential, and ultimately the clinical safety profile.

MMAE (monomethyl auristatin E) and DM1 (emtansine) became the dominant first-generation payloads because they were well-characterized and compatible with early conjugation platforms, but their limitations, including narrow therapeutic windows and resistance mechanisms, have driven the field toward structurally and mechanistically diverse alternatives.

What are the main classes of next-generation ADC payloads beyond MMAE and DM1?

Next-generation ADC payloads span at least four mechanistic classes: topoisomerase I inhibitors, DNA-damaging agents, RNA polymerase inhibitors, and immune-stimulating payloads. Topoisomerase I inhibitors, particularly DXd (deruxtecan) and SN-38, have demonstrated broad activity across tumor types with favorable bystander killing due to their membrane permeability. DNA-damaging agents such as pyrrolobenzodiazepine (PBD) dimers and duocarmycins offer extreme potency at sub-nanomolar concentrations, making them candidates for low-antigen-density targets.

RNA polymerase inhibitors like amatoxins represent an emerging class with a distinct mechanism that avoids cross-resistance with tubulin-targeting payloads. Immune-stimulating payloads, including TLR agonists and STING agonists, are designed not to kill the target cell directly but to remodel the tumor microenvironment, a fundamentally different therapeutic logic from cytotoxic warheads.

What is the difference between a bystander-killing payload and a non-bystander payload, and when does it matter?

Bystander-killing payloads are membrane-permeable after release, allowing them to diffuse into and kill neighboring antigen-negative tumor cells, while non-bystander payloads remain confined to the cell that internalizes the ADC. This distinction is clinically significant because solid tumors frequently display heterogeneous antigen expression: not every cell in a tumor mass expresses the target at sufficient density for direct ADC uptake. Bystander-active payloads such as DXd and MMAE can compensate for this heterogeneity, making them better suited to antigen-heterogeneous solid tumors.

Non-bystander payloads are preferable when the target antigen is expressed on normal tissues adjacent to the tumor, where off-target bystander killing would increase toxicity. Matching payload bystander activity to the antigen expression landscape of the indication is therefore a core element of ADC design strategy.

How do dual-payload ADCs address tumor resistance, and what are the engineering challenges?

Dual-payload ADCs carry two mechanistically distinct cytotoxic warheads on a single antibody, designed to prevent the emergence of resistant tumor subpopulations that survive single-payload treatment. Tumors can develop resistance to a specific cell-killing mechanism, leading to regrowth from resistant clones; delivering two payloads with different mechanisms of action broadens tumor clearance and targets resistant subpopulations simultaneously.

The engineering challenges are substantial: the two payloads must be compatible with the same linker chemistry or require orthogonal conjugation sites, the combined DAR must remain within a range that preserves antibody pharmacokinetics and avoids aggregation, and the differential release kinetics of each payload must be tuned to ensure both reach the intracellular target at effective concentrations. Site-specific conjugation technologies are generally required to achieve the controlled stoichiometry that dual-payload designs demand.

Is there a difference between “fully human” and “humanized” antibodies in the context of ADC development, and does it affect payload delivery?

Fully human antibodies carry 100% human sequence, produced through natural immune selection in platforms such as Harbour Mice® (transgenic mice engineered to produce fully human heavy-chain-only antibodies and conventional H2L2 antibodies), while humanized antibodies are non-human sequences that have been engineered to reduce immunogenicity but still carry residual non-human residues. This distinction matters for ADC development because residual non-human sequences in humanized antibodies carry an inherent immunogenicity risk: anti-drug antibody (ADA) responses can accelerate ADC clearance, reduce payload delivery to the tumor, and trigger infusion reactions that compress the therapeutic window.

Fully human sequences are inherently compatible with human immune tolerance, reducing the ADA risk and supporting more predictable pharmacokinetics for the conjugate as a whole. When the antibody component of an ADC is sourced from a camelid-derived VHH and subsequently humanized, the engineering trade-offs compound: affinity can be lost during humanization, and the residual non-human framework residues remain a liability that fully human HCAb-derived binders avoid by design.

How do antibody format, specifically HCAb versus conventional IgG, affect ADC linker-payload design?

Heavy-chain-only antibodies (HCAbs), the format produced by Harbour Mice®, are approximately half the molecular weight of conventional IgG antibodies, which has direct consequences for ADC design. Their smaller size improves tumor penetration, particularly relevant for solid tumors with high interstitial pressure that limits access by full-size IgG-based ADCs. For ADC discovery and conjugation, HCAbs also offer a reduced number of conjugation sites, which simplifies site-specific conjugation and produces more homogeneous DAR distributions compared to conventional IgG, where lysine or cysteine conjugation generates heterogeneous mixtures.

The absence of a light chain eliminates the chain mispairing problem that complicates bispecific ADC manufacturing, where conventional IgG formats require engineered heavy-chain heterodimerization strategies to prevent incorrect chain assembly. Nona’s HCAb platform supports both conventional H2L2 and HCAb formats, allowing the antibody format to be matched to the specific requirements of the ADC program.

When should a program consider an antibody-oligonucleotide conjugate (AOC) instead of a traditional small-molecule ADC payload?

AOCs replace the small-molecule cytotoxic payload with a therapeutic oligonucleotide, such as siRNA or antisense oligonucleotide, enabling targeted gene silencing in specific cell populations rather than direct cytotoxicity. The case for AOCs is strongest when the therapeutic goal is modulation of a disease-driving gene in a cell type that is difficult to reach with systemic oligonucleotide delivery, such as neurons in CNS indications or specific immune cell subsets in metabolic disease.

Traditional small-molecule ADC payloads are optimized for oncology, where cytotoxicity is the desired endpoint; AOCs extend the antibody-conjugate concept to non-oncology indications where killing the target cell is not the goal. The antibody component of an AOC must internalize efficiently and traffic to the appropriate intracellular compartment for oligonucleotide release, placing specific demands on the antibody’s epitope and internalization kinetics that differ from cytotoxic ADC requirements.

What role does linker chemistry play in next-generation payload strategies?

The linker is the structural bridge between antibody and payload, and its cleavage mechanism determines where, when, and how efficiently the payload is released. Cleavable linkers, including protease-sensitive (valine-citrulline), pH-sensitive (hydrazone), and disulfide linkers, release the payload in response to specific intracellular conditions, enabling bystander killing when the released payload is membrane-permeable. Non-cleavable linkers require complete lysosomal degradation of the antibody before the payload is active, producing charged metabolites that are less membrane-permeable and therefore more confined to the target cell.

Next-generation payload classes have driven innovation in linker design: DXd-based ADCs use a tetrapeptide cleavable linker optimized for lysosomal cathepsin activity, while PBD-based ADCs have required careful linker tuning to manage the extreme potency of the warhead and reduce off-target toxicity. Nona’s ADC linker and payload design capabilities, including proprietary patents in this area, address the integrated optimization of linker-payload combinations rather than treating them as independent variables.

How does Nona Biosciences support next-generation ADC programs from antibody discovery through IND?

Nona’s Idea toward IND (I-to-I®) (the integrated end-to-end service pathway from ideation through IND filing) covers the full ADC development continuum, from antigen preparation and antibody discovery using Harbour Mice® through lead engineering, developability assessment, and preclinical pharmacology. With over 300 antibody discovery programs completed and 19+ clinical-stage molecules, including the Pfizer MesoC2 ADC program presented at ASCO, Nona brings clinically validated antibody components to ADC programs rather than untested binders.

The AstraZeneca strategic collaboration further anchors Nona’s position as a partner for complex modalities requiring both antibody quality and manufacturing de-risking. For ADC programs specifically, the combination of fully human HCAb and H2L2 antibody formats, site-specific conjugation expertise, and integrated CMC and toxicology services means that payload and linker decisions are made in the context of the full development pathway, not in isolation.

If your ADC program requires a next-generation payload strategy grounded in fully human antibody discovery and integrated preclinical development, speak with Nona’s ADC discovery team to map the right antibody format, conjugation approach, and payload class to your target biology.

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