Antibody-drug conjugates (ADCs) have emerged as a powerful class of targeted cancer therapies, combining the specificity of monoclonal antibodies with the cytotoxic potency of small-molecule payloads. Among the three essential components of an ADC – antibody, linker, and payload – the payload plays the ultimate role in executing cell death.
These payloads, also known as cytotoxic warheads, vary widely in mechanism and structure. Some disrupt microtubule dynamics to halt cell division, while others directly damage DNA or interfere with transcription. Choosing the right payload is critical: it determines not only the potency and safety profile of the ADC but also influences linker design, conjugation strategy, and clinical applicability.
Classification of ADC Payloads
1. Microtubule inhibitors
Microtubule inhibitors are among the earliest and most widely used ADC payloads. They disrupt the microtubule network essential for cell division, leading to mitotic arrest and apoptosis.
· Monomethyl auristatin E (MMAE): A synthetic antineoplastic agent that inhibits tubulin polymerization. Used in FDA-approved ADCs such as Brentuximab vedotin (Adcetris®).
· Monomethyl auristatin F (MMAF): Similar to MMAE but with a charged C-terminal group reducing cell permeability, which can reduce off-target effects. Used in Belantamab mafodotin (Blenrep®).
· Maytansinoid DM1: A derivative of maytansine that binds to tubulin and inhibits microtubule assembly. It is conjugated to antibodies via a non-cleavable linker in Trastuzumab emtansine (Kadcyla®), where it remains active only after intracellular degradation of the ADC.
· Maytansinoid DM4: Another maytansinoid with a cleavable linker commonly used in preclinical and clinical -stage ADCs. Compared to DM1, DM4 enables a bystander effect due to its ability to diffuse across membranes once released. Example includes Mirvetuximab soravtansine Elahere®.
2. DNA Damaging Agents
These payloads induce DNA damage through crosslinking, alkylation, or strand breaks, leading to cell death. They are especially useful against tumors resistant to microtubule inhibitors.
· Pyrrolobenzodiazepine (PBD) Dimers: Form covalent DNA interstand crosslinks preventing replication and transcription. Example includes Loncastuximab tesirine (Zynlonta®).
· Duocarmycins: Alkylate the minor groove of DNA causing irreversible damage; used in experimental ADCs like SYD985.
· Calicheamicin: Causes DNA double-strand breaks and is used in Gemtuzumab ozogamicin (Mylotarg®).
3. Topoisomerase I Inhibitors
Topoisomerase I inhibitors are a newer class of ADC payloads that interfere with DNA replication by stabilizing the Topo I-DNA cleavage complex, causing DNA damage and cell death.
· DXd (Deruxtecan): A camptothecin derivative based on DX-8951f, used in Trastuzumab deruxtecan (Enhertu®). DXd is linked via an enzyme-cleavable linker and can cross cell membranes, allowing a bystander killing effect.
· SHR9265: A novel camptothecin-derived Topo I inhibitor used in the China-approved ADC SHR-A1811 艾维达®. It is linked via a cleavable tetrapeptide linker and features improved stability and reduced off-target toxicity.
· SN-38: The active metabolite of irinotecan, employed in Sacituzumab govitecan (Trodelvy®). SN-38 is released via hydrolysable linkers and is highly potent but requires careful toxicity management.
· T030: A novel camptothecin-derived Topo I inhibitor used in the China-approved ADC sacituzumab tirumotecan 佳泰莱®. T030 is conjugated via a cleavable CL2A linker and incorporates a methylsulfonyl moiety to enhance plasma stability and reduce off-target toxicity.
4. Photoactivated Payloads
Photoactivated payloads represent a novel class of ADC mechanisms, where cytotoxicity is triggered only upon light activation, offering high spatial control and reduced systemic toxicity.
· IRDye® 700DX: A near-infrared (NIR) photosensitizer used in cetuximab sarotalocan Akalux®. Upon NIR light exposure, IRDye® 700DX induces rapid disruption of the cell membrane, leading to necrotic cell death. This Photoimmunotherapy strategy provides a non-traditional, targeted cytotoxic approach with minimal dark toxicity.
5. Emerging and Non-Traditional Payloads
In addition to classical cytotoxic payloads, several novel ADC payloads are being explored to expand therapeutic windows, reduce resistance, and target previously inaccessible cancer vulnerabilities, including
· RNA Polymerase II Inhibitors
· Spliceosome Inhibitors
· Protein Translation Inhibitors
· Targeted Protein Degraders (PROTAC-like Payloads).
Payloads in Approved ADC Drugs
|
Payload Class |
Payload Name |
Approved Drug Name |
Payload Structure |
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Tubulin inhibitors |
MMAE |
Telisotuzumab vedotin Emrelis® |
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MMAE |
Tisotumab vedotin Tivdak® |
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MMAE |
Enfortumab vedotin Padcev® |
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MMAE |
Polatuzumab vedotin Polivy® |
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MMAE |
Brentuximab vedotin Adcetris® |
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MMAE |
Disitamab vedotin Aidixi® 爱地希® |
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MMAF |
Belantamab mafodotin Blenrep® |
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DM1 |
Trastuzumab emtansine Kadcyla® |
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DM4 |
Mirvetuximab soravtansine Elahere® |
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DNA damaging agents |
PBD dimer |
Loncastuximab tesirine Zynlonta®
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Calicheamicin |
Inotuzumab ozogamicin Besponsa® |
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Calicheamicin |
Gemtuzumab ozogamicin Mylotarg® |
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Top I inhibitors |
DXd |
Datopotamab deruxtecan Datroway®
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DXd |
Trastuzumab deruxtecan Enhertu® |
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SHR9265 |
Trastuzumab rezetecan SHR-A1811 艾维达® |
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SN-38 |
Sacituzumab govitecan Trodelvy® |
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T030 |
Sacituzumab Tirumotecan 佳泰莱® |
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Photosensitizer |
IR700 |
Cetuximab saratolacan Akalux® |
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Key Considerations When Selecting Payloads
The selection of a payload is a pivotal decision in ADC development, directly influencing the therapeutic index, clinical efficacy, and safety profile. Several factors must be carefully considered:
· Potency: ADC payloads must exhibit high cytotoxicity at nanomolar or picomolar concentrations. This is essential because each antibody molecule can carry only a limited number of payloads.
· Membrane Permeability: Payloads with good membrane permeability can diffuse into neighboring tumor cells, enabling a bystander killing effect. This can be advantageous in tumors with heterogeneous antigen expression but must be balanced against potential off-target toxicity.
· Stability: The payload must remain stable during systemic circulation to prevent premature release, which could lead to systemic toxicity. Once internalized into the target cell, however, the payload should be efficiently released in its active form.
· Compatibility with Linker Chemistry: Payloads must possess functional groups compatible with linker chemistries (cleavable or non-cleavable), as these interactions influence release kinetics, solubility, and ADC pharmacokinetics.
· Manufacturing Feasibility and Cost: Some highly potent compounds may be synthetically challenging, unstable, or expensive to manufacture. Scalable and robust production is essential for clinical and commercial viability.
Payload and Linker Synergy: A Critical Relationship
The success of an ADC depends not only on the potency of the payload or the specificity of the antibody, but also on the finely turned interplay between the payload and its linker. This synergy affects the pharmacokinetics, stability, and therapeutic index of the final drug.
· Cleavable linkers are designed to release the payload inside target cells in response to specific intracellular cues such as low pH, enzymatic activity, or redox potential. These linkers are ideal for payloads that must be released in their active form to exert cytotoxicity. Their design must balance stability in circulation with sensitivity to the tumor microenvironment.
· Non-cleavable linkers, on the other hand, are chemically stable and rely on the complete degradation of the antibody within the lysosome to release an active drug-linker complex. While this can reduce off-target effects, the payload must remain potent and bioactive even when conjugated to a linker remnant.
· Hydrophobicity of the payload significantly affects ADC solubility and aggregation risk. Highly hydrophobic payloads can lead to poor pharmacokinetics and faster clearance. In such cases, hydrophilic linkers are often introduced to counterbalance the hydrophobic nature of the payload and improve aqueous solubility and manufacturability.
· Site-specific conjugation strategies are increasingly favored to achieve uniform drug-to-antibody ratios (DAR) and consistent performance. These strategies often require the payload to have accessible and chemically compatible conjugation handles (e.g., thiols, amines, etc.), and may involve engineered antibodies with specific reactive sites. Matching the conjugation chemistry to the payloads is essential for minimizing loss of activity and improving therapeutic index.
Conclusion
The success of ADCs relies on the integration of three critical components: the antibody, the linker, and the cytotoxic payload. Among these, the selection of the payload largely determines the cell-killing efficacy and clinical performance of the ADC, making it a core focus in ADC development. However, the full potential of a payload can only be realized with the support of high-quality linkers. The design of linkers not only governs the stability and timely release of the payload but also directly impacts the safety and therapeutic index of the ADC.
At Precise PEG, we specialize in the development and production of ADC linkers and conjugation technologies to meet diverse conjugation needs. Our products ensure optimal stability, controlled payload release, and efficient conjugation, helping our clients accelerate the development of ADC therapies.
Reference
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