ADC Toxicities

ADC toxicities fall into a few predictable buckets, driven by target expression, linker stability, and payload class.

Major toxicity categories

• Hematologic toxicity – Neutropenia, anemia, and thrombocytopenia are the most common class-wide adverse events, reflecting bone marrow exposure to released payload (especially topo I and DNA-damaging payloads).

• Gastrointestinal toxicity – Nausea, vomiting, diarrhea, and mucositis are frequent, particularly with topoisomerase I inhibitor ADCs such as trastuzumab deruxtecan and sacituzumab govitecan.

• Hepatotoxicity – Transaminase elevations and, with some earlier DNA-damaging ADCs (e.g., calicheamicin-based), veno-occlusive disease/sinusoidal obstruction syndrome.

• On-target, off-tumor toxicity – Damage to normal tissues that express the antigen at low levels (e.g., HER2-related cardiotoxicity, Nectin-4 skin toxicity) is a key limitation, especially as ADCs move into earlier lines and broader indications.

Payload- and drug-specific toxicities

• Microtubule inhibitor payloads (MMAE/DM1/DM4/MMAF) – Peripheral neuropathy, rash, and alopecia are characteristic; MMAE-based ADCs (Adcetris, Padcev, Polivy) frequently cause sensory neuropathy and skin reactions.

• Topoisomerase I inhibitor payloads (SN-38, DXd, exatecan) – High rates of neutropenia, diarrhea, nausea, and a distinctive risk of interstitial lung disease (ILD)/pneumonitis, particularly with trastuzumab deruxtecan.

• Ocular toxicity – Microtubule inhibitor ADCs like belantamab mafodotin (Blenrep) and tisotumab vedotin (Tivdak) cause keratopathy, dry eye, and vision changes due to off-target effects on corneal epithelium.

Mechanistic drivers

• Linker instability / premature cleavage – Leads to systemic release of payload and "chemotherapy-like" side effects, reducing the therapeutic window.

• Bystander effect – Cleavable linkers plus membrane-permeable payloads enable killing of antigen-negative tumor cells but also increase risk of collateral damage to adjacent healthy cells.

• Fc-mediated uptake and aggregation – ADC aggregates can be internalized by Fcγ receptor–bearing immune cells, causing unexpected off-target cytotoxicity.

Because of this, modern ADC design and clinical practice heavily emphasize: more stable and tumor-selective linkers, hydrophilic high-DAR designs to reduce aggregation, careful antigen selection, and close monitoring for ILD, ocular events, and hematologic toxicity with drug- and payload-specific mitigation strategies.

ADC toxicities largely arise from how, where, and in what form the payload is released and taken up, rather than from the antibody itself.

1. On-target, off-tumor toxicity
   Many ADC targets (HER2, TROP2, Nectin-4, BCMA, etc.) are also expressed at low levels in normal tissues.
   When the ADC binds these normal cells, it is internalized and processed in the same way as in tumors, releasing payload and damaging otherwise healthy tissue (e.g., HER2 expression in cardiomyocytes, Nectin-4 in skin, TROP2 in GI epithelium).
   Mechanistically, this is "correct" ADC pharmacology occurring in the wrong tissue.

2. Off-target toxicity from premature payload release
   If cleavable linkers are too labile (e.g., hydrazones or overly protease-sensitive peptides), they can be cleaved in plasma or non-tumor tissues.
   This releases free payload systemically, causing classic chemotherapy-like toxicities (myelosuppression, GI toxicity, hepatotoxicity) and narrowing the therapeutic window.
   Here the mechanism is linker failure: systemic small-molecule exposure rather than targeted delivery.

3. Bystander effect–driven collateral damage
   Cleavable linkers plus membrane-permeable, lipophilic payloads (MMAE, DXd, SN-38, PBDs) allow payload to diffuse out of target cells into neighbors.
   In tumors this is beneficial (kills antigen-low cells), but in normal tissues or at tumor–normal interfaces, diffusing payload can injure adjacent non-target cells.
   Mechanistically, the same diffusion that gives bystander killing in heterogeneous tumors produces "spillover" toxicity in healthy tissue.

4. Payload-specific mechanisms

• Microtubule inhibitors (MMAE, DM1/DM4, MMAF): disrupt microtubule dynamics in neurons and rapidly dividing normal cells, causing peripheral neuropathy and myelosuppression.

• DNA-damaging agents (calicheamicin, PBD dimers): cause double-strand breaks and crosslinks in any cell exposed, including hematopoietic and hepatic cells, explaining marrow suppression and veno-occlusive disease.

• Topoisomerase I inhibitors (SN-38, DXd): stabilize Topo I–DNA cleavage complexes; rapidly dividing GI epithelium and bone marrow are particularly affected (diarrhea, neutropenia). ILD/pneumonitis likely reflects off-target exposure of lung parenchyma to topo-I payload and associated inflammatory cell death.

So even with perfect targeting, payload mechanism dictates which normal tissues are most vulnerable.

5. Fcγ receptor–mediated uptake of ADC aggregates
   Conjugation and high DAR increase hydrophobicity and aggregation risk.
   Aggregated ADCs can crosslink Fcγ receptors (FcγRs) on immune cells, get internalized, and release payload inside FcγR-positive, antigen-negative cells, causing off-target cytotoxicity.
   Blocking FcγRs or engineering Fc-silent antibodies reduces this aggregate-driven, Fc-mediated toxicity.
   Mechanistically, this is non-specific uptake driven by Fc–FcγR interactions, not antigen binding.

6. Catabolite permeability and linker catabolism
   For non-cleavable linkers, payload remains attached to an amino acid (e.g., Lys–SMCC–DM1), yielding a charged, poorly permeable catabolite that stays within the target cell and limits bystander toxicity.
   For cleavable linkers, neutral, lipophilic catabolites (e.g., free DXd or MMAE) readily cross membranes; their permeability largely determines how far and into what tissues they spread.
   Design-wise, toxicity is therefore a function of: where the catabolite is generated, how stable the linker is in circulation, and how permeable the catabolite is.

In summary, ADC toxicity arises from a combination of antigen expression in normal tissues, linker instability or over-cleavability, payload pharmacology, aggregate formation and FcγR uptake, and the permeability of released catabolites — hence modern designs focus on tumor-selective linkers, hydrophilic high-DAR architectures, Fc-silent backbones, and "tuned" catabolites to decouple efficacy from systemic toxicity.