Antibody-drug conjugates (ADCs) are a novel class of cancer treatment drugs and are composed of a small molecule cytotoxin linked to a monoclonal antibody.¹ Binding of the circulating ADC to the antigen on the cell surface is the first step required for the internalization of the drug molecule.¹ The drug is then internalized via endocytosis by clathrin-coated endosomes.¹ The endosome is then fused with lysosomes where proteolytic enzymes cleave the linkers between the antibody and the drug is released.¹ Resistance to ADC may be derived from 3 possible processes including antigen expression, ADC processing and the payload itself.¹ During the European Society of Gynaecological Oncology (ESGO) 2024 Congress, Professor Toon Van Gorp from the University Hospital Leuven, Belgium, presented the mechanisms of ADC resistance, as well as several approaches currently being developed to overcome this resistance.¹

The first mechanism of resistance is related to tumor antigen expression and ADC binding. In the DESTINY-PanTumor02 trial, patients with endometrial, cervical or ovarian cancer who had a higher expression of HER2 as measured by immunohistochemistry (IHC) consistently showed better response to trastuzumab deruxtecan (T-DXd).¹ Moreover, cervical cancer patients with a HER2 IHC score of 3+ showed significantly better progression-free survival (PFS) and overall survival (OS) outcomes.¹ The selection pressure resulting from treatment may select for clones that do not display the target antigen as a result of the natural heterogeneity of antigen expression.¹ In vitro experiments showed that tumor cells with extended exposure to trastuzumab-ADC could also lead to the downregulation of HER2 protein.¹ Another mechanism affecting tumor antigen expression is proteases such as ADAM10/17 which may truncate the extracellular domain of the antigen, preventing the recognition and binding of the antibody.¹ For example, p95HER2, a truncated form of HER2, is not recognized by trastuzumab.¹

The second mechanism of resistance is related to the downstream processes after the ADC is bound to the receptor.¹ Increased caveolae-mediated endocytosis bypasses clathrin-coated endocytosis and the fusion with lysosomes.¹ Higher expression of caveolin-1 (CAV1) co-localized with trastuzumab-ADC was observed in ADC-resistant cell lines but not in ADC-sensitive cell lines.¹ Furthermore, HER2-positive gastric cancer patients treated with trastuzumab had poorer survival outcomes if their tumors expressed higher levels of CAV1 compared to those with low CAV1 levels.¹ In caveolae-mediated endocytosis, vesicles are directed to non-proteolytic compartments, preventing drug release from the ADC.¹ Even with clathrin-coated endocytosis, lysosome dysfunction caused by reduced proteolytic activity of lysosomal enzymes may also affect the release of the drug payload into the cell.¹

Thirdly, resistance to the payload itself is another mechanism of ADC resistance.¹ These drug resistance mechanisms include increased drug efflux by upregulating the expression of the MRP1 drug transporter, changes in cell-cycle dynamics, activation of downstream signaling pathways, and dysregulation of apoptosis.¹

One approach to overcoming resistance is by designing ADCs that can bind to two distinct epitopes of the same antigen. The affinity for binding is thereby improved allowing for a more efficient payload delivery to biomarker-low cancers.¹ ADCs that bind to two distinct antigens can also be designed to overcome heterogeneity in antigen expression.¹ Another method to overcome resistance is through the use of novel payloads.¹ For example, when HER2-positive breast cancer patients who progressed even after treatment with trastuzumab emtansine (T-DM1) were switched to T-DXd, significant improvements in PFS were seen.¹ Dual-drug ADCs which combine two payloads in a single antibody are also under development and have shown greater efficacy at inhibiting tumor growth in mice compared to single-drug ADCs.¹ Lastly, statins may also be a potential approach to reducing caveolae-mediated endocytosis, enabling proper cleavage by lysosomal enzymes and facilitating the release of the drug inside the cell.¹

In summary, cancer cells may develop resistance against ADCs through various mechanisms along the processes of antigen presentation, ADC processing, and payload resistance.¹ Novel ADCs which may be bispecific, have novel payloads, or are linked to dual drugs, are currently being developed to overcome these resistance mechanisms.¹