Innovative approaches to combat ADC resistance.

Antibody drug conjugates (ADCs) have surged in popularity in recent years, with over 200 in clinical trials and 11 approved by the FDA.¹ These targeted therapies—comprising a monoclonal antibody (mAb), a chemotherapeutic agent, and a chemical linker—are designed to deliver a cytotoxic payload directly into cancer cells while minimizing the impact on healthy tissues.

To exert their effect, ADCs bind to a specific antigen on the surface of cancer cells. Once bound, the ADC is internalized, releasing the cytotoxic payload to induce apoptosis.

Currently, ADC payloads fall into three main classes: DNA-targeting agents, tubulin-binding agents, and topoisomerase 1 (TOP1) inhibitors. Among these, TOP1 inhibitors have gained the most attention. However, the emergence of resistance to first-generation ADCs has highlighted the need for alternative payloads and innovative targeting strategies to ensure their continued success in clinical applications.

Mechanisms of ADC resistance

An emerging challenge associated with ADC therapy is the development of resistance, which can occur at the level of the antibody, the payload, or both.

Resistance to the antibody can arise through several mechanisms:

• Antigen loss: The target antigen may be entirely lost due to downregulation, deletion, or mutation, reducing the antibody’s ability to bind effectively.
• Clearance of the ADC: The ADC may be cleared from circulation before the payload has had a chance to be released, reducing its therapeutic impact.
• Derangement in internalization: Alternatively, internalization of the ADC into calveolin-1-positive puncta (specialized membrane domains) can disrupt lysosomal trafficking, preventing payload release.

Resistance to the payload can occur through various pathways:

• Alterations in the payload target: Mutations may occur in the payload target, such as TOP1, preventing the payload from exerting its cytotoxic effects.
• Anti-apoptotic protein upregulation: Upregulation of anti-apoptotic proteins, such as PLK1, can block apoptotic pathways, rendering the payload less effective.
• Activation of survival pathways: Activation of cell signaling pathways, such as the PI3K/Akt pathway, can promote cell growth and proliferation, countering the cytotoxic effects of the payload.
• Clearance of the payload: Finally, efflux of the payload might occur through ABC drug transporters, reducing its efficacy. 

Understanding these mechanisms of resistance is critical for improving ADC efficacy and driving the development of next-generation therapies.

Emergence of TOP1 resistance

The increasing reliance on TOP1 inhibitors as ADC payloads has highlighted an important limitation: the emergence of cross resistance. This has been observed in various studies of metastatic breast cancer, where sequential treatments with ADCs containing TOP1 inhibitors have resulted in reduced therapeutic responses. The reduced efficacy is most pronounced when the ADCs share identical payloads, suggesting that repeated exposure to the same class of cytotoxic agents can lead to resistance.

Strategies to overcome resistance

To address resistance in ADC therapy, researchers are exploring innovative strategies to improve efficacy and durability. These efforts focus on refining ADC design and leveraging combination therapies to target multiple pathways simultaneously.

One promising approach is the sequential use of ADCs targeting the same antigen but with different payloads. By avoiding repeated exposure to the same class of cytotoxic agent, this strategy reduces the risk of cross resistance.

Another strategy involves combining ADCs with complementary therapeutic agents, such as tyrosine kinase inhibitors (TKIs) to disrupt signaling pathways, or immune checkpoint inhibitors to promote immunogenic cell death. Synthetic lethal approaches, such as pairing ADCs with DNA damage and repair (DDR) inhibitors like PARP1 or ATR inhibitors, exploit cancer cell vulnerabilities, while the combination of ADCs with statins offers a novel method to disrupt cholesterol-dependent pathways.

Future directions

Building on these approaches, researchers are exploring next-generation strategies to overcome resistance and enhance the efficacy of ADCs. These include the development of dual-targeting ADCs, which are designed to bind two different antigens simultaneously, and the modification of payloads to enhance their cytotoxic effects while reducing the likelihood of resistance.

At Revvity, we are dedicated to accelerating your preclinical research. By partnering with us, you can streamline in vitro cell engineering and screening processes for ADC therapeutic candidates using our comprehensive suite of services and platforms to develop your pipeline.

To find out more about the evolution of ADC design, manufacturing techniques, and mechanisms of resistance, watch our webinar: Antibody drug conjugates: A novel targeted approach to cancer therapy.