The Promise of ADCs in Modern Oncology
Antibody-drug conjugates (ADCs) have emerged as one of the most promising therapeutic modalities in oncology, combining the specificity of monoclonal antibodies with the cytotoxic potency of chemotherapy. By delivering highly potent payloads directly to tumour cells, ADCs aim to maximise anti-tumour activity while minimising systemic toxicity. This targeted approach has already transformed treatment paradigms in several haematological malignancies and solid tumours, and is increasingly demonstrating clinical value across different settings.
In recent years, both early and late-phase clinical trials evaluating ADCs have shown significant activity in tumour types traditionally considered difficult to treat, including heavily pretreated urothelial cancer, breast cancer, lung cancer, colorectal cancer and other advanced solid tumours. These studies are critical not only for evaluating safety and efficacy, but also for identifying which patients are most likely to benefit from these increasingly sophisticated therapies.
Despite these advances, important challenges remain. Clinical outcomes are often constrained by narrow therapeutic windows, off-target toxicity, tumour heterogeneity, and the emergence of resistance. At the same time, the complexity and cost of ADC manufacturing continue to limit global access. Addressing these challenges requires continued innovation, not only in ADC design, but also in clinical development strategies and healthcare delivery.
Understanding the Complexity Behind ADC Design
ADCs are composed of three interconnected elements: a monoclonal antibody targeting a tumour-associated antigen, a cytotoxic payload, and a linker that connects the two. The success of an ADC depends on the precise coordination of these components. However, this complexity also introduces vulnerabilities.
Many ADCs operate within a narrow therapeutic index, where modest increases in dose can lead to disproportionate toxicity. Off-target effects may arise from premature and post-action payload release or low-level antigen expression in normal tissues. There are also other challenges that are particularly evident in solid tumours, where heterogeneous antigen expression and limited tumour penetration can reduce efficacy.
In clinical trials, these limitations are often reflected in variability in response rates across patient populations, reinforcing the importance of thoughtful study design and patient selection.
Expanding the Therapeutic Window Through Innovation
A central focus of current research is the expansion of the therapeutic window, enabling ADCs to deliver greater anti-tumour activity without increasing toxicity. Advances in payload design have been critical in this effort. New generations of cytotoxic agents exhibit activity at nanomolar or even picomolar concentrations, allowing for effective tumour cell killing with minimal systemic exposure in the context of more stable linkage to the monoclonal antibody.
At the same time, innovation is extending beyond traditional cytotoxic approaches. Radioligand-based payloads are being explored to deliver targeted radiation directly to tumour cells, while immune-stimulatory molecules as novel payloads aim to activate anti-tumour immune responses. Payloads based on proteolysis-targeting chimeras (PROTACs) represent another emerging strategy, designed to degrade oncogenic proteins rather than simply inhibit them. These approaches are increasingly being evaluated in preclinical settings—as well as early-phase clinical trials—where investigators can assess not only tumour response, but also how these novel mechanisms interact with the tumour microenvironment.
Equally important are advances in linker technology. Early-generation linkers were often unstable in circulation, leading to unintended payload release and systemic toxicity. Modern linkers are engineered to remain stable in the bloodstream and release their payload selectively within the tumour microenvironment, often in response to enzymatic activity or specific physiological conditions such as pH.
Advances in antibody engineering are also contributing to improved performance. Enhanced internalisation properties and the development of dual- or multi-targeting antibodies are helping to address tumour heterogeneity, while the so-called bystander effect—where the released payload diffuses to neighbouring tumour cells—extends therapeutic activity beyond antigen-positive cells.
Clinical Trials as the Engine of ADC Progress
The progress seen in ADC development is fundamentally driven by clinical trials. Early-phase studies, in particular, play a critical role in translating scientific innovation into patient benefit. These trials not only establish dosing and safety parameters, but also provide the first signals of efficacy in populations with limited treatment options.
In practice, participation in early-phase ADC trials often represents an important opportunity for patients with advanced or treatment-refractory disease. For many, these trials provide access to therapies that are not otherwise available, reinforcing the importance of expanding clinical trial availability beyond traditional academic centres.
Global, community-based research networks are increasingly playing a key role in this evolution. By bringing trials closer to where patients receive care, these networks can improve enrollment timelines while also ensuring broader and more representative patient participation. This is particularly important for ADCs, where understanding how these, in principle, active therapies perform across diverse populations and less restrictive settings is critical to both clinical development and regulatory decision-making.
Overcoming Resistance and Enhancing Durability
Resistance remains an important barrier to long-term efficacy. Tumours may adapt through a range of mechanisms, including downregulation of target antigen expression, impaired internalisation, increased drug efflux, or alterations in intracellular trafficking pathways.
Strategies to overcome resistance are therefore a critical component of ongoing ADC development. Sequential treatment approaches using ADCs with different targets or payloads are being explored, alongside rational combination strategies. Combining ADCs with immune modulators, targeted therapies, or other ADCs may enhance efficacy by addressing multiple pathways simultaneously. In clinical trials, these combination approaches are increasingly being evaluated to determine whether they can improve durability of response without additional detrimental side effects and extend patient benefit.
The Role of Biomarkers in Patient Selection
Another key area of progress lies in biomarker-guided patient selection. Ensuring that ADCs are delivered to patients most likely to benefit is essential for maximising clinical outcomes.
Biomarkers such as target antigen expression levels, genomic alterations, and characteristics of the tumour microenvironment are increasingly used to inform treatment decisions. However, biomarker development for ADCs presents unique challenges. Unlike therapies targeting a single molecular alteration, ADC efficacy depends on multiple biological factors, including antigen density, internalisation efficiency, and intracellular processing.
Advances in diagnostic technologies, including liquid biopsies and spatial profiling, are expected to enhance the precision of patient selection. These tools are also being integrated into clinical trials, enabling more adaptive and data-driven approaches to patient enrollment.
Expanding Access and Participation in Clinical Trials
Despite these advances, several biological and operational challenges continue to shape the clinical impact of ADCs. Among these, access to clinical trials remains one of the most important and addressable barriers.
The manufacturing of ADCs is complex and resource-intensive, requiring specialised facilities, advanced technologies, and stringent quality controls. These factors contribute to high costs and limit availability in many parts of the world. However, access challenges are not limited to manufacturing alone. Geographic barriers, limited trial site availability, and referral patterns can all restrict patient participation.
Expanding access to clinical trials is therefore critical. Increasing the number of sites capable of delivering early-phase studies, particularly in community settings, can significantly broaden participation. In turn, this enables more patients to access innovative therapies earlier in their disease course, while also generating more robust and inclusive clinical data, which is key for the expedited drug approval and potentially broader access to these therapies.
Efforts to improve referral pathways, streamline trial activation, and integrate clinical research into routine care settings are all contributing to a more accessible clinical trial ecosystem.
Looking Ahead: The Future of ADC Development
Looking ahead, the next generation of ADCs is expected to build on these advances, incorporating multifunctional designs that combine cytotoxic, immune-modulatory, and targeted mechanisms within a single platform.
Greater integration of biomarker-driven strategies will enable more precise patient selection, while advances in digital health technologies may support real-time monitoring and adaptive treatment approaches. Importantly, continued investment in clinical trial infrastructure and access, as well as decentralized platforms, will be essential to ensure that these innovations reach the patients who stand to benefit most.
By strengthening clinical trial design, expanding access to participation, and aligning scientific progress with patient-centred strategies, ADCs can deliver meaningful improvements in outcomes and broaden access to effective cancer therapies for patients worldwide.
Reference
1. Mansinho A, Albuquerque J, Barigazzi C, Calvo, E. Antibody-drug conjugates in cancer treatment: from molecular design to clinical implementation. The Lancet Regional Health – Europe, 2026; 64
