How Do Antibody-Drug Conjugates Work?
Antibody-drug conjugates (ADCs) are targeted therapeutics designed to deliver a highly potent cytotoxic agent to tumor cells. Each ADC contains three components: a monoclonal antibody (mAb) targeting a tumor-associated antigen, a highly potent cytotoxic agent, and a chemical linker connecting the antibody and the drug. In this way, the ADC serves as a "guided missile" and selectively targets tumor cells while avoiding exposure to healthy tissue.
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After binding to a tumor-associated antigen on the cancer cell surface, the ADC is taken up by the target cell via receptor-mediated endocytosis and transported to the lysosome, where the linker is cleaved, either by enzyme or by the acidic pH in the lysosome, releasing the cytotoxic drug. The released drug can interfere with DNA replication (e.g. DNA topoisomerase I inhibitors), disrupt microtubule dynamics (e.g. auristatins and maytansinoids), or target other processes within the cell, leading to cell death by apoptosis.
Figure 1. Overview of Tumor-Targeted ADCs. ADCs utilize blood circulation to reach the tumor microenvironment (TME), which is composed of cancer-associated fibroblasts (CAFs) and other cell types, and interact with malignant cells that expose tumor-associated/specific antigens on their surfaces[1].
How Do Dual-Loaded and Bispecific ADCs Enhance Therapeutic Efficacy?
Tumor heterogeneity and acquired resistance remain significant challenges to the clinical utility of single-agent ADCs. As such, dual-drug ADCs and bispecific antibody-based ADCs have been developed to enhance treatment outcomes.
Dual-drug ADCs incorporate two distinct cytotoxic payloads—such as MMAE and MMAF—into a single molecule. MMAE, a membrane-permeable drug, exhibits a pronounced bystander killing effect, while MMAF, which is membrane-impermeable, can target resistant cell populations by avoiding drug efflux mechanisms. When tested in HER2-positive and HER2-negative xenograft models, these dual-loaded ADCs showed superior antitumor activity and durable remission compared to their mono-loaded counterparts.
Figure 2. Preparation of dual-drug ADCs. Dual-drug ADCs composed of MMAE and MMAF were prepared using MTGase-mediated conjugation of a bifunctional branched linker followed by orthogonal click reactions with MMAE (magenta circles) and MMAF (yellow triangles)[2].
Bispecific ADCs are ADCs that target two different antigens or epitopes on the same antigen, leading to enhanced selectivity, internalization, and coverage. The following table summarizes these mechanisms of action and their respective bispecific antibody examples.
Table 1. Mechanisms of Action and Examples of Bispecific Antibodies[2].
| Mechanism of Action | Examples |
| Activation of immune cells | RG7769, a bispecific CrossMabVH–VL, induces IFN-γ secretion, enhancing tumor-specific T-cell responses. It increases ex vivo effector function in T cells from melanoma patients and improves the activity of tumor-infiltrating lymphocytes compared to a monospecific PD-1 antibody. Additionally, a PD-1/TIM-3 bispecific antibody has entered Phase 1 clinical trials in advanced metastatic solid tumors. |
| Blocking of immune checkpoints | AK112, an anti-PD-1/VEGF bispecific antibody, is specifically engineered to concurrently inhibit PD-1-mediated immunosuppression and tumor angiogenesis within the tumor microenvironment. |
| Blocking of inflammatory factors | ABT-122, a dual-variable domain immunoglobulin, features two distinct binding sites targeting TNF and IL-17A, effectively modulating inflammatory responses in autoimmune and oncological settings. |
| Blocking of dual signaling pathways | EMB-01, a novel bispecific antibody, simultaneously targets EGFR and c-MET, disrupting dual oncogenic signaling pathways critical for tumor proliferation and survival. |
For instance, the HER2×PRLR bispecific ADC (carrying MMAE) has been found to exhibit enhanced internalization and cytotoxicity in breast cancer cells compared to HER2-targeted ADCs. Another construct, the MET×MET bispecific ADC (e.g., REGN5093-M114), has been found to be effective in MET-expressing non-small cell lung cancer, including in cells resistant to tyrosine kinase inhibitors. These novel ADCs represent a new wave of ADCs designed to harness the synergistic interaction of targeted engagement and combinatorial drug action to overcome resistance and enhance therapeutic index.
Figure 3. Generation of bispecific ADCs targeting human epidermal growth factor receptor 2 and PRLR[2].
What Is the Role of the Tumor Microenvironment in ADC Efficacy?
The tumor microenvironment (TME) plays a critical role in modulating ADC performance. Unlike hematologic malignancies, solid tumors exist within a complex and dynamic TME composed of extracellular matrix, cancer-associated fibroblasts (CAFs), neovasculature, and immune infiltrates. Targeting tumor microenvironment-associated antigens (TMAs) offers a novel approach to ADC therapy, especially in cancers lacking highly tumor-specific surface markers.
A key phenomenon in this context is the "bystander effect," wherein cleavable linkers allow the payload to diffuse into neighboring antigen-negative cells, broadening the cytotoxic reach of the ADC. This is particularly valuable in heterogeneous tumors where not all cancer cells express the target antigen uniformly. ADCs designed to exploit the bystander effect are more effective in infiltrating the TME and suppressing tumor progression.
Figure 4. Potential drug targets in tumor cells, immune cells, stromal cells, and the extracellular matrix (ECM) compartment of the TME[3].
How Do ADCs Impact Cancer-Associated Cellular Senescence?
Cellular senescence in cancer is characterized by irreversible growth arrest and the secretion of pro-inflammatory factors collectively known as the senescence-associated secretory phenotype (SASP). Traditional chemotherapeutics may inadvertently induce senescence in normal tissues, promoting chronic inflammation and possibly tumor recurrence. In contrast, ADCs offer the advantage of localized drug delivery, thereby reducing the risk of systemic senescence induction.
Moreover, ADCs are being investigated not only for their cytotoxic potential but also for their capacity to induce "therapeutic senescence" in cancer cells. This strategy could lead to tumor suppression by halting cell proliferation and activating immune-mediated clearance of senescent cells. Certain payloads, such as microtubule inhibitors, are preferred over DNA-damaging agents to limit off-target senescence and avoid long-term tissue damage.
Figure 5. Role of the SASP in tumor promotion and suppression[4].
What Are the Benefits of Combining ADCs with Other Cancer Therapies?
The combination of ADCs with other treatment modalities, particularly immune checkpoint inhibitors (e.g., anti-PD-1/PD-L1), represents a powerful synergy. ADCs can cause immunogenic cell death, leading to the release of tumor antigens and danger-associated molecular patterns (DAMPs) that enhance immune priming. In this context, PD-1 blockade can relieve T-cell exhaustion, unleashing a robust antitumor immune response. Preclinical studies combining ADCs with nivolumab have demonstrated improved efficacy in refractory tumors.
ADC co-administration with kinase inhibitors or low-dose chemotherapy is another promising avenue. These combinations can sensitize tumors to ADCs or vice versa, allowing for dose reductions and toxicity minimization. Innovations in linker chemistry, such as tumor-cleavable disulfide and enzymatically labile linkers, further facilitate the selective release of payloads in combination regimens.
FAQs About Antibody-Drug Conjugates
1. What types of linkers are used in ADCs?
Linkers can be cleavable (e.g., enzyme-sensitive, pH-sensitive) or non-cleavable. Cleavable linkers allow payload release under tumor-specific conditions, while non-cleavable linkers rely on lysosomal degradation for payload activation.
2. Can ADCs target intracellular antigens?
No, ADCs generally target extracellular or membrane-bound antigens, as the antibody component must bind on the cell surface to trigger internalization.
3. Are ADCs effective in treating brain tumors?
Currently, ADC efficacy in brain tumors is limited due to the blood-brain barrier. However, strategies such as receptor-mediated transcytosis and bispecific ADCs are being explored.
4. What makes a good ADC target antigen?
An ideal target is highly expressed on tumor cells, minimally expressed on normal tissues, capable of mediating internalization, and not subject to rapid antigen shedding.
5. How are ADCs manufactured?
Manufacturing involves antibody production, linker-payload synthesis, conjugation, and purification. Strict quality control is essential to ensure batch-to-batch consistency in drug-to-antibody ratio (DAR) and stability.
6. What analytical tools are used for ADC characterization?
Tools include LC-MS, ELISA, HIC, and CE-SDS to assess DAR, aggregation, free drug content, and stability.
7. Are there off-target effects associated with ADCs?
Yes, though reduced compared to traditional chemotherapy, off-target effects may occur due to premature drug release or low-level expression of the antigen on healthy tissues.
8. Can ADCs be used in combination with radiotherapy?
This approach is under investigation. Some ADCs may sensitize tumor cells to radiation or help reduce the required radiation dose.
References
- Riccardi F, et al. A comprehensive overview on antibody-drug conjugates: from the conceptualization to cancer therapy. Frontiers in Pharmacology. 2023, 14, 1274088.
- Fong J-Y, et al. Advancements in antibody-drug conjugates as cancer therapeutics. Journal of the National Cancer Center. 2025.
- Zhang L, et al. Targets of tumor microenvironment for potential drug development. MedComm – Oncology. 2024, 3(1), e68.
- Jo H, et al. The Potential of Senescence as a Target for Developing Anticancer Therapy. Int. J. Mol. Sci. 2023, 24(4), 3436.