Antibodies (Part 2. Applications in Antibody-Drug Conjugates)

Antibody-drug conjugates (ADCs) are an innovative class of targeted cancer therapeutics that harness the high specificity of monoclonal antibodies to deliver cytotoxic drugs directly to cancer cells. By combining three key components – a targeting antibody, a chemical linker, and a potent payload, ADCs aim to maximize anti-tumor activity while minimizing systemic toxicity.

Each of these components plays a crucial role in determining the safety and efficacy of the ADC. Among them, the antibody is not only responsible for guiding the drug to its target but also contributes to pharmacokinetics, internalization, and even immune modulation.

 

Figure 1. Structure of antibody-drug conjugates

Antibody Function in ADCs

In an ADC, the antibody does far more than simply guide the payload. It plays a critical role in:

 

·   Targeted delivery. By specifically recognizing tumor-associated antigens, the antibody directs the cytotoxic payload precisely to cancer cells, reducing off-target effects and systemic toxicity.

·   Internalization. After binding its target, the entire ADC-antigen complex is internalized via receptor-mediated endocytosis. Inside the cell, the linker is cleaved in the lysosome, releasing the active drug where it can exert its cytotoxic effect.

·       Pharmacokinetics. IgG-based antibodies (especially IgG1) engage the neonatal Fc receptor (FcRn), which protects them from degradation and enables recycling. This extends the ADC’s half-life in circulation, allowing for prolonged exposure to the tumor.

·       Immune effector function. In certain ADC designs, the antibody’s Fc region can recruit immune cells through mechanisms such as antibody-dependent cellular cytotoxicity (ADCC) or complement-dependent cytotoxicity (CDC), providing additional anti-tumor activity.

In summary, the antibody in an ADC is not just a delivery system – it actively shapes specificity, stability, circulation time, and immune engagement, making it essential to ADC success.

 

Figure 2. General mechanism of action of ADC

Antibody Targeting and Selection in ADCs

Table 1 summarized selected ADCs that have received regulatory approval in recent years. Here are several key trends in antibody origin, isotype choice and antigen targeting strategy.

•  Antibody origin. Most ADCs use humanized antibodies, which offer an optimal balance between antigen specificity and reduced immunogenicity. A few ADCs, such as cetuximab saratolacan and brentuximab vedotin use chimeric antibodies, while others like enfortumab vedotin and tisotumab vedotin are based on fully human antibodies.
•  Antibody isotype. The vast majority adopt the IgG1 subclass, valued for its long serum half-life, structural stability, and optional effector functions. A few ADCs (e.g.,  inotuzumab ozogamicin, gemtuzumab ozogamicin) use IgG4, which is Fc-silent and less immunostimulatory.
• Target antigens. The target antigens chosen for approved ADCs, such as HER2, CD30, CD22, and Trop 2 – are highly and selectively expressed on tumor cells, with limited presence in normal tissue. Ideal targets also support internalization, enabling effective payload delivery.

Together, these trends reflect the critical importance of choosing antibodies with optimal human compatibility, Fc properties, and antigen specificity to support ADC performance and safety.

 

 Table 1. Approved antibody-drug conjugates.

 

 

Antibody Targeting and Selection in ADCs

The antibody component of an ADC is responsible for directing the cytotoxic payload to tumor cells with high precision. However, effective targeting requires much more than antigen recognition; it depends on a combination of thoughtful antibody design and rigorous selection.

Figure 3. Key factors for ADC antibody targeting and selection

•  Antigen specificity. An ideal target antigen is highly and uniformly expressed on tumor cells while having minimal or no expression in normal tissues. Examples include HER2 (breast cancer), CD30 (Hodgkin lymphoma), and CD33 (acute myeloid leukemia). Clinical success such as trastuzumab emtansine (Kadcyla®) and gemtuzumab ozogamicin (Mylotarg®) underscores the importance of well-chosen targets.
•  Affinity. While high binding affinity may seem advantageous, excessively tight binding can limit tumor penetration – a phenomenon known as the binding-site barrier. In solid tumors, antibodies with moderate-to-high affinity often perform better by allowing deeper tissue diffusion while ensuring stable target engagement.
• Internalization. Efficient internalization is essential for payload release. After binding to the antigen, the ADC-antigen complex must undergo receptor-mediated endocytosis, delivering the payload into the cell’s lysosomal compartment for release. Antibodies that fail to internalize limit the therapeutic efficacy of ADCs.
• Isotype. Among the IgG subclasses, IgG1 is the most commonly used in ADCs due to its long half-life, FcRn-mediated recycling, and capacity to mediate immune effector functions like ADCC and CDC, when desired. Other isotypes like IgG2, IgG3, and IgG4 have less favorable profiles for most ADC applications.
• Pharmacokinetics. Antibody choice also influences pharmacokinetics. IgG1-based antibodies benefit from FcRn-mediated recycling, which extends serum half-life and supports sustained systemic exposure. Their large molecular weight (~150 kDa) limits distribution into healthy tissues, but the tumor’s leaky vasculature (EPR effect) enables preferential accumulation, enhancing tumor selectivity and reducing off-target toxicity.
•         Immunogenicity. Minimizing immunogenicity is essential for clinical success. Fully human or humanized antibodies are preferred to reduce the risk of anti-drug antibody (ADA) responses, which can shorten circulation time or trigger unwanted immune reactions.

Taken together, ideal ADC antibodies should not only bind specifically to the tumor target, but also possess optimized isotype, binding kinetics, internalization behavior, and low immunogenicity. All of which contribute to therapeutic precision, stability, and safety.

 

Conclusion

In ADCs, the antibody does much more than just deliver the payload – it plays key roles in targeting precision, payload uptake, circulation time, and even immune activation. As the field advances, continued refinement in antibody engineering will be essential to unlock the full potential of next-generation ADCs.

While the antibody defines where the drug goes, the linker determines how and when it is released. At PrecisePEG, we specialize in ADC linkers and linker attachment technologies, helping our partners enhance the precision, stability and efficacy of their ADC candidates.

 

Reference

1.     He, J.; Zeng, X.; Wang, C.; Wang, E.; Li, Y. Antibody-drug conjugates in cancer therapy: mechanisms and clinical studies. MedComm 2024, 5 (8), e671. https://doi.org/10.1002/mco2.671.

2.     Baah, S.; Laws, M.; Rahman, K. M. Antibody–Drug Conjugates—A Tutorial Review. Molecules 2021, 26 (10), 2943. https://doi.org/10.3390/molecules26102943.

3.     Fu, Z.; Li, S.; Han, S.; Shi, C.; Zhang, Y. Antibody drug conjugate: the “biological missile” for targeted cancer therapy. Sig. Transduct. Target. Ther. 2022, 7, 93. https://doi.org/10.1038/s41392-022-00947-7.

4.     Ruan, D. Y.; Wu, H. X.; Meng, Q.; Xu, R. H. Development of antibody-drug conjugates in cancer: Overview and prospects. Cancer Commun. 2024, 44 (1), 3–22. https://doi.org/10.1002/cac2.12517.