How much do you know about the molecular design and mechanism of action of ADC drugs?

1、ADC brief introduction ：

In the field of cancer therapy, antibody-drug conjugates (ADCs) are often referred to as "targeted biological missiles." This analogy stems from the structural design of ADCs: a highly specific and high-affinity monoclonal antibody is responsible for precisely directing the toxin toward tumor sites that express specific surface markers; the linker facilitates the precise release of the toxin at the tumor site; and the toxin itself is responsible for killing the cancer cells. The design of ADCs aims to efficiently eliminate cancer cells while minimizing damage to healthy tissues as much as possible. However, the molecular design of ADCs is far more complex than commonly perceived; an ADC is not merely a simple combination of an antibody and a drug. Its clinical safety and efficacy are determined by a multitude of factors, including the specificity and payload capacity of the antibody, the selection and design of the linker, the choice of the therapeutic payload, the conjugation method, and the disease microenvironment, among other factors.

2、ADC trimeric structure: antibody, linker, and payload

An ADC consists of three core components: a monoclonal antibody that targets a specific antigen, a linker that connects the antibody to the drug, and a highly active cytotoxic payload.

(1) The ideal antibody: The most commonly used antibody subtype in clinical practice is humanized IgG1; IgG1 exhibits a long half-life, stronger ADCC and CDC effects, superior tumor-killing activity, and good stability under low-pH conditions. ADC antibodies must simultaneously possess high affinity, high specificity, low immunogenicity, and effective receptor-mediated internalization to effectively kill tumor cells while minimizing toxicity to normal tissues as much as possible.

(2) Linkers: These are primarily categorized into cleavable linkers and non-cleavable linkers. Linkers play a pivotal role in the stability and safety of ADCs, directly influencing their half-life, toxin release efficiency, and potential premature off-target toxicity. Cleavable linkers primarily rely on intracellular proteases or pH-dependent mechanisms to release the payload; consequently, they carry the risk of premature toxin release that may damage normal tissues, but they also exhibit a bystander effect. In contrast, non-cleavable linkers require complete degradation of the antibody by lysosomes before the payload can be released; thus, they offer relatively higher stability but lack the bystander effect.

(3) The payload of ADCs—acting as the "warhead" —is primarily divided into two categories: microtubule inhibitors and DNA damage agents. Since only a very small fraction of ADCs can reach the interior of tumor cells, the payload must exhibit extremely high cytotoxicity (typically with an IC?? ranging from nanomolar to picomolar levels); however, premature release of the payload can trigger cytotoxic reactions that damage normal tissue cells.

3、ADC's multi-targeted mechanism of action

The antitumor mechanism of ADCs encompasses at least three aspects:

(1) Direct cytotoxicity (primary mechanism of action): After the ADC binds to tumor cell surface antigens, it is internalized via endocytosis, releases the payload within the lysosome, and induces tumor cell death.

(2) The bystander effect: kills adjacent tumor cells that do not express the target antigen; this mechanism is crucial for eliminating tumor cells with antigenic heterogeneity—specifically, by enabling the payload to spread from the targeted tumor cells to surrounding tumor cells with antigenic heterogeneity, thereby exerting a cytotoxic effect.

(3) Immunomodulatory effects: ADCs can interact with the immune system directly or indirectly through mechanisms such as immunogenic cell death, Fc effector functions, Fc-mediated cargo uptake, and bystander cargo uptake.

图1 免疫刺激性ADC作用机制.png

Figure 1: Mechanism of action of immunostimulatory ADCs

4、Challenges in ADC design:

Antibody-drug conjugates (ADCs) possess immense therapeutic potential; however, realizing this potential still requires overcoming several key challenges, such as drug resistance, intratumoral and intertumoral heterogeneity, and the risk of treatment-related adverse events (TRAEs). Emerging ADC modalities, including bispecific ADCs and dual-ligand ADCs, demonstrate potential in addressing drug resistance and tumor heterogeneity; whereas peptide-drug conjugates (PDCs) may enhance tumor specificity and reduce the incidence of adverse events. Integrating ADC platforms with other intervention strategies—such as immunomodulation and degradation of traditionally "undruggable" targets—provides opportunities for developing multimodal cancer therapies that combine chemotherapy, radiotherapy, immunotherapy, and other targeted therapies.

However, without comprehensive patient stratification and biomarker identification, the full potential of ADCs cannot be fully realized—a point that is often overlooked in the early stages of novel ADC development. Biomarkers are crucial for identifying patient populations most likely to benefit from a specific ADC, thereby enabling personalized medicine. Given that many solid tumors exhibit heterogeneity, reliable biomarkers are particularly critical for optimizing patient selection.

5、Challenges in the design of novel ADCs:

Many researchers are exploring various novel ADC designs. Dual-ligand ADCs link two payloads with different mechanisms of action to the same antibody. Their core rationale is to address tumor resistance: if tumor cells require dual mutations to simultaneously evade the cytotoxic effects of both payloads, the probability of developing resistance will be significantly reduced. However, optimizing dual-ligand systems is considerably more complex than optimizing single-ligand systems, as it requires ensuring that the two mechanisms do not interfere with each other while simultaneously optimizing the release efficiency of both ligands. Bispecific ADCs enhance binding specificity against tumor cells by simultaneously recognizing two distinct targets and hold potential to overcome resistance arising from antigen heterogeneity. Immunostimulatory ADCs and protein-degrading ADCs introduce entirely new mechanisms of action: immunostimulatory ADCs activate the immune system within the tumor microenvironment while delivering cytotoxic payloads, whereas protein-degrading ADCs exert their antitumor effects by inducing the degradation of target proteins, thereby enabling multimodal cancer therapy.

6、Epilogue ：

In an ADC molecule, the targeting specificity of the antibody, the stability of the linker, and the potency and release efficiency of the payload collectively determine the therapeutic efficacy and safety of the ADC. As our understanding of ADC molecules and their immunological drivers continues to deepen, coupled with ongoing breakthroughs in novel linker chemistry, payload types, and antibody engineering technologies, ADCs are increasingly being adopted in clinical practice for greater precision, higher efficacy, and improved safety. Tek Biotech(Tianjin) Co., Ltd. has established a comprehensive ADC drug development platform leveraging phage display and yeast display technologies, enabling it to provide clients with high-quality one-stop services encompassing ADC drug development and druggability assessment. Regarding ADC conjugation, Tek Biotech also possesses extensive experience and expertise, offering a wide range of linker designs (including both cleavable and non-cleavable linkers) and customized conjugation solutions (such as site-specific conjugation, non-site-specific conjugation, and customized conjugation modifications) to meet the diverse research needs of our clients.

References

[1] Zippelius, A., Tolaney, S.M., Tarantino, P. [1] Zippelius, A., Tolaney, S.M., Tarantino, P. et al. Unveiling the molecular and immunological drivers of antibody–drug conjugates in cancer treatment. Nat Rev Cancer 25, 925–944 (2025).

[2] Tsuchikama, K., Anami, Y., Ha, S.Y.Y. [2] Tsuchikama, K., Anami, Y., Ha, S.Y.Y. et al. Exploring the next generation of antibody–drug conjugates. Nat Rev Clin Oncol 21, 203–223 (2024).

[3] Valle, I., Grinda, T., Antonuzzo, L. [3] Valle, I., Grinda, T., Antonuzzo, L. et al. Antibody–drug conjugates in breast cancer: mechanisms of resistance and future therapeutic perspectives. npj Breast Cancer 11, 102 (2025).

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