Antibody-Drug Conjugates: Microtubule Inhibitors, DNA Binders, and Topoisomerase Inhibitors

Antibody-drug conjugates (ADCs) combine the specificity of monoclonal antibodies with potent cytotoxic payloads. This strategy allows for selective delivery of toxic agents to tumor cells while minimizing harm to healthy tissues. ADCs have three critical components: a monoclonal antibody for tumor targeting, a cytotoxic payload for killing cancer cells, and a linker to bind them. Among the diverse payloads, microtubule inhibitors, DNA binders, and topoisomerase inhibitors have gained significant attention due to their potent mechanisms of action.

Microtubule Inhibitors

Mechanism of Action

Microtubule inhibitors disrupt the cytoskeletal structure critical for cell division by targeting tubulin, a protein essential for forming microtubules. These agents arrest tumor cells in the G2/M phase of the cell cycle, leading to apoptosis. However, when used systemically, they often cause toxicities such as myelosuppression and peripheral neuropathy. As ADC payloads, microtubule inhibitors like maytansinoids and auristatins allow precise targeting of tumor cells while reducing off-target effects. 

• Maytansinoids (e.g., DM1, DM4): These derivatives of maytansine bind tubulin and inhibit microtubule assembly. They are commonly used in ADCs like trastuzumab emtansine (T-DM1), approved for HER2-positive breast cancer. 

• Auristatins (e.g., MMAE, MMAF): These synthetic peptides disrupt microtubule polymerization and are featured in brentuximab vedotin, used for CD30-positive lymphomas. 

Both payload types are exceptionally potent, with IC50 values in the picomolar range, making them ideal for targeting low-antigen-expressing tumors.

Limitations and Innovations

While effective against rapidly dividing tumor cells, microtubule inhibitors show limited efficacy against quiescent cancer cells and those in hypoxic environments. Efforts to address these challenges include developing novel linkers that enhance payload release and optimizing conjugation strategies to improve therapeutic indices.

Related: Types of Payloads for Antibody-Drug Conjugates (ADCs)

DNA Binders

Mechanism of Action

DNA-binding agents function by causing irreversible damage to the DNA of tumor cells. Payloads in this category include alkylators, cross-linkers, and intercalators, which disrupt DNA replication and transcription, leading to cell death. Unlike microtubule inhibitors, DNA binders are effective across all phases of the cell cycle, making them suitable for targeting both actively dividing and dormant tumor cells.

• Pyrrolobenzodiazepines (PBDs): These alkylating agents form cross-links in the DNA double helix, preventing strand separation and replication.

• Calicheamicin: A DNA-cleaving agent that induces double-strand breaks with high potency.

Limitations and Innovations

ADCs like gemtuzumab ozogamicin, which incorporates calicheamicin, have shown success in treating acute myeloid leukemia (AML). However, DNA binders can cause off-target toxicity, including myelosuppression and hepatotoxicity. Advances in linker technology aim to enhance the tumor-specific release of these agents, mitigating systemic side effects.

Related: What are Antibody-Drug Conjugates?

Topoisomerase Inhibitors

Mechanism of Action

Topoisomerases are enzymes that resolve DNA supercoiling during replication and transcription. Topoisomerase inhibitors trap these enzymes in a cleavage complex with DNA, causing double-strand breaks and triggering apoptosis. ADCs incorporating these payloads, such as camptothecin derivatives, offer potent anticancer activity.

• Camptothecin Derivatives (e.g., SN-38): These inhibitors selectively target topoisomerase I (TOP1), disrupting DNA repair and replication in cancer cells.

• Irinotecan and Derivatives: Commonly used in ADCs, these agents demonstrate strong efficacy in solid tumors.

Limitations and Innovations

The development of topoisomerase inhibitor-based ADCs like trastuzumab deruxtecan (T-DXd) has shown promise in treating HER2-positive breast cancer, even in patients with low HER2 expression. Enhancements in linker stability and payload potency are expanding their applicability to diverse tumor types.

Overcoming ADC Payload Challenges

• Drug Resistance: Tumor cells may develop resistance to ADCs through mechanisms such as drug efflux or mutations in target antigens. 

• Toxicity: Off-target effects remain a concern, especially in normal tissues with antigen expression. 

• Stability: Premature payload release in circulation can lead to reduced efficacy and systemic side effects. 

Addressing these challenges involves developing novel payloads with unique mechanisms, such as RNA inhibitors, immune-modulating agents, and phototoxic molecules. Additionally, integrating dual-payload ADCs—combining agents with complementary mechanisms—offers a strategy to overcome resistance and improve efficacy.

Innovations in linker chemistry, payload stability, and targeted delivery are paving the way for next-generation ADCs. The integration of immune-stimulating agents and PROTACs (proteolysis-targeting chimeras) into ADCs holds promise for expanding their therapeutic potential.

Biointron’s catalog products for in vivo research can be found at Abinvivo, where we have a wide range of Benchmark Positive Antibodies, Isotype Negative Antibodies, Anti-Mouse Antibodies, Bispecific Antibodies, and Antibody-Drug Conjugates. Contact us to find out more at info@biointron.com or +86 400-828-8830 / +1(732)790-8340.

References:

1. Wang, Z., Li, H., Gou, L., Li, W., & Wang, Y. (2023). Antibody–drug conjugates: Recent advances in payloads. Acta Pharmaceutica Sinica. B, 13(10), 4025. https://doi.org/10.1016/j.apsb.2023.06.015

2. Han, S., Lim, K. S., Blackburn, B. J., Yun, J., Putnam, C. W., Bull, D. A., & Won, W. (2022). The Potential of Topoisomerase Inhibitor-Based Antibody–Drug Conjugates. Pharmaceutics, 14(8), 1707. https://doi.org/10.3390/pharmaceutics14081707

3. Klute, K., Nackos, E., Tasaki, S., Nguyen, D. P., Bander, N. H., & Tagawa, S. T. (2014). Microtubule inhibitor-based antibody–drug conjugates for cancer therapy. OncoTargets and Therapy, 7, 2227. https://doi.org/10.2147/OTT.S46887