Monomethyl auristatin E[HRP]

Derived from the natural product dolastatin 10, which was initially isolated from the marine sea hare Dolabella auricularia, MMAE has demonstrated remarkable efficacy in disrupting cell division. It achieves this by binding to tubulin, a protein that constitutes microtubules, which are essential for mitotic spindle formation and function during cell division. MMAE blocks microscopic assembly by binding to tubulin, leading to cell cycle arrest in the G2/M phase, which triggers apoptosis of the affected cells. Because MMAE is extremely sensitive and has effective cytotoxicity, it has become a powerful cancer chemotherapy drug. However, this high efficiency also brings great clinical challenges, especially in terms of drug side effects. The inherent potency of MMAE necessitates a targeted delivery approach to maximize its therapeutic benefits while minimizing adverse effects. This challenge has been addressed through the development of antibody-drug conjugates (ADCs), which are sophisticated biopharmaceuticals designed to deliver cytotoxic agents specifically to cancer cells. An ADC is composed of three main components: a monoclonal antibody (mAb) that specifically targets a tumor-associated antigen, a cytotoxic drug (in this case, MMAE), and a linker that connects the drug to the antibody.The monoclonal antibody guides the ADC to the cancer cells by binding to specific antigens expressed on their surface. Once bound, ADC is internalized by cancer cells and releases the cytotoxic drug, exerting its lethal effects within the cell. This targeted delivery system enhances the therapeutic index of MMAE by reducing its impact on healthy, non-cancerous cells. Among the therapeutic drugs developed by MMAE through ADC technology, brentuximab vedotin (an ADC, in which the antibody attaches to a protein called CD30, which is present in certain lymphoma cells and anticancer drugs may help kill cancer cells) is a more successful application and has been approved for the treatment of Hodgkin lymphoma and systemic anaplastic large cell lymphoma. Brentuximab vedotin contains a protease-cleavable linker, MMAE, and an anti-CD30 monoclonal antibody. Because CD30 is highly expressed on the surface of certain lymphoma cancer cells, brentuximab vedotin can selectively deliver MMAE to these cancer cells, reducing the toxicity of MMAE to normal cells. Using a linker to connect the antibody and MMAE can keep it stable under physiological conditions, but after internalization into cancer cells, MMAE is released by cysteine protease cleavage, ensuring that the drug's cytotoxicity acts only on target cells. Clinical trials demonstrated that brentuximab vedotin significantly improved patient outcomes, demonstrating the potential of MMAE-based ADCs in oncology. The development and optimization of ADCs involving MMAE require meticulous consideration of several factors to ensure their efficacy and safety. The selection of an appropriate monoclonal antibody is critical, as it must have high specificity and affinity for a tumor-associated antigen that is minimally expressed on normal tissues. Additionally, the linker chemistry is crucial; it must balance stability in the circulation with efficient release of the drug in the target cell environment. Various linker technologies have been explored, including acid-labile linkers, disulfide linkers, and peptide linkers, each with unique properties that influence ADC pharmacokinetics and pharmacodynamics. The site of drug conjugation on the antibody is another important factor, as it can affect the ADC's stability, efficacy, and immunogenicity. Currently, more mature site-specific conjugation methods have been developed, such as engineering cysteine residues and enzymatic conjugation applications, which can produce ADCs with more homogeneity.

Figure 1. The drug release process of MMAE- and Cys-linker-MMAE-based ADCs. (Sources: Wang Y, et al. 2020)

In addition to brentuximab vedotin, other MMAE-based ADCs are in development or approved for clinical use. For example, polatuzumab vedotin targets CD79b, a protein expressed on B-cell lymphomas and has shown promising effects in clinical trials. Enfortumab vedotin targets Nectin-4, a protein found on urothelial cancer cells, and has been approved for the treatment of advanced urothelial carcinoma. The successful marketing or clinical development of these ADCs demonstrates the versatility and effectiveness of MMAE in treating different cancer types and advanced ADC technology. Despite the success of MMAE-based ADCs, their therapeutic efficacy and minimization of potential side effects still need to be optimized. In the clinical application of ADC, the most worrying issue is the possibility of off-target toxicity. This toxicity is caused by the binding of monoclonal antibodies to antigens on normal cells, the combination of antibodies with non-specific antigens, or the linker being cleaved in the blood. Caused by reasons such as premature release of MMAE. Currently, strategies to mitigate these risks mainly include improving the specificity of mAbs, designing more stable linkers, and optimizing ADCs to enhance their internalization and processing by cancer cells. In addition, ADC-MMAE will also have drug resistance issues, because cancer cells may evade drug toxicity by downregulating target antigens, upregulating drug efflux pumps, or generating mutations that affect the ADC mechanism of action. To overcome the above problems, researchers are exploring combination therapies that combine ADCs with other treatment modalities, such as immune checkpoint inhibitors or small molecule inhibitors, to overcome drug resistance and enhance treatment efficacy. Research suggests that advances in antibody engineering, linker chemistry, and conjugation technology may lead to next-generation ADCs with improved efficacy, safety, and patient tolerability. As our understanding of cancer biology and drug delivery mechanisms continues to deepen, the potential of MMAE and other cytotoxic drugs will continue to be realized with the help of ADC technology, bringing new hope to patients with difficult and refractory diseases.

Monomethyl-AE
Monomethyl auristatin-E
Auristatin E, monomethyl
Monomethyl-Auristatin E