Kadcyla, also known as Trastuzumab Emtansine, is a breakthrough drug for the treatment of breast cancer. This innovative drug is a targeted therapy that combines the efficacy of two well-known anticancer drugs, Trastuzumab and Emtansine, to specifically target and destroy cancer cells that overexpress human epidermal growth factor receptor 2 (HER2). This unique dual mechanism of action makes Kadcyla a key treatment option for patients with HER2-positive breast cancer.

What is Kadcyla?

Kadcyla is a second-generation antibody-drug conjugates (ADCs) that uses humanized monoclonal antibodies and is a more effective cytotoxic drug, which reduces immunogenicity and improves the activity of toxic drugs. It is an ADC targeting HER2, consisting of humanized IgG1 trastuzumab coupled to DM1 through an MCC linker (MCC-DM1). MCC is a stable thioether linker 4-[N-maleimidomethyl]cyclohexane-1-carboxylate, which is a non-cleaving linker and has no bystander effect. DM1 is a maytansine alkaloid. Maytansine is a natural alkaloid that was first isolated from Maytansium ovalifolium in 1972 and exists in the genus Maytansium of the family Celastraceae and its relatives. This type of alkaloid acts on microtubules and tubulin, inhibits the depolymerization of cell microtubules, blocks the formation of spindles during cell mitosis, and thus inhibits tumor cell proliferation. Therefore, this type of cytotoxic drug also belongs to the class of tubulin inhibitors. Currently, the most commonly used ADCs are DM1 and DM4. Kadcyla has an average of 3.5 DM1s attached to each antibody, with a molecular weight of approximately 148,781 Da. Below is a list of maytansine and its analogs:

How Does Kadcyla Work?

Kadcyla (Trastuzumab Emtansine) has multiple mechanisms of action, including the anti-tumor effects of trastuzumab and the anti-tumor effects of DM1. The antibody portion is humanized anti-HER2 IgG1 trastuzumab. The small molecule cytotoxin DM1 is a microtubule inhibitor. After binding to the HER2 receptor IV subdomain, emtansine trastuzumab releases cytotoxic decomposition products containing DM1 in the cell through receptor-mediated internalization and lysosomal degradation. DM1 binds to tubulin and interferes with the intracellular microtubule network, leading to cell cycle arrest and apoptosis. In addition, in vitro studies have shown that, similar to trastuzumab, emtansine trastuzumab inhibits HER2 receptor signaling, exerts antibody-dependent cell-mediated cytotoxicity, and inhibits the shedding of the HER2 extracellular domain in human breast cancer cells that overexpress HER2. Emtansine trastuzumab has been approved for the adjuvant treatment of patients with HER-2-positive early breast cancer who still have residual invasive lesions after receiving neoadjuvant therapy based on taxanes and trastuzumab. In addition, the drug has also shown potential therapeutic effects on a variety of other cancers such as melanoma, non-small cell lung cancer, etc.

Fig. 1. Structure of Kadcyla (Polym Adv Technol. 2019, 1-17).

Enhertu vs Kadcyla

Enhertu (Trastuzumab Deruxtecan, DS-8201) is an ADC jointly developed and commercialized by AstraZeneca and Daiichi Sankyo. It was approved in the United States in December 2019 and is a third-generation ADC drug consisting of an anti-HER2 IgG1 monoclonal antibody and a topoisomerase I inhibitor Dxd (10 times more potent than irinotecan) through a linker. The Enhertu antibody and drug are connected by a cleavable tetrapeptide linker, with a drug antibody ratio (DAR) of up to 8. Currently, clinical research results from AstraZeneca and Daiichi Sankyo have shown that Enhertu has certain efficacy in treating biliary tract cancer, bladder cancer, cervical cancer, endometrial cancer, ovarian cancer, pancreatic cancer, colon cancer and rare cancers.

Both Enhertu and Kadcyla have shown significant efficacy in clinical trials for the treatment of HER2-positive breast cancer. However, multiple clinical results have shown that Enhertu is more effective than Kadcyla. Compared with Kadcyla, Enhertu significantly reduced the risk of disease progression or death by 72%, and more than doubled the objective response rate (ORR). In addition, Enhertu showed impressive response rates and prolonged progression-free survival in patients who had previously received HER2-targeted therapies such as trastuzumab and pertuzumab. Kadcyla also showed efficacy in patients who had previously received trastuzumab and taxanes. The choice between Enhertu and Kadcyla may depend on factors such as previous treatment and individual patient characteristics.

ADC Drug

In addition to Kadcyla, the FDA has approved 14 ADC drugs. ADC drugs are targeted biological agents that couple cytotoxic drugs to monoclonal antibodies. They can efficiently transport small molecule cytotoxic drugs to target tumor cells, achieving the effect of precisely killing tumor cells. Compared with traditional antibody drugs, ADC products have both the potent effects of traditional small molecule drugs and the targeting properties of antibody drugs, reducing systemic toxicity and more selectively delivering payloads to tumor cells, tumor microenvironment or other target cells. In recent years, with the rapid development of antibodies, cytotoxins, linkers, conjugation technologies and analytical technologies, ADC products have higher uniformity, stability and therapeutic index, which has greatly promoted the development of ADC products.

ADC Drug Development

The main mechanisms of ADC drugs in treating tumors include antibody cytotoxicity, receptor interference, and effector cytotoxicity. Among them, antibody cytotoxicity includes antibody-dependent cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and antibody-mediated endocytosis (ADCP). The killing effect of effector molecules can be achieved through two pathways: direct killing and bystander killing. In short, ADC drugs target tumor cells through the synergistic effect of multiple mechanisms, thereby achieving the effect of treating tumors. Generally speaking, the successful development of ADC drugs is inseparable from the following aspects:

• Antibody screening and optimization: The antibodies in ADC drugs are key molecules that bind to the surface antigens of targeted cancer cells, so a large number of antibodies need to be screened and optimized to obtain antibodies with high affinity and specificity.
• Selection and modification of toxins: The toxins in ADC drugs are key components for killing cancer cells, so it is necessary to select suitable toxins and modify them to improve the safety and efficacy of the drugs.
• Innovation in linker technology: ADC drugs need to connect antibodies and toxins, so innovative connection technology is needed to achieve close binding of antibodies and toxins, and to ensure the stability of the connection and the biological stability of the drug.
• Design and implementation of clinical trials: Clinical trials of ADC drugs are an important part of the research and development process. It is necessary to design a reasonable trial plan and conduct scientific data analysis and interpretation to evaluate the efficacy and safety of the drug and provide a basis for the drug's marketing.
• Optimization of production process: ADC drugs are highly complex biological products, and their production processes need to be optimized to ensure the quality and consistency of the drug, as well as the feasibility and economy of production.

ADC Drug Testing

ADC drug testing covers a wide range of activities designed to evaluate the safety, efficacy and quality of antibody-drug conjugates for cancer treatment. By integrating preclinical and clinical studies with analytical testing and regulatory compliance, researchers can advance the ADC field and provide innovative cancer treatments to patients in need. ADC product quality studies include small molecule part, naked antibody part and ADC bulk/formulation part.

• Small Molecule Analysis

In addition to conventional impurity analysis, the quality study of small molecules should also be combined with the structure of impurities to analyze the impact on subsequent processes and final product quality. Based on the type, structure and other characteristics of impurities, combined with the destination (whether the impurity participates in the coupling reaction) and removal (whether the impurity is removed through the subsequent process) research, a comprehensive risk assessment is conducted to formulate a reasonable impurity control strategy. Since the coupling impurities are extremely difficult to quantify and remove after the coupling reaction is completed, the risks of coupling impurities need to be focused on, strict control standards should be formulated, and reasonable basis should be provided. For non-coupling impurities, reasonable control strategies can be established based on the removal capacity of subsequent processes to provide sufficient research data.

• Naked Antibody Analysis

In principle, the quality research requirements for naked antibodies are basically the same as those for antibody drugs. In addition, it is necessary to fully study and properly control the quality attributes that may affect the coupling process, such as the oxidation level of the antibody. For antibody modification by introducing cysteine or non-natural amino acids containing active reactive groups at specific sites of the antibody through genetic engineering technology, the modification site needs to be confirmed. Although ADCC, CDC or ADCP are generally not the main mechanism of action of ADC products, they may also have certain efficacy. Fc function research should be carried out based on the type of antibody, structural modification, etc. If it affects the safety or effectiveness of the final product, appropriate control should be carried out.

• ADC Stock Solution/Formulation Analysis

ADC products combine the advantages of the targeting of antibody drugs and the potent effects of small molecule drugs (such as cytotoxicity, etc.), but the coupling of the two also changes the physical and chemical properties of each other, which may cause changes in the molecular structure and charge of the drug. Therefore, in addition to paying attention to the quality attributes of large molecule proteins and small molecule-related quality attributes, ADC products also need to increase the research and control of key quality attributes caused by coupling, such as DAR, drug distribution, coupling sites (including non-target coupling sites), uncoupled naked antibodies, free small molecules and their derivatives (such as degradation products and/or reaction products with quenchers), heterogeneity, etc. Appropriate and advanced analytical techniques should be used to conduct comprehensive characterization from the perspectives of structural confirmation, physicochemical properties, biological activity and impurity research, and combined with the analysis of the characteristics of naked antibodies to fully understand the relevant changes in characteristics before and after coupling (such as advanced structure, post-translational modification, molecular size variants, charge variants, antigen binding activity, Fc functional activity, etc.), and provide as detailed information as possible to reflect the quality attributes of the final product.