The past decade has been a decade of accumulation and harvest for ADC. After the second new ADC drug Adcetris (Brentuximab vedotin) was approved by the FDA in 2011, 10 ADC drugs have been approved so far, 7 of which have been approved in the last three or four years. Moreover, about 90 candidate ADC therapies in the R&D pipeline are in the clinical development stage, and more than 200 candidate drugs are in the clinical research stage.
The tissue specificity and cytotoxicity of the new generation ADCs are improved compared to the previous generation products, allowing them to show amazing activity in the treatment of refractory cancers. However, there are still many obstacles preventing the wide application of ADC, including systemic toxicity, insufficient biomarkers for selecting patients, and acquired drug resistance.
The evidence obtained so far shows that the effectiveness of ADC is based on the complex and subtle interactions between the various components of antibodies, linkers, and cytotoxins, and the tumor’s and its microenvironment. In an in-depth review published by Nature Reviews Clinical Oncology, a comprehensive description of the mechanisms affecting ADC efficacy and the limitations of ADC drugs was made. The authors also proposed a variety of strategies to maximize ADC’s anti-cancer potential.
1 ADC requires cell handling and metabolism to exert its ultimate anti-cancer activity
The traditional mechanism of ADC is the following description: the monoclonal antibody part of ADC binds to the target antigen, and then is swallowed into the cell interior, and then the linker is decomposed, resulting in the release of the load and the effect of killing cancer cells. Although this simple description provides the basic framework of the role of ADC, the process of ADC’s role is more complicated. The author emphasizes that an important point worth considering is that if ADC is to treat tumors, it usually requires tumors to have an effect on ADC. From this perspective, ADCs can be regarded as a kind of prodrugs, which require the handling and metabolism of target cells to be able to finally exert their full activity. In this review, the author elaborated on the complexity of the steps involved in ADC’s effect.
Due to insufficient purification during the manufacturing process, low linker stability, or metabolic problems, ADC drug formulations will contain three main components in the blood circulation after entering the human body. Antibody conjugated drug, antibody without load, and load without strong antibody binding.
The relative proportions of these three components are very different between different types of ADC, and with the metabolism of ADC in the human body, these three The proportion of each component in the body will also change dynamically. It is necessary to establish a pharmacokinetic and pharmacodynamic model for this feature to determine the clinical characteristics of ADC.
Compared with traditional cytotoxic drugs, monoclonal antibodies are macromolecules, which means that they have a limited number of penetration into tumors. Current studies have shown that only a small part of the ADC input into patients can reach tumor cells, which means that The strength of the load toxicity needs to be considered when designing the ADC.
After the ADC binds to the antigen, the internalization of the ADC-antigen complex is the key to many ADC drug delivery loads. This is usually dependent on an endocytosis process of antigen-dependent , or not an antigen-independent tosis of pinocytosis.
After internalization, the ADC-antigen complex will be transported to the endosome or lysosomal pathway according to the degree of acidification of the organelle. The load connected by the acid-cleavable linker is likely to be released in the early endosome. The load that is designed to require specific proteases or protein degradation processes will be released in the late endosomes or lysosomes.
Regardless of the release path of the load, some ADCs have the “bystander effect” that can affect surrounding cells, regardless of whether the neighboring cells express the target antigen.For internalized ADCs, it is considered to be an important factor in ADC activity against tumors with highly heterogeneous target antigen expression. This action requires the load to cross the cell membrane, the “bystander effect” requires the cleavable linker to release non-polar load molecules. Polar loads are more likely to remain in the cell.
2 Ways of tumor develop resistance to ADC
A deep understanding of the emergence of tumor resistance can provide insights into the fundamental mechanism of drug action and promote further drug development. Although the mechanism of resistance to ADC drugs has not been fully clarified, current evidence shows that tumors can evade ADC activity in many ways.
The effect of antigen expression level and patient selection on curative effect
A characteristic of solid tumors is that the expression of target antigens is highly heterogeneous and may be constantly changing. Therefore, selecting the patient population most likely to benefit from ADC therapy requires measuring the level of target antigen expression in tumor tissues. Taking breast cancer as an example, the expression level of HER2 protein may range from almost no expression to high expression due to high-level ERBB2 gene amplification, and the difference in expression level may reach 1000 to 10000 times. Currently, the detection of HER2 positive in the guidelines Including immunohistochemistry and fluorescence in situ hybridization. However, the ability of these tests to predict the effectiveness of HER2 targeting ADC has not been confirmed.
The current early results of the anti-TROP2 antibody conjugate drug sacituzumab govitecan showed that there is a direct correlation between TROP2 expression level and patient response. In the clinical trials supporting the approval, 26% of patients had disease progression, and 37% of the patients’ best response was stable disease, which means that the tumor is resistant to this therapy.
These results indicate that the use of biomarkers to select patients is an important part of ADC drug development.
The underlying mechanism of ADC acquired resistance
The resistance to tyrosine kinase inhibitors usually revolves around escape mutations in the drug target, while ADCs are more complex and diverse due to the complexity of the mechanism of action. The current main drug resistance mechanisms can be divided into three categories, which are reducing the expression level of antigens, changing the intracellular transport pathway, and developing drug resistance to the payload. These potential mechanisms have been verified in preclinical in vitro and animal studies, and clinical evidence to confirm these mechanisms is still limited.
For example, long-term exposure to breast cancer cells with a HER2 targeting antibody-conjugated drug will reduce the expression of HER2 receptors, reduce lysosomal acidification and slow down protein degradation. At the same time, they increase the expression of ATP-binding cassette transporter proteins.
Some ATP-binding cassette transporters (MDR1/MRP1/BCRP) have long been considered to play an important role in actively expelling traditional anti-cancer drugs. The load of some common ADC drugs can be transported by the ATP binding cassette transporter. Examples include MMAE, DMA, and ozogamicin. This makes these ADCs more susceptible to this drug resistance mechanism. However, the load of some ADCs is not easily affected by the ATP-binding cassette transporter. Enhertu still exhibits anticancer activity in HER2-positive cancer cells with high transporter expression. This may be one of the reasons why it still shows activity in refractory tumors.
3 Strategies to maximize the anti-cancer potential of ADC
Research in the past few decades has allowed ADC research and development to focus on the development of ADC drugs that target tumor-associated antigens, carry cleavable linkers and powerful microtubule inhibitors or genotoxic loads. The author predicts that in the next decade, ADC design and clinical applications will usher in more innovations. Strategies to explore the anti-cancer potential of ADCs can be divided into R&D strategies for ADC drug design and R&D strategies other than ADC drug design.
R&D strategies for ADC drug design
The ADC itself is composed of three parts: monoclonal antibody, linker, and load, which means that replacing any of these three parts has the potential to improve the efficiency of the ADC. Nowadays, small-scale drug screening usually uses in vitro or xenograft models to optimize the design of ADCs. They usually compare the effects of combinations of the same monoclonal antibody with different linkers and loads.
Different antibodies that target the same antigen may have different binding abilities and have very different effects on receptor dimerization and antigenization. Current studies have shown that ADC internalization and intracellular delivery pathways have a critical impact on the cytotoxic activity of ADCs. Therefore, monoclonal antibodies optimized for other clinical applications may not be the best choice as the backbone of ADCs.
Compared with the wild-type protein, the mutant protein has a higher level of ubiquitination and is easily internalized and degraded. This means that if ADC is used to target the mutant protein, it may bring about a significant clinical response. It is conceivable that targeting ADCs carrying oncogenic mutant proteins (EGFR mutants) may maximize the tumor specificity of the therapy and reach the level of highly selective TKI.
The progress of bispecific antibody technology has brought more possibilities for innovation. These ADC designs may increase antibody internalization or improve tumor specificity. Current research therapies are already exploring these possibilities. For example, bispecific ADCs that target different sites on the same antigen can increase receptor aggregation and lead to rapid internalization of the target. In addition, a bispecific ADC targeting HER2 and the lysosomal membrane protein CD63 showed better lysosomal aggregation and load delivery in preclinical experiments.
There are still plenty of opportunities for innovation in load selection. The choice of load is no longer limited to standard cytotoxic drugs.
R&D strategies beyond ADC drug design
In addition to ADC drug design and clinical research, clinical researchers shoulder the responsibility of exploring the clinical potential of ADC through rational design of clinical trials. This includes two aspects: discovering the patient groups most likely to benefit from ADC therapy, and studying combination therapy options that can have a synergistic effect with ADC therapy, thereby enhancing their clinical efficacy.
For the first task, predictive biomarkers for improved ADC therapy are clearly needed. At present, in clinical trials, immunohistochemical detection is a commonly used method to measure the expression of target proteins. However, IHC is a semi-quantitative test, and there is no clear theoretical basis for how to define what is positive. For different ADC therapies, the strength of the IHC signal intensity, required to produce anti-cancer activity may vary greatly, and once the threshold is reached, the cytotoxicity may not necessarily increase as the expression level of the target antigen increases.
A variety of factors may affect the therapeutic window and efficacy of ADC, such as target metabolism rate, expression heterogeneity, expression in non-tumor tissues, and characteristics of the tumor microenvironment. Therefore, in addition to quantifying target expression levels, the development of other biomarkers that represent tumor sensitivity to ADC provides important benefits in this area.
In terms of rational development of combination therapies, a number of early clinical trials are already underway. One of the strategies is to use drugs that target the target antigen to change the dynamic balance of the target antigen, thereby enhancing the sensitivity of cancer cells to ADC. This can be accomplished by stimulating the overexpression of the target antigen or promoting the degradation of the target. For example, the use of irreversible inhibitors against ADC targets can stimulate antigen internalization and ADC endocytosis or activity. Other methods can use feedback mechanisms. For example, inhibiting the MAPK signaling pathway can lead to an increase in AXL expression, thereby increasing the activity of the AXL-targeting antibody-conjugated drug enapotamab vedotin in the treatment of melanoma cancer cell lines.
In addition to the use of kinase inhibitors, the combination of ADC with other antibody therapies. For example, the combination of ADC with the anti-VEGFA monoclonal antibody bevacizumab has also shown activity in preclinical models. This may be because bevacizumab enhances the delivery efficiency of the drug by changing the vascular innervation of the tumor. Research on the mechanism of tumor resistance may also help discover potential combination therapy targets.
Currently, more than 20 clinical studies are testing the effect of ADC in combination with approved or under-development immunotherapies. The scientific basis of this combination is that ADC-mediated cell death may trigger an immune response and recruit tumor-infiltrating lymphocytes, thereby promoting the recognition of “cold” tumors by immune cells.
The author of the article also pointed out that the degree of clinical trials of all combination therapies needs to consider whether the combination therapy can bring more benefits in the case of potential toxicity. Therefore, the rational design of combination therapy should be based on preclinical data to the greatest extent. Just combining two independently effective drugs does not necessarily bring synergy, but may reduce the therapeutic window due to overlapping toxicity.
4 Conclusion
After decades of research and error correction, technological advancement and a better understanding of the mechanism of ADC activity have brought a variety of ADC therapies that have benefited cancer patients. The authors emphasize that the rules that apply to the development of standard chemotherapy or antibody therapies do not necessarily apply to predicting the clinical characteristics of ADC. The conceptual model that simplifies ADC into targeted drug delivery may require further improvement to describe the complexity of the ADC’s mechanism of action. On the whole, after the antigen and antibody are combined, a more nuanced understanding of ADC processing and activity will help the development of the ADC development field. If we have a better understanding and utilization of the subtleties of the interaction between ADCs and tumors, it will give full play to the true potential of this technology platform and may bring far-reaching and even revolutionary impacts to the treatment of cancer patients.