As we all known, antibody-drug conjugate (ADC) is a drug obtained by chemical conjuagtion of antibody, linker and cytotoxin. Compared with simple monoclonal antibodies, ADC has better targeting and more efficient killing effect. Because of the complexity of ADC structure, many factors need to be considered to design a reasonable ADC. We classify it into four aspects: target, antibody and linker, cytotoxin.
Work with creative biolabs to analyze the key elements of ADC design.
First of all, we need to understand how to screen targets. The specific binding of antibodies to target antigens is the first step in the development of successful ADC drugs. The ideal antigen target is high expression on the surface of tumor cells, but low expression or no expression in normal tissues, or expression in limited tissues, such as CD37, which mainly expressed in mature B cells and bone marrow cells, and low expression in T cells, granulocytes and monocytes. The antigen targets expressed in normal tissues will combine with ADC drugs, resulting in “off-target” toxic effects, and reduce the dose of ADC enriched in tumor tissues, reducing the therapeutic effect.
Whether ADC drugs play an active role or not has a certain relationship with the number of antigens on the cell surface. It has been found that at least 10^4 antigen targets are needed on the surface of tumor cells to ensure that a lethal dose of cytotoxic drugs could be delivered into the cells and make ADC drugs bioactive. The ideal tumor cell surface should have a target antigen higher than 10^4 and evenly distributed on the cell surface. However, in practice, the number of antigens on the surface of tumor cells is limited, with an average of about 5000 to 10^6 antigens per cell surface. Moreover, the average DAR of most ADC drugs is 3.5 to 4. As a result, the number of small molecular cytotoxin that ADC actually delivers to tumor cells is very limited. Of course, this is also one of the reasons for the clinical failure of ADC to deliver general toxic drugs.
The ideal therapeutic effect can be achieved only when ADC delivers cytotoxic drugs to tumor cells and plays a toxic role. Therefore, the process of ADC entering tumor cells from the tissue environment is also an important step, which called internalization. The best target antigen can trigger the internalization pathway of the antibody-antigen complex into the cell after binding with the antibody, thus realizing the intracellular delivery of the drug. However, at present, there is also a kind of ADC that can work without internalization process, which is called non-internalization ADC. The principle underlying this new approach is based on the fact that because of the reducing conditions, the payload can be released extracellularly, such as in the tumor microenvironment, where it diffuses inside the tumor cells provoking their death. Indeed, many reports have documented that potent therapeutic activity can be obtained by targeting tumor or stroma cells components by non-internalizing ADC in different tumor models.
At present, the main target of ADC in clinical development stage is leukocyte surface antigen, such as CD33, CD30, CD79b, CD22, CD19, CD56, CD138, CD74. To a large extent, these antigens are high expression in tumor tissues, no expression or very low expression in normal hematopoietic tissues. In addition, some solid tumor surface receptor molecules have been gradually found to be suitable clinical ADC targets, such as PSMA, EGFR, HER2.
The screening factors of tumor target antigens can be summarized as follows：
- Cell surface localization: to allow efficient antibody/ADC binding.
- Tumor specific with decent protein expression: to ensure ADC specificity and efficacy.
- Higher tumor surface expression: if the identification of a tumor specific antigen is proven to be difficult, a shared cell surface antigen can also be considered under the condition that it shows a much higher expression at cancer cell surfaces comparing to that on normal cells to reduce collateral damages.
- Internalization: to ensure the ADC incorporation via receptor mediated endocytosis upon ADC binding.
- Desired turnover time: to facilitate maximizing ADC efficiency and efficacy.