A New Force in Tumor Immunotherapy- Antibody-drug Conjugate

Posted by beauty33 on November 29th, 2019

What is antibody-drug conjugate?

Antibody-drug conjugate (ADC) links small biologically active drugs to monoclonal antibodies via a chemical link, and monoclonal antibodies act as carriers to deliver small molecular drugs to target cells. Monoclonal antibody and small molecule are both drugs, which can be used to treat diseases alone. The design idea of ADC is to make use of their advantages to make up for deficiencies. Monoclonal antibody is of high specificity, but usually the efficacy is not strong, and often need to be used together with small molecule drugs; small molecule drugs are of strong activity, but the disadvantage is that the specificity is poor, so there may be toxicity, which leads to side effects. Some small molecule drugs, especially peptide drugs, have a short half-life in the blood. By combining with monoclonal antibody, this defect can be improved. ADC combines the two and releases small molecules of drugs when it reaches the target cells, which can not only sensitively distinguish healthy and diseased tissues with limited effects on non-target cells and reduced toxicity, but also significantly improve pharmacokinetics and delivery to target tissues.

Problems in ADC Construction

The process of transporting ADC safely and effectively to the target organization is full of challenges, and a series of problems is involved as follows.

(1) Stability: Links must be stable in the blood circulation to reduce damage to healthy tissues, and premature splitting will release small molecules of drugs that cannot reach the target tissue.

(2) Non-specific immune response: ADC must maintain high immune affinity. Combining small molecular drugs with monoclonal antibodies shall not destroy the specific binding ability of monoclonal antibodies.

(3) Internalization and drug release: ADC usually releases small molecule drugs in cells through the combination of monoclonal antibodies and target cells. When the effective concentration of small molecule drugs in cells reaches a certain level, they can work well. This requires a sufficient number of small molecule drugs on each monoclonal antibody, but too many of them will cause changes in the nature of monoclonal antibodies. It is also challenging to control the constant number of small molecules. In addition, the number of antigens on the cell surface and the binding pattern of antibody antigens may also affect the internalization of ADC.

(4) Drug action: The release of small molecule drugs shall to be able to kill target cells at very low concentrations, so the activity of small molecule drugs is a factor that needs to be fully considered.

By adjusting the number of drug molecules linked to monoclonal antibodies, the desired efficacy can be controlled. According to (3) and (4), in order to obtain sufficient drug activity, multiple drug molecules need to be linked on a monoclonal antibody, but if too many drug molecules are linked, ADC will accelerate the aggregation, lose its affinity to the antigen, and be identified as harmful substances in the body and quickly removed, thus failing to meet the requirements of (1) and (2). The degree of drug molecule substitution also affects pharmacokinetics. Experiments show that in general, 2-4 drug molecules linked by each monoclonal antibody can balance the pros and cons of all aspects and have the best therapeutic window. In addition, in order to avoid interfering with antigen recognition, drug molecules should be linked to the heavy chain region of monoclonal antibodies, which does not affect the binding of monoclonal antibodies to antigens. The splittable links can release the drug molecules in a complete form in the target cells, thus obtaining the complete pharmacodynamics of the drug molecules. Influencing the stability of ADC structure is the biggest disadvantage of this link. Therefore, a balance must be found between blood stability and effective drug release.

Selection of appropriate antibodies

In general, monoclonal antibodies shall be of high affinity and selectivity to antigens, so that ADC can easily identify and enter target cells, and can maximize its efficacy. The selected monoclonal antibody should be aimed at the highly expressed antigens on the target cells, preferably human antibodies, so that the target cells can bind more ADC to enhance the efficacy, and the concentration of other parts of the body is relatively reduced to eliminate the toxic side effects. Other properties are of the same importance, such as longer half-lives, immune response and inhibition of target cells. At present, about 15% of ADCs use antibodies in clinical stage, and many modified antibodies such as bispecific antibodies and antibody fragments are frequently used. Many of the ADCs being developed have also saved many pre-clinical antibodies that do not have sufficient activity against cells when used alone, probably because the antigens they target are not necessary for cell survival. Once these antigens are internalized, the targeted antibodies need to be re-selected. Although some new antibodies are also used, there is no conflict between old and new antibodies, and any of them is potentially useful for ADC.

Selection of appropriate small molecule drugs

In the field of small molecule drug development, the research competition for the same target is very fierce. The research range of ADC targets, such as CD19, CD22, CD70 and EGFR (epidermal growth factor receptor), will be expanded, while more and more companies are focusing on the wider range of targets without competition. Although some target biological nodes have been established, such as HER2 and EGFR, many targets still do not have corresponding small molecule drugs.

Selection of the right link

It is very important to study the appropriate link and connection methods in the development of ADC. Links between monoclonal antibodies and small molecules are destroyed by endogenous proteases in the blood, releasing small molecules ahead of time, resulting in the same side effects as when chemotherapy is used alone. A new generation of links needs to be able to effectively reduce splitting, thereby releasing drug molecules when they reach the target cell. The selection of links is influenced by both small molecule drugs and monoclonal antibodies. Different small molecule structures require different links. Proteins, including monoclonal antibodies, provide fewer chemical sites for modification, mainly on the amino group of lysine or the sulfhydryl group of cysteine. Theoretically, any amino acid, lysine or cysteine of the monoclonal antibody can be modified. However, the coupling reaction yields a mixture of various ADCs. But if batch and quality control are qualified, the proportion of mixture can be mass-produced. The polarity of links also plays an important role in pharmacokinetics, pharmacodynamics and the number of links of small molecular drugs. Most commonly used links are hydrophobic.

ADC development trend

ADC links small molecule drugs to antibodies. It not only makes full use of the specificity, affinity and potency of small molecule drugs, but also reduces the potential toxicity of small molecule drugs by targeting small molecule drugs to target cells. Linking technology has a great impact on the efficacy, affinity and safety of ADC.

With the development of monoclonal antibodies and small molecule drugs, chemical linking technology has also made great progress. New types of links such as peptide bonds and disulfide bonds with steric hindrance have good stability in vivo. Non-splitting link ADC can avoid affecting normal cells, and its link is the most stable. This opens up a new direction for the development of ADC, that is, to modify small molecule drugs with a suitable non-splitting link, while small molecule drugs and non-splitting links as a whole can still maintain the original activity. Non-cleavage-linked ADCs must enter cells and degrade in cells. Even if it is difficult to enter cells, cleavage-linked ADCs can exert their efficacy by releasing small molecules of drugs through extracellular cleavage interpretation and reaching targets. Small molecules of drugs in cells can also exude cells and kill adjacent non-expressing antigen cells. 

During the coupling process, some antibodies are not coupled with small molecules, and the number and location of small molecules in antibodies that have been coupled are different. Therefore, it is necessary to find a method to control the number and location of small molecules on antibodies, so that more consistent ADC can be obtained, which will have unique pharmacokinetic, pharmacodynamic and toxicological properties. The structure-function relationship can be studied by changing different binding positions.