The Complete Guide to Linker Selection in Antibody-drug Conjugate Development

An antibody-drug conjugate (ADC) connects a biologically active small molecule drug to a monoclonal antibody via a chemical link, and the monoclonal antibody acts as a carrier to transport the small molecule drug to the target cell. The use of cytotoxicity of small molecule drugs to kill tumor cells is the mechanism of many chemotherapeutic drugs. However, the cytotoxicity of small molecule drugs is too strong to pursue better therapeutic effects. Due to insufficient selectivity, it is also harmful to normal cells, which in turn causes damage to the body. The dose range between the maximum tolerated dose and the minimum effective dose is the therapeutic window, which may be scaled down due to the lack of drug selectivity, bringing inconvenience to clinical medication.

The basic requirement for an antibody drug conjugate linker is stability mainly in the water environment and in vivo. Such conjugates cannot be degraded in the systemic circulation to avoid reduction in efficacy and side reactions. In addition, the linking group can also be degraded under certain conditions to complete the drug release.

1 Amide bond-type linker groupsThis linker group forms an amide bond after the reaction to finish the antibody conjugation. Microorganisms use carbohydrate fermentation to produce a variety of industrial solvents and chemical raw materials, such as ethanol, acetone-butanol, butanol-isopropanol, acetone-ethanol, 2,3-butanediol, and glycerol fermentation. The amide bond has an advantage over the relatively simple reaction and stable conjugation, so that the therapeutic window can be enlarged to improve the tolerance of the body to the drug. However, the shortcomings are also obvious, that is, the conjugate is stable to most conditions, resulting in a single drug release mechanism and often relying on lysosome degradation, and it is difficult to design a complicated delivery strategy.

2 Hydrazine linker groupsHydrazine is formed after the reaction of drug conjugation. It can be hydrolyzed under acidic condition. Since the hydrogen ion concentration index in the human body circulation is close to neutral, the conjugate is stable in plasma. The tumor microenvironment and the strong acidity inside the tumor cells can help release the drug. For example, the design of the drug BR96-Doxorubicin is based on this principle. However, the disadvantage is that the sputum is not stable enough to occur in the blood. This drug release outside the range of the lesion is detrimental and can lead to drug intolerance and immune response, so the use of such linking groups should be extraordinarily cautious for safety assessment.

1. A disulfide linker The conjugation of the drug is completed after the formation of the disulfide linker, and the disulfide linker is stable under normal conditions without fracture in the systemic circulation. Since the reduced glutathione of the glutathione reduction system in the tumor reaches 1000 times that of the normal cell cytoplasm, it can be broken in the tumor cells to complete drug release. In addition, tumor cells also contain enzymes in the protein disulfide isomerase family, which can also promote the cleavage of disulfide linkers. Using this strategy, controlled cleavage of the conjugate can be achieved, thereby enabling targeted delivery of the drug.
2. A Short Peptide Linker GroupA polypeptide-based linker group is designed to maintain conjugate stability in the systemic circulation, and to allow cytotoxic drugs to be released under specific enzyme cleavage. Since sequences of different functions can be designed, they can be stabilized against pH, stabilized against some enzymes, and sensitive only to specific enzymes such as cathepsin B. The targeted release of the drug in a particular cell is then completed.

Antibody drug conjugates are a class of drugs that are very useful in the treatment of tumors. In the construction of such conjugates, in addition to focusing on the small molecule drugs themselves, the choice of monoclonal antibody species should also be concerned with the choice of linking groups. The main idea is to achieve targeted delivery of the drug through a reasonable fracture strategy, thereby improving the therapeutic effect.

About Creative BiolabsEstablished in 2004, Creative Biolabs is highly specialized in advanced antibody biochemistry and engineering, such as ADC payload. With more than a decade of exploration and expansion, our current research and service capacity covers the entire new drug discovery and development pipeline, ranging from early discovery, pre-clinical evaluations, cGMP manufacturing, to clinical trials. As an international cooperation, we have established offices all around the globe with more than 200 well-trained full-time scientists and technicians, who work closely with our customers and research partners to develop new medicines for a better, healthier world.

Creative Biolabs was founded by scientists who are dedicated to conquering cancer, optimizing the drug development process, leveraging accessible resources.