Antibodies are the precise guidance components of ADCs. In theory, molecules that bring effector molecules to the surface of tumor cells can play the role of antibody guidance.

The choice of antibody depends on the target of the disease. Targets can be divided into tumor-specific antigens (TSA) and tumor-associated antigens (TAA) according to their expression. Antigens that are only expressed in tumor cells but not in normal cells are tumor-specific antigens and the most ideal targets; antigens that are lowly expressed in normal tissues but highly expressed in tumor tissues are tumor-related antigens and can also be used as suitable candidate targets. Tumor-specific antigens only exist on the cell membrane proteins or membrane protein complexes on the surface of tumor cells, mainly including pMHC mutations on the surface of tumor cells and membrane proteins with mutations in the extracellular region; tumor-related antigens can be subdivided into cell proliferation-related and carcinoembryonic antigens, Leukocyte differentiation antigen, etc.

Table 1: Tumor target antigen types.

Factors to consider in antibody screening

• High specificity and high affinity are the main characteristics of antibodies in ADC drugs. In addition, the antibody should also have low immunogenicity, low cross-reactivity, and appropriate connection and binding properties.
• Antigen-antibody complex mediates the endocytosis efficiency of ADC drugs and the accurate localization in cells after endocytosis (such as lysosomes).
• In addition, the modification of antibodies, such as glycosylation modification of the constant region of the antibody or modification of the attachment site, can help to improve the characteristics and efficacy of drugs.

The high specificity of antibody molecules is the basic requirement to achieve the efficacy of ADC drugs, and lgG1 is the most commonly used subtype.

After the target antigen is selected, the antibody needs to specifically bind to the antigen for precise guidance, so the high targeting and potential immunogenicity of the antibody must be considered. At present, ADC drugs use humanized monoclonal antibodies to modify the Fc fragment to enhance its ADCC (antibody-dependent cell-mediated cytotoxicity) effects and CDC (complement-dependent cytotoxicity) effects.

At present, ADC drugs mostly use IgG1 molecules in humanized monoclonal antibody subtypes. Its advantage is that it has a higher affinity for the target antigen and a longer half-life in the blood. Compared with other IgG molecules, IgG1 has multiple natural sites for coupling, has better binding activity and is easier to produce, and is mostly the first choice for ADC drug development. With the improvement of technology, IgG4 has also begun to be used in the development of ADC drugs, but the content of IgG4 in plasma is low, the structural stability is not as good as IgG1, and it is easy to aggregate under low pH conditions.

Table 2: Differences in structure and function of different humanized IgG subclasses.

a: Depends on allotype; b: A/A isomer; c: Association constant (×10⁶ M^-1) for monovalent binding; d: After repeated encounters with protein antigens, often allergens.

Considering that different subtypes of IgG have their own unique effects, the focus of subsequent research and development may be to select heavy and light chains of different subtypes for recombination to produce new monoclonal antibodies.

Konitzer et al. replaced IgG1 heavy chains with IgG2 or IgG4 heavy chains to reconstruct Rituximab, which significantly improved its ability to induce apoptosis in vitro. The determinants of this behavior are the hinge area and CH1 area of the heavy chain. Therefore, in vitro modification of antibodies to improve cytotoxicity or enhance antibody specificity and other capabilities is a new direction for antibody development in ADC drugs.