Fig. 5

Classification of linkers in ADCs. Linkers of ADCs are classified into two categories: cleavable and non-cleavable linkers. Cleavable linkers consist of seven subtypes, which can be further divided into chemical cleavable and enzymatic cleavable linkers. In chemically cleavable linkers, the C-terminus of the hydrazone linker is conjugated to the cysteine residue of the antibody via an acetylbutyryl (AcBut) group, while its hydrazine terminus (NH-NH-R₃) is directly attached to the cytotoxic agent. For disulfide linkers, one sulfur atom originates from the cysteine residue of the antibody, whereas the other sulfur atom in the disulfide bridge stems from the thiol group of the cytotoxic agent. In Val-Cit and Val-Ala peptide linkers, the N-terminus (-NH₂) is covalently linked to antibody cysteine residues via a maleimide moiety, while the C-terminus (-COOH) is tethered to the cytotoxic agent through a PABC group. The N-terminus (-NH₂) of Gly-Gly-Phe-Gly is covalently conjugated to antibody cysteine residues via a maleimide moiety, while the C-terminus (-COOH) is connected to the cytotoxic agent through a PABC linker. In Glucosidase cleavable linkers, the C₁ hydroxyl group (-OH) is covalently attached to the PABC moiety, which cennects the cytotoxic agent to the antibody. The spacer serves as a crucial component of this linker type, meticulously engineered to ensure optimal length and flexibility for maintaining linker stability. Common spacer designs include alkyl chains, PEG, amino acid/peptide sequences, or aromatic moieties. In maleimidocaproyl linkers, the maleimide group facilitates site-specific conjugation to cysteine residues on the antibody and to the amino group of cytotoxic agents via the carboxyl group of the caproyl moiety. (http://pubchem.ncbi.nlm.nih.gov)