[20] Preparation of antibody-linked cytotoxic agents

This chapter presents the preparation of antibody-linked cytotoxic agents. It discusses the strategy of linking cytotoxic agents to immunoglobulins so that biologically active conjugates retaining agent and antibody activities are produced. Immunoglobulins share a number of distinctive structural features, while exhibiting enormous structural diversity compared to other biologically active proteins. Distinctive features include (a) a tetrameric arrangement of two light (L) chains (Mr 23,000) and two heavy (H) chains (Mr 50,000-70,000) linked by disulfide bridges and (b) the division of each dimer made up of one L and one H chain into variable (V) (antigen binding) and constant (C) regions. The V region occurs near the amino terminal end of the immunoglobulin chain. Immunoglobulins of different classes differ considerably in the amino acid sequence of the C segment of their H chains, in chain length, in the number of homologous domains in the chain, in the disulfide bridge pattern, in the degree of polymerization, and in the position, numbers, and kind of oligosaccharides. When L or H chains from the same class and species are compared, the C region is found to contain an invariant sequence characteristic of the class and species, whereas the V region shows multiple substitutions ranging from 10 to 60 in the first 110 residues.