As a representative of the development direction of the new generation of antibody technology, antibody drug conjugates (ADCs) have high specificity of antibodies and high toxicity of cytotoxic drugs to tumors. Cytotoxic Agents play an anti-tumor role in ADCs, which can affect the expected efficacy of target tumors. Its chemical structure, coupling mode and coupling number also affect the biological activity of the antibody. In theory, cytotoxic substances such as chemotherapeutic drugs, toxins and radionuclides that have great killing effects on tumor cells can be used as cytotoxic molecules of ADCs.

Chemical characteristics and mechanism of action

The choice of cytotoxins is crucial for ADCs. The cytotoxin used for coupling must have clear mechanism of action, small molecular weight, high activity, and no immunogenicity, and they can still retain anti-tumor activity after chemically coupled to the antibody. At present, the mechanism of anticancer drugs used in clinic is mostly to interfere with or block the proliferation of cells.  According to the mechanism, it can be roughly divided into four categories.

• Inhibition of DNA synthesis. The main structural characteristics of this anticancer drugs are as follows: containing a pyrimidine or heterocyclic compound similar to pyrimidine, and a glucose analogue with five-membered ring. It interferes with DNA synthesis by blocking the synthesis and exchange of deoxypurine nucleosides or deoxypyrimidine nucleosides. Such as, 5-Fluorouracil, which mainly exerts anticancer effects through antimetabolic effects.
• Destruction of DNA structure or DNA synthesis. Anticancer drugs that directly destroy DNA structures or synthesize DNA are mostly alkylating agents and anticancer antibiotics. The structure and function of DNA are destroyed by cross-linking with DNA through alkylation. For example, nitrogen mustard alkylating agent betamerphalan mainly exerts anticancer effect through alkylation.
• Inhibition of protein synthesis. Most of these drugs contain antifolate whose core is a pyrrole or pyrazine pyrimidine group, which affect the normal replication process of proteins by destroying the activity of intracellular folate reductase, thus inhibiting the growth of tumors.  Such as, pemetrexed disodium and methotrexate.
• Inhibition of cell mitosis. These anticancer drugs are mainly characterized by multi-hydroxyl and multi-thick ring structural. Alkaloids derived from plants account for the majority of active ingredients. Such as, vincristine and vindesine.

Application

The selection of cytotoxic is very wide. In principle, compounds with high enough inhibitory activity against tumor cells can be used as coupling drugs. At present, the most cytotoxic used in ADCs are Auristatins, Maytansine and Calicheamicins. Majority of clinical research involving ADCs projects use tubulin inhibitors, and both subtoxoids have approved products on the market (Adcetris uses oristatin MMAE and Kadcyla uses maytansinoid DM1). Among them, Auristatin is dominating the market, accounting for more than 50% of ADC drugs under investigation.