An antibody-coupled drug (ADC) has the ability to deliver cytotoxicity to a specific tumor region with precision. It consists of three key components: a monoclonal antibody, a cytotoxic drug payload for delivery, and a junction molecule to connect the two. The monoclonal antibody is engineered to bind specifically to cancer cells. The junction molecule attaches the cytotoxic drug to the antibody, and its stability is critical for controlling the release of the drug in the target cell. When it interacts with a tumor antigen, it is taken up into the cell via a receptor-mediated endocytosis process. During this process, the core function of the junction is to employ a variety of mechanisms, such as chemical cleavage, in order to release the active ingredients of the cytotoxic drug. Once released, these components achieve therapeutic benefits by disrupting core cellular functions and triggering apoptotic mechanisms. Many pharmaceutical entities are currently utilizing ADC therapies to advance new drugs and progress through clinical trials. ADC therapies have come a long way in the last two decades, gaining approval for a variety of cancer targets. Over the past five years, there has been a significant increase in R&D concentration and patent filings related to ADC therapies.

Antibody is an important component of ADC with requirements. It should have high affinity for the target and affinity antigen, but no binding or insignificant binding to the off-target site, and selective binding to the target antigen is important to the accumulation and retention of ADCs in the target site. For example, the Fab region ADC of an antibody can be degraded by blocking ligand binding, interfering with concomitant dimerization or endocytosis, and target protein degradation. The large size of the antibody can also lead to target penetration problems solid tumors. The targeting ability of antibodies is achieved by their small variable loop structure (VH) present in the terminal portion of the Fab fragment; therefore, native antibodies can be used to develop smaller binding motifs such as F(ab)2, Fab′, Fab, and Fv fragments. In addition humanized fragments of uncommon IgGs, such as heavy chain variable based structural domain (VHH) and heavy chain variable structural domain antibody fragments. Due to their small size, ease of production, ease of handling, coupling, high solubility, stability and delayed serum clearance, these antibody fragment couplers or FDCs with their antibody precursors.

Site-specific antibody coupling has been introduced into ADC development to improve therapeutic efficacy. Antibodies can be engineered using genes for modification engineering, chemical enzyme modification or metabolic labeling of their Fab or Fc regions for easy introduction of site-specific reaction site conjugation. In addition to natural amino acids, unnatural amino acids (UAA) (also known as non-classical amino acids) containing orthogonal side chain functional groups are also being used to generate antibodies that stabilize the coupling site. For example, the microtubule protein inhibitor payload AS269 was doped with HER2-targeted coupled mAb with UAA, pAF. The anti-HER2ADC (ARX788) obtained had a DAR of 1.9 and exhibited high serum stability and half-life.ARX788 showed strong anti-tumor activity in mice and is currently undergoing phase III clinical trials. Recently, other ligands such as short peptide tags, modified free oligosaccharides and small molecule based affinity ligands have also been used for generating coupling site loading of antibodies.

The active molecule of an ADC is attached to an antibody, and this antibody is responsible for recognizing the specific target of the drug. To ensure effectiveness, the junction must be stable in the plasma environment. The linker should be hydrophilic enough to moderate or mitigate the solubility effect of the slug, which in most cases is lipophilic. The linker should not aggregate; as aggregation may impair antibody activity and may reduce ADC stability. Finally, the linker should completely and selectively release the drug under appropriate conditions. The choice of linker will be determined by the function present on the drug and antibody. Connectors can be categorized into two main types: one is cleavable and the other is non-cleavable. Cleavable connectors allow easy separation of the drug from the antibody without the need for cleavage by antibody protein hydrolysis; in contrast, non-cleavable linkers are dependent on the protein hydrolysis process of the antibody in order for the drug to diffuse to its site of action. Cleavable connectors allow the drug attached to the antibody to be released from the ADC without damaging the antibody. The common types are those cleavable junctions that can be broken by acids, reducing agents, or enzymes. Non-cleavable junctions, on the other hand, are designed to remain structurally intact after the antibody is internalized into the lysosome and before it undergoes protein hydrolysis. Since conditional cleavage of ADCs containing non-cleavable junctions is rare or absent, premature release of the drug fraction should be limited to cleavage of the antibody itself, an unlikely event. Therefore, off-target toxicity of ADCs with non-cleavable junctions should be minimized.

The payload is capable of shipping cytotoxicity, connecting the drug to the antibody, and its mechanism of action will determine the efficacy of the resulting ADC as an anticancer compound and its possible indications. First generation ADCs coupled to conventional chemotherapeutic agents (paclitaxel, anthracycline) lacked efficacy because the payload was not effective enough. Tumor penetration, copy number of the target on the cell surface will payload. There are two main targets involved in payload: one is cell cycle-dependent (target DNA) and the other selection criterion focuses on cell cycle-dependent (involving microtubule inhibition with G2/M-phase blockade) payloads covering stability, solubility and their coupling properties. Especially critical is the need for the payload to maintain a steady state in the somatic circulation before reaching the target site. Drug molecules containing olefins, epoxides, or disulfide bonds may be subject to conversion or reduction by cytosolic enzymes, whereas acid-sensitive drugs may be degraded in the lysosome and lose their efficacy before they reach the cytoplasmic target. Effective payloads must be sufficiently soluble to allow antibody coupling in aqueous buffers, as higher concentrations of organic solvents may cause denaturation of the antibody scaffold.