Three generations of ADC: Nearly a decade of development-PII  

Kadcyla and second-generation non-cleavable linkers

Alternative linker design methodologies are still under development. Immunogen discovered a very powerful ADC by an accidental discovery. The lysine residue of trastuzumab is linked to DM1 via a non-cleavable linker containing N-succinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SMCC), an ADC (T-DM1, Kadcyla) authorized by the FDA in 2013 for treatment in HER2-positive breast cancer patients. This novel ADC is very effective in HER2-positive breast cancer models, where the original structure is active only after ADC is internalized and completely digested by enzymes in the lysosome to obtain the active metabolite Lys-MCC-DM1.

Adcetris, Polivy, and second-generation cleavable linkers

Meanwhile, Seagen (formerly Seattle Genetics) designed its own coupling technology to bioconjugate marigold (MMAE) to the cysteine residues of anti-CD30 antibodies via the cleavable linker mc-VC-PABC, which contains a maleimide-based spacer, a standard Val Cit dipeptide sequence as a histoproteinase substrate, and a PABC self-degrading spacer, an ADC (Adcetris) that was approved by the FDA in 2011 for the treatment of mesenchymal large cell lymphoma and Hodgkin's lymphoma.

3rd Generation ADC

Internalization of monoclonal antibodies, transport or recirculation-related disturbances, antigen shedding, and defective lysosomal degradation of ADCs have all been linked to lower drug release, which has a negative impact on ADC efficacy. As a result, new technologies linked to bioconjugation, carrier forms, linkers, or toxic drugs are urgently needed to expand the application areas of ADCs, and third-generation ADCs have emerged.

Site-specific ADCs

Despite the rising success of ADCs, every licensed ADC on the market till 2019 is a heterogeneous mixture with varying levels and distributions of cytotoxic drugs on monoclonal antibodies, causing analytical issues throughout manufacture. Indeed, the DAR is unregulated, and the pharmacokinetics and efficacy of ADCs can be considerably influenced by this complicated mixture.

Naked antibodies may be competitive inhibitors. Weak DAR couples are less efficacious, and antibodies with high DAR are rapidly eliminated in plasma, thus compromising the therapeutic window for ADCs. To broaden the therapeutic index of ADCs, site-specific bioconjugation approaches have been extensively developed since 2008, which can be classified into natural or unnatural amino acid bioconjugation, enzyme-based bioconjugation, or linker-based bioconjugation.

The first method is to use antibody engineering to add certain amino acids. The second strategy employs enzyme-mediated region-specific bioconjugation. Finally, natural monoclonal antibodies can also be used to create region-specific ADCs.

Other types of ADC

Despite its efficacy, most ADCs targeting solid tumors have not progressed beyond phase 2 clinical trials, suggesting that other parameters need to be optimized in order to reach the market. The size of ADC (150 kDa), which is linked to tumor penetration and malabsorption, limits its efficiency. ADC's prolonged half-life, generated by FcRn, increases exposure to healthy tissue, whereas FcR cross-reacts with endothelial cells and the immune system, all of which are linked to off-target toxicity. Smaller binding forms, such as peptides, single-domain antibody fragments (sdAb or VHH), single-chain antibodies (scFv), antigen-binding fragments (Fab), or small immune proteins (SIP) dimerized using the CH4 structural domain, have been explored to compensate for these limitations.

New targets and associated release systems

In solid tumors rich in interstitial cells, targeting internalized antigens on the surface of cancer cells is very difficult. Therefore, a new approach was developed where the cancer cells are replaced by the tumor microenvironment (stroma or vascular system). In this strategy, extracellular proteases and other components of the extracellular matrix (such as acidic mediators or reduced glutathione) can be used for effective extracellular release for the cytotoxicity of non-internalized ADCs.