Main elements of antibody-drug conjugate (ADC): Linker (cleavable/non-cleavable, structure and mechanism)

Val-Ala dipeptide is also widely used. Seven molecules are in the clinical stage. The fastest progress is loncastuximab tesirine, which includes a PEGylated spacer to balance the lipophilicity of the payload sg3199 belonging to the PBD dimer family.

Research shows that Val CIT is difficult to achieve high Dar due to precipitation and aggregation. In contrast, the Val ala linker allows Dar up to 7.4 with limited aggregation (< 10%). Compared with Val CIT, Val ala has lower hydrophobicity, which explains why this linker is excellent in lipophilic payload (such as PBD dimer). Val ala linkers of seven clinical candidate ADCs are connected to PBD.

Some studies have compared the structure of Val CIT and val ala dipeptides with the payload connection of MMAE. In the case of non internalized antibodies, Val CIT and val ala linkers bound to engineered cysteine showed similar characteristics and better performance than Val Lys and val Arg analogues. In the case of anti HER2 ADC with random cysteine binding, Val ala showed less aggregation in the high Dar structure than Val CIT. On the other hand, the two linkers showed similar buffer stability, cathepsin B release efficiency, cell activity and histopathological characteristics. Tetrapeptide Gly-Gly-Phe-Gly shows all the characteristics of stable and effective cleavable linkers, which are used by the listed ADC drug enhertu. The first trimester of enhertu is a plasma stable ADC with a dar of 7.7. Protease degradation occurs in lysosomes and dx-8951f is released. It is an effective topoisomerase I inhibitor derived from exatecan. Since the linker does not contain solubilizers, reaching such a high Dar is very considerable because it contradicts the widely established principle that high Dar conjugates may have poor pharmacokinetic characteristics. The self degradation spacer used here is simple and compact hemiamination, rather than the PABC used by Val-CIT linker.