Linker and Conjugation Site Synergy in Antibody–Drug Conjugates: Impacts on Biological Activity

Antibody–drug conjugates (ADCs) produced using general conjugation methods yield heterogeneous products containing mixtures of species with different numbers of payloads per antibody (drug–antibody ratios) conjugated at multiple sites. This heterogeneity affects the stability, efficacy, and safety of ADCs. Thus, various site-specific conjugation methods have been developed to achieve homogeneity in ADCs. It was reported that linker structures and conjugation sites generally affected the characteristics of site-specific ADCs such as stability, efficacy, and safety. However, the combined effects of conjugation sites and linker structures on the physicochemical and biological characteristics of site-specific ADCs have remained unclear. In this study, we generated 30 homogeneous site-specific ADCs with a combination of six conjugation sites and five linker structures using THIOMAB technology and evaluated the characteristics of these homogeneous ADCs. We found that both conjugation sites and linker structures affected characteristics unique to ADCs (linker stability as well as target-dependent and target-independent cytotoxicity) in site-specific ADCs. Especially, conjugation to the constant regions of the light chain and the presence of polyethylene glycol structures in the linker are important for those ADC-specific characteristics. Interestingly, we also found that the effects of linker structures on the target-independent cytotoxicity of homogeneous ADCs at certain conjugation sites differed from those seen in conventional heterogeneous ADCs. Our results suggest that optimizing linker structures based on the conjugation site may be necessary for site-specific ADCs.