Elucidating Critical Factors of Internalization and Drug Release of Antibody-Drug Conjugates (ADCs) Using Kinetic Parameters Evaluated by a Novel Tool Named TORCH

During the past decade, antibody-drug conjugates (ADCs) have emerged as new drugs in cancer therapy with 15 ADCs already approved such as Kadcyla, Enhertu, and Adcetris. ADCs contain a cytotoxic drug that is linked to an antibody, allowing for specific delivery of the warhead to tumor cells. Typically, the antibody targets a tumor-specific antigen expressed on the cell surface. After the internalization of ADCs into cells, the linker is often cleaved by enzymes in the lysosomal compartment of the cell, releasing the warhead and thereby allowing for its interaction with, for example, the DNA or the tubulin cytoskeleton, which finally leads to cell death. Consequently, binding, internalization, and drug release are key attributes for the efficacy of ADCs. Here, we describe a novel molecule named TORCH (Turn On after Release by CatHepsins) that contains a fluorescence quencher system that is separated by a cathepsin B-cleavable linker. When conjugated to an antibody, the TORCH molecule allows one to gain valuable insights on the internalization and drug release of ADCs. While we cannot exclude the influence of other factors such as receptor recycling, we have found that the receptor density is directly related to the amount of payload released intracellularly, meaning that the internalization per receptor is very similar for all investigated antibodies and cell lines.