CTSB and cancer: We successfully synthesized four quaternary ammonium ADCs by linking camptothecin derivatives CPTS-1 and WL-14 to trastuzumab via Val-Ala and Val-Cit dipeptide quaternary ammonium linkers, which can be cleaved by cathepsin B. In comparison to the positive control DXd, both CPTS-1 and WL-14 demonstrated superior cytotoxic activity against HER2-positive cancer cells.