Used in the development of antibody-drug conjugates (ADCs) as a cleavable linker system. The Val-Cit-PAB motif serves as a substrate for cathepsin B, an enzyme overexpressed in tumor cells, enabling targeted release of cytotoxic drugs inside cancer cells. The PEG4 spacer improves solubility and provides flexibility, enhancing the efficiency of drug release. The maleimide (Mal) group allows for stable conjugation to cysteine residues on monoclonal antibodies. This linker system is particularly valued for its stability in circulation and precise intracellular activation, improving the therapeutic index of ADCs.