Used in the development of antibody-drug conjugates (ADCs) as a cleavable linker system. The Val-Cit-PAB motif is a substrate for cathepsin B, an enzyme overexpressed in many tumor cells, enabling targeted release of cytotoxic drugs inside cancer cells. The PEG4 spacer improves solubility and reduces aggregation, enhancing the pharmacokinetic profile of the conjugate. The terminal Mal (maleimide) group allows stable conjugation to cysteine residues on monoclonal antibodies. This linker system is particularly valued for its stability in circulation and efficient intracellular drug release, making it a key component in next-generation ADCs for cancer therapy.