Used in antibody-drug conjugates (ADCs) as a cleavable linker to connect cytotoxic drugs to monoclonal antibodies. The Val-Cit-PAB segment is a substrate for cathepsin B, an enzyme overexpressed in tumor cells, enabling selective drug release inside cancer cells. The PNP (para-nitrophenol) acts as a self-immolative spacer that facilitates drug release upon enzymatic cleavage. The Mal-amido-PEG2 group provides a maleimide functionality for conjugation to thiol groups on antibodies and includes a short polyethylene glycol chain to improve solubility and stability of the conjugate. This linker system enhances the therapeutic index of ADCs by minimizing off-target toxicity and promoting targeted drug delivery.