Used in antibody-drug conjugates (ADCs) as a cleavable linker-payload system. The Val-Cit-PAB motif is sensitive to cathepsin B, an enzyme overexpressed in tumor cells, enabling targeted release of the cytotoxic agent MMAE inside cancer cells. The amino-PEG4 spacer enhances solubility and provides flexibility, improving conjugation efficiency and stability in circulation. This construct allows selective delivery of the potent microtubule-disrupting agent MMAE to tumor tissues, minimizing systemic toxicity and increasing therapeutic index in cancer treatments. Commonly employed in the development of targeted oncology therapies.