CPP and neoplasm: Upon arrival at the tumor site, CPP would be deshielded in response to the acidic extracellular pH and the overexpressed MMP-2 to promote the penetration of nanoparticles into tumor cells.500,501 In a similar way, a polyarginine CPP was covalently attached to a polyanionic inhibitory peptide through the HSSKYQ peptide linker, which is a cleavable substrate of the serine protease prostate-specific antigen (PSA).