NRP1 and neoplasm: The underlying mechanisms for the enhancement of tumor accumulation and penetration are summarized as follows: (1) the conjugate is long-circulating and has an applicable size; (2) iRGD peptides coupled to PPCD activate tumor-targeted properties to further improve the accumulation; and (3) the penetration of cells and tissues for iRGD-mediated PPCD is activated by proteolytic cleavage to expose the cryptic CendR, which binds to NRP-1 on the cell surface [87].