A Cyclic\nPeptide Targets Glioblastoma by Binding to\nAberrantly Exposed SNAP25

Disease-specific changes in tumors and other diseased\ntissues are\nan important target of research because they provide clues about the\npathophysiology of the disease as well as uncover potentially useful\nmarkers for diagnosis and treatment. Here, we report a new cyclic\npeptide, CESPLLSEC (CES), that specifically accumulated (homed) in\nintracranial U87MG and the WT-GBM model of glioblastoma (GBM) from\nintravenous (IV) injection, and associated with the vasculature. Affinity\nchromatography of U87MG tumor extracts on insolubilized CES peptide\nidentified Synaptosomal Associated Protein 25 (SNAP25) as a candidate\ntarget molecule (receptor) for CES. Several results supported the\nidentification of SNAP25 as the CES receptor. IV-injected FAM-CES\ncolocalized with SNAP25 in the tumors, and direct binding studies\nshowed specific binding of the CES peptide to recombinant human SNAP25.\nA CES peptide–drug conjugate designed for photodynamic therapy\nshowed selective cytotoxicity to SNAP25<sup>+</sup> glioblastoma cell\nlines. Specific accumulation of systemically injected anti-SNAP25\nantibody in U87MG glioblastoma and labeling of intact U87MG cells\nwith anti-SNAP in flow cytometry showed that SNAP25 is available from\nthe circulation but not in normal tissues and that it is present at\nthe cell surface. Using an array of ECM proteins and surface plasmon\nresonance revealed that SNAP25 binds moderately to collagen V and\nstrongly to collagen VI. Modeling studies suggested that CES and collagen\nVI compete for the same binding site on SNAP25. Our results introduce\nCES as a valuable targeting peptide for drug delivery and its receptor\nSNAP25 as a possible molecular marker of interest for glioblastoma.