Improved positron emission tomography imaging of glioblastoma cancer using novel 68 Ga-labeled peptides targeting the urokinase-type plasminogen activator receptor (uPAR)

Amino Acids. 2017 Jun;49(6):1089-1100. doi: 10.1007/s00726-017-2407-4. Epub 2017 Mar 18.

Abstract

The urokinase-type plasminogen activator receptor (uPAR) is overexpressed in several cancers including glioblastoma (GBM) and is an established biomarker for metastatic potential. The uPAR-targeting peptide AE105-NH2 (Ac-Asp-Cha-Phe-(D)Ser-(D)Arg-Tyr-Leu-Trp-Ser-CONH2) is a promising candidate for non-invasive positron emission tomography (PET) imaging of uPAR. Despite the optimal physical properties of 68Ga for peptide-based PET imaging, low tumor uptakes have previously been reported using 68Ga-labeled AE105-NH2-based tracers. In an attempt to optimize the tumor uptake, we developed three novel tracers with alkane (AOC) and polyethylene glycol (PEG) spacers inserted between AE105-NH2 and the radio metal chelator 2-(4,7-bis(carboxymethyl)-1,4,7-triazonan-1-yl)pentanedioic acid (NODAGA). The resulting tracers NODAGA-AOC-AE105-NH2, NODAGA-PEG3-AE105-NH2 and NODAGA-PEG8-AE105-NH2 were compared to the non-spacer version, NODAGA-AE105-NH2. Following radiolabeling with 68Ga, we evaluated the in vitro and in vivo performance in mice bearing subcutaneous tumors derived from the uPAR-expressing human GBM cell line U87MG. In vivo PET/CT imaging showed that introduction of PEG spacers more than doubled the in vivo tumor uptake after 1 h compared with the non-spacer version: 68Ga-NODAGA-PEG3-AE105-NH2 (2.08 ± 0.37%ID/g) and 68Ga-NODAGA-PEG8-AE105-NH2 (2.01 ± 0.22%ID/g) vs. 68Ga-NODAGA-AE105-NH2 (0.70 ± 0.40%ID/g), p < 0.05. In addition, 68Ga-NODAGA-PEG8-AE105-NH2 showed significantly higher (p < 0.05) tumor-to-background contrast (3.68 ± 0.23) than the other tracers. The specific tumor-targeting property of 68Ga-NODAGA-PEG8-AE105-NH2 was established by effectively blocking the tumor uptake with co-injection of unlabeled AE105-NH2 (1 h: unblocked 2.01 ± 0.22%ID/g vs. blocked 1.24 ± 0.09%ID/g, p < 0.05). Ex vivo biodistribution confirmed the improved tumor uptakes of the PEG-modified tracers. 68Ga-NODAGA-PEG8-AE105-NH2 is thus a promising candidate for human translation for PET imaging of GBM.

Keywords: Glioblastoma; PET; PET/CT; Radiotracer; uPAR.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Cell Line, Tumor
  • Contrast Media* / chemistry
  • Contrast Media* / pharmacology
  • Gallium / chemistry
  • Gallium / pharmacology
  • Glioblastoma / diagnostic imaging*
  • Glioblastoma / metabolism
  • Humans
  • Peptides* / chemistry
  • Peptides* / pharmacology
  • Positron-Emission Tomography*
  • Receptors, Urokinase Plasminogen Activator / agonists*

Substances

  • Contrast Media
  • Peptides
  • Receptors, Urokinase Plasminogen Activator
  • Gallium