title: [⁶⁸Ga]NS₃-RGD and [⁶⁸Ga] Oxo-DO3A-RGD for imaging α(v)β₃ integrin expression: synthesis, evaluation, and comparison.
abstract: BACKGROUND
  ⁶⁸Ga-labeled RGD peptides in combination with PET allow non-invasive determination of α(v)β₃ integrin expression which is highly increased during tumor-induced angiogenesis. The aim of this study was to synthesize and evaluate two RGD peptides containing alternative chelating systems, namely [⁶⁸Ga]NS₃-RGD-RGD and [⁶⁸Ga]Oxo-DO3A-RGD and to compare their in vitro and in vivo properties with [⁶⁸Ga]DOTA- and [⁶⁸Ga]NODAGA-RGD.
  
  
  METHODS
  Syntheses of both radiotracers followed standard SPPS protocols. For in vitro characterization distribution coefficients, protein binding abilities, serum stabilities, and α(v)β₃ integrin binding affinities were determined. For in vitro tests as well as for the biodistribution assay α(v)β₃ positive human melanoma M21 and α(v)β₃ negative M21-L cells were used.
  
  
  RESULTS
  ⁶⁸Ga-labeling of NS₃-RGD resulted in good radiochemical purity, whereas HPLC analysis showed two peaks with a ratio of 1:6 for [⁶⁸Ga]Oxo-DO3A-RGD. Distribution coefficients were -3.4 for [⁶⁸Ga]Oxo-DO3A-RGD and -2.9 for [⁶⁸Ga]NS₃-RGD. Both radiotracers were stable in PBS solution at 37°C for 2 h but lack stability in human serum. Protein binding was approximately 40% of the total activity for [⁶⁸Ga]NS₃-RGD and 70% for [⁶⁸Ga]Oxo-DO3A-RGD, respectively, resulting in high blood pool activities. Biodistribution assays confirmed these findings and showed an additional high uptake in liver and kidneys, especially for [⁶⁸Ga]NS₃-RGD. Furthermore, [⁶⁸Ga]Oxo-DO3A-RGD showed nearly the same activity concentrations in α(v)β₃ positive and α(v)β₃ negative tumors.
  
  
  CONCLUSIONS
  [⁶⁸Ga]Oxo-DO3A-RGD and [⁶⁸Ga]NS₃-RGD have inferior characteristics compared to already existing ⁶⁸Ga-labeled RGD peptides and thus, both are not suited to image α(v)β₃ integrin expression. Of all our tested RGD peptides, [⁶⁸Ga]NODAGA-RGD still possesses the most favorable imaging properties. Moreover this study shows that the use of appropriate chelators to achieve good targeting properties of ⁶⁸Ga-labeled biomolecules and careful in vitro and in vivo evaluation including comparative studies of different strategies are essential components in designing an effective imaging agent for PET.
authors.affiliation: Department of Nuclear Medicine, Innsbruck Medical University, Innsbruck, Austria.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Knetsch
authors.forename: Milos
authors.initials: M
authors.lastname: Petrik
authors.forename: Christine
authors.initials: C
authors.lastname: Rangger
authors.forename: Gesine
authors.initials: G
authors.lastname: Seidel
authors.forename: Hans-Jürgen
authors.initials: HJ
authors.lastname: Pietzsch
authors.forename: Irene
authors.initials: I
authors.lastname: Virgolini
authors.forename: Clemens
authors.initials: C
authors.lastname: Decristoforo
authors.forename: Roland
authors.initials: R
authors.lastname: Haubner
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D002614:Chelating Agents / Q000737:chemistry*; D060326:Chemistry Techniques, Synthetic; D056831:Coordination Complexes / Q000138:chemical synthesis* / Q000737:chemistry / Q000493:pharmacokinetics; D005710:Gallium Radioisotopes; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D039302:Integrin alphaVbeta3 / Q000378:metabolism*; D007553:Isotope Labeling; D051379:Mice; D009842:Oligopeptides / Q000138:chemical synthesis* / Q000737:chemistry / Q000493:pharmacokinetics; D010456:Peptides, Cyclic / Q000138:chemical synthesis* / Q000737:chemistry / Q000493:pharmacokinetics; D049268:Positron-Emission Tomography / Q000379:methods*
chemical_list: C581139:68Ga-NS3-RGD; C581140:68Ga-oxo-DO3A-RGD; D002614:Chelating Agents; D056831:Coordination Complexes; D005710:Gallium Radioisotopes; D039302:Integrin alphaVbeta3; D009842:Oligopeptides; D010456:Peptides, Cyclic; C047981:arginyl-glycyl-aspartic acid