title: Synthesis and evaluation of a99mTc tricarbonyl-labeled somatostatin receptor-targeting antagonist peptide for imaging of neuroendocrine tumors.
abstract: BACKGROUND
  A somatostatin receptor (SSTR)-targeting antagonist peptide (sst2-ANT) was radiolabeled with99mTc tricarbonyl via a tridentate [N,S,N]-type ligand (L) to develop a radiodiagnostic agent,99mTcL-sst2-ANT, for imaging of SSTR-expressing neuroendocrine tumors.
  
  
  METHODS
  Receptor affinity was assessed in vitro with the nonradioactive analogue, ReL-sst2-ANT, via a challenge experiment in AR42J cells with125I-SS-14 as the competing radioligand. Preparation of99mTcL-sst2-ANT was achieved via reaction of [99mTc(CO)3(H2O)3]+with L-sst2-ANT. To test the stability of the radiolabeled complex, challenge experiments were performed in phosphate-buffered saline solutions containing cysteine or histidine and also in mouse serum. Biodistribution and micro-SPECT/CT imaging studies were performed in AR42J tumor-bearing female ICR SCID mice.
  
  
  RESULTS
  The half maximal inhibitory concentration (IC50value) of ReL-sst2-ANT in AR42J cells was 15nM. Preparation of99mTcL-sst2-ANT was achieved with ≥97% radiochemical yield (RCY) and was verified by HPLC co-elution with the ReL-sst2-ANT analogue. The radiolabeled complex remained intact for up to 24h in high concentration solutions of cysteine and histidine at 37°C. Furthermore, the radiotracer was 90% stable for 1h at 37°C in mouse serum. Micro-SPECT/CT images showed clear uptake in tumors and were supported by the biodistribution data, in which the 3.2% ID/g tumor uptake at 4h was significantly blocked by co-administration of nonradioactive SS-14.
  
  
  CONCLUSIONS
  A [99mTc(CO)3(N,S,N)]+chelate was employed for radiolabeling of an SSTR-targeting antagonist peptide. Synthesis of99mTcL-sst2-ANT was achieved in high RCY, and the resulting complex displayed high in vitro stability. Somatostatin receptor affinity was retained in both cells and in tumor-bearing mice, where the complex successfully targeted SSTR-positive tumors via a receptor-mediated process. Advances in Knowledge and Implications for Patient Care. This first99mTc-tricarbonyl-labeled SSTR antagonist peptide showed promising in vivo tumor targeting in mice. Future studies may lead to translation of a similar design into the clinic.
authors.affiliation: Department of Chemistry, University of Missouri, Columbia, MO 65211, USA.
authors.forename: Lauren
authors.initials: L
authors.lastname: Radford
authors.affiliation: Structural Biology Core, University of Missouri, Columbia, MO 65211, USA.
authors.forename: Fabio
authors.initials: F
authors.lastname: Gallazzi
authors.affiliation: Research Service, Harry S. Truman Memorial Veterans' Hospital, Columbia, MO 65201, USA.
authors.forename: Lisa
authors.initials: L
authors.lastname: Watkinson
authors.affiliation: Research Service, Harry S. Truman Memorial Veterans' Hospital, Columbia, MO 65201, USA.
authors.forename: Terry
authors.initials: T
authors.lastname: Carmack
authors.affiliation: Research Service, Harry S. Truman Memorial Veterans' Hospital, Columbia, MO 65201, USA.
authors.forename: Ashley
authors.initials: A
authors.lastname: Berendzen
authors.affiliation: Research Service, Harry S. Truman Memorial Veterans' Hospital, Columbia, MO 65201, USA; Department of Veterinary Medicine and Surgery, University of Missouri, Columbia, MO 65211, USA.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Lewis
authors.affiliation: Department of Chemistry, University of Missouri, Columbia, MO 65211, USA.
authors.forename: Silvia S
authors.initials: SS
authors.lastname: Jurisson
authors.affiliation: School of Pharmacy, Aristotle University of Thessaloniki, 54124 Thessaloniki, GR.
authors.forename: Dionysia
authors.initials: D
authors.lastname: Papagiannopoulou
authors.affiliation: Research Reactor Center, University of Missouri, Columbia, MO 65211, USA. Electronic address: hennkensh@missouri.edu.
authors.forename: Heather M
authors.initials: HM
authors.lastname: Hennkens
mesh_terms: D000818:Animals; D060326:Chemistry Techniques, Synthetic; D005260:Female; D007553:Isotope Labeling; D051379:Mice; D018358:Neuroendocrine Tumors / Q000000981:diagnostic imaging*; D010455:Peptides / Q000138:chemical synthesis* / Q000737:chemistry / Q000378:metabolism* / Q000493:pharmacokinetics; D011854:Radiochemistry; D017481:Receptors, Somatostatin / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D012211:Rhenium; D000072098:Single Photon Emission Computed Tomography Computed Tomography / Q000379:methods*; D013667:Technetium / Q000737:chemistry*; D014018:Tissue Distribution
chemical_list: D010455:Peptides; D017481:Receptors, Somatostatin; D012211:Rhenium; D013667:Technetium
keywords: Neuroendocrine tumors; SPECT imaging; Somatostatin receptor antagonist; Technetium; Tricarbonyl complexes