Lauren Radford1, Fabio Gallazzi2, Lisa Watkinson3, Terry Carmack3, Ashley Berendzen3, Michael R Lewis4, Silvia S Jurisson1, Dionysia Papagiannopoulou5, Heather M Hennkens6. 1. Department of Chemistry, University of Missouri, Columbia, MO 65211, USA. 2. Structural Biology Core, University of Missouri, Columbia, MO 65211, USA. 3. Research Service, Harry S. Truman Memorial Veterans' Hospital, Columbia, MO 65201, USA. 4. 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. 5. School of Pharmacy, Aristotle University of Thessaloniki, 54124 Thessaloniki, GR. 6. Research Reactor Center, University of Missouri, Columbia, MO 65211, USA. Electronic address: hennkensh@missouri.edu.
Abstract
INTRODUCTION: A somatostatin receptor (SSTR)-targeting antagonist peptide (sst2-ANT) was radiolabeled with 99mTc 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 with 125I-SS-14 as the competing radioligand. Preparation of 99mTcL-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 (IC50 value) of ReL-sst2-ANT in AR42J cells was 15nM. Preparation of 99mTcL-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 of 99mTcL-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 first 99mTc-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.
INTRODUCTION: A somatostatin receptor (SSTR)-targeting antagonist peptide (sst2-ANT) was radiolabeled with 99mTc 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 with 125I-SS-14 as the competing radioligand. Preparation of 99mTcL-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 SCIDmice. RESULTS: The half maximal inhibitory concentration (IC50 value) of ReL-sst2-ANT in AR42J cells was 15nM. Preparation of 99mTcL-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 of 99mTcL-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 first 99mTc-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: Keelara Abiraj; Samer Ursillo; Maria Luisa Tamma; Svetlana N Rylova; Beatrice Waser; Edwin C Constable; Melpomeni Fani; Guillaume P Nicolas; Jean Claude Reubi; Helmut R Maecke Journal: EJNMMI Res Date: 2018-08-02 Impact factor: 3.138
Authors: Raghuvir Haridas Gaonkar; Fabius Wiesmann; Luigi Del Pozzo; Lisa McDougall; Sandra Zanger; Renata Mikołajczak; Rosalba Mansi; Melpomeni Fani Journal: Pharmaceuticals (Basel) Date: 2021-03-28