Synthesis and evaluation of a 99mTc tricarbonyl-labeled somatostatin receptor-targeting antagonist peptide for imaging of neuroendocrine tumors

Nucl Med Biol. 2017 Apr:47:4-9. doi: 10.1016/j.nucmedbio.2016.12.002. Epub 2016 Dec 9.

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.

Keywords: Neuroendocrine tumors; SPECT imaging; Somatostatin receptor antagonist; Technetium; Tricarbonyl complexes.

MeSH terms

  • Animals
  • Chemistry Techniques, Synthetic
  • Female
  • Isotope Labeling
  • Mice
  • Neuroendocrine Tumors / diagnostic imaging*
  • Peptides / chemical synthesis*
  • Peptides / chemistry
  • Peptides / metabolism*
  • Peptides / pharmacokinetics
  • Radiochemistry
  • Receptors, Somatostatin / antagonists & inhibitors*
  • Receptors, Somatostatin / metabolism*
  • Rhenium
  • Single Photon Emission Computed Tomography Computed Tomography / methods*
  • Technetium / chemistry*
  • Tissue Distribution

Substances

  • Peptides
  • Receptors, Somatostatin
  • Rhenium
  • Technetium