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

IntroductionA 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.MethodsReceptor 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.ResultsThe half maximal inhibitory concentration (IC50 value) of ReL-sst2-ANT in AR42J cells was 15 nM. 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 24 h in high concentration solutions of cysteine and histidine at 37 °C. Furthermore, the radiotracer was 90% stable for 1 h 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 4 h was significantly blocked by co-administration of nonradioactive SS-14.ConclusionsA [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.