Synthesis, radiolabeling, and preclinical evaluation of a new octreotide analog for somatostatin receptor-positive tumor scintigraphy

Radiolabeled somatostatin analogs have become powerful tools in the diagnosis and staging of neuroendocrine tumors, which express somatostatin receptors. The aim of this study was to evaluate a new somatostatin analog, 6-hydrazinopyridine-3-carboxylic acid-Ser(3)-octreotate (HYNIC-SATE) radiolabeled with Tc-99m, using ethylenediamine-N,N'-diacetic acid and tricine as coligands, to be used as a radiopharmaceutical for the in vivo imaging of somatostatin receptor subtype 2 (SSTR2)-positive tumor. Synthesis of the peptide was carried out on a solid phase using a standard Fmoc strategy. Peptide conjugate affinities for SSTR2 were determined by receptor binding affinity on rat brain cortex and C6 cell membranes. Internalization rate of Tc-99m-HYNIC-SATE was studied in SSTR2-expressing C6 cells that were used for intracranial tumor studies in rat brain. A reproducible in vivo C6 glioma model was developed in Sprague-Dawley rat and confirmed by histopathology and immunohistochemical analysis. Biodistribution and imaging properties of this new radiopeptide were also studied in C6 tumor-bearing rats. Radiolabeling was performed at high specific activities, with a radiochemical purity of >96%. Peptide conjugate showed high affinity binding for SSTR2 (HYNIC-SATE IC50 = 1.60 +/- 0.05 nM) and specific internalization into rat C6 cells. After administration of Tc-99m-HYNIC-SATE in C6 glioma-bearing rats, a receptor specific uptake of radioactivity was observed in SSTR-positive organs and in the implanted intracranial tumor and rapid excretion from nontarget tissues via kidneys. Tc-99m-HYNIC-SATE is a new receptor-specific radiopeptide for targeting SSTR2-positive brain tumor and might be of great promise in the scintigraphy of SSTR2-positive tumors. Copyright (C) 2012 European Peptide Society and John Wiley & Sons, Ltd.