Synthesis, characterization, conformational analysis of a cyclic conjugated octreotate peptide and biological evaluation of 99mTc-HYNIC-His3-Octreotate as novel tracer for the imaging of somatostatin receptor-positive tumors

Peptides are attracting increasing interest in nuclear oncology for targeted tumor diagnosis and therapy. We therefore synthesized new cyclic octapeptides conjugated with HYNIC by Fmoc solid-phase peptide synthesis. These were purified and analyzed by RP-HPLC, MALDI mass, ¹H NMR, ¹³C NMR, HSQC, HMBC, COSY and IR spectroscopy. Conformational analysis of the peptides was performed by circular dichroism spectroscopy, in pure water and trifluoroethanol–water (1:1), revealed the presence of strong secondary structural features like β-sheet and random coils. Labeling was performed with ^99mTc using Tricine and EDDA as coligands by SnCl₂ method to get products with excellent radiochemical purity >99.5 %. Metabolic stability analysis did not show any evidence of breaking of the labeled compounds and formation of free ^99mTc. Internalization studies were done and IC₅₀ values were determined in somatostatin receptor-expressing C6 glioma cell line and rat brain cortex membrane, and the results compared with HYNIC-TOC as standard. The IC₅₀ values of ^99mTc-HYNIC-His³-Octreotate (21 ± 0.93 nM) and ^99mTc-HYNIC-TOC (2.87 ± 0.41 nM) proved to be comparable. Biodistribution and image study on normal rat under gamma camera showed very high uptake in kidney and urine, indicating kidney as primary organ for metabolism and route of excretion. Biodistribution and image study on rats bearing C6 glioma tumor found high uptake in tumor (1.27 ± 0.15) and pancreas (1.71 ± 0.03). Using these findings, new derivatives can be prepared to develop ^99mTc radiopharmaceuticals for imaging somatostatin receptor-positive tumors.