SynthesisofNewSomatostatinReceptorBindingRadio-labelledPeptides: Physicochemical and Biological Evaluation and Scintigraphic Studies

Peptides are important regulators of growth and cellular functions not only in normal tissue but also in tumors. Peptides are becoming of increasing interest in nuclear oncology for targeted tumor diagnosis and therapy. 111In-diethylenetriaminepentaacetic acid (111In-DTPA) labelled Octreotide (Octreoscan; Mallinckrodt / Inc.) was the first routinely used tracer in nuclear medicine for the localization and staging of somatostatin receptor positive tumors. Stable somatostatin derivatives such as Octreotide / Lantrotide are the most frequently used radiopharmaceuticals acting through specific binding to somatostatin receptors; however, they do not bind with high affinity to all five receptor subtypes. 111In-DTPA-Octreotide based imaging served as a sensitive and specific technique for somatostatin receptor scintigraphy that can detect in-vivo somatostatin receptors in various tumors mainly of endocrine origin. 111In-DTPA-Octreotide is commercially available for imaging; however the use of 111In as a radioisotope entails high cost, limited availability, suboptimal image quality and a high radiation burden to the patient. These disadvantages were overcome by the use of 99mTc labelled somatostatin analogs. Thus newly developed somatostatin analogs labelled with 99mTc were intensively studied in the proposed research work.New somatostatin analogs (octreotide derivatives) like HYNIC-Tyr3-Octreotide (TOC), HYNIC-His3-Octreotate and HYNIC-Met3-Octreotate (TATE) were synthesized by standard Fmoc Solid Phase Peptide Synthesis (SPPS) on O-t-butylthreoninol-2-chloro trityl resin for TOC derivatives and H-Thr(tBu)-2-Cl trityl resin for TATE derivatives on a semi automated peptide synthesizer according to our laboratory protocol. Coupling of prochelator to peptide was performed in the presence of excess of HYNIC-BOC in solid phase. BOC protected HYNIC was also synthesized and characterized in our laboratory. Then the peptide chains were cyclized by formation of cys-cys disulfide bond through Iodine oxidation method. These peptides were deblocked and deprotected from resin and lateral protecting groups by using trifluoro acetic acid cocktail. Then purification was achieved by semi-preparative HPLC. The structures of the peptides were then elucidated by 1D NMR (1H NMR, 13C NMR, DEPT 90, 135), 2D NMR (HSQC, HMBC, COSY), MALDI mass and FT-IR spectroscopy. The peptides were again quantitated by amino acid analysis of a 24 h acid hydrolysate using Waters Accu-Tag chemistry together with a Waters HPLC system. Conformational analysis of the peptides was performed by circular dichorism spectroscopy in pure water and trifluroethanol-water (1:1)