The effects of spacer on radiochemical and biological properties of new radiolabeled bombesin(7-14) derivative.

PURPOSE The aim of this study was to develop 99mTc-[HYNIC-X-D-Phe13]-BBN(7-14)NH2 derivatives using two different tripeptidic spacer groups (X=GGG and X=SSS) in order to improve its pharmacokinetic, in vitro stability, specific binding and affinity. BACKGROUND Bombesin (BBN), a 14-aminoacid amphibian peptide homologue of mammalian gastrin-releasing peptide (GRP), has demonstrated the ability to bind with high affinity and specificity to GRP receptor, which are overexpressed on a variety of human cancers. METHODS Peptide conjugates labeled with 99mTc using tricine-EDDA and radiochemical purity was assessed by TLC and HPLC. The stability of radioconjugates was evaluated in the presence of saline and human serum. Affinity, internalization and also dissociation Constant was evaluated using MDA-MB-231 and PC-3 cell line. Biodistribution study was performed in BALB/C mice. RESULTS Labeling yield of ˃95% was obtained. The change introduced in the BBN sequence increased plasma stability. In vitro blocking studies showed that, binding and internalization of both radiolabeled peptides are mediated by their receptors on the surface of MDA-MB-231 and PC-3 cells. Biodistribution results demonstrated a rapid blood clearance, with predominantly renal excretion. Specific binding in GRP receptor-positive tissues such as pancreas was confirmed with blocking study. CONCLUSIONS The introduction of spacer sequence between chelator and BBN(7-14) led to improved bidistribution. Analogue with tri-Gly spacer is the more promising radiopeptide for targeting GRP receptor than Ser conjugate. Therefore, these analogs can be considered as a candidate for the identification of bombesin-positive tumors.