Objective: 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
pharmacokinetics, in vitro stability, specific binding, and affinity.
Background: Bombesin (BBN), a 14-aminoacid amphibian peptide homolog of mammalian gastrinreleasing
peptide (GRP), has demonstrated the ability to bind with high affinity and specificity to GRP
receptor, which is 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 radio conjugates was evaluated in the presence of saline
and human serum. Affinity, internalization, and also dissociation Constant was evaluated using MDAMB-
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 a blocking study.
Conclusion: The introduction of the spacer sequence between chelator and BBN(7-14) led to improved
bidistribution. Analog with tri-Gly spacer is the more promising radiopeptide for targeting GRP receptors
than Ser conjugates.
Therefore, these analogs can be considered as a candidate for the identification of bombesin-positive