HYNIC and DOMA conjugated radiolabeled bombesin analogs as receptor-targeted probes for scintigraphic detection of breast tumor.

Background: Among the many peptide receptor systems, gastrin-releasing-peptide (GRP) receptors, the mammalian equivalent of bombesin (BN) receptors, are potential targets for diagnosis and therapy of breast tumors due to their overexpression in various frequently occurring human cancers. The aim of this study was to synthesize and comparative evaluation of Tc-labeled new BN peptide analogs. Four new BN analogs, HYNIC-Asp[PheNle]BN(7-14)NH, BN1; HYNIC-Pro-Asp[TyrMet]BN(7-14)NH, BN2; HYNIC-Asp-Asn[Lys-CHAla-Nle]BN(7-14)NH, BN3; and DOMA-GABA[Pro-Tyr-Nle]BN(7-14)NH, BN4 were synthesized and biologically evaluated for targeted imaging of GRP receptor-positive breast-tumors.

Methods: Solid-phase synthesis using Fmoc-chemistry was adopted for the synthesis of peptides. BN1-BN4 analogs were better over the standard Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH (BNS). Lipophilicity, serum stability, internalization, and binding affinity studies were carried out using Tc-labeled analogs. Biodistribution and imaging analyses were performed on MDA-MB-231 cell-induced tumor-bearing mice. BN-analogs induced angiogenesis; tumor formation and GRP-receptor-expression were confirmed by histology and immunohistochemistry analyses of tumor sections and important tissue sections.

Results: All the analogs displayed ≥ 97% purity after the HPLC purification. BN-peptide-conjugates exhibited high serum stability and significant binding affinity to GRP-positive tumor; rapid internalization/externalization in/from MDA-MB-231 cells were noticed for the BN analogs. BN4 and BN3 exhibited higher binding affinity, stability than BN1 and BN2. Highly specific in vivo uptakes to the tumor were clearly visualized by scintigraphy; rapid excretion from non-target tissues via kidneys suggests a higher tumor-to-background ratio. BN4, among all the analogs, stimulates the expression of angiogenic markers to a maximum.

Conclusion: Considering its most improved pharmacological characteristics, BN4 is thus considered as most promising probes for early non-invasive diagnosis of GRP receptor-positive breast tumors.