Bombesin analogues for gastrin-releasing peptide receptor imaging

Objectives: The present study describes the design and development of a series of new bombesin (BBN) antagonist peptide ligands of the form [64Cu-(NO2A-X-D-Phe6-BBN(6-13)NHEt)], where Cu-64=a positron emitting radiometal; NO2A=1,4,7-triazacyclononane-1,4-diacetic acid; X=6-amino hexanoic acid, 8-amino octanoic acid or 9-Aminononanoic acid; and BBN(6-13)NHEt=Gln-Trp-Ala-Val-Gly-His-Leu-NHEt, an antagonist analogue of bombesin peptide for specific targeting of the gastrin-releasing peptide receptor (GRPR). Methods: [NO2A-X-D-Phe6-BBN(6-13)NHEt] conjugates were manually conjugated with NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid), and the resulting conjugates were labeled with 64Cu to yield [64Cu-(NO2A-X-D-Phe6-BBN(6-13)NHEt)]. The metallated and nonmetallated conjugates were purified via reversed-phase high-performance liquid chromatography and characterized by electrospray ionization–mass spectrometry. Results: Competitive displacement binding assays displayed nanomolar binding affinities toward human GRPR for all of the newly formed peptide analogues. Biodistribution studies showed very high uptake and retention of tumor-associated radioactivity in PC-3 (a prostate tumor model known to express the GRPR) tumor-bearing rodent models. The radiolabeled conjugates also exhibited rapid urinary excretion and very high tumor to background ratios. Micro-positron emission tomography (PET) molecular imaging investigations showed clear visualization of tumors in female PC-3 tumor-bearing mice 15 h postinjection. Conclusion: The biodistribution and molecular imaging study suggests that these conjugates can be considered as potential PET tracer candidates for the diagnosis of GRPR-positive tumors in human patients.