[64Cu-NOTA-8-Aoc-BBN(7-14)NH2] targeting vector for positron-emission tomography imaging of gastrin-releasing peptide receptor-expressing tissues

Radiolabeled peptides hold promise as diagnostic/therapeutic targeting vectors for specific human cancers. We report the design and development of a targeting vector, [Cu-64-NOTA-8-Aoc-BBN(7-14)]NH2] (NOTA = 1,4,7-triazacyclononane-1,4,7-triacetic acid, 8-Aoc = 8-aminooctanoic acid, and BBN = bombesin), having very high selectivity and affinity for the gastrin-releasing peptide receptor (GRPr). GRPrs are expressed on a variety of human cancers, including breast, lung, pancreatic, and prostate, making this a viable approach toward site-directed localization or therapy of these human diseases. In this study, [NOTA-X-BBN(7-14)NH2] conjugates were synthesized, where X = a specific pharmacokinetic modifier. The IC50 Of [NOTA-8-Aoc-BBN(7-14)NH2 was determined by a competitive displacement cell-binding assay in PC-3 human prostate cancer cells using I-125-[Tyr(4)]-BBN as the displacement ligand. An IC50 of 3.1 +/- 0.5 nM was obtained, demonstrating high binding affinity of [NOTA-8-Aoc-BBN] for the GRPr. [Cu-64-NOTA-X-BBN] conjugates were prepared by the reaction of (CuCl2)-Cu-64 with peptides in buffered aqueous solution. In vivo studies of [Cu-64-NOTA-8-Aoc-BBN(7-14)NH2] in tumor-bearing PC-3 mouse models indicated very high affinity of conjugate for the GRPr. Uptake of conjugate in tumor was 3.58 +/- 0.70% injected dose (ID) per g at 1 h postintra-venous injection (p.i.). Minimal accumulation of radioactivity in liver tissue (1.58 +/- 0.40% ID per g, 1 h p.i.) is indicative of rapid renal-urinary excretion and suggests very high in vivo kinetic stability of [Cu-64-NOTA-8-Aoc-BBN(7-14)NH2] with little or no in vivo dissociation of Cu-64(2+) from the NOTA chelator. Kidney accumulation at 1 h p.i. was 3.79 +/- 1.09% ID per g. Molecular imaging studies in GRPr-expressing tumor models produced high-contrast, high-quality micro-positron-emission tomography images.