[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, [⁶⁴Cu-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 ¹²⁵I-[Tyr⁴]-BBN as the displacement ligand. An IC ₅₀ of 3.1 ± 0.5 nM was obtained, demonstrating high binding affinity of [NOTA-8-Aoc-BBN] for the GRPr. [⁶⁴Cu-NOTA-X-BBN] conjugates were prepared by the reaction of ⁶⁴CuCl₂ with peptides in buffered aqueous solution. In vivo studies of [⁶⁴Cu-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 postintravenous 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 [⁶⁴Cu-NOTA-8-Aoc-BBN(7-14) NH₂] with little or no in vivo dissociation of ⁶⁴Cu 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.