In vitro and in vivo evaluation of a ⁶⁴Cu-labeled NOTA-Bn-SCN-Aoc-bombesin analogue in gastrin-releasing peptide receptor expressing prostate cancer

Introduction: Bombesin (BN) is an amphibian peptide that binds to the gastrin-releasing peptide receptor (GRPR). It has been demonstrated that BN analogues can be radiolabeled for potential diagnosis and treatment of GRPR-expressing malignancies. Previous studies have conjugated various chelators to the eight C-terminal amino acids of BN [BN(7-14)] for radiolabeling with ⁶⁴Cu. Recently, (1,4,7-triazacyclononane-1,4,7-triacetic acid) (NOTA) has been evaluated as the five-coordinate ⁶⁴Cu complex, with results indicating GRPR-specific tumor uptake. This study aimed to conjugate S-2-(4-isothiocyanatobenzyl)-NOTA (p-SCN-Bn-NOTA) to BN(7-14) such that it could form a six-coordinate complex with ⁶⁴Cu and to evaluate the resulting peptide. Methods: p-SCN-NOTA was conjugated to 8-aminooctanoic acid (Aoc)-BN(7-14) in solution to yield NOTA-Bn-SCN-Aoc-BN(7-14). The unlabeled peptide was evaluated in a cell binding assay using PC-3 prostate cancer cells and ¹²⁵I-Tyr⁴-BN to determine the IC₅₀ value. The peptide was radiolabeled with ⁶⁴Cu and evaluated for internalization into PC-3 cells and for tumor uptake in mice bearing PC-3 xenografts using biodistribution and micro-positron emission tomography imaging studies. Results: The binding assay demonstrated that NOTA-Bn-SCN-Aoc-BN(7-14) bound with high affinity to GRPR with an IC₅₀ of 1.4 nM. The radiolabeled peptide demonstrated time-dependent internalization into PC-3 cells. In vivo, the peptide demonstrated tumor-specific uptake and imaging that were comparable to those of previously reported ⁶⁴Cu-labeled BN analogues. Conclusions: These studies demonstrate that ⁶⁴Cu-NOTA-Bn-SCN-Aoc-BN(7-14) binds to GRPR-expressing cells and that it can be used for imaging of GRPR-expressing prostate cancer.