MicroPET imaging of a gastrin-releasing peptide receptor-positive tumor in a mouse model of human prostate cancer using a 64Cu-labeled bombesin analogue

The gastrin-releasing peptide receptor (GRPR) is overexpressed on a variety of carcinomas and has been the target for detection and treatment of these neoplasms in animals. In particular, analogues of the tetradecapeptide bombesin (BN) have been radiolabeled with Tc-99m and In-111 for detection of GRPR-positive tumors by gamma ray scintigraphy. The goal of this study was to evaluate the potential of the bombesin analogue, DOTA-Aoc-BN(7-14), for positron-emission tomographic (PET) imaging after radiolabeling with the positron-emitter Cu-64. A saturation binding assay on PC-3 human prostate cancer cells showed that Cu-64-DOTA-Aoc-BN(7-14) had an equilibrium binding constant (K-d) of 6.1 +/-2.5 nM and a receptor concentration (B-max) of 2.7 +/- 0.6 x 10(5) receptors/cell. The radiolabeled analogue also showed rapid internalization with 18.2% internalized into 10(5) PC-3 cells by 2 h. The tumor localization of Cu-64-DOTA-Aoc-BN(7-14) was 5.5% injected dose per gram in athymic nude mice bearing PC-3 xenografts at 2 h postinjection. The tumor retention with respect to the 2 h value was 76% and 45% at 4 and 24 h, respectively, and was GRPR-mediated as shown by inhibition with a coinjection of excess peptide. MicroPET imaging of Cu-64-DOTA-Aoc-BN(7-14) in athymic nude mice bearing subcutaneous PC-3 tumors showed good tumor localization. Further studies with Cu-64 pyruvaldehyde-bis(N-4-methylthiosemicarbazone) (Cu-64-PTSM) suggested that low blood flow to the PC-3 tumors may have limited the localization of Cu-64-DOTA-Aoc-BN(7-14). This study demonstrates that Cu-64-DOTA-Aoc-BN(7-14) can be used to detect GRPR-positive tumors by PET imaging.