Preparation and evaluation of 67Ga-DOTA-Bombesin (7-14) as a tumor scintigraphic agent

Bombesin is a 14-aminoacid peptide isolated from frog skin. The mammalian counterparts of the frog peptide are neuromedin B (NMB) and gastrin-releasing peptide (GRP). Bombesin (BBN) is a peptide showing high affinity for the gastrin releasing peptide receptor (GRPr). Prostate, small cell lung cancer, breast, gastric, and colon cancers are known to over express receptors to bombesin (BBN) and gastrin releasing peptide (GRP). In this study a new 67Ga radiolabeled BBN analogue evaluated based upon the bifunctional chelating ligand DOTA (1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetic acid) that can be used as a tool for diagnosis of GRP receptor-positive tumors. DOTA-BBN (7-14) NH2 was synthesized using a standard Fmoc strategy. Labeling with 67Ga was performed at 95°C for 30 minutes in ammonium acetate buffer (pH = 4.8). Radiochemical analysis involved ITLC and HPLC methods. The stability of radiopeptide was examined in the presence of human serum at 37°C up to 24 hours. The receptor-bound internalization and externalization rates were studied in GRP receptor expressing PC-3 cells. Biodistribution of radiopeptide was studied in nude mice bearing PC-3 tumor. Labeling yield of >90% was obtained corresponding to a specific activity of ≈ 2.48 MBq/nmol. Peptide conjugate showed good stability in the presence of human serum. The radioligand showed a good and specific internalization into PC-3 cells (14.13±0.61% at 4 h). In animal biodistribution studies, a receptor-specific uptake of radioactivity was observed in GRP-receptor-positive organs. After 4 h, uptake in mouse pancreas was 1.08 ± 0.29% ID/g (percentage of injected dose per gram of tissue). These data show that [67Ga]-DOTA-Bombesin (7-14) NH2 is a specific radioligand for gastrin-releasing peptide receptor positive tumors.