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

ABSTRACT

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