Evaluation of a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-conjugated bombesin-based radioantagonist for the labeling with single-photon emission computed tomography, positron emission tomography, and therapeutic radionuclides

Abstract

Purpose: G protein-coupled receptor agonists are being used as radiolabeled vectors for in vivo localization and therapy of tumors. Recently, somatostatin-based antagonists were shown to be superior to agonists. Here, we compare the new [111In/68Ga]-labeled bombesin-based antagonist RM1 with the agonist [111In]-AMBA for targeting the gastrin-releasing peptide receptor (GRPR). Experimental Design: IC50, Kd values, and antagonist potency were determined using PC-3 and HEK-GRPR cells. Biodistribution and imaging studies were done in nude mice transplanted with the PC-3 tumor. The antagonist potency was assessed by evaluating the effects on calcium release and on receptor internalization monitored by immunofluorescence microscopy. Results: The IC50 value of [natIn]-RM1 was 14 ± 3.4 nmol/L. [nat/111In]-RM1 was found to bind to the GRPR with a Kd of 8.5 ± 2.7 nmol/L compared with a Kd of 0.6 ± 0.3nmol/L of [111In]- AMBA. A higher maximum number of binding site value was observed for [ 111In]-RM1 (2.4 ± 0.2 nmol/L) compared with [ 111In]-AMBA(0.7 ± 0.1nmol/L). [natLu]-AMBA is a potent agonist in the immunofluorescence-based internalization assay,whereas [natIn]-RM1 is inactive alone but efficiently antagonizes the bombesin effect. These data are confirmed by the calcium release assay. The pharmacokinetics showed a superiority of the radioantagonist with regard to the high tumor uptake (13.4 ± 0.8% IA/g versus 3.69 ± 0.75% IA/g at 4 hours after injection. as well as to all tumor-to-normal tissue ratios. Conclusion: Despite their relatively low GRPR affinity, the antagonists [111In/ 68Ga]-RM1 showed superior targeting properties compared with [ 111In]-AMBA. As found for somatostatin receptor-targeting radiopeptides, GRP-based radioantagonists seem to be superior to radioagonists for in vivo imaging and potentially also for targeted radiotherapy of GRPR-positive tumors. © 2009 American Association for Cancer Research.