Bombesin antagonist-based radioligands for translational nuclear imaging of gastrin-releasing peptide receptor-positive tumors

Abstract

Bombesin receptors are overexpressed on a variety of human tumors. In particular, the gastrin-releasing peptide receptor (GRPr) has been identified on prostate and breast cancers and on gastrointestinal stromal tumors. The current study aims at developing clinically translatable bombesin antagonist-based radioligands for SPECT and PET of GRPr-positive tumors. Methods: A potent bombesin antagonist (PEG 4-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu- NH 2 [AR]) was synthesized; conjugated to the chelators DOTA, 6-carboxy-1,4,7,11-tetraazaundecane (N4), 1,4,7-triazacyclononane, 1-glutaric acid-4,7 acetic acid (NODAGA), and 4,11-bis(carboxymethyl)-1,4,8, 11-tetraazabicyclo[6.6.2]hexadecane (CB-TE2A); and radiolabeled with 111In, 99mTc, 68Ga, and 64Cu, respectively. The radioconjugates were evaluated in vitro and in vivo in PC-3 tumor-bearing nude mice. Antagonist potency was determined by Ca 2+-flux measurements and immunofluorescence. Results: All the conjugates showed high binding affinity to GRPr (inhibitory concentration of 50% [IC 50], 2.5-25 nmol/L). The immunofluorescence and Ca 2+-flux assays confirmed the antagonist properties of the conjugates. Biodistribution revealed high and specific uptake in PC-3 tumor and in GRPr-positive tissues. Tumor uptake of 64Cu-CB-TE2A-AR (31.02 ± 3.35 percentage injected activity per gram [%IA/g]) was higher than 99mTc-N4-AR (24.98 ± 5.22%IA/g), 111In-DOTA-AR (10.56 ± 0.70%IA/g), and 68Ga-NODAGA-AR (7.11 ± 3.26%IA/g) at 1 h after injection. Biodistribution at later time points showed high tumor-to-background ratios because of the fast washout of the radioligand from normal organs, compared with tumor. High tumor-to-background ratios were further illustrated by PET and SPECT images of PC-3 tumor-bearing nude mice acquired at 12 h after injection showing high tumor uptake, clear background, and negligible or no radioactivity in the abdomen. Conclusion: The chelators do influence the affinity, antagonistic potency, and pharmacokinetics of the conjugates. The promising preclinical results warrant clinical translation of these probes for SPECT and PET. Copyright © 2011 by the Society of Nuclear Medicine, Inc.