Positron Emission Tomography Imaging of the Gastrin-Releasing Peptide Receptor with a Novel Bombesin Analogue

The gastrin-releasing peptide receptor (GRPR), a G protein-coupled receptor, is overexpressed in solid malignancies and particularly in prostate cancer. We synthesized a novel bombesin derivative, [Ga-68] Ga-ProBOMB1, evaluated its pharmacokinetics and potential to image GRPR expression with positron emission tomography (PET), and compared it with [Ga-68] Ga-NeoBOMB1. ProBOMB1 (DOTA-pABzA-DIG-D-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-psi(CH2N)-Pro-NH2) was synthesized by solid-phase peptide synthesis. The polyaminocarboxylate chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was coupled to the N-terminal and separated from the GRPR-targeting sequence by a p-aminomethylaniline-diglycolic acid (pABzA-DIG) linker. The binding affinity to GRPR was determined using a cell-based competition assay, whereas the agonist/antagonist property was determined with a calcium efflux assay. ProBOMB1 was radiolabeled with (GaCl3)-Ga-68. PET imaging and biodistribution studies were performed in male immunocompromised mice bearing PC-3 prostate cancer xenografts. Blocking experiments were performed with coinjection of [D-Phe(6), Leu-NHEt13, desMet(14)] bombesin(6-14). Dosimetry calculations were performed with OLINDA software. ProBOMB1 and the nonradioactive Ga-ProBOMB were obtained in 1.1 and 67% yield, respectively. The K-i value of Ga-ProBOMB1 for GRPR was 3.97 +/- 0.76 nM. Ga-ProBOMB1 behaved as an antagonist for GRPR. [Ga-68] Ga-ProBOMB1 was obtained in 48.2 +/- 10.9% decay-corrected radiochemical yield with 121 +/- 46.9 GBq/mu mol molar activity and >95% radiochemical purity. Imaging/biodistribution studies showed that the excretion of [Ga-68] Ga-ProBOMB1 was primarily through the renal pathway. At 1 h postinjection (p.i.), PC-3 tumor xenografts were clearly delineated in PET images with excellent contrast. The tumor uptake for [Ga-68] Ga-ProBOMB1 was 8.17 +/- 2.57 percent injected dose per gram (% ID/g) and 9.83 +/- 1.48% ID/g for [68Ga] Ga-NeoBOMB1, based on biodistribution studies at 1 h p.i. This corresponded to tumor-to-blood and tumor-to-muscle uptake ratios of 20.6 +/- 6.79 and 106 +/- 57.7 for [Ga-68] Ga-ProBOMB1 and 8.38 +/- 0.78 and 39.0 +/- 12.6 for [Ga-68] Ga-NeoBOMB1, respectively. Blockade with [D-Phe(6), Leu-NHEt13, des-Met(14)] bombesin(6-14) significantly reduced the average uptake of [Ga-68] Ga-ProBOMB1 in tumors by 62%. The total absorbed dose was lower for [Ga-68] Ga-ProBOMB1 in all organs except for bladder compared with [Ga-68] Ga-NeoBOMB1. Our data suggest that [68Ga] Ga-ProBOMB1 is an excellent radiotracer for imaging GRPR expression with PET. [Ga-68] Ga-ProBOMB1 achieved a similar uptake as [Ga-68] Ga-NeoBOMB1 in tumors, with enhanced contrast and lower whole-body absorbed dose.