18F-labeled bombesin analogs for targeting GRP receptor-expressing prostate cancer

Molecular Imaging Program at Stanford (MIPS), Department of Radiology and Bio-X Program, Stanford University School of Medicine, Stanford, California 94305-5484, USA.
Journal of Nuclear Medicine (Impact Factor: 6.16). 03/2006; 47(3):492-501.
Source: PubMed


The gastrin-releasing peptide receptor (GRPR) is found to be overexpressed in a variety of human tumors. The aim of this study was to develop 18F-labeled bombesin analogs for PET of GRPR expression in prostate cancer xenograft models.
[Lys3]Bombesin ([Lys3]BBN) and aminocaproic acid-bombesin(7-14) (Aca-BBN(7-14)) were labeled with 18F by coupling the Lys3 amino group and Aca amino group, respectively, with N-succinimidyl-4-18F-fluorobenzoate (18F-SFB) under slightly basic condition (pH 8.5). Receptor-binding affinity of FB-[Lys3]BBN and FB-Aca-BBN(7-14) was tested in PC-3 human prostate carcinoma cells. Internalization and efflux of both radiotracers were also evaluated. Tumor-targeting efficacy and in vivo kinetics of both radiotracers were examined in male athymic nude mice bearing subcutaneous PC-3 tumors by means of biodistribution and dynamic microPET imaging studies. 18F-FB-[Lys3]BBN was also tested for orthotopic PC-3 tumor delineation. Metabolic stability of 18F-FB-[Lys3]BBN was determined in mouse blood, urine, liver, kidney, and tumor homogenates at 1 h after injection.
The typical decay-corrected radiochemical yield was about 30%-40% for both tracers, with a total reaction time of 150 +/- 20 min starting from 18F-. 18F-FB-[Lys3]BBN had moderate stability in the blood and PC-3 tumor, whereas it was degraded rapidly in the liver, kidneys, and urine. Both radiotracers exhibited rapid blood clearance. 18F-FB-[Lys3]BBN had predominant renal excretion. 18F-FB-Aca-BBN(7-14) exhibited both hepatobiliary and renal clearance. Dynamic microPET imaging studies revealed that the PC-3 tumor uptake of 18F-FB-[Lys3]BBN in PC-3 tumor was much higher than that of 18F-FB-Aca-BBN(7-14) at all time points examined (P < 0.01). The receptor specificity of 18F-FB-[Lys3]BBN in vivo was demonstrated by effective blocking of tumor uptake in the presence of [Tyr4]BBN. No obvious blockade was found in PC-3 tumor when 18F-FB-Aca-BBN(7-14) was used as radiotracer under the same condition. 18F-FB-[Lys3]BBN was also able to visualize orthotopic PC-3 tumor at early time points after tracer administration, at which time minimal urinary bladder activity was present to interfere with the receptor-mediated tumor uptake.
This study demonstrates that 18F-FB-[Lys3]BBN and PET are suitable for detecting GRPR-positive prostate cancer in vivo.

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    • "This result demonstrates the value of [ 18 F]SFB as a prosthetic group for highly efficient peptide labeling with 18 F. [ 18 F]BBN-2 was synthesized within 80–96 min including HPLC purification. The obtained decay-corrected radiochemical yield of 24%–30% was comparable to [ 18 F]SFB-labeling of full-length BBN peptide [Lys 3 ]BBN and Aca-BBN(7–14) reported by Zhang et al. [23] "
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    ABSTRACT: Introduction: Bombesin (BBN) and BBN analogues have attracted much attention as high-affinity ligands for selective targeting of the gastrin-releasing peptide (GRP) receptor. GRP receptors are overexpressed in a variety of human cancers including prostate cancer. Radiolabeled BBN derivatives are promising diagnostic probes for molecular imaging of GRP receptor-expressing prostate cancer. This study describes the synthesis and radiopharmacological evaluation of various metabolically stabilized fluorobenzoylated bombesin analogues (BBN-1, BBN-2, BBN-3). Methods: Three fluorobenzoylated BBN analogues containing an aminovaleric (BBN-1, BBN-2), or an aminooctanoic acid linker (BBN-3) were tested in a competitive binding assay against (125)I-[Tyr(4)]-BBN for their binding potency to the GRP receptor. Intracellular calcium release in human prostate cancer cells (PC3) was measured to determine agonistic or antagonistic profiles of fluorobenzoylated BBN derivatives. Bombesin derivative BBN-2 displayed the highest inhibitory potency toward GRP receptor (IC50 = 8.7 ± 2.2 nM) and was subsequently selected for radiolabeling with fluorine-18 ((18)F) through acylation with N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB). The radiopharmacological profile of (18)F-labeled bombesin [(18)F]BBN-2 was evaluated in PC3 tumor-bearing NMRI nude mice involving metabolic stability studies, biodistribution experiments and dynamic small-animal PET studies. Results: All fluorobenzoylated BBN derivatives displayed high inhibitory potency toward the GRP receptor (IC50=8.7-16.7 nM), and all compounds exhibited antagonistic profiles as determined in an intracellular calcium release assay. The (18)F-labeled BBN analogue [(18)F]BBN-2 was obtained in 30% decay-corrected radiochemical yield with high radiochemical purity >95% after semi-preparative HPLC purification. [(18)F]BBN-2 showed high metabolic stability in vivo with 65% of the radiolabeled peptide remaining intact after 60 min p.i. in mouse plasma. Biodistribution experiments and dynamic small-animal PET studies demonstrated high tumor uptake of [(18)F]BBN-2 in PC3 xenografts (2.75 ± 1.82 %ID/g after 5 min and 2.45 ± 1.25 %ID/g after 60 min p.i.). Specificity of radiotracer uptake in PC3 tumors was confirmed by blocking experiments. Conclusion: The present study demonstrates that (18)F-labeled BBN analogue [(18)F]BBN-2 is a suitable PET radiotracer with favorable metabolic stability in vivo for molecular imaging of GRP receptor-positive prostate cancer.
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    • "A few 18F-BN derivatives (both agonists and antagonists) have been developed and evaluated [103, 112–119]. Zhang et al. radiofluorinated a full-length ([Lys3]BN) and a truncated derivative Aca-BN(7–14) (where Aca is aminocaproic acid) using N-succinimidyl-4-18F-fluorobenzoate (18F-SFB) reacted at the Lys or the Aca amino groups, respectively. "
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