Gastrin-releasing peptide receptor-based targeting using bombesin analogues is superior to metabolism-based targeting using choline for in vivo imaging of human prostate cancer xenografts

Abstract

PurposeProstate cancer (PC) is a major health problem. Overexpression of the gastrin-releasing peptide receptor (GRPR) in PC, but not in the hyperplastic prostate, provides a promising target for staging and monitoring of PC. Based on the assumption that cancer cells have increased metabolic activity, metabolism-based tracers are also being used for PC imaging. We compared GRPR-based targeting using the 68Ga-labelled bombesin analogue AMBA with metabolism-based targeting using 18F-methylcholine (18F-FCH) in nude mice bearing human prostate VCaP xenografts.MethodsPET and biodistribution studies were performed with both 68Ga-AMBA and 18F-FCH in all VCaP tumour-bearing mice, with PC-3 tumour-bearing mice as reference. Scanning started immediately after injection. Dynamic PET scans were reconstructed and analysed quantitatively. Biodistribution of tracers and tissue uptake was expressed as percent of injected dose per gram tissue (%ID/g).ResultsAll tumours were clearly visualized using 68Ga-AMBA. 18F-FCH showed significantly less contrast due to poor tumour-to-background ratios. Quantitative PET analyses showed fast tumour uptake and high retention for both tracers. VCaP tumour uptake values determined from PET at steady-state were 6.7 ± 1.4%ID/g (20–30 min after injection, N = 8) for 68Ga-AMBA and 1.6 ± 0.5%ID/g (10–20 min after injection, N = 8) for 18F-FCH, which were significantly different (p <0.001). The results in PC-3 tumour-bearing mice were comparable. Biodistribution data were in accordance with the PET results showing VCaP tumour uptake values of 9.5 ± 4.8%ID/g (N = 8) for 68Ga-AMBA and 2.1 ± 0.4%ID/g (N = 8) for 18F-FCH. Apart from the GRPR-expressing organs, uptake in all organs was lower for 68Ga-AMBA than for 18F-FCH.ConclusionTumour uptake of 68Ga-AMBA was higher while overall background activity was lower than observed for 18F-FCH in the same PC-bearing mice. These results suggest that peptide receptor-based targeting using the bombesin analogue AMBA is superior to metabolism-based targeting using choline for scintigraphy of PC.