[99mTc]Demobesin 1, a novel potent bombesin analogue for GRP receptor-targeted tumour imaging

Demobesin 1 is a potent new GRP-R-selective bombesin (BN) analogue containing an open chain tetraamine chelator for stable technetium-99m binding. Following a convenient labelling protocol, the radiopeptide, [Tc-99m]Demobesin 1, formed in nearly quantitative yields and with high specific activities. Both unlabelled and labelled peptide demonstrated high-affinity binding in membrane preparations of the human androgen-independent prostate adenocarcinoma PC-3 cell line. The IC50 values determined for Demobesin 1 and [Tyro]BN were 0.70+/-0.08 nM and 1.5+/-0.20 nM, respectively, while the K-d defined for [Tc-99m/Tc-99g]Demobesin 1 was 0.67+/-0.10 nM. [Tc-99m]Demobesin 1 was rather stable in murine plasma, whereas it degraded rapidly in kidney and liver homogenates. After injection in healthy Swiss albino mice, [Tc-99m]Demobesin 1 accumulated very efficiently in the target organs (pancreas, intestinal tract) via a GRP-R-mediated process, as shown by in vivo receptor blocking experiments. An equally high and GRP-R-mediated uptake was exhibited by [Tc-99m]Demobesin 1 after injection in PC-3 tumour-bearing athymic mice. The initial high radioligand uptake of 16.2+/-3.1%ID/g in the PC-3 xenografts at 1 h p.i. remained at a similar level (15.61+/-1.19%ID/g) at 4 h p.i. Even after 24 h p.i., when the radioactivity had cleared from all other tissues, a value of 5.24+/-0.67%ID/g was still observed in the tumour. The high and prolonged localization of [Tc-99m]Demobesin 1 at the tumour site and its rapid background clearance are very promising qualities for GRP-R-targeted tumour imaging in man.