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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, [^99mTc]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 [Tyr⁴]BN were 0.70±0.08 nM and 1.5±0.20 nM, respectively, while the K _d defined for [^99mTc/^99gTc]Demobesin 1 was 0.67±0.10 nM. [^99mTc]Demobesin 1 was rather stable in murine plasma, whereas it degraded rapidly in kidney and liver homogenates. After injection in healthy Swiss albino mice, [^99mTc]Demobesin 1 accumulated very efficiently in the target organs (pancreas …