DOTA-PESIN, a DOTA-conjugated bombesin derivative designed for the imaging and targeted radionuclide treatment of bombesin receptor-positive tumours

Purpose: We aimed at designing and developing a novel bombesin analogue, DOTA-PEG₄-BN(7-14) (DOTA-PESIN), with the goal of labelling it with ^67/68Ga and ¹⁷⁷Lu for diagnosis and radionuclide therapy of prostate and other human cancers overexpressing bombesin receptors. Methods: The 8-amino acid peptide bombesin (7-14) was coupled to the macrocyclic chelator DOTA via the spacer 15-amino-4,7,10,13-tetraoxapentadecanoic acid (PEG ₄). The conjugate was complexed with Ga(III) and Lu(III) salts. The GRP receptor affinity and the bombesin receptor subtype profile were determined in human tumour specimens expressing the three bombesin receptor subtypes. Internalisation and efflux studies were performed with the human GRP receptor cell line PC-3. Xenografted nude mice were used for biodistribution. Results: [Ga^III/Lu^III]-DOTA-PESIN showed good affinity to GRP and neuromedin B receptors but no affinity to BB3. [⁶⁷Ga/ ¹⁷⁷Lu]-DOTA-PESIN internalised rapidly into PC-3 cells whereas the efflux from PC-3 cells was relatively slow. In vivo experiments showed a high and specific tumour uptake and good retention of [⁶⁷Ga/ ¹⁷⁷Lu]-DOTA-PESIN. [⁶⁷Ga/¹⁷⁷Lu]-DOTA-PESIN highly accumulated in GRP receptor-expressing mouse pancreas. The uptake specificity was demonstrated by blocking tumour uptake and pancreas uptake. Fast clearance was found from blood and all non-target organs except the kidneys. High tumour-to-normal tissue ratios were achieved, which increased with time. PET imaging with [⁶⁸Ga]-DOTA-PESIN was successful in visualising the tumour at 1 h post injection. Planar scintigraphic imaging showed that the ¹⁷⁷Lu-labelled peptide remained in the tumour even 3 days post injection. Conclusion: The newly designed ligands have high potential with regard to PET and SPECT imaging with ^68/67Ga and targeted radionuclide therapy with ¹⁷⁷Lu.