Development of a 68Ga/177Lu-labeled bombesin peptide analog derived from the bombesin universal sequence for imaging and therapy of bombesin receptor-expressing tumors

Objectives The aim of the study was to develop and evaluate a bombesin (BN) peptide analog based on the universal BN sequence (capable of targeting all four BN receptor subtypes), and radiolabeled it with both diagnostic (Ga-68) and therapeutic radionuclide (Lu-177), thus making this peptide useful for both diagnosis and therapy of BN receptor-expressing tumors.

Methods DOTA-Glu-Gln-Trp-Ala-Val-βAla-His-Phe-Nle-NH₂ was prepared by solid-phase peptide synthesis following Fmoc/HBTU methodology. Glu was used as a spacer function. Labeling with Ga-68 and Lu-177 was achieved in the presence of 2.5M NaOAc and 0.1M NH₄OAc buffer solution, respectively. In vitro cell-binding was conducted on BN receptor-positive T47D breast cancer cell line. In vivo biodistribution and clearance kinetics was performed on Balb/c mice at different time points.

Results The structure and purity of DOTA-coupled BN peptide was confirmed by mass spectrometry and HPLC. Radio-HPLC analysis revealed that the BN analog radiolabeled well with both radionuclides with high labeling efficiency (98%). The radiopeptide showed a high chemical stability (90%) in excess of DTPA and a high metabolic stability (95%) when incubated with human plasma. In vitro cell-binding assay indicated the high affinity and specificity of the radiopeptide towards breast cancer cells (Kd=5 nM) and also significant internalization (20%) into the breast cancer cells. In mice, the radiopeptide displayed a fast clearance from the blood and excretion predominantly by the renal route, with some clearance through the hepatobiliary pathway. The uptake in the major organs (lungs, stomach, liver, intestines kidneys, etc.) was low (<3% ID/g).

Conclusions This initial study towards the development of a potent BN-based peptide suggests that the BN peptide under investigation possesses favorable characteristics that need to be further explored in order to determine the real potential of this peptide for targeting tumors expressing BN receptor types.

Research Support This study was partially supported by the International Atomic Energy Agency-Vienna.