Research PaperSynthesis and exploration of novel radiolabeled bombesin peptides for targeting receptor positive tumor

- Synthesis and biological interpretation of two new bombesin analogs HYNIC-Asp[Phe13]BBN(7-13)-NH-CH2-CH2-CH3 (BA1) and HYNIC-Pro[Tyr13,Met14]BBN- (7-14)-NH2 (BA2) were synthesized and compared to BBN(7-14)-NH2 (BS) with the goal of improving GRP receptor specific cellular internalization and retention of radioactivity in cancer cells for targeting molecular imaging.
- These BBN peptides could efficiently be radiolabeled with 99mTc via the ligand exchange method which exhibited improved radiochemical and in vitro metabolic stability compared to the previously reported analogs BS.
- Binding assay of radioconjugates confirmed its high binding affinity (affinity sequences were BA1 > BA2 > BS) towards GRP receptor specific breast cancer cell line MDA-MB-231. Potential usefulness of these analogs as diagnostic imaging agents was assessed by biodistribution and imaging studies and compare with 99mTc-BS.
- In view of these results, 99mTc-BA1 and 99mTc-BA2 were efficient radiotracer for targeting GRP receptor positive tumor than 99mTc-BS. The 99mTc-BA1 exhibited two different entities: specific for extra and an intracellular target in the same radioconjugate which seems to be a promising candidate for detection and therapy of GRP receptor positive lesions in the clinical phase. However, during scintigraphic studies 99mTC-BA1 showed improved and distinct delineation of the tumor than 99mTC-BA2 and 99mTC-BS.
- These studies demonstrate the potential of using the bombesin analog, BA1 possessing antagonist C-terminal vectors as a potential diagnostic as well as antagonistic agent for specific targeting of GRP-receptors over-expressing tumors.

Increasing evidence of peptide receptor overexpression in various cancer cells, warrant the development of receptor specific radiolabeled peptides for molecular imaging and therapy in nuclear medicine. Gastrin-releasing-peptide (GRP) receptor, are overexpressed in a variety of human cancer cells. The present study report the synthesis and biological evaluation of new bombesin (BBN) analogs, HYNIC-Asp-[Phe13]BBN(7-13)-NH-CH2-CH2-CH3:BA1, HYNIC-Pro-[Tyr13Met14]BBN(7-14)NH2:BA2 as prospective tumor imaging agent with compare to BBN(7-14)NH2:BS as standard. The pharmacophores were radiolabeled in high yields with 99mTc, characterized for their stability in serum and saline, cysteine/histidine and were found to be substantially stable. Internalization/externalization and receptor binding studies were assessed using MDA-MB-231 cells and showed high receptor binding-affinity and favourable internalization. Fluorescence studies revealed that BA1 changed the morphology of the cells and could localize in the nucleus more effectively than BA2/BS. Cell-viability studies displayed substantial antagonistic and nuclear-internalization effect of BA1. BA1 also exhibited antiproliferative effect on MDA-MB-231 cell by inducing apoptosis. In vivo behaviour of the radiopeptides was evaluated in GRP receptor positive tumor bearing mice. The 99mTc-BA1/99mTc-BA2 demonstrated rapid blood/urinary clearance through the renal pathway and comparatively more significant tumor uptake image and favourable tumor-to-non-target ratios provided by 99mTc-BA1. The specificity of the in vivo uptake was confirmed by co-injection with BS. Moreover, 99mTc-BA1 provided a much clearer tumor image in scintigraphic studies than others. Thus the combination of favourable in vitro and in vivo properties renders BA1 as more potential antagonist bombesin-peptide for targeting GRP-receptor positive tumor. These properties are encouraging to carry out further experiments for non-invasive receptor targeting potential diagnostinc and therapeutic agent for tumors.

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