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

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.