SYNTHESIS OF BOMBESIN ANALOGS FOR RADIOLABELING WITH RE-188

BACKGROUND. Gastrin-releasing peptide receptors (GRPR) are overexpress ed in small cell lung carcinoma and some other human cancers. Small mo lecule peptides with antagonistic activities toward these receptors ar e potential radiotherapeutic agents. METHODS. A 7-amino acid analogue of bombesin (BBN) was synthesized through solid-phase techniques. The peptide was conjugated to trisuccin prior to cleavage from the resin. The conjugate was hydrogenated to remove the hydroxamate-protecting be nzyl groups followed by purification through reversed-phase high perfo rmance liquid chromatography (RP-HPLC). Rhenium-188 (Re-188)-labeling of the trisuccin-peptide conjugate was performed by a SnCl2-reduced ra dioisotope and the labeled product was purified by RP-HPLC. The labele d conjugate was incubated with BNR-11 (3T3 mouse fibroblast cells stab ly transfected with murine GRPR) and PC-3 human prostate carcinoma GRP R positive cells. The nonradioactive peptide analogue was used as a co mpetitive inhibitor and I-125-[Tyr(4)]-BBN was used as a positive cont rol. RESULTS. Solid-phase and solution phase synthesis afforded the co njugates of the hydroxamate ligand trisuccin with the 7-amino acid BBN analogue. The molecules differed by either a direct attachment of the trisuccin to the peptide (TrisBBN) or connection through a B-carbon l inker (TrisC6BBN). The overall yield for each synthesis was approximat ely 20%. Both conjugates showed the correct molecular weights on mass spectroscopy. Radiolabeling of the conjugates with Re-188 were perform ed in greater than or equal to 90% yield. Cell-binding assays performe d with BNR-11 (TrisBBN and TrisC6BBN) and PC-3 (TrisBBN) cell lines re sulted in positive binding. CONCLUSIONS. The synthesis and radiolabeli ng of Tris-BBN conjugates with Re-188 were shown to be feasible. The y ields of chemical syntheses and radiolabeling and positive binding of the radiolabeled conjugates to GRPR-positive tumor cells reveal promis e in the use of these molecules for cancer imaging and therapy. More w ork is needed and is in progress to optimize the cell-binding properti es. (C) 1997 American Cancer Society.