Radiochemical Investigations of Gastrin-releasing Peptide Receptor-specific [Tc(X)(CO)3-Dpr-Ser-Ser-Ser-Gln-Trp-Ala-Val-Gly-His-Leu-Met-(NH2)] in PC-3, Tumor-bearing, Rodent Models: Syntheses, Radiolabeling, and in Vitro/in Vivo

Bombesin (BBN), a 14 amino acid peptide, is an analogue of human gastrin-releasing peptide (GRP) that binds to GRP receptors (GRPrs) with high affinity and specificity. The GRPr is overexpressed on a variety of human cancer cells, including prostate, breast, lung, and pancreatic cancers. The specific aim of this study was to develop Tc(I)-radiolabled BBN analogues that maintain high specificity for the GRPr in vivo. A preselected synthetic sequence via solid phase peptide synthesis was designed to produce 2,3-diaminopropionic acid (Dpr)-BBN conjugates with the following general structure: Dpr-Ser-Ser-Ser-Gln-Trp-Ala-Val-GlyHis-Leu-Met-(NH2). The new BBN constructs were purified by reversed phase high-performance liquid chromatography. Electrospray mass spectrometry was used to characterize the nonmetallated BBN conjugates. Re(I)-BBN conjugates were prepared by the reaction of [Re(Br)3(CO)3] 2 and Dpr-Ser-Ser-Ser-Gln-Trp-Ala-Val-Gly-His-Leu-Met-(NH2) with gentle heating. Electrospray mass spectrometry was used to determine the molecular constitution of the new Re(I) conjugates. The Tc conjugates were prepared at the tracer level by preconjugation, postlabeling approach from the reaction of [Tc(H2O)3(CO)3] and corresponding ligand. The Tc and Re(I) conjugates behaved similarly under identical reversed phase high-performance liquid chromatography conditions. Results from in vitro and in vivo models demonstrated the ability of these derivatives to specifically target GRPrs on human, prostate, cancerous PC-3 cells.