Radiochemical investigations of [188Re(H2O)(CO)3-diaminopropionic acid-SSS-bombesin(7-14)NH2]: syntheses, radiolabeling and in vitro/in vivo GRP receptor targeting studies.

Background: Bombesin (BBN), a 14 amino acid peptide, is an analogue of human gastrin-releasing peptide (GRP) that binds to GRP receptors (GRPr) with high affinity and specificity. The GRPr is over-expressed on a variety of human cancer cells including prostate, breast, lung, and pancreatic cancers. The specific aim of this study was to develop a 188Re(I)-radiolabeled BBN analogue that maintains high specificity for the GRPr in vivo.
Materials and Methods: A preselected synthetic sequence via solid phase peptide synthesis (SPPS) was designed to produce a Dpr-BBN (Dpr = Diaminopropionic acid) conjugate with the following general structure: Dpr-X-Q-W-A-V-G-H-L-M-(NH2), where the spacer group, X = Serylserylserine. The new BBN-construct was purified by reversed phase-HPLC (RP-HPLC). The non-radioactive Re(I)-BBN conjugate was prepared by the reaction of [Re(Br)3(CO)3]2- and Dpr-SSS-bombesin(7-14)NH2 with heating. ES-MS was used to determine the molecular constitution of the non-metallated and metallated Re (I)--conjugates. The 188 Re-conjugate was prepared at the tracer level by the pre-conjugation, postlabeling approach from the reaction of [188Re(H2O)3(CO)3]+ and corresponding ligand.
Results: The 188Re- and non-radioactive Re(I)conjugate behaved similarly under identical RP-HPLC conditions. In vitro cell displacement assays showed that the new conjugate has an IC50 value of approximately 1 nM. In vitro cell binding assays showed that the new conjugate is rapidly internalized and exhibits long-term retention, demonstrating the agonistic efficacy of the radiolabel. In vivo targeting of human prostate, PC-3 tumor xenografts indicated uptake and retention of the new radioconjugate for time-point < or = 24 hours.
Conclusion: Results from in vitro and in vivo models demonstrated the ability of these derivatives to specifically target GRP receptors on human, prostate and cancerous PC-3 cells. This new construct holds potential for the development of a therapeutic entity for the treatment of prostate cancer.