Bombesin Receptor Antagonists May Be Preferable to Agonists for Tumor Targeting

Two bombesin analogs, Demobesin 4 and Demobesin 1, were characterized in vitro as gastrin-releasing peptide (GRP) receptor agonist and antagonist, respectively, and were compared as 99mTc-labeled ligands for their in vitro and in vivo tumor-targeting properties. Methods: N4-[Pro1,Tyr4,Nle14]Bombesin (Demobesin 4) and N4-[d-Phe6,Leu-NHEt13,des-Met14]bombesin(6–14) (Demobesin 1) were characterized in vitro for their binding properties with GRP receptor autoradiography using GRP receptor–transfected HEK293 cells, PC3 cells, and human prostate cancer specimens. Their ability to modulate calcium mobilization in PC3 and transfected HEK293 cells was analyzed as well as their ability to trigger internalization of the GRP receptor in transfected HEK293 cells, as determined qualitatively by immunofluorescence microscopy and quantitatively by enzyme-linked immunosorbent assay (ELISA). Further, their internalization properties as 99mTc-labeled radioligands were tested in vitro in both cell lines. Finally, their biodistribution was analyzed in PC3 tumor–bearing mice. Results: A comparable binding affinity with the 50% inhibitory concentration (IC50) in the nanomolar range was measured for Demobesin 4 and Demobesin 1 in all tested tissues. Demobesin 4 behaved as an agonist by strongly stimulating calcium mobilization and by triggering GRP receptor internalization. Demobesin 1 was ineffective in stimulating calcium mobilization and in triggering GRP receptor internalization. However, in these assays, it behaved as a competitive antagonist as it reversed completely the agonist-induced effects in both systems. 99mTc-Labeled Demobesin 1 was only weakly taken up by PC3 cells or GRP receptor–transfected HEK293 cells (10% and 5%, respectively, of total added radioactivity) compared with 99mTc-labeled Demobesin 4 (45% of total added radioactivity in both cell lines). Remarkably, the biodistribution study revealed a much more pronounced uptake at 1, 4, and 24 h after injection of 99mTc-labeled Demobesin 1 in vivo into PC3 tumors than 99mTc-labeled Demobesin 4. In vivo competition experiments demonstrated a specific uptake in PC3 tumors and in physiologic GRP receptor–expressing tissues. The tumor-to-kidney ratios were 0.7 for Demobesin 4 and 5.2 for Demobesin 1 at 4 h. Conclusion: This comparative in vitro/in vivo study with Demobesin 1 and Demobesin 4 indicates that GRP receptor antagonists may be superior targeting agents to GRP receptor agonists, suggesting a change of paradigm in the field of bombesin radiopharmaceuticals.