Small-Animal PET of Tumors with 64Cu-Labeled RGD-Bombesin Heterodimer

The overexpression of gastrin-releasing peptide receptor (GRPR) in various tumor types suggests that GRPR is an attractive target for cancer imaging and therapy with radiolabeled bombesin analogs. We recently reported the ability of F-18-labeled RGD-bombesin heterodimer to be used for dual integrin alpha(v)beta(3)- and GRPR-targeted imaging. To further investigate the synergistic effect of the dual-receptor targeting of peptide heterodimers, we evaluated Cu-64-labeled RGD-bombesin for PET imaging of tumors. Methods: RGD-bombesin was coupled with 1,4,7,10-tetraazacyclododecane-N, N ', N '', N '''-tetraacetic acid (DOTA) and 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA), and the conjugates were labeled with Cu-64. The in vitro and in vivo characteristics of Cu-64-NOTA-RGD-bombesin were compared with those of Cu-64-NOTA-RGD, Cu-64-NOTA-bombesin, and Cu-64-DOTA-RGD-bombesin. Results: Cu-64-NOTA-RGD-bombesin and Cu-64-DOTA-RGD-bombesin had comparable dual integrin alpha(v)beta(3)- and GRPR-binding affinities in vitro, both of which were slightly lower than RGD for integrin binding and bombesin for GRPR binding. Cu-64-NOTA-RGD-bombesin possessed significantly higher tumor uptake than did Cu-64-NOTA-RGD, Cu-64-NOTA-bombesin, the mixture of Cu-64-NOTA-RGD and Cu-64-NOTA-bombesin, or Cu-64-DOTA-RGD-bombesin in PC-3 prostate cancer. Cu-64-NOTA-RGD-bombesin also showed improved in vivo kinetics such as lower liver and intestinal activity accumulation than did the bombesin tracers. Cu-64-NOTA-RGD-bombesin also outperformed Cu-64-NOTA-RGD in a 4T1 murine mammary carcinoma model that expresses integrin on tumor vasculature but no GRPR in tumor tissue, which had no uptake of Cu-64-NOTA-bombesin. Conclusion: Compared with other tracers, Cu-64-NOTA-RGD-bombesin showed favorable in vivo kinetics and enhanced tumor uptake, which warrants its further investigation for targeting tumors that express integrin or GRPR or that coexpress integrin and GRPR for imaging and therapeutic applications. The synergistic effect of RGD-bombesin heterodimers observed in this study also encourages further investigations of novel heterodimers recognizing other cell surface receptors for tumor targeting.