RT Journal Article
SR Electronic
T1 In Vitro and In Vivo Evaluation of <sup>64</sup>Cu-Labeled SarAr-Bombesin Analogs in Gastrin-Releasing Peptide Receptor–Expressing Prostate Cancer
JF Journal of Nuclear Medicine
JO J Nucl Med
FD Society of Nuclear Medicine
SP 470
OP 477
DO 10.2967/jnumed.110.082826
VO 52
IS 3
A1 Lears, Kimberly A.
A1 Ferdani, Riccardo
A1 Liang, Kexian
A1 Zheleznyak, Alexander
A1 Andrews, Rebecca
A1 Sherman, Christopher D.
A1 Achilefu, Samuel
A1 Anderson, Carolyn J.
A1 Rogers, Buck E.
YR 2011
UL http://jnm.snmjournals.org/content/52/3/470.abstract
AB Bombesin is a 14–amino-acid amphibian peptide that binds with high affinity to the gastrin-releasing peptide receptor (GRPR), which is overexpressed on a variety of solid tumors. It has been demonstrated that bombesin analogs can be radiolabeled with a variety of radiometals for potential diagnosis and treatment of GRPR-positive tumors. In this regard, several studies have used different chelators conjugated to the 8 C-terminal amino acids of bombesin(7-14) for radiolabeling with 64Cu. These analogs have demonstrated GRPR-specific small-animal PET of tumors but have various advantages and disadvantages. The objective of this study was to conjugate the previously described (1-N-(4-aminobenzyl)-3,6,10,13,16,19-hexaazabicyclo[6.6.6]-eicosane-1,8-diamine) (SarAr) chelator to bombesin(7-14), radiolabel the conjugate with 64Cu, and evaluate in vitro and in vivo. Methods: SarAr was synthesized as previously published and conjugated to bombesin(7-14) by solid-phase peptide synthesis using standard Fmoc chemistry. Succinic acid (SA), 8-aminooctanoic acid (Aoc), and Gly-Ser-Gly (GSG) were used as linkers between SarAr and bombesin(7-14) to yield the resulting SarAr-SA-Aoc-bombesin(7-14) and SarAr-SA-Aoc-GSG-bombesin(7-14) peptides. The unlabeled peptides were evaluated in a competitive binding assay using PC-3 prostate cancer cells and 125I-Tyr4-bombesin to determine the inhibitory concentration of 50%. The peptides were radiolabeled with 64Cu and evaluated for internalization into PC-3 cells in vitro and for in vivo tumor uptake in mice bearing PC-3 xenografts using biodistribution and small-animal PET/CT studies. Results: The competitive binding assay demonstrated that both SarAr-SA-Aoc-bombesin(7-14) and SarAr-SA-Aoc-GSG-bombesin(7-14) bound with high affinity to GRPR with an inhibitory concentration of 50% of 3.5 and 4.5 nM, respectively. Both peptides were radiolabeled with 64Cu at room temperature without further purification and demonstrated similar internalization into PC-3 cells. In vivo, the radiolabeled peptides demonstrated tumor-specific uptake (13.0 and 8.5 percentage injected dose per gram for 64Cu-SarAr-SA-Aoc-bombesin(7-14) and 64Cu-SarAr-SA-Aoc-GSG-bombesin(7-14), respectively, at 1 h) and imaging that was comparable to, or better than, that of the previously reported 64Cu-labeled bombesin analogs. The 64Cu-SarAr-SA-Aoc-GSG-bombesin(7-14) had more rapid blood clearance and lower tumor and normal-tissue uptake than 64Cu-SarAr-SA-Aoc-bombesin(7-14), resulting in similar tumor-to-blood ratios for each analog (15.1 vs. 11.3 for 64Cu-SarAr-SA-Aoc-bombesin(7-14) and 64Cu-SarAr-SA-Aoc-GSG-bombesin(7-14), respectively, at 1 h). Conclusion: These studies demonstrate that 64Cu-SarAr-SA-Aoc-bombesin(7-14) and 64Cu-SarAr-SA-Aoc-GSG-bombesin(7-14) bound with high affinity to GRPR-expressing cells and that these peptides can be used for PET of GRPR-expressing prostate cancer.