TY - JOUR
T1 - Evolution of Bombesin Conjugates for Targeted PET Imaging of Tumors
AU - Zhang, Hanwen
AU - Abiraj, Keelara
AU - Thorek, Daniel L.J.
AU - Waser, Beatrice
AU - Smith-Jones, Peter M.
AU - Honer, Michael
AU - Reubi, Jean Claude
AU - Maecke, Helmut R.
PY - 2012/9/14
Y1 - 2012/9/14
N2 - Bombesin receptors are under intense investigation as molecular targets since they are overexpressed in several prevalent solid tumors. We rationally designed and synthesized a series of modified bombesin (BN) peptide analogs to study the influence of charge and spacers at the N-terminus, as well as amino acid substitutions, on both receptor binding affinity and pharmacokinetics. This enabled development of a novel 64/67Cu-labeled BN peptide for PET imaging and targeted radiotherapy of BN receptor-positive tumors. Our results show that N-terminally positively charged peptide ligands had significantly higher affinity to human gastrin releasing peptide receptor (GRPr) than negatively charged or uncharged ligands (IC50: 3.2±0.5 vs 26.3±3.5 vs 41.5±2.5 nM). The replacement of Nle14 by Met, and deletion of D-Tyr6, further resulted in 8-fold higher affinity. Contrary to significant changes to human GRPr binding, modifications at the N-terminal and at the 6th, 11th, and 14th position of BN induced only slight influences on affinity to mouse GRPr. [CuII]-CPTA-[βAla11] BN(7-14) ([CuII]-BZH7) showed the highest internalization rate into PC-3 cells with relatively slow efflux because of its subnanomolar affinity to GRPr. Interestingly, [64/67Cu]-BZH7 also displayed similar affinities to the other 2 human BN receptor subtypes. In vivo studies showed that [64/67Cu]-BZH7 had a high accumulation in PC-3 xenografts and allowed for clear-cut visualization of the tumor in PET imaging. In addition, a CPTA-glycine derivative, forming a hippurane-type spacer, enhanced kidney clearance of the radiotracer. These data indicate that the species variation of BN receptor plays an important role in screening radiolabeled BN. As well, the positive charge from the metallated complex at the N-terminal significantly increases affinity to human GRPr. Application of these observations enabled the novel ligand [64/67Cu]-BZH7 to clearly visualize PC-3 tumors in vivo. This study provides a strong starting point for optimizing radiopeptides for targeting carcinomas that express any of the BN receptor subtypes.
AB - Bombesin receptors are under intense investigation as molecular targets since they are overexpressed in several prevalent solid tumors. We rationally designed and synthesized a series of modified bombesin (BN) peptide analogs to study the influence of charge and spacers at the N-terminus, as well as amino acid substitutions, on both receptor binding affinity and pharmacokinetics. This enabled development of a novel 64/67Cu-labeled BN peptide for PET imaging and targeted radiotherapy of BN receptor-positive tumors. Our results show that N-terminally positively charged peptide ligands had significantly higher affinity to human gastrin releasing peptide receptor (GRPr) than negatively charged or uncharged ligands (IC50: 3.2±0.5 vs 26.3±3.5 vs 41.5±2.5 nM). The replacement of Nle14 by Met, and deletion of D-Tyr6, further resulted in 8-fold higher affinity. Contrary to significant changes to human GRPr binding, modifications at the N-terminal and at the 6th, 11th, and 14th position of BN induced only slight influences on affinity to mouse GRPr. [CuII]-CPTA-[βAla11] BN(7-14) ([CuII]-BZH7) showed the highest internalization rate into PC-3 cells with relatively slow efflux because of its subnanomolar affinity to GRPr. Interestingly, [64/67Cu]-BZH7 also displayed similar affinities to the other 2 human BN receptor subtypes. In vivo studies showed that [64/67Cu]-BZH7 had a high accumulation in PC-3 xenografts and allowed for clear-cut visualization of the tumor in PET imaging. In addition, a CPTA-glycine derivative, forming a hippurane-type spacer, enhanced kidney clearance of the radiotracer. These data indicate that the species variation of BN receptor plays an important role in screening radiolabeled BN. As well, the positive charge from the metallated complex at the N-terminal significantly increases affinity to human GRPr. Application of these observations enabled the novel ligand [64/67Cu]-BZH7 to clearly visualize PC-3 tumors in vivo. This study provides a strong starting point for optimizing radiopeptides for targeting carcinomas that express any of the BN receptor subtypes.
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U2 - 10.1371/journal.pone.0044046
DO - 10.1371/journal.pone.0044046
M3 - Article
C2 - 23024746
AN - SCOPUS:84866427436
SN - 1932-6203
VL - 7
JO - PloS one
JF - PloS one
IS - 9
M1 - e44046
ER -