Optimization, biological evaluation and microPET imaging of copper-64-labeled bombesin agonists, [64Cu-NO2A-(X)-BBN(7-14)NH2], in a prostate tumor xenografted mouse model.
Gastrin-releasing peptide receptors (GRPr) are a member of the bombesin (BBN) receptor family. GRPr are expressed in high numbers on specific human cancers, including human prostate cancer. Therefore, copper-64 ((64)Cu) radiolabeled BBN(7-14)NH(2) conjugates could have potential for diagnosis of human prostate cancer via positron-emission tomography (PET). The aim of this study was to produce [(64)Cu-NO2A-(X)-BBN(7-14)NH(2)] conjugates for prostate cancer imaging, where X=pharmacokinetic modifier (beta-alanine, 5-aminovaleric acid, 6-aminohexanoic acid, 8-aminooctanoic acid, 9-aminonanoic acid or para-aminobenzoic acid) and NO2A=1,4,7-triazacyclononane-1,4-diacetic acid [a derivative of NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid)].


METHODS
[(X)-BBN(7-14)NH(2)] Conjugates were synthesized by solid-phase peptide synthesis (SPPS), after which NOTA was added via manual conjugation. The new peptide conjugates were radiolabeled with (64)Cu radionuclide. The receptor-binding affinity was determined in human prostate PC-3 cells, and tumor-targeting efficacy was determined in PC-3 tumor-bearing severely combined immunodeficient (SCID) mice. Whole-body maximum intensity microPET/CT images of PC-3 tumor-bearing SCID mice were obtained 18 h postinjection (pi).


RESULTS
Competitive binding assays in PC-3 cells indicated high receptor-binding affinity for the [NO2A-(X)-BBN(7-14)NH(2)] and [(nat)Cu-NO2A-(X)-BBN(7-14)NH(2)] conjugates. In vivo biodistribution studies of the [(64)Cu-NO2A-(X)-BBN(7-14)NH(2)] conjugates at 1, 4 and 24 h pi showed very high uptake of the tracer in GRPr-positive tissue with little accumulation and retention in nontarget tissues. High-quality, high-contrast microPET images were obtained, with xenografted tumors being clearly visible at 18 h pi.


CONCLUSIONS
NO2A chelator sufficiently stabilizes copper(II) radiometal under in vivo conditions, producing conjugates with very high uptake and retention in targeted GRPr. Preclinical evaluation of these new peptide conjugates in tumor-bearing mice provides some impetus for clinical evaluation in human patients.
Lane
Stephanie R
SR
Department of Radiology, University of Missouri-Columbia School of Medicine, Columbia, MO 65211, USA. srlf36@mail.missouri.edu
Nanda
Prasanta
P
Rold
Tammy L
TL
Sieckman
Gary L
GL
Figueroa
Said D
SD
Hoffman
Timothy J
TJ
Jurisson
Silvia S
SS
Smith
Charles J
CJ
D000818:Animals; D001839:Bombesin / Q000737:chemistry; D003301:Copper Radioisotopes; D006573:Heterocyclic Compounds, 1-Ring; D006801:Humans; D008297:Male; D051379:Mice; D008819:Mice, Nude; D049268:Positron-Emission Tomography; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism; D019275:Radiopharmaceuticals; D014018:Tissue Distribution; D014407:Tumor Cells, Cultured; D023041:Xenograft Model Antitumor Assays
D003301:Copper Radioisotopes; D006573:Heterocyclic Compounds, 1-Ring; D019275:Radiopharmaceuticals; D001839:Bombesin