Molecular imaging investigations of a Ga-67/Cu-64 labeled bivalent ligand, [RGD-Glu-(DO3A)-6-Ahx-RM2], targeting GRPR/alpha(v)beta(3) biomarkers: a comparative study

摘要

In the present study, we report the metallation, characterization, in vitro and in vivo studies comparing Ga-67/Cu-64-radiolabeled bivalent peptide ligands as targeting probes with the capability of targeting the alpha(v)beta(3) integrin or the gastrin releasing peptide receptor (GRPR) for preclinical and clinical use in single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging of prostate tumors. The ligand precursor, [RGD-Glu-6-Ahx-RM2] (RGD: Arg-GlyAsp; Glu: glutamic acid; 6-Ahx: 6-amino hexanoic acid; RM2: D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH 2), was conjugated to a DOTA (1,4,7,10-tetraazacyclododecane1,4,7,10- tetraacetic acid) bifunctional chelator (BFCA), purified by reversed-phase high-performance liquid chromatography (RP-HPLC), characterized via electrospray ionization-mass spectrometry (ESI-MS), and radiolabeled with Ga-67 or Cu-64. The in vitro investigations of the binding affinities of the natural-metallated ligands for the GRPR or the alpha(v)beta(3) integrin were conducted via competitive displacement binding assays in human prostate PC-3 and glioblastoma U87-MG cell lines. Following stability investigations via RP-HPLC, the in vivo evaluations of the Ga-67/Cu-64-radiolabeled ligands were performed in CF-1 mice and SCID mice bearing PC-3 tumors. The in vitro studies of the natural-metallated ligands showed high binding affinities for the GRPR (7.78 +/- 2.42, 8.64 +/- 2.16nM; Ga, Cu respectively) and moderate binding affinity for the alpha(v)beta(3) integrin receptor (307 +/- 40.0, 308 +/- 42.6 nM; Ga, Cu respectively). In vivo biodistribution studies displayed high tumor uptake (7.44 +/- 1.09, 10.85 +/- 4.02% ID/g at 1 h post-intravenous injection; 67 Ga, 64 Cu respectively) and prolonged tumor retention (4.89 +/- 1.11, 4.09 +/- 0.96% ID/g at 24 h post-intravenous injection; 67 Ga, 64 Cu respectively) in PC-3 tumor-bearing mice. Micro-single photon emission computed tomography (microSPECT) andmicropositron emission computed tomography (microPET) molecular imaging studies produced high-quality, high-contrast images in PC-3 tumor-bearing mice at 18 h post-intravenous injection. Both radiolabeled ligands show satisfactory tumor uptake and retention in PC-3 tumor-bearing mice. However, [RGD-Glu-(67 Ga-DO3A)-6-Ahx-RM2] demonstrates superior pharmacokinetic profiles to [RGD-Glu-(64 Cu-DO3A)-6-Ahx-RM2], presumably due to more favorable in vivo stability.