title: In vitro and in vivo evaluation of a (18)F-labeled high affinity NOTA conjugated bombesin antagonist as a PET ligand for GRPR-targeted tumor imaging.
abstract: Expression of the gastrin-releasing peptide receptor (GRPR) in prostate cancer suggests that this receptor can be used as a potential molecular target to visualize and treat these tumors. We have previously investigated an antagonist analog of bombesin (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2, RM26) conjugated to 1,4,7-triazacyclononane-N,N',N''-triacetic acid (NOTA) via a diethylene glycol (PEG2) spacer (NOTA-P2-RM26) labeled with (68)Ga and (111)In. We found that this conjugate has favorable properties for in vivo imaging of GRPR-expression. The focus of this study was to develop a (18)F-labelled PET agent to visualize GRPR. NOTA-P2-RM26 was labeled with (18)F using aluminum-fluoride chelation. Stability, in vitro binding specificity and cellular processing tests were performed. The inhibition efficiency (IC50) of the [(nat)F]AlF-NOTA-P2-RM26 was compared to that of the (nat)Ga-loaded peptide using (125)I-Tyr(4)-BBN as the displacement radioligand. The pharmacokinetics and in vivo binding specificity of the compound were studied. NOTA-P2-RM26 was labeled with (18)F within 1 h (60-65% decay corrected radiochemical yield, 55 GBq/µmol). The radiopeptide was stable in murine serum and showed high specific binding to PC-3 cells. [(nat)F]AlF-NOTA-P2-RM26 showed a low nanomolar inhibition efficiency (IC50=4.4±0.8 nM). The internalization rate of the tracer was low. Less than 14% of the cell-bound radioactivity was internalized after 4 h. The biodistribution of [(18)F]AlF-NOTA-P2-RM26 demonstrated rapid blood clearance, low liver uptake and low kidney retention. The tumor uptake at 3 h p.i. was 5.5±0.7 %ID/g, and the tumor-to-blood, -muscle and -bone ratios were 87±42, 159±47, 38±16, respectively. The uptake in tumors, pancreas and other GRPR-expressing organs was significantly reduced when excess amount of non-labeled peptide was co-injected. The low uptake in bone suggests a high in vivo stability of the Al-F bond. High contrast PET image was obtained 3 h p.i. The initial biological results suggest that [(18)F]AlF-NOTA-P2-RM26 is a promising candidate for PET imaging of GRPR in vivo.
authors: Preclinical PET Platform, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden.
authors: Zohreh
authors: Z
authors: Varasteh
authors: Ola
authors: O
authors: Aberg
authors: Irina
authors: I
authors: Velikyan
authors: Gunnar
authors: G
authors: Lindeberg
authors: Jens
authors: J
authors: Sörensen
authors: Mats
authors: M
authors: Larhed
authors: Gunnar
authors: G
authors: Antoni
authors: Mattias
authors: M
authors: Sandström
authors: Vladimir
authors: V
authors: Tolmachev
authors: Anna
authors: A
authors: Orlova
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001839:Bombesin / Q000037:antagonists & inhibitors*; D045744:Cell Line, Tumor; D002471:Cell Transformation, Neoplastic; D004355:Drug Stability; D005026:Ethylene Glycols / Q000737:chemistry; D005462:Fluorine Radioisotopes; D006571:Heterocyclic Compounds / Q000737:chemistry*; D006573:Heterocyclic Compounds, 1-Ring; D006801:Humans; D008024:Ligands; D008297:Male; D051379:Mice; D009842:Oligopeptides / Q000737:chemistry / Q000493:pharmacokinetics / Q000494:pharmacology; D049268:Positron-Emission Tomography / Q000379:methods*; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology; D018004:Receptors, Bombesin / Q000378:metabolism*
chemical_list: D005026:Ethylene Glycols; D005462:Fluorine Radioisotopes; D006571:Heterocyclic Compounds; D006573:Heterocyclic Compounds, 1-Ring; D008024:Ligands; D009842:Oligopeptides; D018004:Receptors, Bombesin; C048993:1,4,7-triazacyclononane-N,N',N''-triacetic acid; C013484:diethylene glycol; D001839:Bombesin