title: The effect of macrocyclic chelators on the targeting properties of the 68Ga-labeled gastrin releasing peptide receptor antagonist PEG2-RM26.
abstract: BACKGROUND
  Overexpression of gastrin-releasing peptide receptors (GRPR) has been reported in several cancers. Bombesin (BN) analogs are short peptides with a high affinity for GRPR. Different BN analogs were evaluated for radionuclide imaging and therapy of GRPR-expressing tumors. We have previously investigated an antagonistic analog of BN (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH(2), RM26) conjugated to NOTA via a PEG(2) spacer (NOTA-PEG(2)-RM26) labeled with (68)Ga, (111)In and Al(18)F. (68)Ga-labeled NOTA-PEG(2)-RM26 showed high tumor-to-organ ratios.
  
  
  METHODS
  The influence of different macrocyclic chelators (NOTA, NODAGA, DOTA and DOTAGA) on the targeting properties of (68)Ga-labeled PEG(2)-RM26 was studied in vitro and in vivo.
  
  
  RESULTS
  All conjugates were labeled with generator-produced (68)Ga with high yields and demonstrated high stability and specific binding to GRPR. The IC(50) values of (nat)Ga-X-PEG(2)-RM26 (X = NOTA, DOTA, NODAGA, DOTAGA) were 2.3 ± 0.2, 3.0 ± 0.3, 2.9 ± 0.3 and 10.0 ± 0.6 nM, respectively. The internalization of the conjugates by PC-3 cells was low. However, the DOTA-conjugated analog demonstrated a higher internalization rate compared to other analogs. GRPR-specific uptake was found in receptor-positive normal tissues and PC-3 xenografts for all conjugates. The biodistribution of the conjugates was influenced by the choice of the chelator moiety. Although all radiotracers cleared rapidly from the blood, [(68)Ga]Ga-NOTA-PEG(2)-RM26 showed significantly lower uptake in lung, muscle and bone compared to the other analogs. The uptake in tumors (5.40 ± 1.04 %ID/g at 2 h p.i.) and the tumor-to-organ ratios (25 ± 3, 157 ± 23 and 39 ± 4 for blood, muscle and bone, respectively) were significantly higher for the NOTA-conjugate than the other analogs.
  
  
  CONCLUSIONS
  Chelators had a clear influence on the biodistribution and targeting properties of (68)Ga-labeled antagonistic BN analogs. Positively charged [(68)Ga]Ga-NOTA-PEG(2)-RM26 provided a low kidney radioactivity uptake, high affinity, high tumor uptake and high image contrast.
authors: Preclinical PET Platform, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden.
authors: Zohreh
authors: Z
authors: Varasteh
authors: Preclinical PET Platform, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden.
authors: Bogdan
authors: B
authors: Mitran
authors: Organic Pharmaceutical Chemistry, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden.
authors: Ulrika
authors: U
authors: Rosenström
authors: Preclinical PET Platform, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden.
authors: Irina
authors: I
authors: Velikyan
authors: Preclinical PET Platform, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden.
authors: Maria
authors: M
authors: Rosestedt
authors: Organic Pharmaceutical Chemistry, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden.
authors: Gunnar
authors: G
authors: Lindeberg
authors: Nuclear Medicine and PET, Uppsala University, Uppsala, Sweden.
authors: Jens
authors: J
authors: Sörensen
authors: Science for Life Laboratory, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden.
authors: Mats
authors: M
authors: Larhed
authors: Department of Immunology, Genetics and Pathology, Faculty of Medicine, Uppsala University, Uppsala, Sweden.
authors: Vladimir
authors: V
authors: Tolmachev
authors: Preclinical PET Platform, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden. Electronic address: anna.orlova@pet.medchem.uu.se.
authors: Anna
authors: A
authors: Orlova
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport; D001839:Bombesin / Q000737:chemistry* / Q000378:metabolism* / Q000493:pharmacokinetics / Q000494:pharmacology; D045744:Cell Line, Tumor; D002471:Cell Transformation, Neoplastic; D002614:Chelating Agents / Q000737:chemistry*; D005260:Female; D005710:Gallium Radioisotopes; D006571:Heterocyclic Compounds / Q000737:chemistry; D006801:Humans; D007553:Isotope Labeling; D047028:Macrocyclic Compounds / Q000737:chemistry*; D008297:Male; D051379:Mice; D011092:Polyethylene Glycols / Q000737:chemistry*; D011471:Prostatic Neoplasms / Q000378:metabolism / Q000473:pathology; D018004:Receptors, Bombesin / Q000037:antagonists & inhibitors*; D013329:Structure-Activity Relationship; D014018:Tissue Distribution
chemical_list: D002614:Chelating Agents; D005710:Gallium Radioisotopes; D006571:Heterocyclic Compounds; D047028:Macrocyclic Compounds; D018004:Receptors, Bombesin; D011092:Polyethylene Glycols; D001839:Bombesin
keywords: Bombesin antagonistic analog; DOTA; DOTAGA; Molecular imaging; NODAGA; NOTA