title: Synthesis and preclinical evaluation of 68Ga-labeled collagelin analogs for imaging and quantification of fibrosis.
abstract: OBJECTIVE
  Fibrosis affecting functionality of vital organs such as liver, lung, heart, and kidney, is involved in many chronic diseases. Positron emission tomography (PET) would not only provide precise localization and extent of the affected tissue but also allow the accurate quantification of the fibrotic process for the subsequent prognosis.
  
  
  METHODS
  A cyclic peptide c[CPGRVMHGLHLGDDEGPC] conjugated either to 2-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazonan-1-yl)acetic acid (NOTA(tBu)2) or 4-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazacyclononan-1-yl)-5-(tert-butoxy)-5-oxopentanoic acid (NODAGA(tBu)3) via polyethylene glycol link (PEG2) was synthesized and labeled with (68)Ga. Non-specific organ distribution, blood clearance, and excretion were investigated ex vivo in healthy rats. The binding specificity of the radioligands was assessed in vitro using autoradiography on cryosections of dog fibrotic heart tissue.
  
  
  RESULTS
  The yield of NOTA-PEG2-c[CPGRVMHGLHLGDDEGPC] and NODAGA-PEG2-c[CPGRVMHGLHLGDDEGPC] was 56% and 41%, respectively. Non-decay-corrected radiochemical yield was 80 ± 5% with radiochemical purity of 95 ± 4%. Pharmacokinetic studies in healthy male Sprague-Dawley rats showed fast blood clearance and renal excretion. Lower uptake in liver, spleen, and kidney was found for [[(68)Ga]Ga-NOTA](+1)-PEG2-c[CPGRVMHGLHLGDDEGPC] as compared to [[(68)Ga]Ga-NODAGA](0)-PEG2-c[CPGRVMHGLHLGDDEGPC]. Histologic evaluation of the left ventricle (LV) myocardium from a dog with severe mitral regurgitation (MR), revealed mild to moderate perivascular and subendocardial, and mild diffuse interstitial fibrosis. The tracer binding to the cryosections of the tissue was specific with the equilibrium Kd of 2.3 ± 0.8 μM and 2.1 ± 0.9 μM, respectively for [(68)Ga]Ga-NO2A-Col and [(68)Ga]Ga-NODAGA-Col.
  
  
  CONCLUSIONS
  Two novel peptide based agents for the imaging of fibrosis by PET were developed. Moderation of the biodistribution could be achieved by variation of the charge on the complex moiety of the agents. The combination of the fast clearance from non-target organs as well as organs of interest such as lung, heart, and liver and binding specificity to the target tissue suggests the potential of the analogs for the imaging of fibrosis.
authors.affiliation: Department of Medicinal Chemistry, Preclinical PET Platform, Uppsala University, SE-75183 Uppsala, Sweden. Electronic address: irina.velikyan@bms.uu.se.
authors.forename: Irina
authors.initials: I
authors.lastname: Velikyan
authors.affiliation: Department of Medicinal Chemistry, Preclinical PET Platform, Uppsala University, SE-75183 Uppsala, Sweden.
authors.forename: Ulrika
authors.initials: U
authors.lastname: Rosenström
authors.affiliation: Department of Medicinal Chemistry, Preclinical PET Platform, Uppsala University, SE-75183 Uppsala, Sweden.
authors.forename: Sergio
authors.initials: S
authors.lastname: Estrada
authors.affiliation: Department of Clinical Sciences, Faculty of Veterinary Medicine and Animal Science, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden.
authors.forename: Ingrid
authors.initials: I
authors.lastname: Ljungvall
authors.affiliation: Department of Clinical Sciences, Faculty of Veterinary Medicine and Animal Science, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden.
authors.forename: Jens
authors.initials: J
authors.lastname: Häggström
authors.affiliation: Department of Medicinal Chemistry, Preclinical PET Platform, Uppsala University, SE-75183 Uppsala, Sweden.
authors.forename: Olof
authors.initials: O
authors.lastname: Eriksson
authors.affiliation: Department of Medicinal Chemistry, Preclinical PET Platform, Uppsala University, SE-75183 Uppsala, Sweden. Electronic address: gunnar.antoni@akademiska.se.
authors.forename: Gunnar
authors.initials: G
authors.lastname: Antoni
mesh_terms: D000818:Animals; D004285:Dogs; D004353:Drug Evaluation, Preclinical; D005355:Fibrosis / Q000000981:diagnostic imaging / Q000378:metabolism; D005710:Gallium Radioisotopes / Q000737:chemistry / Q000493:pharmacokinetics*; D007553:Isotope Labeling / Q000379:methods; D008297:Male; D008657:Metabolic Clearance Rate; D009206:Myocardium / Q000378:metabolism* / Q000473:pathology*; D009928:Organ Specificity; D010456:Peptides, Cyclic / Q000493:pharmacokinetics*; D049268:Positron-Emission Tomography / Q000379:methods; D019275:Radiopharmaceuticals / Q000138:chemical synthesis; D051381:Rats; D017207:Rats, Sprague-Dawley; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014018:Tissue Distribution
chemical_list: D005710:Gallium Radioisotopes; D010456:Peptides, Cyclic; D019275:Radiopharmaceuticals
keywords: (68)Ga; Fibrosis; Molecular imaging; PET