title: Multimodal Molecular Imaging Reveals High Target Uptake and Specificity of 111In- and 68Ga-Labeled Fibrin-Binding Probes for Thrombus Detection in Rats.
abstract: We recently showed the high target specificity and favorable imaging properties of 64Cu and Al18F PET probes for noninvasive imaging of thrombosis. Here, our aim was to evaluate new derivatives labeled with either with 68Ga, 111In, or 99mTc as thrombus imaging agents for PET and SPECT. In this study, the feasibility and potential of these probes for thrombus imaging was assessed in detail in 2 animal models of arterial thrombosis. The specificity of the probes was further evaluated using a triple-isotope approach with multimodal SPECT/PET/CT imaging.
  
  
  METHODS
  Radiotracers were synthesized using a known fibrin-binding peptide conjugated to 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid monoamide (DOTA-MA), or a diethylenetriamine ligand (DETA-propanoic acid [PA]), followed by labeling with 68Ga (FBP14, 68Ga-NODAGA), 111In (FBP15, 111In-DOTA-MA), or 99mTc (FBP16, 99mTc(CO)3-DETA-PA), respectively. PET or SPECT imaging, biodistribution, pharmacokinetics, and metabolic stability were evaluated in rat models of mural and occlusive carotid artery thrombosis. In vivo target specificity was evaluated by comparing the distribution of the SPECT and PET probes with preformed 125I-labeled thrombi and with a nonbinding control probe using SPECT/PET/CT imaging.
  
  
  RESULTS
  All 3 radiotracers showed affinity similar to soluble fibrin fragment DD(E) (inhibition constant=0.53-0.83 μM). After the kidneys, the highest uptake of 68Ga-FBP14 and 111In-FBP15 was in the thrombus (1.0±0.2 percentage injected dose per gram), with low off-target accumulation. Both radiotracers underwent fast systemic elimination (half-life, 8-15 min) through the kidneys, which led to highly conspicuous thrombi on PET and SPECT images. 99mTc-FBP16 displayed low target uptake and distribution consistent with aggregation or degradation. Triple-isotope imaging experiments showed that both 68Ga-FBP14 and 111In-FBP15, but not the nonbinding derivative 64Cu-D-Cys-FBP8, detected the location of the 125I-labeled thrombus, confirming high target specificity.
  
  
  CONCLUSIONS
  68Ga-FBP14 and 111In-FBP15 have high fibrin affinity and thrombus specificity and represent useful PET and SPECT probes for thrombus detection.
authors.affiliation: Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Harvard Medical School, Massachusetts General Hospital, Charlestown, Massachusetts; and.
authors.forename: Bruno L
authors.initials: BL
authors.lastname: Oliveira
authors.affiliation: Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Harvard Medical School, Massachusetts General Hospital, Charlestown, Massachusetts; and.
authors.forename: Francesco
authors.initials: F
authors.lastname: Blasi
authors.affiliation: Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Harvard Medical School, Massachusetts General Hospital, Charlestown, Massachusetts; and.
authors.forename: Tyson A
authors.initials: TA
authors.lastname: Rietz
authors.affiliation: Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Harvard Medical School, Massachusetts General Hospital, Charlestown, Massachusetts; and.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Rotile
authors.affiliation: Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Harvard Medical School, Massachusetts General Hospital, Charlestown, Massachusetts; and.
authors.forename: Helen
authors.initials: H
authors.lastname: Day
authors.affiliation: Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Harvard Medical School, Massachusetts General Hospital, Charlestown, Massachusetts; and Institute for Innovation in Imaging, Massachusetts General Hospital, Boston, Massachusetts caravan@nmr.mgh.harvard.edu.
authors.forename: Peter
authors.initials: P
authors.lastname: Caravan
mesh_terms: D000818:Animals; D005337:Fibrin / Q000031:analogs & derivatives* / Q000378:metabolism*; D005710:Gallium Radioisotopes / Q000493:pharmacokinetics; D006207:Half-Life; D007205:Indium Radioisotopes / Q000493:pharmacokinetics; D007457:Iodine Radioisotopes; D007668:Kidney / Q000000981:diagnostic imaging / Q000378:metabolism; D008297:Male; D008958:Models, Molecular; D057054:Molecular Imaging / Q000379:methods*; D064847:Multimodal Imaging / Q000379:methods*; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals / Q000737:chemistry / Q000378:metabolism / Q000493:pharmacokinetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D013927:Thrombosis / Q000000981:diagnostic imaging*; D014018:Tissue Distribution; D015899:Tomography, Emission-Computed, Single-Photon
chemical_list: D005710:Gallium Radioisotopes; D007205:Indium Radioisotopes; D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals; D005337:Fibrin
keywords: PET; SPECT; fibrin; thrombosis