title: Activatable magnetic resonance imaging agent reports myeloperoxidase activity in healing infarcts and noninvasively detects the antiinflammatory effects of atorvastatin on ischemia-reperfusion injury.
abstract: BACKGROUND
  Ischemic injury of the myocardium causes timed recruitment of neutrophils and monocytes/macrophages, which produce substantial amounts of local myeloperoxidase (MPO). MPO forms reactive chlorinating species capable of inflicting oxidative stress and altering protein function by covalent modification. We have used a small-molecule, gadolinium-based activatable sensor for magnetic resonance imaging of MPO activity (MPO-Gd). MPO-Gd is first radicalized by MPO and then either spontaneously oligomerizes or binds to matrix proteins, all leading to enhanced spin-lattice relaxivity and delayed washout kinetics. We hypothesized that MPO imaging could be used to measure inflammatory responses after myocardial ischemia locally and noninvasively in a murine model.
  
  
  RESULTS
  We injected 0.3 mmol/kg MPO-Gd (or Gd-DTPA as control) and performed magnetic resonance imaging up to 120 minutes later in mice 2 days after myocardial infarction. The contrast-to-noise ratio (infarct versus septum) after Gd-DTPA injection peaked at 10 minutes and returned to preinjection values at 60 minutes. After injection of MPO-Gd, the contrast-to-noise ratio peaked later and was higher than Gd-DTPA (40.8+/-10.4 versus 10.5+/-0.2; P<0.05). MPO imaging was validated by magnetic resonance imaging of MPO-/- mice and correlated well with immunoreactive staining (r2=0.92, P<0.05), tissue activity by guaiacol assay (r2=0.65, P<0.001), and immunoblotting. In time course imaging, activity peaked 2 days after coronary ligation. Flow cytometry of digested infarcts detected MPO in neutrophils and monocytes/macrophages. Furthermore, serial MPO imaging accurately tracked the antiinflammatory effects of atorvastatin therapy after ischemia-reperfusion injury.
  
  
  CONCLUSIONS
  MPO-Gd enables in vivo assessment of MPO activity in injured myocardium. This approach allows noninvasive evaluation of the inflammatory response to ischemia and has the potential to guide the development of novel cardioprotective therapies.
authors.affiliation: Center for Systems Biology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA. MNahrendorf@mgh.harvard.edu
authors.forename: Matthias
authors.initials: M
authors.lastname: Nahrendorf
authors.forename: David
authors.initials: D
authors.lastname: Sosnovik
authors.forename: John W
authors.initials: JW
authors.lastname: Chen
authors.forename: Peter
authors.initials: P
authors.lastname: Panizzi
authors.forename: Jose-Luiz
authors.initials: JL
authors.lastname: Figueiredo
authors.forename: Elena
authors.initials: E
authors.lastname: Aikawa
authors.forename: Peter
authors.initials: P
authors.lastname: Libby
authors.forename: Filip K
authors.initials: FK
authors.lastname: Swirski
authors.forename: Ralph
authors.initials: R
authors.lastname: Weissleder
mesh_terms: D000818:Animals; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000627:therapeutic use; D000069059:Atorvastatin; D005682:Gadolinium / Q000032:analysis*; D006538:Heptanoic Acids / Q000627:therapeutic use*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D051379:Mice; D008810:Mice, Inbred C57BL; D009203:Myocardial Infarction / Q000175:diagnosis / Q000188:drug therapy / Q000201:enzymology*; D015428:Myocardial Reperfusion Injury / Q000175:diagnosis / Q000188:drug therapy / Q000201:enzymology*; D009195:Peroxidase / Q000032:analysis* / Q000378:metabolism; D011758:Pyrroles / Q000627:therapeutic use*
chemical_list: D000894:Anti-Inflammatory Agents, Non-Steroidal; D006538:Heptanoic Acids; D011758:Pyrroles; D000069059:Atorvastatin; D005682:Gadolinium; D009195:Peroxidase