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.

Methods and 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.