Ischemic myocardial injury causes timed recruitment of neutrophils and monocyte/macrophages, which produce significant amounts of myeloperoxidase (MPO). MPO leads to the formation of reactive chlorinating species capable of oxidizing proteins. We developed a small molecule based MPO substrate for MRI, Gd-bis-5-HT-DPTA, which is first radicalized, and then oligomerized and covalently bound to matrix proteins, all leading to enhanced R1-relaxivity and delayed wash out kinetics. Mice were subjected to coronary artery ligation and injected with 0.3mmol/kg Gd-bis-5-HT-DPTA (or Gd-DTPA as control). We performed T1-weighted cardio-respiratory gated MRI 10–120min later, followed by immunoreactive staining for MPO. 3 mice each were studied at day 1, 2, 4, 8, and >1 month after MI. Subsequently, MPO tissue activity was determined with the guaiacol method. MPO activity peaked 2 days after MI (contrast-to-noise-ratio (CNR) day 1, 26+/−4; day 2, 39+/−10; day 4, 29+/−3), and tissue levels of MPO over time correlated well with probe activity in vivo (r2=0.65, p<0.01). CNR following Gd-DTPA peaked ten minutes after injection (10.5+/−0.2), and returned to pre-injection values at 60min. In contradistinction, following injection Gd-bis-5-HT-DPTA, CNR was higher and peaked later (p<0.05 vs. Gd-DTPA, arrows depict MI in figure
). Immunoreactive staining for MPO correlated well with enhancement (r2=0.92, p<0.05). Gd-bis-5-HT-DPTA facilitates in-vivo assessment of MPO activity in injured myocardium. This approach allows non-invasive probing of the inflammatory response to ischemia and has the potential to guide the development and application of novel cardioprotective therapies.
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