Near-infrared fluorescent imaging of matrix metalloproteinase activity after myocardial infarction

Background - We used a molecular probe activated by protease cleavage to image expression of matrix metalloproteinases (MMPs) in the heart after myocardial infarction. Methods and Results - We synthesized and characterized a near-infrared fluorescent (NIRF) probe that is activated by proteolytic cleavage by MMP2 and MMP9. The NIRF probe was injected into mice at various time points up to 4 weeks after myocardial infarction induced by ligation of the left anterior descending coronary artery. NIRF imaging of MMP activity increased in the infarct region, with maximal expression at 1 to 2 weeks, persisting to 4 weeks. Zymography and real-time polymerase chain reaction analysis showed that MMP9 expression is increased at 2 to 4 days, and MMP2 expression is increased at 1 to 2 weeks. Dual-label confocal microscopy showed colocalization of NIRF imaging with neutrophils on day 2, and flow cytometric analysis confirmed that NIRF signal is associated with leukocytes in the infarct zone. Conclusions - This study demonstrates that the activity of MMPs in the myocardium may be imaged by use of specific activity-dependent molecular probes.