Myeloperoxidase activity predicts atherosclerotic plaque disruption and atherothrombosis

Background Unstable atherosclerotic plaque is characterized by increased myeloperoxidase (MPO) activity. As unstable plaque is vulnerable to disruption and ensuing thrombosis, we examined whether plaque MPO activity predicts atherothrombosis in a pre-clinical model and correlates with ruptured human atheroma. Methods To assess if plaque MPO activity predicts atherothrombosis, rabbits were subjected to aortic endothelial denudation, cholesterol feeding, in vivo magnetic resonance imaging (MRI) of MPO activity using MPO-Gd (gadolinium), followed by pharmacological triggering of atherothrombosis, histology, and MPO activity determined by liquid chromatography tandem mass spectrometry (LC-MSMS) by quantifying the MPO-specific product of hydroethidine, 2-chloroethidium. To correlate plaque MPO activity to ruptured human atheroma, ex vivo determination of MPO activity by MPO-Gd retention in carotid endarterectomy (CEA) specimens was correlated with in vivo MRI plaque phenotyping in patients, histology, and MPO activity determined by LC-MSMS. Results In rabbits, pre-trigger in vivo MPO activity, validated by LC-MSMS and histology, was higher in thrombosis-prone than thrombosis-resistant plaques and lesion-free segments (R1 relaxation rate = 2.2 ± 0.4 versus 1.6 ± 0.2 and 1.5 ± 0.2 s-[1][1], respectively, p<0.0001), and it predicted atherothrombosis. In CEA specimens, MPO-Gd retention was greater in histologic and MRI-graded American Heart Association (AHA) type VI than types III, IV and V plaques (ΔR1 relaxation rate from baseline = 48 ± 6 versus 16 ± 7, 17 ± 8 and 23 ± 8%, respectively, p<0.0001). This association was confirmed by comparing AHA grade to MPO activity determined by LC-MSMS (277 ± 338 versus 7 ± 6, 11 ± 12 and 42 ± 39 pmol 2-chloroethidium/mg protein for type VI versus type III-V plaques, respectively, p=0.0008). Conclusions MPO activity is elevated in thrombosis-prone rabbit and ruptured human atheroma. Non-invasive molecular imaging of MPO activity predicts atherothrombosis, highlighting the potential of arterial MPO activity to detect vulnerable, destabilized atherosclerosis. ### Competing Interest Statement The authors have declared no competing interest. * 2-Cl-E+ : 2-chloroethidium AHA : American Heart Association AWA : aortic wall area CEA : carotid endarterectomy DOTA : gadolinium-gadoteric acid DTPA : diethylenetriaminepentaacetic acid FDG : 18F-fluorodeoxyglucose IPH : intraplaque hemorrhage LC-MSMS : liquid chromatography tandem mass spectrometry LGE : late MPO-Gd-enhanced MPO : myeloperoxidase MPO-Gd : gadolinium- bis -5-hydroxytryptamide diethylenetriaminepentaacetic acid MRI : magnetic resonance imaging PET : positron emission tomography R1 : relaxation rate ROC : receiver operating characteristic T1BB : 2D T1-weighted black blood T1w-IR : 3D inversion recovery T1w TMCR : tissue-to-muscle contrast ratio USPIO : ultra-small superparamagnetic iron oxide [1]: #ref-1