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Introduction Atherosclerotic plaque rupture and erosion underlie myocardial infarction and stroke. Yet, current imaging lacks specificity for identifying plaques at high-risk for a clinical event. Extracellular arterial activity of the pro-inflammatory enzyme myeloperoxidase (MPO) destabilises atherosclerotic plaque and associates with future atherothrombosis. A clinically translatable method to identify MPO-driven inflammation could improve risk stratification and guide anti-inflammatory therapies. To support clinical translation and first-in-human studies, we developed and validated the NODAGA-based positron emission tomography (PET) radiotracer [ 68Ga]Ga-IEMA (Index for Extracellular MPO Activity), designed to image extracellular MPO activity in vivo.Methods IEMA was synthesised in four steps and radiolabelled in a single complexation step without pre-processing of the 68Ge/68Ga generator eluate or purification steps. Radiochemical purity was assessed by reversed-phase radio-HPLC and thin layer chromatography (ITLC); lipophilicity by LogP and LogD7.4; and serum stability by size exclusion radio-HPLC. Reactivity with enzymatically active MPO was evaluated in solution and gel assays. Translational relevance was assessed in a murine model of plaque instability and a rabbit model of atherothrombosis. Intravenous injection of 8.2 ± 3.8 MBq [68Ga]Ga-IEMA in Apoe–/– mice (n=8) with tandem stenosis producing stable and unstable plaque was used to assess tracer kinetics and plaque uptake by in vivo PET/CT imaging. Rabbits (n=10) underwent MRI before and after pharmacological plaque disruption, followed by intravenous administration of non-radioactive [natGa]Ga-IEMA. Tracer retention in stable versus ruptured/eroded plaques was quantified using inductively coupled plasma mass spectrometry.Results Synthesis of [ 68Ga]Ga-IEMA (figure 1) was achieved with rapid (<11 min) and efficient radiolabelling (95–100% radiochemical purity). [68Ga]Ga-IEMA is moderately hydrophilic, does not cross cell membranes, and is stable in human serum. Enzymatic activation led to probe oligomerisation and covalent protein binding. In vivo, [68Ga]Ga-IEMA showed favourable blood kinetics (half-lives fast=3.1 min and slow=37.8 min) with mostly renal clearance. In vivo imaging with [68Ga]Ga-IEMA PET/CT selectively identified unstable plaque in the mouse model of plaque instability (figure 1a). Consistently, [natGa]Ga-IEMA retention was higher in ruptured/eroded than stable plaques in rabbits (figure 1b).Conclusions [ 68Ga]Ga-IEMA enables non-invasive PET imaging of extracellular MPO activity and demonstrates robust performance across small and large animal models. These findings support its translational potential for identifying high-risk atherosclerotic plaques and for monitoring therapeutic interventions targeting vascular inflammation.