Macrophage PET Imaging in Mouse Models of Cardiovascular Disease and Cancer with an Apolipoprotein-inspired Radiotracer

Macrophages are key inflammatory mediators in many pathological conditions, including cardiovascular disease (CVD) and cancer, the leading causes of morbidity and mortality worldwide. This makes macrophage burden a valuable diagnostic marker and several strategies to monitor these cells have been reported. However, such strategies are often high-priced, non-specific, invasive, and/or not quantitative. Here, we developed a positron emission tomography (PET) radiotracer based on apolipoprotein A1 (ApoA1), the main protein component of high-density lipoprotein (HDL), which has an inherent affinity for macrophages. We radiolabeled an ApoA1-mimetic peptide (mA1) with zirconium-89 (Zr) to generate a lipoprotein-avid PET probe (Zr-mA1). We first characterized Zr-mA1's affinity for lipoproteins in vitro by size exclusion chromatography. To study Zr-mA1's in vivo behavior and interaction with endogenous lipoproteins, we performed extensive studies in wildtype C57BL/6 and hypercholesterolemic mice. Subsequently, we used in vivo PET imaging to study macrophages in melanoma and myocardial infarction using mouse models. The tracer's cell specificity was assessed by histology and mass cytometry (CyTOF). Our data show that Zr-mA1 associates with lipoproteins in vitro. This is in line with our in vivo experiments, in which we observed longer Zr-mA1 circulation times in hypercholesterolemic mice compared to C57BL/6 controls. Zr-mA1 displayed a tissue distribution profile similar to ApoA1 and HDL, with high kidney and liver uptake as well as substantial signal in the bone marrow and spleen. The tracer also accumulated in tumors of melanoma-bearing mice and in the ischemic myocardium of infarcted animals. In these sites, CyTOF analyses revealed that Zr-mA1 was predominantly taken up by macrophages. Our results demonstrate that Zr-mA1 associates with lipoproteins and hence accumulates in macrophages in vivo. Zr-mA1's high uptake in these cells makes it a promising radiotracer for non-invasively and quantitatively studying conditions characterized by marked changes in macrophage burden.