In vivo near-infrared-II photoacoustic molecular imaging using a myeloperoxidase-targeted nanoprobe to identify vulnerable atherosclerotic plaques

Introduction Inflammation is a pivotal driver of atherosclerotic plaque progression. Current imaging modalities face significant limitations in the molecular assessment of inflamed vulnerable plaques. Photoacoustic imaging (PAI) within the second near-infrared window (NIR-II, 1000–1700 nm) has emerged as a promising alternative, offering reduced tissue autofluorescence and deeper penetration. In this study, we aimed to develop a novel NIR-II PAI platform operating at 1064 nm to specifically detect and characterize vulnerable plaques by targeting myeloperoxidase (MPO), a potential biomarker of plaque inflammation and vulnerability. Methods Human carotid plaques were analyzed for MPO correlation with vulnerability markers. An MPO-targeted nanoprobe (5-HT-MNPs) was synthesized via self-assembly of IR1061 dye and 5-HT-conjugated PEG-DSPE. Vulnerable plaque were established in ApoE−/− mice fed a high-fat diet for 12 weeks and subjected to tandem stenosis (TS) surgery. TS-operated mice received either colchicine (anti-inflammatory treatment) or vehicle for six weeks post-surgery. Additionally, an atherosclerotic rabbit model was generated by abdominal aortic balloon deendothelialization followed by a high-fat diet. Following intravenous administration of 5-HT-MNPs, NIR-II PAI at 1064 nm was performed using a commercial photoacoustic tomography system (mice) or a custom-built intravascular ultrasound (IVUS) and photoacoustic (IVPA) imaging system (rabbits). Histopathological analysis was conducted to evaluate MPO expression and plaque vulnerability features. Results MPO correlated strongly with human plaque vulnerability markers. The synthesized 5-HT-MNPs probe demonstrated excellent stability, robust NIR-II photoacoustic signal generation, high specificity for MPO, and favorable biocompatibility. In ApoE−/− mice with TS-induced vulnerable plaques, the 5-HT-MNPs probe selectively accumulated within carotid lesions, enabling clear visualization by NIR-II PAI. Following colchicine treatment, a marked reduction in photoacoustic signal was observed, reflecting therapeutic monitoring capacity in vivo. In atherosclerotic rabbits, IVUS/IVPA imaging revealed strong PAI signals co-localizing with IVUS-detected atheromata only in animals injected with 5-HT-MNPs. Quantitative analysis confirmed significantly higher PAI signals in the targeted group compared to saline-injected controls (P < 0.01). Histopathological correlation demonstrated that high PAI signal regions corresponded to elevated MPO expression and features of unstable plaque phenotype, including increased macrophage infiltration, larger necrotic cores, and reduced collagen content. Conclusion We have developed a MPO-targeted NIR-II PAI platform which enables molecular imaging of plaque inflammation and therapeutic response. Strong signal-histopathology correlation supports its clinical potential for vulnerable plaque detection and risk stratification.