P2X7 Receptor-Specific Radioligand 18F-FTTM for Atherosclerotic Plaque PET Imaging

Introduction: Site-specific imaging agents play a key role in evaluation of vulnerable atherosclerotic plaques, but only few agents are currently available for non-invasive measurement of inflammation reaction intensity, which is strongly associated with the progression of atherosclerosis. Since P2X7 receptors have been considered as a promising biomarker for inflammation reaction in vulnerable atherosclerotic plaques, therefore, we aim to evaluated the potential value of ¹⁸F-labeled tracer ¹⁸F-FTTM ((2-chloro-3-[¹⁸F]fluorophenyl)[1,4,6,7-tetrahydro-1-(2-pyrimidinyl)-5H-1,2,3-triazolo-[4,5-c] pyridin-5-yl]methanone) for targeting P2X7 receptors, and thus identifying vulnerable atherosclerotic plaques.

Methods: The radioligand ¹⁸F-FTTM was achieved based on the copper-mediated radiofluorination of arylstannane. In vitro and in vivo experiments were performed to verify the biochemical properties. Dynamic ¹⁸F-FTTM Micro-PET/CT imagings were performed for 1 hour on ApoE^-/- mice (10, 20 30 weeks on high fat diet) and wild type C57BL/6J mice on normal diet. Ex vivo PET imagings were conducted to verify the specificity of the radioligand. In addition, ¹⁸F-FDG as the gold standard was also used for evaluation of atherosclerotic plaque models. Serum inflammatory cytokines, lipids and lipoproteins profiles were detected by ELISA. The lipid distribution and morphology of plaques were evaluated by Oil Red O, HE, Masson and immunofluorescence stainings.

Results: The overall radiochemical yield of ¹⁸F-FTTM was 5 %-10 %, the specific activity was 269-320 MBq/nmol (n = 8, EOS), and the radiochemical purity was over 99 %. ¹⁸F-FTTM showed excellent biochemical properties and in vivo and ex vivo imagings showed that increased ¹⁸F-FTTM signal was observed in atherosclerotic lesions, and the ratios of plaque to background value in ApoE^-/- mice (on 20 weeks high-fat diet) were 1.6 times higher than those of the control group (P < 0.05 ,45 min post injection). ¹⁸F-FTTM plaque signal correlates with the duration of a high-fat diet (r = 0.94, P < 0.05). In addition, the ¹⁸F-FTTM SUV_max at the aortic arch of ApoE^-/- mice fed with high fat for 20 and 30 weeks were 43% and 53% higher than those of the control group, respectively. Histological analysis and flow cytometry confirmed that the ¹⁸F-FTTM specific imaging results.

Conclusions: Together, the current study provides compelling data that ¹⁸F-FTTM is a novel radioligand targeting to P2X7 receptors and has potential as a radiotracer to identify vulnerable atherosclerotic plaques, to detect the inflammatory response of atherosclerosis. ¹⁸F-FTTM PET imaging would be a powerful non-invasive method for the diagnosis of atherosclerotic lesions and new drug screening for accurate treatment.

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