Synthesis and preclinical evaluation of a novel fluorine-18 labeled small-molecule PET radiotracer for imaging of CXCR3 receptor in mouse models of atherosclerosis

<p>Background CXCR3 is a chemokine receptor and is expressed in innate and adaptive immune cells. It promotes<br> the recruitment of T-lymphocytes and other immune cells to the inflammatory site in response to the binding of cognate<br> chemokines. Upregulation of CXCR3 and its chemokines has been found during atherosclerotic lesion formation.<br> Therefore, detection of CXCR3 by positron emission tomography (PET) radiotracer can be a useful tool for detecting the development of atherosclerosis in a noninvasive manner. Herein, we report the synthesis, radiosynthesis, and characterization of a novel fluorine-18 (F-18, ¹⁸F) labeled small-molecule radiotracer for the imaging of the CXCR3 receptor in mouse models of atherosclerosis.<br> Results The reference standard <strong>1</strong> and its precursor <strong>9</strong> were synthesized over 5 steps from starting materials in good to moderate yields. The measured K_i values of CXCR3A and CXCR3B were 0.81 &plusmn; 0.02 nM and 0.31 &plusmn; 0.02 nM, respectively. [¹⁸F]<strong>1</strong> was prepared by a two-step radiosynthesis with a decay-corrected radiochemical yield of 13 &plusmn; 2%, radiochemical purity &gt; 99%, and specific activity of 44.4 &plusmn; 3.7 GBq/&mu;mol at the end of synthesis (n = 6). The baseline studies showed that [¹⁸F]<strong>1</strong> displayed high uptake in the atherosclerotic aorta and brown adipose tissue in Apolipoprotein E (ApoE) knockout (KO) mice fed with a high-fat diet over 12 weeks. The uptake of [¹⁸F]<strong>1</strong> in these regions was reduced significantly in self-blocking studies, demonstrating CXCR3 binding specificity. Contrary to this, no significant differences in uptake of [¹⁸F]<strong>1</strong> in the abdominal aorta of C57BL/6 control mice fed with a normal diet were observed in both baseline and blocking studies, indicating increased CXCR3 expression in atherosclerotic lesions. Immunohistochemistry studies demonstrated that [¹⁸F]<strong>1</strong>-positive regions were correlated with CXCR3 expression, but some<br> atherosclerotic plaques with significant size were not detected by [¹⁸F]<strong>1</strong>, and their CXCR3 expressions were minimal.<br> Conclusion [¹⁸F]<strong>1</strong> was synthesized with good radiochemical yield and high radiochemical purity. In PET imaging<br> studies, [¹⁸F]<strong>1</strong> displayed CXCR3-specific uptake in the atherosclerotic aorta in ApoE KO mice. [¹⁸F]<strong>1</strong> visualized CXCR3<br> expression in different regions in mice aligned with the tissue histology studies. Taken together, [¹⁸F]<strong>1</strong> is a potential<br> PET radiotracer for imaging CXCR3 in atherosclerosis.</p>