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

Abstract

Background: CXCR3 is a chemokine receptor and is expressed in innate and adaptive immune cells. It promotes the recruitment of T-lymphocytes and other immune cells to the inflammatory site in response to the binding of cognate chemokines. Upregulation of CXCR3 and its chemokines has been found during atherosclerotic lesion formation. 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.

Results: The reference standard 1 and its precursor 9 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 ± 0.02 nM and 0.31 ± 0.02 nM, respectively. [¹⁸F]1 was prepared by a two-step radiosynthesis with a decay-corrected radiochemical yield of 13 ± 2%, radiochemical purity > 99%, and specific activity of 44.4 ± 3.7 GBq/µmol at the end of synthesis (n = 6). The baseline studies showed that [¹⁸F]1 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]1 in these regions was reduced significantly in self-blocking studies, demonstrating CXCR3 binding specificity. Contrary to this, no significant differences in uptake of [¹⁸F]1 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]1-positive regions were correlated with CXCR3 expression, but some atherosclerotic plaques with significant size were not detected by [¹⁸F]1, and their CXCR3 expressions were minimal.

Conclusion: [¹⁸F]1 was synthesized with good radiochemical yield and high radiochemical purity. In PET imaging studies, [¹⁸F]1 displayed CXCR3-specific uptake in the atherosclerotic aorta in ApoE KO mice. [¹⁸F]1 visualized CXCR3 expression in different regions in mice aligned with the tissue histology studies. Taken together, [¹⁸F]1 is a potential PET radiotracer for imaging CXCR3 in atherosclerosis.

Keywords: Atherosclerosis; CXCR3 chemokine receptor; Inflammation; PET imaging; Small-molecule radiotracer.