The Biological Research of Novel CXCR4 Agents [68Ga]-APD for Evaluating the Therapeutic Efficacy of Bromelain on Atherosclerotic Apolipoprotein E-deficient (ApoE-/-) mice

Introduction: Atherosclerotic plaque-induced stroke and coronary heart disease (CHD) are the worldwide leading causes of mortality. Owing to the unstable plaques by the pathologic mechanisms of inflammation, atherosclerotic lesions can be diagnosis from the mechanism of inflammation. C-X-C motif chemokine receptor 4 (CXCR4) and stromal cell–derived factor 1a (SDF-1a)-induced monocyte recruitment to the injured endothelial cells and subsequent plaque formation are the key steps for pathogenesis of atherosclerosis. CXCR4 was been proved to be intensively expressed on monocytes/macrophage. Therefore, CXCR4 represents a promising target for molecular imaging in atherosclerotic lesions. [⁶⁸Ga]-APD, based on the structure of CXCR4 antagonists TIQ-15, had designed by computer simulation as a PET tracer for imaging activated macrophages. The aim of this study was to compare the biological characteristics of [⁶⁸Ga]-APD with [⁶⁸Ga]-Pentixafor and evaluated the therapeutic efficacy of Bromelain by [⁶⁸Ga]-APD.

Methods: The novel tracer 6-Aminomethyl- pyridin-2-ylmethyl)-pyridin-2-ylmethyl-[3-(1,4,7,10tetraaza-cyclododec-1-yl)-propyl]-amine (APD) has been designed from computer simulation by Institute of Nuclear Energy Research. Chemical characteristics of APD were been determined by NMR, Mass spectra and HPLC. After been labeled with Ga-68, the radiochemical purity (RCP) was been analyzed by iTLC/citrate buffer and HPLC. Biological characteristics of [⁶⁸Ga]-APD was compared with [⁶⁸Ga]-Pentixafor in chow diet-fed atherosclerotic ApoE^-/- mice for 16 weeks and evaluated its efficacy in the same ApoE^-/- mice model with or without bromelain treatment for 4 weeks.

Results: After being labeled with Ga-68 under acetate buffer (pH≒5.5), radiochemical purity was over 90% and stable for more than 4 hours in 37℃human serum. [⁶⁸Ga]-APD could quickly eliminate from the kidney and accumulated in CXCR4 expression organs. The highest target/background ratio (TBR) of [⁶⁸Ga]-APD and [⁶⁸Ga]-Pentixafor on atherosclerotic lesions were 12.26 ± 0.68 (n=3) and 1.89 ± 0.53 (n=3) in ApoE^-/- mice after injection within 1 hour, respectively. However, the TBR decreased to 4.51 ± 0.49 (n=3) after the bromelain treatment.

Conclusions: In vivo evaluation of CXCR4 expression in ApoE^-/- mice revealed the uptake of [⁶⁸Ga]-APD mainly accumulated in the atherosclerotic lesion and better than [⁶⁸Ga]-Pentixafor for the imaging of atherosclerosis. The therapeutic efficacy of bromelain could be evaluated by non-invasive [⁶⁸Ga]-APD imaging. This novel tracer [⁶⁸Ga]-APD is more feasible as a surrogate marker for inflammatory atherosclerosis.