Spirocyclic iodonium ylide mediated radiofluorination of 2-arylquinolin-4-yl oxypropanamide derivatives for imaging of translocator protein (18 kDa) with insensitivity to the rs6971 polymorphism

Introduction: The translocator protein 18 kDa (TSPO), whose expression is upregulated by the activation of glia cells, is a continuously attractive biomarker for exploring neuroinflammation and neurologic disorders such as AD, PD and multiple sclerosis by PET. However, most TSPO PET tracers are sensitive to rs6971 polymorphism in human. In this study, we aim to identify some potential ¹⁸F-labeled PET tracers with high affinities, good brain uptake as well as reasonable insensitivity to rs6971 polymorphism.

Methods: Through the structure-activity relationship study, two novel candidates with aryl fluorine moiety, namely LW1 and LW2, were screened with high TSPO binding affinities. The corresponding ¹⁸F-isotopologues, ¹⁸F-LW1 and ¹⁸F-LW2, were synthesized automatically via spirocyclic iodonium ylide (SCIDY) strategy by the FX2N module. Rodent metabolic stability in vitro and biodistribution (biod) in vivo were studied to determine biological distribution and drug metabolism. The specificities and sensitivities to rs6971 polymorphism were evaluated by autoradiography (ARG) with LAB and HAB postmortem human brain slices. Dynamic microPET was used to evaluate the in vivo imaging potential in AD and MCAO models. PET-MRI imaging and arterial blood sampling were performed on non-human primates (NHP) to evaluate the in vivo imaging effect and dynamic characteristics.

Results: Both LW1 and LW2 showed excellent binding affinity with K_i < 0.1 nM. For radiofluorination, the corresponding SCIDY precursors were successfully obtained as colorless solids. The radiochemical yields were 16 ± 5% (¹⁸F-LW1) and 15 ± 7% (¹⁸F-LW2), respectively. Ex vivo biodistribution of the two tracers showed high radioactivity accumulation in the lungs, heart and kidney as well as urinary and hepatobiliary excretion. The ARG results from human brain slices of ¹⁸F-LW1 showed a certain level of sensitivity to rs6971 (ratio_LAB/HAB = 0.58) and nonspecific binding. ¹⁸F-LW2 showed reasonable insensitivity (ratio_LAB/HAB = 0.87) and excellent in vitro specific binding. The distributed volume (V_T) values of AD mice in hippocampus and other brain areas were higher than those of control mice, and the lesion areas of stroke rats also showed specific binding of ¹⁸F-LW2. PET imaging results of rhesus monkey brain showed that ¹⁸F-LW2 could successfully penetrate the blood-brain barrier and elute quickly. Although uptake was higher in the brain with the pretreatment with PK11195 (10 mg/kg), uptake in peripheral organs, such as lung, was decreased by more than 80%. The pharmacokinetic properties of ¹⁸F-LW2 were verified by arterial blood collection and can be well explained by a two-compartment model (2TCM). In addition, we evaluated SUVr and V_T(70-80min) after injection, and showed a high positive correlation.

Conclusions: We have efficiently synthesized a series of novel TSPO ligands with aryl fluorine skeleton. LW1 and LW2 showed good binding affinity to TSPO, which were successfully radiolabeled for imaging study. The PET images, in vitro ARG and ex vivo bioD study revealed that ¹⁸F-LW2 showed good specificity, pharmacokinetics as well as reasonable insensitivity to rs6971 polymorphism, which is a potential PET tracer for visualizing TSPO in human.

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