Radiosynthesis and Preliminary Evaluation of A Styryl Pyridine Derivative ¹⁸F-SPy5 as Aβ Imaging Agents

¹⁸F-AV45 was the first β-amyloid (Aβ) imaging drug approved by the U.S. Food and Drug Administration and the first positron emission tomography drug for Alzheimer’s disease (AD) diagnosis in the world. In this work, ¹⁸F-SPy5, a styryl pyridine derivative and the isomer of ¹⁸F-AV45, was preliminary studied. First, ^18/19F-SPy5 and ^18/19F-AV45 were synthesized by nucleophilic substitution. The structures of ¹⁹F-SPy5 and ¹⁹F-AV45 were confirmed by HRMS, ¹H NMR and ¹³C NMR. The radiochemical purity and the molar activity of ¹⁸F-SPy5 and ¹⁸F-AV45 were determined by HPLC. And then, the preliminary evaluation of ¹⁸F-SPy5 was carried out, which was compared with ¹⁸F-AV45. For ¹⁸F-SPy5, the radiochemical purity was more than 99%, the molar activity was 3.90×10⁹ MBq/mol, and it had suitable lipophilicity and good in vitro stability. ¹⁹F-SPy5 displayed high binding affinity to Aβ aggregates (K_i = 25.9±7.9 nmol) in vitro and ¹⁸F-SPy5 displayed specific labeling of Aβ plaques in the brain sections of the AD model mice. ¹⁸F-SPy5 exhibited relatively high initial brain uptake, rapid brain clearance and slight defluorination in normal mice. The pharmacokinetics of ¹⁸F-SPy5 in normal mice was accorded with two-compartment model. Compared with ^18/19F-AV45, ¹⁹F-SPy5 had a slightly higher affinity to Aβ aggregates, ¹⁸F-SPy5 had a slightly higher initial brain uptake level and a slightly faster clearance from normal brain tissue and a slightly lower skull bone uptake level, but the results of in vitro fluorescence staining by ¹⁹F-SPy5 and in vitro autoradiography by ¹⁹F-SPy5 were slightly worse. These preliminary results suggested that ¹⁸F-SPy5 might serve as a potential Aβ imaging agent, although more in-depth evaluation and further structural modification were required to improve its performance.