Developmentand Preclinical Characterization of [18F]H3-2406 and [18F]H3-2407 for Positron EmissionTomography Imaging of the Histamine Subtype-3 Receptor

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The histamine subtype 3 receptor (H3R) is a G protein-coupled receptor involved in various central nervous system (CNS) disorders. We herein describe the identification and preclinical evaluation of two H3R antagonists: compounds 3 (H3-2406, Ki = 2.87 nM) and 4 (H3-2407, Ki = 3.15 nM) as potential PET radioligands. Both were radiolabeled with fluorine-18 using a copper-mediated method. Among them, [18F]3 showed high radiochemical yield (32.4%), molar activity (103 GBq/μmol), and moderate brain uptake (SUV = 2.4), with regional distribution matching known H3R expression. [18F]3 also exhibited favorable pharmacokinetics, high metabolic stability and negligible efflux by transporter proteins. However, relatively high cerebellar uptake, along with dedicated blocking studies, suggested off-target binding to the sigma-1 receptor, likely due to limited selectivity over sigma-1 (69-fold). These findings highlight [18F]3 as a promising lead for H3R imaging, with future work aimed at improving its selectivity and minimizing off-target binding.