Structure–Affinity Relationships of 2,3,4,5-Tetrahydro‑1H‑3-benzazepine and 6,7,8,9-Tetrahydro‑5H‑benzo[7]annulen-7-amine Analogues and the Discovery of a Radiofluorinated 2,3,4,5-Tetrahydro‑1H‑3-benzazepine Congener for Imaging GluN2B Subunit-Containing N‑Methyl‑d‑aspartate Receptors

Aspiring to develop a positron emission tomography (PET) imaging agent for the GluN2B subunits of the N-methyl-d-aspartate receptor (NMDAR), a key therapeutic target for drug development toward several neurological disorders, we synthesized a series of 2,3,4,5-tetrahydro-1H-3-benzazepine and 6,7,8,9-tetrahydro-5H-benzo­[7]­annulen-7-amine analogues. After in vitro testing via competition binding assay and autoradiography, [18F]­PF-NB1 emerged as the best performing tracer with respect to specificity and selectivity over σ1 and σ2 receptors and was thus selected for further in vivo evaluation. Copper-mediated radiofluorination was accomplished in good radiochemical yields and high molar activities. Extensive in vivo characterization was performed in Wistar rats comprising PET imaging, biodistribution, receptor occupancy, and metabolites studies. [18F]­PF-NB1 binding was selective to GluN2B-rich forebrain regions and was specifically blocked by the GluN2B antagonist, CP-101,606, in a dose-dependent manner with no brain radiometabolites. [18F]­PF-NB1 is a promising fluorine-18 PET tracer for imaging the GluN2B subunits of the NMDAR and has utility for receptor occupancy studies.