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

Abstract

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.

Preclinical Development of 18F-OF-NB1 for Imaging GluN2B-Containing N-Methyl-d-Aspartate Receptors and Its Utility as a Biomarker for Amyotrophic Lateral Sclerosis

“…At the time, it was also reported that 3-(4-phenylbutyl)-2,3,4,5-tetrahydro-1H-benzazepine-1,7-diol exhibits 18-fold higher GluN2B subunit affinity than its methoxy analog (15). Remarkably, we showed in our previous study that the loss of high binding affinity and selectivity of the para-fluorinated 3-benzazepin-1ol (PF-Me-NB1) could be restored on cleavage of the methyl ether group (16). More importantly, we found that the (S)-and (R)enantiomers of PF-NB1 did not differ in their in vitro properties with regard to their binding affinity and selectivity toward GluN2B subunits.…”

“…Chemicals and materials were purchased from ABCR, Ametek Scientific, Armar, Merck, Sigma-Aldrich, Acros Organics, and PerkinElmer and were used directly without any further purification. Detection, characterization, and binding affinity experiments were performed in accordance with previously published procedures (16).…”

“…The 18 F-OF-NB1 was radiosynthesized using boronic ester 6 in analogy to a previously published procedure by our group (Supplemental Fig. 3) (16,18). The 18 F-fluspidine was radiosynthesized as previously reported (19).…”

“…In vitro autoradiography was performed as previously described by our group (14,16). For blocking experiments, 1 mM of GluN2B ligands of either CERC-301 or EVT-101 were used.…”

A Review of Molecular Imaging of Glutamate Receptors

“…Seeking to develop a fluorine-18 labelled analogue of [ 11 C]Me-NB1 due to the inherently brief physical half-life of carbon-11, which limits its use only to centers with an on-site cyclotron, the Ametamey group evaluated a series of fluorinated analogues of Me-NB1 [ 74 , 75 , 76 ]. Two fluorine-18 labelled compounds, [ 18 F]OF-NB1 ( 42b ) and [ 18 F]PF-NB1) ( 42c ) ( Figure 4 ) emerged as promising radioligands for imaging the GluN2B receptor.…”