Fluorinated GluN2B Receptor Antagonists with a 3‐Benzazepine Scaffold Designed for PET Studies

Abstract

To analyze the N-methyl-d-aspartate (NMDA) receptor distribution in the central nervous system, fluorinated ligands that selectively address the ifenprodil binding site of GluN2B-subunit-containing NMDA receptors were developed. Various strategies to introduce a fluorine atom into the potent GluN2B ligand 2 (3-(4-phenylbutyl)-2,3,4,5-tetrahydro-1H-3-benzazepin-1,7-diol) were pursued, including replacement of the benzylic OH moiety with a fluorine atom (13) and introduction of fluoroethoxy moieties at various positions (14 (7-position), 17 (9-position), 18a-c (1-position)). With respect to GluN2B affinity and selectivity over related receptors, the fluoroethoxy derivatives 14 and 18a are the most promising ligands. Radiosynthesis of fluoroethoxy derivative [ F]14 was performed by nucleophilic substitution of the phenol 2 with 2-[ F]fluoroethyl tosylate. On rat brain slices the fluorinated PET tracer [ F]14 accumulated in regions with high density of NMDA receptors containing GluN2B subunits. The bound radioactivity could not be replaced by (S)-glutamate. However, the GluN2B ligands eliprodil, Ro 25-6981, and the non-labeled 3-benzazepine 14 were able to abolish the specific binding of [ F]14.

Synthesis and Pharmacological Evaluation of Enantiomerically Pure GluN2B Selective NMDA Receptor Antagonists

“…The desired enantiomers of 3-benzazepin-1-ols 2-4 were synthesized from racemic secondary amine rac-1, [20] which was alkylated with 1-chloro-4-phenylbutane in the presence of tetrabutylammonium iodide (TBAI) to give rac-2.T he enantiomers (R)-2 and (S)-2 were separated by preparative chiral HPLC using the DaicelC hiralpak IA chiral column. Afterward, the benzyle thers of (R)-2 and (S)-2 werec leaved with H 2 and Pd/C to afford the phenols (R)-3 and (S)-3.F inally,t he enantiomerically pure phenols (R)-3 and (S)-3 were methylated with methyli odide to yield methyle thers (R)-4 and (S)-4 ( Figure 1).…”

Synthesis and characterization of 11C‐labeled benzyl amidine derivatives as PET radioligands for GluN2B subunit of the NMDA receptors

“…Conversely, the specific binding values in the cerebellum were very low (21%-32% of total). The control binding of all 11 C-ligands to the forebrain regions were significantly reduced by coincubation with cold ligands, although no similar SCHEME 1 Synthesis of 11 Figure 2D-F). These results were similar to those of [ 3 H]CP-101,606, which is known to be a GluN2B subunit selective antagonist.…”

“…7 Although these ligands showed an extremely high in vitro selectivity for the GluN2B subunit, none of them showed specific binding in vivo. [8][9][10][11] Claiborne et al reported N-(benzyl) amidine derivatives as orally efficacious GluN2B-selective NMDAR antagonists. 12 In addition, Curtis et al have reported that cinnamyl and 2-naphthyl analogs of benzyl amidine (CBA and NBA) have high-GluN2B selectivity, extremely high-binding affinity (the K i values of CBA, and NBA for [ 3 H] ifenprodil binding are 0.7 nM and 1.3 nM, respectively), and methoxy groups, which can be labeled with carbon-11.…”

“…13 Arstad evaluated several 11 C-labeled benzyl amidines ([ 11 C] CBA, [ 11 C] NBA, and [ 11 C]1) that showed excellent specific binding and a similar localization to that of GluN2B. However, these 11 C ligands are unsuitable imaging agents because of its metabolic instability. 14 Alternatively, the FIGURE 1 Chemical structure of reported PET radioligands for the GluN2B subunit of NMDA receptors A, and 11 Clabeled benzyl amidine derivatives in this study B detailed in vitro and in vivo properties of 11 C-benzyl amidine derivatives are still unclear, and prospective benzyl amidine type GluN2B imaging agents have not yet been developed.…”

“…However, these 11 C ligands are unsuitable imaging agents because of its metabolic instability. 14 Alternatively, the FIGURE 1 Chemical structure of reported PET radioligands for the GluN2B subunit of NMDA receptors A, and 11 Clabeled benzyl amidine derivatives in this study B detailed in vitro and in vivo properties of 11 C-benzyl amidine derivatives are still unclear, and prospective benzyl amidine type GluN2B imaging agents have not yet been developed. In this study, we prepared 11 C-benzyl amidines, [ 11 C]CBA and [ 11 C]NBA, and evaluated their in vitro binding properties and selectivity for NMDARs.…”