Candidate 3-benzazepine-1-ol type GluN2B receptor radioligands ((11)C-NR2B-Me enantiomers) have high binding in cerebellum but not to σ1 receptors

INTRODUCTION: We recently reported (11)C-NR2B-SMe ([S-methyl-(11)C](R,S)-7-thiomethoxy-3-(4-(4-methyl-phenyl)butyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepin-1-ol) and its enantiomers as candidate radioligands for imaging the GluN2B subunit within rat N-methyl-D-aspartate receptors. However, these radioligands gave unexpectedly high and displaceable binding in rat cerebellum, possibly due to cross-reactivity with sigma-1 (σ1) receptors. This study investigated (11)C-labeled enantiomers of a close analogue (7-methoxy-3-(4-(p-tolyl)butyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepin-1-ol; NR2B-Me) of (11)C-NR2B-SMe as new candidate GluN2B radioligands. PET was used to evaluate these radioligands in rats and to assess potential cross-reactivity to σ1 receptors. METHODS: NR2B-Me was assayed for binding affinity and selectivity to GluN2B in vitro. (11)C-NR2B-Me and its enantiomers were prepared by Pd-mediated treatment of boronic ester precursors with (11)C-iodomethane. Brain PET scans were conducted after radioligand intravenous injection into rats. Various ligands for GluN2B receptors or σ1 receptors were administered at set doses in pre-blocking or displacement experiments to assess their impact on imaging data. (18)F-FTC146 and enantiomers of (11)C-NR2B-SMe were used for comparison. Radiometabolites from brain and plasma were measured ex vivo and in vitro. RESULTS: NR2B-Me enantiomers showed high GluN2B affinity and selectivity in vitro. (11)C-NR2B-Me enantiomers gave high early whole rat brain uptake of radioactivity, including high uptake in cerebellum, followed by slower decline. Radioactivity in brain at 30 min ex vivo was virtually all unchanged radioligand. Only less lipophilic radiometabolites appeared in plasma. When (11)C-(R)-NR2B-Me was used, three high-affinity GluN2B ligands—NR2B-SMe, Ro25-6981, and CO101,244—showed increasing pre-block of whole brain radioactivity retention with increasing dose. Two σ1 receptor antagonists, FTC146 and BD1407, were ineffective pre-blocking agents. Together, these results strongly resemble those obtained with (11)C-NR2B-SMe enantiomers, except that (11)C-NR2B-Me enantiomers showed faster reversibility of binding. When (18)F-FTC146 was used as a radioligand, FTC146 and BD1407 showed strong pre-blocking effects whereas GluN2B ligands showed only weak blocking effects. CONCLUSION: (11)C-NR2B-Me enantiomers showed specific binding to GluN2B receptors in rat brain in vivo. High unexpected specific binding in cerebellum was not due to σ1 receptors. Additional investigation is needed to identify the source of the high specific binding. GRAPHICAL ABSTRACT: [Image: see text] SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13550-023-00975-6.