Comparison of [¹¹C](R)-NR2B-SMe and [¹¹C](R)-NR2B-Me as PET radioligands for imaging NR2B subunits in NMDA receptors

Objectives: NR2B is the most studied NMDA receptor subunit within the NMDA receptor complex and its expression is largely limited to forebrain regions and dorsal horn of the spinal cord [1]. NR2B is considered to be a therapeutic target for schizophrenia, stroke, and neurodegenerative diseases, especially neuro-pain. Therapeutics targeting NR2B rather than the NMDA channel may have fewer side-effects [1]. The quantification of NR2B subunits could help to elucidate the involvement of this subunit and the NMDA receptor in neuropsychiatric disorders and also assist in drug development [2]. Previously, we reported [¹¹C](R)-NR2B-SMe as a PET radioligand for NR2B subunits [3]. (R)-NR2B-SMe has affinity for NR2B in the nM range, moderate lipophilicity (logD_7.4 = 3.41± 0.08; n = 6), and ease of labeling with carbon-11. However, in PET experiments with [¹¹C](R)-NR2B-SMe in rodent and monkey, decline of brain radioactivity from peak values was very slow. We reasoned that this may be a consequence of too high binding affinity. Subsequently, we prepared a lower affinity radioligand, [¹¹C](R)-NR2B-Me. Here we report a comparison of this radioligand with [¹¹C](R)-NR2B-SMe for PET imaging of brain NR2B subunit in rats. Methods: PET imaging of brain was performed after intravenous administration of [¹¹C](R)-NR2B-SMe or [¹¹C](R)-NR2B-Me to rats at baseline and after administration with (R)-NR2B-SMe, (R)-NR2B-Me, an NR2B-selective ligand, either Ro 25 6981 or Co101244, a sigma-1 receptor-selective agonist, either SA4503 or TC1, or a sigma-1 receptor-selective antagonist, either FTC146 or BD1047 (each at 0.01−3 mg/kg, i.v. at 10 min before radioligand injection). Displacement experiments, where (R)-NR2B-SMe, (R)-NR2B-Me or Ro 25 6981, was given at 10 min after radioligand, were also performed.

Results: The precursors were obtained in 38 and 40% overall yields, respectively. [¹¹C](R)-NR2B-SMe and [¹¹C](R)-NR2B-Me were obtained in 25 and 20% yields from cyclotron-produced [¹¹C]CO₂ and with a radiochemical purity of >98% and a mean molar activity of 58 GBq/μmol. PET imaging of [¹¹C](R)-NR2B-SMe and [¹¹C](R)-NR2B-Me in rats at baseline showed similar moderately high brain radioactivity uptakes, reaching 3.5 SUV at 5 min. [¹¹C](R)-NR2B-Me gave much faster decline of brain radioactivity from the peak value than [¹¹C](R)-NR2B-SMe (Figure panel A). Displacing agents also gave faster declines in radioactivity for [¹¹C](R)-NR2B-Me than for [¹¹C](R)-NR2B-SMe. Pre-blocking with self, a NR2B-selective ligand, either Ro 25 6981 or Co101244, or the sigma-1 receptor-selective agonist, either SA4503 or TC1, were similarly effective (up to 95%) (Figure panel B). The sigma-1 receptor-selective antagonists FTC146 and BD1047 showed no blocking effect, except at high dose (1.25 mg/kg).

Conclusions: [¹¹C](R)-NR2B-SMe and [¹¹C](R)-NR2B-Me were readily synthesized and they showed high brain uptakes in rat, which could be pre-blocked by self, NR2B-selective ligands, or either of two sigma-1 receptor agonists. Sigma-1 antagonists showed no blocking effect at low dose, but some blocking effect at high dose. More studies are necessary to elucidate the selectivity of the two radioligands and the blocking effects of some sigma-1 receptor agonists. Research Support: Intramural Research Program of the National Insitutes of Health (NIMH). References: [1] Zhuo M, Neuropharmacology, 2017, 112, 228. [2] Kassenbrock A, Vasdev N, Liang SH. Curr. Top. Med. Chem. 2016; 16, 1830. [3] Cai L, Liow J-S, Morse CL, Davies R, Frankland MP, Zoghbi SS, Innis RB, Pike VW. J. Nucl. Med. 2018, 59, 543. Figure. Panel A - whole brain PET time activity curves in rats at baseline, showing different washout rate for the 2 radioligands, [¹¹C](R)-NR2B-SMe (n = 1) and [¹¹C](R)-NR2B-Me (n = 5). Panel B - Average axial PET images near eye ball of [¹¹C](R)-NR2B-Me in rat brain without (left) and with (right) pre-blocker Ro 25 6981 at the dose of 0.25 mg/kg.

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