Evaluation of (<i>rac</i>)-, (<i>R</i>)-, and (<i>S</i>)-¹⁸F-OF-NB1 for Imaging GluN2B Subunit–Containing<i>N</i>-Methyl-d-Aspartate Receptors in Nonhuman Primates

Abstract

Despite 2 decades of research, no <i>N</i>-methyl-d-aspartate (NMDA) glutamate receptor (GluN) subtype 2B (GluN1/2B) radioligand is yet clinically validated. Previously, we reported on (<i>rac</i>)-¹⁸F-OF-NB1 as a promising GluN1/2B PET probe in rodents and its successful application for the visualization of GluN2B-containing NMDA receptors in postmortem brain tissues of patients with amyotrophic lateral sclerosis. In the current work, we report on the <i>in&nbsp;vivo</i> characterization of (<i>rac</i>)-, (<i>R</i>)-, and (<i>S</i>)-¹⁸F-OF-NB1 in nonhuman primates. <b>Methods:</b> PET scans were performed on rhesus monkeys. Plasma profiling was used to obtain the arterial input function. Regional brain time–activity curves were generated and fitted with the 1- and 2-tissue-compartment models and the multilinear analysis 1 method, and the corresponding regional volumes of distribution were calculated. Blocking studies with the GluN1/2B ligand Co 101244 (0.25 mg/kg) were performed for the enantiopure radiotracers. Receptor occupancy, nonspecific volume of distribution, and regional binding potential (<i>BP</i>_ND) were obtained. Potential off-target binding toward σ₁ receptors was assessed for (<i>S</i>)-¹⁸F-OF-NB1 using the σ₁ receptor ligand FTC-146. <b>Results:</b> Free plasma fraction was moderate, ranging from 12% to 16%. All radiotracers showed high and heterogeneous brain uptake, with the highest levels in the cortex. (<i>R</i>)-¹⁸F-OF-NB1 showed the highest uptake and slowest washout kinetics of all tracers. The 1-tissue-compartment model and multilinear analysis 1 method fitted the regional time–activity curves well for all tracers and produced reliable regional volumes of distribution, which were higher for (<i>R</i>)- than (<i>S</i>)-¹⁸F-OF-NB1. Receptor occupancy by Co 101244 was 85% and 96% for (<i>S</i>)-¹⁸F-OF-NB1 and (<i>R</i>)-¹⁸F-OF-NB1, respectively. Pretreatment with FTC-146 at both a low (0.027 mg/kg) and high (0.125 mg/kg) dose led to a similar reduction (48% and 49%, respectively) in specific binding of (<i>S</i>)-¹⁸F-OF-NB1. Further, pretreatment with both Co 101244 and FTC-146 did not result in a further reduction in specific binding compared with Co 101244 alone in the same monkey (82% vs. 81%, respectively). Regional <i>BP</i>_ND values ranged from 1.3 in the semiovale to 3.4 in the cingulate cortex for (<i>S</i>)-¹⁸F-OF-NB1. <b>Conclusion:</b> Both (<i>R</i>)- and (<i>S</i>)-¹⁸F-OF-NB1 exhibited high binding specificity to GluN2B subunit–containing NMDA receptors. The fast washout kinetics, good regional <i>BP</i>_ND values, and high plasma free fraction render (<i>S</i>)-¹⁸F-OF-NB1 an attractive radiotracer for clinical translation.

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