Evaluation of (rac)-, (R)- and (S)-¹⁸F-OF-NB1 for imaging GluN2B subunit-containing N-methyl-D-aspartate receptors in non-human primates

Despite two decades of research, no GluN1/2B radioligand is yet clinically validated. Previously, we reported on (rac)-18F-OF-NB1 as a promising GluN1/2B PET probe in rodents, and its successful application for the visualization of GluN2B-containing N-methyl-D-aspartate receptors (NMDARs) in postmortem brain tissues of patients with amyotrophic lateral sclerosis. In the current work, we report on the in vivo characterization of (rac)-, (R)- and (S)-18F-OF-NB1 in non-human primates. Methods: PET scans were performed in rhesus monkeys. Plasma profiling was used to obtain the arterial input function. Regional brain time-activity curves (TACs) were generated and fitted with the one-tissue and two-tissue compartment models (1TCM and 2TCM) and the multilinear analysis-1 (MA1) method, and the corresponding regional volumes of distribution (VT) were calculated. Blocking studies with the GluN1/2B ligand Co 101244 (0.25 mg/kg) were performed for the enantiopure radiotracers. Receptor occupancy, non-specific volume of distribution, and regional binding potential (BPND) values were obtained. Potential off-target binding towards sigma-1 receptors (σ1Rs) was assessed for (S)-18F-OF-NB1 using the σ1R ligand FTC-146. Results: Free plasma fraction was moderate, ranging from 12% to 15%. All radiotracers showed high and heterogeneous brain uptake with highest levels in the cortex. (R)-18F-OF-NB1 showed the highest uptake and slowest washout kinetics out of all tracers. The 1TCM and MA1 method fitted the regional TACs well for all tracers and produced reliable regional VT values, which were higher for (R)- than (S)-18F-OF-NB1. Receptor occupancy by Co 101244 was 85% and 96% for (S)-18F-OF-NB1 and (R)-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 similar reduction (48% and 49%, respectively) in specific binding of (S)-18F-OF-NB1. Further, pretreatment with both Co 101244 and FTC-146 did not result in further reduction of specific binding than Co 101244 alone in the same monkey (82% vs 81%, respectively). Regional BPND values ranged from 1.32 in the semiovale to 3.44 in the cingulate cortex for (S)-18F-OF-NB1. Conclusion: Both (R)- and (S)-18F-OF-NB1 exhibited high binding specificity to GluN2B subunit-containing NMDARs. The fast washout kinetics, good regional BPND values, and high plasma free fraction render (S)-18F-OF-NB1 an attractive radiotracer for clinical translation.