Abstract
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Introduction: The GluN2B subunit-containing NMDA receptors are involved in numerous pathologic conditions of the central nervous system, including Alzheimer’s disease, ischemic brain injury, and schizophrenia, yet there are currently no suitable PET radioligands for imaging the GluN2B subunits in humans. Previously we have reported (R)-[11C]OF-Me-NB1 and (R)-[18F]OF-Me-NB1 as promising radioligands in rodents (1). This abstract reports the synthesis of the phenolic analog [18F]OF-NB1, and its evaluation in rhesus monkey brain.
Methods: OF-NB1 was assayed in vitro for binding to the GluN2B subunit of the NMDA receptor. Both racemic (±)-[18F]OF-NB1 and enantiopure (R)-[18F]OF-NB1 were prepared by copper catalyzed radiofluorination of the boronic ester precursor followed by simultaneous cleavage of the ether- and ester protecting groups with BBr3. PET scans of up to 180 min each in rhesus monkeys were conducted on the Focus 220 scanner. Metabolite analysis was performed by HPLC and the arterial input function was subsequently calculated. Regional brain time-activity curves (TACs) were generated and analyzed with one-tissue (1T) and two-tissue (2T) compartment models and multilinear analysis-1 (MA1) method to obtain the respective regional volumes of distribution (VT, mL/cm3). Results: The novel ligands displayed high binding affinity for the NMDA GluN2B subunit with enantioselectivity (Ki values of 10.4, 3.9 and 15.3 nM, respectively, for racemic, (S)- and (R)-OF-NB1), and good selectivity over sigma-1 receptors. Both racemic [18F]OF-NB1 and (R)-[18F]OF-NB1 (after chiral HPLC separation of the racemic product) were successfully prepared in good molar activities (3.5 ± 1.2 mCi/nmol, n=5). Interestingly, metabolism appeared to display enantioselectivity, with 26% and 57% of parent compound for the racemic and (R)-[18F]OF-NB1, respectively, remaining at 30 min post-injection in the same monkey. Further Inter-subject difference in metabolism was also observed. Both compounds showed plasma free fraction of ~12% and exhibited high brain uptake. Specifically, tracer uptake was highest in the putamen, cerebellum, and occipital cortex, moderate in the hippocampus and thalamus, and lowest in the white matter region of centrum semiovale. Racemic [18F]OF-NB1 showed faster tissue clearance than (R)-[18F]OF-NB1. Both the 1TC model and MA1 method fitted the TACs well and gave reliable VT estimates, ranging from 33.7 in the centrum semiovale to 72.9 in the cingulate cortex for racemic [18F]OF-NB1. Calculated regional VT values of (R)-[18F]OF-NB1 were similar to those of the racemic form.
Conclusions: We have successfully synthesized and evaluated novel 18F-labeled radioligands targeting the GluN2B subunits of the NMDA receptor in non-human primates. Blocking studies are currently ongoing to assess their binding specificity and selectivity in vivo. Research support: This work is supported by the Swiss National Science Foundation Grant Nos. 310030E_160403/1 and 310030E_182872/1. References: 1. Haider A, Iten I, Ahmed H et al. Identification and Preclinical Evaluation of a Radiofluorinated Benzazepine Derivative for Imaging the GluN2B Subunit of the Ionotropic NMDA Receptor. J Nucl Med 2019; 60:259-266.