Characterization in nonhuman primates of (R)-[18F]OF-Me-NB1 and (S)-[18F]OF-Me-NB1 for imaging the GluN2B subunits of the NMDA receptor

Abstract

Purpose GluN2B containing N-methyl-D-aspartate receptors (NMDARs) play an essential role in neurotransmission and are a potential treatment target for multiple neurological and neurodegenerative diseases, including stroke, Alzheimer’s disease, and Parkinson’s disease. (R)-[18F]OF-Me-NB1 was reported to be more specific and selective than (S)-[18F]OF-Me-NB1 for the GluN2B subunits of the NMDAR based on their binding affinity to GluN2B and sigma-1 receptors. Here we report a comprehensive evaluation of (R)-[18F]OF-Me-NB1 and (S)-[18F]OF–Me-NB1 in nonhuman primates. Methods The radiosynthesis of (R)-[18F]OF-Me-NB1 and (S)-[18F]OF-Me-NB1 started from 18F-fluorination of the boronic ester precursor, followed by removal of the acetyl protecting group. PET scans in two rhesus monkeys were conducted on the Focus 220 scanner. Blocking studies were performed after treatment of the animals with the GluN2B antagonist Co101,244 or the sigma-1 receptor antagonist FTC-146. One-tissue compartment (1TC) model and multilinear analysis-1 (MA1) method with arterial input function were used to obtain the regional volume of distribution (VT, mL/cm3). Occupancy values by the two blockers were obtained by the Lassen plot. Regional non-displaceable binding potential (BPND) was calculated from the corresponding baseline VT and the VND derived from the occupancy plot of the Co101,244 blocking scans. Results (R)- and (S)-[18F]OF-Me-NB1 were produced in > 99% radiochemical and enantiomeric purity, with molar activity of 224.22 ± 161.69 MBq/nmol at the end of synthesis (n = 10). Metabolism was moderate, with ~ 30% parent compound remaining for (R)-[18F]OF-Me-NB1 and 20% for (S)-[18F]OF-Me-NB1 at 30 min postinjection. Plasma free fraction was 1–2%. In brain regions, both (R)- and (S)-[18F]OF-Me-NB1 displayed fast uptake with slower clearance for the (R)- than (S)-enantiomer. For (R)-[18F]OF-Me-NB1, both the 1TC model and MA1 method gave reliable estimates of regional VT values, with MA1 VT (mL/cm3) values ranging from 8.9 in the cerebellum to 12.8 in the cingulate cortex. Blocking with 0.25 mg/kg of Co101,244 greatly reduced the uptake of (R)-[18F]OF-Me-NB1 across all brain regions, resulting in occupancy of 77% and VND of 6.36, while 0.027 mg/kg of FTC-146 reduced specific binding by 30%. Regional BPND, as a measure of specific binding signals, ranged from 0.40 in the cerebellum to 1.01 in the cingulate cortex. Conclusions In rhesus monkeys, (R)-[18F]OF-Me-NB1 exhibited fast kinetics and heterogeneous uptake across brain regions, while the (S)-enantiomer displayed a narrower dynamic range of uptake across regions. A Blocking study with a GluN2B antagonist indicated binding specificity. The value of BPND was > 0.5 in most brain regions, suggesting good in vivo specific binding signals. Taken together, results from the current study demonstrated the potential of (R)-[18F]OF-Me-NB1 as a useful radiotracer for imaging the GluN2B receptors.

Comparison of three novel radiotracers for GluN2B-containing NMDA receptors in non-human primates: (R)-[11C]NR2B-Me, (R)-[18F]of-Me-NB1, and (S)-[18F]of-NB1

“…Detailed descriptions of radiochemistry, metabolite analysis, and initial non-human primate imaging results for [ 18 F]OF-Me-NB1 and [ 18 F]OF-NB1 are described in separate manuscripts. 22,23 Here, we present direct comparisons of imaging characteristics, kinetic modeling methods, and blocking studies for (R) -[ 11 C]NR2B-Me, (R) -[ 18 F]OF-Me-NB1, and (S) -[ 18 F]OF-NB1. A minimum of three bolus injection scans were performed in each of two animals for each of the three radiotracers, comprising scans at baseline and after pre-blocking with the GluN2B antagonist Co-101,244 24 (0.25 mg/kg) or the σ 1 receptor antagonist FTC-146 (0.027 or 0.125 mg/kg).…”

“…Radiosyntheses were performed as previously described for (R) -[ 11 C]NR2B-Me, 17,27 (R) -[ 18 F]OF-Me-NB1, 20,22 and (S) -[ 18 F]OF-NB1. 13,23 All PET scans were performed on the Focus 220 scanner (Siemens Medical Solutions, Knoxville, TN) after radiotracer injection.…”

Translational molecular imaging and drug development in Parkinson’s disease

“…Due to the possible side effects that arise from global inhibition of NMDA receptors, recent drug discovery efforts are focused on the identification of subtype-selective NMDA antagonists, for instance by targeting the GluN2B subunit of the NMDA receptor, which is believed to be a key contributor to excitotoxicity-induced neuronal cell death [289][290][291]. The recent breakthroughs in translational molecular imaging of GluN2B-containing NMDA receptors may help with the development of subtype-selective NMDA antagonists for the treatment of PD [292][293][294][295][296][297][298][299][300]. The most advanced class of GluN2B-targeted PET probes is based on a benzazepine scaffold and includes the carbon-11 derivative, (R)-11 C-Me-NB1 [295,300], which has recently been translated to humans, as well as the radiofluorinated analogs, (R)-18 F-OF-Me-NB1 [292,298], and (R)/(S)-18 F-OF-NB1 [293,296], which have recently been advanced to non-human primate studies and exhibited favorable performance characteristics, providing effective tools for the non-invasive quantification of GluN2B-carrying NMDA receptors.…”

A Molecular Toolbox of Positron Emission Tomography Tracers for General Anesthesia Mechanism Research

“…In vivo biodistribution in mice showed that [ 18 F] 33 penetrates the BBB with moderate brain uptake (3.60%ID/g at 2 min) and rapid elution. Further detailed in vivo studies, including clinical studies, are needed to confirm the feasibility of these PET tracers. − Their physicochemical properties are described in Table .…”