Abstract:S-Methyl-11 C](±)-7-methoxy-3-(4-(4-(methylthio)phenyl)butyl)-2,3,4,5-tetrahydro-1H-benzo[d]azepin-1-ol (11 C-NR2B-SMe) and its enantiomers were synthesized as candidates for imaging the NR2B subunit within the N-methyl-D-aspartate receptor with positron emission tomography (PET). Methods: Brains were scanned with PET for 90 min after intravenous injection of one of the candidate radioligands into rats. To detect any NR2B specific binding of radioligand in brain, various pre-blocking or displacing agents were … Show more
“…The blocking studies were conducted using the GluN2B antagonist Co101,244 and the selective sigma-1 receptor antagonist FTC-146 [ 25 , 30 , 31 ]. Pretreatment with Co101,244 (0.25 mg/kg) reduced the uptake of ( R )-[ 18 F]OF-Me-NB1 in all brain regions to nearly homogenous levels (Fig.…”
Section: Resultsmentioning
confidence: 99%
“…In this animal, a higher dose of FTC-146 (0.125 mg/kg) gave a similar level of occupancy, 33%, to the low dose in the male monkey. This indicates that the sigma-1 receptor antagonist, at both low (0.027 mg/kg) and high dose (0.125 mg/kg), produced a similar reduction in the binding of ( R )-[ 18 F]OF-Me-NB1 in the monkey brain [ 25 ]. Fig.…”
Section: Resultsmentioning
confidence: 99%
“…Each monkey underwent one baseline scan with ( R )-[ 18 F]OF-Me-NB1, and two blocking scans at 10 min after the injection of the GluN2B antagonist Co-101,244 (0.25 mg/kg) or the sigma-1 antagonist FTC-146 (0.027 and 0.125 mg/kg). Co-101,244 and FTC-146 were previously used by Cai et al to determine the in vivo binding specificity and selectivity of a radiotracer from the same scaffold, [ 11 C]NR2B-SMe, in rodents [ 25 ]. In addition, a baseline scan with ( S )-[ 18 F]OF-Me-NB1 was also performed in one monkey.…”
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.
“…The blocking studies were conducted using the GluN2B antagonist Co101,244 and the selective sigma-1 receptor antagonist FTC-146 [ 25 , 30 , 31 ]. Pretreatment with Co101,244 (0.25 mg/kg) reduced the uptake of ( R )-[ 18 F]OF-Me-NB1 in all brain regions to nearly homogenous levels (Fig.…”
Section: Resultsmentioning
confidence: 99%
“…In this animal, a higher dose of FTC-146 (0.125 mg/kg) gave a similar level of occupancy, 33%, to the low dose in the male monkey. This indicates that the sigma-1 receptor antagonist, at both low (0.027 mg/kg) and high dose (0.125 mg/kg), produced a similar reduction in the binding of ( R )-[ 18 F]OF-Me-NB1 in the monkey brain [ 25 ]. Fig.…”
Section: Resultsmentioning
confidence: 99%
“…Each monkey underwent one baseline scan with ( R )-[ 18 F]OF-Me-NB1, and two blocking scans at 10 min after the injection of the GluN2B antagonist Co-101,244 (0.25 mg/kg) or the sigma-1 antagonist FTC-146 (0.027 and 0.125 mg/kg). Co-101,244 and FTC-146 were previously used by Cai et al to determine the in vivo binding specificity and selectivity of a radiotracer from the same scaffold, [ 11 C]NR2B-SMe, in rodents [ 25 ]. In addition, a baseline scan with ( S )-[ 18 F]OF-Me-NB1 was also performed in one monkey.…”
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.
“…Enantiomers of ( 44 ) [S-Methyl- 11 C](±)-7-methoxy-3-(4-(4-(methylthio)phenyl)butyl)-2,3,4,5-tetrahydro-benzo[d]azepin-1-ol ([ 11 C]GluN2B-SMe) were prepared and tested as GluN2Bsubunit ligands [ 80 ]. The compound exhibited 2 nM K D for GluN2B receptors expressed in mouse fibroblasts.…”
Molecular imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) is a well-established and important in vivo technique to evaluate fundamental biological processes and unravel the role of neurotransmitter receptors in various neuropsychiatric disorders. Specific ligands are available for PET/SPECT studies of dopamine, serotonin, and opiate receptors, but corresponding development of radiotracers for receptors of glutamate, the main excitatory neurotransmitter in mammalian brain, has lagged behind. This state of affairs has persisted despite the central importance of glutamate neurotransmission in brain physiology and in disorders such as stroke, epilepsy, schizophrenia, and neurodegenerative diseases. Recent years have seen extensive efforts to develop useful ligands for molecular imaging of subtypes of the ionotropic (N-methyl-D-aspartate (NMDA), kainate, and AMPA/quisqualate receptors) and metabotropic glutamate receptors (types I, II, and III mGluRs). We now review the state of development of radioligands for glutamate receptor imaging, placing main emphasis on the suitability of available ligands for reliable in vivo applications. We give a brief account of the radiosynthetic approach for selected molecules. In general, with the exception of ligands for the GluN2B subunit of NMDA receptors, there has been little success in developing radiotracers for imaging ionotropic glutamate receptors; failure of ligands for the PCP/MK801 binding site in vivo doubtless relates their dependence on the open, unblocked state of the ion channel. Many AMPA and kainite receptor ligands with good binding properties in vitro have failed to give measurable specific binding in the living brain. This may reflect the challenge of developing brain-penetrating ligands for amino acid receptors, compounded by conformational differences in vivo. The situation is better with respect to mGluR imaging, particularly for the mGluR5 subtype. Several successful PET ligands serve for investigations of mGluRs in conditions such as schizophrenia, depression, substance abuse and aging. Considering the centrality and diversity of glutamatergic signaling in brain function, we have relatively few selective and sensitive tools for molecular imaging of ionotropic and metabotropic glutamate receptors. Further radiopharmaceutical research targeting specific subtypes and subunits of the glutamate receptors may yet open up new investigational vistas with broad applications in basic and clinical research.
“…In parallel, Cai et al. 17 described synthesis and evaluation of (R) -[ 11 C]NR2B-Me in rodents. Here, we present a detailed comparison of the imaging properties, kinetic modeling methods, quantitative parameters, and evidence for selectivity of (R) -[ 11 C]NR2B-Me, (R) -[ 18 F]OF-Me-NB1, and (S) -[ 18 F]OF-NB1 (Figure 1) in non-human primates.…”
The NMDA receptor GluN2B subunit is a target of interest in neuropsychiatric disorders but to date there is no selective radiotracer available to quantify its availability in vivo. Here we report direct comparisons in non-human primates of three GluN2B-targeting radioligands: (R)-[11C]NR2B-Me, (R)-[18F]OF-Me-NB1, and (S)-[18F]OF-NB1. Plasma free fraction, metabolism, tissue distribution and kinetics, and quantitative kinetic modeling methods and parameters were evaluated in two adult rhesus macaques. Free fraction in plasma was <2% for (R)-[11C]NR2B-Me and (R)-[18F]OF-Me-NB1 and higher for (S)-[18F]OF-NB1 (15%). All radiotracers showed good brain uptake and distribution throughout grey matter, with substantial (>68%) blockade across the brain by the GluN2B-targeting drug Co-101,244 (0.25 mg/kg), including in the cerebellum. Time-activity curves were well-fitted by the one-tissue compartment model, with volume of distribution values of 20–40 mL/cm3 for (R)-[11C]NR2B-Me, 8–16 mL/cm3 for (R)-[18F]OF-Me-NB1, and 15–35 mL/cm3 for (S)-[18F]OF-NB1. Estimates of regional non-displaceable binding potential were in the range of 2–3 for (R)-[11C]NR2B-Me and (S)-[18F]-OF-NB1, and 0.5-1 for (R)-[18F]OF-Me-NB1. Altogether, each radiotracer showed an acceptable profile for quantitative imaging of GluN2B. (S)-[18F]OF-NB1 has particularly promising imaging characteristics for potential translation into humans. However, the source of unexpected displaceable binding in the cerebellum for each of these compounds requires further investigation.
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