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PurposeExploring synaptic density changes during brain growth is crucial to understanding brain development. Previous studies in nonhuman primates report a rapid increase in synapse number between the late gestational period and the early neonatal period, such that synaptic density approaches adult levels by birth. Prenatal synaptic development may have an enduring impact on postnatal brain development, but precisely how synaptic density changes in utero are unknown because current methods to quantify synaptic density are invasive and require post-mortem brain tissue.MethodsWe used synaptic vesicle glycoprotein 2A (SV2A) positron emission tomography (PET) radioligands [11C]UCB-J and [18F]Syn-VesT-1 to conduct the first assessment of synaptic density in the developing fetal brain in gravid rhesus monkeys. Eight pregnant monkeys were scanned twice during the third trimester at two imaging sites. Fetal post-mortem samples were collected near term in a subset of subjects to quantify SV2A density by Western blot.ResultsImage-derived fetal brain SV2A measures increased during the third trimester. SV2A concentrations were greater in subcortical regions than in cortical regions at both gestational ages. Near term, SV2A density was higher in primary motor and visual areas than respective associative regions. Post-mortem quantification of SV2A density was significantly correlated with regional SV2A PET measures.ConclusionWhile further study is needed to determine the exact relationship of SV2A and synaptic density, the imaging paradigm developed in the current study allows for the effective in vivo study of SV2A development in the fetal brain.

Background: Females are twice as likely to experience post-traumatic stress disorder (PTSD) than males, yet specific factors contributing to this greater risk are not fully understood. Our clinical and recent preclinical findings suggest a role for the metabotropic glutamate receptor 5 (mGlu5) in PTSD and differential involvement between males and females. Methods: Here, we further investigate whether mGlu5 receptor availability may contribute to individual and sex differences in PTSD susceptibility by quantifying receptor availability using the mGlu5 receptor-specific radiotracer, [18F]FPEB, and positron emission tomography in male (n = 16) and female (n = 16) rats before and after traumatic footshock exposure (FE) and assessment of stress-enhanced fear learning (SEFL) susceptibility, as compared with no-shock controls (CON; n = 7 male; n = 8 female). Results: Overall, FE rats displayed greater fear generalization as compared with CON (p <.001). Further, greater mGlu5 receptor availability at baseline (p =.003) and post-test (p =.005) was significantly associated with expression of the SEFL phenotype. Notably, FE female rats displayed a shift to more passive coping (ie, freezing), and displayed greater SEFL susceptibility (p =.01), and had lower baseline mGlu5 availability (p =.03) relative to their FE male rat counterparts. Conclusion: Results are consistent with clinical findings of higher mGlu5 receptor availability in PTSD, and add to growing evidence implicating these receptors in the pathophysiology of PTSD and sex-differences in susceptibility for this disorder. [ABSTRACT FROM AUTHOR]

Background: Females are twice as likely to experience post-traumatic stress disorder (PTSD) than males, yet specific factors contributing to this greater risk are not fully understood. Our clinical and recent preclinical findings suggest a role for the metabotropic glutamate receptor 5 (mGlu5) in PTSD and differential involvement between males and females. Methods: Here, we further investigate whether mGlu5 receptor availability may contribute to individual and sex differences in PTSD susceptibility by quantifying receptor availability using the mGlu5 receptor-specific radiotracer, [18F]FPEB, and positron emission tomography in male (n =16) and female (n =16) rats before and after traumatic footshock exposure (FE) and assessment of stress-enhanced fear learning (SEFL) susceptibility, as compared with no-shock controls (CON; n=7 male; n= 8 female). Results: Overall, FE rats displayed greater fear generalization as compared with CON (p < .001). Further, greater mGlu5 receptor availability at baseline (p=.003) and post-test (p=.005) was significantly associated with expression of the SEFL phenotype. Notably, FE female rats displayed a shift to more passive coping (ie, freezing), and displayed greater SEFL susceptibility (p=.01), and had lower baseline mGlu5 availability (p=.03) relative to their FE male rat counterparts. Conclusion: Results are consistent with clinical findings of higher mGlu5 receptor availability in PTSD, and add to growing evidence implicating these receptors in the pathophysiology of PTSD and sex-differences in susceptibility for this disorder. [ABSTRACT FROM AUTHOR]

Purpose: The descending raphespinal serotonin (5-HT) system contributes to neural activities required for locomotion. The presynaptic serotonin transporter (SERT) is a marker of 5-HT innervation. In this study, we explored the use of PET imaging with the SERT radioligand [11C]AFM as a biomarker of 5-HT axon damage after spinal cord injury (SCI) in a rodent model and its translation to imaging SCI in humans. Procedures: PET imaging with [11C]AFM was performed in healthy rats under baseline and citalopram blocking conditions and a mid-thoracic transection rat model of SCI. The lumbar-to-cervical activity (L/C) ratio was calculated for the healthy and SCI animals to assess SERT binding decrease after SCI. Finally, translation of [11C]AFM PET was attempted to explore its potential to image SCI in humans. Results: Intense uptake in the brain and intact spinal cord was observed at 30–60 min post-injection of [11C]AFM in healthy rats. About 65% of [11C]AFM uptake in the spinal cord was blocked by citalopram. In the SCI rat model, the cervical uptake of [11C]AFM was similar to that in healthy rats, but the lumbar uptake was dramatically reduced, resulting in about half the L/C ratio in SCI rats compared to healthy rats. In contrast, [11C]AFM uptake in the human spinal cord showed no obvious decrease after treatment with citalopram. In the human subjects with SCI, decreases in [11C]AFM uptake were also not obvious in the section of spinal cord caudal to the injury point. Conclusion: [11C]AFM PET imaging of SERT provides a useful preclinical method to non-invasively visualize the rodent spinal cord and detect SERT changes in SCI rodent models. However, there appears to be little detectable specific binding signal for [11C]AFM in the human spinal cord. An SERT tracer with higher affinity and lower non-specific binding signal is needed to image the spinal cord in humans and to assess the axonal status in SCI patients. [ABSTRACT FROM AUTHOR]

Purpose: To quantify the synaptic vesicle glycoprotein 2A (SV2A) changes in the whole central nervous system (CNS) under pathophysiological conditions, a high affinity SV2A PET radiotracer with improved in vivo stability is desirable to minimize the potential confounding effect of radiometabolites. The aim of this study was to develop such a PET tracer based on the molecular scaffold of UCB-A, and evaluate its pharmacokinetics, in vivo stability, specific binding, and nonspecific binding signals in nonhuman primate brains, in comparison with [11C]UCB-A, [11C]UCB-J, and [18F]SynVesT-1. Methods: The racemic SDM-16 (4-(3,5-difluorophenyl)-1-((2-methyl-1H-imidazol-1-yl)methyl)pyrrolidin-2-one) and its two enantiomers were synthesized and assayed for in vitro binding affinities to human SV2A. We synthesized the enantiopure [18F]SDM-16 using the corresponding enantiopure arylstannane precursor. Nonhuman primate brain PET scans were performed on FOCUS 220 scanners. Arterial blood was drawn for the measurement of plasma free fraction (fP), radiometabolite analysis, and construction of the plasma input function. Regional time-activity curves (TACs) were fitted with the one-tissue compartment (1TC) model to obtain the volume of distribution (VT). Nondisplaceable binding potential (BPND) was calculated using either the nondisplaceable volume of distribution (VND) or the centrum semiovale (CS) as the reference region. Results: SDM-16 was synthesized in 3 steps with 44% overall yield and has the highest affinity (Ki = 0.9 nM) to human SV2A among all reported SV2A ligands. [18F]SDM-16 was prepared in about 20% decay-corrected radiochemical yield within 90 min, with greater than 99% radiochemical and enantiomeric purity. This radiotracer displayed high specific binding in monkey brains and was metabolically more stable than the other SV2A PET tracers. The fP of [18F]SDM-16 was 69%, which was higher than those of [11C]UCB-J (46%), [18F]SynVesT-1 (43%), [18F]SynVesT-2 (41%), and [18F]UCB-H (43%). The TACs were well described with the 1TC. The averaged test–retest variability (TRV) was 7 ± 3%, and averaged absolute TRV (aTRV) was 14 ± 7% for the analyzed brain regions. Conclusion: We have successfully synthesized a novel SV2A PET tracer [18F]SDM-16, which has the highest SV2A binding affinity and metabolical stability among published SV2A PET tracers. The [18F]SDM-16 brain PET images showed superb contrast between gray matter and white matter. Moreover, [18F]SDM-16 showed high specific and reversible binding in the NHP brains, allowing for the reliable and sensitive quantification of SV2A, and has potential applications in the visualization and quantification of SV2A beyond the brain. [ABSTRACT FROM AUTHOR]

Purpose: Cholesterol 24-hydroxylase (CH24H) is a brain-specific enzyme that plays a major role in brain cholesterol homeostasis by converting cholesterol into 24S-hydroxycholesterol. The selective CH24H inhibitor soticlestat (TAK-935) is being pursued as a drug for treatment of seizures in developmental and epileptic encephalopathies. Herein, we describe the successful discovery and the preclinical validation of the novel radiolabeled CH24H ligand (3-[18F]fluoroazetidin-1-yl){1-[4-(4-fluorophenyl)pyrimidin-5-yl]piperidin-4-yl}methanone ([18F]T-008) and its tritiated analog, [3H]T-008. Methods: In vitro autoradiography (ARG) studies in the CH24H wild-type (WT) and knockout (KO) mouse brain sections were conducted using [3H]T-008. PET imaging was conducted in two adult rhesus macaques using [18F]T-008. Each macaque received two test–retest baseline scans and a series of two blocking doses of soticlestat administered prior to [18F]T-008 to determine the CH24H enzyme occupancy. PET data were analyzed with Logan graphical analysis using plasma input. A Lassen plot was applied to estimate CH24H enzyme occupancy by soticlestat. Results: In ARG studies, binding of [3H]T-008 was specific to CH24H in the mouse brain sections, which was not observed in CH24H KO or in wild-type mice after pretreatment with soticlestat. In rhesus PET studies, the rank order of [18F]T-008 uptake was striatum > cortical regions > cerebellum, which was consistent with CH24H distribution in the brain. Pre-blocking with soticlestat reduced the maximum uptake and increased the washout in all brain regions in a dose-dependent manner. Calculated global occupancy values for soticlestat at a dose of 0.89 mg/kg were 97–98%, indicating maximum occupancy. Conclusion: The preclinical in vitro and in vivo evaluation of labeled T-008 demonstrates that [18F]T-008 is suitable for imaging CH24H in the brain and warrants further studies in humans. [ABSTRACT FROM AUTHOR]

Background: The 18-kDa translocator protein (TSPO) is an important target for assessing neuroimmune function in brain with positron-emission tomography (PET) imaging. The goal of this work was to assess two [C]PBR28 imaging paradigms for measuring dynamic microglia changes in Macaca mulatta. Methods: Dynamic [C]PBR28 PET imaging data with arterial blood sampling were acquired to quantify TSPO levels as [C]PBR28 V . Scans were acquired at three timepoints: baseline, immediately post-drug, and prolonged post-drug. Results: In one animal , a colony-stimulating factor 1 receptor kinase inhibitor, previously shown to deplete brain microglia, reduced [C]PBR28 V in brain by 46 ± 3% from baseline, which recovered after 12 days to 7 ± 5% from baseline. In a different animal , acute lipopolysaccharide administration, shown to activate brain microglia, increased [C]PBR28 V in brain by 39 ± 9% from baseline, which recovered after 14 days to −11 ± 3% from baseline. Conclusions: These studies provide preliminary evidence of complementary paradigms to assess microglia dynamics via in vivo TSPO imaging . [ABSTRACT FROM AUTHOR]

Although [11C]-(+)-PHNO has enabled quantification of the dopamine-D3 receptor (D3R) in the human brain in vivo, its selectivity for the D3R is not sufficiently high to allow us to disregard its binding to the dopamine-D2 receptor (D2R). We quantified the affinity of [11C]-(+)-PHNO for the D2R and D3R in the living primate brain. Two rhesus monkeys were examined on four occasions each, with [11C]-(+)-PHNO administered in a bolus + infusion paradigm. Varying doses of unlabeled (+)-PHNO were coadministered on each occasion (total doses ranging from 0.09 to 5.61 μg kg−1). The regional binding potential (BPND) and the corresponding doses of injected (+)-PHNO were used as inputs in a model that quantified the affinity of (+)-PHNO for the D2R and D3R, as well as the regional fractions of the [11C]-(+)-PHNO signal attributable to D3R binding. (+)-PHNO in vivo affinity for the D3R ( Kd/ fND ∼ 0.23-0.56 nM) was 25- to 48-fold higher than that for the D2R ( Kd/ fND ∼ 11-14 nM). The tracer limits for (+)-PHNO (dose associated with D3R occupancy ∼ 10%) were estimated at ∼0.02-0.04 μg kg−1 injected mass for anesthetized primate and at 0.01-0.02 μg kg−1 for awake human positron emission tomography (PET) studies. Our data enabled a rational design and interpretation of future PET studies with [11C]-(+)-PHNO. Synapse, 2012. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

A positron emission tomography (PET) radioligand, [18F]FMH3, has been developed to interrogate histamine receptor subtype 3 (H3R), where dysfunction at this site is linked with obesity, sleep abnormality, and cognitive disorders. [18F]FMH3 was evaluated for imaging central H3R sites in non‐human primates through test–retest (TRT) and dose‐receptor occupancy studies with two selective H3R antagonists in order to support clinical investigations. Two adult female baboons underwent [18F]FMH3 PET brain scans in the HR+, at repeated baseline (n = 7) and following administration of escalating doses of ABT‐239 (0.003–0.1m/kg, n = 4) and ciproxifan (0.5–2.1 mg/kg, n = 7). Volume of distribution (VT) in brain regions was estimated using the 2‐tissue compartment model. TRT variability of VT across repeated baseline scans was reported as % coefficient of variation (COV). ABT‐239 and ciproxifan occupancy at H3R was estimated using the occupancy plot, and the relationship of occupancy with dose and plasma levels was determined. In baboons, distribution of [18F]FMH3 was high in the striatum, intermediate in cortical regions, and low in the brain stem. COV of baseline VT was 7.0 ± 3.5%, averaged across regions and animals. Dose‐dependent effects of ABT‐239 and ciproxifan measured the brain. ED50 and EC50, respectively, were 0.011 mg/kg and 0.942 ng/ml for ABT‐239 and 0.73 mg/kg and 208.3 ng/ml for ciproxifan. [18F]FMH3 demonstrated high TRT reliability and can be used to measure occupancy of H3R‐targeted drugs. Validation in non‐human primates support [18F]FMH3 PET studies toward clinical investigations of H3R. [ABSTRACT FROM AUTHOR]

Abstract: Introduction: A new radiotracer for imaging the serotonin 4 receptors (5-HT4) in brain, [18F]MNI-698, was recently developed by our group. Evaluation in nonhuman primates indicates the novel radiotracer holds promise as an imaging agent of 5-HT4 in brain. This paper aims to describe the whole-body biodistribution and dosimetry estimates of [18F]MNI-698. Methods: Whole-body positron emission tomography (PET) images were acquired over 240minutes after intravenous bolus injection of [18F]MNI-698 in adult rhesus monkeys. Different models were investigated for quantification of radiation absorbed and effective doses using OLINDA/EXM 1.0 software. Results: The radiotracer main elimination route was found to be urinary and the critical organ was the urinary bladder. Modeling of the urinary bladder voiding interval had a considerable effect on the estimated effective dose. Normalization of rhesus monkeys’ organs and whole-body masses to human equivalent reduced the calculated dosimetry values. The effective dose ranged between 0.017 and 0.027mSv/MBq. Conclusion: The dosimetry estimates, obtained when normalizing organ and whole-body weights and applying the urinary bladder model, indicate that the radiation doses from [18F]MNI-698 comply with limits and guidelines recommended by key regulatory authorities that govern the translation of radiotracers to human clinical trials. The timing of urinary bladder emptying should be considered when designing future clinical protocols with [18F]MNI-698, in order to minimize the subject absorbed doses. [Copyright &y& Elsevier]

Abstract: [11C]MRB is one of the most promising radioligands used to measure brain norepinephrine transporters (NET) with positron emission tomography (PET). The objective of this study was to evaluate the suitability of [11C]MRB for drug occupancy studies of NET using atomoxetine (ATX), a NET uptake inhibitor used in the treatment of depression and attention-deficit hyperactivity disorder (ADHD). A second goal of the study was identification of a suitable reference region. Ten PET studies were performed in three anesthetized rhesus monkeys following an infusion of ATX or placebo. [11C]MRB arterial input functions and ATX plasma levels were also measured. A dose-dependent reduction of [11C]MRB volume of distribution was observed after correction for [11C]MRB plasma free fraction. ATX IC 50 was estimated to be 31±10ng/mL plasma. This corresponds to an effective dose (ED 50) of 0.13mg/kg, which is much lower than the therapeutic dose of ATX in ADHD (1.0–1.5mg/kg). [11C]MRB binding potential BP ND in the thalamus was estimated to be 1.8±0.3. Defining a reference region for a NET radiotracer is challenging due to the widespread and relatively uniform distribution of NET in the brain. Three regions were evaluated for use as reference region: caudate, putamen and occipital cortex. Caudate was found to be the most suitable for preclinical drug occupancy studies in rhesus monkeys. The IC 50 estimate obtained using MRTM2 BP ND without arterial blood sampling was 21±3ng/mL (using caudate as the reference region). This study demonstrated that [11C]MRB is suitable for drug occupancy studies of NET. [Copyright &y& Elsevier]