Opposing Effects of Fasting Metabolism on Tissue Tolerance in Bacterial and Viral Inflammation.

Acute infections are associated with a set of stereotypic behavioral responses, including anorexia, lethargy, and social withdrawal. Although these so-called sickness behaviors are the most common and familiar symptoms of infections, their roles in host defense are largely unknown. Here, we investigated the role of anorexia in models of bacterial and viral infections. We found that anorexia was protective while nutritional supplementation was detrimental in bacterial sepsis. Furthermore, glucose was necessary and sufficient for these effects. In contrast, nutritional supplementation protected against mortality from influenza infection and viral sepsis, whereas blocking glucose utilization was lethal. In both bacterial and viral models, these effects were largely independent of pathogen load and magnitude of inflammation. Instead, we identify opposing metabolic requirements tied to cellular stress adaptations critical for tolerance of differential inflammatory states. VIDEO ABSTRACT.

The regional pattern of age-related synaptic loss in the human brain differs from gray matter volume loss: in vivo PET measurement with [11C]UCB-J.

PURPOSE: Aging is a major societal concern due to age-related functional losses. Synapses are crucial components of neural circuits, and synaptic density could be a sensitive biomarker to evaluate brain function. [11C]UCB-J is a positron emission tomography (PET) ligand targeting synaptic vesicle glycoprotein 2A (SV2A), which can be used to evaluate brain synaptic density in vivo. METHODS: We evaluated age-related changes in gray matter synaptic density, volume, and blood flow using [11C]UCB-J PET and magnetic resonance imaging (MRI) in a wide age range of 80 cognitive normal subjects (21-83 years old). Partial volume correction was applied to the PET data. RESULTS: Significant age-related decreases were found in 13, two, and nine brain regions for volume, synaptic density, and blood flow, respectively. The prefrontal cortex showed the largest volume decline (4.9% reduction per decade: RPD), while the synaptic density loss was largest in the caudate (3.6% RPD) and medial occipital cortex (3.4% RPD). The reductions in caudate are consistent with previous SV2A PET studies and likely reflect that caudate is the site of nerve terminals for multiple major tracts that undergo substantial age-related neurodegeneration. There was a non-significant negative relationship between volume and synaptic density reductions in 16 gray matter regions. CONCLUSION: MRI and [11]C-UCB-J PET showed age-related decreases of gray matter volume, synaptic density, and blood flow; however, the regional patterns of the reductions in volume and SV2A binding were different. Those patterns suggest that MR-based measures of GM volume may not be directly representative of synaptic density.

Adaptive data-driven motion detection and optimized correction for brain PET.

Head motion during PET scans causes image quality degradation, decreased concentration in regions with high uptake and incorrect outcome measures from kinetic analysis of dynamic datasets. Previously, we proposed a data-driven method, center of tracer distribution (COD), to detect head motion without an external motion tracking device. There, motion was detected using one dimension of the COD trace with a semiautomatic detection algorithm, requiring multiple user defined parameters and manual intervention. In this study, we developed a new data-driven motion detection algorithm, which is automatic, self-adaptive to noise level, does not require user-defined parameters and uses all three dimensions of the COD trace (3DCOD). 3DCOD was first validated and tested using 30 simulation studies (18F-FDG, N = 15; 11C-raclopride (RAC), N = 15) with large motion. The proposed motion correction method was tested on 22 real human datasets, with 20 acquired from a high resolution research tomograph (HRRT) scanner (18F-FDG, N = 10; 11C-RAC, N = 10) and 2 acquired from the Siemens Biograph mCT scanner. Real-time hardware-based motion tracking information (Vicra) was available for all real studies and was used as the gold standard. 3DCOD was compared to Vicra, no motion correction (NMC), one-direction COD (our previous method called 1DCOD) and two conventional frame-based image registration (FIR) algorithms, i.e., FIR1 (based on predefined frames reconstructed with attenuation correction) and FIR2 (without attenuation correction) for both simulation and real studies. For the simulation studies, 3DCOD yielded -2.3 ± 1.4% (mean ± standard deviation across all subjects and 11 brain regions) error in region of interest (ROI) uptake for 18F-FDG (-3.4 ± 1.7% for 11C-RAC across all subjects and 2 regions) as compared to Vicra (perfect correction) while NMC, FIR1, FIR2 and 1DCOD yielded -25.4 ± 11.1% (-34.5 ± 16.1% for 11C- RAC), -13.4 ± 3.5% (-16.1 ± 4.6%), -5.7 ± 3.6% (-8.0 ± 4.5%) and -2.6 ± 1.5% (-5.1 ± 2.7%), respectively. For real HRRT studies, 3DCOD yielded -0.3 ± 2.8% difference for 18F-FDG (-0.4 ± 3.2% for 11C-RAC) as compared to Vicra while NMC, FIR1, FIR2 and 1DCOD yielded -14.9 ± 9.0% (-24.5 ± 14.6%), -3.6 ± 4.9% (-13.4 ± 14.3%), -0.6 ± 3.4% (-6.7 ± 5.3%) and -1.5 ± 4.2% (-2.2 ± 4.1%), respectively. In summary, the proposed motion correction method yielded comparable performance to the hardware-based motion tracking method for multiple tracers, including very challenging cases with large frequent head motion, in studies performed on a non-TOF scanner.

An objective evaluation method for head motion estimation in PET-Motion corrected centroid-of-distribution.

Head motion presents a continuing problem in brain PET studies. A wealth of motion correction (MC) algorithms had been proposed in the past, including both hardware-based methods and data-driven methods. However, in most real brain PET studies, in the absence of ground truth or gold standard of motion information, it is challenging to objectively evaluate MC quality. For MC evaluation, image-domain metrics, e.g., standardized uptake value (SUV) change before and after MC are commonly used, but this measure lacks objectivity because 1) other factors, e.g., attenuation correction, scatter correction and parameters used in the reconstruction, will confound MC effectiveness; 2) SUV only reflects final image quality, and it cannot precisely inform when an MC method performed well or poorly during the scan time period; 3) SUV is tracer-dependent and head motion may cause increases or decreases in SUV for different tracers, so evaluating MC effectiveness is complicated. Here, we present a new algorithm, i.e., motion corrected centroid-of-distribution (MCCOD) to perform objective quality control for measured or estimated rigid motion information. MCCOD is a three-dimensional surrogate trace of the center of tracer distribution after performing rigid MC using the existing motion information. MCCOD is used to inform whether the motion information is accurate, using the PET raw data only, i.e., without PET image reconstruction, where inaccurate motion information typically leads to abrupt changes in the MCCOD trace. MCCOD was validated using simulation studies and was tested on real studies acquired from both time-of-flight (TOF) and non-TOF scanners. A deep learning-based brain mask segmentation was implemented, which is shown to be necessary for non-TOF MCCOD generation. MCCOD is shown to be effective in detecting abrupt translation motion errors in slowly varying tracer distribution caused by the motion tracking hardware and can be used to compare different motion estimation methods as well as to improve existing motion information.

Age, gender and body-mass-index relationships with in vivo CB1 receptor availability in healthy humans measured with [11C]OMAR PET.

Brain cannabinoid 1 receptors (CB1Rs) contribute importantly to the regulation of autonomic tone, appetite, mood and cognition. Inconsistent results have been reported from positron emission tomography (PET) studies using different radioligands to examine relationships between age, gender and body mass index (BMI) and CB1R availability in healthy individuals. In this study, we examined these variables in 58 healthy individuals (age range: 18-55 years; 44 male; BMI=27.01±5.56), the largest cohort of subjects studied to date using the CB1R PET ligand [11C]OMAR. There was a significant decline in CB1R availability (VT) with age in the pallidum, cerebellum and posterior cingulate. Adjusting for BMI, age-related decline in VT remained significant in the posterior cingulate among males, and in the cerebellum among women. CB1R availability was higher in women compared to men in the thalamus, pallidum and posterior cingulate. Adjusting for age, CB1R availability negatively correlated with BMI in women but not men. These findings differ from those reported using [11C]OMAR and other radioligands such as [18F]FMPEP-d2 and [18F]MK-9470. Although reasons for these seemingly divergent findings are unclear, the choice of PET radioligand and range of BMI in the current dataset may contribute to the observed differences. This study highlights the need for cross-validation studies using both [11C]OMAR and [18F]FMPEP-d2 within the same cohort of subjects.

Feasibility study of PET dynamic imaging of [18F]DHMT for quantification of reactive oxygen species in the myocardium of large animals.

OBJECTIVES: We aimed to develop a dynamic imaging technique for a novel PET superoxide tracer, [18F]DHMT, to allow for absolute quantification of myocardial reactive oxygen species (ROS) production in a large animal model. METHODS: Six beagle dogs underwent a single baseline dynamic [18F]DHMT PET study, whereas one animal underwent three serial dynamic studies over the course of chronic doxorubicin administration (1 mg·kg-1·week-1 for 15 weeks). During the scans, sequential arterial blood samples were obtained for plasma metabolite correction. The optimal compartment model and graphical analysis method were identified for kinetic modeling. Values for the left ventricular (LV) net influx rate, Ki, were reported for all the studies and compared with the LV standard uptake values (SUVs) and the LV-to-blood pool SUV ratios from the 60 to 90 minute static images. Parametric images were also generated. RESULTS: [18F]DHMT followed irreversible kinetics once oxidized within the myocardium in the presence of superoxide, as evidenced by the fitting generated by the irreversible two-tissue (2Ti) compartment model and the linearity of Patlak analysis. Myocardial Ki values showed a weak correlation with LV SUV (R2 = 0.27), but a strong correlation with LV-to-blood pool SUV ratio (R2 = 0.92). Generation of high-quality parametric images showed superior myocardial to blood contrast compared to static images. CONCLUSIONS: A dynamic PET imaging technique for [18F]DHMT was developed with full and simplified kinetic modeling for absolute quantification of myocardial superoxide production in a large animal model.

Partial volume correction analysis for 11C-UCB-J PET studies of Alzheimer's disease.

PURPOSE: 11C-UCB-J PET imaging, targeting synaptic vesicle glycoprotein 2A (SV2A), has been shown to be a useful indicator of synaptic density in Alzheimer's disease (AD). For SV2A imaging, a decrease in apparent tracer uptake is often due to the combination of gray-matter (GM) atrophy and SV2A decrease in the remaining tissue. Our aim is to reveal the true SV2A change by performing partial volume correction (PVC). METHODS: We performed two PVC algorithms, Müller-Gärtner (MG) and 'iterative Yang' (IY), on 17 AD participants and 11 cognitive normal (CN) participants using the brain-dedicated HRRT scanner. Distribution volume VT, the rate constant K1, binding potential BPND (centrum semiovale as reference region), and tissue volume were compared. RESULTS: In most regions, both PVC algorithms reduced the between-group differences. Alternatively, in hippocampus, IY increased the significance of between-group differences while MG reduced it (VT, BPND and K1 group differences: uncorrected: 20%, 27%, 17%; MG: 18%, 22%, 14%; IY: 22%, 28%, 17%). The group difference in hippocampal volume (10%) was substantially smaller than any PET measures. MG increased GM binding values to a greater extent than IY due to differences in algorithm assumptions. CONCLUSION: 11C-UCB-J binding is significantly reduced in AD hippocampus, but PVC is important to adjust for significant volume reduction. After correction, PET measures are substantially more sensitive to group differences than volumetric MRI measures. Assumptions of each PVC algorithm are important and should be carefully examined and validated. For 11C-UCB-J, the less stringent assumptions of IY support its use as a PVC algorithm over MG.

Separating dopamine D2 and D3 receptor sources of [11C]-(+)-PHNO binding potential: Independent component analysis of competitive binding.

Development of medications selective for dopamine D2 or D3 receptors is an active area of research in numerous neuropsychiatric disorders including addiction and Parkinson's disease. The positron emission tomography (PET) radiotracer [11C]-(+)-PHNO, an agonist that binds with high affinity to both D2 and D3 receptors, has been used to estimate relative receptor subtype occupancy by drugs based on a priori knowledge of regional variation in the expression of D2 and D3 receptors. The objective of this work was to use a data-driven independent component analysis (ICA) of receptor blocking scans to separate D2-and D3-related signal in [11C]-(+)-PHNO binding data in order to improve the precision of subtype specific measurements of binding and occupancy. Eight healthy volunteers underwent [11C]-(+)-PHNO PET scans at baseline and at two time points following administration of the D3-preferring antagonist ABT-728 (150-1000 â€‹mg). Parametric binding potential (BPND) images were analyzed as four-dimensional image series using ICA to extract two independent sources of variation in [11C]-(+)-PHNO BPND. Spatial source maps for each component were consistent with respective regional patterns of D2-and D3-related binding. ICA-derived occupancy estimates from each component were similar to D2-and D3-specific occupancy estimated from a region-based approach (intraclass correlation coefficients â€‹> â€‹0.95). ICA-derived estimates of D3 receptor occupancy improved quality of fit to a single site binding model. Furthermore, ICA-derived estimates of the regional fraction of [11C]-(+)-PHNO binding related to D3 receptors was generated for each subject and values showed good agreement with region-based model estimates and prior literature values. In summary, ICA successfully separated D2-and D3-related components of the [11C]-(+)-PHNO binding signal, establishing this approach as a powerful data-driven method to quantify distinct biological features from PET data composed of mixed data sources.

[11C]Methionine and [11C]PBR28 as PET Imaging Tracers to Differentiate Metastatic Tumor Recurrence or Radiation Necrosis.

PURPOSE: As stereotactic radiosurgery (SRS) and immunotherapy are increasingly used to treat brain metastases, incidence of radiation necrosis (RN) is consequently rising. Differentiating tumor regrowth (TR) from RN is vital in management but difficult to assess using MRI. We hypothesized that tumor methionine levels would be elevated given increased metabolism and high amino acid uptake, whereas RN would increase inflammation marked by upregulated translocator protein (PBR-TSPO), which can be quantified with specific PET tracers. PROCEDURES: We performed a feasibility study to prospectively evaluate [11C]methionine and [11C]PBR28 using PET in 5 patients with 7 previously SRS-treated brain metastases demonstrating regrowth to differentiate TR from RN. RESULTS: Sequential imaging with dual tracers was well-tolerated. [11C]methionine was accurate for detecting pathologically confirmed TR in 7/7 lesions, whereas [11C]PBR28 was only accurate in 3/7 lesions. Tumor PBR-TSPO expression was elevated in both melanoma and lung cancer cells, contributing to lack of specificity of [11C]PBR28-PET. CONCLUSION: Sequential use of PET tracers is safe and effective. [11C]Methionine was a reliable TR marker, but [11C]PBR28 was not a reliable marker of RN. Studies are needed to determine the causes of post-radiation inflammation and identify specific markers of RN to improve diagnostic imaging.

Norepinephrine transporter availability in brown fat is reduced in obesity: a human PET study with [11C] MRB.

The energy-dissipating properties of brown adipose tissue (BAT) have been proposed as therapeutic targets for obesity and diabetes. Little is known about basal BAT activity. Capitalizing on the dense sympathetic innervation of BAT, we have previously shown that BAT can be detected in humans under resting room temperature (RT) conditions by using (S,S)-11C-O-methylreboxetine (MRB), a selective ligand for the norepinephrine transporter (NET). In this study, we determine whether MRB labeling of human BAT is altered by obesity. Fifteen healthy, nondiabetic Caucasian women (nine lean, age 25.6 ± 1.7, BMI 21.8 ± 1.3 kg/m2; six obese age 30.8 ± 8.8 BMI 37.9 ± 6.6 kg/m2) underwent PET-CT imaging of the neck/supraclavicular region using 11C-MRB under RT conditions. The distribution volume ratio (DVR) for 11C-MRB was estimated via multilinear reference tissue model 2 (MRTM2) referenced to the occipital cortex. Two women (one lean and one with obesity) had no detectable BAT. Of the women with detectable BAT, women with obesity had lower 11C-MRB DVR (0.80 ± 0.12 BAT DVR) compared to lean (1.15 ± 0.19 BAT DVR) (p = 0.004). Our findings are consistent with reports that NET is decreased in obesity and suggest that the sympathetic innervation of BAT is altered in obesity.

Reduced synaptic vesicle protein 2A binding in temporal lobe epilepsy: A [11 C]UCB-J positron emission tomography study.

OBJECTIVE: In this positron emission tomography (PET) study with [11 C]UCB-J, we evaluated synaptic vesicle glycoprotein 2A (SV2A) binding, which is decreased in resected brain tissues from epilepsy patients, in subjects with temporal lobe epilepsy (TLE) and compared the regional binding pattern to [18 F]fluorodeoxyglucose (FDG) uptake. METHODS: Twelve TLE subjects and 12 control subjects were examined. Regional [11 C]UCB-J binding potential (BPND ) values were estimated using the centrum semiovale as a reference region. [18 F]FDG uptake in TLE subjects was quantified using mean radioactivity values. Asymmetry in outcome measures was assessed by comparison of ipsilateral and contralateral regions. Partial volume correction (PVC) with the iterative Yang algorithm was applied based on the FreeSurfer segmentation. RESULTS: In 11 TLE subjects with medial temporal lobe sclerosis (MTS), the hippocampal volumetric asymmetry was 25 ± 11%. After PVC, [11 C]UCB-J BPND asymmetry indices were 37 ± 19% in the hippocampus, with very limited asymmetry in other brain regions. Reductions in [11 C]UCB-J BPND values were restricted to the sclerotic hippocampus when compared to control subjects. The corresponding asymmetry in hippocampal [18 F]FDG uptake was 22 ± 7% and correlated with that of [11 C]UCB-J BPND across subjects (R2  = .38). Hippocampal asymmetries in [11 C]UCB-J binding were 1.7-fold larger than those of [18 F]FDG uptake. SIGNIFICANCE: [11 C]UCB-J binding is reduced in the seizure onset zone of TLE subjects with MTS. PET imaging of SV2A may be a promising biomarker approach in the presurgical selection and evaluation of TLE patients and may improve the sensitivity of molecular imaging for seizure focus detection.

Effects of age, BMI and sex on the glial cell marker TSPO - a multicentre [11C]PBR28 HRRT PET study.

PURPOSE: The purpose of this study was to investigate the effects of ageing, sex and body mass index (BMI) on translocator protein (TSPO) availability in healthy subjects using positron emission tomography (PET) and the radioligand [11C]PBR28. METHODS: [11C]PBR28 data from 140 healthy volunteers (72 males and 68 females; N = 78 with HAB and N = 62 MAB genotype; age range 19-80 years; BMI range 17.6-36.9) were acquired with High Resolution Research Tomograph at three centres: Karolinska Institutet (N = 53), Turku PET centre (N = 62) and Yale University PET Center (N = 25). The total volume of distribution (VT) was estimated in global grey matter, frontal, temporal, occipital and parietal cortices, hippocampus and thalamus using multilinear analysis 1. The effects of age, BMI and sex on TSPO availability were investigated using linear mixed effects model, with TSPO genotype and PET centre specified as random intercepts. RESULTS: There were significant positive correlations between age and VT in the frontal and temporal cortex. BMI showed a significant negative correlation with VT in all regions. Additionally, significant differences between males and females were observed in all regions, with females showing higher VT. A subgroup analysis revealed a positive correlation between VT and age in all regions in male subjects, whereas age showed no effect on TSPO levels in female subjects. CONCLUSION: These findings provide evidence that individual biological properties may contribute significantly to the high variation shown in TSPO binding estimates, and suggest that age, BMI and sex can be confounding factors in clinical studies.

Quantitative PET Imaging in Drug Development: Estimation of Target Occupancy.

Positron emission tomography, an imaging tool using radiolabeled tracers in humans and preclinical species, has been widely used in recent years in drug development, particularly in the central nervous system. One important goal of PET in drug development is assessing the occupancy of various molecular targets (e.g., receptors, transporters, enzymes) by exogenous drugs. The current linear mathematical approaches used to determine occupancy using PET imaging experiments are presented. These algorithms use results from multiple regions with different target content in two scans, a baseline (pre-drug) scan and a post-drug scan. New mathematical estimation approaches to determine target occupancy, using maximum likelihood, are presented. A major challenge in these methods is the proper definition of the covariance matrix of the regional binding measures, accounting for different variance of the individual regional measures and their nonzero covariance, factors that have been ignored by conventional methods. The novel methods are compared to standard methods using simulation and real human occupancy data. The simulation data showed the expected reduction in variance and bias using the proper maximum likelihood methods, when the assumptions of the estimation method matched those in simulation. Between-method differences for data from human occupancy studies were less obvious, in part due to small dataset sizes. These maximum likelihood methods form the basis for development of improved PET covariance models, in order to minimize bias and variance in PET occupancy studies.

Imaging robust microglial activation after lipopolysaccharide administration in humans with PET.

Neuroinflammation is associated with a broad spectrum of neurodegenerative and psychiatric diseases. The core process in neuroinflammation is activation of microglia, the innate immune cells of the brain. We measured the neuroinflammatory response produced by a systemic administration of the Escherichia coli lipopolysaccharide (LPS; also called endotoxin) in humans with the positron emission tomography (PET) radiotracer [11C]PBR28, which binds to translocator protein, a molecular marker that is up-regulated by microglial activation. In addition, inflammatory cytokines in serum and sickness behavior profiles were measured before and after LPS administration to relate brain microglial activation with systemic inflammation and behavior. Eight healthy male subjects each had two 120-min [11C]PBR28 PET scans in 1 d, before and after an LPS challenge. LPS (1.0 ng/kg, i.v.) was administered 180 min before the second [11C]PBR28 scan. LPS administration significantly increased [11C]PBR28 binding 30-60%, demonstrating microglial activation throughout the brain. This increase was accompanied by an increase in blood levels of inflammatory cytokines, vital sign changes, and sickness symptoms, well-established consequences of LPS administration. To our knowledge, this is the first demonstration in humans that a systemic LPS challenge induces robust increases in microglial activation in the brain. This imaging paradigm to measure brain microglial activation with [11C]PBR28 PET provides an approach to test new medications in humans for their putative antiinflammatory effects.

Regional and source-based patterns of [11C]-(+)-PHNO binding potential reveal concurrent alterations in dopamine D2 and D3 receptor availability in cocaine-use disorder.

Dopamine type 2 and type 3 receptors (D2R/D3R) appear critical to addictive disorders. Cocaine-use disorder (CUD) is associated with lower D2R availability and greater D3R availability in regions primarily expressing D2R or D3R concentrations, respectively. However, these CUD-related alterations in D2R and D3R have not been concurrently detected using available dopaminergic radioligands. Furthermore, receptor availability in regions of mixed D2R/D3R concentration in CUD remains unclear. The current study aimed to extend investigations of CUD-related alterations in D2R and D3R availability using regional and source-based analyses of [11C]-(+)-PHNO positron emission tomography (PET) of 26 individuals with CUD and 26 matched healthy comparison (HC) participants. Regional analysis detected greater binding potential (BPND) in CUD in the midbrain, consistent with prior [11C]-(+)-PHNO research, and lower BPND in CUD in the dorsal striatum, consistent with research using non-selective D2R/D3R radiotracers. Exploratory independent component analysis (ICA) identified three sources of BPND (striatopallidal, pallidonigral, and mesoaccumbens sources) that represent systems of brain regions displaying coherent variation in receptor availability. The striatopallidal source was associated with estimates of regional D2R-related proportions of BPND (calculated using independent reports of [11C]-(+)-PHNO receptor binding fractions), was lower in intensity in CUD and negatively associated with years of cocaine use. By comparison, the pallidonigral source was associated with estimates of regional D3R distribution, was greater in intensity in CUD and positively associated with years of cocaine use. The current study extends previous D2R/D3R research in CUD, demonstrating both lower BPND in the D2R-rich dorsal striatum and greater BPND in the D3R-rich midbrain using a single radiotracer. In addition, exploratory ICA identified sources of [11C]-(+)-PHNO BPND that were correlated with regional estimates of D2R-related and D3R-related proportions of BPND, were consistent with regional differences in CUD, and suggest receptor alterations in CUD may also be present in regions of mixed D2R/D3R concentration.

Quantitative projection of human brain penetration of the H3 antagonist PF-03654746 by integrating rat-derived brain partitioning and PET receptor occupancy.

1. Unbound brain drug concentration (Cb,u), a valid surrogate of interstitial fluid drug concentration (CISF), cannot be directly determined in humans, which limits accurately defining the human Cb,u:Cp,u of investigational molecules. 2. For the H3R antagonist (1R,3R)-N-ethyl-3-fluoro-3-[3-fluoro-4-(pyrrolidin-1-lmethyl)phenyl]cyclobutane-1-carboxamide (PF-03654746), we interrogated Cb,u:Cp,u in humans and nonhuman primate (NHP). 3. In rat, PF-03654746 achieved net blood-brain barrier (BBB) equilibrium (Cb,u:Cp,u of 2.11). 4. In NHP and humans, the PET receptor occupancy-based Cp,u IC50 of PF-03654746 was 0.99 nM and 0.31 nM, respectively, which were 2.1- and 7.4-fold lower than its in vitro human H3 Ki (2.3 nM). 5. In an attempt to understand this higher-than-expected potency in humans and NHP, rat-derived Cb,u:Cp,u of PF-03654746 was integrated with Cp,u IC50 to identify unbound (neuro) potency of PF-03654746, nIC50. 6. The nIC50 of PF-03654746 was 2.1 nM in NHP and 0.66 nM in human which better correlated (1.1- and 3.49-fold lower) with in vitro human H3 Ki (2.3 nM). 7. This correlation of the nIC50 and in vitro hH3 Ki suggested the translation of net BBB equilibrium of PF-03654746 from rat to NHP and humans, and confirmed the use of Cp,u as a reliable surrogate of Cb,u. 8. Thus, nIC50 quantitatively informed the human Cb,u:Cp,u of PF-03654746.

Age-related changes in binding of the D2/3 receptor radioligand [(11)C](+)PHNO in healthy volunteers.

OBJECTIVE: Previous imaging studies with positron emission tomography (PET) have reliably demonstrated an age-associated decline in the dopamine system. Most of these studies have focused on the densities of dopamine receptor subtypes D2/3R (D2R family) in the striatum using antagonist radiotracers that are largely nonselective for D2R vs. D3R subtypes. Therefore, less is known about any possible age effects in D3-rich extrastriatal areas such as the substantia nigra/ventral tegmental area (SN/VTA) and hypothalamus. This study sought to investigate whether the receptor availability measured with [(11)C](+)PHNO, a D3R-preferring agonist radiotracer, also declines with age. METHODS: Forty-two healthy control subjects (9 females, 33 males; age range 19-55 years) were scanned with [(11)C](+)PHNO using a High Resolution Research Tomograph (HRRT). Parametric images were computed using the simplified reference tissue model (SRTM2) with cerebellum as the reference region. Binding potentials (BPND) were calculated for the amygdala, caudate, hypothalamus, pallidum, putamen, SN/VTA, thalamus, and ventral striatum and then confirmed at the voxel level with whole-brain parametric images. RESULTS: Regional [(11)C](+)PHNO BPND displayed a negative correlation between receptor availability and age in the caudate (r=-0.56, corrected p=0.0008) and putamen (r=-0.45, corrected p=0.02) in healthy subjects (respectively 8% and 5% lower per decade). No significant correlations with age were found between age and other regions (including the hypothalamus and SN/VTA). Secondary whole-brain voxel-wise analysis confirmed these ROI findings of negative associations and further identified a positive correlation in midbrain (SN/VTA) regions. CONCLUSION: In accordance with previous studies, the striatum (an area rich in D2R) is associated with age-related declines of the dopamine system. We did not initially find evidence of changes with age in the SN/VTA and hypothalamus, areas previously found to have a predominantly D3R signal as measured with [(11)C](+)PHNO. A secondary analysis did find a significant positive correlation in midbrain (SN/VTA) regions, indicating that there may be differential effects of aging, whereby D2R receptor availability decreases with age while D3R availability stays unchanged or is increased.

Test-retest reliability of the novel 5-HT1B receptor PET radioligand [11C]P943.

PURPOSE: [(11)C]P943 is a novel, highly selective 5-HT1B PET radioligand. The aim of this study was to determine the test-retest reliability of [(11)C]P943 using two different modeling methods and to perform a power analysis with each quantification technique. METHODS: Seven healthy volunteers underwent two PET scans on the same day. Regions of interest (ROIs) were the amygdala, hippocampus, pallidum, putamen, insula, frontal, anterior cingulate, parietal, temporal and occipital cortices, and cerebellum. Two multilinear radioligand quantification techniques were used to estimate binding potential: MA1, using arterial input function data, and the second version of the multilinear reference tissue model analysis (MRTM2), using the cerebellum as the reference region. Between-scan percent variability and intraclass correlation coefficients (ICC) were used to assess test-retest reliability. We also performed power analyses to determine the method that would allow the least number of subjects using within-subject or between-subject study designs. A voxel-wise ICC analysis for MRTM2 BPND was performed for the whole brain and all the ROIs studied. RESULTS: Mean percent variability between two scans across regions ranged between 0.4 % and 12.4 % for MA1 BPND, 0.5 % and 11.5 % for MA1 BPP, 16.7 % and 28.3 % for MA1 BPF, and between 0.2 % and 5.4 % for MRTM2 BPND. The power analyses showed a greater number of subjects were required using MA1 BPF compared with other outcome measures for both within-subject and between-subject study designs. ICC values were the highest using MRTM2 BPND and the lowest with MA1 BPF in ten ROIs. Small regions and regions with low binding had lower ICC values than large regions and regions with high binding. CONCLUSION: Reliable measures of 5-HT1B receptor binding can be obtained using the novel PET radioligand [(11)C]P943. Quantification of 5-HT1B receptor binding with MRTM2 BPND and with MA1 BPP provided the least variability and optimal power for within-subject and between-subject designs.

Test-retest reproducibility of the metabotropic glutamate receptor 5 ligand [¹8F]FPEB with bolus plus constant infusion in humans.

PURPOSE: [(18)F]FPEB is a promising PET radioligand for the metabotropic glutamate receptor 5 (mGluR5), a potential target for the treatment of neuropsychiatric diseases. The purpose of this study was to evaluate the test-retest reproducibility of [(18)F]FPEB in the human brain. METHODS: Seven healthy male subjects were scanned twice, 3 - 11 weeks apart. Dynamic data were acquired using bolus plus infusion of 162 ± 32 MBq [(18)F]FPEB. Four methods were used to estimate volume of distribution (V T): equilibrium analysis (EQ) using arterial (EQA) or venous input data (EQV), MA1, and a two-tissue compartment model (2 T). Binding potential (BP ND) was also estimated using cerebellar white matter (CWM) or gray matter (CGM) as the reference region using EQ, 2 T and MA1. Absolute test-retest variability (aTRV) of V T and BP ND were calculated for each method. Venous blood measurements (C V) were compared with arterial input (C A) to examine their usability in EQ analysis. RESULTS: Regional V T estimated by the four methods displayed a high degree of agreement (r (2) ranging from 0.83 to 0.99 among the methods), although EQA and EQV overestimated V T by a mean of 9 % and 7 %, respectively, compared to 2 T. Mean values of aTRV of V T were 11 % by EQA, 12 % by EQV, 14 % by MA1 and 14 % by 2 T. Regional BP ND also agreed well among the methods and mean aTRV of BP ND was 8 - 12 % (CWM) and 7 - 9 % (CGM). Venous and arterial blood concentrations of [(18)F]FPEB were well matched during equilibrium (C V = 1.01 · C A, r (2) = 0.95). CONCLUSION: [(18)F]FPEB binding shows good TRV with minor differences among analysis methods. Venous blood can be used as an alternative for input function measurement instead of arterial blood in EQ analysis. Thus, [(18)F]FPEB is an excellent PET imaging tracer for mGluR5 in humans.

(11)C-PBR28 imaging in multiple sclerosis patients and healthy controls: test-retest reproducibility and focal visualization of active white matter areas.

PURPOSE: Activated microglia play a key role in inflammatory demyelinating injury in multiple sclerosis (MS). Microglial activation can be measured in vivo using a positron emission tomography (PET) ligand (11)C-PBR28. We evaluated the test-retest variability (TRV) and lesion detectability of (11)C-PBR28 binding in MS subjects and healthy controls (HCs) with high-resolution PET. METHODS: Four clinically and radiologically stable relapsing-remitting MS subjects (age 41 ± 7 years, two men/two women) and four HCs (age 42 ± 8 years, 2 two men/two women), matched for translocator protein genotype [two high- and two medium-affinity binders according to DNA polymorphism (rs6971) in each group], were studied for TRV. Another MS subject (age 41 years, male) with clinical and radiological activity was studied for lesion detectability. Dynamic data were acquired over 120 min after injection of 634 ± 101 MBq (11)C-PBR28. For the TRV study, subjects were scanned twice, on average 1.4 weeks apart. Volume of distribution (V T) derived from multilinear analysis (MA1) modeling (t* = 30 min, using arterial input data) was the main outcome measure. RESULTS: Mean test V T values (ml cm(-3)) were 3.9 ± 1.4 in the whole brain gray matter (GM), 3.6 ± 1.2 in the whole brain white matter (WM) or normal-appearing white matter (NAWM), and 3.3 ± 0.6 in MS WM lesions; mean retest V T values were 3.7 ± 1.0 in GM, 3.3 ± 0.9 in WM/NAWM, and 3.3 ± 0.7 in MS lesions. Test-retest results showed a mean absolute TRV ranging from 7 to 9 % across GM, WM/NAWM, and MS lesions. High-affinity binders demonstrated 30 % higher V T than medium-affinity binders in GM. Focal (11)C-PBR28 uptake was detected in two enhancing lesions of the active MS patient. CONCLUSION: High-resolution (11)C-PBR28 PET can visualize focal areas where microglial activation is known to be present and has good test-retest reproducibility in the human brain. (11)C-PBR28 PET is likely to be valuable for monitoring both MS disease evolution and response to therapeutic strategies that target microglial activation.