Designing drug occupancy studies with PET neuroimaging: Sample size, occupancy ranges and analytical methods.

Molecular neuroimaging is today considered essential for evaluation of novel CNS drugs; it is used to quantify blood-brain barrier permeability, verify interaction with key target and determine the drug dose resulting in 50% occupancy, IC50. In spite of this, there has been limited data available to inform on how to optimize study designs. Through simulations, we here evaluate how IC50 estimation is affected by the (i) range of drug doses administered, (ii) number of subjects included, and (iii) level of noise in the plasma drug concentration measurements. Receptor occupancy is determined from PET distribution volumes using two different methods: the Lassen plot and Likelihood estimation of occupancy (LEO). We also introduce and evaluate a new likelihood-based estimator for direct estimation of IC50 from PET distribution volumes. For estimation of IC50, we find very limited added benefit in scanning individuals who are given drug doses corresponding to less than 40% receptor occupancy. In the range of typical PET sample sizes (5-20 subjects) each extra individual clearly reduces the error of the IC50 estimate. In all simulations, likelihood-based methods gave more precise IC50 estimates than the Lassen plot; four times the number of subjects were required for the Lassen plot to reach the same IC50 precision as LEO.

[11 C]raclopride positron emission tomography study of dopamine-D2/3 receptor binding in patients with severe major depressive episodes before and after electroconvulsive therapy and compared to control subjects.

AIM: The aim of the study was to test: (i) if D2 /D3 binding in three functional subsections of striatum is different in patients with severe major depressive episodes than in controls; and (ii) if this difference is normalized after electroconvulsive therapy (ECT). METHODS: Nine inpatients were examined with positron emission tomography (PET) and the radioligand [11 C]raclopride before and after an average of 8.4 ECT sessions. Treatment response was assessed using the Montgomery-Åsberg Depression Rating Scale. Nine age- and sex-matched controls were examined twice with PET and [11 C]raclopride. RESULTS: [11 C]raclopride binding was significantly lower in all three subsections of striatum in patients compared to controls (Cohen's dz , 1.14-1.68; P = 0.003-0.027). Montgomery-Åsberg Depression Ratings decreased significantly after ECT (P < 0.001; Cohen's dz , 2.9). ECT had no statistically significant effect on [11 C]raclopride binding, although post-ECT binding estimates were more similar to those obtained in controls in all subsections of striatum. CONCLUSION: Using PET and [11 C]raclopride, we found support for the notion that severe major depressive episodes are associated with significantly lower dopamine D2 /D3 binding in all three subsections of striatum compared to controls. We noted no significant effect on D2 /D3 binding in the patient group after response to ECT.

Accuracy and reliability of [11C]PBR28 specific binding estimated without the use of a reference region.

[11C]PBR28 is a positron emission tomography radioligand used to examine the expression of the 18 kDa translocator protein (TSPO). TSPO is located in glial cells and can function as a marker for immune activation. Since TSPO is expressed throughout the brain, no true reference region exists. For this reason, an arterial input function is required for accurate quantification of [11C]PBR28 binding and the most common outcome measure is the total distribution volume (VT). Notably, VT reflects both specific binding and non-displaceable binding. Therefore, estimates of specific binding, such as binding potential (e.g. BPND) and specific distribution volume (VS) should theoretically be more sensitive to underlying differences in TSPO expression. It is unknown, however, if unbiased and accurate estimates of these outcome measures are obtainable for [11C]PBR28. The Simultaneous Estimation (SIME) method uses time-activity-curves from multiple brain regions with the aim to obtain a brain-wide estimate of the non-displaceable distribution volume (VND), which can subsequently be used to improve the estimation of BPND and VS. In this study we evaluated the accuracy of SIME-derived VND, and the reliability of resulting estimates of specific binding for [11C]PBR28, using a combination of simulation experiments and in vivo studies in healthy humans. The simulation experiments, based on data from 54 unique [11C]PBR28 examinations, showed that VND values estimated using SIME were both precise and accurate. Data from a pharmacological competition challenge (n = 5) showed that SIME provided VND values that were on average 19% lower than those obtained using the Lassen plot, but similar to values obtained using the Likelihood-Estimation of Occupancy technique. Test-retest data (n = 11) showed that SIME-derived VS values exhibited good reliability and precision, while larger variability was observed in SIME-derived BPND values. The results support the use of SIME for quantifying specific binding of [11C]PBR28, and suggest that VS can be used in complement to the conventional outcome measure VT. Additional studies in patient cohorts are warranted.

Validity and reliability of extrastriatal [11C]raclopride binding quantification in the living human brain.

[11C]raclopride is a well established PET tracer for the quantification of dopamine 2/3 receptors (D2/3R) in the striatum. Outside of the striatum the receptor density is up to two orders of magnitude lower. In contrast to striatal binding, the characteristics of extrastriatal [11C]raclopride binding quantification has not been thoroughly described. Still, binding data for e.g., neocortex is frequently reported in the scientific literature. Here we evaluate the validity and reliability of extrastriatal [11C]raclopride binding quantification. Two sets of healthy control subjects were examined with HRRT and [11C]raclopride: (i) To assess the validity of extrastriatal [11C]raclopride binding estimates, eleven subjects were examined at baseline and after dosing with quetiapine, a D2/3R antagonist. (ii) To assess test-retest repeatability, nine subjects were examined twice. Non displaceable binding potential (BPND) was quantified using the simplified reference tissue model with cerebellum as reference. Quetiapine dosing was associated with decrease in [11C]raclopride BPND in temporal cortex (18 ±â€¯17% occupancy) and thalamus (20 ±â€¯17%), but not in frontal cortex. Extrastriatal occupancy was lower than in putamen (51 ±â€¯4%). The mean absolute variation was 4-7% in the striatal regions, 17% in thalamus, and 13-59% in cortical regions. Our data indicate that [11C]raclopride PET, quantified using cerebellum as reference, is not a suitable tool to measure D2/3R in extrastriatal regions.

Likelihood estimation of drug occupancy for brain PET studies.

Neuroimaging with PET is unique in its capability to measure in vivo the occupancy of a drug. The occupancy is typically obtained by conducting PET measurements before and after administration of the drug. For radioligands for which no reference region exists, however, the only established procedure to estimate the occupancy from these data is via linear regression analysis, forming the basis for the so-called Lassen plot. There are several reasons why simple linear regression analysis is not ideal for analyzing these data, including regression attenuation and correlated errors. Here, we propose the use of Likelihood Estimation of Occupancy (LEO) in such a situation. Similar to the Lassen plot, LEO uses the total distribution volume estimates at baseline and at block condition as input, but estimates the non-displaceable distribution volume (VND) and fractional occupancy (Δ) via direct maximum likelihood estimation (MLE). This study outlines the rationale for using MLE to estimate Δ and VND from PET data, and evaluates its performance in relation to the Lassen Plot via two separate simulation experiments. Finally, LEO and Lassen plot are applied to a PET dataset acquired with [11C]WAY-100635. LEO can exploit the covariance structure of the data to improve the accuracy and precision of the estimates of Δ and VND. Theoretically, the covariance matrix can be extracted from a test-retest dataset for the radioligand at hand. Several procedures to estimate the covariance matrix were considered as part of the simulation experiments, and the effect of the test-retest sample size was also assessed. The results are conclusive in that MLE can be used to estimate Δ and VND from PET data, avoiding the limitations associated with linear regression. The performance of LEO was, naturally, dependent on the procedure used to estimate the covariance matrix, and the test-retest sample size. Given a test-retest sample size of at least 5, but preferably 10 individuals, LEO provides higher accuracy and precision than Lassen plot in the estimation of Δ and VND. We conclude that LEO is valuable in drug occupancy studies.

Non-invasive estimation of [11C]PBR28 binding potential.

[11C]PBR28 is a PET radioligand used to estimate densities of the 18 kDa translocator protein (TSPO) in vivo. Since there is no suitable reference region, arterial blood samples are required for full quantification. Here, we evaluate a methodology for full quantification of [11C]PBR28 PET data that does not require either a reference region or blood samples. Simultaneous estimation (SIME) uses time-activity curves from several brain regions to estimate binding potential (BPND), a theoretically more sensitive outcome measure than total distribution volume. SIME can be employed with either a measured arterial input function (AIF) or a template input function (tIF) that has similar shape as the AIF, but with arbitrary amplitude. We evaluated the ability of SIME to detect group differences in TSPO densities using PET and arterial plasma data from 21 Alzheimer's disease (AD) patients and 15 controls that underwent [11C]PBR28 imaging. Regional BPND obtained with tIFs were compared to those obtained using measured AIFs. Standard kinetic modeling was also employed for comparison. The sensitivity of each method to detect group differences in TSPO densities were assessed by comparing estimated effect sizes between AD patients and controls. For this purpose, BPND estimated for one region with high pathological burden (inferior temporal cortex), and for one region with low pathological burden (cerebellum) was used. BPND estimates obtained with SIME and tIFs were close to identical to those obtained with AIF (3.0 ± 21% difference, r2 = 0.78). In this dataset, the effect sizes between AD patients and controls for both SIME with AIF and SIME with tIF were similar (30.3%, p = 0.001 and 31.0%, p = 0.004, respectively) and were each greater than the effect size observed using the two-tissue compartment model (16.1%, p = 0.12). None of the tested methods showed difference in TSPO binding in cerebellum. These results demonstrate that BPND can be estimated for [11C]PBR28 using SIME, and may be useful in clinical studies. In addition, arterial sampling may not be necessary if tIFs can be reliably estimated.

Estimation of the binding potential BPND without a reference region or blood samples for brain PET studies.

Binding potential (BPND) is a commonly used PET outcome measure because it can be estimated without blood sampling if a brain reference region (RR) devoid of the target of interest exists. For many radioligands, however, no RR exists, and the total distribution volume (VT), whose estimation requires arterial blood sampling, is normally considered as the outcome measure. Here, we present a method that allows calculation of BPND without requiring either blood samples or a RR. The method extends our previous algorithm for estimating non-displaceable distribution volumes (VND) without using a RR. Here we show that if a template input function, with arbitrary amplitude but a shape similar to the actual arterial input function, is used in the algorithm, estimation of VT and VND are both proportionally biased, and thus this bias cancels out in the estimation of BPND. The method is evaluated using simulated data, human data acquired with the serotonin 1A receptor radioligand [11C]WAY-100635, and blocking data acquired in baboons using the serotonin 1A receptor radioligand [11C]CUMI-101. We evaluated two versions of template input functions: an arbitrarily downscaled version of the actual arterial input function, and an unscaled population-based input function. In addition, we evaluated how shape modifications of the template input function impact the estimates of BPND. With the downscaled input function, BPND values close to the gold standard were obtained. When the unscaled population-based based input function was used, greater variability was observed but no discernable bias was introduced. When the input function shape was modified, a systematic but small bias in BPND was introduced. We conclude that, provided the shape of the arterial input function is adequately described, determination of its amplitude is not necessary for estimation of BPND.

Patterns of age related changes for phosphodiesterase type-10A in comparison with dopamine D2/3 receptors and sub-cortical volumes in the human basal ganglia: A PET study with 18F-MNI-659 and 11C-raclopride with correction for partial volume effect.

Phosphodiesterase 10A enzyme (PDE10A) is an important striatal target that has been shown to be affected in patients with neurodegenerative disorders, particularly Huntington´s disease (HD). PDE10A is expressed on striatal neurones in basal ganglia where other known molecular targets are enriched such as dopamine D2/3 receptors (D2/3 R). The aim of this study was to examine the availability of PDE10A enzyme in relation with age and gender and to compare those changes with those related to D2/3 R and volumes in different regions of the basal ganglia. As a secondary objective we examined the relative distribution of D2/3 R and PDE10A enzyme in the striatum and globus pallidus. Forty control subjects (20F/20M; age: 44±11y, age range 27-69) from an ongoing positron emission tomography (PET) study in HD gene expansion carriers were included. Subjects were examined with PET using the high-resolution research tomograph (HRRT) and with 3T magnetic resonance imaging (MRI). The PDE10A radioligand 18F-MNI-659 and D2/3 R radioligand 11C-raclopride were used. The outcome measure was the binding potential (BPND) estimated with the two-tissue compartment model (18F-MNI-659) and the simplified reference tissue model (11C-raclopride) using the cerebellum as reference region. The PET data were corrected for partial volume effects. In the striatum, PDE10A availability showed a significant age-related decline that was larger compared to the age-related decline of D2/3 R availability and to the age-related decline of volumes measured with MRI. In the globus pallidus, a less pronounced decline of PDE10A availability was observed, whereas D2/3 R availability and volumes seemed to be rather stable with aging. The distribution of the PDE10A enzyme was different from the distribution of D2/3 R, with higher availability in the globus pallidus. These results indicate that aging is associated with a considerable physiological reduction of the availability of PDE10A enzyme in the striatum. Moreover as result of the analysis, in the striatum for both the molecular targets, we observed a gender effect with higher BPND the female group.

Increased basal ganglia binding of 18 F-AV-1451 in patients with progressive supranuclear palsy.

BACKGROUND: Progressive supranuclear palsy (PSP) is difficult to diagnose accurately. The recently developed tau PET tracers may improve the diagnostic work-up of PSP. METHODS: Regional tau accumulation was studied using 18 F-AV-1451 PET in 11 patients with PSP and 11 age-matched healthy controls in the Swedish BioFinder study. RESULTS: 18 F-AV-1451 standard uptake volume ratios were significantly higher in the basal ganglia in PSP patients when compared with controls (globus pallidus 1.75 vs 1.50; putamen 1.51 vs 1.35). Retention in the basal ganglia was correlated with age in both groups (r = .43-.78, P < .05). In PSP, we observed a significant correlation between clinical deterioration measured with the PSP rating scale and standard uptake volume ratios in the globus pallidus (r = .74, P < .05). However, no 18 F-AV-1451 retention was observed in the cerebral cortex or white matter of either PSP patients or controls, and autoradiography did not reveal any specific binding of AV-1451 to PSP tau aggregates. CONCLUSION: We found higher 18 F-AV-1451 retention in the basal ganglia of PSP patients when compared with healthy elderly controls, but also increases with age in both controls and patients. As a result of the overlap in retention between diagnostic groups and the age-dependent increase present also in controls, 18 F-AV-1451 PET might not reliably distinguish individual patients with PSP from controls. However, further studies are needed to evaluate whether 18 F-AV-1451 PET might be useful as a progression marker in clinical PSP trials. © The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Neuroinflammation in Neurodegenerative Disorders-a Review.

The potential for positron emission tomography (PET) to detect neuroinflammation in vivo has sparked a remarkable interest in various disciplines of neuroscience. Early PET radioligands, such as [11C]PK(R)-11195 for the 18-kDa translocator protein (TSPO) and [11C]L-deprenyl for monoamine oxidase B, have been used in studies designed to clarify the role of neuroinflammation in a variety of psychiatric and neurological disorders. Recent years have witnessed the development of several second-generation PET radioligands for TSPO and radioligands to measure endogenous targets that are active in various stages of the inflammatory cascade, such as cyclooxygenase and arachidonic acid. Here, we discuss some of the biomarkers for neuroinflammation that are available for quantification with PET, as well as recent findings from studies where neuroinflammation has been assessed in neurodegenerative disorders. In addition, we highlight the challenges to accurate interpretation of PET studies of neuroinflammation.

In vivo evidence of a functional association between immune cells in blood and brain in healthy human subjects.

Microglia, the resident macrophages in the central nervous system, are thought to be maintained by a local self-renewal mechanism. Although preclinical and in vitro studies have suggested that the brain may contain immune cells also from peripheral origin, the functional association between immune cells in the periphery and brain at physiological conditions is poorly understood. We examined 32 healthy individuals using positron emission tomography (PET) and [(11)C]PBR28, a radioligand for the 18-kDa translocator protein (TSPO) which is expressed both in brain microglia and blood immune cells. In 26 individuals, two measurements were performed with varying time intervals. In a subgroup of 19 individuals, of which 12 had repeat examinations, leukocyte numbers in blood was measured on each day of PET measurements. All individuals were genotyped for TSPO polymorphism and categorized as high, mixed, and low affinity binders. We assessed TSPO binding expressed as total distribution volume of [(11)C]PBR28 in brain and in blood cells. TSPO binding in brain was strongly and positively correlated to binding in blood cells both at baseline and when analyzing change between two PET examinations. Furthermore, there was a significant correlation between change of leukocyte numbers and change in TSPO binding in brain, and a trend-level correlation to change in TSPO binding in blood cells. These in vivo findings indicate an association between immunological cells in blood and brain via intact BBB, suggesting a functional interaction between these two compartments, such as interchange of peripherally derived cells or a common regulatory mechanism. Measurement of radioligand binding in blood cells may be a way to control for peripheral immune function in PET studies using TSPO as a marker of brain immune activation.

Diurnal and seasonal variation of the brain serotonin system in healthy male subjects.

The mammalian circadian clock underlies both diurnal and seasonal changes in physiology, and its function is thought to be disturbed in both seasonal and non-seasonal depression. In humans, molecular imaging studies have reported seasonal changes in the serotonin system. Despite the role of the circadian clock in generating seasonal physiological changes, however, diurnal variation of serotonin receptors and transporters has never been directly studied in humans. We used positron emission tomography to examine diurnal and seasonal changes in the serotonin 5-HT1A receptor and serotonin transporter in two large cohorts of healthy male subjects, employing a cross-sectional design. In 56 subjects measured with [(11)C]WAY-100635, we observed diurnal increases in the availability of 5-HT1A receptors in the cortex. In 40 subjects measured with [(11)C]MADAM, a decrease in 5-HTT was observed in the midbrain across the day. We also found seasonal changes in the 5-HT1A receptor in serotonin projection regions, with higher availability on days with a longer duration of daylight. Our observation that serotonin receptor and transporter levels may change across the day in humans is corroborated by experimental research in rodents. These findings have important implications for understanding the relationship between the circadian and serotonin systems in both the healthy brain and in affective disorders, as well as for the design of future molecular imaging studies.

Test-retest reliability of [11C]AZ10419369 binding to 5-HT(1B) receptors in human brain.

PURPOSE: [(11)C]AZ10419369 is a recently developed 5-HT1B receptor radioligand that is sensitive to changes in endogenous serotonin concentrations in the primate brain. Thus, [(11)C] AZ10419369 may serve as a useful tool in clinical studies of the pathophysiology and pharmacological treatment of diseases related to the serotonin system, such as depression and anxiety disorders. The aim of this study was to evaluate the test-retest reliability of [(11)C]AZ10419369. METHODS: Eight men were examined with PET and [(11)C] AZ10419369 twice on the same day. The binding potentials (BPND) of [(11)C]AZ10419369 in selected serotonergic projection areas and in the raphe nuclei (RN) were determined using the simplified reference tissue model, and for comparison also using a wavelet-aided parametric imaging approach. The BPND values obtained from the first and second PET scans were compared by means of descriptive statistics, difference, absolute variability and intraclass correlation coefficient. RESULTS: Similar BPND values were obtained with the two methods. The absolute mean differences in BPND between PET 1 and PET 2 were less than 3% in all serotonergic projection regions. Absolute variabilities were low in cortical regions (5 - 7%), low to moderate (7 - 14%) in subcortical regions, but higher (20%) in the RN. CONCLUSION: The BPND of [(11)C]AZ10419369 is highly reproducible in cortical regions and satisfactory in subcortical projection areas. The variability in the RN is higher. Thus larger sample sizes or larger divergences are required to assess a potential difference between subjects or between experimental conditions in this region.

Improved mapping and quantification of serotonin transporter availability in the human brainstem with the HRRT.

PURPOSE: The serotonin system is involved in many physiological functions and clinical conditions. Serotonergic neurons originate from the raphe nuclei in the brainstem, and reliable estimates of receptor/transporter availability in the raphe in vivo are thus of interest. Though positron emission tomography (PET) can be used to quantify receptor distribution in the brain, high noise levels prevent reliable estimation of radioligand binding in small regions such as the raphe. For this purpose, parametric imaging in combination with high-resolution PET systems may provide images with reduced noise levels and sufficient contrast for reliable quantification. This study examined the potential to evaluate radioligand binding in brainstem nuclei, and assessed the effect of improved resolution on the outcome measures. METHODS: For comparative purposes, radioligand binding was measured with an ECAT EXACT HR PET system (resolution about 4.5 mm FWHM) and a high-resolution research tomograph (HRRT) system (resolution about 1.5 mm FWHM). Six subjects were examined with both systems on the same day using the serotonin transporter radioligand [(11)C]MADAM. Parametric images of binding potential (BP (ND)) were obtained using a wavelet-aided approach. Regions of interest (ROIs) were delineated using a threshold-based semiautomatic delineation procedure for five brainstem structures. Regional BP (ND) values were estimated by applying the ROIs to the parametric images, and the percentage difference in BP (ND) between the systems was calculated. RESULTS: Signals for [(11)C]MADAM binding were obtained for all five brainstem structures. Overall, the HRRT provided 30-40 % higher BP (ND) values than the HR (p = 0.0017), independent of thresholds used in the ROI delineation procedure. CONCLUSION: The methodology used enabled the estimation of [(11)C]MADAM binding in the small nuclei of the brainstem. Differences in the BP (ND) values calculated using data from the two systems were mainly attributable to their differing resolutions. The estimated BP (ND) values provided lower across-subject variability than those previously obtained using compartment analysis. This procedure may therefore facilitate quantitative studies of receptor/transporter availability in the brainstem.

Kinetic models for estimating occupancy from single-scan PET displacement studies.

The traditional design of PET target engagement studies is based on a baseline scan and one or more scans after drug administration. We here evaluate an alternative design in which the drug is administered during an on-going scan (i.e., a displacement study). This approach results both in lower radiation exposure and lower costs. Existing kinetic models assume steady state. This condition is not present during a drug displacement and consequently, our aim here was to develop kinetic models for analysing PET displacement data. We modified existing compartment models to accommodate a time-variant increase in occupancy following the pharmacological in-scan intervention. Since this implies the use of differential equations that cannot be solved analytically, we developed instead one approximate and one numerical solution. Through simulations, we show that if the occupancy is relatively high, it can be estimated without bias and with good accuracy. The models were applied to PET data from six pigs where [11C]UCB-J was displaced by intravenous brivaracetam. The dose-occupancy relationship estimated from these scans showed good agreement with occupancies calculated with Lassen plot applied to baseline-block scans of two pigs. In summary, the proposed models provide a framework to determine target occupancy from a single displacement scan.

The α-synuclein PET tracer [18F] ACI-12589 distinguishes multiple system atrophy from other neurodegenerative diseases.

A positron emission tomography (PET) tracer detecting α-synuclein pathology will improve the diagnosis, and ultimately the treatment of α-synuclein-related diseases. Here we show that the PET ligand, [18F]ACI-12589, displays good in vitro affinity and specificity for pathological α-synuclein in tissues from patients with different α-synuclein-related disorders including Parkinson's disease (PD) and Multiple-System Atrophy (MSA) using autoradiography and radiobinding techniques. In the initial clinical evaluation we include 23 participants with α-synuclein related disorders, 11 with other neurodegenerative disorders and eight controls. In vivo [18F]ACI-12589 demonstrates clear binding in the cerebellar white matter and middle cerebellar peduncles of MSA patients, regions known to be highly affected by α-synuclein pathology, but shows limited binding in PD. The binding statistically separates MSA patients from healthy controls and subjects with other neurodegenerative disorders, including other synucleinopathies. Our results indicate that α-synuclein pathology in MSA can be identified using [18F]ACI-12589 PET imaging, potentially improving the diagnostic work-up of MSA and allowing for detection of drug target engagement in vivo of novel α-synuclein targeting therapies.

Quantification of serotonin transporter availability with [11C]MADAM--a comparison between the ECAT HRRT and HR systems.

UNLABELLED: The High Resolution Research Tomograph (HRRT) is the PET system providing the highest resolution for imaging of the human brain. In this study, the improved quantitative performance of the HRRT was evaluated in comparison with a previously developed lower resolution PET system, the ECAT HR. The radioligand [(11)C]MADAM was chosen for the purpose since it provides a signal for serotonin transporter (5-HTT) binding in cortical and sub-cortical brain regions of different sizes and expressing different 5-HTT densities. A secondary objective was to assess the effect of partial volume effect (PVE) correction on the cross-comparability between the two systems. METHOD: Six male control subjects (ages 20-35 yr) were examined twice using the HRRT and the HR system, respectively. Regions of interest (ROIs) included cortical regions (frontal cortex, temporal cortex, insula, anterior cingulate cortex, and hippocampus), sub-cortical regions (caudate, putamen, thalamus, dorsal brainstem and ventral midbrain) and cerebellum. The ROIs were manually delineated on T1-weighted MRI-images and subsequently applied to both HRRT and HR images. Regional binding potential (BP(ND)) values were calculated with the simplified reference tissue model (SRTM) using cerebellum as the reference region. The percent difference in BP(ND) between the systems was calculated for each ROI. In addition, both HRRT and HR data were corrected for PVE using established MRI-based methods described by Meltzer and Müller-Gärtner. The effect of PVE correction (PVEc) on the agreement between the systems was assessed via percent difference calculation and linear regression analysis. RESULTS: Quantification with SRTM showed that regional BP(ND) values for [(11)C]MADAM were on average 23% higher for the HRRT than those obtained by the HR system. More specifically, BP(ND) measured with HRRT was 31.1±48.1% higher in neocortical/limbic regions and 14.6±20.9% higher in sub-cortical regions. The effect of PVEc varied between regions. After correction according to Müller-Gärtner, the agreement between systems was best in the neocortical/limbic regions (3.7±22.5%). With the exception of the caudate, in which the agreement was improved by approximately 17% using the Meltzer method, the effect of PVEc in sub-cortical regions was less pronounced. Linear regression analysis showed improved correlation between the two systems after PVEc, particularly in the neocortical/limbic regions. CONCLUSION: As expected, BP(ND) values measured with the HRRT were higher than those measured with the HR due to higher resolution and recovery. The difference in BP(ND) between the two systems was approximately 30% in the neocortical/limbic regions. PVEc improved the agreement between the systems in particular for the neocortical/limbic regions. In these regions, the best agreement was found after applying Müller-Gärtner's PVEc. The demonstrated agreement provides an opportunity for combining data between the two systems in clinical studies aimed at evaluating receptor/transporter availability in cortical brain regions.

An Automated Quantification Tool for Angiogenic Sprouting From Endothelial Spheroids.

The process of sprouting angiogenesis can be measured in vitro using endothelial cells in sprouting assays such as the fibrin bead assay and the spheroid-based assay. While the technical aspects of these sprouting assays have been well-optimized, the analysis aspects have been limited to manual methods, which can be time-consuming and difficult to reproduce. Here, we developed an automated analysis tool called AQuTAS to quantify sprouting parameters from the spheroid-based sprouting assay. We trained and validated the algorithm on two subsets of data, and tested its sensitivity by measuring changes in sprouting parameters over a range of concentrations of pro- and antiangiogenic compounds. Our results demonstrate that the algorithm detects known differences in sprouting parameters in endothelial spheroids treated with pro- and antiangiogenic compounds. Moreover, it is sensitive to biological changes that are ≥40%. Among the five quantified parameters, cumulative sprout length is likely the most discriminative parameter for measuring differences in sprouting behavior because it had the highest effect size (>1.5 Cohen's d). In summary, we have generated an automated tool that quantifies sprouting parameters from the spheroid-based assay in a reproducible and sensitive manner.

In vivo correlation of serotonin transporter and 1B receptor availability in the human brain: a PET study.

Synaptic serotonin levels in the brain are regulated by active transport into the bouton by the serotonin transporter, and by autoreceptors, such as the inhibitory serotonin (5-HT) 1B receptor which, when activated, decreases serotonin release. Animal studies have shown a regulatory link between the two proteins. Evidence of such coupling could translate to an untapped therapeutic potential in augmenting the effect of selective serotonin reuptake inhibitors through pharmacological modulation of 5-HT1B receptors. Here we will for the first time in vivo examine the relationship between 5-HT1B receptors and serotonin transporters in the living human brain. Seventeen healthy individuals were examined with PET twice, using the radioligands [11C]AZ10419369 and [11C]MADAM for quantification of the 5-HT1B receptor and the 5-HT transporter, respectively. The binding potential was calculated for a set of brain regions, and the correlations between the binding estimates of the two radioligands were studied. [11C]AZ10419369 and [11C]MADAM binding was positively correlated in all examined brain regions. In most cortical regions the correlation was strong, e.g., frontal cortex, r(15) = 0.64, p = 0.01 and parietal cortex, r(15) = 0.8, p = 0.0002 while in most subcortical regions, negligible correlations was observed. Though the correlation estimates in cortex should be interpreted with caution due to poor signal to noise ratio of [11C]MADAM binding in these regions, it suggests a link between two key proteins involved in the regulation of synaptic serotonin levels. Our results indicate a need for further studies to address the functional importance of 5-HT1B receptors in treatment with drugs that inhibit serotonin reuptake.