Translocator Protein Genotype Research Articles

Brain inflammation co-localizes highly with tau in mild cognitive impairment due to early-onset Alzheimer's disease.

Brain inflammation, with an increased density of microglia and macrophages, is an important component of Alzheimer's disease (AD) and a potential therapeutic target. However, it is incompletely characterized, particularly in patients whose disease begins before the age of 65 years and, thus, have few co-pathologies. Inflammation has been usefully imaged with translocator protein (TSPO) positron emission tomography (PET), but most inflammation PET tracers cannot image subjects with a low-binder TSPO rs6971 genotype. In an important development, participants with any TSPO genotype can be imaged with a novel tracer, [11C]ER176, that has a high binding potential and a more favorable metabolite profile than other TSPO tracers currently available. We applied [11C]ER176 to detect brain inflammation in mild cognitive impairment (MCI) caused by early-onset AD. Furthermore, we sought to correlate the brain localization of inflammation, volume loss, elevated Aβ and tau. We studied brain inflammation in 25 patients with early-onset amnestic MCI (average age 59 ± 4.5 years, 10 women) and 23 healthy controls (average age 65 ± 6.0 years, 12 women), both groups with a similar proportion of all three TSPO-binding affinities. [11C]ER176 total distribution volume (VT), obtained with an arterial input function, was compared across patients and controls using voxel-wise and region-wise analyses. In addition to inflammation PET, most MCI patients had Aβ (n=23), and tau PET (n=21). For Aβ and tau tracers, standard uptake value ratios (SUVRs) were calculated using cerebellar grey matter as region of reference. Regional correlations among the three tracers were determined. Data were corrected for partial volume effect. Cognitive performance was studied with standard neuropsychological tools. In MCI caused by early-onset AD, there was inflammation in the default network, reaching statistical significance in precuneus and lateral temporal and parietal association cortex bilaterally, and in the right amygdala. Topographically, inflammation co-localized most strongly with tau (r= 0.63 ± 0.24). This correlation was higher than the co-localization of Aβ with tau (r= 0.55±0.25) and of inflammation with Aβ (0.43±0.22). Inflammation co-localized least with atrophy (-0.29±0.26). These regional correlations could be detected in participants with any of the three rs6971 TSPO polymorphisms. Inflammation in AD-related regions correlated with impaired cognitive scores. Our data highlight the importance of inflammation, a potential therapeutic target, in the AD process. Furthermore, they support the notion that, as shown in experimental tissue and animal models, the propagation of tau in humans is associated with brain inflammation.

Translocator protein (TSPO) genotype does not change cerebrospinal fluid levels of glial activation, axonal and synaptic damage markers in early Alzheimer’s disease

BackgroundPET imaging of the translocator protein (TSPO) is used to assess in vivo brain inflammation. One of the main methodological issues with this method is the allelic dependence of the radiotracer affinity. In Alzheimer’s disease (AD), previous studies have shown similar clinical and patho-biological profiles between TSPO genetic subgroups. However, there is no evidence regarding the effect of the TSPO genotype on cerebrospinal-fluid biomarkers of glial activation, and synaptic and axonal damage. MethodWe performed a trans-sectional study in early AD to compare cerebrospinal-fluid levels of GFAP, YKL-40, sTREM2, IL-6, IL-10, NfL and neurogranin between TSPO genetic subgroups. ResultsWe recruited 33 patients with early AD including 16 (48%) high affinity binders, 13 (39%) mixed affinity binders, and 4/33 (12%) low affinity binders. No difference was observed in terms of demographics, and cerebrospinal fluid levels of each biomarker for the different subgroups. ConclusionTSPO genotype is not associated with a change in glial activation, synaptic and axonal damage in early AD. Further studies with larger numbers of participants will be needed to confirm that the inclusion of specific TSPO genetic subgroups does not introduce selection bias in studies and trials of AD that combine TSPO imaging with cerebrospinal fluid biomarkers.

Pilot study with [18F]DPA-714 PET-CT to explore tumor-associated-macrophages in triple negative breast cancer.

e12557 Background: Triple-negative breast cancer (TNBC) tends to exhibit aggressive behavior lacking from targeted therapies. Tumor-associated-macrophages (TAMs as M2 and M1 macrophages) are interest targets in TNBC to approach the tumor patient immunity. The mitochondrial translocator protein (TSPO), a sensitive marker for macrophages, could be interesting for TNBC micro-environment stratification. We performed a multicenter imaging pilot study (NCT04320030) using non-invasive [18F]DPA-714 (DPA), a TSPO PET radioligand, aiming at assessing immunity to define the immunotherapy possibilities in TNBC patients. Methods: All patients underwent TSPO genotyping (LAB: Low Affinity Binder, MAB: Mixed and HAB: High), [18F]FDG (FDG) and DPA PET-CT, macrophages immunochemistry and in vitro TSPO autoradiography using both tritiated PK11195 (PK) and DPA. Thorax PET-CT was acquired just after injection (DPA1) to 30 min, then from 45 min (DPA2) to 60 min, followed by a whole body PET-CT. Groups of interest (Macrophages immunochemistry and Genotype) were compared using Kruskal-Wallis test. Correlation between continuous parameters were determined using Spearman’s Rho. Results: A total of 13 TNBC patients were included. No adverse events occurred after DPA injection. For TSPO genotyping, 2/13 (15.5%) patients were LAB, 6/13 (46%) MAB and 5/13 (38.5%) HAB. All of them showed FDG and DPA uptake regardless of the patient's genotype without significant difference between the 3 groups. The DPA kinetics showed a tracer accumulation between DPA1 and DPA2, whatever patient genotypes, observed on SUVmax (p = 0.0015), SUVmean (p = 0.0015) and TL-DPA (p = 0.0024). A correlation between FDG and DPA1 either DPA2 tumor volume was shown, respectively (p = 0.0252 and 0.0067) as well as between the TLG (FDG) and the TL-DPA1 (p = 0.0346) regardless of the patient's genotype. Macrophages immunochemistry showed a tendency difference for M2 percentage (p = 0.09) between the 3 genotypes groups. In vitro binding ratio (PK/DPA), measured for both radioligands, on adjacent tumor slices, was 1.77 (1.35-2.21). A difference in this binding ratio was observed between MAB and HAB (Mean 1.68±0.36) compared to LAB 3.04±0.23 (p = 0.0367), suggesting that DPA binding is sensitive to TSPO genotype, unlike PK binding. Conclusions: Despite the limited population, all TNBC tumor patients were DPA positive with an accumulation over time. The total glycolysis and total DPA uptake in the tumor were correlated, which may link tumor aggressiveness and DPA uptake. These data were confirmed by a better sensitivity for HAB and MAB vs LAB. DPA seemed to be a promising tracer to explore non-invasively TNBC patient’s immunity. Clinical trial information: NCT04320030.

Reduced microglia activity in patients with long-term immunosuppressive therapy after liver transplantation

PurposeCalcineurin inhibitors (CNI) can cause long-term impairment of brain function. Possible pathomechanisms include alterations of the cerebral immune system. This study used positron emission tomography (PET) imaging with the translocator protein (TSPO) ligand 18F-GE-180 to evaluate microglial activation in liver-transplanted patients under different regimens of immunosuppression.MethodsPET was performed in 22 liver-transplanted patients (3 CNI free, 9 with low-dose CNI, 10 with standard-dose CNI immunosuppression) and 9 healthy controls. The total distribution volume (VT) estimated in 12 volumes-of-interest was analyzed regarding TSPO genotype, CNI therapy, and cognitive performance.ResultsIn controls, VT was about 80% higher in high affinity binders (n = 5) compared to mixed affinity binders (n = 3). Mean VT corrected for TSPO genotype was significantly lower in patients compared to controls, especially in patients in whom CNI dose had been reduced because of nephrotoxic side effect.ConclusionOur results provide evidence of chronic suppression of microglial activity in liver-transplanted patients under CNI therapy especially in patients with high sensitivity to CNI toxicity.

Neuroinflammatory and functional connectivity signatures in radicular and axial chronic low back pain

We recently showed elevated levels of the 18kDa translocator protein (TSPO), a marker of neuroinflammation, in chronic low back pain patients (cLBP) compared to healthy controls(Loggia et al., 2015). Here, we test whether TSPO signal 1) can further subtype cLBP patients based on their clinical presentation, and 2) is associated with functional connectivity measures (because neuroinflammation may affect neuronal communication(Clark et al., 2015)). Patients with axial (cLBPAX; n=26; 43.7±16.6 y.o.) or radicular cLBP (cLBPRAD; n=28; 48.3±13.2 y.o.) received an integrated PET/MRI scan with the TSPO ligand [11C]PBR28. TSPO signal was quantified using standardized uptake values normalized by whole-brain signal (SUVR). Functional connectivity from primary somatosensory cortex (S1) (i.e., a region significant in the PET group comparison) was calculated from BOLD resting-state fMRI data. Connectivity and PET measures were compared across groups, correlated with and against each other and against the Fibromyalgia Screening Questionnaire (FSQ) scores, measure of pain “centralization”. Because two different scanners were used (BrainPET: 10 cLBPAX, 15 cLBPRAD; Biograph mMR: 16 cLBPAX, 13 cLBPRAD), scanner was included as a covariate in all analyses. Moreover, PET analyses also included Ala147Thr TSPO genotype, which predicts binding affinity to [11C]PBR28. In S1, TSPO signal and functional connectivity to the thalamus were: 1) higher in cLBPRAD compared to cLBPAX; 2) positively correlated with each other and 3) positively correlated with FSQ scores. Our data support the existence of different “neuroinflammatory signatures” in patients with different clinical presentation and that S1 neuroinflammatory signal is more pronounced in patients with higher pain “centralization”. Further, because S1 TSPO signal was correlated to S1-thalamus connectivity, our data support an association between neuroinflammatory changes and changes in neuronal communication, possibly indicating that the observed alterations reflect “neurogenic neuroinflammation”. Future studies are needed to determine whether neuroinflammation and related connectivity changes are possible therapeutic targets for chronic pain. 1R21NS087472-01A1; 1R01NS095937-01A; 1R01NS094306-01A1; W81XWH-14-1-0543.

Association of Early β-Amyloid Accumulation and Neuroinflammation Measured With [11C]PBR28 in Elderly Individuals Without Dementia.

ObjectiveTo examine whether early β–amyloid (Aβ) accumulation and metabolic risk factors are associated with neuroinflammation in elderly individuals without dementia.MethodsWe examined 54 volunteers (mean age 70.0 years, 56% women, 51% APOE ɛ4 carriers) with the translocator protein (TSPO) tracer [11C]PBR28 to assess neuroinflammation and with [11C] Pittsburgh compound B (PiB) to assess cerebral Aβ accumulation. [11C]PBR28 and [11C]PiB standardized uptake value ratios (SUVRs) were quantified in 6 regions of interests by using the cerebellar cortex as a pseudo-reference and reference region, respectively. Fasting venous glucose, insulin, and high-sensitivity C-reactive protein (hs-CRP) values were determined. Homeostatic model assessment of insulin resistance (HOMA-IR) was calculated. A subset of individuals (n = 11) underwent CSF sampling, and Aβ40, Aβ42, total tau, phospho-tau, soluble TREM2, and YKL-40 levels were measured.ResultsAmong the whole study group, no significant association was found between [11C]PiB and [11C]PBR28 SUVR composite scores (slope 0.02, p = 0.30). However, higher [11C]PiB binding was associated with higher [11C]PBR28 binding among amyloid-negative ([11C]PiB composite score ≤1.5) (TSPO genotype–, age- and sex-adjusted slope 0.26, p = 0.008) but not among amyloid-positive (slope −0.004, p = 0.88) participants. Higher CSF soluble TREM2 (rs = 0.72, p = 0.01) and YKL-40 (rs = 0.63, p = 0.04) concentrations were associated with a higher [11C]PBR28 composite score. Higher body mass index, HOMA-IR, and hs-CRP were associated with higher [11C]PBR28 binding in brain regions where Aβ accumulation is first detected in Alzheimer disease.ConclusionsWhile there was no association between amyloid and neuroinflammation in the overall study group, neuroinflammation was associated with amyloid among the subgroup at early stages of amyloid pathology.

TSPO polymorphism in individuals with alcohol use disorder: Association with cholesterol levels and withdrawal severity.

The translocator protein (TSPO) transports cholesterol into mitochondria and is involved in steroidogenesis. The TSPO polymorphism rs6971 influences binding of cholesterol and other TSPO ligands including positron-emission tomography (PET) imaging radiotracers. Although it is recognized that alcohol increases plasma high-density lipoproteins (HDLs), its effects on total cholesterol and triglycerides along with its relationship to TSPO genotype have not been assessed. Here, we evaluated whether plasma cholesterol and triglycerides are disrupted in alcohol use disorder (AUD) and their association with rs6971 in 932 AUD participants (DSM IV or 5) and 546 controls. AUD participants compared with controls had significantly higher plasma levels of total cholesterol, HDL, and triglycerides, but not of low-density lipoprotein (LDL). In the AUD group only, TSPO rs6971 had a significant effect on plasma levels of cholesterol, LDL, and triglycerides (AA (n = 62) > AG (n = 319) > GG (n = 551)), but not on HDL levels. Additionally, we showed a significant effect of TSPO rs6971 on withdrawal scores (Clinical Institute Withdrawal Assessment for Alcohol [CIWA]), with higher scores in AA (n = 50) compared with AG (n = 238) and GG (n = 428). CIWA scores in AUD participants correlated negatively with LDL and positively with HDL, but not with total cholesterol or triglycerides. These findings corroborate elevated plasma cholesterol and HDL levels in AUD and document significant increases in triglycerides. We also reveal for the first time an association in AUD participants between TSPO rs6971 genotype and plasma cholesterol, LDL, and triglyceride levels (not for HDL) and with withdrawal severity. Mediation analyses revealed that LDL (but not HDL) influenced the association between TSPO and alcohol withdrawal severity.

Effects of age, BMI and sex on the glial cell marker TSPO \u2014 a multicentre [11C]PBR28 HRRT PET study

PurposeThe 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.ResultsThere 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.ConclusionThese 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.

Disease activity in rheumatoid arthritis is inversely related to cerebral TSPO binding assessed by [11C]PBR28 positron emission tomography

Reumatoid Arthritis (RA) is an autoimmune disorder characterized by peripheral joint inflammation. Recently, an engagement of the brain immune system has been proposed. The aim with the current investigation was to study the glial cell activation marker translocator protein (TSPO) in a well characterized cohort of RA patients and to relate it to disease activity, peripheral markers of inflammation and autonomic activity.Fifteen RA patients and fifteen healthy controls matched for age, sex and TSPO genotype (rs6971) were included in the study. TSPO was measured using Positron emission tomography (PET) and the radioligand [11C]PBR28. The outcome measure was total distribution volume (VT) estimated using Logan graphical analysis, with grey matter (GM) as the primary region of interest. Additional regions of interest analyses as well as voxel-wise analyses were also performed. Clinical evaluation of disease activity, symptom assessments, serum analyses of cytokines and heart rate variability (HRV) analysis of 24 h ambulatory ECG were performed in all subjects.There were no statistically significant group differences in TSPO binding, either when using the primary outcome VT or when normalizing VT to the lateral occipital cortex (p > 0.05). RA patients had numerically lower VT values than healthy controls (Cohen's D for GM = −0.21). In the RA group, there was a strong negative correlation between [11C]PBR28 VT in GM and disease activity (DAS28)(r = −0.745, p = 0.002, corrected for rs6971 genotype).Higher serum levels of IFNγ and TNF-α were found in RA patients compared to controls (p < 0.05) and several measures of autonomic activity showed significant differences between RA and controls (p < 0.05). However, no associations between markers of systemic inflammation or autonomic activity and cerebral TSPO binding were found.In conclusion, no statistically significant group differences in TSPO binding as measured with [11C]PBR28 PET were detected. Within the RA group, lower cerebral TSPO binding was associated with higher disease activity, suggesting that cerebral TSPO expression may be related to disease modifying mechanisms in RA. In light of the earlier confirmed neuro-immune features of RA, these results warrant further investigations regarding neuro-immune joint-to-CNS signalling to open up for potentially new treatment strategies.

PET imaging of [11C]PBR28 in Parkinson\u2019s disease patients does not indicate increased binding to TSPO despite reduced dopamine transporter binding

PurposeTo examine the hypothesis that cerebral binding to the 18 kDa translocator protein (TSPO), a marker of microglia activation, is elevated in Parkinson’s disease (PD), and to assess the relationship between brain TSPO binding and dopaminergic pathology in PD.MethodsThe radioligand [11C]PBR28 was used for quantitative assessment of brain TSPO in 16 control subjects and 16 PD patients. To analyse the relationship between dopaminergic pathology and brain TSPO binding, PET studies of the dopamine transporter (DAT) were undertaken in PD patients using the DAT radioligand [18F]FE-PE2I. The total distribution volume of [11C]PBR28 was quantified in nigrostriatal regions, limbic cortices and thalamus, and the binding potential of [18F]FE-PE2I was quantified in nigrostriatal regions.ResultsBased on genotype analysis of the TSPO rs6971 polymorphism, 16 subjects (8 control subjects and 8 PD patients) were identified as high-affinity binders, and the remaining subjects were identified as mixed-affinity binders. A two-way ANOVA showed a strong main effect of TSPO genotype on the cerebral binding of [11C]PBR28, whereas no statistically significant main effect of diagnostic group, or a group by genotype interaction was found for any of the regions analysed. [18F]FE-PE2I PET studies in patients indicated a marked reduction in nigrostriatal binding to DAT. However, no correlations between the binding parameters were found for [11C]PBR28 and [18F]FE-PE2I.ConclusionThe findings do not support the hypothesis of elevated cerebral TSPO binding or a relationship between TSPO binding and dopaminergic pathology in PD.

Influence of alcoholism and cholesterol on TSPO binding in brain: PET [11C]PBR28 studies in humans and rodents

Neuroinflammation appears to contribute to neurotoxicity observed with heavy alcohol consumption. To assess whether chronic alcohol results in neuroinflammation we used PET and [11C]PBR28, a ligand that binds to the 18-kDa translocator protein (TSPO), to compare participants with an alcohol use disorder (AUD: n = 19) with healthy controls (HC: n = 17), and alcohol-dependent (n = 9) with -nondependent rats (n = 10). Because TSPO is implicated in cholesterol’s transport for steroidogenesis, we investigated whether plasma cholesterol levels influenced [11C]PBR28 binding. [11C]PBR28 binding did not differ between AUD and HC. However, when separating by TSPO genotype rs6971, we showed that medium-affinity binders AUD participants showed lower [11C]PBR28 binding than HC in regions of interest (whole brain, gray and white matter, hippocampus, and thalamus), but no group differences were observed in high-affinity binders. Cholesterol levels inversely correlated with brain [11C]PBR28 binding in combined groups, due to a correlation in AUD participants. In rodents, we observed no differences in brain [11C]PBR28 uptake between alcohol-dependent and -nondependent rats. These findings, which are consistent with two previous [11C]PBR28 PET studies, may indicate lower activation of microglia in AUD, whereas failure to observe alcohol effects in the rodent model indicate that species differences do not explain the discrepancy with prior rodent autoradiographic studies reporting increases in TSPO binding with chronic alcohol. However, reduced binding in AUD participants could also reflect competition from endogenous TSPO ligands such as cholesterol; and since the rs6971 polymorphism affects the cholesterol-binding domain of TSPO this could explain why differences were observed only in medium-affinity binders.

Lower levels of the glial cell marker TSPO in drug-naive first-episode psychosis patients as measured using PET and [11C]PBR28.

Several lines of evidence are indicative of a role for immune activation in the pathophysiology of schizophrenia. Nevertheless, studies using positron emission tomography (PET) and radioligands for the translocator protein (TSPO), a marker for glial activation, have yielded inconsistent results. Whereas early studies using a radioligand with low signal-to-noise in small samples showed increases in patients, more recent studies with improved methodology have shown no differences or trend-level decreases. Importantly, all patients investigated thus far have been on antipsychotic medication, and as these compounds may dampen immune cell activity, this factor limits the conclusions that can be drawn. Here, we examined 16 drug-naive, first-episode psychosis patients and 16 healthy controls using PET and the TSPO radioligand [11C]PBR28. Gray matter (GM) volume of distribution (VT) derived from a two-tissue compartmental analysis with arterial input function was the main outcome measure. Statistical analyses were performed controlling for both TSPO genotype, which is known to affect [11C]PBR28 binding, and gender. There was a significant reduction of [11C]PBR28 VT in patients compared with healthy controls in GM as well as in secondary regions of interest. No correlation was observed between GM VT and clinical or cognitive measures after correction for multiple comparisons. The observed decrease in TSPO binding suggests reduced numbers or altered function of immune cells in brain in early-stage schizophrenia.

Test-retest analysis of a non-invasive method of quantifying [(11)C]-PBR28 binding in Alzheimer's disease.

PurposeIn order to maximise the utility of [11C]-PBR28 for use in longitudinal studies and clinical trials in Alzheimer’s disease (AD), there is a need to develop non-invasive metrics of tracer binding that do not require arterial cannulation. Recent work has suggested that standardised uptake value (SUV)-based methods may be sensitive to changes in translocator protein (TSPO) levels associated with neurodegeneration. However, the test-retest reliability of these approaches in AD over a time period relevant for clinical trials is unknown. In this study, the test-retest reliability of three SUV-based metrics was assessed in AD patients over 12 weeks.MethodsFive patients with mild AD and the high-affinity binding TSPO genotype underwent two [11C]-PBR28 PET scans approximately 12 weeks apart. The test-retest reliability (TRR) of the unadjusted SUV, SUV relative to cerebellar grey matter (SUVRC) and SUV normalised to whole brain activity (SUVRWB) in nine cortical and limbic regions of interest was assessed using the absolute variability and the intraclass correlation coefficient.ResultsOf the three measures, SUVRWB performed best overall, showing low absolute variability (mean −0.13 %, SD 2.47 %) and high reliability (mean ICC = 0.83). Unadjusted SUV also performed well, with high reliability (ICC = 0.94) but also high variability (mean −1.24 %, SD 7.28 %). By comparison, the SUVRC showed higher variability (mean −3.98 %, SD 7.07 %) and low reliability (ICC = 0.65).ConclusionsIn this AD sample, we found that SUV-derived metrics of [11C]-PBR28 binding showed high stability over 12 weeks. These results compare favourably with studies reporting TRR of absolute quantification of [11C]-PBR28. Pending further validation of SUV-based measures of [11C]-PBR28, semi-quantitative methods of [11C]-PBR28 analysis may prove useful in longitudinal studies of AD.Electronic supplementary materialThe online version of this article (doi:10.1186/s13550-016-0226-3) contains supplementary material, which is available to authorized users.

Abstract # 1847 Cross-sectional and longitudinal associations between immune cells in blood and brain – A TSPO PET study in healthy control subjects

The interaction between immune cells in the periphery and brain in humans is poorly understood, partly due to the lack of in vivo studies. Here, we examined 32 healthy individuals using positron emission tomography (PET) and [11C]PBR28, a radioligand for the 18-kDa translocator protein (TSPO) which is expressed in immune cells both in brain and blood. In 26 individuals, two measurements were performed. In a subgroup of 19 individuals, of which 12 had repeat examinations, leukocyte numbers in blood was measured on each day of PET measurements. We assessed TSPO binding expressed as total distribution volume of [11C]PBR28 in brain and in blood cells. TSPO binding in brain was strongly and positively correlated to binding in blood cells at baseline (r = .85, p = 2.1 × 10−9, corrected for TSPO genotype). A correlation was also observed when analyzing change in TSPO levels in brain and blood between two PET examinations (r = .60, p = .002). Finally, there was a significant correlation between change of leukocyte numbers and change in brain TSPO binding, and a trend-level correlation to TSPO change in blood cells (r = .63, p = .038; r = .6, p = .052). These findings indicate a functional association between immunological cells in blood and brain at physiological conditions, such as interchange of peripherally derived cells or a common regulatory mechanism.

Can Studies of Neuroinflammation in a TSPO Genetic Subgroup (HAB or MAB) Be Applied to the Entire AD Cohort?

Neuroinflammation plays a significant role in Alzheimer disease (AD), and translocator protein (TSPO) PET imaging allows us to quantify this process. However, the binding of second-generation TSPO tracers depends on the TSPO genotype coded by the rs6971 single-nucleotide polymorphism, with a 40%-50% increase in BP in high-affinity binders (HABs) compared with mixed-affinity binders (MABs), whereas low-affinity binders (LABs) are unsuitable for evaluation. Hence, several studies are using either HAB alone or HAB and MAB subjects. To translate the findings of neuroinflammation studies to the entire population, it is crucial to establish the influence of TSPO genotypes on AD. Here, we investigated whether different TSPO genotypes influence cognitive function, amyloid load, and disease progression over time. We evaluated 798 subjects (225 control, 388 with mild cognitive impairment [MCI], and 185 with AD) from the Alzheimer's Disease Neuroimaging Initiative database at baseline and during follow-up. All subjects were screened for TSPO genotype and underwent detailed clinical and neuropsychologic assessments yearly for 4 y. Of the 798 subjects, 255 also had T1- and T2-weighted MR imaging and amyloid PET with (11)C-Pittsburgh compound B or (18)F-florbetapir. We demonstrated that all TSPO binding groups (HAB, MAB, and LAB) have same level of amyloid load in AD and MCI subjects. We also demonstrated that the prevalence is 50.3% for HAB, 41.2% for MAB, and 8.5% for LAB, without a statistical difference among the AD, MCI, and control groups. During longitudinal follow-up, the mean change in neuropsychometric test scores on the Mini-Mental State Examination, the cognitive and modified Alzheimer Disease Assessment Scales (ADASs), and the Geriatric Depression Scale over time were similar in AD and MCI subjects among the 3 TSPO binding groups. Analysis of the covariates showed that diagnostic group (control, MCI, AD), apolipoprotein E4 status, and sex had a significant effect on decline on the modified Alzheimer Disease Assessment Scale (>3 points of the scale), but age and TSPO genotype did not. This study suggests that information obtained from evaluating a subgroup of AD or MCI subject using second-generation TSPO tracers can be translated to the entire AD and MCI population. Thus, we can study fewer AD subjects in evaluating new antineuroinflammatory and antimicroglial agents in intervention studies and in observational studies evaluating the role of neuroinflammation.

In vivo quantification of cerebral translocator protein binding in humans using 6-chloro-2-(4'-123I-iodophenyl)-3-(N,N-diethyl)-imidazo[1,2-a]pyridine-3-acetamide SPECT.

This study provides the first comprehensive quantification of translocator protein (TSPO) binding using SPECT and 6-chloro-2-(4'-(123)I-iodophenyl)-3-(N,N-diethyl)-imidazo[1,2-a]pyridine-3-acetamide ((123)I-CLINDE) in neurologic patients. (123)I-CLINDE is structurally related to well-known PET ligands such as (18)F-PBR111 and (18)F-DPA-714. Six patients with cerebral stroke and 4 patients with glioblastoma multiforme (GBM) underwent 150-min dynamic SPECT scans with arterial blood sampling. Four of the patients were rescanned. All patients were genotyped for the rs6971 polymorphism. Volumes of interest were delineated on the individual SPECT scans and the coregistered MR images. Compartmental and graphical models using arterial input or the cerebellum as a reference region were used to quantify (123)I-CLINDE binding. Among the 6 models investigated, the 2-tissue-compartment model with arterial input described the time-activity data best. Time-stability analyses suggested that acquisition time should be at least 90 min. Intersubject variation in the cerebellar distribution volume (VT) was clearly related to the TSPO genotype. In the stroke patients the VT in the periinfarction zone, compared with VT in the ipsilateral cerebellum, ranged from 1.4 to 3.4, and in the GBM patients the VT in the tumor, compared with the VT in the cerebellum, ranged from 1.8 to 3.4. In areas of gadolinium extravasation, (123)I-CLINDE binding parameters were not significantly changed. Thus, (123)I-CLINDE binding does not appear to be importantly affected by blood-brain barrier disruption. As demonstrated within a group of stroke and GBM patients, (123)I-CLINDE SPECT can be used for quantitative assessment of TSPO expression in vivo. Because of the absence of a region devoid of TSPO, reference tissue models should be used with caution. The 2-tissue-compartment kinetic analysis of a 90-min dynamic scan with arterial blood sampling is recommended for the quantification of (123)I-CLINDE binding with SPECT.

145. Translocator protein (TSPO) as a biomarker of microglia activation: PET imaging using [11C]PBR28 in Parkinson’s disease

Positron emission tomography (PET) studies in Parkinson’s’ disease (PD) using the radioligand [11C]PK11195 have demonstrated increased binding to TSPO, in particular in the nigrostriatal system. However, due to high non-specific binding [11C]PK11195 is not ideal for quantitative analysis. In the present study we used the new radioligand [11C]PBR28 in 24 patients with PD, following confirmation of the diagnosis by PET imaging of the dopamine transporter ([18F]FE-PE2I). [11C]PBR28 data was quantified by linear graphic analysis, using an arterial input function and wavelet aided parametric imaging. DNA polymorphism for TSPO (A147T) was controlled for by genetic analysis. The total distribution volume, VT, was similar across brain regions with slightly higher levels in the thalamus. The mean VT values ranged from 2.60 in the substantia nigra to 3.54 in the thalamus. The inter-subject variability in VT was high and could be related to TSPO genotype. There were no correlations between [11C]PBR28 and [18F]FE-PE2I binding. The radioligand [11C]PBR28 shows suitable characteristics for quantitative imaging of TSPO binding in brain. For clinical applications, i.e. studies on disease pathophysiology, TSPO genotyping is needed. Preliminary analysis suggests that patients with PD may not have conspicuous upregulation of TSPO in the nigrostriatal system. Note: The data presented are a subset of results from a double-blind, placebo-controlled study to assess the effect of treatment with AZD3241 on microglia activation using PET.

A Genetic Polymorphism for Translocator Protein 18 Kda Affects bothin Vitroandin VivoRadioligand Binding in Human Brain to this Putative Biomarker of Neuroinflammation

Second-generation radioligands for translocator protein (TSPO), an inflammation marker, are confounded by the codominant rs6971 polymorphism that affects binding affinity. The resulting three groups are homozygous for high-affinity state (HH), homozygous for low-affinity state (LL), or heterozygous (HL). We tested if in vitro binding to leukocytes distinguished TSPO genotypes and if genotype could affect clinical studies using the TSPO radioligand [(11)C]PBR28. In vitro binding to leukocytes and [(11)C]PBR28 brain imaging were performed in 27 human subjects with known TSPO genotype. Specific [(3)H]PBR28 binding was measured in prefrontal cortex of 45 schizophrenia patients and 47 controls. Leukocyte binding to PBR28 predicted genotype in all subjects. Brain uptake was ∼40% higher in HH than HL subjects. Specific [(3)H]PBR28 binding in LL controls was negligible, while HH controls had ∼80% higher binding than HL controls. After excluding LL subjects, specific binding was 16% greater in schizophrenia patients than controls. This difference was insignificant by itself (P=0.085), but was significant after correcting for TSPO genotype (P=0.011). Our results show that TSPO genotype influences PBR28 binding in vitro and in vivo. Correcting for this genotype increased statistical power in our postmortem study and is recommended for in vivo positron emission tomography studies.