Objective-To determine whether microglial activity, measured using translocator-protein positron emission tomographic imaging (PET), is increased in unmedicated subjects presenting with sub-clinical symptoms indicating they are at ultra high risk of psychosis, and to determine if it is elevated in schizophrenia after controlling for a translocator specific genetic polymorphism.Method-Here we use the second generation radioligand [ 11 C]PBR28 and PET to image microglial activity in the brains of subjects at ultra high risk for psychosis. Subjects were recruited from early intervention centres. We also imaged a cohort of patients with schizophrenia and healthy controls for comparison, in total 56 subjects completed the study. At screening, subjects were genotyped to account for the rs6971 polymorphism in the gene encoding the 18Kd Translocator Protein. The main outcome measure was total grey matter [ 11 C]PBR28 binding ratio, representing microglial activity. Conclusion-Microglial activity is elevated in schizophrenia and in subjects with sub-clinical symptoms who are at ultra high risk of psychosis, and is related to at risk symptom severity. This indicates that neuroinflammation is linked to the risk of psychosis and related disorders, and the expression of sub-clinical symptoms. Follow up of ultra high risk subjects will determine whether this is specific to the later development of schizophrenia or risk factors in general. Results-[

The 18-kDA translocator protein (TSPO) is consistently elevated in activated microglia of the central nervous system (CNS) in response to a variety of insults as well as neurodegenerative and psychiatric conditions. It is therefore a target of interest for molecular strategies aimed at imaging neuroinflammation in vivo. For more than 20 years, positron emission tomography (PET) has allowed the imaging of TSPO density in brain using [11C]-(R)-PK11195, a radiolabelled-specific antagonist of the TSPO that has demonstrated microglial activation in a large number pathological cohorts. The significant clinical interest in brain immunity as a primary or comorbid factor in illness has sparked great interest in the TSPO as a biomarker and a surprising number of second generation TSPO radiotracers have been developed aimed at improving the quality of TSPO imaging through novel radioligands with higher affinity. However, such major investment has not yet resulted in the expected improvement in image quality. We here review the main methodological aspects of TSPO PET imaging with particular attention to TSPO genetics, cellular heterogeneity of TSPO in brain tissue and TSPO distribution in blood and plasma that need to be considered in the quantification of PET data to avoid spurious results as well as ineffective development and use of these radiotracers.

These findings are consistent with a transdiagnostic role for dopamine dysfunction in the pathoetiology of psychosis and suggest dopamine synthesis capacity as a potential novel drug target for bipolar disorder and schizophrenia.

The positron emission tomography radioligand [ 11 C]PBR28 targets translocator protein (18 kDa) (TSPO) and is a potential marker of neuroinflammation. [ 11 C]PBR28 binding is commonly quantified using a two-tissue compartment model and an arterial input function. Previous studies with [ 11 C]-(R)-PK11195 demonstrated a slow irreversible binding component to the TSPO proteins localized in the endothelium of brain vessels, such as venous sinuses and arteries. However, the impact of this component on the quantification of [ 11 C]PBR28 data has never been investigated. In this work we propose a novel kinetic model for [ 11 C]PBR28. This model hypothesizes the existence of an additional irreversible component from the blood to the endothelium. The model was tested on a data set of 19 healthy subjects. A simulation was also performed to quantify the error generated by the standard twotissue compartmental model when the presence of the irreversible component is not taken into account. Our results show that when the vascular component is included in the model the estimates that include the vascular component (2TCM-1K) are more than three-fold smaller, have a higher time stability and are better correlated to brain mRNA TSPO expression than those that do not include the model (2TCM).

BackgroundConverging lines of evidence implicate an important role for the immune system in schizophrenia. Microglia are the resident immune cells of the central nervous system and have many functions including neuroinflammation, axonal guidance and neurotrophic support. We aimed to provide a quantitative review of in vivo PET imaging studies of microglia activation in patients with schizophrenia compared with healthy controls.MethodsDemographic, clinical and imaging measures were extracted from each study and meta-analysis was conducted using a random-effects model (Hedge's g). The difference in 18-kDa translocator protein (TSPO) binding between patients with schizophrenia and healthy controls, as quantified by either binding potential (BP) or volume of distribution (VT), was used as the main outcome. Sub-analysis and sensitivity analysis were carried out to investigate the effects of genotype, ligand and illness stage.ResultsIn total, 12 studies comprising 190 patients with schizophrenia and 200 healthy controls met inclusion criteria. There was a significant elevation in tracer binding in schizophrenia patients relative to controls when BP was used as an outcome measure, (Hedge's g = 0.31; p = 0.03) but no significant differences when VT was used (Hedge's g = −0.22; p = 0.29).ConclusionsIn conclusion, there is evidence for moderate elevations in TSPO tracer binding in grey matter relative to other brain tissue in schizophrenia when using BP as an outcome measure, but no difference when VT is the outcome measure. We discuss the relevance of these findings as well as the methodological issues that may underlie the contrasting difference between these outcomes.

Psychotic illnesses show variable responses to treatment. Determining the neurobiology underlying this is important for precision medicine and the development of better treatments. It has been proposed that dopaminergic differences underlie variation in response, with striatal dopamine synthesis capacity (DSC) elevated in responders and unaltered in nonresponders. We therefore aimed to test this in a prospective cohort, with a nested case-control comparison. 40 volunteers (26 patients with first-episode psychosis and 14 controls) received an 18 F-DOPA Positron Emission Tomography scan to measure DSC (Ki cer ) prior to antipsychotic treatment. Clinical assessments (Positive and Negative Syndrome Scale, PANSS, and Global Assessment of Functioning, GAF) occurred at baseline and following antipsychotic treatment for a minimum of 4 weeks. Response was defined using improvement in PANSS Total score of >50%. Patients were followed up for at least 6 months, and remission criteria applied. There was a significant effect of group on Ki cer in associative striatum (F (2, 37) = 7.9, p = 0.001). Ki cer was significantly higher in responders compared with non-responders (Cohen's d = 1.55, p = 0.01) and controls (Cohen's d = 1.31, p = 0.02). Ki cer showed significant positive correlations with improvements in PANSS-positive (r = 0.64, p < 0.01), PANSS negative (rho = 0.51, p = 0.01), and PANSS total (rho = 0.63, p < 0.01) ratings and a negative relationship with change in GAF (r = −0.55, p < 0.01). Clinical response is related to baseline striatal dopaminergic function. Differences in dopaminergic function between responders and non-responders are present at first episode of psychosis, consistent with dopaminergic and non-dopaminergic sub-types in psychosis, and potentially indicating a neurochemical basis to stratify psychosis.

Objective:To image β-amyloid (Aβ) plaque burden in long-term survivors of traumatic brain injury (TBI), test whether traumatic axonal injury and Aβ are correlated, and compare the spatial distribution of Aβ to Alzheimer disease (AD).Methods:Patients 11 months to 17 years after moderate–severe TBI underwent 11C-Pittsburgh compound B (11C-PiB)-PET, structural and diffusion MRI, and neuropsychological examination. Healthy aged controls and patients with AD underwent PET and structural MRI. Binding potential (BPND) images of 11C-PiB, which index Aβ plaque density, were computed using an automatic reference region extraction procedure. Voxelwise and regional differences in BPND were assessed. In TBI, a measure of white matter integrity, fractional anisotropy, was estimated and correlated with 11C-PiB BPND.Results:Twenty-eight participants (9 with TBI, 9 controls, 10 with AD) were assessed. Increased 11C-PiB BPND was found in TBI vs controls in the posterior cingulate cortex and cerebellum. Binding in the posterior cingulate cortex increased with decreasing fractional anisotropy of associated white matter tracts and increased with time since injury. Compared to AD, binding after TBI was lower in neocortical regions but increased in the cerebellum.Conclusions:Increased Aβ burden was observed in TBI. The distribution overlaps with, but is distinct from, that of AD. This suggests a mechanistic link between TBI and the development of neuropathologic features of dementia, which may relate to axonal damage produced by the injury.

BackgroundQuantitative in vivo imaging of myelin loss and repair in patients with multiple sclerosis (MS) is essential to understand the pathogenesis of the disease and to evaluate promyelinating therapies. Selectively binding myelin in the central nervous system white matter, Pittsburgh compound B ([11C]PiB) can be used as a positron emission tomography (PET) tracer to explore myelin dynamics in MS.MethodsPatients with active relapsing‐remitting MS (n = 20) and healthy controls (n = 8) were included in a longitudinal trial combining PET with [11C]PiB and magnetic resonance imaging. Voxel‐wise maps of [11C]PiB distribution volume ratio, reflecting myelin content, were derived. Three dynamic indices were calculated for each patient: the global index of myelin content change; the index of demyelination; and the index of remyelination.ResultsAt baseline, there was a progressive reduction in [11C]PiB binding from the normal‐appearing white matter to MS lesions, reflecting a decline in myelin content. White matter lesions were characterized by a centripetal decrease in the tracer binding at the voxel level. During follow‐up, high between‐patient variability was found for all indices of myelin content change. Dynamic remyelination was inversely correlated with clinical disability (p = 0.006 and beta‐coefficient = –0.67 with the Expanded Disability Status Scale; p = 0.003 and beta‐coefficient = –0.68 with the MS Severity Scale), whereas no significant clinical correlation was found for the demyelination index.Interpretation[11C]PiB PET allows quantification of myelin dynamics in MS and enables stratification of patients depending on their individual remyelination potential, which significantly correlates with clinical disability. This technique should be considered to assess novel promyelinating drugs. Ann Neurol 2016;79:726–738