Nicola Kalk

Determination of [11C]PBR28 binding potential in vivo: a first human TSPO blocking study

Positron emission tomography (PET) targeting the 18 kDa translocator protein (TSPO) is used to quantify neuroinflammation. Translocator protein is expressed throughout the brain, and therefore a classical reference region approach cannot be used to estimate binding potential (BP ND ). Here, we used blockade of the TSPO radioligand [11C]PBR28 with the TSPO ligand XBD173, to determine the non-displaceable volume of distribution (V ND ), and hence estimate the BP ND . A total of 26 healthy volunteers, 16 high-affinity binders (HABs) and 10 mixed affinity binders (MABs) underwent a [11C]PBR28 PET scan with arterial sampling. Six of the HABs received oral XBD173 (10 to 90 mg), 2 hours before a repeat scan. In XBD173-dosed subjects, V ND was estimated via the occupancy plot. Values of BP ND for all subjects were calculated using this V ND estimate. Total volume of distribution (V T ) of MABs (2.94 ± 0.31) was lower than V T of HABs (4.33 ± 0.29) (P<0.005). There was dose-dependent occupancy of TSPO by XBD173 (ED50 = 0.34 ± 0.13 mg/kg). The occupancy plot provided a V ND estimate of 1.98 (1.69, 2.26). Based on these V ND estimates, BP ND for HABs is approximately twice that of MABs, consistent with predictions from in vitro data. Our estimates of [11C]PBR28 V ND and hence BP ND in the healthy human brain are consistent with in vitro predictions. XBD173 blockade provides a practical means of estimating V ND for TSPO targeting radioligands.

The role of central noradrenergic dysregulation in anxiety disorders: evidence from clinical studies

The nature of the noradrenergic dysregulation in clinical anxiety disorders remains unclear. In panic disorder, the predominant view has been that central noradrenergic neuronal networks and/or the sympathetic nervous system was normal in patients at rest, but hyper-reactive to specific stimuli, for example carbon dioxide. These ideas have been extended to other anxiety disorders, which share with panic disorder characteristic subjective anxiety and physiological symptoms of excess sympathetic activity. For example, Generalized Anxiety Disorder is characterized by chronic free-floating anxiety, muscle tension, palpitation and insomnia. It has been proposed that there is chronic central hypersecretion of noradrenaline in Generalized Anxiety Disorder, with consequent hyporesponsiveness of central post-synaptic receptors. With regards to other disorders, it has been suggested that there is noradrenergic involvement or derangement, but a more specific hypothesis has not been enunciated. This paper reviews the evidence for noradrenergic dysfunction in anxiety disorders, derived from indirect measures of noradrenergic function in clinical populations.

Determination of |[lsqb]|11C|[rsqb]|PBR28 binding potential in vivo: a first human TSPO blocking study

Positron emission tomography (PET) targeting the 18 kDa translocator protein (TSPO) is used to quantify neuroinflammation. Translocator protein is expressed throughout the brain, and therefore a classical reference region approach cannot be used to estimate binding potential (BP ND ). Here, we used blockade of the TSPO radioligand [11C]PBR28 with the TSPO ligand XBD173, to determine the non-displaceable volume of distribution (V ND ), and hence estimate the BP ND . A total of 26 healthy volunteers, 16 high-affinity binders (HABs) and 10 mixed affinity binders (MABs) underwent a [11C]PBR28 PET scan with arterial sampling. Six of the HABs received oral XBD173 (10 to 90 mg), 2 hours before a repeat scan. In XBD173-dosed subjects, V ND was estimated via the occupancy plot. Values of BP ND for all subjects were calculated using this V ND estimate. Total volume of distribution (V T ) of MABs (2.94 ± 0.31) was lower than V T of HABs (4.33 ± 0.29) (P<0.005). There was dose-dependent occupancy of TSPO by XBD173 (ED50 = 0.34 ± 0.13 mg/kg). The occupancy plot provided a V ND estimate of 1.98 (1.69, 2.26). Based on these V ND estimates, BP ND for HABs is approximately twice that of MABs, consistent with predictions from in vitro data. Our estimates of [11C]PBR28 V ND and hence BP ND in the healthy human brain are consistent with in vitro predictions. XBD173 blockade provides a practical means of estimating V ND for TSPO targeting radioligands.

Acute increases in synaptic GABA detectable in the living human brain: A [11C]Ro15-4513 PET study

The inhibitory γ-aminobutyric acid (GABA) neurotransmitter system is associated with the regulation of normal cognitive functions and dysregulation has been reported in a number of neuropsychiatric disorders including anxiety disorders, schizophrenia and addictions. Investigating the role of GABA in both health and disease has been constrained by difficulties in measuring acute changes in synaptic GABA using neurochemical imaging. The aim of this study was to investigate whether acute increases in synaptic GABA are detectable in the living human brain using the inverse agonist GABA-benzodiazepine receptor (GABA-BZR) positron emission tomography (PET) tracer, [(11)C]Ro15-4513. We examined the effect of 15 mg oral tiagabine, which increases synaptic GABA by inhibiting the GAT1 GABA uptake transporter, on [(11)C]Ro15-4513 binding in 12 male participants using a paired, double blind, placebo-controlled protocol. Spectral analysis was used to examine synaptic α1 and extrasynaptic α5 GABA-BZR subtype availability in brain regions with high levels of [(11)C]Ro15-4513 binding. We also examined the test-retest reliability of α1 and a5-specific [(11)C]Ro15-4513 binding in a separate cohort of 4 participants using the same spectral analysis protocol. Tiagabine administration produced significant reductions in hippocampal, parahippocampal, amygdala and anterior cingulate synaptic α1 [(11)C]Ro15-4513 binding, and a trend significance reduction in the nucleus accumbens. These reductions were greater than test-retest reliability, indicating that they are not the result of chance observations. Our results suggest that acute increases in endogenous synaptic GABA are detectable in the living human brain using [(11)C]Ro15-4513 PET. These findings have potentially major implications for the investigation of GABA function in brain disorders and in the development of new treatments targeting this neurotransmitter system.

Neuroinflammation in treated HIV-positive individuals A TSPO PET study

Objective: To explore the effects of microglial activation on brain function and structure, and its relationship with peripheral inflammatory markers, in treated, HIV-positive individuals, using in vivo [11C]PBR28 PET (to measure the 18 kDa translocator protein [TSPO]). Methods: Cognitively healthy HIV-positive individuals on suppressive antiretroviral therapy and HIV-negative individuals (controls) underwent brain [11C]PBR28 PET and MRI. HIV-positive patients completed neuropsychological testing and CSF testing for chemokines. The concentration of bacterial ribosomal 16sDNA in plasma was measured as a marker of microbial translocation. Results: HIV-positive individuals showed global increases in TSPO expression compared to controls (corrected p < 0.01), with significant regional increases in the parietal (p = 0.001) and occipital (p = 0.046) lobes and in the globus pallidus (p = 0.035). TSPO binding in the hippocampus, amygdala, and thalamus were associated with poorer global cognitive performance in tasks assessing verbal and visual memory (p < 0.05). Increased TSPO binding was associated with increased brain white matter diffusion MRI mean diffusivity in HIV-positive individuals, a lower CD4/CD8 ratio, and both high pretreatment HIV RNA and plasma concentration ribosomal 16s DNA (p < 0.05). Conclusions: Cognitively healthy HIV-positive individuals show evidence for a chronically activated brain innate immune response and elevated blood markers of microbial translocation despite effective control of plasma viremia. Increased brain inflammation is associated with poorer cognitive performance and white matter microstructural pathology, suggesting a possible role in cognitive impairments found in some HIV-positive patients despite effective treatment.

The clinical pharmacology of acamprosate.

Acamprosate is one of the few medications licensed for prevention of relapse in alcohol dependence, and over time it has proved to be significantly, if moderately, effective, safe and tolerable. Its use is now being extended into other addictions and neurodevelopmental disorders. The mechanism of action of acamprosate has been less clear, but in the decade or more that has elapsed since its licensing, a body of translational evidence has accumulated, in which preclinical findings are replicated in clinical populations. Acamprosate modulates N‐methyl‐d‐aspartic acid receptor transmission and may have indirect effects on γ‐aminobutyric acid type A receptor transmission. It is known to decrease brain glutamate and increase β‐endorphins in rodents and man. Acamprosate diminishes reinstatement in ethanolized rodents and promotes abstinence in humans. Although acamprosate has been called an anticraving drug, its subjective effects are subtle and relate to diminished arousal, anxiety and insomnia, which parallel preclinical findings of decreased withdrawal symptoms in animals treated with acamprosate. Further understanding of the pharmacology of acamprosate will allow appropriate targeting of therapy in individuals with alcohol dependence and extension of its use to other addictions.

Measurement of GABA using J‐difference edited 1H‐MRS following modulation of synaptic GABA concentration with tiagabine

Though GABA is the major inhibitory neurotransmitter in the brain, involved in a wide variety of brain functions and many neuropsychiatric disorders, its intracellular and metabolic presence provides uncertainty in the interpretation of the GABA signal measured by 1H‐MRS. Previous studies demonstrating the sensitivity of this technique to pharmacological manipulations of GABA have used nonspecific challenges that make it difficult to infer the exact source of the changes. In this study, the synaptic GABA reuptake inhibitor tiagabine, which selectively blocks GAT1, was used to test the sensitivity of J‐difference edited 1H‐MRS to changes in extracellular GABA concentrations. MEGA‐PRESS was used to obtain GABA‐edited spectra in 10 male individuals, before and after a 15‐mg oral dose of tiagabine. In the three voxels measured, no significant changes were found in GABA+ concentration after the challenge compared to baseline. This dose of tiagabine is known to modulate synaptic GABA and neurotransmission through studies using other imaging modalities, and significant increases in self‐reported sleepiness scales were observed. Therefore, it is concluded that recompartmentalization of GABA through transport block does not have a significant impact on total GABA concentration. Furthermore, it is likely that the majority of the magnetic resonance spectroscopy (MRS)‐derived GABA signal is intracellular. It should be considered, in individual interpretation of GABA MRS studies, whether it is appropriate to attribute observed effects to changes in neurotransmission. Synapse 68:355–362, 2014.

Characterisation of the contribution of the GABA-benzodiazepine α1 receptor subtype to [11C]Ro15-4513 PET images

This positron emission tomography (PET) study aimed to further define selectivity of [11C]Ro15-4513 binding to the GABARα5 relative to the GABARα1 benzodiazepine receptor subtype. The impact of zolpidem, a GABARα1-selective agonist, on [11C]Ro15-4513, which shows selectivity for GABARα5, and the nonselective benzodiazepine ligand [11C]flumazenil binding was assessed in humans. Compartmental modelling of the kinetics of [11C]Ro15-4513 time-activity curves was used to describe distribution volume (VT) differences in regions populated by different GABA receptor subtypes. Those with low α5 were best fitted by one-tissue compartment models; and those with high α5 required a more complex model. The heterogeneity between brain regions suggested spectral analysis as a more appropriate method to quantify binding as it does not a priori specify compartments. Spectral analysis revealed that Zolpidem caused a significant VT decrease (~10%) in [11C]flumazenil, but no decrease in [11C]Ro15-4513 binding. Further analysis of [11C]Ro15-4513 kinetics revealed additional frequency components present in regions containing both α1 and α5 subtypes compared with those containing only α1. Zolpidem reduced one component (mean ± s.d.: 71% ± 41%), presumed to reflect α1-subtype binding, but not another (13% ± 22%), presumed to reflect α5. The proposed method for [11C]Ro15-4513 analysis may allow more accurate selective binding assays and estimation of drug occupancy for other nonselective ligands.

Noradrenergic function in generalized anxiety disorder: impact of treatment with venlafaxine on the physiological and psychological responses to clonidine challenge.

Serotonin and noradrenaline reuptake inhibitor (SNRI) antidepressants have evidence of efficacy in the treatment of generalized anxiety disorder (GAD); however, it is not clear whether there is an advantage over selective serotonin reuptake inhibitor (SSRI) medicines and there is limited evidence for noradrenergic dysfunction in GAD. We tested whether a dysfunctional alpha-2 adrenoceptor system is present in patients with GAD and the effects of SNRI treatment on this system. The method used was an infusion of clonidine (a selective alpha-2 adrenergic receptor agonist) on psychological and physiological outcomes in three subject groups: 10 untreated GAD patients, five SNRI-treated GAD patients and seven normal controls. The clonidine challenge elicited sedation, a rise in growth hormone, decrease in blood pressure, decline in saccadic eye movement (SEM) variables, and improvement in verbal fluency as anticipated in the 22 subjects examined. Lower cortisol levels were found in controls and higher blood pressure readings in GAD-treated subjects, as well as evidence that GAD-treated subjects had SEMs that were intermediate between control and GAD subjects’ scores and have less clonidine-induced sedation. The implications of these findings with reference to the study hypothesis in this small study are discussed.

The Imperial College Cambridge Manchester (ICCAM) platform study: An experimental medicine platform for evaluating new drugs for relapse prevention in addiction. Part A: Study description.

Drug and alcohol dependence are global problems with substantial societal costs. There are few treatments for relapse prevention and therefore a pressing need for further study of brain mechanisms underpinning relapse circuitry. The Imperial College Cambridge Manchester (ICCAM) platform study is an experimental medicine approach to this problem: using functional magnetic resonance imaging (fMRI) techniques and selective pharmacological tools, it aims to explore the neuropharmacology of putative relapse pathways in cocaine, alcohol, opiate dependent, and healthy individuals to inform future drug development. Addiction studies typically involve small samples because of recruitment difficulties and attrition. We established the platform in three centres to assess the feasibility of a multisite approach to address these issues. Pharmacological modulation of reward, impulsivity and emotional reactivity were investigated in a monetary incentive delay task, an inhibitory control task, and an evocative images task, using selective antagonists for µ-opioid, dopamine D3 receptor (DRD3) and neurokinin 1 (NK1) receptors (naltrexone, GSK598809, vofopitant/aprepitant), in a placebo-controlled, randomised, crossover design. In two years, 609 scans were performed, with 155 individuals scanned at baseline. Attrition was low and the majority of individuals were sufficiently motivated to complete all five sessions (n=87). We describe herein the study design, main aims, recruitment numbers, sample characteristics, and explain the test hypotheses and anticipated study outputs.