First evaluation of PET-based human biodistribution and radiation dosimetry of 11C-BU99008, a tracer for imaging the imidazoline2 binding site

BACKGROUND

We measured whole body distribution of 11C-BU99008, a new PET biomarker for non-invasive identification of the imidazoline2 binding site. The purpose of this phase I study was to evaluate the biodistribution and radiation dosimetry of 11C-BU99008 in healthy human subjects.

METHODS

A single bolus injection of 11C-BU99008 (296 ± 10.5 MBq) was administered to four healthy subjects who underwent whole-body PET/CT over 120 min from the cranial vertex to the mid-thigh. Volumes of interest were drawn around visually identifiable source organs to generate time-activity curves (TAC). Residence times were determined from time-activity curves. Absorbed doses to individual organs and the whole body effective dose were calculated using OLINDA/EXM 1.1 for each subject.

RESULTS

The highest measured activity concentration was in the kidney and spleen. The longest residence time was in the muscle at 0.100 ± 0.023 h, followed by the liver at 0.067 ± 0.015 h and lungs at 0.052 ± 0.010 h. The highest mean organ absorbed dose was within the heart wall (0.028 ± 0.002 mGy/MBq), followed by the kidneys (0.026 ± 0.005 mGy/MBq). The critical organ was the heart wall. The total mean effective dose averaged over subjects was estimated to be 0.0056 ± 0.0004 mSv/MBq for an injection of 11C-BU99008.

CONCLUSIONS

The biodistribution of 11C-BU99008 has been shown here for the first time in humans. Our dosimetry data showed the total mean effective dose over all subjects was 0.0056 ± 0.0004 mSv/MBq, which would result in a total effective dose of 1.96 mSv for a typical injection of 350 MBq of 11C-BU99008. The effective dose is not appreciably different from those obtained with other 11C tracers.

11C-DPA-713 has much greater specific binding to translocator protein 18 kDa (TSPO) in human brain than 11C-( R)-PK11195

Positron emission tomography (PET) radioligands for translocator protein 18 kDa (TSPO) are widely used to measure neuroinflammation, but controversy exists whether second-generation radioligands are superior to the prototypical agent ¹¹C-( R)-PK11195 in human imaging. This study sought to quantitatively measure the "signal to background" ratio (assessed as binding potential ( BP_ND)) of ¹¹C-( R)-PK11195 compared to one of the most promising second-generation radioligands, ¹¹C-DPA-713. Healthy subjects had dynamic PET scans and arterial blood measurements of radioligand after injection of either ¹¹C-( R)-PK11195 (16 subjects) or ¹¹C-DPA-713 (22 subjects). To measure the amount of specific binding, a subset of these subjects was scanned after administration of the TSPO blocking drug XBD173 (30-90 mg PO). ¹¹C-DPA-713 showed a significant sensitivity to genotype in brain, whereas ¹¹C-( R)-PK11195 did not. Lassen occupancy plot analysis revealed that the specific binding of ¹¹C-DPA-713 was much greater than that of ¹¹C-( R)-PK11195. The BP_ND in high-affinity binders was about 10-fold higher for ¹¹C-DPA-713 (7.3) than for ¹¹C-( R)-PK11195 (0.75). Although the high specific binding of ¹¹C-DPA-713 suggests it is an ideal ligand to measure TSPO, we also found that its distribution volume increased over time, consistent with the accumulation of radiometabolites in brain.

Translocator Protein as an Imaging Marker of Macrophage and Stromal Activation in Rheumatoid Arthritis Pannus

PET radioligands targeted to translocator protein (TSPO) offer a highly sensitive and specific means of imaging joint inflammation in rheumatoid arthritis (RA). Through high expression of TSPO on activated macrophages, TSPO PET has been widely reported in several studies of RA as a means of imaging synovial macrophages in vivo. However, this premise does not take into account the ubiquitous expression of TSPO. This study aimed to investigate TSPO expression in major cellular constituents of RA pannus-monocytes, macrophages, fibroblastlike synoviocytes (FLS cells), and CD4-positive (CD4+) T lymphocytes (T cells)-to more accurately interpret TSPO PET signal from RA synovium. Methods: Three RA patients and 3 healthy volunteers underwent PET of both knees using the TSPO radioligand ¹¹C-PBR28. Through ³H-PBR28 autoradiography and immunostaining of synovial tissue in 6 RA patients and 6 healthy volunteers, cellular expression of TSPO in synovial tissue was evaluated. TSPO messenger RNA expression and ³H-PBR28 radioligand binding was assessed using in vitro monocytes, macrophages, FLS cells, and CD4+ T cells. Results:¹¹C-PBR28 PET signal was significantly higher in RA joints than in healthy joints (average SUV, 0.82 ± 0.12 vs. 0.03 ± 0.004; P < 0.01). Further, ³H-PBR28-specific binding in synovial tissue was approximately 10-fold higher in RA patients than in healthy controls. Immunofluorescence revealed TSPO expression on macrophages, FLS cells, and CD4+ T cells. The in vitro study demonstrated the highest TSPO messenger RNA expression and ³H-PBR28-specific binding in activated FLS cells, nonactivated M0 macrophages, and activated M2 reparative macrophages, with the least TSPO expression being in activated and nonactivated CD4+ T cells. Conclusion: To our knowledge, this study was the first evaluation of cellular TSPO expression in synovium, with the highest TSPO expression and PBR28 binding being found on activated synovial FLS cells and M2 macrophages. TSPO-targeted PET may therefore have a unique sensitivity in detecting FLS cells and macrophage-predominant inflammation in RA, with potential utility for assessing treatment response in trials using novel FLS-cell-targeted therapies.

Evidence for GABA-A receptor dysregulation in gambling disorder: correlation with impulsivity

As a behavioural addiction, gambling disorder (GD) provides an opportunity to characterize addictive processes without the potentially confounding effects of chronic excessive drug and alcohol exposure. Impulsivity is an established precursor to such addictive behaviours, and GD is associated with greater impulsivity. There is also evidence of GABAergic dysregulation in substance addiction and in impulsivity. This study therefore investigated GABA_A receptor availability in 15 individuals with GD and 19 healthy volunteers (HV) using [¹¹C]Ro15‐4513, a relatively selective α5 benzodiazepine receptor PET tracer and its relationship with impulsivity. We found significantly higher [¹¹C]Ro15‐4513 total distribution volume (V_T) in the right hippocampus in the GD group compared with HV. We found higher levels of the ‘Negative Urgency’ construct of impulsivity in GD, and these were positively associated with higher [¹¹C]Ro15‐4513 V_T in the amygdala in the GD group; no such significant correlations were evident in the HV group. These results contrast with reduced binding of GABAergic PET ligands described previously in alcohol and opiate addiction and add to growing evidence for distinctions in the neuropharmacology between substance and behavioural addictions. These results provide the first characterization of GABA_A receptors in GD with [¹¹C]Ro15‐4513 PET and show greater α5 receptor availability and positive correlations with trait impulsivity. This GABAergic dysregulation is potential target for treatment.

Comparison of four 11C-labeled PET ligands to quantify translocator protein 18 kDa (TSPO) in human brain: (R)-PK11195, PBR28, DPA-713, and ER176-based on recent publications that measured specific-to-non-displaceable ratios

Translocator protein (TSPO) is a biomarker for detecting neuroinflammation by PET. ¹¹C-(R)-PK11195 has been used to image TSPO since the 1980s. Here, we compared the utility of four ¹¹C-labeled ligands-(R)-PK11195, PBR28, DPA-713, and ER176-to quantify TSPO in healthy humans. For all of these ligands, BP _ND (specific-to-non-displaceable ratio of distribution volumes) was measured by partially blocking specific binding with XNBD173 administration. In high-affinity binders, DPA-713 showed the highest BP _ND of 7.3 followed by ER176 (4.2), PBR28 (1.2), and PK11195 (0.8). Only ER176 allows the inclusion of low-affinity binders because of little influence of radiometabolites and high BP _ND. If inclusion of all three genotypes is important for study logistics, ER176 is the best of these four radioligands for studying neuroinflammation.

Investigation of the Brain Biodistribution of the Lipoprotein-Associated Phospholipase A2 (Lp-PLA2) Inhibitor [18F]GSK2647544 in Healthy Male Subjects

Purpose

GSK2647544 is a potent and specific inhibitor of lipoprotein-associated phospholipase A₂ (Lp-PLA₂), which was in development as a potential treatment for Alzheimer’s disease (AD). In order to refine therapeutic dose predictions and confirm brain penetration, a radiolabelled form of the inhibitor, [¹⁸F]GSK2647544, was manufactured for use in a positron emission tomography (PET) biodistribution study.

Procedures

[¹⁸F]GSK2647544 was produced using a novel, copper iodide (Cu(I)) mediated, [¹⁸F]trifluoromethylation methodology. Healthy male subjects (n = 4, age range 34–42) received an oral dose of unlabelled GSK2647544 (100 mg) and after 2 h an intravenous (iv) injection of [¹⁸F]GSK2647544 (average injected activity and mass were 106 ± 47 MBq and 179 ± 55 μg, respectively) followed by dynamic PET scans for 120 min. Defined regions of interest (ROI) throughout the brain were used to obtain regional time-activity curves (TACs) and compartmental modelling analysis used to estimate the primary outcome measure, whole brain volume of distribution (V_T). Secondary PK and safety endpoints were also recorded.

Results

PET dynamic data were successfully obtained from all four subjects and there were no clinically significant variations of the safety endpoints. Inspection of the TACs indicated a relatively homogenous uptake of [¹⁸F]GSK2647544 across all the ROIs examined. The mean whole brain V_T was 0.56 (95 % CI, 0.41–0.72). Secondary PK parameters, C_max (geometric mean) and T_max (median), were 354 ng/ml and 1.4 h, respectively. Metabolism of GSK2647544 was relatively consistent across subjects, with 20–40 % of the parent compound [¹⁸F]GSK2647544 present after 120 min.

Conclusions

The study provides evidence that GSK2647544 is able to cross the blood brain barrier in healthy male subjects leading to a measurable brain exposure. The administered doses of GSK2647544 were well tolerated. Exploratory modelling suggested that a twice-daily dose of 102 mg, at steady state, would provide ~80 % trough inhibition of brain Lp-PLA₂ activity.

Trial Registration

Clintrials.gov: NCT01924858.

Electronic supplementary material

The online version of this article (doi:10.1007/s11307-016-0982-5) contains supplementary material, which is available to authorized users.

Pro-inflammatory activation of primary microglia and macrophages increases 18 kDa translocator protein expression in rodents but not humans

The 18kDa Translocator Protein (TSPO) is the most commonly used tissue-specific marker of inflammation in positron emission tomography (PET) studies. It is expressed in myeloid cells such as microglia and macrophages, and in rodent myeloid cells expression increases with cellular activation. We assessed the effect of myeloid cell activation on TSPO gene expression in both primary human and rodent microglia and macrophages in vitro, and also measured TSPO radioligand binding with ³H-PBR28 in primary human macrophages. As observed previously, we found that TSPO expression increases (∼9-fold) in rodent-derived macrophages and microglia upon pro-inflammatory stimulation. However, TSPO expression does not increase with classical pro-inflammatory activation in primary human microglia (fold change 0.85 [95% CI 0.58-1.12], p = 0.47). In contrast, pro-inflammatory activation of human monocyte-derived macrophages is associated with a reduction of both TSPO gene expression (fold change 0.60 [95% CI 0.45-0.74], p = 0.02) and TSPO binding site abundance (fold change 0.61 [95% CI 0.49-0.73], p < 0.0001). These findings have important implications for understanding the biology of TSPO in activated macrophages and microglia in humans. They are also clinically relevant for the interpretation of PET studies using TSPO targeting radioligands, as they suggest changes in TSPO expression may reflect microglial and macrophage density rather than activation phenotype.

Loss of phosphodiesterase 4 in Parkinson disease

Objective:

To assess in vivo the expression of phosphodiesterase 4 (PDE4) and its relevance to cognitive symptoms in patients with Parkinson disease (PD) using [11C]rolipram PET.

Methods:

We studied 12 levodopa-treated patients with PD with no concurrent diagnosis of mild cognitive impairment or dementia. Their data were compared with those from 12 healthy controls. All participants underwent neuropsychiatric and cognitive assessment using the Cambridge Neuropsychological Test Automated Battery. Parametric images of [11C]rolipram volume of distribution (VT) values were determined with the Logan plot.

Results:

Patients with PD performed worse than healthy controls in cognitive examinations assessing psychomotor speed, episodic memory, and spatial working memory and executive function. Patients with PD showed reductions in [11C]rolipram VT compared to healthy controls, in the caudate (28%), thalamus (23%), hypothalamus (32%), and cortex (16%). Within thalamic subregions, [11C]rolipram VT values in patients with PD were decreased by 12%–32%, with most marked decreases observed in prefrontal and temporal thalamic nuclei, whereas motor nuclei were less affected. Within the cortex, [11C]rolipram VT values in patients with PD were decreased by 11%–20%, with most marked decreases observed in posterior dorsolateral frontal cortex, medial frontal cortex, and supplementary motor area, whereas orbitofrontal cortex was less affected. Worse performance in spatial working memory correlated with lower [11C]rolipram VT values in posterior dorsolateral frontal cortex, medial frontal cortex, supplementary motor area, precentral gyrus, caudate, and prefrontal thalamic nuclei.

Conclusions:

Our findings demonstrate loss of PDE4 expression in the striato-thalamo-cortical circuit, which is associated with deficits of spatial working memory in patients with PD.

Kisspeptin modulates sexual and emotional brain processing in humans

BACKGROUND. Sex, emotion, and reproduction are fundamental and tightly entwined aspects of human behavior. At a population level in humans, both the desire for sexual stimulation and the desire to bond with a partner are important precursors to reproduction. However, the relationships between these processes are incompletely understood. The limbic brain system has key roles in sexual and emotional behaviors, and is a likely candidate system for the integration of behavior with the hormonal reproductive axis. We investigated the effects of kisspeptin, a recently identified key reproductive hormone, on limbic brain activity and behavior.

METHODS. Using a combination of functional neuroimaging and hormonal and psychometric analyses, we compared the effects of kisspeptin versus vehicle administration in 29 healthy heterosexual young men.

RESULTS. We demonstrated that kisspeptin administration enhanced limbic brain activity specifically in response to sexual and couple-bonding stimuli. Furthermore, kisspeptin’s enhancement of limbic brain structures correlated with psychometric measures of reward, drive, mood, and sexual aversion, providing functional significance. In addition, kisspeptin administration attenuated negative mood.

CONCLUSIONS. Collectively, our data provide evidence of an undescribed role for kisspeptin in integrating sexual and emotional brain processing with reproduction in humans. These results have important implications for our understanding of reproductive biology and are highly relevant to the current pharmacological development of kisspeptin as a potential therapeutic agent for patients with common disorders of reproductive function.

FUNDING. National Institute for Health Research (NIHR), Wellcome Trust (Ref 080268), and the Medical Research Council (MRC).

Decreased hippocampal translocator protein (18 kDa) expression in alcohol dependence: a [11C]PBR28 PET study

Repeated withdrawal from alcohol is clinically associated with progressive cognitive impairment. Microglial activation occurring during pre-clinical models of alcohol withdrawal is associated with learning deficits. We investigated whether there was microglial activation in recently detoxified alcohol-dependent patients (ADP), using [¹¹C]PBR28 positron emission tomography (PET), selective for the 18kDa translocator protein (TSPO) highly expressed in activated microglia and astrocytes. We investigated the relationship between microglial activation and cognitive performance. Twenty healthy control (HC) subjects (45±13; M:F 14:6) and nine ADP (45±6, M:F 9:0) were evaluated. Dynamic PET data were acquired for 90 min following an injection of 331±15 MBq [¹¹C]PBR28. Regional volumes of distribution (V_T) for regions of interest (ROIs) identified a priori were estimated using a two-tissue compartmental model with metabolite-corrected arterial plasma input function. ADP had an ~20% lower [¹¹C]PBR28 V_T, in the hippocampus (F(1,24) 5.694; P=0.025), but no difference in V_T in other ROIs. Hippocampal [¹¹C]PBR28 V_T was positively correlated with verbal memory performance in a combined group of HC and ADP (r=0.720, P<0.001), an effect seen in HC alone (r=0.738; P=0.001) but not in ADP. We did not find evidence for increased microglial activation in ADP, as seen pre-clinically. Instead, our findings suggest lower glial density or an altered activation state with lower TSPO expression. The correlation between verbal memory and [¹¹C]PBR28 V_T, raises the possibility that abnormalities of glial function may contribute to cognitive impairment in ADP.

Translocator positron-emission tomography and magnetic resonance spectroscopic imaging of brain glial cell activation in multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is characterised by a diffuse inflammatory response mediated by microglia and astrocytes. Brain translocator protein (TSPO) positron-emission tomography (PET) and [myo-inositol] magnetic resonance spectroscopy (MRS) were used together to assess this.

OBJECTIVE

To explore the in vivo relationships between MRS and PET [¹¹C]PBR28 in MS over a range of brain inflammatory burden.

METHODS

A total of 23 patients were studied. TSPO PET imaging with [¹¹C]PBR28, single voxel MRS and conventional magnetic resonance imaging (MRI) sequences were undertaken. Disability was assessed by Expanded Disability Status Scale (EDSS) and Multiple Sclerosis Functional Composite (MSFC).

RESULTS

[¹¹C]PBR28 uptake and [ myo-inositol] were not associated. When the whole cohort was stratified by higher [¹¹C]PBR28 inflammatory burden, [ myo-inositol] was positively correlated to [¹¹C]PBR28 uptake (Spearman's ρ = 0.685, p = 0.014). Moderate correlations were found between [¹¹C]PBR28 uptake and both MRS creatine normalised N-acetyl aspartate (NAA) concentration and grey matter volume. MSFC was correlated with grey matter volume (ρ = 0.535, p = 0.009). There were no associations between other imaging or clinical measures.

CONCLUSION

MRS [ myo-inositol] and PET [¹¹C]PBR28 measure independent inflammatory processes which may be more commonly found together with more severe inflammatory disease. Microglial activation measured by [¹¹C]PBR28 uptake was associated with loss of neuronal integrity and grey matter atrophy.

Sleep problems and hypothalamic dopamine D3 receptor availability in Parkinson disease

OBJECTIVE

To investigate the relationship between hypothalamic D3 dopamine receptor availability and severity of sleep problems in Parkinson disease (PD).

METHODS

Twelve patients were assessed with PET and the high-affinity dopamine D3 receptor radioligand [¹¹C]-propyl-hexahydro-naphtho-oxazin ([¹¹C]-PHNO). Severity of sleep problems was rated with appropriate subitems of the Unified Parkinson's Disease Rating Scale part I (patient questionnaire) and the Epworth Sleepiness Scale.

RESULTS

We found that lower dopamine D3 receptor availability measured with [¹¹C]-PHNO PET was associated with greater severity of excessive daytime sleepiness but not with problems of falling asleep or insomnia.

CONCLUSION

In our cohort of patients with PD, the occurrence of excessive daytime sleepiness was linked to reductions in hypothalamic dopamine D3 receptor availability. If these preliminary findings are confirmed in larger cohorts of patients with polysomnographic characterization, selective pharmacologic modulation of the dopaminergic D3 system could be used to increase daytime alertness in patients with PD.

Imaging in Central Nervous System Drug Discovery

The discovery and development of central nervous system (CNS) drugs is an extremely challenging process requiring large resources, timelines, and associated costs. The high risk of failure leads to high levels of risk. Over the past couple of decades PET imaging has become a central component of the CNS drug-development process, enabling decision-making in phase I studies, where early discharge of risk provides increased confidence to progress a candidate to more costly later phase testing at the right dose level or alternatively to kill a compound through failure to meet key criteria. The so called "3 pillars" of drug survival, namely; tissue exposure, target engagement, and pharmacologic activity, are particularly well suited for evaluation by PET imaging. This review introduces the process of CNS drug development before considering how PET imaging of the "3 pillars" has advanced to provide valuable tools for decision-making on the critical path of CNS drug development. Finally, we review the advances in PET science of biomarker development and analysis that enable sophisticated drug-development studies in man.

Loss of extra-striatal phosphodiesterase 10A expression in early premanifest Huntington's disease gene carriers

Huntington's disease (HD) is a monogenic neurodegenerative disorder with an underlying pathology involving the toxic effect of mutant huntingtin protein primarily in striatal and cortical neurons. Phosphodiesterase 10A (PDE10A) regulates intracellular signalling cascades, thus having a key role in promoting neuronal survival. Using positron emission tomography (PET) with [(11)C]IMA107, we investigated the in vivo extra-striatal expression of PDE10A in 12 early premanifest HD gene carriers. Image processing and kinetic modelling was performed using MIAKAT™. Parametric images of [(11)C]IMA107 non-displaceable binding potential (BPND) were generated from the dynamic [(11)C]IMA107 scans using the simplified reference tissue model with the cerebellum as the reference tissue for nonspecific binding. We set a threshold criterion for meaningful quantification of [(11)C]IMA107 BPND at 0.30 in healthy control data; regions meeting this criterion were designated as regions of interest (ROIs). MRI-based volumetric analysis showed no atrophy in ROIs. We found significant differences in mean ROIs [(11)C]IMA107 BPND between HD gene carriers and healthy controls. HD gene carriers had significant loss of PDE10A within the insular cortex and occipital fusiform gyrus compared to healthy controls. Insula and occipital fusiform gyrus are important brain areas for the regulation of cognitive and limbic function that is impaired in HD. Our findings suggest that dysregulation of PDE10A-mediated intracellular signalling could be an early phenomenon in the course of HD with relevance also for extra-striatal brain areas.

Evidence of Brain Inflammation in Patients with Human T-Lymphotropic Virus Type 1-Associated Myelopathy (HAM): A Pilot, Multimodal Imaging Study Using 11C-PBR28 PET, MR T1-Weighted, and Diffusion-Weighted Imaging

HTLV-1-associated myelopathy (HAM; HTLV-1 is human T-lymphotropic virus type 1) is a chronic debilitating neuroinflammatory disease with a predilection for the thoracic cord. Tissue damage is attributed to the cellular immune response to HTLV-1-infected lymphocytes. The brains of HTLV-1-infected patients, with and without HAM but no clinical evidence of brain involvement, were examined using a specific 18-kDa translocator protein ligand, ¹¹C-PBR28, and T1-weighted and diffusion-weighted MRI.

METHODS

Five subjects with HAM and 2 HTLV-1 asymptomatic carriers were studied. All underwent clinical neurologic assessment including cognitive function and objective measures of gait, quantification of HTLV-1 proviral load in peripheral blood mononuclear cells, and human leukocyte antigen-antigen D related expression on circulating CD8+ lymphocytes. ¹¹C-PBR28 PET and MRI were performed on the same day. ¹¹C-PBR28 PET total volume of distribution and distribution volume ratio (DVR) were estimated using 2-tissue-compartment modeling. MRI data were processed using tools from the FMRIB Software Library to estimate mean diffusivity (MD) and gray matter (GM) fraction changes. The results were compared with data from age-matched healthy volunteers.

RESULTS

Across the whole brain, the total volume of distribution for the subjects with HAM (5.44 ± 0.84) was significantly greater than that of asymptomatic carriers (3.44 ± 0.80). The DVR of the thalamus in patients with severe and moderate HAM was higher than that in the healthy volunteers, suggesting increased translocator protein binding (z > 4.72). Subjects with more severe myelopathy and with high DR expression on CD8+ lymphocytes had increased DVR and MD (near-significant correlation found for the right thalamus MD: P = 0.06). On the T1-weighted MRI scans, the GM fraction of the brain stem was reduced in all HTLV-1-infected patients compared with controls (P < 0.001), whereas the thalamus GM fraction was decreased in patients with HAM and correlated with the disease severity. There was no correlation between neurocognitive function and these markers of central nervous system inflammation.

CONCLUSION

This pilot study suggests that some patients with HAM have asymptomatic inflammation in the brain, which can be detected and monitored by ¹¹C-PBR28 PET together with structural and diffusion-weighted MRI.

Hippocampal Neuroinflammation, Functional Connectivity, and Depressive Symptoms in Multiple Sclerosis

BACKGROUND

Depression, a condition commonly comorbid with multiple sclerosis (MS), is associated more generally with elevated inflammatory markers and hippocampal pathology. We hypothesized that neuroinflammation in the hippocampus is responsible for depression associated with MS. We characterized the relationship between depressive symptoms and hippocampal microglial activation in patients with MS using the 18-kDa translocator protein radioligand [(18)F]PBR111. To evaluate pathophysiologic mechanisms, we explored the relationships between hippocampal neuroinflammation, depressive symptoms, and hippocampal functional connectivities defined by resting-state functional magnetic resonance imaging.

METHODS

The Beck Depression Inventory (BDI) was administered to 11 patients with MS and 22 healthy control subjects before scanning with positron emission tomography and functional magnetic resonance imaging. We tested for higher [(18)F]PBR111 uptake in the hippocampus of patients with MS relative to healthy control subjects and examined the correlations between [(18)F]PBR111 uptake, BDI scores, and hippocampal functional connectivities in the patients with MS.

RESULTS

Patients with MS had an increased hippocampal [(18)F]PBR111 distribution volume ratio relative to healthy control subjects (p = .024), and the hippocampal distribution volume ratio was strongly correlated with the BDI score in patients with MS (r = .86, p = .006). Hippocampal functional connectivities to the subgenual cingulate and prefrontal and parietal regions correlated with BDI scores and [(18)F]PBR111 distribution volume ratio.

CONCLUSIONS

Our results provide evidence that hippocampal microglial activation in MS impairs the brain functional connectivities in regions contributing to maintenance of a normal affective state. Our results suggest a rationale for the responsiveness of depression in some patients with MS to effective control of brain neuroinflammation. Our findings also lend support to further investigation of the role of inflammatory processes in the pathogenesis of depression more generally.

Phosphodiesterase 10A in Schizophrenia: A PET Study Using [(11)C]IMA107

OBJECTIVE

Phosphodiesterase 10A (PDE10A) is an enzyme present in striatal medium spiny neurons that degrades the intracellular second messengers triggered by dopamine signaling. The pharmaceutical industry has considerable interest in PDE10A inhibitors because they have been shown to have an antipsychotic-like effect in animal models. However, the status of PDE10A in schizophrenia is unknown. Using a newly developed and validated radioligand, [(11)C]IMA107, the authors report the first in vivo assessment of PDE10A brain expression in patients with schizophrenia.

METHOD

The authors compared PDE10A availability in the brains of 12 patients with chronic schizophrenia and 12 matched healthy comparison subjects using [(11)C]IMA107 positron emission tomography (PET). Regional estimates of the binding potential (BPND) of [(11)C]IMA107 were generated from dynamic PET scans using the simplified reference tissue model with the cerebellum as the reference tissue for nonspecific binding.

RESULTS

There was no significant difference in [(11)C]IMA107 BPND between schizophrenia patients and comparison subjects in any of the brain regions studied (thalamus, caudate, putamen, nucleus accumbens, globus pallidus, and substantia nigra). There was also no significant correlation between [(11)C]IMA107 BPND and the severity of psychotic symptoms or antipsychotic dosage.

CONCLUSIONS

Patients with schizophrenia have normal availability of PDE10A in brain regions thought to be involved in the pathophysiology of this disorder. The findings do not support the proposal of an altered PDE10A availability in schizophrenia. The implication of this finding for future drug development is discussed.

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 [(11)C]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 [(11)C]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.