Imaging of Dopamine and Serotonin Transporters. Pre-clinical Studies with Radiotracers for Positron Emission Tomography

The action of the neurotransmitters dopamine (DA) and serotonin (5-HT) at synapses is terminated by their rapid reuptake into presynaptic nerve endings via plasma membrane dopamine (DAT) and serotonin (SERT) transporters. Alterations in the function of these transporters have been suggested as a feature of several neurological and neuropsychiatric diseases, such as Parkinson’s disease (PD), depression, and anxiety. A suitable clinical method for studying these transporters non-invasively in vivo is positron emission tomography (PET) utilizing radiopharmaceuticals (tracers) labelled with short-lived positron-emitting radionuclides. The aim of this study was to evaluate in rats two novel radiotracers, [18F]beta -CFT-FP and 18FFMe-McN, for imaging DAT and SERT, respectively, using in vitro, ex vivo and in vivo methods. Substituting an N-methyl in [18F]beta-CFT, a well known DAT tracer, with a 18Ffluoropropyl group significantly changed the properties of the tracer. [18F]beta- CFT showed slow kinetics and metabolism, and a high specific uptake in the striatum, whereas [18F]beta-CFT-FP showed fast kinetics and metabolism, and a moderate specific uptake in the striatum. [18F]betaCFT-FP was selective for DAT; but [18F]beta-CFT also bound to the noradrenaline transporter. [18F]beta-CFT-FP may be a suitable PET tracer for imaging the striatal DAT sites, but a tracer with a higher affinity is needed for imaging extrastriatal DAT sites. In rats, 18FFMe-McN showed high target-to-non-target ratios, specificity and selectivity for SERT, but slow kinetics. However, 18FFMe-McN reveals potential for imaging SERT, at least in pre-clinical studies. In addition, the sensitivities of [18F]beta CFT and [18 F]FDOPA (a precursor of DA) for detecting mild nigrostriatal hypofunction were compared in an animal model of PD. The uptake of [18F]FDOPA was significantly affected by compensatory effects in dopaminergic cells, whereas [18F]beta-CFT was more sensitive and therefore more suitable for PET studies of mild dopaminergic symptoms. In conclusion, both novel tracers, [18F]-CFT-FP and 18FFMe-McN, have potential, but are not optimal PET tracers for DAT and SERT imaging in rats, respectively. [18F]beta-CFT is superior to [18F]FDOPA for imaging mild nigral lesions in rat brains.Siirretty Doriast

Association of neuroinflammation with episodic memory : a [C-11]PBR28 PET study in cognitively discordant twin pairs

Alzheimer's disease is associated with chronic response of innate immune system, referred as neuroinflammation. PET radioligands binding to the 18kDa translocator protein are potential biomarkers of neuroinflammation. Translocator protein PET studies in mild cognitive impairment and Alzheimer's disease have indicated controversial results, possibly reflecting interindividual variation and heterogeneity of study populations. We controlled for genetic and environmental effects by studying twin pairs discordant for episodic memory performance. Episodic memory impairment is a well-known cognitive hallmark of early Alzheimer's disease process. Eleven same-sex twin pairs (four monozygotic pairs, six female pairs, age 72-77 years) underwent [C-11]N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine ([C-11]PBR28) PET imaging, structural magnetic resonance imaging and neuropsychological testing in 2014-17. Main PET outcome was the volume-weighted average standardized uptake value of cortical regions vulnerable to Alzheimer's disease pathology. Ten pairs were discordant for episodic memory performance. In the eight pairs with identical translocator protein genotype, twins with poorer episodic memory had similar to 20% higher cortical [C-11]PBR28 binding compared with their better-performing co-twins (mean intra-pair difference 0.21 standardized uptake value, 95% confidence interval 0.05-0.37, P = 0.017). The result remained the same when including all discordant pairs and controlling for translocator protein genotype. Increased translocator protein PET signal suggests that increased microglial activation is associated with poorer episodic memory performance. Twins with worse episodic memory performance compared with their co-twins had on average 20% higher uptake of the neuroinflammatory marker translocator protein PET tracer (11)[C-11]PBR28. The findings support a negative association between neuroinflammation and episodic memory and the use of translocator protein positron emission tomography as a useful indicator of Alzheimer's disease process.Peer reviewe

Association of neuroinflammation with episodic memory: a [11C]PBR28 PET study in cognitively discordant twin pairs

Alzheimer’s disease is associated with chronic response of innate immune system, referred as neuroinflammation. PET radioligands binding to the 18 kDa translocator protein are potential biomarkers of neuroinflammation. Translocator protein PET studies in mild cognitive impairment and Alzheimer’s disease have indicated controversial results, possibly reflecting interindividual variation and heterogeneity of study populations. We controlled for genetic and environmental effects by studying twin pairs discordant for episodic memory performance. Episodic memory impairment is a well-known cognitive hallmark of early Alzheimer’s disease process. Eleven same-sex twin pairs (four monozygotic pairs, six female pairs, age 72–77 years) underwent [11C]N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine ([11C]PBR28) PET imaging, structural magnetic resonance imaging and neuropsychological testing in 2014–17. Main PET outcome was the volume-weighted average standardized uptake value of cortical regions vulnerable to Alzheimer’s disease pathology. Ten pairs were discordant for episodic memory performance. In the eight pairs with identical translocator protein genotype, twins with poorer episodic memory had ∼20% higher cortical [11C]PBR28 binding compared with their better-performing co-twins (mean intra-pair difference 0.21 standardized uptake value, 95% confidence interval 0.05–0.37, P = 0.017). The result remained the same when including all discordant pairs and controlling for translocator protein genotype. Increased translocator protein PET signal suggests that increased microglial activation is associated with poorer episodic memory performance. Twins with worse episodic memory performance compared with their co-twins had on average 20% higher uptake of the neuroinflammatory marker translocator protein PET tracer 11[11C]PBR28. The findings support a negative association between neuroinflammation and episodic memory and the use of translocator protein positron emission tomography as a useful indicator of Alzheimer’s disease process.</p

[18F]SPA-RQ/PET Study of NK1 receptors in the Whole Body of Guinea Pig and Rat

There is a substantial interest in the development of NK1 substance P antagonists as potential treatments for various neuropsychiatric and somatic disorders. The aim of this study was to determine whether [18F]SPA-RQ can be utilized as a tool for studying the whole body distribution and function of NK1 receptors in preclinical settings. The compound was injected into guinea pigs with or without premedication with a NK1 receptor antagonist (NK1A-2). For comparison, we included two rats in the study, as the affinity of antagonists for NK1 receptors is known to vary between species. The whole body biodistribution of the tracer was determined at several time points. The tracer showed specific binding in organs compatible with the known location of NK1-receptors. Premedication with a NK1 antagonist led to an inhibited uptake of [18F]SPA-RQ in several organs of guinea pigs, notably intestine, pancreas, urinary bladder, uterus, skin and lung. Specific binding was also seen in both cortex and striatum. In contrast, negligible specific binding was observed in the rat brain with [18F]SPA-RQ, whereas the tracer uptake in peripheral tissues was similar to that seen in guinea pigs. We conclude that [18F]SPA-RQ/PET is a useful tool to study the distribution and function of peripherally located NK1 receptors e.g. in different disease models.</div

Application of the PET ligand [C-11]ORM-13070 to examine receptor occupancy by the alpha(2C)-adrenoceptor antagonist ORM-12741: translational validation of target engagement in rat and human brain

BackgroundAvailability of the α2C-adrenoceptor (α2C-AR) positron emission tomography (PET) tracer, [11C]ORM-13070, and the α2C-AR antagonist ORM-12741 allows probing of the roles of this G-protein coupled receptor subtype in brain function, both in healthy humans and in patients with various brain disorders. This translational study employed [11C]ORM-13070 autoradiography and PET to determine α2C-AR occupancy by ORM-12741 in rat and human brain, respectively.ResultsORM-12741 has high affinity (Ki: 0.08 nM) and potent antagonist activity (Kb: 0.04 nM) as well as selectivity (Ki estimates for the humanα2A-AR and α2B-AR were 8.3 nM and 0.8 nM, respectively) for the human α2C-AR subtype. [11C]ORM-13070 had highest uptake in the basal ganglia of rat and human brain. Pretreatment with ORM-12741 inhibited [11C]ORM-13070 binding in rat striatum in a time- and dose-dependent manner at 10 and 50 µg/kg (s.c.) with an EC50 estimate of 1.42 ng/mL in rat plasma, corresponding to protein-free drug concentration of 0.23 nM. In the living human brain, time- and dose-related α2C-AR occupancy was detected with EC50 estimates of 24 ng/mL and 31 ng/mL for the caudate nucleus and putamen, respectively, corresponding to protein-free concentrations in plasma of 0.07 nM and 0.1 nM. Modelling-based maximum α2C-AR occupancy estimates were 63% and 52% in the caudate nucleus and the putamen, respectively.ConclusionsORM-12741 is a selective α2C-AR antagonist which penetrates the rat and human brain to occupy α2C-ARs in a manner consistent with its receptor pharmacology.Trial registration number and date of registration: ClinicalTrial.cov NCT00829907. Registered 11 December 2008. https://clinicaltrials.gov/.</div

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

OBJECTIVE: To examine whether early β-amyloid (Aβ) accumulation and metabolic risk factors are associated with neuroinflammation in elderly individuals without dementia. METHODS: We 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. RESULTS: Among 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. CONCLUSIONS: While 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. Copyright © 2021 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American Academy of Neurology.</p

Folate Receptor β Targeted PET Imaging of Macrophages in Autoimmune Myocarditis

Rationale: Currently available imaging techniques have limited specificity for the detection of active myocardial inflammation. Aluminum fluoride-18-labeled 1,4,7-triazacyclononane-N,N′,N″-triacetic acid conjugated folate (18F-FOL) is a positron emission tomography (PET) tracer targeting folate receptor β (FR-β) that is expressed on activated macrophages at sites of inflammation. We evaluated 18F-FOL PET for the detection of myocardial inflammation in rats with autoimmune myocarditis and studied expression of FR-β in human cardiac sarcoidosis specimens. Methods: Myocarditis was induced by immunizing rats (n = 18) with porcine cardiac myosin in complete Freund’s adjuvant. Control rats (n = 6) were injected with Freund’s adjuvant alone. 18F-FOL was intravenously injected followed by imaging with a small animal PET/computed tomography (CT) scanner and autoradiography. Contrast-enhanced high-resolution CT or 2-deoxy-2-18F-fluoro-D-glucose (18F-FDG) PET images were used for co-registration. Rat tissue sections and myocardial autopsy samples of 6 patients with cardiac sarcoidosis were studied for macrophages and FR-β. Results: The myocardium of 10 out of 18 immunized rats showed focal macrophage-rich inflammatory lesions with FR-β expression occurring mainly in M1-polarized macrophages. PET images showed focal myocardial 18F-FOL uptake co-localizing with inflammatory lesions (SUVmean, 2.1 ± 1.1), whereas uptake in the remote myocardium of immunized rats and controls was low (SUVmean, 0.4 ± 0.2 and 0.4 ± 0.1, respectively; P </p

Aluminum fluoride-18 labeled folate enables in vivo detection of atherosclerotic plaque inflammation by positron emission tomography

Inflammation plays an important role in the development of atherosclerosis and its complications. Because the folate receptor beta (FR-beta) is selectively expressed on macrophages, an FR targeted imaging agent could be useful for assessment of atherosclerotic inflammation. We investigated aluminum fluoride-18-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid conjugated folate (F-18-FOL) for the detection of atherosclerotic plaque inflammation. We studied atherosclerotic plaques in mice, rabbits, and human tissue samples using F-18-FOL positron emission tomography/computed tomography (PET/CT). Compound 2-deoxy-2-[F-18]fluoro-D-glucose (F-1(8)-FDG) was used as a comparison. Firstly, we found that the in vitro binding of F-18-FOL co-localized with FR-beta-positive macrophages in carotid endarterectomy samples from patients with recent ischemic symptoms. We then demonstrated specific accumulation of intravenously administered F-18-FOL in atherosclerotic plaques in mice and rabbits using PET/CT. We noticed that the F-18-FOL uptake correlated with the density of macrophages in plaques and provided a target-to-background ratio as high as F-18-FDG, but with considerably lower myocardial uptake. Thus, F-18-FOL PET/CT targeting of FR-beta-positive macrophages presents a promising new tool for the in vivo imaging of atherosclerotic inflammation

Fibroblast Growth Factor 21 Drives Dynamics of Local and Systemic Stress Responses in Mitochondrial Myopathy with mtDNA Deletions

AbstractMitochondrial dysfunction elicits stress responses that safeguard cellular homeostasis against metabolic insults. Mitochondrial integrated stress response (ISRmt) is a major response to mitochondrial (mt)DNA expression stress (mtDNA maintenance, translation defects), but the knowledge of dynamics or interdependence of components is lacking. We report that in mitochondrial myopathy, ISRmt progresses in temporal stages and development from early to chronic and is regulated by autocrine and endocrine effects of FGF21, a metabolic hormone with pleiotropic effects. Initial disease signs induce transcriptional ISRmt (ATF5, mitochondrial one-carbon cycle, FGF21, and GDF15). The local progression to 2nd metabolic ISRmt stage (ATF3, ATF4, glucose uptake, serine biosynthesis, and transsulfuration) is FGF21 dependent. Mitochondrial unfolded protein response marks the 3rd ISRmt stage of failing tissue. Systemically, FGF21 drives weight loss and glucose preference, and modifies metabolism and respiratory chain deficiency in a specific hippocampal brain region. Our evidence indicates that FGF21 is a local and systemic messenger of mtDNA stress in mice and humans with mitochondrial disease.</div

Elevated serum chemokine CCL22 levels in first-episode psychosis: associations with symptoms, peripheral immune state and in vivo brain glial cell function

Several lines of research support immune system dysregulation in psychotic disorders. However, it remains unclear whether the immunological marker alterations are stable and how they associate with brain glial cell function. This longitudinal study aimed at investigating whether peripheral immune functions are altered in the early phases of psychotic disorders, whether the changes are associated with core symptoms, remission, brain glial cell function, and whether they persist in a one-year follow-up. Two independent cohorts comprising in total of 129 first-episode psychosis (FEP) patients and 130 controls were assessed at baseline and at the one-year follow-up. Serum cyto-/chemokines were measured using a 38-plex Luminex assay. The FEP patients showed a marked increase in chemokine CCL22 levels both at baseline (p < 0.0001; Cohen's d = 0.70) and at the 12-month follow-up (p = 0.0007) compared to controls. The group difference remained significant (p = 0.0019) after accounting for relevant covariates including BMI, smoking, and antipsychotic medication. Elevated serum CCL22 levels were significantly associated with hallucinations (rho = 0.20) and disorganization (rho = 0.23), and with worse verbal performance (rho = -0.23). Brain glial cell activity was indexed with positron emission tomography and the translocator protein radiotracer [C-11]PBR28 in subgroups of 15 healthy controls and 14 FEP patients with serum CCL22/CCL17 measurements. The distribution volume (V-T) of [C-11]PBR28 was lower in patients compared to controls (p = 0.026; Cohen's d = 0.94) without regionally specific effects, and was inversely associated with serum CCL22 and CCL17 levels (p = 0.036). Our results do not support the over-active microglia hypothesis of psychosis, but indicate altered CCR4 immune signaling in early psychosis with behavioral correlates possibly mediated through cross-talk between chemokine networks and dysfunctional or a decreased number of glial cells