1,721,033 research outputs found
Molecular Imaging Studies of Second Messenger Pathways: Looking Deeper than the Membrane
No full textThe most successful applications of molecular imaging by PET include studies of energy metabolism, dopamine synthesis, transporters, and membrane-bound receptors. The metabotropic neurotransmitter receptors mediate their intracellular effects via second messenger systems, but methods for imaging these pathways are poorly developed. Agonist-evoked activation of G-protein coupled receptors (GPCRs) has effects on the activity of adenylate cyclase, an enzyme which can be detected by autoradiography in vitro with [3H]-forskolin. However, the very few efforts to detect adenylate cyclase in living brain have been unsuccessful. Whereas activation of adenylate cyclase increases intracellular levels of cyclic AMP, phosphodiesterase IV (PDE4) attenuates signaling by this pathway. In a number of preclinical imaging studies, PDE4 has been detected with [11C]-rolipram. There have been no clinical PET studies with this ligand, despite theoretical grounds for predicting PDE4 changes in mood disorders and neurodegenerative disease. Phosphodiesterase V, the target of Sidafenil, has been detected in pig myocardium using [11C]-RAL-01. Some GPCRs mediate their intracellular effects through phospholipases, which increase intracellular levels of arachidonic acid. The influx to brain of [11C]-arachidonic acid thus reveals the net turnover of membrane phospholipids, which decreased with healthy aging but increased in Alzheimer’s disease, suggesting a relationship with inflammatory processes. Activated protein kinase C can be labeled in vitro with phorbol esters, and [11C]-diacylglycerol has been tested in PET studies. Although glycogen levels are low in brain, the enzyme glycogen synthase kinase-3β is highly abundant, presenting a target for tracer development, and likewise fatty acid amide hydrolase (FAAH), the enzyme responsible for metabolizing anandamide. Charting the second messenger systems in living brain presents a molecular imaging project for the coming decades
Biodistribution of a Mitochondrial Metabolic Tracer, [(18)F]F-AraG, in Healthy Volunteers
Full text linkPURPOSE: [(18)F]F-AraG is a radiolabeled nucleoside analog that shows relative specificity for activated T cells. The aim of this study was to investigate the biodistribution of [(18)F]F-AraG in healthy volunteers and assess the preliminary safety and radiation dosimetry. METHODS: Six healthy subjects (three female and three male) between the ages of 24 and 60 participated in the study. Each subject received a bolus venous injection of [(18)F]F-AraG (dose range: 244.2–329.3 MBq) prior to four consecutive PET/MR whole-body scans. Blood samples were collected at regular intervals and vital signs monitored before and after tracer administration. Regions of interest were delineated for multiple organs, and the area under the time-activity curves was calculated for each organ and used to derive time-integrated activity coefficient (TIAC). TIACs were input for absorbed dose and effective dose calculations using OLINDA. RESULTS: PET/MR examination was well tolerated, and no adverse effects to the administration of [(18)F]F-AraG were noted by the study participants. The biodistribution was generally reflective of the expression and activity profiles of the enzymes involved in [(18)F]F-AraG's cellular accumulation, mitochondrial kinase dGK, and SAMHD1. The highest uptake was observed in the kidneys and liver, while the brain, lung, bone marrow, and muscle showed low tracer uptake. The estimated effective dose for [(18)F]F-AraG was 0.0162 mSv/MBq (0.0167 mSv/MBq for females and 0.0157 mSv/MBq for males). CONCLUSION: Biodistribution of [(18)F]F-AraG in healthy volunteers was consistent with its association with mitochondrial metabolism. PET/MR [(18)F]F-AraG imaging was well tolerated, with a radiation dosimetry profile similar to other commonly used [(18)F]-labeled tracers. [(18)F]F-AraG's connection with mitochondrial biogenesis and favorable biodistribution characteristics make it an attractive tracer with a variety of potential applications
De-Risking PET Radioligand Discovery for Neurodegenerative Diseases
Full text linkThis work sought to characterize novel and existing radioligands, assessing their utility and effectiveness for positron emission tomography (PET) imaging of neurodegeneration. Three proteins were targeted: 1) colony stimulating factor-1 receptor (CSF-1R); 2) glycogen synthase kinase-3 (GSK-3); and 3) transactivation response DNA-binding protein with molecular weight of 43 kDa (TDP-43). Utilizing autoradiography, a putative CSF-1R binding radioligand, [3H]CPPC, did not localize to microglia in the brain tissue of a preclinical model of neuroinflammation and percent displacement did not exceed 50% of signal, requiring use of spleen in screening assays. Selectivity assessment showed numerous binding sites which prevents an interpretation of signal contribution. Using similar methods, two radioligands targeting GSK-3, a kinase implicated in Alzheimer’s disease (AD) pathogenesis, were shown to quantify similar target densities (Bmax) with similar affinity (Kd) in human and rodent post-mortem homogenates. Investigation of human AD brain revealed possible differences in GSK-3 density compared to controls within females. In P301L mice, GSK-3 density in males was elevated compared to control, and a trend was recapitulated by in vivo PET, while females remained unchanged by both methods. GSK-3 PET research could inform the biological role of this critical kinase in pathogenesis of AD. Lastly, autoradiography identified two potential lead structures following evaluation of existing tau PET radioligands in post-mortem amyotrophic lateral sclerosis (ALS) tissues. Colocalization of specific binding with aggregate TDP-43 immunostaining was evaluated for isotopologues of 6 existing tau PET radioligands in varying structural classes. Three radioligands, [3H]MK-6240, [3H]GTP-1, and [3H]JNJ-067, showed no binding in ALS tissues. [3H]CBD-2115 specific binding was elevated but did not correlate to pathology. [3H]Flortaucipir binding was increased in the motor cortex compared to the cerebellum. Immunofluorescent staining with APN-1607 revealed no colocalization with a TDP-43 inclusion but significant binding to -amyloid. Despite revealing both positive and negative results an overarching objective of de-risking radioligands was achieved in each case by providing important information including selectivity, target density, specificity, or affinity. The results presented here are not unexpected due to challenges in developing imaging probes which has led to most putative radioligands failing prior to translation as successful clinical imaging ligands.Ph.D
Evaluating PET Imaging Biomarkers in Chronic Traumatic Encephalopathy
No full textThis work aims to characterize novel and existing radioligands to assess their utility for positron emission tomography (PET) imaging of chronic traumatic encephalopathy (CTE). Two pathological mechanisms were targeted: 1) neuroinflammation and 2) tau protein aggregates. Neuroinflammatory PET biomarkers were evaluated in Chapter 2 in CTE post-mortem tissues compared to controls and no significance in target density (Bmax) of the 18 kDa translocator protein (TSPO) or monoamine oxidase-B (MAO-B) was determined. Further validation of TSPO as a PET imaging biomarker in CTE was performed in a young cohort (19-29 year-olds). The activated microglial marker, CD68, and TSPO cell density was significantly higher in the calcarine cortex compared to the dorsolateral frontal cortex (DLFC), in individuals with repetitive head impacts (RHIs), regardless of CTE pathology. There was no significant difference found in TPSO density or in the percentage of CD68+ cells co-expressing TPSO in CTE or RHI groups.
Upon evaluation of five tau-PET radiotracers in CTE in Chapter 3, a Bmax and affinity (Kd) for CTE-tau was determined with [3H]flortaucipir and off-target binding to MAO-A with this radiotracer was identified. [3H]MK-6240, [3H]PI-2620 and [3H]APN-1607 bound in late-stage CTE cases with mixed Alzheimer’s disease (AD) pathology and [3H]APN-1607 demonstrated off-target binding to amyloid-b. In a case study of a cognitively impaired former National Football League player, [18F]MK-6240 PET imaging was performed, and radiotracer binding was observed in regions that generally show CTE-related tau pathology at autopsy. Evaluation of PI-2620 derivatives toward the identification of a radioligand suitable for imaging tau beyond AD was performed.
Radiotracer development including autoradiography and in vivo PET imaging was performed in Chapter 4 to image other neurodegenerative targets, namely metabotropic glutamate receptor subtype 5 (mGluR5) with [18F]FPEB and glycogen synthase kinase-3 (GSK-3) in which [18F]OCM-50 was identified as a potent, selective GSK-3 targeting radiotracer displaying good brain uptake, quick washout and retention in non-human primate. Kinetic modelling of this radiotracer has now been performed and first-in-human studies are planned.
This thesis has contributed to understanding the underlying pathological mechanisms associated with head injuries with PET imaging towards the goal of imaging CTE in life.Ph.D
Synthesis of Fluorine-18 Labelled Radiotracers for Positron Emission Tomography
Full text linkThis work improved the radiosynthesis of a known M2 muscarinic receptor imaging agent, [18F]FP-TZTP, and subsequent syntheses and in vitro evaluation of several novel TZTP derivatives highlighted a lead compound which exhibited M4 potency and selectivity, the thioether fluoro-polyethyleneglycol, which was then adapted for radiolabelling (23% radiochemical yield (uncorrected), >99% radiochemical purity, reaction time of 37 minutes). The present study also seeked to utilize aziridines as intermediates in [18F]-radiolabelling chemistry for the facile radiosynthesis of [18F]-labelled beta-blockers. Novel [18F]-labelled amines were synthesized via ring-opening and deprotection to yield the [18F]-1-fluoro-2-propanamine moiety (85%) favourably over the regioisomer [18F]-2-fluoro-1-propanamine (15%). Subsequent attempts to use these amine synthons in the synthesis of the beta-blocker [18F]Exaprolol resulted in poor radiochemical yields (1-3%). The chemistry of aziridine ring-opening with 19F-fluoride sources was thoroughly explored in order to understand the fundamentals of this chemistry, and the 1-fluoro-2-propanamine moiety was characterized by X-ray crystallography and NMR spectroscopy.MAS
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
PET of signal transduction pathways in cancer
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In this era of systems biology, the tide of information derived from “omic” technologies (genomics, proteomics, etc.) has sparked a revolution in drug design, with many industrial and academic programs now embracing the concepts of molecular medicine (i.e., targeting changes in specific proteins or pathways) as measures of treatment efficacy and outcome. This approach has yielded a plethora of new preclinical therapeutics directed at novel targets within oncology. In many ways, the evolution of molecular imaging agents as diagnostic probes mirrors that of chemotherapeutics; yet despite an increasing number of PET and SPECT radiotracers being evaluated in human trials, relatively few agents have found widespread use in clinical oncology. In light of this observation, is it time to reevaluate our strategies for radiopharmaceutical design and use? In this article, we argue that PET has enormous potential to deliver clinically relevant information on disease dynamics that extends beyond mapping the density and spatial distribution of a target. Recent developments in targeting pharmacodynamic biomarkers aim to exploit better the advantages of functional PET by detecting changes in signal transduction pathways, particularly in response to disease progression or treatment in cancer
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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