Impact de l’exposition aux inhibiteurs réversibles (Donépézil) et irréversibles (l’organophosphoré NIMP) des acétylcholinestérases sur les mécanismes cérébraux étudié par imagerie moléculaire

Acetylcholinesterases (AchE) are enzymes widely spread in the central nervous system (CNS) that allow the recycling of acetylcholine (Ach). The use of reversible/irreversible inhibitors of these enzymes prevents the degradation of Ach. Depending on the context of their use, AchE inhibitors can have a beneficial or detrimental impact on brain mechanisms. In the last decades, a class of irreversible AchE inhibitors considered as warfare nerve agents such as sarin gas have been used in various military conflicts and/or in several attacks against civilian populations. They cause an excessive accumulation of Ach in the SNC of people who have been in contact with them, which is considered as intoxication. The hyperexcitability of the CNS following intoxication generates a succession of symptoms that can be detected either in the short or long term, depending on the dose of the nerve agent. Studies following the Tokyo attacks in 1995 showed that even a low dose of toxic that did not cause the emergence of immediate symptoms was capable of generating cognitive disorders, particularly memory impairment, several years after exposure. However, the mechanisms underlying such disorders are still not well understood. AchE can also be used in a therapeutic context. Indeed, in Alzheimer's disease, a decrease in Ach levels are observed. Therefore, reversible AchE inhibitors such as Donepezil (DPZ) are used to maintain normal levels of Ach in the brain, thus allowing a proper brain functioning. The studies carried out in this thesis aimed to determine the effect of these AchE inhibitors on various brain mechanisms. The first study investigated the impact of high and low dose administration of an organophosphate (NIMP; analogue of sarin gas), an irreversible AchE inhibitor, on neuroinflammation using in vivo [¹⁸F]DPA-714 PET imaging. Longitudinal in vivo monitoring of neuroinflammation over several months allowed to identify a therapeutic window for neuroprotective treatment. For the second study a preclinical model of Alzheimer's disease (AD) was employed in order to evaluate in vivo the effects of Donepezil treatment on cerebral glucose metabolism using [¹⁸F]FDG PET. Various pathophysiological mechanisms (neuroinflammation, abnormal accumulation of misfolded proteins (AD)...) are associated with neurological diseases and may impact the blood-brain barrier (BBB) integrity. It is therefore essential to develop tools to quantify BBB permeability in vivo. In this context, a third project combining focused ultrasound and PET was set up in order to investigate the potential of [¹⁸F]FDS PET imaging as a quantitative biomarker of BBB integrity in vivo. In the course of this thesis, PET imaging allowed to investigate different brain functions (neuroinflammation, cerebral glucose metabolism, integrity of the BBB) in order to assess i) changes in brain mechanisms in a longitudinal manner, ii) a therapeutic follow-up and iii) validate a new biomarker.Les acétylcholinestérases (AchE) sont des enzymes largement distribuées au sein du système nerveux central (SNC) permettant le recyclage de l’acétylcholine (Ach). L’utilisation d’inhibiteurs réversibles/irréversibles de ces dernières empêchent la dégradation de l’Ach. Suivant le contexte d’utilisation, les inhibiteurs des AchE peuvent avoir un impact bénéfique ou néfaste sur les mécanismes cérébraux. Ces dernières décennies, une classe d’inhibiteurs irréversibles des AchE considérés comme des neurotoxiques de guerre tel que le gaz sarin ont été utilisés lors de divers conflits militaires et/ou lors d’attaques ciblant les populations civiles. Ils provoquent une accumulation anormalement élevée d’Ach au sein du SNC des personnes ayant été en contact avec ces derniers, on parle alors d’intoxication. L’hyperexcitabilité du SNC suite à l’intoxication engendre une succession de symptômes détectables soit sur le court ou le long terme selon la dose de toxique à laquelle les personnes ont été exposés. Des études suite aux attentats de Tokyo en 1995 ont mis en évidence que même une faible dose de toxique ne provoquant pas l’apparition de symptômes immédiats était capable de générer des troubles cognitifs notamment mnésiques des années post-exposition. Cependant les mécanismes sous-jacents à de tels troubles restent encore méconnus. L’utilisation des inhibiteurs d’AchE est aussi possible dans un contexte thérapeutique. En effet, dans la maladie d’Alzheimer une diminution des taux d’Ach est observée. Des inhibiteurs cette fois-ci réversibles des AchE comme le Donépézil (DPZ) sont alors utilisés pour maintenir des taux normaux d’Ach dans le cerveau permettant ainsi son bon fonctionnement. Les études réalisées au cours de cette thèse ont pour but de déterminer l’effet de ces inhibiteurs d’AchE sur divers mécanismes cérébraux. La première étude a permis d’analyser l’impact de l’administration d’une forte et d’une faible dose d’un organophosphoré (NIMP ; analogue du gaz sarin), un inhibiteur irréversible des AchE, sur la neuroinflammation en imagerie TEP in vivo via l’utilisation du radiotraceur [¹⁸F]DPA-714. Le suivi de la neuroinflammation in vivo sur plusieurs mois a permis de déterminer une fenêtre thérapeutique d’action pour tester un traitement neuroprotecteur. Pour la deuxième étude un modèle préclinique de la maladie d’Alzheimer (MA) a été employé afin d’évaluer in vivo les effets d’un traitement avec le Donépézil sur le métabolisme cérébral du glucose en utilisant l’imagerie TEP au [¹⁸F]FDG. Divers mécanismes pathophysiologiques (neuroinflammation, accumulation anormale de protéines mal conformées (MA)…) sont associés aux maladies neurologiques et peuvent impacter l’étanchéité de la barrière hématoencéphalique (BHE). Il est donc primordial d’avoir des outils permettant la quantification de la perméabilité de la BHE in vivo. Dans ce cadre, un troisième projet utilisant la technique des ultrasons focalisés mis au point au laboratoire a permis de repositionner l’imagerie TEP au [¹⁸F]-2- fluoro-2-désoxy-sorbitol ([¹⁸F]FDS) comme biomarqueur quantitatif de l’intégrité de la BHE in vivo. Au cours de cette thèse l’imagerie TEP a permis d’investiguer différents fonctionnalités cérébrales (neuroinflammation, métabolisme cérébral du glucose, intégrité de la BHE) afin de i) réaliser un suivi longitudinal, ii) un suivi thérapeutique et iii) valider un nouveau biomarqueur

Evaluation of the contrast enhancement performance of gadopiclenol for magnetic resonance angiography in healthy rabbits and pigs

International audienceObjectives Unexpected accumulations of gadolinium in various organs were reported after the administration of gadolinium-based contrast agents, making desirable to reduce the dose while maintaining equivalent diagnostic performance. The aim of this study was to evaluate the contrast enhancement performance of high relaxivity gadopiclenol compared with gadoterate meglumine in abdominal contrast-enhanced magnetic resonance angiography (CE-MRA). Materials and Methods In a first study in healthy rabbits, axial 3D gradient echo sequences were applied at 4.7 T to study arterial enhancement as a function of gadopiclenol dose (0.025, 0.05, 0.075, and 0.1 mmol Gd/kg) or gadoterate meglumine at 0.1 mmol Gd/kg (n = 5–6/group). The increase in signal-to-noise ratio (ΔSNR) in the aorta at the first pass was measured and compared. In a second, crossover study in 6 healthy pigs, abdominal CE-MRA sequences were acquired at 3 T with gadopiclenol at 0.05 mmol Gd/kg or gadoterate meglumine at 0.1 mmol Gd/kg at a 1-week interval. Quantitatively on the maximum intensity projection (MIP) images, the mean MIP SNR within the aorta of both groups was compared. Qualitatively, a blinded comparison of the angiograms was performed by an experienced radiologist to determine the preferred contrast agent. Results In the rabbit, ∆SNR is linearly correlated with the gadopiclenol dose ( P = 0.0010). Compared with gadoterate meglumine 0.1 mmol Gd/kg, an increase in the ∆SNR is observed after 0.05, 0.075, and 0.1 mmol Gd/kg of gadopiclenol (+63% P = 0.0731, +78% P = 0.0081, and +72% P = 0.0773, respectively), whereas at 0.025 mmol Gd/kg, ∆SNR is in the same range as with gadoterate meglumine 0.1 mmol Gd/kg (+15% P > 0.9999). In pigs, contrast enhancement after gadopiclenol at 0.05 mmol/kg is +22% superior to MIP SNR after gadoterate meglumine at 0.1 mmol Gd/kg ( P = 0.3095). Qualitatively, a preference was shown for gadopiclenol images (3/6) over the gadoterate meglumine examinations (1/6), with no preference being shown for the remainder (2/6). Conclusions First-pass CE-MRA is feasible with gadopiclenol at 0.05 mmol Gd/kg with at least the same arterial signal enhancement and image quality as gadoterate meglumine at 0.1 mmol Gd/kg

[18F]2-Fluoro-2-deoxy-sorbitol PET Imaging for Quantitative Monitoring of Enhanced Blood-Brain Barrier Permeability Induced by Focused Ultrasound

International audienceFocused ultrasound in combination with microbubbles (FUS) provides an effective means to locally enhance the delivery of therapeutics to the brain. Translational and quantitative imaging techniques are needed to noninvasively monitor and optimize the impact of FUS on blood-brain barrier (BBB) permeability in vivo. Positron-emission tomography (PET) imaging using [ 18 F]2fluoro-2-deoxy-sorbitol ([ 18 F]FDS) was evaluated as a small-molecule (paracellular) marker of blood-brain barrier (BBB) integrity. [ 18 F]FDS was straightforwardly produced from chemical reduction of commercial [ 18 F]2-deoxy-2-fluoro-D-glucose. [ 18 F]FDS and the invasive BBB integrity marker Evan's blue (EB) were i.v. injected in mice after an optimized FUS protocol designed to generate controlled hemispheric BBB disruption. Quantitative determination of the impact of FUS on the BBB permeability was determined using kinetic modeling. A 2.2 ± 0.5-fold higher PET signal (n = 5; p < 0.01) was obtained in the sonicated hemisphere and colocalized with EB staining observed post mortem. FUS significantly increased the blood-to-brain distribution of [ 18 F]FDS by 2.4 ± 0.8-fold (VT; p < 0.01). Low variability (=10.1%) of VT values in the sonicated hemisphere suggests reproducibility of the estimation of BBB permeability and FUS method. [ 18 F]FDS PET provides a readily available, sensitive and reproducible marker of BBB permeability to noninvasively monitor the extent of BBB disruption induced by FUS in vivo

Evaluation of Myelin Radiotracers in the Lysolecithin Rat Model of Focal Demyelination: Beware of Pitfalls!

International audienceThe observation that amyloid radiotracers developed for Alzheimer's disease bind to cerebral white matter paved the road to nuclear imaging of myelin in multiple sclerosis. e lysolecithin (lysophosphatidylcholine (LPC)) rat model of demyelination proved useful in evaluating and comparing candidate radiotracers to target myelin. Focal demyelination following stereotaxic LPC injection is larger than lesions observed in experimental autoimmune encephalitis models and is followed by spontaneous progressive remyelination. Moreover, the contralateral hemisphere may serve as an internal control in a given animal. However, demyelination can be accompanied by concurrent focal necrosis and/or adjacent ventricle dilation. e in uence of these side e ects on imaging ndings has never been carefully assessed. e present study describes an optimization of the LPC model and highlights the use of MRI for controlling the variability and pitfalls of the model. e prototypical amyloid radiotracer [ 11 C]PIB was used to show that in vivo PET does not provide su cient sensitivity to reliably track myelin changes and may be sensitive to LPC side e ects instead of demyelination as such. Ex vivo autoradiography with a uorine radiotracer should be preferred, to adequately evaluate and compare radiotracers for the assessment of myelin content

Impact of Cytochrome Induction or Inhibition on the Plasma and Brain Kinetics of [11C]metoclopramide, a PET Probe for P-Glycoprotein Function at the Blood-Brain Barrier

[11C]metoclopramide PET imaging provides a sensitive and translational tool to explore P-glycoprotein (P-gp) function at the blood-brain barrier (BBB). Patients with neurological diseases are often treated with cytochrome (CYP) modulators which may impact the plasma and brain kinetics of [11C]metoclopramide. The impact of the CYP inducer carbamazepine or the CYP inhibitor ritonavir on the brain and plasma kinetics of [11C]metoclopramide was investigated in rats. Data obtained in a control group were compared with groups that were either orally pretreated with carbamazepine (45 mg/kg twice a day for 7 days before PET) or ritonavir (20 mg/kg, 3 h before PET) (n = 4 per condition). Kinetic modelling was performed to estimate the brain penetration (VT) of [11C]metoclopramide. CYP induction or inhibition had negligible impact on the plasma kinetics and metabolism of [11C]metoclopramide. Moreover, carbamazepine neither impacted the brain kinetics nor VT of [11C]metoclopramide (p &gt; 0.05). However, ritonavir significantly increased VT (p &lt; 0.001), apparently behaving as an inhibitor of P-gp at the BBB. Our data suggest that treatment with potent CYP inducers such as carbamazepine does not bias the estimation of P-gp function at the BBB with [11C]metoclopramide PET. This supports further use of [11C]metoclopramide for studies in animals and patients treated with CYP inducers

Redox Systems, Antioxidants and Sarcopenia

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