Alain Plenevaux

Breakdown of within- and between-network Resting State Functional Magnetic Resonance Imaging Connectivity during Propofol-induced Loss of Consciousness

Background:Mechanisms of anesthesia-induced loss of consciousness remain poorly understood. Resting-state functional magnetic resonance imaging allows investigating whole-brain connectivity changes during pharmacological modulation of the level of consciousness. Methods:Low-frequency spontaneous blood oxygen level-dependent fluctuations were measured in 19 healthy volunteers during wakefulness, mild sedation, deep sedation with clinical unconsciousness, and subsequent recovery of consciousness. Results:Propofol-induced decrease in consciousness linearly correlates with decreased corticocortical and thalamocortical connectivity in frontoparietal networks (i.e., default- and executive-control networks). Furthermore, during propofol-induced unconsciousness, a negative correlation was identified between thalamic and cortical activity in these networks. Finally, negative correlations between default network and lateral frontoparietal cortices activity, present during wakefulness, decreased proportionally to propofol-induced loss of consciousness. In contrast, connectivity was globally preserved in low-level sensory cortices, (i.e., in auditory and visual networks across sedation stages). This was paired with preserved thalamocortical connectivity in these networks. Rather, waning of consciousness was associated with a loss of cross-modal interactions between visual and auditory networks. Conclusions:Our results shed light on the functional significance of spontaneous brain activity fluctuations observed in functional magnetic resonance imaging. They suggest that propofol-induced unconsciousness could be linked to a breakdown of cerebral temporal architecture that modifies both within- and between-network connectivity and thus prevents communication between low-level sensory and higher-order frontoparietal cortices, thought to be necessary for perception of external stimuli. They emphasize the importance of thalamocortical connectivity in higher-order cognitive brain networks in the genesis of conscious perception.

High-Yield Radiosynthesis and Preliminary In Vivo Evaluation of p-[18F]MPPF, a Fluoro Analog of WAY-100635

No-carrier-added 4-[18F]fluoro-N-[2-[1-(2-methoxyphenyl)-1 piperazinyl]ethyl-N-2-pyridinyl-benzamide (p-[18F]MPPF) was synthesized by nucleophilic substitution of the corresponding nitro compound in the presence of Kryptofix 222 and K2CO3 by microwave heating (3 min, 500 W) using a remotely controlled radiosynthesis. Baseline separation of p-[18F]MPPF from the nitro derivative was performed on a semipreparative HPLC C18 column. After Sep-Pak formulation, the radiopharmaceutical was obtained with a radiochemical yield of 25% (EOS) in about 70 min. Specific radioactivity averaged between 1-5 Ci/micromol EOS. Labelling of the ortho and meta derivatives was also attempted. Brain uptake of p-[18F]MPPF was studied with PET on fluothane-anesthetized cats. Following intravenous injection of p-[18F]MPPF, high accumulation of radioactivity was observed in the hippocampus and cerebral cortex. Low levels of radioactivity were observed in cerebellum. At 30 min, the mean hippocampus/cerebellum and cortex/cerebellum ratios were 5 and 3.8, respectively. The accumulation of the tracer was blocked by prior administration of reference WAY-100635, demonstrating the specificity of the ligand.

Development and evaluation of an automated atlas-based image analysis method for microPET studies of the rat brain

An automated method for placement of 3D rat brain atlas-derived volumes of interest (VOIs) onto PET studies has been designed and evaluated. VOIs representing major structures of the rat brain were defined on a set of digitized cryosectioned images of the rat brain. For VOI placement, each PET study was registered with a synthetic PET target constructed from the VOI template. Registration was accomplished with an automated algorithm that maximized the mutual information content of the image volumes. The accuracy and precision of this method for VOI placement was determined using datasets from PET studies of the striatal dopamine and hippocampal serotonin systems. Each evaluated PET study could be registered to at least one synthetic PET target without obvious failure. Registration was critically dependent upon the initial position of the PET study relative to the synthetic PET target, but not dependent on the amount of synthetic PET target smoothing. An evaluation algorithm showed that resultant radioactivity concentration measurements of selected brain structures had errors=2% due to misalignment with the corresponding VOI. Further, radioligand binding values calculated from these measurements were found to be more precise than those calculated from measurements obtained with manually drawn regions of interest (ROIs). Overall, evaluation results demonstrated that this atlas-derived VOI method can be used to obtain unbiased measurements of radioactivity concentration from PET studies. Its automated features, and applicability to different radioligands and brain regions, will facilitate quantitative rat brain PET assessment procedures.

Serotonin 5HT2 receptor imaging in the human brain using positron emission tomography and a new radioligand, [18F]Altanserin : results in young normal controls

Changes in serotonin-2 receptors have been demonstrated in brain autopsy material from patients with various neurodegenerative and affective disorders. It would be desirable to locate a ligand for the study of these receptors in vivo with positron emission tomography (PET). Altanserin is a 4-benzoylpiperidine derivative with a high affinity and selectivity for S2 receptors in vitro. Dynamic PET studies were carried out in nine normal volunteers with high-specific activity (376–1,680 mCi/μmol) [18F]altanserin. Arterial blood samples were obtained and the plasma time–activity curves were corrected for the presence of labeled metabolites. Thirty minutes after injection, selective retention of the radioligand was observed in cortical areas, while the cerebellum, caudate, and thalamus had low radioactivity levels. Specific binding reached a plateau between 30 and 65 min postinjection at 1.8% of the injected dose/L of brain and then decreased, indicating the reversibility of the binding. The total/nonspecific binding ratio reached 2.6 for times between 50 and 70 min postinjection. The graphical analysis proposed by Logan et al. allowed us to estimate the binding potential (Bmax/KD). Pretreatment with ketanserin was given to three volunteers and brain activity remained uniformly low. An additional study in one volunteer showed that [18F]altanserin can be displaced from the receptors by large doses of ketanserin. At the end of the study, unchanged altanserin was 57% of the total plasma activity. These results suggest that [18F]altanserin is selective for S2 receptors in vivo as it is in vitro. They indicate that [18F]altanserin is suitable for imaging and quantifying S2 receptors with PET in humans.

Solid phase extraction : An alternative to the use of rotary evaporators for solvent removal in the rapid formulation of PET radiopharmaceuticals

Solid phase extraction (SPE) was used for the formulation of several radiopharmaceuticals. The method involves dilution of the previously purified HPLC compound with water, trapping of the activity on an SPE bed, washing off the support, elution of the radiopharmaceutical with a small volume of ethanol (<1 mL) and dilution with sterile isotonic saline solution. Recovery of the radiopharmaceuticals was always higher than 97%. Two different methods of automation were developed for the formulation of [11C] and [18F]radiopharmaceuticals. In all cases, organic solvent levels in the injectable solution were below the recommended limits. This fast (3–6 min.) and easy to automate process can be considered as an alternative to the conventional methods (rotary evaporators). Copyright

Tissue distribution, autoradiography, and metabolism of 4-(2'-methoxyphenyl)-1-[2' -[N-2"-pyridinyl)-p-[(18)F]fluorobenzamido]ethyl]piperazine (p-[(18)F]MPPF), a new serotonin 5-HT(1A) antagonist for positron emission tomography: An In vivo study in rats.

The in vivo behavior of 4‐(2′‐methoxyphenyl)‐1‐[2′‐[N‐(2″‐pyridinyl)‐p‐[18F]fluorobenzamido]ethyl]‐piperazine (p‐[18F]MPPF), a new serotonin 5‐HT1A antagonist, was studied in awake, freely moving rats. Biodistribution studies showed that the carbon‐fluorine bond was stable in vivo, that this compound was able to cross the blood‐brain barrier, and that a general diffusion equilibrium could account for the availability of the tracer. The great quantity of highly polar metabolites found in plasma did not contribute to the small amounts of metabolites found in hippocampus, frontal cortex, and cerebellum. Exvivo p‐[18F]MPPF and in vitro 8‐hydroxy‐2‐(di‐n‐[3H]propylamino)tetralin autoradiography were compared both qualitatively and quantitatively. Qualitative evaluation proved that the same brain regions were labeled and that the p‐[18F]MPPF labeling is (a) in total agreement with the known distribution of 5‐HT1A receptors in rats and (b) characterized by very low nonspecific binding. Quantitative comparison demonstrated that the in vivo labeling pattern obtained with p‐[18F]MPPF cannot be explained by differences in regional blood flow, capillary density, or permeability. The 5‐HT1A specificity of p‐[18F]MPPF and binding reversibility were confirmed in vivo with displacement experiments. Thus, this compound can be used to evaluate parameters characterizing 5‐HT1A binding sites in the brain.

[18F]p-MPPF: A Radiolabeled Antagonist for the Study of 5-HT1A Receptors with PET

Abstract This paper summarizes the present status of the researches conducted with [ 18 F]4-(2′-methoxyphenyl)-1-[2′-[ N -(2′′-pyridinyl)- p -fluorobenzamido]ethyl]-piperazine known as [ 18 F] p -MPPF, a new 5-HT 1A antagonist for the study of the serotonergic neurotransmission with positron emission tomography (PET). This includes chemistry, radiochemistry, animal data (rats, cats, and monkeys) with autoradiography and PET, human data with PET, toxicity, and metabolism.

Biochemical and autoradiographic measurements of brain serotonin synthesis rate in the freely moving rat: a reexamination of the alpha-methyl-L-tryptophan method.

Abstract: Biochemical approaches were used in freely moving rats to determine, under steady‐state conditions, the brain/arterial plasma partition coefficients of L‐tryptophan and α‐[3H]methyl‐L‐tryptophan, from which the lumped constant for the α‐methyl‐L‐tryptophan method of estimating the rate of brain serotonin synthesis is calculated. The lumped constants were significantly different in the various structures examined: 0.149 ± 0.003 in the raphe dorsalis, 0.103 ± 0.002 in the raphe centralis, 0.087 ± 0.003 in the reticular formation, and 0.62 ± 0.08 in the pineal gland. From these data we proposed a two‐compartment model to calculate the rate of serotonin synthesis by quantitative autoradiography using a three‐time point experiment. Rates of synthesis for the raphe dorsalis and the reticular formation (620 ± 57 and 80 ± 35 pmol/g of tissue/min, respectively) were similar to those measured simultaneously by biochemical means, but rates were 50% higher for the raphe centralis (568 ± 90 vs. 381 ± 31 pmol/g of tissue/min). The lack of dynamic equilibrium of the tracer between plasma and tissue pools may explain the discrepancy between the two methods. Our findings did not confirm previous data, indicating that the application of the autoradiographic method to measure the rate of brain serotonin synthesis using α‐methyl‐L‐tryptophan as tracer has limitations.

Production at the Curie Level of No-Carrier-Added 6-18F-Fluoro-l-Dopa

6-18F-fluoro-l-dopa (18F-FDOPA) has proven to be a useful radiopharmaceutical for the evaluation of presynaptic dopaminergic function using PET. In comparison to electrophilic synthesis, the no-carrier-added (NCA) nucleophilic method has several advantages. These include much higher available activity and specific activity. Recently, we have described an NCA enantioselective synthesis using a chiral phase-transfer catalyst. However, some chemicals were difficult to implement into a commercially available synthesizer, restricting access to this radiopharmaceutical to only a few PET centers. Methods: In this paper, 2 important chemical improvements are proposed to simplify production of 18F-FDOPA, resulting in straightforward automation of the synthesis in a commercially available module. Results: First, a fast, simple, and reliable synthesis of 2-18F-fluoro-4,5-dimethoxybenzyl iodide on a solid-phase support was developed. Second, a phase-transfer catalyst alkylation of a glycine derivative at room temperature was used to enable enantioselective carbon–carbon bond formation. After hydrolysis and high-performance liquid chromatography purification, a high enantiomeric excess of 18F-FDOPA (∼97%) was obtained using a chiral catalyst available from a biphenyl 3 substrate. The total synthesis time was 63 min, and the decay-corrected radiochemical yield was 36% ± 3% (n = 8). Conclusion: By exploiting the advantages of this NCA approach, using a starting activity of 185 GBq of NCA 18F-fluoride, high activities of 18F-FDOPA (>45 GBq) with high specific activity (≥753 GBq/μmol) are now available at the end of synthesis for use in clinical investigations.

Resting-state Network-specific Breakdown of Functional Connectivity during Ketamine Alteration of Consciousness in Volunteers

Background:Consciousness-altering anesthetic agents disturb connectivity between brain regions composing the resting-state consciousness networks (RSNs). The default mode network (DMn), executive control network, salience network (SALn), auditory network, sensorimotor network (SMn), and visual network sustain mentation. Ketamine modifies consciousness differently from other agents, producing psychedelic dreaming and no apparent interaction with the environment. The authors used functional magnetic resonance imaging to explore ketamine-induced changes in RSNs connectivity. Methods:Fourteen healthy volunteers received stepwise intravenous infusions of ketamine up to loss of responsiveness. Because of agitation, data from six subjects were excluded from analysis. RSNs connectivity was compared between absence of ketamine (wake state [W1]), light ketamine sedation, and ketamine-induced unresponsiveness (deep sedation [S2]). Results:Increasing the depth of ketamine sedation from W1 to S2 altered DMn and SALn connectivity and suppressed the anticorrelated activity between DMn and other brain regions. During S2, DMn connectivity, particularly between the medial prefrontal cortex and the remaining network (effect size &bgr; [95% CI]: W1 = 0.20 [0.18 to 0.22]; S2 = 0.07 [0.04 to 0.09]), and DMn anticorrelated activity (e.g., right sensory cortex: W1 = −0.07 [−0.09 to −0.04]; S2 = 0.04 [0.01 to 0.06]) were broken down. SALn connectivity was nonuniformly suppressed (e.g., left parietal operculum: W1 = 0.08 [0.06 to 0.09]; S2 = 0.05 [0.02 to 0.07]). Executive control networks, auditory network, SMn, and visual network were minimally affected. Conclusions:Ketamine induces specific changes in connectivity within and between RSNs. Breakdown of frontoparietal DMn connectivity and DMn anticorrelation and sensory and SMn connectivity preservation are common to ketamine and propofol-induced alterations of consciousness.