Longitudinal mouse-PET imaging: a reliable method for estimating binding parameters without a reference region or blood sampling

International audienceLongitudinal mouse PET imaging is becoming increasingly popular due to the large number of transgenic and disease models available but faces challenges. These challenges are related to the small size of the mouse brain and the limited spatial resolution of microPET scanners, along with the small blood volume making arterial blood sampling challenging and impossible for longitudinal studies. The ability to extract an input function directly from the image would be useful for quantification in longitudinal small animal studies where there is no true reference region available such as TSPO imaging.METHODS:Using dynamic, whole-body 18F-DPA-714 PET scans (60 min) in a mouse model of hippocampal sclerosis, we applied a factor analysis (FA) approach to extract an image-derived input function (IDIF). This mouse-specific IDIF was then used for 4D-resolution recovery and denoising (4D-RRD) that outputs a dynamic image with better spatial resolution and noise properties, and a map of the total volume of distribution (VT) was obtained using a basis function approach in a total of 9 mice with 4 longitudinal PET scans each. We also calculated percent injected dose (%ID) with and without 4D-RRD. The VT and %ID parameters were compared to quantified ex vivo autoradiography using regional correlations of the specific binding from autoradiography against VT and %ID parameters.RESULTS:The peaks of the IDIFs were strongly correlated with the injected dose (Pearson R = 0.79). The regional correlations between the %ID estimates and autoradiography were R = 0.53 without 4D-RRD and 0.72 with 4D-RRD over all mice and scans. The regional correlations between the VT estimates and autoradiography were R = 0.66 without 4D-RRD and 0.79 with application of 4D-RRD over all mice and scans.CONCLUSION:We present a FA approach for IDIF extraction which is robust, reproducible and can be used in quantification methods for resolution recovery, denoising and parameter estimation. We demonstrated that the proposed quantification method yields parameter estimates closer to ex vivo measurements than semi-quantitative methods such as %ID and is immune to tracer binding in tissue unlike reference tissue methods. This approach allows for accurate quantification in longitudinal PET studies in mice while avoiding repeated blood sampling

Preparation and evaluation of novel pyrazolo[1,5-a]pyrimidine acetamides, closely related to DPA-714, as potent ligands for imaging the TSPO 18kDa with PET

International audienc

Impact of Endothelial 18-kDa Translocator Protein on the Quantification of 18 F-DPA-714

International audienc

Differential influence of propofol and isoflurane anesthesia in non-human primate on the brain kinetics and binding of [18^{18}F]DPA-714, a PET imaging marker of glial activation

International audienceTranslocator protein 18 kDa (TSPO) expression at the mitochondrial membrane of glial cells is related to glial activation. TSPO radioligands such as [$^{18}$F]DPA‐714 are useful for the non‐invasive study of neuroimmune processes using positron emission tomography (PET). Anesthetic agents were shown to impact mitochondrial function and may influence [$^{18}$F]DPA‐714 binding parameters and PET kinetics. [$^{18}$F]DPA‐714 PET imaging was performed in $Papio\ anubis$ baboons anesthetized using either intravenous propofol ($n$ = 3) or inhaled isoflurane ($n$ = 3). Brain kinetics and metabolite‐corrected input function were measured to estimate [$^{18}$F]DPA‐714 brain distribution ($V_T$). Displacement experiments were performed using PK11195 (1.5 mg/kg). In vitro [$^{18}$F]DPA‐714 binding experiments were performed using baboon brain tissue in the absence and presence of tested anesthetics. Brain radioactivity peaked higher in isoflurane‐anesthetized animals compared with propofol (SUVmax = 2.7 ± 0.5 vs. 1.3 ± 0.2, respectively) but was not different after 30 min. Brain $V_T$ was not different under propofol and isoflurane. Displacement resulted in a 35.8 ± 8.4% decrease of brain radioactivity under propofol but not under isoflurane (0.1 ± 7.0%). In vitro, the presence of propofol increased TSPO density and dramatically reduced its affinity for [$^{18}$F]DPA‐714 compared with control. This in vitro effect was not significant with isoflurane. Exposure to propofol and isoflurane differentially influences TSPO interaction with its specific radioligand [$^{18}F$]DPA‐714 with subsequent impact on its tissue kinetics and specific binding estimated in vivo using PET. Therefore, the choice of anesthetics and their potential influence on PET data should be considered for the design of imaging studies using TSPO radioligands, especially in a translational research context

Imaging the Impact of the P-Glycoprotein (ABCB1) Function on the Brain Kinetics of Metoclopramide

International audienceThe effects of metoclopramide on the central nervous system (CNS) in patients suggest substantial brain distribution. Previous data suggest that metoclopramide brain kinetics may nonetheless be controlled by ATP-binding cassette (ABC) transporters expressed at the blood-brain barrier. We used (11)C-metoclopramide PET imaging to elucidate the kinetic impact of transporter function on metoclopramide exposure to the brain

Optimized quantification of translocator protein radioligand 18^{18}F-DPA-714 uptake in the brain of genotyped healthy volunteers

International audienceTranslocator protein (TSPO) is expressed at a low level in healthy brain and is upregulated during inflammatory processes that may occur in neurodegenerative diseases. Thus, TSPO may be a suitable in vivo indicator of neurodegeneration. Here, we quantified the $^{18}$F-DPA-714 radioligand in healthy TSPO-genotyped volunteers and developed a method to eliminate the need for invasive arterial blood sampling. Methods: Ten controls (7 high-affinity binders [HABs] and 3 mixed-affinity binders [MABs]) underwent $^{18}$F-DPA-714 PET with arterial and venous sampling. $^{18}$F-DPA-714 binding was quantified with a metabolite-corrected arterial plasma input function, using the 1-and 2-tissue-compartment models (TCMs) as well as the Logan analysis to estimate total volume distribution (V$_T$) in the regions of interest. Alternative quantification methods were tested, including tissue-to-plasma ratio or population-based input function approaches normalized by late time points of arterial or venous samples. Results: The distribution pattern of $^{18}$F-DPA-714 was consistent with the known distribution of TSPO in humans, with the thalamus displaying the highest binding and the cerebellum the lowest. The 2-TCM best described the regional kinetics of 18 F-DPA-714 in the brain, with good identifiability (percentage coefficient of variation , 5%). For each region of interest, V$_T$ was 47.6% ± 6.3% higher in HABs than in MABs, and estimates from the 2-TCM and the Logan analyses were highly correlated. Equilibrium was reached at 60 min after injection. V$_T$ calculated with alternative methods using arterial samples was strongly and significantly correlated with that calculated by the 2-TCM. Replacement of arterial with venous sampling in these methods led to a significant but lower correlation. Conclusion: Gen-otyping of subjects is a prerequisite for a reliable quantification of $^{18}$F-DPA-714 PET images. The 2-TCM and the Logan analyses are accurate methods to estimate $^{18}$F-DPA-714 V T in the human brain of both HAB and MAB individuals. Population-based input function and tissue-to-plasma ratio with a single arterial sample are promising alternatives to classic arterial plasma input function. Substitution with venous samples is promising but still requires methodologic improvements

[ 18 F]DPA‐C5yne, a novel fluorine‐18‐labelled analogue of DPA‐714: radiosynthesis and preliminary evaluation as a radiotracer for imaging neuroinflammation with PET

International audienceAbstract DPA‐C5yne, the lead compound of a novel series of DPA‐714 derivatives in which the fluoroethoxy chain linked to the phenylpyrazolopyrimidine scaffold has been replaced by a fluoroalkyn‐1‐yl moiety, is a high affinity (K i : 0.35 nM) and selective ligand targeting the translocator protein 18 kDa. In the present work, DPA‐C5yne was labelled with no‐carrier‐added [ 18 F]fluoride based on a one‐step tosyloxy‐for‐fluorine nucleophilic substitution reaction, purified by cartridge and HPLC, and formulated as an i.v. injectable solution using a TRACERLab FX N Pro synthesizer. Typically, 4.3–5.2 GBq of [ 18 F]DPA‐C5yne, ready‐to‐use, chemically and radiochemically pure (> 95%), was obtained with specific radioactivities ranging from 55 to 110 GBq/µmol within 50–60 min, starting from a 30 GBq [ 18 F]fluoride batch (14–17%). LogP and LogD of [ 18 F]DPA‐C5yne were measured using the shake‐flask method and values of 2.39 and 2.51 were found, respectively. Autoradiography studies performed on slices of ((R,S)‐α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolopropionique (AMPA)‐lesioned rat brains showed a high target‐to‐background ratio (1.9 ± 0.3). Selectivity and specificity of the binding for the translocator protein was demonstrated using DPA‐C5yne (unlabelled), PK11195 and Flumazenil (central benzodiazepine receptor ligand) as competitors. Furthermore, DPA‐C5yne proved to be stable in plasma at 37°C for at least 90 min

Increased microglial activation in patients with Parkinson disease using [18F]-DPA714 TSPO PET imaging

International audienceIntroduction: Increasing evidence suggests that neuroinflammation is active in Parkinson disease (PD) and contributes to neurodegeneration. This process can be studied in vivo with PET and radioligands targeting TSPO, upregulated in activated microglia. Initial PET studies investigating microglial activation in PD with the [$^{11}$C]-PK11195 have provided inconclusive results. Here we assess the presence and distribution of neuroinflammatory response in PD patients using [$^{18}$F]-DPA714 and to correlate imaging biomarkers to dopamine transporter imaging and clinical status.Methods: PD patients (n = 24, Hoehn and Yahr I-III) and 28 healthy controls were scanned with [$^{18}$F]-DPA714 and [$^{11}$C]-PE2I and analyzed. They were all genotyped for TSPO polymorphism. Regional binding parameters were estimated (reference Logan graphical approach with supervised cluster analysis). Impact of TSPO genotype was analyzed using Wilcoxon signed-rank test. Differences between groups were investigated using a two-way ANOVA and Tukey post hoc tests.Results: PD patients showed significantly higher [$^{18}$F]-DPA714 binding compared to healthy controls bilaterally in the midbrain (p < 0.001), the frontal cortex (p = 0.001), and the putamen contralateral to the more clinically affected hemibody (p = 0.038). Microglial activation in these regions did not correlate with the severity of motor symptoms, disease duration nor putaminal [$^{11}$C]-PE2I uptake. However, there was a trend toward a correlation between cortical TSPO binding and disease duration (p = 0.015 uncorrected, p = 0.07 after Bonferroni correction).Conclusion: [$^{18}$F]-DPA714 binding confirmed that there is a specific topographic pattern of microglial activation in the nigro-striatal pathway and the frontal cortex of PD patients

From Structure–Activity Relationships on Thiazole Derivatives to the In Vivo Evaluation of a New Radiotracer for Cannabinoid Subtype 2 PET Imaging

International audienceAbstract Nowadays, high‐resolution mass spectrometry is widely used for metabolomic studies. Thanks to its high sensitivity, it enables the detection of a large range of metabolites. In metabolomics, the continuous quest for a metabolite identification as complete and accurate as possible has led during the last decade to an ever increasing development of public MS databases (including LC‐MS data) concomitantly with bioinformatic tool expansion. To facilitate the annotation process of MS profiles obtained from biological samples, but also to ease data sharing, exchange, and exploitation, the standardization and harmonization of the way to describe and annotate mass spectra seemed crucial to us. Indeed, under electrospray (ESI) conditions, a single metabolite does not produce a unique ion corresponding to its protonated or deprotonated form but could lead to a complex mixture of signals. These MS signals result from the existence of different natural isotopologues of the same compound and also to the potential formation of adduct ions, homomultimeric and heteromultimeric ions, fragment ions resulting from “prompt” in‐source dissociations. As a joint reflection process within the French Infrastructure for Metabolomics and Fluxomics (MetaboHUB) and with the purpose of developing a robust and exchangeable annotated MS database made from pure reference compounds (chemical standards) analysis, it appeared to us that giving the metabolomics community some clues to standardize and unambiguously annotate each MS feature was a prerequisite to data entry and further efficient querying of the mass spectral database. The use of a harmonized notation is also mandatory for interlaboratory MS data exchange. Additionally, thorough description of the variety of MS signals arising from the analysis of a unique metabolite might provide greater confidence on its annotation