Pharmacological Characterization of [18F]-FNM and Evaluation of NMDA Receptors Activation in a Rat Brain Injury Model.

PURPOSE: NMDA receptors (NMDARs) dysfunction plays a central role in the physiopathology of psychiatric and neurodegenerative disorders whose mechanisms are still poorly understood. The development of a PET (positron emission tomography) tracer able to selectively bind to the NMDARs intra-channel PCP site may make it possible to visualize NMDARs in an open and active state. We describe the in vitro pharmacological characterization of [18F]-fluoroethylnormemantine ([18F]-FNM) and evaluate its ability to localize activated NMDA receptors in a rat preclinical model of excitotoxicity. PROCEDURES: The affinity of the non-radioactive analog for the intra-channel PCP site was determined in a radioligand competition assay using [3H]TCP ([3H]N-(1-[thienyl]cyclohexyl)piperidine) on rat brain homogenates. Selectivity was also investigated by the displacement of specific radioligands targeting various cerebral receptors. In vivo brain lesions were performed using stereotaxic quinolinic acid (QA) injections in the left motor area (M1) of seven Sprague Dawley rats. Each rat was imaged with a microPET/CT camera, 40 min after receiving a dose of 30 MBq + / - 20 of [18F]-FNM, 24 and 72 h after injury. Nine non-injured rats were also imaged using the same protocol. RESULTS: FNM displayed IC50 value of 13.0 ± 8.9 µM in rat forebrain homogenates but also showed significant bindings on opioid receptors. In the frontal and left somatosensory areas, [18F]FNM PET detected a mean of 37% and 41% increase in [18F]FNM uptake (p < 0,0001) 24 and 72 h after QA stereotaxic injection, respectively, compared to the control group. CONCLUSIONS: In spite of FNM's poor affinity for NMDAR PCP site, this study supports the ability of this tracer to track massive activation of NMDARs in neurological diseases.

Intravenous route for folate supplementation in a patient with celiac disease treated by pemetrexed-based chemotherapy for non-small-cell lung cancer.

INTRODUCTION: Oral folic acid supplementation is essential for patients treated with pemetrexed, to prevent the risk of severe hematologic toxicity. In case of intestinal absorption disorder, no recommendations exist for intravenous folic acid supplementation. CASE REPORT: We describe a 74-year-old patient with multimetastatic non-small-cell lung adenocarcinoma, receiving first-line chemotherapy with carboplatin AUC5, pemetrexed 500 mg/m2 and pembrolizumab 200 mg intravenously every 3 weeks. The patient presented neglected celiac disease, resulting in malabsorption syndrome with iron and folic acid deficiency. The question was how to administer folic acid supplementation during the pemetrexed-based chemotherapy. MANAGEMENT AND OUTCOMES: Intravenous injection of 200 mg levoleucovorin on day 1 of cycle 1 of pemetrexed-based chemotherapy was administered and well tolerated. During the second cycle, the levoleucovorin perfusion was not renewed by omission. The patient was hospitalized for 7 days because of febrile aplasia. Piperacillin-tazobactam was started, and then switched to amoxicillin-clavulanate plus ciprofloxacin. After this episode of post-chemotherapy febrile aplasia, it was decided to systematically supplement the patient with intravenous levoleucovorin, with blood folate concentration monitoring at each cycle. At 16 months after start of treatment, the patient was in complete remission, indicating that the immune-chemotherapy was effective, with no further febrile neutropenia. DISCUSSION/CONCLUSION: This case report highlights intravenous levoleucovorin supplementation as an alternative to oral folic acid if needed during pemetrexed-antifolate-based chemotherapy.

Reduced Regional Cerebral Blood Flow Measured by 99mTc-Hexamethyl Propylene Amine Oxime Single-Photon Emission Computed Tomography in Microgravity Simulated by 5-Day Dry Immersion.

Microgravity induces a cephalad fluid shift that is responsible for cephalic venous stasis that may increase intracranial pressure (ICP) in astronauts. However, the effects of microgravity on regional cerebral blood flow (rCBF) are not known. We therefore investigated changes in rCBF in a 5-day dry immersion (DI) model. Moreover, we tested thigh cuffs as a countermeasure to prevent potential microgravity-induced modifications in rCBF. Around 18 healthy male participants underwent 5-day DI with or without a thigh cuffs countermeasure. They were randomly allocated to a control (n=9) or cuffs (n=9) group. rCBF was measured 4days before DI and at the end of the fifth day of DI (DI5), using single-photon emission computed tomography (SPECT) with radiopharmaceutical 99mTc-hexamethyl propylene amine oxime (99mTc-HMPAO). SPECT images were processed using statistical parametric mapping (SPM12) software. At DI5, we observed a significant decrease in rCBF in 32 cortical and subcortical regions, with greater hypoperfusion in basal ganglia (right putamen peak level: z=4.71, p uncorr<0.001), bilateral occipital regions (left superior occipital peak level: z=4.51, p uncorr<0.001), bilateral insula (right insula peak level: 4.10, p uncorr<0.001), and bilateral inferior temporal (right inferior temporal peak level: 4.07, p uncorr<0.001). No significant difference was found between the control and cuffs groups on change in rCBF after 5days of DI. After a 5-day DI, we found a decrease in rCBF in cortical and subcortical regions. However, thigh cuffs countermeasure failed to prevent hypoperfusion. To date, this is the first study measuring rCBF in DI. Further investigations are needed in order to better understand the underlying mechanisms in cerebral blood flow (CBF) changes after exposure to microgravity.

Innovative Molecular Imaging for Clinical Research, Therapeutic Stratification, and Nosography in Neuroscience.

Over the past few decades, several radiotracers have been developed for neuroimaging applications, especially in PET. Because of their low steric hindrance, PET radionuclides can be used to label molecules that are small enough to cross the blood brain barrier, without modifying their biological properties. As the use of 11C is limited by its short physical half-life (20 min), there has been an increasing focus on developing tracers labeled with 18F for clinical use. The first such tracers allowed cerebral blood flow and glucose metabolism to be measured, and the development of molecular imaging has since enabled to focus more closely on specific targets such as receptors, neurotransmitter transporters, and other proteins. Hence, PET and SPECT biomarkers have become indispensable for innovative clinical research. Currently, the treatment options for a number of pathologies, notably neurodegenerative diseases, remain only supportive and symptomatic. Treatments that slow down or reverse disease progression are therefore the subject of numerous studies, in which molecular imaging is proving to be a powerful tool. PET and SPECT biomarkers already make it possible to diagnose several neurological diseases in vivo and at preclinical stages, yielding topographic, and quantitative data about the target. As a result, they can be used for assessing patients' eligibility for new treatments, or for treatment follow-up. The aim of the present review was to map major innovative radiotracers used in neuroscience, and explain their contribution to clinical research. We categorized them according to their target: dopaminergic, cholinergic or serotoninergic systems, ß-amyloid plaques, tau protein, neuroinflammation, glutamate or GABA receptors, or α-synuclein. Most neurological disorders, and indeed mental disorders, involve the dysfunction of one or more of these targets. Combinations of molecular imaging biomarkers can afford us a better understanding of the mechanisms underlying disease development over time, and contribute to early detection/screening, diagnosis, therapy delivery/monitoring, and treatment follow-up in both research and clinical settings.

Evaluation of [18F]FNM biodistribution and dosimetry based on whole-body PET imaging of rats.

INTRODUCTION: The aim of this work was to study the biodistribution, metabolism and radiation dosimetry of rats injected with [18F]FNM using PET/CT images. This novel radiotracer targeting NMDA receptor has potential for investigation for neurological and psychiatric diseases. METHODS: Free fraction and stability in fresh human plasma were determined in vitro. PET/CT was performed on anesthetized rats. Organs were identified and 3D volumes of interest (VOIs) were manually drawn on the CT in the center of each organ. Time activity curves (TACs) were created with these VOIs, enabling the calculation of residence times. To confirm these values, ex vivo measurements of organs were performed. Plasma and urine were also collected to study in vivo metabolism. Data was extrapolated to humans, effective doses were estimated using ICRP-60 and ICRP-89 dosimetric models and absorbed doses were estimated using OLINDA/EXM V1.0 and OLINDA/EXM V2.0 (which use weighting factors from ICRP-103 to do the calculations). RESULTS: The [18F]FNM was stable in human plasma and the diffusible free fraction was 53%. As with memantine, this tracer is poorly metabolized in vivo. Ex vivo distributions validated PET/CT data as well as demonstrating a decrease of radiotracer uptake in the brain due to anesthesia. Total effective dose was around 6.11 µSv/MBq and 4.65 µSv/MBq for female and male human dosimetric models, respectively. CONCLUSIONS: This study shows that the presented compound exhibits stability in plasma and plasma protein binding very similar to memantine. Its dosimetry shows that it is suitable for use in humans due to a low total effective dose compared to other PET radiotracers.

Imaging grafted cells with [18F]FHBG using an optimized HSV1-TK mammalian expression vector in a brain injury rodent model.

INTRODUCTION: Cell transplantation is an innovative therapeutic approach after brain injury to compensate for tissue damage. To have real-time longitudinal monitoring of intracerebrally grafted cells, we explored the feasibility of a molecular imaging approach using thymidine kinase HSV1-TK gene encoding and [18F]FHBG as a reporter probe to image enzyme expression. METHODS: A stable neuronal cell line expressing HSV1-TK was developed with an optimised mammalian expression vector to ensure long-term transgene expression. After [18F]FHBG incubation under defined parameters, calibration ranges from 1 X 104 to 3 X 106 Neuro2A-TK cells were analysed by gamma counter or by PET-camera. In parallel, grafting with different quantities of [18F]FHBG prelabelled Neuro2A-TK cells was carried out in a rat brain injury model induced by stereotaxic injection of malonate toxin. Image acquisition of the rats was then performed with PET/CT camera to study the [18F]FHBG signal of transplanted cells in vivo. RESULTS: Under the optimised incubation conditions, [18F]FHBG cell uptake rate was around 2.52%. In-vitro calibration range analysis shows a clear linear correlation between the number of cells and the signal intensity. The PET signal emitted into rat brain correlated well with the number of cells injected and the number of surviving grafted cells was recorded via the in-vitro calibration range. PET/CT acquisitions also allowed validation of the stereotaxic injection procedure. Technique sensitivity was evaluated under 5 X 104 grafted cells in vivo. No [18F]FHBG or [18F]metabolite release was observed showing a stable cell uptake even 2 h post-graft. CONCLUSION: The development of this kind of approach will allow grafting to be controlled and ensure longitudinal follow-up of cell viability and biodistribution after intracerebral injection.