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

Abstract

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.

Targeting N-Methyl-d-Aspartate Receptors in Neurodegenerative Diseases

“…Its specific distribution matched the one of the GluN1 subunit, and a low non-specific binding level was seen after pre-injection with ketamine at anesthetic doses. Moreover, FENM competed in vitro with 3 H-TCP in rat brain membranes with a Ki of 3.5 µM [193]. As observed for memantine, FENM was poorly metabolized in vivo with a good stability in plasma and the level of plasma protein binding.…”

“…However, as compared with other PET radiotracers, it showed a low effective dosimetry dose [192]. Moreover, the tracer was used recently in a preclinical model of excitotoxicity induced in Sprague Dawley rats by stereotaxic quinolinic acid injections into the left motor area [193]. PET imaging detected a significant increase in the 18 F-FENM uptake 24 h and 72 h after excitotoxic lesions compared to the control group.…”