The positron emission tomography (PET) tracer [18F]GE-179 binds to the phencyclidine (PCP) site in the open N-methyl-D-aspartate receptor ion channel (NMDAR-IC). To demonstrate that PET can visualise increased [18F]GE-179 uptake by active NMDAR-ICs and that this can be blocked by the PCP antagonist S-ketamine, 15 rats had an electrode unilaterally implanted in their ventral hippocampus. Seven rats had no stimulation, five received pulsed 400 µA supra-threshold 60 Hz stimulation alone, and three received intravenous S-ketamine injection prior to stimulation. Six other rats were not implanted. Each rat had a 90 min [18F]GE-179 PET scan. Stimulated rats had simultaneous depth-EEG recordings of induced seizure activity. [18F]GE-179 uptake (volume of distribution, VT) was compared between hemispheres and between groups. Electrical stimulation induced a significant increase in [18F]GE-179 uptake at the electrode site compared to the contralateral hippocampus (mean 22% increase in VT, p =  0.0014) and to non-stimulated comparator groups. Rats injected with S-ketamine prior to stimulation maintained non-stimulated levels of [18F]GE-179 uptake during stimulation. In conclusion, PET visualisation of focal [18F]GE-179 uptake during electrically activated NMDAR-ICs and the demonstration of specificity for PCP sites by blockade with S-ketamine support the in vivo utility of [18F]GE-179 PET as a use-dependent marker of NMDAR-IC activation.

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SummaryObjectiveImaging activated glutamate N‐methyl‐D‐aspartate receptor ion channels (NMDAR‐ICs) using positron emission tomography (PET) has proved challenging due to low brain uptake, poor affinity and selectivity, and high metabolism and dissociation rates of candidate radioligands. The radioligand [18F]GE‐179 is a known use‐dependent marker of NMDAR‐ICs. We studied whether interictal [18F]GE‐179 PET would detect foci of abnormal NMDAR‐IC activation in patients with refractory focal epilepsy.MethodsTen patients with refractory focal epilepsy and 18 healthy controls had structural magnetic resonance imaging (MRI) followed by a 90‐min dynamic [18F]GE‐179 PET scan with simultaneous electroencephalography (EEG). PET and EEG findings were compared with MRI and previous EEGs. Standard uptake value (SUV) images of [18F]GE‐179 were generated and global gray matter uptake was measured for each individual. To localize focal increases in uptake of [18F]GE‐179, the individual SUV images were interrogated with statistical parametric mapping in comparison to a normal database. Additionally, individual healthy control SUV images were compared with the rest of the control database to determine their prevalence of increased focal [18F]GE‐179 uptake.ResultsInterictal [18F]GE‐179 PET detected clusters of significantly increased binding in eight of 10 patients with focal epilepsy but none of the controls. The number of clusters of raised [18F]GE‐179 uptake in the patients with epilepsy exceeded the focal abnormalities revealed by the simultaneously recorded EEG. Patients with extensive clusters of raised [18F]GE‐179 uptake showed the most abnormal EEGs.SignificanceDetection of multiple foci of abnormal NMDAR‐IC activation in 80% of our patients with refractory focal epilepsy using interictal [18F]GE‐179 PET could reflect enhanced neuronal excitability due to chronic seizure activity. This indicates that chronic epileptic activity is associated with abnormal NMDAR ion channel activation beyond the initial irritative zones. [18F]GE‐179 PET could be a candidate marker for identifying pathological brain areas in patients with treatment‐resistant focal epilepsy.