PET Brain imaging of α7-nAChR with [18F]ASEM

The α7 nicotinic acetylcholine receptor (nAChR) increasingly has been implicated in normal brain physiology, as well as in neuropsychiatric disorders. The α7-nAChR primarily is located in cerebral cortex and sub-cortical regions, compared to the α4β2 nAChR subtype that has a more subcortical distribution. The highly cortical distribution suggests a role of α7-nAChR in cognition. We expanded the first-in-human PET imaging of α7-nAChR with [18F]ASEM from five to 21 healthy non-smoking volunteers and added preliminary evidence of binding in six male patients with schizophrenia. Study aims included 1) confirmation of test-retest reproducibility of [18F]ASEM binding in normal volunteers, 2) demonstration of specificity of [18F]ASEM binding by competition with DMXB-A, an α7-nAChR partial agonist previously tested in clinical trials of patients with schizophrenia, 3) estimation of [18F]ASEM binding potentials and α7-nAChR density in vivo in humans, and 4) α7-nAChR binding in patients with schizophrenia compared to healthy volunteers. Test-retest PET confirmed reproducibility (>90%) (variability ≤ 7%) of [18F]ASEM volume of distribution (VT) estimates in healthy volunteers. Repeated sessions of PET in five healthy subjects included baseline and effect of inhibition after oral administration of 150 mg DMXB-A. From reduction of binding potentials, we estimated the dose-dependent occupancy of α7-nAChR by DMXB-A at 17–49% for plasma concentrations at 60–200 nM DMXB-A. In agreement with evidence post-mortem, α7-nAChR density (Bmax) averaged 0.67–0.82 nM and inhibitor affinity constant (KI) averaged 170–385 nM. Median VT in the six patients with schizophrenia was lower than in healthy volunteers in cingulate cortex, frontal cortex, and hippocampus. Mann-Whitney test identified cingulate cortex and hippocampus as regions with significantly lower median VT in patients than in healthy volunteers when a single outlier patient was excluded from analysis (P = 0.02, corrected for multiple comparisons).