HRH3 and early-onset autosomal dominant Alzheimer disease: GSK189254, a benazepin derivative, is an inverse H3R agonist developed to improve cognitive functions in Alzheimer’s disease [5] and was labeled with 11C for PET imaging of H3Rs. [11C]GSK189254 could visualize H3Rs in the human brains, but its slow kinetics impeded accurate quantification of the available binding sites in brain regions with high-level H3R expression [11].