HTR4 and Alzheimer disease: In the 5-HT4R class though, one PET ligand was evaluated on AD patients: [1-[11C]methylpiperidin-4-yl]methyl 8-amino-7-chloro-2,3-dihydro-1,4-benzodioxine-5-carboxylate ([11C]SB207145), a 5-HT4R antagonist (Figure 8).