Structure-Activity Relationship Study of Heterocyclic Phenylethenyl and Pyridinylethenyl Derivatives as Tau-Imaging Agents That Selectively Detect Neurofibrillary Tangles in Alzheimer's Disease Brains

In order to explore novel tau-imaging agents that can selectively detect neurofibrillary tangles in Alzheimer's disease (AD) brains, we designed and synthesized a series of heterocyclic phenylethenyl and (3-pyridinyl)ethenyl derivatives with or without a dimethyl amino group. In in vitro autoradiography using AD brain sections, all radioiodinated ligands with a dimethyl amino group bound to A beta deposits in the sections. In contrast, the ligands without a dimethyl amino group showed different patterns of radioactivity accumulation in the sections depending on the kind of heterocycle contained in their molecules. Particularly, a phenylethenyl benzimidazole derivative ([I-125]64) showed marked radioactivity accumulation in the temporal lobe which corresponded with the distribution of tau deposits. [I-125]64 also showed the most favorable pharmacokinetics in normal mouse brains (3.69 and 0.06% ID/g at 2 and 60 min postinjection, respectively) among all ligands in this study. Taken together, these results suggest that [I-123]64 may be a new candidate tau-imaging agent.