Synthesis and Structure–Affinity Relationships of Selective High-Affinity 5-HT<sub>4</sub> Receptor Antagonists: Application to the Design of New Potential Single Photon Emission Computed Tomography Tracers
The work described herein aims at finding new potential ligands for the brain imaging of 5-HT4receptors (5-HT4Rs) using single-photon emission computed tomography (SPECT). Starting from the nonsubstituted phenanthridine compound 4a, exhibiting a Ki value of 51 nM on the 5-HT4R, we explored the structure–affinity in this series. We found that substitution in position 4 of the tricycle with a fluorine