With the aim to develop a suitable radiotracer for the brain imaging of the serotonin 4 receptor subtype (5-HT4R) using single photon emission computed tomography (SPECT), we synthesized and evaluated a library of di- and triazaphenanthridines with lipophilicity values which were in the range expected to favour brain penetration, and which demonstrated specific binding to the target of interest. Adding additional nitrogen atoms to previously described phenanthridine ligands exhibiting a high unspecific binding, we were able to design a radioiodinated compound [I-125]14. This compound exhibited a binding affinity value of 0.094 nM toward human 5-HT4R and a high selectivity over other serotonin receptor subtypes (5-HTR). In vivo SPECS imaging studies and competition experiments demonstrated that the decreased lipophilicity (in comparison with our previously reported compounds 4 and 5) allowed a more specific labelling of the 5-HT4R brain-containing regions. (C) 2015 Elsevier Masson SAS. All rights reserved.
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