Radiosynthesis and radiopharmacological evaluation of fluorinated PET radioligands for the 5-HT6 receptors

Objectives The serotonin 5-HT6 receptors are potent therapeutic targets for psychiatric and neurological diseases, i.e., schizophrenia and Alzheimer’s disease. However, the understanding of their pharmacology is currently partial and limited to animal models, particularly because of the lack of fluorinated PET 5-HT6 radiopharmaceuticals. We therefore developed several 5-HT6 radioligands and evaluated their suitability for PET imaging.

Methods Various quinoline and pyridine-based ligands have been synthesized, inspired by the 5-HT6 receptor pharmacophore. The molecules were synthesized by using coupling reactions in 3 steps. Only ligands with high affinities toward 5-HT6 receptors and low affinities toward 5-HT2A receptors (a close pharmacophore) were radiolabeled via 18F-nucleophilic aromatic substitution. These candidates were evaluated by in vitro autoradiography in rat brain. The most interesting radioligands of these were evaluated by in vivo microPET in rat.

Results Eight molecules were initially synthesised and three molecules with low in vitro affinity for 5-HT6 receptors were removed (Ki >10 nM). The chemical and radiochemical purities of the five remaining fluorine radiotracers were > 99%, with a radiochemical yield of 5-45% (EOB) and specific activities in the range of 40-104 GBq/µmole (EOS). Three radiolabelled molecules presented a high in vitro binding in the striatum area rich in 5-HT6 receptors and a dose-dependent displacement by a 5-HT6 receptor antagonist. The microPET studies showed that one among these three molecules had a good brain penetration and a striatal fixation.

Conclusions Our studies allowed to select a radiotracer-candidate with suitable characteristics for PET imaging of 5-HT6 receptors, justifying further evaluations in post-mortem human tissues and the development of chemical analogues.