Synthesis of [11C]granisetron, a possible positron emission tomography ligand for 5-HT3 receptor studies

Positron Emission Tomography (PET) Imaging Tracers for Serotonin Receptors

Serotonin (5-hydroxytryptamine, 5-HT) and 5-HT receptors (5-HTRs) have crucial roles in various neuropsychiatric disorders and neurodegenerative diseases, making them attractive diagnostic and therapeutic targets. Positron emission tomography (PET) is a noninvasive nuclear molecular imaging technique and is an essential tool in clinical diagnosis and drug discovery. In this context, numerous PET ligands have been developed for "visualizing" 5-HTRs in the brain and translated into human use to study disease mechanisms and/or support drug development. Herein, we present a comprehensive repertoire of 5-HTR PET ligands by focusing on their chemotypes and performance in PET imaging studies. Furthermore, this Perspective summarizes recent 5-HTR-focused drug discovery, including biased agonists and allosteric modulators, which would stimulate the development of more potent and subtype-selective 5-HTR PET ligands and thus further our understanding of 5-HTR biology.

Synthesis and Pharmacological Evaluation of [11C]Granisetron and [18F]Fluoropalonosetron as PET Probes for 5-HT3 Receptor Imaging

Serotonin-gated ionotropic 5-HT₃ receptors are the major pharmacological targets for antiemetic compounds. Furthermore, they have become a focus for the treatment of irritable bowel syndrome (IBS) and there is some evidence that pharmacological modulation of 5-HT₃ receptors might alleviate symptoms of other neurological disorders. Highly selective, high-affinity antagonists, such as granisetron (Kytril) and palonosetron (Aloxi), belong to a family of drugs (the "setrons") that are well established for clinical use. To enable us to better understand the actions of these drugs in vivo, we report the synthesis of 8-fluoropalonosetron (15) that has a binding affinity (K_i = 0.26 ± 0.05 nM) similar to the parent drug (K_i = 0.21 ± 0.03 nM). We radiolabeled 15 by nucleophilic ¹⁸F-fluorination of an unsymmetrical diaryliodonium palonosetron precursor and achieved the radiosynthesis of 1-(methyl-¹¹C)-N-granisetron ([¹¹C]2) through N-alkylation with [¹¹C]CH₃I, respectively. Both compounds [¹⁸F]15 (chemical and radiochemical purity >95%, specific activity 41 GBq/μmol) and [¹¹C]2 (chemical and radiochemical purity ≥99%, specific activity 170 GBq/μmol) were evaluated for their utility as positron emission tomography (PET) probes. Using mouse and rat brain slices, in vitro autoradiography with both [¹⁸F]15 and [¹¹C]2 revealed a heterogeneous and displaceable binding in cortical and hippocampal regions that are known to express 5-HT₃ receptors at significant levels. Subsequent PET experiments suggested that [¹⁸F]15 and [¹¹C]2 are of limited utility for the PET imaging of brain 5-HT₃ receptors in vivo.