Radiosynthesis and Preclinical Evaluation of 18F-F13714 as a Fluorinated 5-HT1A Receptor Agonist Radioligand for PET Neuroimaging

Abstract

PET brain imaging of the serotonin 1A (5-hydroxytryptamine 1A [5-HT(1A)]) receptor has been widely used in clinical studies. Currently, only a few well-validated radiolabeled antagonist tracers are available for in vivo imaging of this central receptor. 5-HT(1A) receptors exist in high- and low-affinity states, depending on their coupling to G proteins. Agonists bind preferentially to receptors in the high-affinity state and thereby could provide a measure of functional 5-HT(1A) receptors. Therefore, it is of great interest to develop an (18)F-labeled full agonist 5-HT(1A) receptor radiotracer. In this study, we radiolabeled the high-affinity 5-HT(1A) receptor agonist (18)F-F13714 and investigated its potential as a PET tracer. F13714 nitro precursor was synthesized and radiolabeled via a fluoronucleophilic substitution. In vitro binding assays were performed using established protocols. Radiopharmacologic evaluations included in vitro autoradiography in rat brain and PET scans on anesthetized cats. The chemical and radiochemical purities of (18)F-F13714 were greater than 98%. F13714 has a high affinity (0.1 nM) and selectivity for 5-HT(1A) receptors. In vitro (18)F-F13714 binding in rats was consistent with the known 5-HT(1A) receptors distribution (hippocampus and cortical areas) and was particularly high in the dorsal raphe. In vitro binding of (18)F-F13714 was blocked in a dose-dependent fashion by WAY100635, the prototypical 5-HT(1A) antagonist, and by the endogenous agonist, serotonin (5-HT). Addition of Gpp(NH)p also inhibited in vitro (18)F-F13714 binding, consistent with a preferential binding of the compound to G-protein-coupled receptors. Ex vivo tissue measurements in rat revealed an absence of brain radioactive metabolites. In vivo studies showed that the radiotracer entered the cat brain readily and displayed a preferential labeling of 5-HT(1A) receptors located in cingulate cortex. In vivo labeling was prevented by preinjection of WAY100635. (18)F-F13714 is a radiofluorinated agonist that presents suitable characteristics for probing the high-affinity states of the 5-HT(1A) receptors in vitro and in vivo. Thus, it is a promising tool for investigation of 5-HT(1A) agonist binding in the living human brain.