Radiosynthesis and characterization of 11C-GSK215083 as a PET radioligand for the 5-HT6 receptor.

UNLABELLED The development of a PET radioligand for imaging 5-hydroxytryptamine (5-HT) 6 receptors in the brain would, for the first time, enable in vivo imaging of this target along with assessment of its involvement in disease pathophysiology. In addition, such a tool would assist in the development of novel drugs targeting the 5-HT6 receptor. METHODS On the basis of in vitro data, GSK215083 was identified as a promising 5-HT6 radioligand candidate and was radiolabeled with (11)C via methylation. The in vivo properties of (11)C-GSK215083 were evaluated first in pigs (to investigate brain penetration and specific binding), second in nonhuman primates (to confirm brain penetration, specific binding, selectivity, and kinetics), and third in human subjects (to confirm brain penetration and biodistribution). RESULTS (11)C-GSK215083 readily entered the brain in all 3 species, leading to a heterogeneous distribution (striatum > cortex > cerebellum) consistent with reported 5-HT6 receptor densities and distribution determined by tissue-section autoradiography in preclinical species and humans. In vivo saturation studies using escalating doses of GSK215083 in primates demonstrated saturable, dose-dependent binding to the 5-HT6 receptor in the striatum. Importantly, (11)C-GSK215083 also exhibited affinity for the 5-HT2A receptor; however, given the differential localization of these 2 receptors in the central nervous system, the discrete 5-HT6 binding properties of this radioligand were able to be determined. CONCLUSION These data demonstrate the utility of (11)C-GSK215083 as a promising PET radioligand for probing the 5-HT6 receptor in vivo in both preclinical and clinical species.