[3H]-F13640, a novel, selective and high-efficacy serotonin 5-HT1A receptor agonist radioligand

F13640 is a selective and high-efficacy serotonin 5-HT_1A receptor agonist that demonstrates outstanding analgesic potential in different animal models. Here, we use the radiolabelled compound to further characterise its binding properties at 5-HT_1A receptors. F13640 was tritium-labelled to 47 and 64 Ci/mmol specific activity and used as radioligand at membrane preparations of CHO cells expressing human (h) 5-HT_1A receptors. The K _d of [³H]-F13640 was 1.8 nM at h5-HT_1A receptors as determined from saturation binding experiments. In association time-course experiments, k _obs of [³H]-F13640 was 0.06 min^?1. Dissociation experiments performed in the presence of unlabelled F13640 as competing ligand yielded a k _off value of 0.05 min^?1, resulting in a calculated K _d of 1.4 nM. In comparison, [³H]-8-OH-DPAT had a k _obs of 0.50 min^?1, a k _off of 0.25 min^?1 and a calculated K _d of 0.37 nM. Surprisingly, [³H]-F13640 dissociation kinetics were distinctly slower in the presence of WAY-100635 and spiperone as competing ligands when compared with the agonist competitors, F13640 and (+)8-OH-DPAT. The competitive binding profile of [³H]-F13640 with eight chemically diverse 5-HT_1A receptor agonists and antagonists correlated highly (r?=?0.996) with that of [³H]-8-OH-DPAT. In conclusion, [³H]-F13640 is a potent agonist radioligand at 5-HT_1A receptors and may be a useful tool in pharmacological studies at native and recombinant 5-HT_1A receptors. In addition, [³H]-F13640 dissociates more slowly from h5-HT_1A receptors than [³H]-8-OH-DPAT, a kinetic property that might be related to its powerful analgesic effects as observed in vivo.