Characterization of Ro 04‐6790 and Ro 63‐0563: potent and selective antagonists at human and rat 5‐HT₆ receptors

In rats habituated to observation cages, Ro 04‐6790 produced a behavioural syndrome similar to that seen following treatment with antisense oligonucleotides designed to reduce the expression of 5‐HT6 receptors, suggesting that the compounds are neither agonists nor inverse agonists at the 5‐ HT6 receptor.

Abstract

1. This study describes the in vitro characterization of two potent and selective 5-HT6 receptor antagonists at the rat and human recombinant 5-HT6 receptor. 2. In binding assays with [3H]-LSD, 4-amino-N-(2,6 bis-methylamino-pyrimidin-4-yl)-benzene sulphonamide (Ro 04-6790) and 4-amino-N-(2,6 bis-methylamino-pyridin-4-yl)-benzene sulphonamide (Ro 63-0563) had mean pKi values +/-s.e.mean at the rat 5-HT6 receptor of 7.35+/-0.04 and 7.83+/-0.01, respectively and pKi values at the human 5-HT6 receptor of 7.26+/-0.06 and 7.91+/-0.02, respectively. 3 .Both compounds were found to be over 100 fold selective for the 5-HT6 receptor compared to 23 (Ro 04-6790) and 69 (Ro 63-0563) other receptor binding sites. 4. In functional studies, neither compound had any significant effect on basal levels of cyclicAMP accumulation in Hela cells stably expressing the human 5-HT6 receptor, suggesting that the compounds are neither agonists nor inverse agonists at the 5-HT6 receptor. However, both Ro 04-6790 and Ro 63-0563 behaved as competitive antagonists with mean +/-s.e.mean pA2 values of 6.75+/-0.07 and 7.10+/-0.09, respectively. 5. In rats habituated to observation cages, Ro 04-6790 produced a behavioural syndrome similar to that seen following treatment with antisense oligonucleotides designed to reduce the expression of 5-HT6 receptors. This behavioural syndrome consisted of stretching, yawning and chewing. 6. Ro 04-6790 and Ro 63-0563 represent valuable pharmacological tools for the identification of 5-HT6 receptors in natural tissues and the study of their physiological function.