Characterization of Ro 04‐6790 and Ro 63‐0563: potent and selective antagonists at human and rat 5‐HT 6 receptors

This study describes the in vitro characterization of two potent and selective 5‐HT 6 receptor antagonists at the rat and human recombinant 5‐HT 6 receptor. In binding assays with [ 3 H]‐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 pK i values ±s.e.mean at the rat 5‐HT 6 receptor of 7.35±0.04 and 7.83±0.01, respectively and pK i values at the human 5‐HT 6 receptor of 7.26±0.06 and 7.91±0.02, respectively. Both compounds were found to be over 100 fold selective for the 5‐HT 6 receptor compared to 23 (Ro 04‐6790) and 69 (Ro 63‐0563) other receptor binding sites. In functional studies, neither compound had any significant effect on basal levels of cyclicAMP accumulation in Hela cells stably expressing the human 5‐HT 6 receptor, suggesting that the compounds are neither agonists nor inverse agonists at the 5‐HT 6 receptor. However, both Ro 04‐6790 and Ro 63‐0563 behaved as competitive antagonists with mean ±s.e.mean pA 2 values of 6.75±0.07 and 7.10±0.09, respectively. 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‐HT 6 receptors. This behavioural syndrome consisted of stretching, yawning and chewing. Ro 04‐6790 and Ro 63‐0563 represent valuable pharmacological tools for the identification of 5‐HT 6 receptors in natural tissues and the study of their physiological function.