Novel (4-piperazin-1-ylquinolin-6-yl) arylsulfonamides with high affinity and selectivity for the 5-HT(6) receptor

Efficacy of selective 5-HT6 receptor ligands determined by monitoring 5-HT6 receptor-mediated cAMP signaling pathways

1. Two novel selective 5-HT6 receptor ligands E-6801 (6-chloro-N-(3-(2-(dimethylamino)ethyl)-1H-indol-5-yl)imidazo[2,1-b]thiazole-5-sulfonamide) and E-6837 (5-chloro-N-(3-(2-(dimethylamino)ethyl)-1H-indol-5-yl)naphthalene-2-sulfonamide) were investigated and compared to the putative 5-HT6 receptor antagonists SB-271046 (5-chloro-N-(4-methoxy-3-(piperazin-1-yl)phenyl)-3-methylbenzo[b]thiophene-2-sulfonamide) and Ro 04-06790 (N-(2,6-bis(methylamino)pyrimidin-4-yl)-4-aminobenzenesulfonamide) using a cAMP-mediated pathway. 2. Forskolin stimulation, to increase the magnitude of agonist cAMP responses, and site-directed mutagenesis of the 5-HT6 receptor, in order to yield constitutively active receptor, were applied. 3. 5-HT (E(max), % over basal: 200), E-6801 (120) and E-6837 (23) induced cAMP formation at the rat 5-HT6 receptor. In the copresence of forskolin, cAMP responses were more potent and enhanced to 294 (5-HT, % over forskolin), 250 (E-6801) and 207 (E-6837), respectively. 5-HT-mediated cAMP formation was dose-dependently blocked by SB-271046 (pA(2): 8.76+/-0.22) and Ro 04-6790 (pA(2): 7.89+/-0.10) and not affected by the copresence of forskolin. Both E-6801 and E-6837 yielded partial antagonism of the 5-HT response in the absence of forskolin, whereas antagonism was either completely absent (E-6801) or attenuated (E-6837) in the copresence of forskolin. Intrinsic activity of these 5-HT6 receptor ligands at a constitutively active human S267K 5-HT6 receptor in Cos-7 cells indicated similar efficacy (E(max), % over basal) for 5-HT (97), E-6801 (91) and E-6837 (100), while Ro 04-6790 (-33) and SB-271046 (-39) were equi-efficacious inverse agonists. 4. The use of either forskolin or a constitutively active S267K 5-HT6 receptor enhances the resolution for monitoring the efficacy of 5-HT6 receptor ligands. E-6801 and E-6837 are potent partial agonists at the 5-HT6 receptor. Ro 04-6790 and SB-271046 appear to act as inverse agonists/antagonists.

Hologram quantitative structure activity relationship studies on 5-HT6 antagonists

Predictive hologram quantitative structure activity relationship (HQSAR) models were developed for a series of arylsulfonamide compounds acting as specific 5-HT6 antagonists. A training set containing 48 compounds served to establish the model. The best HQSAR model was generated using atoms, bond, and connectivity as fragment distinction and 4-7 as fragment size showing cross-validated r2(q2) value of 0.702 and conventional r2 value of 0.971. The predictive ability of the model was validated by an external test set of 20 compounds giving satisfactory predictive r2 value of 0.678. The efficiency of HQSAR approach was further evidenced by the generation of predictive models for a training set containing 30 highly diverse, both specific and nonspecific 5-HT6 antagonists. The best HQSAR model for this training set was generated using atoms, bond, and connectivity as fragment distinction and 4-7 as fragment size showing cross-validated r2(q2) value of 0.693 and conventional r2 value of 0.923. This model was also validated by using an external test set of 10 compounds giving satisfactory predictive r2 value of 0.692. The contribution maps obtained from these models were used to explain the individual atomic contributions to the overall activity.

Bicyclic piperazinylbenzenesulphonamides are potent and selective 5-HT6 receptor antagonists

The synthesis of novel 3-(octahydropyrido[1,2-a]pyrazin-2-yl)- and 3-(hexahydropyrrolo[1,2-a]pyrazin-2-yl)phenyl-2-benzo[b]thiophene sulphonamide derivatives 3, (S)-4 and (R)-4 is described. The compounds show high affinity for the 5-HT6 receptor, excellent selectivity against a range of other receptors and good brain penetration.