Stephen F. Moss

Characterization of SB-271046 : A potent, selective and orally active 5-HT6 receptor antagonist

SB‐271046, potently displaced [3H]‐LSD and [125I]‐SB‐258585 from human 5‐HT6 receptors recombinantly expressed in HeLa cells in vitro (pKi 8.92 and 9.09 respectively). SB‐271046 also displaced [125I]‐SB‐258585 from human caudate putamen and rat and pig striatum membranes (pKi 8.81, 9.02 and 8.55 respectively). SB‐271046 was over 200 fold selective for the 5‐HT6 receptor vs 55 other receptors, binding sites and ion channels. In functional studies on human 5‐HT6 receptors SB‐271046 competitively antagonized 5‐HT‐induced stimulation of adenylyl cyclase activity with a pA2 of 8.71. SB‐271046 produced an increase in seizure threshold over a wide‐dose range in the rat maximal electroshock seizure threshold (MEST) test, with a minimum effective dose of 0.1 mg kg−1 p.o. and maximum effect at 4 h post‐dose. The level of anticonvulsant activity achieved correlated well with the blood concentrations of SB‐271046 (EC50 of 0.16 μM) and brain concentrations of 0.01–0.04 μM at Cmax. These data, together with the observed anticonvulsant activity of other selective 5‐HT6 receptor antagonists, SB‐258510 (10 mg kg−1, 2–6 h pre‐test) and Ro 04‐6790 (1–30 mg kg−1, 1 h pre‐test), in the rat MEST test, suggest that the anticonvulsant properties of SB‐271046 are likely to be mediated by 5‐HT6 receptors. Overall, these studies demonstrate that SB‐271046 is a potent and selective 5‐HT6 receptor antagonist and is orally active in the rat MEST test. SB‐271046 represents a valuable tool for evaluating the in vivo central function of 5‐HT6 receptors.

Characterization of [125I]-SB-258585 binding to human recombinant and native 5-HT6 receptors in rat, pig and human brain tissue

SB‐258585 (4‐Iodo‐N‐[4‐methoxy‐3‐(4‐methyl‐piperazin‐1‐yl)‐phenyl]‐benzenesulphonamide) is a high affinity ligand at 5‐HT6 receptors. It displays over 100 fold selectivity for the 5‐HT6 receptor over all other 5‐HT receptors tested so far. SB‐258585 has been radiolabelled, to high specific activity, for its characterization as a 5‐HT6 receptor selective radioligand. [125I]‐SB‐258585 bound, with high affinity, to a single population of receptors in a cell line expressing human recombinant 5‐HT6 receptors. Kinetic and saturation binding experiments gave pKD values of 9.01±0.09 and 9.09±0.02, respectively. In membranes derived from rat or pig striatum and human caudate putamen, [125I]‐SB‐258585 labelled a single site with high levels (>60%) of specific binding. Saturation analysis revealed pKD values of 8.56±0.07 for rat, 8.60±0.10 for pig and 8.90±0.02 for human. Bmax values for the tissues ranged from 173±23 and 181±25 fmol mg−1 protein in rat and pig striatum, respectively, to 215±41 fmol mg−1 protein in human caudate putamen. The pKi rank order of potency for a number of compounds, determined in competition binding assays with [125I]‐SB‐258585, at human caudate putamen membranes was: SB‐271046>SB‐258585>SB‐214111>methiothepin>clozapine>5‐Me‐OT>5‐HT>Ro 04‐6790>mianserin>ritanserin=amitriptyline>5‐CT>mesulergine. Similar profiles were obtained from pig and rat striatal membranes and recombinant 5‐HT6 receptors; data from the latter correlated well with [3H]‐LSD binding. Thus, [125I]‐SB‐258585 is a high affinity, selective radioligand which can be used to label both recombinant and native 5‐HT6 receptors and will facilitate further characterization of this receptor subtype in animal and human tissues.

Phenyl benzenesulfonamides are novel and selective 5-HT6 antagonists: identification of N-(2,5-dibromo-3-fluorophenyl)-4-methoxy-3-piperazin-1-ylbenzenesulfonamide (SB-357134)

Substituted N-phenyl-4-methoxy-3-piperazin-1-ylbenzenesulfonamides and conformationally restricted analogues have been identified as high affinity and selective 5-HT6 antagonists. Compounds from this series had a range of pharmacokinetic profiles in rat and in general there was a correlation between clearance and CNS penetration. Based on its overall biological profile 2 (SB-357134) was selected for further pre-clinical evaluation.

Novel (4-piperazin-1-ylquinolin-6-yl) arylsulfonamides with high affinity and selectivity for the 5-HT6 receptor

The discovery of (4-piperazin-1-ylquinolin-6-yl) arylsulfonamides and their binding affinities for a selection of 5-HT and dopamine subreceptors is described. Many compounds show high affinity (pK(i)>8) for the 5-HT(6) receptor and >100-fold selectivity against a range of other receptors. Structure-activity relationships of these compounds are discussed.