Peter J. Lovell

Characterization of SB-269970-A, a selective 5-HT7 receptor antagonist

The novel 5‐HT7 receptor antagonist, SB‐269970‐A, potently displaced [3H]‐5‐CT from human 5‐HT7(a) (pKi 8.9±0.1) and 5‐HT7 receptors in guinea‐pig cortex (pKi 8.3±0.2). 5‐CT stimulated adenylyl cyclase activity in 5‐HT7(a)/HEK293 membranes (pEC50 7.5±0.1) and SB‐269970‐A (0.03–1 μM) inhibited the 5‐CT concentration‐response with no significant alteration in the maximal response. The pA2 (8.5±0.2) for SB‐269970‐A agreed well with the pKi determined from [3H]‐5‐CT binding studies. 5‐CT‐stimulated adenylyl cyclase activity in guinea‐pig hippocampal membranes (pEC50 of 8.4±0.2) was inhibited by SB‐269970‐A (0.3 μM) with a pKB (8.3±0.1) in good agreement with its antagonist potency at the human cloned 5‐HT7(a) receptor and its binding affinity at guinea‐pig cortical membranes. 5‐HT7 receptor mRNA was highly expressed in human hypothalamus, amygdala, thalamus, hippocampus and testis. SB‐269970‐A was CNS penetrant (steady‐state brain : blood ratio of ca. 0.83 : 1 in rats) but was rapidly cleared from the blood (CLb=ca. 140 ml min−1 kg−1). Following a single dose (3 mg kg−1) SB‐269970 was detectable in rat brain at 30 (87 nM) and 60 min (58 nM). In guinea‐pigs, brain levels averaged 31 and 51 nM respectively at 30 and 60 min after dosing, although the compound was undetectable in one of the three animals tested. 5‐CT (0.3 mg kg−1 i.p.) induced hypothermia in guinea‐pigs was blocked by SB‐269970‐A (ED50 2.96 mg kg−1 i.p.) and the non‐selective 5‐HT7 receptor antagonist metergoline (0.3–3 mg kg−1 s.c.), suggesting a role for 5‐HT7 receptor stimulation in 5‐CT induced hypothermia in guinea‐pigs. SB‐269970‐A (30 mg kg−1) administered at the start of the sleep period, significantly reduced time spent in Paradoxical Sleep (PS) during the first 3  h of EEG recording in conscious rats.

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.

[3H]‐SB‐269970 – A selective antagonist radioligand for 5‐HT7 receptors

Binding of the 5‐HT7 receptor antagonist radioligand [3H]‐SB‐269970 to human 5‐HT7(a) receptors expressed in HEK293 cell membranes (h5‐HT7(a)/293) and to guinea‐pig cerebral cortex membranes, was characterized and compared with [3H]‐5‐CT binding. [3H]‐SB‐269970 (1 nM) showed full association with h5‐HT7(a)/293 membranes after 40 min. Specific binding at equilibrium represented >90% of total binding and was fully reversible by methiothepin (10 μM), full dissociation occurring by 100 min. The association (k+1) and dissociation (k−1) rate constants were 0.05 nM−1min−1 and 0.05 min−1 respectively, giving a KD (k−1/k+1) of 1.0 nM. [3H]‐SB‐269970 bound saturably and apparently monophasically to both h5‐HT7(a)/293 and guinea‐pig cortex membranes, with KD values of 1.25±0.05 and 1.7±0.3 nM respectively. The Bmax for [3H]‐SB‐269970 to both h5‐HT7(a)/293 and guinea‐pig cortex membranes (5780±380 and 125±8.2 fmoles mg protein−1 respectively) was similar to that for [3H]‐5‐CT (6190±940 and 143±19 fmoles mg protein−1 respectively). These data suggest that, in each tissue, both radioligands labelled the same population of receptors, which appear to be present in an agonist high affinity state. The profile of compound inhibition of [3H]‐SB‐269970 binding to h5‐HT7(a)/293 and guineapig cortex membranes correlated well (corr. coeff. 0.98) with those for [3H]‐5‐CT binding and were consistent with the profiles reported previously for the human 5‐HT7(a) and guinea‐pig cortex 5‐HT7 receptors using [3H]‐5‐CT. Hill slopes for inhibition of [3H]‐SB‐269970 and [3H]‐5‐CT binding were close to 1, consistent with binding to a single receptor population in both tissues. [3H]‐SB‐269970 represents the first selective 5‐HT7 antagonist radioligand, which should aid further characterization of 5‐HT7 receptors in recombinant and native tissues and help establish their role in brain function.

New quinoline NK3 receptor antagonists with CNS activity.

Lead optimisation starting from the previously reported selective quinoline NK(3) receptor antagonists talnetant 2 (SB-223412) and 3 (SB-222200) led to the identification of 3-aminoquinoline NK(3) antagonist 10 (GSK172981) with excellent CNS penetration. Investigation of a structurally related series of sulfonamides with reduced lipophilicity led to the discovery of 20 (GSK256471). Both 10 and 20 are high affinity, potent NK(3) receptor antagonists which despite having different degrees of CNS penetration produced excellent NK(3) receptor occupancy in an ex vivo binding study in gerbil cortex.

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.

Novel 5-HT1A/1B/1D receptors antagonists with potent 5-HT reuptake inhibitory activity

Novel 2-methyl-5-quinolinyl-1-piperazinylalkyl-3,4-dihydro-2H-1,4-benzoxazin-3-ones showing high affinities for the 5-HT(1A/1B/1D) receptors coupled with potent 5-HT reuptake inhibitory activity have been discovered. This is the first report describing docking of the lead compound 6-{2-[4-(2-methyl-5-quinolinyl)-1-piperazinyl]ethyl}-2H-1,4-benzoxazin-3(4H)-one 1, into a model of the 5-HT transporter and the 5-HT(1A) receptor model.

Discovery of potent, orally bioavailable, selective 5-HT1A/B/D receptor antagonists

5-HT1 receptor antagonists have been discovered with good selectivity over the 5-HT transporter. This is the first report of highly potent, selective ligands for the 5-HT1A/B/D receptors with low intrinsic activity, which represent a useful set of molecules for further understanding the roles of the 5-HT1 receptor subtypes and providing new approaches for the treatment of depression.