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

1 The novel 5-HT7 receptor antagonist, SB-269970-A, potently displaced [H-3 ]-5-CT from human 5-HT7(a) (pK(i) 8.9+/-0.1) and 5-HT7 receptors in,guinea- pig cortex (pK(i) 8.3+/-0.2). 2 5-CT stimulated adenylyl cyclase activity in 5-HT7(a)/HEK293 membranes (p EC(50) 7.5+/-0.1) and SB-269970-A (0.03-1 mu M) inhibited the 5-CT concentr ation-response with no significant alteration in the maximal response. The pA(2) (8.5+/-0.2) for SB-269970-A agreed well with the pK(i) determined fro m [H-3]-5-CT binding studies. 3 5-CT-stimulated adenylyl cyclase activity in guinea-pig hippocampal membr anes (pEC(50) of 8.4+/-0.2) was inhibited by SB-269970-A (0.3 mu M) with a pK(B) (8.3+/-0.1) in good agreement with its antagonist potency at the huma n cloned 5-HT7(a) receptor and its binding affinity at guinea-pig cortical membranes. 4 5-HT7 receptor mRNA was highly expressed in human hypothalamus, amygdala, thalamus, hippocampus and testis. 5 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 i n 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. 6 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 recep tor 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. 7 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 firs t 3 h of EEG recording in conscious rats.