Wd. Hirst et al., Characterization of [I-125]-SB-258585 binding to human recombinant and native 5-HT6 receptors in rat, pig and human brain tissue, BR J PHARM, 130(7), 2000, pp. 1597-1605
1 SB-258585 (4-Iodo-N-[4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-benzen
esulphonamide) is a high affinity ligand at 5-HT6 receptors. It displays ov
er 100 fold selectivity for the 5-HT6 receptor over all other 5-HT receptor
s tested so far. SB-258585 has been radiolabelled, to high specific activit
y, for its characterization as a 5-HT6 receptor selective radioligand.
2 [I-125]-SB-258585 bound, with high affinity, to a single population of re
ceptors in a cell line expressing human recombinant 5-HT6 receptors. Kineti
c and saturation binding experiments gave pK(D) values of 9.01+/-0.09 and 9
.09+/-0.02, respectively.
3 In membranes derived from rat or pig striatum and human caudate putamen,
[I-125]-SB-258585 labelled a single site with high levels (>60%) of specifi
c binding. Saturation analysis revealed pK(D) values of 8.56+/-0.07 for rat
, 8.60+/-0.10 for pig and 8.90+/-0.02 for human. B-max values for the tissu
es ranged from 173+/-23 and 181+/-25 fmol mg(-1) protein in rat and pig str
iatum, respectively, to 215+/-41 fmol mg(-1) protein in human caudate putam
en.
4 The pK(i) rank order of potency for a number of compounds, determined in
competition binding assays with [I-125]-SB-258585, at human caudate putamen
membranes was: SB-271046 > SB-258585 > SB-214111 > methiothepin > clozapin
e > 5-Me-OT > 5-HT > Po 04-6790 > mianserin > ritanserin = amitriptyline >
5-CT > mesulergine. Similar profiles were obtained from pig and rat striata
l membranes and recombinant 5-HT6 receptors; data from the latter correlate
d well with [H-3]-LSD binding.
5 Thus, [I-125]-SB-258585 is a high affinity, selective radioligand which c
an be used to label both recombinant and native 5-HT6 receptors and will fa
cilitate further characterization of this receptor subtype in animal and hu
man tissues.