Assessment of Affinity and Dissociation Ability of a Newly Synthesized 5-HT2 Antagonist, AT-1015: Comparison With Other 5-HT2 Antagonists

Abstract

ABSTRACT-This study investigated the binding affinities of a newly synthesized 5-HT2 antagonist, AT-1015 (N-[2-[4-(5H-dibenzo[a,d]cyclohepten-5-ylidene)-piperidino]ethyl]-1-formyl-4-piperidinecarboxamide monohydrochloride monohydrate) for [3 H]ketanserin bindings to 5-HT2 receptors in the rabbit cerebral cortex membranes using the radioligand binding assay method. The affinity of this compound was also compared with other 5-HT2-selective antagonists such as ketanserin, sarpogrelate, cyproheptadine and ritanserin, and the results showed that AT-1015 has a high pKi value for the 5-HT2 receptor. The rank order of these antagonists are: ritanserin > ketanserin @ AT-1015 > cyproheptadine @ sarpogrelate. We also evaluated the dissociation ability (slow or rapid) of AT-1015 in the rabbit cerebral cortex membrane and compared it with other 5-HT2 antagonists using the radioligand binding assay method. The blockade of [3 H]ketanserin binding sites in the rabbit cerebral cortex induced by ketanserin and sarpogrelate was readily reversed by washing, whereas the inhibition by AT-1015, cyproheptadine and ritanserin was not readily reversed by washing. The % of control after washing are 76.10% and 49.55% for AT-1015 at 10 -7.5 and 10 -7.0 M, 67.32% and 50.17% for cyproheptadine at 10 -7.5 and 10 -7.0 M, and 72.38% and 39.80% for ritanserin at 10 -9.5 and 10 -9.0 M concentrations, respectively. Thus, these findings suggest that AT-1015 has antagonistic properties towards the 5-HT2 receptor and also shows that AT-1015 slowly dissociates from the 5-HT2 receptor, whereas, ketanserin and sarpogrelate dissociate rapidly from the 5-HT2 receptor, which do not correlate with their affinity.Keywords: AT-1015, 5-HT2-receptor antagonist, Cerebral cortex (rabbit) Through its interactions with different receptor subtypes in the central and peripheral nervous system, 5-hydroxytryptamine (5-HT), a neurotransmitter, has diverse physiological functions (1). These diverse responses are elicited through the activation of a large family of 5-HT-receptor subtypes. So far, more than 14 kinds of 5-HT-receptor subtypes have been confirmed (2). Ketanserin, the first selective [3 H]-ligand for labelling the 5-HT2 receptors, contributed to further investigations to determine the localization of 5-HT2 receptors in the central nervous system and peripheral tissues (3). 5-HT2 receptors are demonstrated on both platelet membranes and brains by radioligand binding assay methods (4 -6).Previously we (7) reported that AT-1015-piperidinecarboxamide monohydrochloride monohydrate) was a strong noncompetitive 5-HT2 antagonist, comparing it with ketanserin, and it inhibited the contraction response of porcine coronary arteries induced by 5-HT and a-methylserotonin. Kihara et al. (8) also reported that AT-1015, which is newly synthesized as a 5-HT2 antagonist, blocked vascular and platelet 5-HT2A receptors and prevented laurate-induced peripheral vascular lesion in rats. The activity of AT-1015 on selective inhibition of 5-HT2A-mediated platelet aggregation was almos...

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