On the Binding of N1-substituted Tryptamines at h5-HT6 Receptors

Abstract

ON THE BINDING OF N1-SUBSTITUTED TRYPTAMINES AT h5-HT6 SEROTONIN RECEPTORS By Abner Nyamwaro Nyandege, M.S. A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science at Virginia Commonwealth University. Virginia Commonwealth University, 2007 Major Director: Richard A. Glennon, Professor Department of Medicinal Chemistry Serotonin was first discovered in the late 1940s as a vasotonic factor and is now considered a principal neurotransmitter in the nervous system. 5-HT6 receptors are one of the most recently identified members of the serotonin receptor family which consists of seven classes (5-HT1-5-HT7). 5-HT6 receptors are G-protein coupled, positively coupled to an adenylate cyclase second messenger system and are primarily found in the central nervous system (CNS). The exact functional role of 5-HT6 receptors has not been determined, but is implicated to have possible involvement in certain neuropsychiatric disorders and cognition. To investigate the functional role of these receptors, it is useful to identify 5-HT6 selective ligands as pharmacological tools. Our laboratory identified one of the first 5-HT6 receptor antagonists: the arylsulfonamide MS-245 (14a). It has been assumed that a sulfonyl (i.e., SO2) moiety is important for the binding of arylsulfonamides at 5-HT6 receptors. We now have identified benzyl analog 33 (R=H) as a single example of a non-sulfonyl analog that retains affinity. This questions the importance of the SO2 group and whether an aryl moiety or other hydrophobic groups (of equal or greater hydrophibicity) is required for binding. The purpose of the present investigation was to determine if the SO2, and the aryl moieties are required for high affinity binding. N1-Alkylsulfonyl78, and N1-benzylsubstituted tryptamines 33 were synthesized and affinities compared with their corresponding N1-benzenesulfonyl-substituted counterparts 31 at h5-HT6 receptors. None of the alkylsulfonyl or benzyl analogs displayed and/or retained the affinity of the simple benzenesulfonyl tryptamine analog (31a) (Ki = 4.1 nM). The results show that an arylsulfonyl group at the tryptamine N1 position is optimal, relative to an alkylsulfonyl group, for 5-HT6 receptor affinity. In a comparative analysis utilizing six pairs of tryptamines, it was found that there was little correspondence (r = 0.048) between the 5HT6 receptor affinities of the examined benzyl and benzenesulfonyl pairs. Current findings indicate that an aryl (or substituted aryl)sulfonyl (rather than benzyl) moiety is optimal for high affinity binding, and further suggest that N1-benzenesulfonyland their corresponding N1-benzyltryptamine counterparts bind in a different fashion.