Based on a medicinal chemistry guided hypothetical pharmacophore model, novel series of indolyl sulfonamides have been designed and prepared as selective and high-affinity serotonin 5-HT(6) receptor ligands. Furthermore, based on a screening approach of a discovery library, a series of benzoxazinepiperidinyl sulfonamides were identified as selective 5-HT(6) ligands. Many of the compounds described in this paper possess excellent affinities, displaying pK(i) values greater than 8 (some even >9) and high selectivities against a wide range (>50) of other CNS relevant receptors. First, structure-affinity relationships of these ligands are discussed. In terms of functionality, high-affinity antagonists, as well as agonists and even partial agonists, were prepared. Compounds 19c and 19g represent the highest-affinity 5-HT(6) agonists ever reported in the literature. These valuable tool compounds should allow for the detailed study of the role of the 5-HT(6) receptor in relevant animal models of disorders such as cognition deficits, depression, anxiety, or obesity.
1 E-6837 is a novel, selective and high-affinity 5-HT 6 receptor ligand (pK i : 9.13) which in vitro demonstrates partial agonism at a presumably silent rat 5-HT 6 receptor and full agonism at a constitutively active human 5-HT 6 receptor by monitoring the cAMP signaling pathway. 2 The effects of chronic treatment with E-6837 were determined in diet-induced obese (DIO)-rats on changes in body weight, food and water intake, plasma indices of comorbid risk factors, and weight regain on compound withdrawal. The centrally acting antiobesity drug, sibutramine, was used as the reference comparator. 3 Sustained body weight loss and decreased cumulative food intake of DIO-rats was observed with E-6837 (30 mg kg À1 , p.o., twice a day) during the 4-week treatment period. The onset of the E-6837 effect on body weight was slower than that of sibutramine (5 mg kg À1 , p.o.), while its maximal effect was greater, that is À15.7 versus À11.0%. 4 E-6837-induced weight loss was exclusively mediated by a decrease (31.7%) in fat mass, with a concomitant reduction (49.6%) in plasma leptin. Reduced obesity was also reflected in improved glycemic control. 5 Although weight regain occurred after withdrawal from either compound, the body weights after E-6837 (À6.6%) remained lower than after sibutramine (À3.8%) indicating that the greater efficacy of the former did not result in profound rebound hyperphagia/weight gain. 6 These results show that the 5-HT 6 receptor partial agonist, E-6837, is a promising new approach to the management of obesity with the potential to produce greater sustained weight loss than sibutramine.
A series of novel indene derivatives designed by a scaffold selection gave access to several examples of (Z)-arylmethylideneindenes and indenylsulfonamides that acted as serotonin 5-HT(6) receptor ligands. Different synthetic multistep routes could be applied to these target compounds, each with their own complexity and limitations. A reasonable route involved the (3-indenyl)acetic acids as the key intermediates, and two alternatives were also examined. The first protocol used was a two-step sequence employing a modified Horner-Wadsworth-Emmons reaction, but better results were obtained with a procedure based on the condensation of indanones with the lithium salt of ethyl acetate, followed immediately by dehydration with acid and hydrolysis/isomerization under basic catalysis. (3-Indenyl)acetic acids were transformed to the corresponding acetamides, which were effectively reduced to indenylsulfonamides using an optimized procedure with AlH(3)-NMe(2)Et. The binding at the 5-HT(6) receptor was with moderate affinity (K(i) = 216.5 nM) for the (Z)-benzylideneindenylsulfonamide and enhanced affinity for the simple indenylsulfonamide counterpart (K(i) = 50.6 nM). Selected indenylsulfonamides were then tested, showing K(i) values as low as 20.2 nM.
Scaffold selection involving an indole-to-indene core change led to the discovery of a series of indenylsulfonamides that act as 5-HT6 serotonin receptor agonists. The variety of the targeted ligands and their synthetic complexity required multistep synthetic approaches. The novel indenylsulfonamides exhibited variable binding affinities for the 5-HT6 receptor, and the in vitro primary binding profiles of the preferred compounds revealed them to be 5-HT6 receptor agonists with Ki values > or =4.5 nM. The structural changes responsible for enhancing the affinities indicated a directing effect modulated by the nature of the indene core, the substitution at the aminoethyl side chain, and especially by the aryl(heteroaryl)sulfonyl group on the indene 5-position. A representative of the family, the N-(inden-5-yl)imidazothiazole-5-sulfonamide (43), exhibited a high affinity and functioned as a potent full agonist for the 5-HT6 receptor (Ki = 4.5 nM, EC50 = 0.9 nM, Emax = 98%).
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