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Series of piperazine analogs of naphthyridine-3-carboxamides and indole-2-carboxamides were designed using a ligand-based approach with consideration of the pharmacophoric requirements for 5-HT3 receptor antagonists. The title carboxamides were synthesized using appropriate synthetic routes. Initially, the 5-HT3 receptor antagonistic activity of all the compounds was determined on isolated guinea pig ileum tissue against the 5-HT3 agonist, 2-methyl-5-hydroxytryptamine, which was denoted in the form of pA2 values. The structure-activity relationship regarding the influence of the aromatic part and basic moiety as features in the 5-HT3 pharmacophore was derived. Among all the compounds screened, the piperazine derivatives of indole-2-carboxamide 13i and naphthyridine-3-carboxamide 8h exhibited prominent 5-HT3 receptor antagonism with pA2 values of 7.5 and 7.3, respectively. Subsequent investigation of the antidepressant activities of selected compounds in the mouse forced swim test (FST) led to the identification of the piperazine analogs of indole-2-carboxamide 13i and naphthyridine-3-carboxamide 8h as the most promising compounds. Both 13i and 8h demonstrated significant reduction in the duration of immobility as compared to the control. Importantly, none of the tested compounds affected the baseline locomotion of mice at the tested dose levels.

Employing a ligand-based approach, 3-methoxyquinoxalin-2-carboxamides were designed as serotonin type-3 (5-HT(3) ) receptor antagonists and synthesized from the starting material o-phenylenediamine in a sequence of reactions. The structures of the synthesized compounds were confirmed by spectral data. These carboxamides were investigated for their 5-HT(3) receptor antagonisms in longitudinal muscle myenteric plexus preparations from guinea-pig ileum against a standard 5-HT(3) agonist, 2-methy-5-HT, and their antagonism activities are expressed as pA(2) values. Compounds 6a (pA(2) : 7.2), 6e (pA(2) : 7.0), 6f (pA(2) : 7.5), 6g (pA(2) : 7.5), 6n (pA(2) : 7.0), and 6o (pA(2) : 7.2) exhibited antagonism greater than that of the standard 5-HT(3) antagonist, ondansetron (pA(2) : 6.9).