A number of substituted indole carbazimidamides were prepared and evaluated as 5-HT4 receptor agonists by using the isolated field-stimulated guinea pig ileum preparation. Their selectivity for the 5-HT4 receptor was established by examining their affinity for other 5-HT receptors using radioligand-binding techniques. Several selective and highly potent full as well as partial agonists emerged from this study. For example, 1b,d were found to be the most potent, full 5-HT4 receptor agonist described so far (EC50 = 0.5 and 0.8 nM, respectively), being 6 and 4 times more potent than serotonin itself. On the other hand, 5b and 1h appeared as partial 5-HT4 receptor agonists in the nonstimulated guinea pig ileum preparation with potencies, evaluated against serotonin action, respectively similar (5b, Ki = 12 nM) to and 300-fold higher (1h, Ki = 0.04 nM) than serotonin.

The design and synthesis of a new class of potent and selective 5-HT4 receptor agonists containing an indole nucleus linked to a carbazimidamide are presented. A conformational study of the 5-HT4 receptor agonists serotonin and zacopride led to the identification of an initial pharmacophore and to the definition of a three-dimensional map of the 5-HT4 agonist recognition site. 1, a representative member of our new class of 5-HT4 receptor agonists, incorporates all reference structural features and matched perfectly with these models. 1 is a highly potent, full agonist at 5-HT4 receptors present in the isolated electrically stimulated guinea pig ileum preparation, with a pD2 value of 8.8, displaying selectivity (ranging from 40- to over 10,000-fold) versus other members of the serotonin receptor family.