The serotonin 5-HT4 receptor: Part 3: Design and pharmacological evaluation of a new class of antagonists

New esters of 4-amino-5-chloro-2-methoxybenzoic acid as potent agonists and antagonists for 5-HT4 receptors

A number of benzoates derived from 4-amino-5-chloro-2-methoxybenzoic acid and substituted 1-piperidineethanol were synthesized and found to be potent 5-HT4 receptor agonists in the electrically-stimulated myenteric plexus and longitudinal muscle of the guinea pig ileum and the rat esophagus muscle. Monosubstitution of the piperidine ring with Me, OH, NH-Ac, or CONH2 groups gave compounds equipotent to 7a (ML 10302), a 5-HT4 receptor agonist previously reported to have nanomolar affinity. 7a,k were as potent as serotonin (5-HT) but had maximal responses which were only 60-80% of that of 5-HT, suggesting a partial agonist profile for these compounds. Binding assays were performed with [3H]GR 113808 in the rat striatum, and several of these compounds were found to have nanomolar affinity for 5-HT4 receptors (7a, Ki = 1.07 +/- 0.5 nM; 7k, Ki = 1.0 +/- 0.3 nM). The introduction of two methyl groups on the piperidine ring brought about a dramatic change in the pharmacological profile of 2-[(cis- and trans-3,5-dimethylpiperidinyl)ethyl]-4-amino-5-chloro-2- methoxybenzoate, 7g,h. 7g (Ki = 0.26 +/- 0.06 nM) inhibited the relaxant action of 5-HT in the rat esophagus muscle with a pA2 value of 8.6. The advantage of the ester function was demonstrated by comparing the activity of several such compounds at 5-HT4 receptors with those of the corresponding amidic derivatives. This difference was less marked when the basic moiety was sterically constrained as in the quinuclidine and tropane moieties. Structural analyses of 7a,g were performed by determining their X-ray crystal structures and by molecular modeling (SYBYL). A relatively limited number of minimum energy conformers was found for both compounds. They were characterized by the cis folded conformation of the ethyl chain and by the orientation of the lone pair of the nitrogen atom pointing out of the molecule as seen in conformationally-constrained benzamides such as zacopride and renzapride. A hypothetical model for the 5-HT4 receptor with two sites for the binding of agonist and antagonist molecules was proposed.