Pyrrolizidine esters and amides as 5-HT4 receptor agonists and antagonists

A series of pyrrolizidine esters, amides, and ureas was prepared and tested for 5-HT4 and 5-HT3 receptor binding, 5-HT4 receptor agonism in the rat tunica muscularis mucosae (TMM) assay, and for 5-HT3 receptor-mediated functional antagonism in the Bezold-Jarisch reflex assay. Several pyrrolizidine derivatives were identified with high affinity for the 5-HT4 receptor, including benzamide 12a (SC-53116), a potent and selective 5-HT4 partial agonist that exhibits efficacy in promoting antral contractions and activity in promoting gastric emptying in canine models. Also discovered were 5-HT4 receptor antagonists, including imidazopyridine amide 12h (SC-53606), which is a potent and selective 5-HT4 receptor antagonist with a pA(2) value of 8.13 in the rat TMM assay. N-Methyl indole ester 13d was identified as a potent 5-HT4 antagonist with a pA(2) value of 8.93. High selectivity was observed for these pyrrolizidine derivatives versus other monoamine receptors, including 5-HT1, 5-HT2, D-1, D-2, alpha(1), alpha(2), and beta receptors.