Novel 5-hydroxytryptamine (5-HT3) receptor antagonists. Synthesis and structure-activity relationships of 9-methyl-2,3,4,9-tetrahydrothiopyrano[2,3-b]indol-4-one derivatives.

Novel 9-methyl-4,9-dihydrothiopyrano[2,3-b]indol-4-one derivatives 2b-e, 3-methylene-9-methyl-2,3,4,9-tetrahydrothiopyrano[2,3-b]indol-4-on e derivatives 3b-e and 9-methyl-2,3,4,9-tetrahydrothiopyrano[2,3-b]indol-4-one derivatives 4a-e were prepared. The 5-hydroxytryptamine (5-HT3) receptor-antagonistic activities of these compounds were evaluated by using the von Bezold-Jarisch reflex test (B. J. reflex, rats) and the contractile response to 5-HT in the isolated distal colon (guinea pig). The 5-ethyl-4-imidazolyl derivative 4d was found to be 79 times more potent than ondansetron 1 in the B. J. reflex test (ID50 = 0.048 microgram/kg, i.v.), and the 5-methyl-4-imidazolyl derivative 4c was found to be 126 times more potent than 1 in the colonic contraction (IC50 = 0.0062 microM) assay.