New 5-HT3 (serotonin-3) receptor antagonists, II. Synthesis and structure-activity relationships of pyrimido[1,6-a]indoles.

A series of pyrimido[1,6-alpha]indol-1(2H)-ones was prepared and evaluated for 5-HT3 receptor antagonist activity. The compounds in this series were regarded as bioisosters of the pyrido[1,2-alpha]indol-6(7H)-ones previously reported. High potency was found for compounds having 5-methyl substituents on both the pyrimido[1,6-alpha]indole ring and the imidazole ring. Optimized members of this series, 8b and (+)-26a, were potent 5-HT3 receptor antagonists as determined by measuring inhibition of the Bezold-Jarisch reflex in anesthetized rats (ED50 0.6 and 0.8 microgram/kg i.v., respectively), being equipotent to or more potent than FK 1052 (1) in the previous paper and 20- to 30-fold more potent than ondansetron (2).