Phenylimidazolidin-2-one derivatives as selective 5-HT₃ receptor antagonists and refinement of the pharmacophore model for 5-HT₃ receptor binding

A possible bioisosterism between the benzamido and the phenylimidazolidin-2-one moieties has been suggested on the basis of the similarity between the molecular electrostatic potential (MEP) of metoclopramide, a D₂ receptor antagonist with weak 5-HT₃ receptor antagonist properties, and zetidoline, a D₂ receptor antagonist. Starting from this premise, a series of phenylimidazolidin-2-one derivatives bearing a basic azabicycloalkyl or an imidazolylalkyl moiety were synthesized and evaluated for 5-HT₃ receptor radioligand binding affinity ([³H]-GR 43694). In vitro 5-HT₃ receptor antagonist activity was tested in the guinea pig ileum assay (GPI). A number of high-affinity ligands were shown to be potent 5-HT₃ receptor antagonists in vivo as determined by inhibition of the Bezold-Jarisch reflex in the anesthetized rat. In general, the imidazolylalkyl derivatives were found to be more active than azabicycloalkyls. 1-(3,5-Dichlorophenyl)-3-[(5-methyl-1H-imidazol-4- yl)methyl]imidazolidin-2-one (58), in particular, displayed very high affinity for the 5-HT₃ receptor (K(i) of 0.038 nM) with a K(b) of 5.62 nM in the GPI assay, being more potent than the reference compounds (ondansetron, tropisetron, granisetron, and BRL 46470) tested. 58 showed an ID₅₀ comparable to that of ondansetron (2.2 μg/kg iv) in the Bezold-Jarisch reflex. A molecular modeling study based on this structurally novel series of compounds allowed the refinement of previously reported 5-HT₃ receptor antagonist pharmacophore models.