In continuation of our previous work on piperazinopyrrolothienopyrazine derivatives, three series of piperazinopyridopyrrolopyrazines, piperazinopyrroloquinoxalines, and piperazinopyridopyrroloquinoxalines were prepared and evaluated as 5-HT3 receptor ligands. The chemical modifications performed within these new series led to structure-activity relationships regarding both high affinity and selectivity for the 5-HT3 receptors that are in agreement with those established previously for the pyrrolothienopyrazine series. The best compound (8a) obtained in these new series is in the picomolar range of affinity for 5-HT3 receptors with a selectivity higher than 10(6). Four of the high-affinity 5-HT3 ligands (8a, 15a,b, and 16d) were selected in both the pyridopyrrolopyrazine and the pyrroloquinoxaline series and were characterized in vitro and in vivo as agonists or partial agonists. Compound 8a was also evaluated in the light/dark test where it showed potential anxiolytic-like activity at very low doses per os.

A series of piperazinopyrrolo[1,2-a]thieno[3,2-e]- and -[2,3-e]pyrazine derivatives were prepared and evaluated in order to determine the necessary requirements for high affinity on the 5-HT3 receptors and high selectivity versus other 5-HT receptor subtypes. Various substitutions on the piperazine and the thiophene ring of the pyrrolothienopyrazine moieties were systematically explored as well as replacement of the piperazine by other cyclic amines. The best compounds are in the nanomolar range of affinity of 5-HT3 receptors with high to very high selectivity (up to 10,000 for 14b). These high-affinity compounds have in common a benzyl- or allylpiperazine substituent with no substitutions on the thiophene ring. Five of these compounds (1a, 4b, 13a,b, and 14b) have been evaluated on the Von Bezold-Jarisch reflex and were characterized as partial agonists. One of them, 13a, has shown in vivo at very low dose a potent anxiolytic-like activity in the light/dark test.