Pyridine- and quinoline-based piperazine nitriles: Potent and selective 5-HT₃ receptor agonists and efficacious...

This work presents a series of pyridine- and quinoline-based piperazine nitriles as potent and selective agonists and efficacious desensitizers of 5-HT₃ serotonin receptors (5-HT₃Rs). A structure-activity relationship study of the functional properties displayed by 33 4,6-dimethyl-2-(piperazin-1-yl)nicotinonitriles and close structural analogs at human 5-HT₃A and 5-HT₃AB receptors in HEK293T cells in a fluorescence-based membrane potential assay identified several potent 5-HT₃R agonists and delineated pharmacophore elements for this activity. The analogs 4,6-dimethyl-2-[4-(2-hydroxyethyl)piperazin-1-yl]pyridine-3-carbonitrile (3b) and 2-[4-(2-methoxyethyl)-1-piperazinyl]-4-methyl-3-quinolinecarbonitrile (6a) displayed high selectivity for 5-HT₃Rs over other serotonin receptors and were as 5-HT₃R-selective as the prototypic reference agonist mCPBG. 3b also mediated robust and concentration-dependent activation of 5-HT₃A in Xenopus oocytes in TEVC recordings, whereas the 6a-evoked responses in these oocytes were characterized by minute current amplitudes and rapid current decays. Both 3b and 6a were found to induce substantial 5-HT₃R desensitization, with low-nanomolar concentrations of 6a mediating pronounced and persistent de facto inhibition of 5-HT₃Rs expressed in Xenopus oocytes and HEK293T cells. These characteristics were contrasted by the rapid recovery of the 5-HT₃R from the desensitization induced by serotonin, mCPBG or SR 57227, whereas the close structurally related agonist quipazine also induced sustained receptor desensitization. Thus, the desensitizer properties were clearly ligand-specific, and the balance between the 5-HT₃R activation and desensitization mediated by the 1-phenylpiperazine and 2-(piperazin-1-yl)quinoline analogs appeared to be tweakable by relatively small structural modifications. In conclusion, the high 5-HT₃R-selectivity and the potent and efficacious receptor desensitization induced by 6a make it a potentially interesting pharmacological tool for studies of these receptors.