Synthesis and Structure−Affinity Relationships of Novel N-(1-Ethyl-4-methylhexahydro-1,4-diazepin-6-yl)pyridine-3-carboxamides with Potent Serotonin 5-HT₃ and Dopamine D₂ Receptor Antagonistic Activity

A structurally original series of N-(1-ethyl-4-methylhexahydro-1,4-diazepin-6-yl)pyridine-3-carboxamides derived from the corresponding benzamide 5 were prepared and evaluated for their binding affinity for the dopamine D(2) and serotonin 5-HT(3) receptors using rat striatum and rat cortical membrane, respectively. Many of the synthesized pyridine-3-carboxamides exhibited nanomolar binding affinity for the serotonin 5-HT(3) receptor along with moderate to high binding affinity for the dopamine D(2) receptor. Introduction of the more lipophilic bromine atom and methylamino group at the 5- and 6-positions of the pyridine ring, respectively, enhanced the affinity for the dopamine D(2) receptor while keeping a potent serotonin 5-HT(3) receptor binding affinity. As a result of structure-affinity relationships, the 5-bromo-2-methoxy-6-methylaminopyridine-3-carboxamide 53 was selected as the most promising product showing a high binding affinity for both receptors. Compound 53 affinity for the dopamine D(2) and serotonin 5-HT(3) receptors was much more potent than that of metoclopramide (dopamine D(2) receptor; 23.3 nM vs 444 nM, serotonin 5-HT(3) receptor; 0.97 nM vs 228 nM). Optical resolution of the racemate 53 brought about a dramatic change in the pharmacological profile with (R)-53 exhibiting a strong affinity for both the dopamine D(2) and serotonin 5-HT(3) receptors, while the corresponding (S)-53 had a potent serotonin 5-HT(3) receptor binding affinity and a moderate dopamine D(2) receptor binding affinity. X-ray crystallographic study of (R)-53 revealed the existence of two energically stable conformers just like two mirror images. This may account for (R)-53 high affinity for both the dopamine D(2) and serotonin 5-HT(3) receptors. Pharmacologically, (R)-53 [AS-8112] showed a potent antagonistic activity for both the dopamine D(2) and serotonin 5-HT(3) receptors in vivo tests and dose-dependently inhibited both the incidence and frequency of emetic episodes induced by cisplatin (ferrets) and morphine (dogs) with ID(50) values of 27.1 microg/kg, po and 136 microg/kg, po, respectively. On the basis of this pharmacological profile, (R)-53 is now under further investigation as a potential broad antiemetic agent.