Novel, Potent, and Selective Quinoxaline-Based 5-HT<sub>3</sub> Receptor Ligands. 1. Further Structure−Activity Relationships and Pharmacological Characterization
We investigated the pharmacological profile of a novel series of quinoxaline-based 5-HT3 receptor ligands bearing an extra basic moiety on the piperazine N-4. High affinity and selectivity were dependent on the electronic properties of the substituents, and at cardiac level 3a and 3c modulated chronotropy but not inotropy. In von Bezold−Jarisch reflex test 3a−c were partial agonists while 3i was a
The synthesis and the biological characterization of novel highly selective pyrroloquinoxaline 5-HT3 receptor (5-HT3R) ligands are described. In functional and in vivo biological studies the novel quinoxalines modulated cardiac parameters by direct interaction with myocardial 5-HT(3)Rs. The potent 5-HT3R ligands 4h and 4n modulate chronotropy (right atrium) but not inotropy (left atrium) at the cardiac level, being antagonist and partial agonist, respectively. Preliminary pharmacokinetic studies indicate that (S)-4n and 4a, representatives of the novel 5-HT3R ligands, possess poor blood-brain barrier permeability, being the prototypes of peripherally acting 5-HT3R modulators endowed with a clear-cut pharmacological activity at the cardiac level. The unique properties of 4h and 4n, compared to their previously described centrally active N-methyl analogue 5a, are mainly due to the hydrophilic groups at the distal piperazine nitrogen. These analogues represent novel pharmacological tools for investigating the role of peripheral 5-HT3R in the modulation of cardiac parameters.