"Methylene Bridge" to 5-HT₃ Receptor Antagonists: Conformationally Constrained Phenylguanidines

Area of Science:

• Medicinal Chemistry
• Pharmacology
• Neuroscience

Background:

• Arylguanidines exhibit diverse activities at serotonin 5-HT3 receptors, acting as partial agonists, agonists, or superagonists.
• The specific aromatic substitution pattern dictates the functional outcome of arylguanidines at these receptors.

Purpose of the Study:

• To investigate the effect of conformational constraint on the activity of arylguanidines at 5-HT3 receptors.
• To explore the structure-activity relationships of dihydroquinazoline derivatives as potential 5-HT3 receptor antagonists.

Main Methods:

• Synthesis and examination of dihydroquinazoline analogs.
• Structure-activity relationship studies involving modifications of the guanidinium moiety.
• Homology modeling, molecular docking, and site-directed mutagenesis studies to support binding data.

Main Results:

• Conformational constraint of arylguanidines into dihydroquinazolines resulted in potent 5-HT3 receptor antagonists.
• Removal or substitution of any guanidinium nitrogen atom significantly decreased binding affinity.
• The presence of all three nitrogen atoms in the guanidinium group is essential for optimal binding.
• Introduction of even a small N2-methyl substituent completely abolished receptor affinity.

Conclusions:

• Introducing a methylene bridge to create dihydroquinazolines converts arylguanidines into 5-HT3 receptor antagonists, irrespective of their original functional activity.
• The three nitrogen atoms of the guanidinium group are critical for high-affinity binding to the 5-HT3 receptor.
• These findings are supported by computational modeling and mutagenesis data, elucidating key structural requirements for antagonism.