Distinctive patterns of G protein coupling induced by the structurally similar 5-HT2A receptor ligands ketanserin and altanserin in human prefrontal cortex

While ketanserin and altanserin are both widely used as 5-HT2A receptor (5-HT2AR) tools and are structurally nearly identical, they often yield discrepant results in clinical and experimental settings. This study investigated whether these differences stem from distinct pharmacological properties, specifically testing the hypothesis that these drugs exhibit biased signaling or inverse agonism in native human brain tissue.

The researchers utilized postmortem human prefrontal cortex membranes to perform radioligand binding assays and functional [35S]GTPγS binding assays. By using immunocapture with specific antibodies, they were able to isolate and measure the activation or inhibition of Gαq/11 and Gαi1 proteins separately. These results were validated using 5-HT2AR knockout mice and molecular dynamics simulations to visualize how subtle structural differences in the ligands influence their binding pocket interactions.

The study revealed that ketanserin and altanserin are not simple neutral antagonists. Ketanserin acts as a partial agonist that stimulates Gq/11 protein coupling, whereas altanserin acts as an inverse agonist that reduces basal Gi1 protein coupling. Molecular dynamics simulations suggest this functional divergence is driven by altanserin's sulfur atom, which alters the ligand's position within the binding pocket and disrupts a critical water-mediated interaction network involving residues S2.61, S5.46, and N6.55. These findings demonstrate that even minor chemical variations in orthosteric ligands can lead to significant differences in G protein pathway modulation.

This research challenges the traditional classification of ketanserin and altanserin as neutral antagonists and highlights the importance of considering biased signaling when using these tools in neuroimaging or drug development. Understanding that these ligands differentially modulate G protein pathways provides a new framework for interpreting past clinical data and designing more precise antipsychotic therapies that target specific 5-HT2AR signaling cascades.