Mechanistic Basis for 5-HT<sub>2A</sub>R Over 5-HT<sub>2B</sub>R Activation.
Abstract
Selective 5-HT<sub>2A</sub>R agonism without 5-HT<sub>2B</sub>R activation is critical for safe psychedelic-inspired therapeutics. Here, we systematically probe the ligand-binding pockets of 5-HT<sub>2A</sub>R and 5-HT<sub>2B</sub>R and identify steric and conformational constraints at the side-extended pocket (SEP) and extended binding pocket (EBP) that dictate ligand orientation and receptor signaling. Guided by these insights, we design derivatives across tryptamine and phenethylamine scaffolds that integrate SEP and EBP engagement with scaffold modification to reinforce 5-HT<sub>2A</sub>R activation while limiting 5-HT<sub>2B</sub>R activity. Cryo-EM structures of 5-HT<sub>2A</sub>R and 5-HT<sub>2B</sub>R bound to IHCH-2330, a selective 5-HT<sub>2A</sub>R agonist and 5-HT<sub>2B</sub>R antagonist, confirm the design principle. These findings show differential activation mechanisms between 5-HT<sub>2A</sub>R and 5-HT<sub>2B</sub>R and provide a rational framework for engineering safer, functionally selective serotonergic compounds, mitigating 5-HT<sub>2B</sub>R-mediated side effects while preserving therapeutic efficacy.
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