Cryo‐ <scp>EM</scp> Structures of the Human 5‐ <scp> HT _2B R </scp> Bound to Three Distinct Ligands Reveal Molecular Determinants of Subtype Selectivity

ABSTRACT The 5‐HT 2B R, a member of the G protein‐coupled receptor (GPCR) family, has been implicated in various diseases, including cardiovascular conditions, fibrotic disorders, cancer, and neuropsychiatric illnesses. Despite its therapeutic potential, the 5‐HT 2B R remains largely underexplored due to the limited availability of subtype‐selective ligands. Additionally, many drugs either exhibit off‐target binding to 5‐HT 2B R or fail to achieve specificity for their intended receptor subtype. Here, we present three cryo‐electron microscopy structures of the human 5‐HT 2B R in complex with the antagonist tegaserod, the inverse agonist ritanserin, and the selective antagonist RS127445, respectively. These structures reveal distinct binding modes for each ligand, and through detailed analysis, we identify residues L362 and V366 as key contributors to 5‐HT 2 subtype selectivity, while E363 and the ECL2 region play critical roles in 5‐HT 2B R subtype selectivity. Our findings offer valuable insights into the molecular mechanisms behind ligand selectivity for 5‐HT 2B R, laying the groundwork for the development of 5‐HT 2B R‐selective ligands.