Structure-based design of a chemical probe set for the 5-HT5A serotonin receptor

The 5-HT5A receptor (5-HT5AR), for which no selective agonists and only a few antagonists exist, remains the least understood serotonin (5-HT) receptor. A single commercial antagonist (SB-699551) has been widely used to investigate central nervous system (CNS) 5-HT5AR function in neurological disorders, including pain. However, because SB-699551 has affinity for many 5-HTRs, lacks inactive property-matched controls, and has assay interference concerns, it has liabilities as a chemical probe. To better illuminate 5-HT5AR function, we developed a probe set through iterative rounds of molecular docking, pharmacological testing, and optimization. Docking over six million lead-like molecules against a 5-HT5AR homology model identified five mid- micromolar ligand starting points with unique scaffolds. Over multiple rounds of structure-based design and testing, a new quinoline scaffold with high affinity and enhanced selectivity for the 5-HT5AR was developed, leading to UCSF678, a 42 nM arrestin-biased partial agonist at the 5-HT5AR with a much more restricted off-target profile and decreased assay liabilities vs. SB-699551. Site-directed mutagenesis supported the docked pose of UCSF678, which was also consistent with recent published 5-HTR structures. Surprisingly, property-matched analogs of UCSF678 that were either inactive across 5-HTRs or retained affinity for UCSF678s off-targets revealed that 5-HT5AR engagement is nonessential for alleviating pain in a mouse model, contrary to previous studies using less-selective ligands. Relative to SB-699551, these molecules constitute a well-characterized and more selective probe set with which to study the function of the 5-HT5A receptor. ### Competing Interest Statement The authors have declared no competing interest.