Novel Bivalent 5-HT _2A Receptor Antagonists Exhibit High Affinity and Potency in Vitro and Efficacy in Vivo

The 5-HT _2A receptor (5-HT _2A R) plays an important role in various neuropsychiatric disorders, including substance use disorder and schizophrenia. Homodimerization of this receptor has been suggested, but tools that allow direct assessment of the relevance of the 5-HT _2A R:5-HT _2A R homodimer in these disorders are necessary. We chemically modified the selective 5-HT _2A R antagonist M100907 to synthesize a series of homobivalent ligands connected by ethylene glycol linkers of varying lengths that may be useful tools for probing 5-HT _2A R:5-HT _2A R homodimer function. We tested these molecules for 5-HT _2A R antagonist activity in a cell line stably expressing the functional 5-HT _2A R and quantified a downstream signaling target, activation (phosphorylation) of extracellular regulated kinases 1/2 (ERK _1/2 ), in comparison to in vivo efficacy of altering spontaneous or cocaine-evoked locomotor activity in rats. All of the synthetic compounds inhibited 5-HT-mediated phosphorylation of ERK _1/2 in the cellular signaling assay; the potency of the bivalent ligands varied as a function of linker length, with the intermediate linker lengths being the most potent. The K _i values for the binding of bivalent ligands to 5-HT _2A R were only slightly lower than the values for the parent (+)-M100907 compound, but significant selectivity for 5-HT _2A R over 5-HT _2B R or 5-HT _2C R binding was retained. In addition, the 11-atom-linked bivalent 5-HT _2A R antagonist (2 mg/kg, intraperitoneally) demonstrated efficacy on par with that of (+)-M100907 in inhibiting cocaine-evoked hyperactivity. As we develop further strategies for ligand-evoked receptor assembly and analyses of diverse signaling and functional roles, these novel homobivalent 5-HT _2A R antagonist ligands will serve as useful in vitro and in vivo probes of 5-HT _2A R structure and function.