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

The 5-HT receptor (5-HTR) 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-HTR:5-HTR homodimer in these disorders are necessary. We chemically modified the selective 5-HTR 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-HTR:5-HTR homodimer function. We tested these molecules for 5-HTR antagonist activity in a cell line stably expressing the functional 5-HTR and quantified a downstream signaling target, activation (phosphorylation) of extracellular regulated kinases 1/2 (ERK), 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 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 values for the binding of bivalent ligands to 5-HTR were only slightly lower than the values for the parent (+)-M100907 compound, but significant selectivity for 5-HTR over 5-HTR or 5-HTR binding was retained. In addition, the 11-atom-linked bivalent 5-HTR 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-HTR antagonist ligands will serve as useful in vitro and in vivo probes of 5-HTR structure and function.