The selective 5-HT2A receptor agonist 25CN-NBOH: structure-activity relationship, in vivo pharmacology, and in vitro and ex vivo binding characteristics of [3H]25CN-NBOH.

Modifications to specific positions on the 25CN-NBOH molecule, a highly selective 5-HT2A receptor agonist, can retain or reduce its activity. Six new analogs were tested; 3′-methyl and fused-ring variants kept high 5-HT2A receptor activity, while 3′-methoxy and 3′-ethyl versions lost binding and potency. All six analogs showed only partial agonism or antagonism. In mice, 25CN-NBOH and a close analog triggered head-twitch responses (a hallmark of 5-HT2A activation) and reduced marble-burying behavior, suggesting potential benefits for cognitive rigidity disorders. A tritium-labeled version of 25CN-NBOH showed high binding affinity and selectivity for 5-HT2A receptors in rat brain tissue, providing a new tool for future receptor studies.

Study at a glance

Abstract

The remarkable effects exhibited by classical psychedelics in recent clinical trials have spawned considerable interest in 5-HT2A receptor (5-HT2AR) activation as a treatment strategy for several psychiatric/cognitive disorders. In this study we have continued our development of 25CN-NBOH, one of the most 5-HT2AR-selective agonists reported to date, as a pharmacological tool for exploration of 5-HT2AR expression and functions. The importance of the 2' and 3' positions in 25CN-NBOH as structural hotspots for its 5-HT2AR activity was investigated by synthesis and characterization of six novel analogs at 5-HT2AR and 5-HT2CR in binding and functional assays. While the 5-HT2AR activity of 25CN-NBOH was retained in 3'-methyl, 2',3'-chroman, 2',3'-dihydrofuran and 2',3'-furan analogs, the 3'-methoxy and 3'-ethyl analogs displayed substantially lower binding affinities and agonist potencies than 25CN-NBOH. Interestingly, the 2',3'-substitution pattern was also a key determinant of agonist efficacy, as all six analogs exhibited pronounced partial agonism or defacto antagonism at the 5-HT2AR in the functional assays. Systemic administration of 25CN-NBOH and its close structural analog 25CN-NBMD induced robust head-twitch response in mice, a well-established behavioural effect of 5-HT2AR activation in vivo, and 25CN-NBOH mediated robust reductions in the activity of mice in an anxiety-related marble burying assay, which supports the proposed beneficial effects of 5-HT2AR activation on disorders characterized by cognitive rigidity. Finally, tritiated 25CN-NBOH exhibited high 5-HT2AR binding affinity (KD ∼1 nM) and selectivity against 5-HT2BR and 5-HT2CR in equilibrium and kinetic binding studies of the recombinant receptors, and in concordance [3H]25CN-NBOH displayed substantial specific, ketanserin-sensitive binding to cortex and small levels of binding to choroid plexus in rat brain slices in autoradiography studies. In conclusion, this work delineates the subtle molecular determinants of 5-HT2AR activity in 25CN-NBOH, substantiates the potential in this compound and its analogs as tools for in vivo studies of 5-HT2AR, and introduces a novel selective agonist radioligand as another valuable tool for future explorations of this receptor.