Scientists design non-hallucinogenic psychedelic treatments potentially accelerating research on mental health benefits

By Tarun Sai LomteReviewed by Danielle Ellis, B.Sc.Dec 19 2023

In a recent study published in Nature Communications, researchers identified 5-hydroxytryptamine 2A (5-HT_2A) receptor (5-HT_2AR) signaling pathways associated with psychedelic potential.

Background

Interest in classical or serotonergic psychedelics has resurged, given their potential to induce rapid, sustained therapeutic effects. Psychedelics are limited by their hallucinogenic effects and can cause confusion and anxiety. Although a recent preclinical study suggested the possibility of disentangling psychedelics from their therapeutic properties, it is unclear which signaling pathways and receptors mediate psychedelic effects.

Serotonergic psychedelics are derivatives of different chemical scaffolds, including lysergamides, tryptamines, and phenethylamines, which all activate the G protein-coupled receptor (GPCR), 5-HT_2AR. GPCR-targeting ligands stabilize some conformations of the receptor, energetically favoring coupling to transducer proteins.

Thus, ligand-dependent bias has implications for clinical pharmacology and drug development. Current psychedelics activate both β-arrestin2 and Gq via 5-HT_2AR; however, the contribution of these pathways in mediating psychedelic effects is uncertain. Although there are non-psychedelic agonists of 5-HT_2AR, there is no explanation for the lack of psychedelic action.

The study and findings

In the present study, researchers developed 5-HT_2AR-biased agonists to unravel a molecular and mechanistic explanation for biased 5-HT_2AR agonism. First, they showed that 5-HT_2AR strongly coupled to arrestin2, Gq/11, G protein subtypes, and β-arrestin 1. Next, classical psychedelics were tested for their effects on Gq dissociation and β-arrestin 2 recruitment.

Psychedelics exhibited dynamic, time-dependent activity profiles, which, in some cases, exceeded the activity of the endogenous agonist, 5-HT, at longer time points. The effects of all tested psychedelics on β-arrestin and Gq activity were comparable at equivalent time points, mirroring the endogenous agonist. Moreover, all psychedelics did not have a preference for β-arrestin 2 or Gq, indicating that they were not biased for either transducer.

Next, the team aimed to develop biased agonists to interrogate 5-HT_2AR-coupled signaling pathways with psychedelic potential. They focused on the phenethylamine scaffold and selected 25N as the core. The researchers confirmed it was a potent, high-affinity 5-HT₂ agonist with weak selectivity for 5-HT_2CR and 5-HT_2AR over 5-HT_2BR.

N-benzylation of 25N generated 25N-NB, which substantially decreased 5-HT_2AR efficacy, but 25N-NB retained potent 5-HT_2CR activity. Further, the team synthesized multiple analogs of 25N-NB to modify the electrostatic properties of the N-benzyl ring. They observed that increasing electron density around the C_5’ position of the ring increased 5-HT_2AR affinity and agonist potency. This led to the discovery of various high-affinity 5-HT_2AR agonists.

Next, they evaluated the effects of substitutions at the N-benzyl 2-position on 5-HT_2AR selectivity. Bulkier/larger 2-iodo or 2-bromo substituent on the ring decreased 5-HT_2BR and 5-HT_2CR activities but retained 5-potent HT_2AR activity, increasing 5-HT_2AR selectivity. 25N-NBI was the most selective 5-HT_2AR agonist from this series, and several assays confirmed its high selectivity. Besides, 25N-NBI induced a head-twitch response (HTR) in mice, confirming its psychedelic potential.

Nevertheless, 25N-NBI lacked preference for Gq or β-arrestin 2 activity. Thus, the team focused on 25N-NB-2-OH-3-Me, another 25N analogy with the highest affinity for 5-HT_2AR. This compound showed a selective decrease in Gq-efficacy but not β-arrestin 2 efficacy, suggesting that steric effects around the 3-position on the ring could influence 5-HT_2AR-biased agonism.

Additionally, the team explored whether Gq signaling could be further reduced by replacing the N-benzyl ring with the N-biphenyl (25N-NBPh) or N-naphthyl (25N-N1-Nap) ring. Interestingly, both 25N-NBPh and 25N-N1-Nap substantially reduced Gq-efficacy but preserved β-arrestin 2 efficacy. These analogs had substantially weaker Gq and β-arrestin 2 activities at 5-HT_2BR and 5-HT_2CR.

Besides, binding assays indicated these compounds had weak affinities for other 5-HT receptors and off-targets. Further analyses showed that β-arrestin 2 -biased agonists in the 25N series (25N-NB-2-OH-3-Me, 25N-NBPh, and 25N-N1-Nap) failed to induce HTR in mice. Additional experiments indicated that a threshold level of Gq activation was required for psychedelic-like effects and that 5-HT_2AR-Gq signaling was necessary for HTR and psychedelic potential.

Next, the team probed whether 25N-NBPh and 25N-N1-Nap induce β-arrestin 2 -dependent receptor internalization. Both compounds induced potent internalization. By contrast, pimavanserin, a 5-HT_2AR antagonist/inverse agonist, failed to induce receptor internalization. Moreover, 25N-N1-Nap induced tachyphylaxis after repeated administration.

Conclusions

In sum, the researchers showed that a rational, structure-based design can help develop 5-HT_2AR-selective compounds. They revealed that various structural and chemical properties of psychedelics could be leveraged to refine 5-HT_2AR activity. The ability of 5-HT_2AR agonists to induce HTR correlated with Gq-activation but not with β-arrestin 2 recruitment.

Although β-arrestin 2 -biased agonists failed to induce psychedelic-like behavioral effects, they blocked psychedelic-like behaviors in vivo. Notably, the findings suggested that a certain Gq-efficacy threshold was needed for psychedelic-like effects, which could predict the psychedelic potential. Overall, the results have implications for understanding the neurobiological basis of psychedelic effects.