5-HT2A/5-HT2C Receptor Pharmacology and Intrinsic Clearance of N-Benzylphenethylamines Modified at the Primary Site of Metabolism

Serotonin 2A Receptor (5-HT2AR) Agonists: Psychedelics and Non-Hallucinogenic Analogues as Emerging Antidepressants

Psychedelics make up a group of psychoactive compounds that induce hallucinogenic effects by activating the serotonin 2A receptor (5-HT2AR). Clinical trials have demonstrated the traditional psychedelic substances like psilocybin as a class of rapid-acting and long-lasting antidepressants. However, there is a pressing need for rationally designed 5-HT2AR agonists that possess optimal pharmacological profiles in order to fully reveal the therapeutic potential of these agonists and identify safer drug candidates devoid of hallucinogenic effects. This Perspective provides an overview of the structure-activity relationships of existing 5-HT2AR agonists based on their chemical classifications and discusses recent advancements in understanding their molecular pharmacology at a structural level. The encouraging clinical outcomes of psychedelics in depression treatment have sparked drug discovery endeavors aimed at developing novel 5-HT2AR agonists with improved subtype selectivity and signaling bias properties, which could serve as safer and potentially nonhallucinogenic antidepressants. These efforts can be significantly expedited through the utilization of structure-based methods and functional selectivity-directed screening.

Structure-Activity Assessment and In-Depth Analysis of Biased Agonism in a Set of Phenylalkylamine 5-HT2A Receptor Agonists

Serotonergic psychedelics are described to have activation of the serotonin 2A receptor (5-HT2A) as their main pharmacological action. Despite their relevance, the molecular mechanisms underlying the psychedelic effects induced by certain 5-HT2A agonists remain elusive. One of the proposed hypotheses is the occurrence of biased agonism, defined as the preferential activation of certain signaling pathways over others. This study comparatively monitored the efficiency of a diverse panel of 4-position-substituted (and N-benzyl-derived) phenylalkylamines to induce recruitment of β-arrestin2 (βarr2) or miniGαq to the 5-HT2A, allowing us to assess structure-activity relationships and biased agonism. All test compounds exhibited agonist properties with a relatively large range of both EC50 and Emax values. Interestingly, the lipophilicity of the 2C-X phenethylamines was correlated with their efficacy in both assays but yielded a stronger correlation in the miniGαq- than in the βarr2-assay. Molecular docking suggested that accommodation of the 4-substituent of the 2C-X analogues in a hydrophobic pocket between transmembrane helices 4 and 5 of 5-HT2A may contribute to this differential effect. Aside from previously used standard conditions (lysergic acid diethylamide (LSD) as a reference agonist and a 2 h activation profile to assess a compound's activity), serotonin was included as a second reference agonist, and the compounds' activities were also assessed using the first 30 min of the activation profile. Under all assessed circumstances, the qualitative structure-activity relationships remained unchanged. Furthermore, the use of two reference agonists allowed for the estimation of both "benchmark bias" (relative to LSD) and "physiology bias" (relative to serotonin).

New Psychoactive Substances: Major Groups, Laboratory Testing Challenges, Public Health Concerns, and Community-Based Solutions

Across communities worldwide, various new psychoactive substances (NPSs) continue to emerge, which worsens the challenges to global mental health, drug rules, and public health risks, as well as combats their usage. Specifically, the vast number of NPSs that are currently available, coupled with the rate at which new ones emerge worldwide, increasingly challenges both forensic and clinical testing strategies. The well-established NPS detection techniques include immunoassays, colorimetric tests, mass spectrometric techniques, chromatographic techniques, and hyphenated types. Nonetheless, mitigating drug abuse and NPS usage is achievable through extensive community-based initiatives, with increased focus on harm reduction. Clinically validated and reliable testing of NPS from human samples, along with community-driven solution, such as harm reduction, will be of great importance, especially in combating their prevalence and the use of other illicit synthetic substances. There is a need for continued literature synthesis to reiterate the importance of NPS, given the continuous emergence of illicit substances in the recent years. All these are discussed in this overview, as we performed another look into NPS, from differentiating the major groups and identifying with laboratory testing challenges to community-based initiatives.

2-Phenethylamines in Medicinal Chemistry: A Review

A concise review covering updated presence and role of 2-phenethylamines in medicinal chemistry is presented. Open-chain, flexible alicyclic amine derivatives of this motif are enumerated in key therapeutic targets, listing medicinal chemistry hits and appealing screening compounds. Latest reports in discovering new bioactive 2-phenethylamines by research groups are covered too.

In vitro assays for the functional characterization of (psychedelic) substances at the serotonin receptor 5-HT2A R

Serotonergic psychedelics are substances that induce alterations in mood, perception, and thought, and have the activation of serotonin (5-HT) 2A receptors (5-HT_2A Rs) as a main pharmacological mechanism. Besides their appearance on the (illicit) drug market, e.g. as new psychoactive substances, their potential therapeutic application is increasingly explored. This group of substances demonstrates a broad structural variety, leading to insufficiently described structure-activity relationships, hence illustrating the need for better functional characterization. This review therefore elaborates on the in vitro molecular techniques that have been used the most abundantly for the characterization of (psychedelic) 5-HT_2A R agonists. More specifically, this review covers assays to monitor the canonical G protein signaling pathway (e.g. measuring G protein recruitment/activation, inositol phosphate accumulation, or Ca²⁺ mobilization), assays to monitor non-canonical G protein signaling (such as arachidonic acid release), assays to monitor β-arrestin recruitment or signaling, and assays to monitor receptor conformational changes. In particular, focus lies on the mechanism behind the techniques, and the specific advantages and challenges that are associated with these. Additionally, several variables are discussed that one should consider when attempting to compare functional outcomes from different studies, both linked to the specific assay mechanism and linked to its specific execution, as these may heavily impact the assay outcome.

A quantitative method for the selective 5-HT2A agonist 25CN-NBOH in rat plasma and brain

In recent years, agonists of the 5-HT_2A receptor have gained increasing attention for their potential therapeutic use to treat psychological disorders such as anxiety and depression. Here, we report the development and validation of an LC-MSMS based analytical method for the quantification of the novel selective 5-HT_2A agonist 25CN-NBOH in rat plasma and brain. As simple and efficient sample clean-up we applied the Phree Phospholipid Removal approach from Phenomenex, which is particularly novel for brain samples. In order to investigate the metabolic stability of 25CN-NBOH in vitro biotransformation studies with recombinant enzymes and human liver microsomes were conducted. Several biotransformation products and pathways could be identified. Based on the in vitro study one of the putative metabolites (2C-CN) was included in the analytical method development. To test the methods applicability 25CN-NBOH was quantified in plasma and brain samples from a pharmacokinetic in vivo study with Wildtype Long Evans rats. Both the in vitro metabolism data as well as the in vivo PK data suggest that 25CN-NBOH is susceptible to metabolism, but is degraded slower and is more stable compared to other NBOMe's investigated to date. The developed analytical method might serve as basis to include further 25CN-NBOH metabolites. It is expected to facilitate further preclinical and clinical investigations of 25CN-NBOH in biological matrices.

Serotonin 2A Receptor (5-HT2AR) Activation by 25H-NBOMe Positional Isomers: In Vitro Functional Evaluation and Molecular Docking

Serotonergic psychedelics are defined as compounds having serotonin 2A receptor (5-HT_2AR) activation as an important pharmacological mechanism. These compounds include the phenylalkylamine class, containing substances with e.g. 2C-X structures (phenethylamines) or their N-methoxybenzyl analogues (NBOMes). Besides their abuse potential, psychedelics are increasingly recognized for having therapeutic benefits. However, many psychedelics remain incompletely characterized, even concerning their structure-activity relationships. Here, five positional isomers of 25H-NBOMe, with two methoxy groups on the different positions of the phenyl ring of the phenethylamine moiety, were subjected to split-nanoluciferase assays assessing the in vitro recruitment of cytosolic proteins to the 5-HT_2AR. Furthermore, molecular docking at the 5-HT_2AR allowed estimation of which residues interact with the specific isomers' methoxy groups. Although the optimal substitution pattern of N-unsubstituted phenylalkylamines has been extensively studied, this is the first comparative evaluation of the functional effects of the positioning of the methoxy groups in the phenethylamine moiety of NBOMes.

In vitro structure-activity relationship determination of 30 psychedelic new psychoactive substances by means of β-arrestin 2 recruitment to the serotonin 2A receptor

Serotonergic psychedelics, substances exerting their effects primarily through the serotonin 2A receptor (5-HT_2AR), continue to comprise a substantial portion of reported new psychoactive substances (NPS). The exact mechanisms of action of psychedelics still remain to be elucidated further, and certain pathways remain largely unexplored on a molecular level for this group of compounds. A systematic comparison of substances belonging to different subclasses, monitoring the receptor-proximal β-arrestin 2 recruitment, is lacking. Based on a previously reported in vitro bioassay employing functional complementation of a split nanoluciferase to monitor β-arrestin 2 recruitment to the 5-HT_2AR, we here report on the setup of a stable HEK 293 T cell-based bioassay. Following verification of the performance of this new stable cell system as compared to a system based on transient transfection, the stable expression system was deemed suitable for the pharmacological characterization of psychedelic NPS. Subsequently, it was applied for the in vitro assessment of the structure-activity relationship of a set of 30 substances, representing different subclasses of phenylalkylamine psychedelics, among which 12 phenethylamine derivatives (2C-X), 7 phenylisopropylamines (DOx) and 11 N-benzylderivatives (25X-NB). The resulting potency and efficacy values provide insights into the structure-activity relationship of the tested compounds, overall confirm findings observed with other reported in vitro assays, and even show a significant correlation with estimated common doses. This approach, in which a large series of psychedelic NPS belonging to different subclasses is comparatively tested, using a same assay setup, monitoring a receptor-proximal event, not only gives pharmacological insights, but may also allow prioritization of legal actions related to the most potent -and potentially dangerous- compounds.

Investigation of the 2,5-Dimethoxy Motif in Phenethylamine Serotonin 2A Receptor Agonists

The 2,5-dimethoxyphenethylamine (2,5-PEA) scaffold is recognized as a motif conferring potent agonist activity at the serotonin 2A receptor (5-HT_2AR). The 2,5-dimethoxy motif is present in several classical phenethylamine psychedelics such as 2,4,5- trimethoxyamphetamine (TMA-2), 2,5-dimethoxy-4-methylamphetamine (DOM), 2,5-dimethoxy-4-iodoamphetamine (DOI), 2,5- dimethoxy-4-bromoamphetamine (DOB), 2,5-dimethoxy-4-bromophenethylamine (2C-B), and 2,5-dimethoxy-4-iodophenethylamine (2C-I), and it has previously been suggested that this structural motif is essential for 5-HT_2AR activation. In the present study, we present data that challenges this assumption. The 2- and 5-desmethoxy derivatives of 2C-B and DOB were synthesized, and their pharmacological profiles were evaluated in vitro at 5-HT_2AR and 5-HT_2CR in binding and functional assays and in vivo by assessing their induction of the head-twitch response in mice. Elimination of either the 2- or 5-methoxy group leads to a modest drop in binding affinity and functional potency at 5-HT_2AR and 5-HT_2CR, which was more pronounced upon removal of the 2-methoxy group. However, this trend was not mirrored in vivo, as removal of either methoxy group resulted in significant reduction in the ability of the compounds to induce the head-twitch response in mice. Thus, the 2,5-dimethoxyphenethylamine motif appears to be important for in vivo potency of phenethylamine 5-HT_2AR agonists, but this does not correlate to the relative affinity and potency of the ligands at the recombinant 5-HT_2AR.

DARK Classics in Chemical Neuroscience: NBOMes

N-Benzylphenethylamines, commonly known as NBOMes, are synthetic psychedelic compounds derived from the phenethylamine class of psychedelics (2C-X compounds), which originally have been derived from the naturally occurring alkaloid mescaline. Analogously to their parent compounds and other classical psychedelics, such as psilocybin and lysergic acid diethylamide (LSD), NBOMes are believed to exert their main pharmacological effects through activation of serotonin 2A (5-HT_2A) receptors. Since their introduction as New Psychoactive Substances (NPSs) in 2010, NBOMes have been widely used for recreational purposes; this has resulted in numerous cases of acute toxicity, sometimes with lethal outcomes, leading to the classification of several NBOMes as Schedule I substances in 2013. However, in addition to their recreational use, the NBOMe class has yielded several important biochemical tools, including [¹¹C]Cimbi-36, which is now being used in positron emission tomography (PET) studies of the 5-HT_2A and 5-HT_2C receptors in the mammalian brain, and 25CN-NBOH, one of the most selective 5-HT_2A receptor agonists developed to date. In this Review, the history, chemistry, structure-activity relationships, ADME (absorption, distribution, metabolism, and excretion) properties, and safety profiles of NBOMes will be outlined and discussed.

Nano liquid chromatography-high-resolution mass spectrometry for the identification of metabolites of the two new psychoactive substances N-(ortho-methoxybenzyl)-3,4-dimethoxyamphetamine and N-(ortho-methoxybenzyl)-4-methylmethamphetamine

Among the emerging new psychoactive substances (NPS), compounds carrying an N-ortho-methoxybenzyl substituent, the so-called NBOMes, represented a highly potent group of new hallucinogens. Recently, 3,4-dimethoxyamphetamine (3,4-DMA)-NBOMe and 4-methylmethamphetamine (4-MMA)-NBOMe occurred, but no data on their pharmacokinetics were available. According to other NBOMes, they are expected to be extensively metabolized. For detection and identification of their phase I and II metabolites, nano liquid chromatography coupled to high resolution tandem mass spectrometry (nanoLC-HRMS/MS) was used. Rat urine was prepared by simple dilution and incubation mixtures with pooled human liver S9 fraction by precipitation. Furthermore, the results concerning detectability using the new nanoLC approach were compared to those obtained by conventional ultra-high performance LC (UHPLC). In addition, the detectability of the compounds by standard urine screening approaches (SUSAs) routinely used by the authors with UHPLC-HRMS/MS, LC-MSⁿ, and GC-MS was tested. Both NBOMes were extensively metabolized mainly by O-demethylation and conjugation with glucuronic acid (3,4-DMA-NBOMe) or oxidation of the tolyl group to the corresponding carboxylic acid (4-MMA-NBOMe). The developed nanoLC-HRMS/MS approach was successfully applied for identification of 38 3,4-DMA-NBOMe metabolites and 33 4-MMA-NBOMe metabolites confirming its detection power. Furthermore, the solvent saving nanoLC system showed comparable results to the UHPLC-HRMS/MS approach. In addition, an intake of an estimated low common user's dose of the compounds was detectable by all SUSAs only via their metabolites. Suggested targets for urine screening procedures were O-demethyl- and O,O-bis-demethyl-3,4-DMA-NBOMe and their glucuronides and carboxy-4-MMA-NBOMe and its glucuronide and N-demethyl-carboxy-4-MMA-NBOMe.

Pharmacology and Toxicology of N-Benzylphenethylamine ("NBOMe") Hallucinogens

Serotonergic hallucinogens induce profound changes in perception and cognition. The characteristic effects of hallucinogens are mediated by 5-HT2A receptor activation. One class of hallucinogens are 2,5-dimethoxy-substituted phenethylamines, such as the so-called 2C-X compounds 2,5-dimethoxy-4-bromophenethylamine (2C-B) and 2,5-dimethoxy-4-iodophenethylamine (2C-I). Addition of an N-benzyl group to phenethylamine hallucinogens produces a marked increase in 5-HT2A-binding affinity and hallucinogenic potency. N-benzylphenethylamines ("NBOMes") such as N-(2-methoxybenzyl)-2,5-dimethoxy-4-iodophenethylamine (25I-NBOMe) show subnanomolar affinity for the 5-HT2A receptor and are reportedly highly potent in humans. Several NBOMEs have been available from online vendors since 2010, resulting in numerous cases of toxicity and multiple fatalities. This chapter reviews the structure-activity relationships, behavioral pharmacology, metabolism, and toxicity of members of the NBOMe hallucinogen class. Based on a review of 51 cases of NBOMe toxicity reported in the literature, it appears that rhabdomyolysis is a relatively common complication of severe NBOMe toxicity, an effect that may be linked to NBOMe-induced seizures, hyperthermia, and vasoconstriction.

18F-Labelling of electron rich iodonium ylides: application to the radiosynthesis of potential 5-HT2A receptor PET ligands

Nucleophilic ¹⁸F-labelling of electron aromatic systems.