Translocation of the 5-alkoxy substituent of 2,5-dialkoxyarylalkylamines to the 6-position: effects on 5-HT(2A/2C) receptor affinity

Serotonin 2C receptors are also important in head-twitch responses in male mice

RATIONALE

Serotonergic psychedelics exert their effects via their high affinity for serotonin (5-HT) receptors, particularly through activating 5-HT2A receptors (5-HT2AR), employing the frontal cortex-dependent head-twitch response (HTR). Although universally believed to be so, studies have not yet fully ascertained whether 5-HT2AR activation is the sole initiator of these psychedelic effects. This is because not all 5-HT2AR agonists exhibit similar pharmacologic properties.

OBJECTIVE

This study aims to identify and discriminate the roles of 5-HT2AR and 5-HT2CR in the HTR induced by Methallylescaline (MAL) and 4-Methyl-2,5,β-trimethoxyphenethylamine (BOD) in male mice. Also, an analysis of their potential neurotoxic properties was evaluated.

METHODS

Male mice treated with MAL and BOD were evaluated in different behavioral paradigms targeting HTR and neurotoxicity effects. Drug affinity, pharmacological blocking, and molecular analysis were also conducted to support the behavioral findings. The HTR induced by DOI has been extensively characterized in male mice, making it a good positive control for this study, specifically for comparing the pharmacological effects of our test compounds.

RESULTS

The activation of 5-HT2CR, alone or in concert with 5-HT2AR, produces a comparable degree of HTRs (at a dose of 1 mg·kg-1), with divergent 5-HT2CR- and 5-HT2AR-Gqα11-mediated signaling and enhanced neurotoxic properties (at a dose of 30 mg·kg-1) coupled with activated pro-inflammatory cytokines. These findings show these compounds' potential psychedelic and neurotoxic effects in male mice.

CONCLUSION

These findings showed that while 5-HT2AR is the main initiator of HTR, the 5-HT2CR also has a distinct property that renders it effective in inducing HTR in male mice.

Receptor Interaction Profiles of 4-Alkoxy-3,5-Dimethoxy-Phenethylamines (Mescaline Derivatives) and Related Amphetamines

3,4,5-Trimethoxyphenethylamine (mescaline) is a psychedelic alkaloid found in peyote cactus. Related 4-alkoxy-3,5-dimethoxy-substituted phenethylamines (scalines) and amphetamines (3C-scalines) are reported to induce similarly potent psychedelic effects and are therefore potential novel therapeutics for psychedelic-assisted therapy. Herein, several pharmacologically uninvestigated scalines and 3C-scalines were examined at key monoamine targets in vitro. Binding affinity at human serotonergic 5-HT_1A, 5-HT_2A, and 5-HT_2C, adrenergic α_1A and α_2A, and dopaminergic D₂ receptors, rat and mouse trace amine-associated receptor 1 (TAAR1), and human monoamine transporters were assessed using target specific transfected cells. Furthermore, activation of human 5-HT_2A and 5-HT_2B receptors, and TAAR1 was examined. Generally, scalines and 3C-scalines bound with weak to moderately high affinity to the 5-HT_2A receptor (K_i = 150–12,000 nM). 3C-scalines showed a marginal preference for the 5-HT_2A vs the 5-HT_2C and 5-HT_1A receptors whereas no preference was observed for the scalines. Extending the 4-alkoxy substituent increased 5-HT_2A and 5-HT_2C receptors binding affinities, and enhanced activation potency and efficacy at the 5-HT_2A but not at the 5-HT_2B receptor. Introduction of fluorinated 4-alkoxy substituents generally increased 5-HT_2A and 5-HT_2C receptors binding affinities and increased the activation potency and efficacy at the 5-HT_2A and 5-HT_2B receptors. Overall, no potent affinity was observed at non-serotonergic targets. As observed for other psychedelics, scalines and 3C-scalines interacted with the 5-HT_2A and 5-HT_2C receptors and bound with higher affinities (up to 63-fold and 34-fold increase, respectively) when compared to mescaline.

Receptor Interaction Profiles of 4-Alkoxy-Substituted 2,5-Dimethoxyphenethylamines and Related Amphetamines

Background: 2,4,5-Trimethoxyamphetamine (TMA-2) is a potent psychedelic compound. Structurally related 4-alkyloxy-substituted 2,5-dimethoxyamphetamines and phenethylamine congeners (2C-O derivatives) have been described but their pharmacology is mostly undefined. Therefore, we examined receptor binding and activation profiles of these derivatives at monoamine receptors and transporters.

Methods: Receptor binding affinities were determined at the serotonergic 5-HT_1A, 5-HT_2A, and 5-HT_2C receptors, trace amine-associated receptor 1 (TAAR1), adrenergic α₁ and α₂ receptors, dopaminergic D₂ receptor, and at monoamine transporters, using target-transfected cells. Additionally, activation of 5-HT_2A and 5-HT_2B receptors and TAAR1 was determined. Furthermore, we assessed monoamine transporter inhibition.

Results: Both the phenethylamine and amphetamine derivatives (K_i = 8–1700 nM and 61–4400 nM, respectively) bound with moderate to high affinities to the 5-HT_2A receptor with preference over the 5-HT_1A and 5-HT_2C receptors (5-HT_2A/5-HT_1A = 1.4–333 and 5-HT_2A/5-HT_2C = 2.1–14, respectively). Extending the 4-alkoxy-group generally increased binding affinities at 5-HT_2A and 5-HT_2C receptors but showed mixed effects in terms of activation potency and efficacy at these receptors. Introduction of a terminal fluorine atom into the 4-ethoxy substituent by trend decreased, and with progressive fluorination increased affinities at the 5-HT_2A and 5-HT_2C receptors. Little or no effect was observed at the 5-HT_1A receptor for any of the substances tested (K_i ≥ 2700 nM). Phenethylamines bound more strongly to the TAAR1 (K_i = 21–3300 nM) compared with their amphetamine analogs (K_i = 630–3100 nM).

Conclusion: As seen with earlier series investigated, the 4-alkyloxy-substituted 2,5-dimethoxyamphetamines and phenethylamines share some trends with the many other phenethylamine pharmacophore containing compounds, such as when increasing the size of the 4-substituent and increasing the lipophilicity, the affinities at the 5-HT_2A/C subtype also increase, and only weak 5-HT_2A/C subtype selectivities were achieved. At least from the binding data available (i.e., high affinity binding at the 5-HT_2A receptor) one may predict mainly psychedelic-like effects in humans, at least for some of the compound investigated herein.

Chemistry and Structure-Activity Relationships of Psychedelics

This chapter will summarize structure-activity relationships (SAR) that are known for the classic serotonergic hallucinogens (aka psychedelics), focusing on the three chemical types: tryptamines, ergolines, and phenethylamines. In the brain, the serotonin 5-HT_2A receptor plays a key role in regulation of cortical function and cognition, and also appears to be the principal target for hallucinogenic/psychedelic drugs such as LSD. It is one of the most extensively studied of the 14 known types of serotonin receptors. Important structural features will be identified for activity and, where possible, those that the psychedelics have in common will be discussed. Because activation of the 5-HT_2A receptor is the principal mechanism of action for psychedelics, compounds with 5-HT_2A agonist activity generally are quickly discarded by the pharmaceutical industry. Thus, most of the research on psychedelics can be related to activation of 5-HT_2A receptors. Therefore, much of the discussion will include not only clinical or anecdotal studies, but also will consider data from animal models as well as a certain amount of molecular pharmacology where it is known.

Some new psychoactive substances: precursor chemicals and synthesis-driven end-products

This paper describes some of the new classes of 'designer drugs' being encountered today by forensic scientists and law enforcement agencies in Europe, the United States, and Australia. In particular, it concentrates on new cathinone derivatives, the tryptamines, new-generation phenethylamines, and some of the synthetic cannabinoids. The synthetic approaches towards many of these designer drugs including a discussion of the chemical precursors used in the syntheses are presented. Many of today's so-called designer drugs exist as a result of legitimate research into medical conditions and the natural product chemistry. A link between synthetic approaches published in the open scientific and medical literature and the exploitation of this research by clandestine manufacture of drugs for illicit purposes is drawn.

Versatile strategy to access fully functionalized benzodifurans: redox-active chromophores for the construction of extended pi-conjugated materials

An efficient synthetic approach to construct a fully substituted benzo[1,2-b:4,5-b']difuran (BDF) 2a via base-catalyzed double annulations is presented. Compound 2a can readily undergo Suzuki, Heck, and Sonogashira coupling reactions to afford in good yields a manifold of extended pi-conjugated BDF derivatives, e.g., with pyridine termini (4-6) and with different spacers. These are highly luminescent materials that undergo two reversible one-electron oxidations. Remarkably, their photophysical and electrochemical properties can be largely tuned by methylation or protonation. Consequently, they can function as pH-dependent fluorescence switches. Finally, the observed electronic properties are explained on the basis of density functional theory.

'Hybrid' benzofuran-benzopyran congeners as rigid analogs of hallucinogenic phenethylamines

Phenylalkylamines that possess conformationally rigidified furanyl moieties in place of alkoxy arene ring substituents have been shown previously to possess the highest affinities and agonist functional potencies at the serotonin 5-HT(2A) receptor among this chemical class. Further, affinity declines when both furanyl rings are expanded to the larger dipyranyl ring system. The present paper reports the synthesis and pharmacological evaluation of a series of 'hybrid' benzofuranyl-benzopyranyl phenylalkylamines to probe further the sizes of the binding pockets within the serotonin 5-HT(2A) agonist binding site. Thus, 4(a-b), 5(a-b), and 6 were prepared as homologs of the parent compound, 8-bromo-1-(2,3,6,7-tetrahydrobenzo[1,2-b:4,5-b']difuran-4-yl)-2-aminopropane 2, and their affinity, functional potency, and intrinsic activity were assessed using cells stably expressing the rat 5-HT(2A) receptor. The behavioral pharmacology of these new analogs was also evaluated in the two-lever drug discrimination paradigm. Although all of the hybrid isomers had similar, nanomolar range receptor affinities, those with the smaller furanyl ring at the arene 2-position (4a-b) displayed a 4- to 15-fold greater functional potency than those with the larger pyranyl ring at that position (5a-b). When the furan ring of the more potent agonist 4b was aromatized to give 6, a receptor affinity similar to the parent difuranyl compound 2 was attained, along with a functional potency equivalent to 2, 4a, and 4b. In drug discrimination experiments using rats trained to discriminate LSD from saline, 4b was more than two times more potent than 5b, with the latter having a potency similar to the classic hallucinogenic amphetamine 1 (DOB).

Assessment of the Roles of Serines 5.43(239) and 5.46(242) for Binding and Potency of Agonist Ligands at the Human Serotonin 5-HT2AReceptor

We assessed the relative importance of two serine residues located near the top of transmembrane helix 5 of the human 5-HT(2A) receptor, comparing the wild type with S5.43(239)A or S5.46(242)A mutations. Using the ergoline lysergic acid diethylamide (LSD), and a series of substituted tryptamine and phenethylamine 5-HT(2A) receptor agonists, we found that Ser5.43(239) is more critical for agonist binding and function than Ser5.46(242). Ser5.43(239) seems to engage oxygen substituents at either the 4- or 5-position of tryptamine ligands and the 5-position of phenylalkylamine ligands. Even when a direct binding interaction cannot occur, our data suggest that Ser5.43(239) is still important for receptor activation. Polar ring-substituted tryptamine ligands also seem to engage Ser5.46(242), but tryptamines lacking such a substituent may adopt an alternate binding orientation that does not engage this residue. Our results are consistent with the role of Ser5.43(239) as a hydrogen bond donor, whereas Ser5.46(242) seems to serve as a hydrogen bond acceptor. These results are consistent with the functional topography and utility of our in silico-activated homology model of the h5-HT(2A) receptor. In addition, being more distal from the absolutely conserved Pro5.50, a strong interaction with Ser5.43(239) may be more effective in straightening the kink in helix 5, a feature that is possibly common to all type A GPCRs that have polar residues at position 5.43.

Synthesis of Tricyclic Heterocycles via a Tandem Aryl Alkylation/Heck Coupling Sequence

A norbornene-mediated palladium-catalyzed sequence is described in which two alkyl-aryl bonds and one alkenyl-aryl bond are formed in one pot with use of microwave irradiation. A variety of symmetrical and unsymmetrical oxygen-, nitrogen-, silicon-, and sulfur-containing tricyclic heterocycles were synthesized from a Heck acceptor and an aryl iodide containing two tethered alkyl halides. This approach was further applied to the synthesis of a tricyclic mescaline analogue.

Molecular interaction of serotonin 5-HT2A receptor residues Phe339(6.51) and Phe340(6.52) with superpotent N-benzyl phenethylamine agonists

Experiments were conducted to examine the molecular basis for the high affinity and potency of a new class of 5-HT(2A) receptor agonists, N-benzyl phenethylamines. Competition binding assays at several serotonin receptors confirmed that an N-arylmethyl substitution was necessary for affinity increases up to 300-fold over simple N-alkyl homologs, as well as enhanced selectivity for 5-HT(2A) versus 5-HT(2C) and 5-HT(1A) receptors. PI hydrolysis functional assays confirmed that these N-benzyl phenethylamines are potent and highly efficacious agonists at the rat 5-HT(2A) receptor. Virtual docking of these compounds into a human 5-HT(2A) receptor homology model indicated that the N-benzyl moiety might be interacting with Phe339((6.51)), whereas the phenethylamine portion was likely to be interacting with Phe340((6.52)). Experiments in h5-HT(2A) receptors with Phe339((6.51))L and Phe340((6.52))L mutations seem to support this hypothesis. Dramatic detrimental effects on affinity, potency, and intrinsic activity were observed with the Phe339((6.51))L mutation for all N-benzyl analogs, whereas most N-unsubstituted phenethylamines and traditional agonists were only weakly affected, if at all. Consistent with other published studies, the Phe340((6.52))L mutation detrimentally affected affinity, potency, and intrinsic activity of nearly all compounds tested, although a strong change in intrinsic activity was not seen with most N-aryl analogs. These data further validate the topology of our h5-HT(2A) receptor homology model. It is noteworthy that this study is the first to identify a hitherto unrecognized role for residue 6.51 in agonist activation of a serotonin G protein-coupled receptor (GPCR), whereas most previous reports have suggested a varied and sometimes contradictory role in homologous GPCRs.

C-(4,5,6-Trimethoxyindan-1-yl)methanamine: A Mescaline Analogue Designed Using a Homology Model of the 5-HT2A Receptor

A conformationally restricted analogue of mescaline, C-(4,5,6-trimethoxyindan-1-yl)-methanamine, was designed using a 5-HT(2A) receptor homology model. The compound possessed 3-fold higher affinity and potency than and efficacy equal to that of mescaline at the 5-HT(2A) receptor. The new analogue substituted fully for LSD in drug discrimination studies and was 5-fold more potent than mescaline. Resolution of this analogue into its enantiomers corroborated the docking experiments, showing the R-(+) isomer to have higher affinity and potency and to have efficacy similar to that of mescaline at the 5-HT(2A) receptor.