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

Serotonin (5-HT) signalling through the 5-HT 2 receptor family (5-HT 2A, 5-HT 2B and 5-HT 2C) is centrally implicated in mood, perception and several psychiatric disorders, and these receptors are key pharmacological targets for both therapeutic and psychoactive drugs. Earlier work characterised many 2,4,5-trisubstituted phenethylamines (the 2C class) as high-affinity 5-HT 2A ligands, whereas 3,4,5-trisubstituted compounds (mescaline and related “scalines” and their α‑methyl amphetamine homologues, “3C‑scalines”) have generally been reported to show lower in vitro affinity and weaker human potencies. However, anecdotal and some pharmacological data indicate that specific 4‑position substituents (longer alkoxy chains, benzyloxy, fluorinated alkoxy groups) can markedly increase potency of mescaline derivatives, suggesting incomplete understanding of structure–activity relationships (SAR) across the 3,4,5-series. Kolaczynska and colleagues set out to characterise a range of 4‑alkoxy-3,5-dimethoxy phenethylamines (scalines) and their α‑methyl amphetamine congeners (3C‑scalines) at key monoaminergic targets. The study aimed to measure binding affinities and functional activities at human serotonergic receptors (5-HT 1A, 5-HT 2A, 5-HT 2B, 5-HT 2C), selected adrenergic and dopaminergic receptors, rodent TAAR1 isoforms and human monoamine transporters, to clarify how 4‑substituent modifications (chain extension, fluorination, bulky groups) and α‑methylation influence receptor interactions that are relevant to psychedelic effects and safety.

The compounds examined were synthesised as racemic phenethylamines and their α‑methyl amphetamine homologues and provided as hydrochloride salts; reported purity for all substances was greater than 98%. Human oral dose and duration information were referenced from existing human data where available. Binding and functional assays used recombinant cell systems overexpressing individual human or rodent receptors or transporters. Radioligand displacement assays determined equilibrium binding affinities (K i ) for human 5-HT 1A, 5-HT 2A, 5-HT 2C, α 1A and α 2A adrenergic, D 2 dopaminergic receptors, human monoamine transporters (hSERT, hDAT, hNET) and rat/mouse TAAR1. Membrane preparations were obtained from cell lines (CHO, CHL, HEK293) transfected with the respective targets. Radioligands were used at concentrations equal to their K d and nonspecific binding was defined using excess competitor; IC 50 values were fitted with a one-site model and K i values calculated via the Cheng–Prusoff equation. The authors defined K i < 50 nM as high affinity, K i < 500 nM as moderate affinity and K i > 1,000 nM as low affinity. Functional activity at 5-HT 2A and 5-HT 2B receptors was measured using calcium mobilisation assays (FLIPR) in NIH-3T3 cells expressing human 5-HT 2A and HEK293 cells expressing human 5-HT 2B, respectively; EC 50 values were derived by nonlinear regression and maximal efficacy was expressed relative to 5-HT = 100% (efficacy < 85% = partial agonist, ≥ 85% = full agonist). Human TAAR1 activity was assessed in HEK293 cells with a cAMP FRET-based assay over a concentration range of 300 pM–30 μM, with reference agonists included on each plate. Monoamine transporter interactions were tested in HEK293 cells stably transfected with hSERT, hDAT or hNET. Uptake inhibition was screened at a single high test concentration (10 μM) with transporter-specific radiolabelled substrates; selective inhibitors and vehicle controls were included. Statistical analyses used nonlinear regression for binding and activation curves and one-way ANOVA with Dunnett's test for uptake assays; activation potency and affinity values were reported from at least three independent experiments where applicable.

Binding at 5-HT 1A receptors was generally weak: only a subset of phenethylamines (including MDFM, DFM, TFM, CP, MAL and BZ) showed measurable affinity in the low micromolar range (K i ~1.6–6.7 μM). None of the examined amphetamine (α‑Me) congeners bound detectably to 5-HT 1A at the concentrations tested (K i > 5,600 nM). At 5-HT 2A, several phenethylamines bearing fluorinated or bulky 4‑alkoxy substituents displayed improved affinity relative to mescaline. Notably, TFM, MAL and BZ bound in the submicromolar range (K i ≈150–550 nM), whereas most other compounds bound in the micromolar range (K i ≈1,300–9,400 nM); two derivatives (FE, DFIP) showed very weak or no measurable binding (K i > 12,000 nM). Functional assays showed a mix of partial and full agonism: several compounds acted as partial 5-HT 2A agonists with EC 50 values spanning ~27–10,000 nM and efficacies 44–78%, while DFM, FE, FP, CP and MAL were reported as full agonists (efficacies 85–94%) with EC 50 values in the ~79–5,700 nM range. Most amphetamine homologues also bound in the micromolar range, with a few (including 3C‑E, 3C‑P and some others) reported as full agonists at 5-HT 2A (efficacy 86–102%). Activation of 5-HT 2B was observed for several fluorinated phenethylamines (MDFM, TFM, TFE) with submicromolar EC 50 values (≈88–210 nM) and for some amphetamine derivatives (e.g. 3C‑DFM, 3C‑DFE, 3C‑FE, 3C‑E) with EC 50 ≈95–800 nM; these 5-HT 2B responses were low-efficacy partial agonism (efficacy ~18–45%). Many other compounds did not activate 5-HT 2B at assay concentrations (EC 50 >10,000 nM). Binding at 5-HT 2C was mostly micromolar for the majority of compounds (K i ~1,200–9,900 nM), with the exceptions TFM, MAL and BZ which showed submicromolar 5-HT 2C affinities (K i ≈290–520 nM). DFIP did not bind to 5-HT 2C at the tested concentrations (K i >10,000 nM). Interactions with non-serotonergic targets were modest or absent. None of the tested compounds activated human TAAR1 (EC 50 >10,000 nM) and none bound human D 2 receptors or the monoamine transporters at pharmacologically relevant concentrations (K i and IC 50 values generally >7,500–10,000 nM). By contrast, several phenethylamines and some amphetamines showed measurable binding to rodent TAAR1 isoforms (rat > mouse > human), with a few compounds binding in the submicromolar to micromolar ranges. Adrenergic interactions were weak to moderate: some phenethylamines bound α 2A with K i ≈450–3,700 nM and MDFM/TFM showed very weak α 1A binding (K i ≈3,200–8,000 nM); most amphetamines had little α 1A affinity and only a few bound α 2A at micromolar levels. Structure–activity trends reported in the results included increased 5-HT 2A and 2C affinity following extension of the 4‑alkoxy chain and proportional increases in affinity with increasing fluorination of the 4‑alkyloxy group. Introduction of an α‑methyl (amphetamine) generally produced only small and mixed effects on affinity and activation, with some amphetamine congeners showing modestly greater 5-HT 2A affinity or activation potency compared with their phenethylamine counterparts. Finally, none of the tested compounds significantly inhibited monoamine uptake in the transporter assays at 10 μM.

Kolaczynska and colleagues interpret the in vitro data as demonstrating that many 4‑alkoxy-3,5-dimethoxy phenethylamines (scalines) and several α‑methyl congeners interact primarily with serotonergic receptors, notably 5-HT 2A and 5-HT 2C, which is consistent with the pharmacology of classical psychedelics. The authors highlight that certain 4‑substituents—particularly trifluoromethoxy, methallyloxy and benzyloxy—substantially increased 5-HT 2A affinity (up to ~17–63-fold relative to mescaline) and, for some compounds, raised activation potency and efficacy at 5-HT 2A. Several of these higher-affinity derivatives (for example TFM and MAL) have been reported as active psychedelics in humans, supporting a link between the observed in vitro receptor interactions and clinical potency. The investigators caution that binding affinity alone does not fully predict in vivo psychedelic potency. They note that mescaline itself is a low-affinity partial 5-HT 2A agonist yet produces strong psychedelic effects at high doses in humans, implying that adequate exposure, pharmacokinetics, additional pharmacological targets, or different intracellular signalling pathways may determine clinical activity. In particular, the authors emphasise that Ki values in their study were derived from antagonist-labelled radioligand assays; because agonists can show higher apparent affinity when measured with agonist radioligands, the antagonist-labelling approach complicates direct correlations between K i and psychoactive doses. They also report instances where activation potency (EC 50 in calcium assays) did not parallel binding affinity, and suggest that alternative functional readouts (e.g. IP formation or β-arrestin recruitment) might more closely reflect clinical potency. Safety considerations are discussed: several derivatives showed low-efficacy partial agonism at 5-HT 2B, and the authors acknowledge the theoretical risk of valvular heart disease from chronic 5-HT 2B activation. They reason that because psychedelics are typically used acutely rather than chronically, clinically relevant 5-HT 2B-mediated valvulopathy is unlikely for most recreational or therapeutic patterns of use, but they flag 5-HT 2B activity as an element to monitor in safety evaluations. The lack of significant activity at human TAAR1, D 2 receptors and monoamine transporters suggests that non-serotonergic monoaminergic targets are unlikely to account for the principal psychedelic effects of these compounds. The authors acknowledge limitations including assay-dependent variability (different cell lines and experimental setups yield somewhat different affinity estimates) and the absence of comprehensive functional profiling across multiple signalling pathways. They recommend further investigations that include alternative functional assays, in vivo pharmacokinetics, and safety assessments to better predict human potency and adverse-effect risks. Finally, the results are positioned as expanding SAR knowledge for the 3,4,5-trisubstituted series and identifying several derivatives whose receptor profiles are compatible with psychedelic effects in humans, while underscoring the need for further mechanistic and safety-oriented work.