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

Serotonergic psychedelics produce their effects largely through activation of the serotonin 2A receptor (5-HT2AR). This chemical family includes diverse scaffolds (ergolines, tryptamines, phenylalkylamines) and both longstanding psychedelics and newer psychoactive substances. Within the phenylalkylamine subgroup, N-benzyl derivatives of 2C-X compounds (NBOMes) have high potency and have been incompletely characterised with respect to structure–activity relationships, despite recent availability of 5-HT2AR structural data that hint at different binding modes for distinct ligands. Pottie and colleagues set out to examine how the positional arrangement of two methoxy groups on the phenethylamine ring of 25H-NBOMe affects functional activation of 5-HT2AR. They compared five positional isomers of 25H-NBOMe using cell-based recruitment assays that monitor cytosolic protein association with the activated receptor, complemented these assays with molecular docking into an adapted cryo-EM 5-HT2AR structure, and aimed to link specific methoxy positions to differential receptor interactions and activation profiles. This is presented as the first direct comparative functional study of methoxy positional isomerism in NBOMes while retaining the N-methoxybenzyl substituent.

The functional experiments used the NanoBiT (split nanoluciferase) system to monitor recruitment of cytosolic proteins to the activated human 5-HT2AR in live HEK293T cells. Two complementary readouts were employed: β-arrestin2 (βarr2) recruitment and a miniGαq recruitment assay, each fused to one half of nanoluciferase while the receptor carried the complementary fragment. Recruitment upon agonist stimulation restores luciferase activity, producing a luminescent signal. HEK293T cells were transfected with plasmids encoding 5-HT2AR and either βarr2 or miniGαq using FuGENE (3:1 FuGENE:DNA). After expression, cells were plated in poly-D-lysine-coated 96-well plates, equilibrated with Nano-Glo substrate and stimulated by addition of concentrated agonist solutions. Luminescence was recorded in real time for up to 2 hours. Concentration–response curves were derived from area under the curve (AUC) measurements calculated either for the first 30 minutes or for the full 2 h activation profile. Each experimental condition was run in duplicate and repeated in at least three independent experiments. Data were normalised to reference agonists (either LSD or serotonin) with the reference set to 100% per plate and fitted using a four-parameter nonlinear regression to obtain EC50 (potency) and Emax (efficacy) values. Molecular docking used an adapted cryo-EM structure of the 5-HT2AR in complex with miniGαq and 25CN‑NBOH (PDB: 6WHA) as the template. Ligand structures (the five NBOMe isomers plus controls) were optimised and docked using induced-fit docking workflows. Complex minimisation employed the OPLS3 force field and Prime MM-GBSA (Molecular Mechanics–Generalized Born Surface Area) calculations were used to estimate interaction energies between ligand moieties (notably methoxy groups) and individual receptor residues. Chemical reagents, compound synthesis and preparation (isomers dissolved in methanol), and cell culture conditions are described, with solvent controls included in all assays.

Pharmacological characterisation in the βarr2 NanoBiT assay showed clear differences in potency (EC50) and more modest differences in efficacy (Emax) across the five 25H-NBOMe positional isomers. Using the authors' standard setup (βarr2 recruitment, LSD as reference, 2 h AUC): 25H‑NBOMe (the conventional 2,5-dimethoxy pattern) had an EC50 of 11.4 nM and an Emax of 164% relative to LSD. 24H‑NBOMe was the most potent isomer (EC50 3.88 nM). 26H‑NBOMe showed similar potency to 25H‑NBOMe (EC50 8.70 nM). 23H‑NBOMe was less potent (EC50 33.6 nM). The two isomers lacking a 2‑methoxy substituent, 34H‑ and 35H‑NBOMe, were substantially less potent, with EC50 values of 248 nM and 343 nM, respectively. Reported efficacies (Emax) were generally high for most isomers: 24H 145%, 26H 156%, 34H 147%, 23H 123%, and 35H 118% (all given relative to LSD set at 100%). LSD itself had an EC50 of 7.43 nM in the same assay. The main potency and efficacy rankings were robust across analytical variations: using serotonin rather than LSD as the reference agonist, using AUC from the first 30 minutes instead of 2 hours, and switching assay readout from βarr2 to miniGαq did not change the relative ordering of isomer potencies or the classification into more-versus-less-efficacious groups. A notable assay-specific observation was that in the miniGαq assay serotonin displayed 2.5–3 fold higher efficacy than LSD; in that assay none of the NBOMes exceeded serotonin in Emax. Overall, the pooled data led to the conclusion that presence of a 2‑methoxy group correlates with higher potency (24H > 26H ≈ 25H > 23H > 34H ≈ 35H), while differences in efficacy were smaller, with 23H and 35H being relatively less efficacious but still approximately on par with LSD. Molecular docking placed all NBOMe isomers in the same orthosteric pocket occupied by 25CN‑NBOH in the template structure. Prime MM-GBSA-derived interaction energies highlighted several consistent interactions: the N‑benzyl methoxy group forms an H‑bond with S159 and hydrophobic contacts with S159, W336 and S373; the ligand amine forms a strong salt bridge with D155 (interaction energies between approximately −18 and −20 kcal/mol). Stabilising hydrophobic and aromatic (pi-stacking) interactions were predicted with W336, F339 and F340. Docking-derived residue interaction patterns suggested that a methoxy at position 2 engages T160, S159 and V156; a methoxy at position 3 enhances contacts with G238 and S242; a 4‑methoxy favours interaction with S239 and V235; the 5‑methoxy strengthens contacts with N343 and L229 (extracellular loop 2); and a 6‑methoxy appears to augment interaction with V156 and D155 and to weakly contact L229. The authors emphasise that these energies are model-derived relative estimates (more negative values imply stronger proposed interactions) and that steric effects from adjacent methoxy substituents likely contribute to the observed trends. The docking model was adapted from a miniGαq-bound state, and no equilibrium binding (affinity) data were obtained in this study to directly corroborate predicted binding strengths.

Pottie and colleagues interpret the data as evidence that the position of methoxy substituents on the phenethylamine ring markedly affects 5-HT2AR activation by NBOMes. The presence of a 2‑methoxy group was associated with substantially higher potency in vitro, with 24H‑NBOMe (2,4‑dimethoxy pattern) the most potent isomer tested and 23H‑NBOMe slightly less potent than the conventional 2,5‑pattern (25H). Isomers lacking the 2‑methoxy (34H and 35H) were much less potent. Efficacy differences were smaller; most isomers produced Emax values equal to or greater than LSD in the βarr2 assay, though they did not exceed serotonin in the miniGαq assay. The authors highlight that these conclusions were stable across assay format, reference agonist and analysis window. The docking results are used to propose mechanistic explanations for these observations: multiple residues in the orthosteric pocket appear to interact differentially with methoxy substituents, and no single residue explains the potency/efficacy differences. Predicted interactions (for example between 2‑methoxy groups and T160/S159/V156) are consistent with the functional prominence of the 2‑substituent. The researchers caution that docking-derived interaction energies are relative and that the model was based on a miniGαq-bound receptor conformation, which may not fully represent βarr2-bound or other physiologically relevant states. Key limitations acknowledged include the absence of direct binding affinity data, which prevents linking potency/efficacy to receptor affinity; the use of artificial cell systems that may not capture in vivo receptor context; and metabolic considerations such as high intrinsic clearance of NBOMes and the known 5‑methoxy metabolic “soft spot”, which could alter in vivo potency. The authors also note broader uncertainties about which molecular events ultimately underlie psychedelic effects (for example biased agonism, receptor dimerisation or engagement of other receptors). Finally, they position this study as the first comparative functional assessment of methoxy positional isomerism in NBOMe compounds and suggest that the findings refine structure–activity understanding for this subclass, while recognising that extrapolation to human in vivo effects remains challenging.