Receptor binding profile suggests multiple mechanisms of action are responsible for ibogaine’s putative anti-addictive activity

Indolealkylamines include centrally active compounds that can produce stimulant and anxiogenic effects. One such derivative, ibogaine, has been reported to reduce behaviours related to opiate, stimulant and alcohol dependence in animal and some human observations. Previous animal studies found that ibogaine decreased morphine self-administration, attenuated morphine- and cocaine-induced dopamine turnover, reduced morphine-induced motor activity, and antagonised cocaine-induced locomotion. These findings pointed to several neurotransmitter systems — bioaminergic (dopamine/serotonin), peptidergic (opioid), and amino acidergic (GABA) — as candidate molecular targets, but no single, clearly defined mechanism has been established that would explain ibogaine’s putative anti-addictive effects. This study set out to characterise ibogaine’s in vitro pharmacological profile across a broad panel of targets. Using over 50 radioligand binding assays, the investigators aimed to identify receptor, transporter, and ion‑channel interactions and to assess potency where activity was detected. The goal was to determine whether ibogaine acts selectively at one target or interacts with multiple sites at concentrations plausibly achieved in brain, thereby informing hypotheses about its mechanisms in addiction models and guiding subsequent medicinal chemistry efforts.

Radioligand competition binding assays were used to survey ibogaine’s interactions with more than 50 neurotransmitter receptors, transporters, ion channels and selected second‑messenger sites. Assays were performed in 250 µl volumes, typically containing receptor preparation, radioligand and cold ligand or test compound; ibogaine and other compounds were solubilised in DMSO and diluted to a final DMSO concentration of 0.4% in the assay. Reactions were terminated by rapid filtration onto glass fibre filters, washed with cold buffer, and radioactivity determined by liquid scintillation or gamma spectrometry. Nonspecific binding was defined in the presence of saturating concentrations of cold ligand. Initial screens were run in duplicate; activity greater than 30% inhibition at a 10 µM screening concentration prompted follow‑up concentration–response studies. Full concentration–response curves were performed in triplicate on three separate days using different tissue preparations; 14‑point reference curves, total and nonspecific binding tubes, and positive control tubes accompanied each assay run. Where feasible, concentration–response curves were verified independently by another laboratory. Apparent Ki values were calculated using the Cheng–Prusoff relationship and KD values established at NovaScreen. Data fitting used an IC50 programme proprietary to NovaScreen or a non‑linear curve‑fitting package from Lundon Associates. Specific assay conditions (tissue sources, buffers, temperatures and incubation times) were referenced in tables in the original text but are not fully reproduced in the extracted material.

Sweetnam and colleagues found that ibogaine displayed inhibitory activity at multiple targets in the low micromolar range (approximately 1–100 µM), with no high‑affinity (nanomolar) interactions detected. The compound displaced the dopamine uptake site ligand [3H]WIN‑35,248 (a putative cocaine receptor) with an apparent IC50 of 3.5 ± 0.6 µM. At related monoamine transporters ibogaine was less potent: serotonin uptake IC50 ≈ 49 ± 3.2 µM and norepinephrine uptake IC50 ≈ 15 ± 4.4 µM. Several serotonergic receptors were affected. Ibogaine inhibited 5‑HT2 receptor binding ([3H]ketanserin; reported Kd ≈ 4.8 ± 1.4 µM) and 5‑HT3 receptor binding ([3H]GR‑65630; IC50 ≈ 3.9 ± 1.1 µM). No inhibition at the 5‑HT1 receptor was observed up to 1 mM. Dopaminergic post‑synaptic receptor subtypes D1, D2, D3 and D4 showed no meaningful inhibition, although a partial (~30%) reduction in [3H]clozapine binding was seen at 100 µM. At opioid sites, ibogaine displaced the κ‑selective ligand [3H]U‑69593 with an apparent IC50 of 16 ± 2.1 µM and had weaker activity at the μ subtype (reported IC50 ≈ 26 µM). Binding at the site labelled by [3H]DTG (historically referred to as a sigma/opiate site) was inhibited with an apparent IC50 of 38 ± 4.0 µM. Ion‑channel and related findings included inhibition of [3H]batrachotoxin B (BTX‑B) binding to the voltage‑gated sodium channel (IC50 ≈ 9 ± 3.0 µM). For NMDA‑associated sites ibogaine inhibited [3H]MK‑801 binding with an IC50 of 5.6 ± 0.8 µM and [3H]TCP binding with an IC50 of 50.5 ± 11 µM, indicating interaction with the NMDA ion channel region rather than the classical competitive glutamate sites. No activity was detected at the glycine (strychnine‑sensitive) receptor, GABAA receptor sites (including benzodiazepine and chloride channel sites), GABAB receptor, GABA uptake site, peripheral benzodiazepine site, or the classical excitatory amino‑acid (AMPA, kainate, NMDA competitive) binding sites. The compound also displaced muscarinic ligands consistent with weak M1 (IC50 ≈ 7.6 ± 0.7 µM) and M2 (IC50 ≈ 5.9 ± 1.4 µM) activity, and inhibited α1‑adrenergic ([3H]prazosin) binding with IC50 ≈ 7.2 ± 3 µM. Many other receptor classes and second‑messenger binding sites tested showed no inhibitory activity in these assays. Overall, the detected interactions clustered in the micromolar concentration range and were reproducible across multiple assay runs.

Sweetnam and colleagues interpret these results as evidence that ibogaine is a relatively non‑selective CNS agent that interacts with multiple receptor, transporter and ion‑channel targets at low micromolar concentrations. The investigators identified 14 distinct sites with measurable inhibition, spanning opioid subtypes, monoamine uptake sites, serotonergic receptors, muscarinic and α1 adrenergic receptors, sodium channels and the NMDA ion channel region. Because no nanomolar‑affinity targets were found, the authors emphasise that a multiplicity of modest‑affinity interactions could underlie ibogaine’s complex behavioural effects. Pharmacokinetic considerations are invoked to argue for potential in vivo relevance: preliminary data cited by the authors indicate that a 10 mg/kg subcutaneous dose produced brain tissue levels on the order of a few micromolar, suggesting that the in vitro micromolar activities documented here could be achieved in vivo. The discussion therefore considers how different interactions might contribute to reported anti‑addictive, locomotor and tremorogenic effects. For example, inhibition of the dopamine uptake site and modulation of 5‑HT3 receptors are proposed as indirect routes to alter dopaminergic synaptic function relevant to cocaine and opioid behaviours, whereas sodium‑channel blockade is offered as a plausible mechanism for ibogaine’s tremorogenic effects and for altering dopamine release/uptake dynamics. The authors also note potential safety concerns and unresolved questions. Interaction with the NMDA ion channel is highlighted because non‑competitive NMDA antagonists have been linked to psychotropic and neurodegenerative outcomes; the paper cites reports of cerebellar Purkinje cell degeneration after ibogaine and suggests further neurotoxicity studies are warranted. Discrepancies between the present binding profile and some prior studies are acknowledged and attributed primarily to methodological differences such as choice of receptor preparation or radioligand. Finally, the investigators propose alternative explanations for ibogaine’s in vivo actions: the observed effects might arise from (a) coordinated actions across multiple modest‑affinity targets, (b) an active, more selective metabolite not assessed here, or (c) an unidentified mechanism beyond the capabilities of the applied in vitro assays. They recommend medicinal‑chemistry efforts to generate more potent and selective ibogaine analogues and suggest using the present receptor profile as a template to guide such work.