Noribogaine, the main human metabolite of the anti-addictive substance ibogaine, reaches brain concentrations up to 20 μM after a therapeutic dose. Binding experiments and computational simulations indicate it may bind to the orthosteric morphinan site of opioid receptors. Noribogaine is a weak mu opioid receptor antagonist (Ke=20 μM at both G-protein and β-arrestin pathways) but a G-protein biased kappa opioid receptor agonist: 75% as efficacious as dynorphin A at stimulating GDP-GTP exchange (EC50=9 μM) yet only 12% as efficacious at recruiting β-arrestin. It also functionally inhibits dynorphin-induced kappa β-arrestin recruitment (IC50=1 μM), more potent than its G-protein agonism.
Noribogaine, the primary metabolite of the anti-addictive substance ibogaine, modulates opioid receptors in ways that may explain its therapeutic effects. At mu-opioid receptors, noribogaine acts as a moderately potent antagonist of both G-protein and β-arrestin signaling pathways. At kappa-opioid receptors, it is a partial agonist of the G-protein pathway, activating at 75% the maximal efficacy of Dynorphin A with a potency of 9 µM, while poorly activating the β-arrestin pathway. Noribogaine functionally inhibits Dynorphin A-induced β-arrestin recruitment at physiologically relevant concentrations, with an IC50 of 1.45 µM. Computational simulations suggest noribogaine binds to the orthosteric morphinan binding site.